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2.4.6: Dataduik - Beverimpakte op vleilande - Biologie

2.4.6: Dataduik - Beverimpakte op vleilande - Biologie


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Oorsig

'n Ekosisteemingenieur is enige dier wat 'n habitat skep, aansienlik verander, onderhou of vernietig. Figuur 2.4.6a hieronder vertoon sommige van die resultate in hierdie studie:

Figuur(PageIndex{a}): Gemiddelde aantal spesies waargeneem in monsterpersele 1-2 jaar en 10-12 jaar nadat bevers ingebring is. Grafiek deur Rachel Schleiger (CC-BY-NC) gewysig vanaf data in Wet A, Graywood MJ, Jones KC, Ramsay P, en Willby NJ 2017.

Vrae

  1. Wat is die onafhanklike (verklarende) veranderlike en die afhanklike (respons) veranderlike?
  2. Watter vraag(s) probeer die skrywers met hierdie grafiek beantwoord?
  3. Watter tendens(e) kan in hierdie grafiek tussen die 1-2 en 10-12 roosters waargeneem word? Ondersteun jou antwoord deur na toepaslike patrone in die grafiek te verwys.
  4. Dink jy dat die skrywers tevrede is met die resultate in die grafiek? Hoekom?
  5. Hoe kan die resultate van hierdie grafiek toekomstige herinvoering van bevers inlig waar vleilandherstel nodig is?
  6. Watter inligting/patrone is nie duidelik uit hierdie grafiek nie?

Vegetatiewe ekologie van natuurlike en gekonstrueerde vleilande langs die Leon-rivier in Comanche County, Texas.

Opsomming.--Die plantegroei gevind in twee riviervleilande, geleë langs die Leonrivier in die West Cross Timbers, Comanche County, Texas, is vir een jaar vergelyk. 'n Vleiland wat gedurende 1999 gebou is en 'n aangrensende natuurlike verwysingsvleiland, gevestig deur bever, is vergelyk deur relatiewe bedekking, digtheid, frekwensie en belangrikheidwaardes te gebruik. Vleilandaanwyserstatus vir plante en diversiteit is ook geassesseer. Oor vier monsternemingsperiodes het die gekonstrueerde vleiland opeenvolging ondergaan om soortgelyk aan die aangrensende natuurlike vleiland te word. Gewone katstert (Typha latifolia), 'n verpligte vleilandspesie, het die binnekern van die gekonstrueerde vleiland oorheers met moerasvlei (Iva annua) wat die droër periferie beset het. Vyf-en-negentig persent van die spesies wat in die verwysingsvleiland gemonster is, was hidrofiete en plantopvolging het 'n sentrifugale model gevolg. Die gekonstrueerde vleiland is oorheers deur vlei- en knopgras (Paspalum distichum) met slegs 67% van spesies wat hidrofiete was. Diversiteit was groter in die gekonstrueerde vleiland omdat dit in die aankoms- en vestigingsfase van vleilandopvolging was.

Vleilande word gedefinieer deur die teenwoordigheid van hidrofitiese plantegroei, periodieke oorstroming en hidriese gronde (United States Army Corps of Engineers (USACOE) 1987 Tiner 1999 van der Valk 2006 Mitsch & Gosselink 2007 Keddy 2010). Texas se vleilande is een van sy waardevolste natuurlike hulpbronne. Hierdie lande bied baie ekonomiese en ekologiese voordele, insluitend vloedbeheer, verbeterde watergehalte, oesbare produkte en habitat vir volop vis, skulpvis en wildlewe hulpbronne (Texas Parks and Wildlife Department (TPWD) 1997 van der Valk 2006 Mitsch & Gosselink 2007 King et al. 2009). Texas bevat 'n verskeidenheid vleilandhabitattipes, beide natuurlik en antropogenies. Alhoewel vleilande in Texas minder as 5% van die staat se totale landoppervlak uitmaak (TPWD 1997), is Texas een van 19 state wat die grootste verliese aan vleiland-ekosisteme getoon het (Tiner 1984) met 'n verlies van 56% gedurende die voorafgaande 200 jaar (TPWD 1997). Texas het ook 40 tot 60% van sy varswaterplantgemeenskappe met matige tot hoë risiko om uitgeskakel te word (Heinz Center 2008). Min vleilande is in die West Cross Timbers-streek van Texas bestudeer, met een ondersoek wat slegs gekonstrueerde vleilande behels (Williams & Hudak 2005) en 'n ander ondersoek 'n gekonstrueerde vleiland met vergelykings met 'n bever-geskepte moeras (Brister 2005), so min is bekend oor vleilandplantgemeenskappe en opvolging.

Die West Cross Timbers-gebied van Texas (Figuur 1) lê suid van die Rooirivier, wes van die Fort Worth Prairie, noord van die Lampasas Cut Plain, en oos van die Rolling Plains. Dit beslaan 'n oppervlakte van 1 665 686 ha en word beskou as 'n mosaïek van prairie-, bosveld- en savanne-plantegroei wat gekenmerk word deur post-eik (Quercus stellata) en swarteik (Quercus marilandica) as die dominante houtsoorte (Dyksterhuis 1948 Diggs et al. 1999 Hoagland et al. al. 1999). Die Rooi-, Drie-eenheid-, Brazos- en Colorado-rivierbekken dreineer almal 'n gedeelte van die West Cross Timbers-gebied van Texas, met talle sytakke wat deur die gebied versprei is. Elkeen van hierdie riviere en hul verwante sytakke dra 'n aansienlike hoeveelheid oppervlakte by tot die Texas-oewervleilandkomponent. Een so sytak is die Leonrivier.

Die bolope van die Leon begin in Eastland County, naby die dorp Eastland, en vloei suidooswaarts deur Comanche, Hamilton en Coryell Counties om deur die Lampasasrivier in Bell County en die San Gabrielrivier in Milam County verbind te word om die Little River te vorm , 'n groot sytak van die Brazosrivier. Twee reservoirs, die Leon-meer en die Proctor-meer, is aan die bolope van die Leon geleë. Die Leonrivier dra water by tot 'n aantal oewervleilandkomplekse soos dit in en uit Proctor-meer vloei.

Een kompleks, ongeveer 32 ha groot, is onder die dam by Proctor Lake geleë. Dit bestaan ​​uit twee vleilandgebiede geleë tussen die Leonrivierkanaal en die uitvloeikanaal wat deur die USACOE gebou is toe die meer gebou is. Een van die vleilandgebiede, 'n varswatermoeras, is geskep deur beweraktiwiteit, wat lei tot damme wat water van die Leonrivier-kanaal vasvang en deur sypeling deur die grondgedeelte van die damdam. Die ouderdom van die vlei is onbekend, maar volgens meerpersoneel is dit vermoedelik so oud soos die meer, wat in 1963 in werking getree het. Vir doeleindes van hierdie studie sal daar na die vleiland verwys word as die verwysingsvleiland.

Die tweede van die vleilandgebiede, 'n gekonstrueerde vleiland wat deur USACOE, TPWD en Ducks Unlimited gebou is om watervoëlhabitat te vergroot, is suidoos (rivieraf) van die verwysingsvleiland geleë. Dit is in 1999 voltooi en het geen noemenswaardige hidrologiese insette ontvang tot die Winter/Lente van 2001 nie. Die aangelegde vleiland is nie gesaai nie, en ook is geen plantegroei in die vleiland ingebring nie. Die gekonstrueerde vleiland lê aangrensend aan die verwysingsvleiland en die twee word deur 'n berm geskei om onafhanklike beheer van watervlakke moontlik te maak. Beide vleilandgebiede sal as binnelandse, rivier-, varswatermoerasse beskou word (Mitsch & Gosselink 2007).

Riviermoerasse grens dikwels aan oewerwoude of beslaan sakke daarin, soms in verlate osseboë en by die bolope van riviere. Daarbenewens bou bevers vleilande deur strome op te dam, wat klein riviermoerasse skep wat wild lok, veral watervoëls (van der Valk 2006 Mitch & Gosselink 2007). Bevers speel dikwels 'n belangrike rol in vleilandontwikkeling en instandhouding. As hul damme hou, skep dit 'n stabiele hidroperiode. As hul damme gereeld uitspoel, veroorsaak dit watervlakskommelings en verhoog dus plantdiversiteit. Verhoogde plantdiversiteit veroorsaak gewoonlik verhoogde watervoëls en soogdierdiversiteit in en om die vleiland. Plantegroei in varswatermoerasse wat deur bevers geskep word, word dikwels gekenmerk deur eensaadlobbige plante soos Typha, vog-aangepaste grasse en sibbe (Mitsch & Gosselink 2007). Die bykomende voedingstowwe wat by die water gevoeg word as gevolg van bevers en ander soogdiere, watervoëls en organiese materiaal van verrottende plantegroei, verhoog die biodiversiteit van die vleiland deur plankton, ongewerwelde diere, skaaldiere, amfibieë, visse en reptiele te voed. Die bever help om die hele ekosisteem van organismes te ondersteun en word dus 'n sleutelsteenspesie (Primack 2010). As gevolg van hul damkonstruksie wat watervlak en stroomvloei reguleer, kan bevers 'n andersins ongunstige gebied omskep in een wat bewoonbaar is deur vleilandbiota (Schmidly 2004).

Verlies van vleilandhabitat is 'n ernstige omgewingsprobleem (Mitsch & Gosselink 2007 Heinz Centre 2008). Namate vleilande aanhou afneem, nie net in Texas nie, maar oor die hele wêreld, word dit al hoe belangriker dat vleiland-ekosisteme beter verstaan ​​word om gehalteversagting en konstruksiepraktyke te fasiliteer. Aangesien min vleilande in die West Cross Timbers-streek van Texas ondersoek is, sal die ekologiese data vir hierdie ondersoek dien as 'n basislyn vir toekomstige studies van vleilande in die streek. Die doel van hierdie ondersoek is om veranderinge in plantegroei-ekologie van die verwysing en gekonstrueerde vleilande by Proctor Lake Reservoir te evalueer.

Die ekologiese vergelykings is gedoen onder die dam by Proctor Lake in die twee voorheen genoemde vleilandgebiede geleë in Comanche County, Texas (Figuur 1). Beide vleilande is gemonster om plantegroei oor vier tydperke te ondersoek. Monsternemingsdatums vir plantegroei is gekies op grond van groeiseisoendata vir die area. Die gemiddelde lengte van die rypvrye tydperk vir Comanche County, Texas is ongeveer 230 dae. Die rypvrye tydperk begin middel Maart en duur tot middel November. Monsterneming is gedoen vanaf Augustus-September 2001, Maart-April 2002, Junie-Julie 2002 en September-Oktober 2002.

Grondmonsters van elke kwadrat waar plantegroei gemonster is, is geneem met 'n 38 cm grondmonsteringssonde en vergelyk met 'n Munsell grondkleurkaart vir hidriese grondkleuraanwysers. Op grond van hierdie toets was alle gronde wat in die verwysingsvleiland gemonster is hidries en twee derdes van die monsters van die gekonstrueerde vleiland is as hidries geklassifiseer.

Gebaseer op visuele waarnemings van oorstroming, grondversadiging en watermerke op omliggende plantegroei, was hidrologie tussen die twee terreine soortgelyk behalwe in die herfs van 2001, aan die begin van die ondersoek, toe ongeveer een derde van die kwadrate in die gekonstrueerde vleiland geen visuele tekens van oorstroming, grondversadiging of watermerke op omliggende plantegroei nie. Dus, toe plantegroei vir die eerste keer in die ondersoek bemonster is, was hidrologie in die gekonstrueerde vleiland meer veranderlik as die hidrologie van die verwysingsvleiland, maar gedurende die drie daaropvolgende monsterperiodes was hidrologie konstant in kwadrate vir beide vleilande.

Die ekologiese vergelyking vir plantegroei is uitgevoer deur eers 'n basislyn vas te stel wat parallel met die Leonrivier geloop het en beide vleilandgebiede ingesluit het. Die basislyn was ongeveer 890 m en is gesny deur 10 transekte wat die vleilandgebiede deurkruis. Die basislyn is verdeel in segmente wat gelykop gespasieer is (107 m uitmekaar) in die natuurlike vleiland en in die gekonstrueerde vleiland (71 m uitmekaar). Gedurende elke steekproefperiode is transekbeginpunt vir elke segment bepaal deur gebruik te maak van 'n ewekansige getalletabel, en die transek was loodreg op die basislyn van die vleiland geleë. Elke transek het twee ewekansige gestratifiseerde waarnemingspunte gehad wat bepaal is deur gebruik te maak van 'n ewekansige getalletabel waar 'n 0.25 [m.sup.2] kwadrat gebruik is om die aantal en persentasie bedekking van plante te bepaal. Taksonomie vir plante het Diggs et al. (1999) en wetenskaplike name met gesag is in tabelle 1-4 ingesluit. Sodra dit geïdentifiseer is, is plantegroei by elke waarnemingspunt langs elke transek gekarakteriseer deur gebruik te maak van die volgende metodologie soos gewysig vanaf USACOE (1987) en Brower et al. (1990).

Kruie is in beide vleilande gemonster. Kruie volgens USACOE (1987) sluit alle nie-houtagtige plante en houtagtige plante < 1 m hoog in. Ten einde kruidagtige plantegroei te meet, is 'n 0.25 [m.sup.2] kwadrat ewekansig op die transek binne die 1 [m.sup.2] waarnemingspunt geplaas. Elke spesie wat binne die kwadrat gevind is, is dan geïdentifiseer, getel en sy persentasie bedekking is beraam. 'n Belangrikheidswaarde (IV) is toegeken aan elke kruidagtige of houtagtige saailingspesie wat by elke waarnemingspunt gevind word, gebaseer op die som van die relatiewe bedekking, relatiewe digtheid en relatiewe frekwensie van elke spesie, terwyl die persentasie relatiewe belangrikheidwaarde (%RIV) is bepaal deur die IV van elke spesifieke spesie te vergelyk met die totale IV van al die spesies (Fredrickson 1979 Brower et al. 1990, Smith 1996). 'n Diversiteitsindeks vir alle kruidagtige en houtagtige saailingspesies is toegepas deur gebruik te maak van die Shannon-Weaver indeks, gebaseer op inligting verkry uit bogenoemde data (Miller 1985 Brower et al. 1990). Boonop is rykheid en egaligheid bereken (Brower et al. 1990).

Daardie plante wat aangepas het by groei in water of in grond, wat ten minste periodiek versadig of oorstroom is, word dikwels na verwys as hidrofitiese plante, of bloot hidrofiete (USACOE 1987). Plante is op grond van hul lewensgeskiedenis gekategoriseer in vyf afsonderlike groepe genoem vleilandaanwyserkategorieë (USACOE 1987 United States Fish and Wildlife Service (USFWS) 1997 Tiner 1999). Dit sluit verpligte vleilandspesies (OBL) in wat meer as 99% van die tyd in vleilande voorkom. Fakultatiewe vleilandplante (FACW-, FACW en FACW+) kom 67-99% van die tyd in vleilande voor. Fakultatiewe plante (FAC-, FAC en FAC+) kom 34-66% van die tyd in vleilande voor en fakultatiewe hooglandplante (FACU+, FACU en FACU-) kom 1-33% van die tyd in vleilande voor. Hooglandplante (UPL) kom minder as 1% van die tyd in vleilande voor. Hierdie klassifikasiestelsel laat mens toe om plantegroei in monsternemingseenhede te kategoriseer, en as meer as 50% van die dominante plantegroei OBL, FACW+, FACW, FACW-, FAC+ of FAC is, word dit as hidrofitiese plantegroei beskou en die maatstaf vir vleilandplantegroei in daardie monsterneming eenheid gestig is (USACOE 1987). Elke komponent van die plantegroeidata is geklassifiseer volgens sy vleilandaanwyserstatus (USFWS 1997) en die 50/20-reël is dan toegepas om die dominansie van spesies te bepaal. Die spesies is in dalende volgorde van dominansie gerangskik en daardie spesies wat onmiddellik 50% oorskry het, plus enige addisionele spesies wat 20% of meer van die totale dominansie uitmaak, is as dominant beskou. Indien meer as 50% van die dominante plantspesies wat binne elke waarnemingspunt gemonster is, verpligte vleiland (OBL), fakultatiewe vleiland (FACW+, FACW, FACW-), of fakultatief (FAC+, FAC) was, is die area wat deur die waarnemingspunt gedek word, oorweeg positief vir hidrofitiese plantegroei (USACOE 1987). Daarbenewens is 'n Mann-Whitney-toets gebruik om te bepaal of daar betekenisvolle verskille tussen die gekonstrueerde en natuurlike vleiland vir kruidagtige en houtagtige saailingspesies was (Brower et al. 1990).

Relatiewe belangrikheid waardes (RIV%), 'n maatstaf van dominansie vir kruidagtige plantegroei in die verwysing en gekonstrueerde vleilande, is vir elke steekproefperiode bepaal. Gedurende die Herfs 2001 monsternemingsperiode was gewone katstert (Typha latifolia 49.8%) en Bermuda-gras (Cynodon dactylon 12.7%) die belangrikste plante wat in die verwysingsvleiland geïdentifiseer is, terwyl paddavrugte (Lippia nodiflora 21.3%), Johnson-gras (Sorghum) halepense 19.2%), en boererfgras (Echinochloa muricata 15.5%) was die belangrikste in die gekonstrueerde vleiland (Tabel 1). Gedurende die Lente 2002 monsternemingsperiode is gevind dat gewone katstert (77.3%) die belangrikste spesie in die verwysingsvleiland is, terwyl dit in die gekonstrueerde vleiland, vleiland (Iva annua 34.0%) en viooldok (Rumex pulcher, Daar is gevind dat 19.6%) dominant is (Tabel 2). Gedurende die Somer 2002 steekproefperiode is die verwysingsvleiland oorheers deur vleiland (42.7%) en Bermuda gras (19.6%), terwyl die gekonstrueerde vleiland vleiland (31.8%) en (vir die eerste keer) gewone kat- stert (24,4%) en knopgras (Paspalum distichum 23,7%) as dominante (Tabel 3). Daar is gevind dat knoopgras (32.1%) en moerasouderling (18.7%) die belangrikste is in die verwysingsvleiland gedurende die Herfs 2002 monsternemingsperiode, en knoopgras (20.0%), waterhennep (Amaranthus rudis 12.7%), knoffelgras (Xanthium strumarium 12.4%), en moeras-ouderling (10.4%) het almal mede-belang in die gekonstrueerde vleiland gedeel (Tabel 4). Totale %RIV vir die verwysingsvleiland, toe die data as 'n gekombineerde totaal ontleed is, het aangedui dat gewone katstert (25.7%) die belangrikste is, gevolg deur moeras-ouderling (25.6%) (Tabel 5). Die gekonstrueerde vleiland is oorheers deur moeras-ouderling (17.6%), maar het relatief hoë belangrikheidswaardes vir knopgras (17.5%), gewone katstert (10.6%) en haanvoël (8.9%) gehad (Tabel 5).

In die herfs van 2001 was die persentasie kruidagtige plantspesies wat in die verwysingsvleiland gemonster is, wat as hidrofiete geklassifiseer is, 70%, terwyl 56% van die plantspesies wat in die gekonstrueerde vleiland gemonster is as hidrofiete geklassifiseer is (Tabel 1 en 5). Honderd persent van die plantspesies is as hidrofiete in die verwysingsvleiland Lente 2002 geklassifiseer, terwyl 88% as hidrofiete in die gekonstrueerde vleiland geklassifiseer is (Tabelle 2 en 5). Gedurende die volgende twee steekproefperiodes was die persentasie plantspesies wat as hidrofiete geklassifiseer is omtrent dieselfde, 80 en 82% in die verwysingsvleiland teenoor onderskeidelik 83 en 78% in die gekonstrueerde vleiland (Tabelle 3, 4 en 5). . In die ontleding van die data as 'n gekombineerde totaal, het die verwysingsvleiland 'n groter persentasie plantspesies gehad wat as hidrofiete geklassifiseer is as die gekonstrueerde vleiland, onderskeidelik 83% en 72% (Tabel 5).

Twee-en-dertig spesies is vanaf 80 kwadrate in die twee vleilandgebiede geïdentifiseer. Van daardie 32 spesies is gevind dat 13 (40.6%) in beide vleilandgebiede teenwoordig is, agt (25%) is uitsluitlik in die verwysingsvleiland gevind en 11 (34.4%) is uitsluitlik in die gekonstrueerde vleiland gevind (Tabel 5). Agt-en-twintig van die 32 spesies was inheems, met vier spesies wat ingebring is. Al vier ingevoerde spesies is in die verwysingsvleiland gevind (Bermuda-gras, knopgras, Johnson-gras en viooldok en drie van die vier (knoopgras uitgesluit) is in die gekonstrueerde vleiland geïdentifiseer. Die spesies wat in die verwysingsvleiland geïdentifiseer is, het bestaan ​​uit agt eenjariges en 13 meerjariges (Tabel 5), terwyl die spesies wat in die gekonstrueerde vleiland geïdentifiseer is, uit 10 eenjariges en 14 meerjariges bestaan ​​het (Tabel 5).

Die rykdom van kruidagtige plantspesies vir Herfs 2001 (Tabel 6) was effens groter in die verwysingsvleiland (10) as in die gekonstrueerde vleiland (nege). Gedurende die Lente 2002 monsternemingsperiode het die gekonstrueerde vleiland egter die verwysingsvleiland oortref in plantspesierykheid (agt en vyf, onderskeidelik) (Tabel 6). Gedurende die Somer 2002 steekproefperiode het die verwysingsvleiland hoër rykdom gehad met 11 spesies in vergelyking met agt in die gekonstrueerde vleiland (Tabel 6). Gedurende Herfs 2002 (Tabel 6) het die gekonstrueerde vleiland die verwysingsvleiland oortref in spesierykheid met 18 spesies, terwyl die verwysingsvleiland 11 gehad het. Toe die data as 'n gekombineerde totaal ontleed is, is gevind dat die gekonstrueerde vleiland 'n effens groter rykdom het van 27 spesies, in vergelyking met 25 vir die verwysingsvleiland (Tabel 6).

Ewerheidswaardes (Tabel 6) was groter in die gekonstrueerde vleiland vir elke monsternemingsperiode. Verwysingsvleilandgelykheidswaardes het gewissel van 'n laagtepunt van 0.38 in die lente van 2002 tot 'n hoogtepunt van 0.67 in die herfs van 2001, vir 'n gekombineerde totale waarde van 0.48 (Tabel 6). Gekonstrueerde vleilandwaardes het gewissel van 'n laagtepunt van 0.53 in die Somer 2002 tot 'n hoogtepunt van 0.78 in Herfs 2001, vir 'n gekombineerde totale waarde van 0.64 (Tabel 6).

Shannon se diversiteitsindeks is vir elke steekproefperiode bereken. Soos gelykheid, is daar gevind dat diversiteit (Tabel 6) groter is in die gekonstrueerde vleiland as in die verwysingsvleiland gedurende elke steekproefperiode. Diversiteitswaardes in die verwysingsvleiland het gewissel van 'n laagtepunt van 0.62 in Lente 2002, tot 'n hoogtepunt van 1.53 gedurende Herfs 2001. Toe die data as 'n gekombineerde totaal ontleed is, was die Shannon se diversiteitswaarde 1.56. Diversiteitswaardes in die gekonstrueerde vleiland het gewissel van 'n laagtepunt van 1.10 gedurende die Somer 2002 steekproefperiode tot 'n hoogtepunt van 1.86 in Herfs 2002. Toe die data as 'n gekombineerde totaal ontleed is, is 'n Shannon se diversiteitswaarde van 2.11 verkry (Tabel 6).

In die Herfs 2001 monsternemingsperiode het 100% van die kwadrate wat in die verwysingsvleiland gemonster is, aan hidrofitiese plantvereistes voldoen (meer as 50% van die dominante spesies was FAC of natter), terwyl slegs 60% van dié in die gekonstrueerde vleiland aan dieselfde vereistes voldoen het. . Gedurende die Lente 2002 monsternemingsperiode het 80% van die verwysingsvleilande se kwadrate voldoen aan hidrofitiese plantegroeivereistes vergeleke met 40% van die gekonstrueerde vleilande se kwadrate. Gedurende die Somer 2002 steekproefperiode het die persentasie kwadrate wat aan die hidrofitiese plantegroeivereistes voldoen in beide die verwysings- en gekonstrueerde vleilande tot 100 en 80% onderskeidelik toegeneem. Gedurende die Herfs 2002 monsternemingstydperk het die persentasie kwadrate wat aan hidrofitiese plantegroeivereistes in die verwysingsvleiland voldoen, op 100% gebly, terwyl die gekonstrueerde vleilande persentasie tot 70 gedaal het. Hierdie resultate, wanneer as 'n geheel ontleed, toon dat die verwysingsvleiland aan sy hidrofiete voldoen het. plantegroeivereistes 95% van die tyd, terwyl die gekonstrueerde vleiland 63% van die tyd aan sy hidrofitiese plantegroeivereistes voldoen het.

Benewens bogenoemde ontledings, is 'n nie-parametriese Mann-Whitney statistiese toets uitgevoer op rykheid, egaligheid, diversiteit, plantdigtheid en persentasie plantbedekking, maar daar is gevind dat nie een betekenisvol verskil in vergelykings tussen die verwysing en gekonstrueerde vleilande nie.

Deur gebruik te maak van die standaarde van hidrofitiese plantegroei, gronde en hidrologie (USACOE 1987), sal die verwysingsgebied regdeur hierdie ondersoek as 'n vleiland beskou word. Die gekonstrueerde vleiland het plantegroei gehad wat deur die hele ondersoek toenemend hidrofities was, het ongeveer twee derdes van sy grondmonsters as hidries geklassifiseer, en het 'n konstante hidrologie gehad, soortgelyk aan die verwysingsvleiland, na die eerste monsternemingsperiode.

Gebaseer op Relatiewe Belangrikheidswaardes (RIV%) vir die Herfs 2001 monsternemingsperiode, was die belangrikste spesies in die verwysingsvleiland gewone katstert- en Bermuda-gras (Tabel 1). Gewone katstert kan 'n indringerspesie wees wat die vermoë het om groot monokulture te skep en ander spesies uit te kompeteer. Dit is as gevolg van sy vinnige groeitempo en die feit dat dit ongeslagtelik voortplant met behulp van risome, wat vinnig versprei. Katstert-indringing kan problematies wees omdat digte monokulture die verhouding van oop water tot plantegroei verminder, die diversiteit van plantegroei verminder en dus die habitatwaarde van die terrein verminder (Brown en Bedford 1997 Mitsch & Gosselink 2007). Daarbenewens is gewone katstert 'n verpligte vleilandspesie (wat in vleilande >99% van die tyd aangetref word) en die meeste van die verwysingsvleiland is gedurende die hele studie vir die grootste deel van die jaar versadig gevind. Daarom sal gewone katstert goed aangepas wees vir hierdie omgewing. Elke individu het die vermoë om duisende sade vry te laat, wat aansienlike afstande in óf die lug óf die water kan dryf, wat dit die vermoë gee om maklik te versprei.

Bermuda-gras is ook gevind as een van die dominante in die verwysingsvleiland. Dit is egter 'n FACU+ spesie, wat dit ongeveer 'n 10% waarskynlikheid gee om in 'n vleiland gevind te word. Die waarskynlikheid is laag, maar vleilande is dinamiese ekosisteme, wat voortdurend verander en Bermuda-gras is 'n ingevoerde spesie wat in 'n verskeidenheid toestande, dikwels as 'n onkruid, in baie dele van Texas kan oorleef (Diggs et al. 1999). Dit het gedurende die droër maande van die groeiseisoen langs die vleilandrand voorgekom omdat die grondoppervlak langs die rand van die vleiland 'n neiging gehad het om te droog, wat spesies wat nie oor die algemeen by vleilande aangepas is 'n kans gegee het om met sommige van die vleilandspesies mee te ding nie, wat moontlik gesterf het of dormant geraak het. Bermuda-gras is ook op groot skaal ingevoer vir veehooi en weiding in landerye langs die Leonrivier.

In die gekonstrueerde vleiland, gedurende Herfs 2001, is daar gevind dat paddavrugte, Johnson-gras en boertuingras die belangrikste spesies teenwoordig was (Tabel 1). Paddavrugte het 'n vleiland-aanwyserstatus van FAC+ (ongeveer 60% kans om in 'n vleiland voor te kom), Johnson-gras is FACU (20% kans om in 'n vleiland te voorkom), en boererfgras is FACW (80% kans om in 'n vleiland te voorkom) , gemiddeld 'n bietjie meer as 50% kans om in die vleiland te voorkom. Hierdie spesies word dikwels as onkruidagtig beskou, wat versteurde gebiede bewoon (Diggs et al. 1999).

Gedurende die Lente 2002 monsternemingseisoen is die verwysingsvleiland deur gewone katstert oorheers, terwyl die gekonstrueerde vleiland mede-oorheers is deur moerasvlei en viooldok (Tabel 2). Hulle is geklassifiseer as FAC+ en FACW- (70% kans om in 'n vleiland voor te kom) onderskeidelik. Moeras-ouderling is inheems en kom in lae, klam habitatte voor, terwyl viooldok ingevoer word en gewoonlik in nat, versteurde gebiede voorkom (Diggs et al. 1999). Die gekonstrueerde vleiland is deur meer vleilandspesies gekoloniseer, wat verwag sou word na 'n herfs en winter van oorstromings deur reservoirpersoneel, om watervoëlhabitatte te skep. Indien die vleilandhidrologie mettertyd stabiliseer, behoort die spesiesamestelling meer hidrofities te word en in 'n sekondêre serale stadium of selfs 'n klimaksgemeenskap te verander.

Gedurende die Somer 2002 monsternemingsperiode is die verwysingsvleiland oorheers deur vlei- en Bermuda-gras (Tabel 3). Die teenwoordigheid van Bermuda-gras as 'n dominante is nie noodwendig die gevolg van 'n verskuiwing in die verwysingsvleiland se plantegroei nie, maar 'n gevolg van ewekansige steekproefneming wat langs die droër marges gedurende daardie steekproefperiode plaasgevind het. Marsh-elder het ook die buitenste kern van die verwysingsvleiland oorheers, waar die grond droër was, terwyl gewone katstert die binnekern van die vleiland oorheers het, wat die meeste van die tyd versadig was.

Gedurende hierdie selfde tydperk (somer 2002) is die gekonstrueerde vleiland ook oorheers deur vleiland, sowel as gewone katstert- en knopgras (Tabel 3), 'n FACW+ spesie (kans om in 'n vleiland voor te kom 90%). Na die herfs/winter-oorstroming wat deur reservoirpersoneel aangehits is, is die aangelegde vleiland gekoloniseer met meer hidrofitiese spesies.

Gedurende die Herfs 2002 monsternemingsperiode is die verwysingsvleiland oorheers deur knopgras en moerasvlei (Tabel 4). Gedurende hierdie tyd van die jaar was daar oor die algemeen min neerslag, wat die rand van die verwysingsvleiland toegelaat het om na die kern terug te trek. Hierdie uitdrogende effek rondom die rande van die vleiland het meer fakultatiewe spesies 'n kans gegee om hulself te hervestig.

Die gekonstrueerde vleiland is oorheers deur knopgras, waterhennep (FAC met 'n 50% kans om in 'n vleiland gevind te word), cocklebur (FAC- met 'n 40% kans om in 'n vleiland voor te kom), en moeras-ouderling (Tabel 4). Water-hennep en kokkelaar was twee nuwe mede-dominante, wat albei die rande van die vleiland gekoloniseer het nadat watervlakke gedaal het.

Oor die algemeen is die verwysingsvleiland oorheers deur gewone katstert en moerasouderling (Tabel 5). Die gewone katstert is regdeur die binnekern van die vleiland aangetref, terwyl vleivlies buite die kern en na die rande gevind is. Hierdie resultate volg die sentrifugale organisasiemodel (Mitsch & Gosselink 2007 Keddy 2010), wat stel dat die kernhabitat in vleilande lae versteuring en hoë vrugbaarheid het en oorheers word deur spesies wat digte blaredakke vorm soos Typha. Perifere habitatte verteenwoordig verskillende soorte en kombinasies van spanninge (onvrugbaarheid, versteuring) en ondersteun kenmerkende plantassosiasies (Mitsch & Gosselink 2007). In hierdie geval was die stressor die periodieke onttrekking van die vleilande, wat die kernhabitat toegelaat het om na binne en uit te puls, afhangende van die hidroperiode.

Die oorheersende plant in die gekonstrueerde vleiland, in die algemeen, was moeras-ouderling (Tabel 5). In 'n studie van uitgegrawe depressies naby 'n gronddam in noord-sentraal Texas, was die dominante kruid ook moeras-ouderling (Williams & Hudak 2005). In die Proctor Lake-gekonstrueerde vleiland is egter relatief hoë belangrikheidswaardes gevind vir knoopgras, gewone katstert en haanvoël. Knoopgras en gewone katstert is nie in opgegrawe holtes gevind nie en haankruid is selde in die uitgegrawe holtes teëgekom (Williams & Hudak 2005). Elkeen van hierdie spesies word as FAC- of natter geklassifiseer. In die algemeen is die gekonstrueerde vleiland oorheers deur meer as 50% hidrofiete en soos die verwysingsvleiland het gelyk of dit sentrifugale organisasie ontwikkel het (Deberry & Perry 2005).

Daar is gevind dat spesierykheid in die geheel effens groter is in die gekonstrueerde vleiland (Tabel 6). Dit kan toegeskryf word aan die feit dat die aangelegde vleiland onlangs onbewoon was as gevolg van die versteuring van konstruksie. Vroeë koloniserende spesies het voordeel getrek uit die onbewoonde ruimte en die gebied gekoloniseer wat 'n groter spesierykdom veroorsaak het, soos wat gerapporteer is vir gekonstrueerde vleilande in Grimes County, Texas (Middag 1996). Suksesvolle vroeë koloniste het aanpassings wat hulle in staat stel om vinnig op oop terreine te vestig. Hulle is oor die algemeen klein en laaggroeiend, het kort lewensiklusse en reproduseer jaarliks ​​deur sade of stuur nuwe lote uit knoppe naby die grond (van der Valk 1981 Mitsch & Gosselink 2007). Hulle produseer groot getalle maklik verspreide klein sade, reageer vinnig op versteuring (veral blootstelling van minerale grond), en bereik vinnig dominansie deur die groei van enige latere serai-stadiumplante wat as saailinge onder hulle kan bestaan, te onderdruk (Smith 1996 Mitsch & Gosselink 2007) . Hierdie vroeë koloniste is verdraagsaam teenoor wisselende omgewingstoestande, veral 'n wye reeks daaglikse temperature op die grondoppervlak, afwisselende benatting en droging, en intense lig (Smith 1996 Mitsch & Gosselink 2007). Hierdie spesies verteenwoordig wat genoem is die aankoms- en vestigingsfase van vleilandopvolging (Noon 1996) en die hoër diversiteit wat in die gekonstrueerde vleiland gevind word, is waarskynlik die gevolg van vroeë opvolgfase. Rykdomswaardes in gerestoureerde en natuurlike prairie-vleilande wat aangrensend aan mekaar in noordelike Iowa was, het drie jaar na herstel, natuurlike vleilande gehad met 46 totale spesies in vergelyking met slegs 27 totale spesies wat in herstelde vleilande voorkom (Galatowitsch & van der Valk 1996). Noon (1996), met behulp van data van agt gekonstrueerde vleilande in Texas, het gevind dat hoe ouer die vleilande geword het, hoe minder spesies het voorgekom. Daarom is dit waarskynlik dat, sodra die gekonstrueerde vleiland uit die aankoms- en vestigingsfase beweeg, spesiegetalle sal afneem.

Spesies teenwoordig tydens die eerste paar jaar van plantegroei-vestiging wat nie geplant of gesaai is nie, word aanvaar as vrywilligers van offsite bronne, en verteenwoordig dus die primêre serale stadium van plantegroei opeenvolging (Reinhartz & Wame 1993 DeBerry & Perry 2004). In addition, vegetation in adjacent wetlands may be a potential seed source particularly from habitats dominated by herbaceous species that are more likely to colonize a young substrate (DeBerry & Perry 2004). Shared species composition between the wetlands was greater than 40% because the reference wetland likely served as a seed source for colonization of the constructed wetland.

The constructed wetland had greater diversity then the reference wetland during each sampling period (Table 6). Data from eight constructed wetlands in Texas (Noon 1996) indicated that diversity was highest at younger sites. Marshes, when compared to savannah type habitat, exhibited low species diversity in a study of wetland habitats along a portion of the North Fork of the Forked Deer River in West Tennessee. The savannah habitat was seasonally flooded, bringing in new seed sources and nutrients, while the marsh habitat retained water year around (Miller 1985). Ice scouring, infertile sandy soils, flooding by beavers, and open shorelines are among the stresses that shift communities to be less vegetatively productive, albeit possibly more diverse, assemblages (Mitsch & Gosselink 2007). Fluctuating hydroperiod in wetlands can promote dominance by annuals and nonclonal perennials in zones or patches within the wetland (Collins & Wein 1995 Mitsch & Gosselink 2007), and this patchiness and annual duration may increase diversity. Because, with the exception of the first sampling period, hydrology of the two wetlands was similar, differences in diversity are likely due to differences in successional stages between the two wetlands. The reference wetland is older and exhibited hydrophytic vegetation 95% of the time whereas the younger constructed wetland exhibited hydrophytic vegetation only 63% of the time. Assuming hydrology remains constant over time, the constructed wetland should develop more hydrophytic vegetation.

The adjacent riverine forest does not invade the marsh because the beaver dam and constructed wetland hydrology maintains a habitat too wet for the establishment of most tree species. Mitchell & Niering (1993) found that 30 year old beaver wetlands decimated the anchored forest wetland and replaced it with an abundance of graminoids in a northwestern Connecticut bog. The beaver-created marsh functions in a similar fashion and maintains separate marsh and upland forest communities. Because of more consistent hydrology and the older age of the reference wetland, common cattail, an obligate wetland species, dominates the inner core area with marsh-elder, a FAC+ species, occupying the drier periphery. Ninety-five percent of the species sampled in the reference wetland were hydrophytes. These results closely resemble a successional model of centrifugal organization that describes the distribution of species and vegetation types along gradients caused by a combination of environmental constraints (Wisheu & Keddy 1992 Mitsch & Gosselink 2007 Keddy 2010). In eastern North America, the core habitats are often dominated by species that form dense canopies such as Typha. Habitats peripheral to the core depend on different kinds and combinations of stress and disturbance. When beavers create disturbance, the peripheral habitats include species of sedge near the Typha core and wet-adapted forb species near the edges (Wisheu & Keddy 1992 Mitsch & Gosselink 2007 Keddy 2010). A similar plant community was observed in the reference wetland at Proctor Lake.

In contrast, the constructed wetland was dominated by marsh-elder and knot grass, both FAC+ species, with a greater diversity of vegetation of which 67% were hydrophytes. Richness, evenness, and Shannon's diversity were greater in the constructed wetland. This was likely due to its earlier successional stage with plants being a part of the arrival and establishment phase (Noon 1996).

Because common cat-tail was more important in later sampling in the constructed wetland it may develop according to the centrifugal organization with a Typha core (Deberry & Perry 2004). Mitsch et al. (2005) describes a naturally colonizing, riverine wetland in Ohio as being dominated by Typha. Noon (1996) proposed two phases in early constructed wetland primary succession called the arrival and establishment phase, which is characterized by random arrival and establishment of species, and the autogenic dominance phase, which is characterized by biomass production and competition between species. Because of its greater diversity, the constructed wetland is probably beginning to finish the first successional stage and is moving into the second phase whereas the reference wetland is in the autogenic dominance phase. In van der Valk's (1981) model of freshwater wetland vegetation dynamics, two basic types of wetland species are recognized: (1) species with long-lived propagules that are in the wetland seedbank and can grow when suitable conditions occur, and (2) species with short-lived propagules that can only grow in the wetland if they reach it during a period when conditions are suitable for germination. This causes the wetland to function as a sieve, allowing only establishment of certain species at any given time and dependent on whether the wetland is flooded or in a drawdown (van der Valk 1981 Mitsch & Gosselink 2007). At Delta Marsh in Canada and marshes of Eagle Lake in Iowa, flooding the marshes after drawdowns resulted in plant communities that contained or were dominated by a species of Typha (van der Valk 1981). Of herbaceous species identified in the Leon River wetlands, there were equal numbers of perennial species, but numbers of annuals differed (Table 5). The constructed wetland had more annual species than the reference (10 vs. 7) likely due to its early successional stages (Noon 1996) and more variable early hydrology, while the older riverine marsh, created by the beavers, was centrifugally organized (Wisheu & Keddy 1992 Deberry & Perry 2004 Mitsch & Gosselink 2007 Keddy 2010) with a perennial common cat-tail core.

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Jeffrey S. Brister (1) and Allan D. Nelson (2)

(1) Natural Resource Conservation Service, United States Department of Agriculture Service Center, 5040 Loop 340, Waco, Texas 76706

(2) Department of Biological Sciences, Box T-0I00, Tarleton State University, Stephenville, Texas 76402


2 MATERIALE EN METODES

2.1 Aquatic plants

We selected E. densa (Hydrocharitaceae) as invader. It is a popular aquarium plant in Europe and world-wide the aquarium trade is considered its main introduction pathway (Yarrow et al., 2009 ). Egeria densa disperses mainly vegetatively for which fragments with only two nodes are enough to establish and develop new stands (Yarrow et al., 2009 ). The root system and shoots can break easily allowing plant fragments to be carried through the water to colonize new areas. This species can grow to over 3 m long and form monospecific stands with closed canopies, that can severely alter the structure of the native communities and local environmental conditions (Yarrow et al., 2009 ). It is well adapted to cold climates and can survive freezing winters by storing starch in its leaves and stems (Thiébaut, Gillard, & Deleu, 2016 ). Egeria densa has caused many problems throughout temperate regions including the United States of America and New Zealand, and has also become a nuisance species in its native range (Bini, Thomaz, Murphy, & Camargo, 1999 ).

The three common native submerged species that we used in the experiments are widely distributed in Northwestern Europe and co-occur in temperate shallow lakes (Van De Haterd & Ter Heerdt, 2007 ). Ceratophyllum demersum (Ceratophylaceae) is a free floating submerged species and M. spicatum (Haloragaceae) and P. perfoliatus (Potamogetonaceae) are rooted species. All three species are perennial and capable of clonal growth. All the plants used in this study were acquired from a commercial plant trader (De Zuurstofplantgigant, Hapert, The Netherlands). The acquired plants were pre-cultivated in 200 L cattle tanks (diameter = 66 cm and height = 60 cm, two tanks per species) under controlled greenhouse conditions with a 16/8 hr light/dark cycle at a temperature of 21 ± 3°C during the day and 16 ± 3°C during the night (Figure S1). The tanks were filled with a 3.4 kg (

2 cm) bottom layer of artificial plant pond sediment (Plant soil Moerings—Velda, organic matter = 34.31%), 44.9 kg (

10 cm) of washed sand on top (0.8–1.0 mm grain size, organic matter content = 0.16%) and filled with water from freshwater Lake Terra Nova (52°12′55.2″N, 5°02′25.7″E). Lake Terra Nova is a shallow peat lake located in the centre of the Netherlands where all three native plant species used in the experiment co-occur (Van De Haterd & Ter Heerdt, 2007 ). The lake is characterized by high nutrient concentrations in the water (water used in the experiment: M ± SD, n = 6 water samples, 0.14 ± 0.05 mg/L P-PO4 0.55 ± 0.46 mg/L N-NO3). The plants were cultivated under the following conditions: water temperature 22.3 ± 0.8°C, dissolved oxygen 12.5 ± 1.3 mg/L, conductivity 263 ± 28 µS/cm, pH 9.8 ± 0.3 and alkalinity 2.37 ± 0.52 mEq/L. Plants were pre-cultivated for at least 20 days before the start of the experiment.

2.2 Generalist herbivore

Lymnaea stagnalis (Gastropoda, Pulmonata, Basommatophora), the great pond snail, is a common and widely distributed generalist herbivore native to the Holarctic region. Most freshwater gastropod species consume mainly algae, bacteria and detritus but large species such as L. stagnalis can consume considerable amounts of aquatic plants having a large impact on aquatic plant abundance (Brönmark, 1989 , 1990 Wood et al., 2017 ). Densities of 10–40 L. stagnalis individuals/m 2 are commonly found under natural conditions (Elger et al., 2007 ), where it occurs in slow flowing and stagnant freshwater systems. This species has also been previous commonly used as model species in aquatic settings (Bakker et al., 2013 Elger & Barrat-Segretain, 2002 , 2004 Grutters et al., 2017 Zhang, Liu, Luo, Dong, & Yu, 2018 ).

Adult snails were collected from a pond located at the Netherlands Institute of Ecology (NIOO-KNAW, 51°59′16.8″N, 5°40′24.7″E, Wageningen, The Netherlands). They were acclimated to laboratory conditions for at least 2 weeks in 15 L buckets filled with groundwater at 20°C and constant aeration and exposed to a 16:8 hr day:night cycle, before being experimentally used. The snails were fed butterhead lettuce (Lactuca sativa L.) 6 days a week. Once a week fish food pellets (Velda, Gold Sticks Basic Food) and chalk were provided to ensure enough nutrients and calcium for shell development (following Grutters et al., 2017 ).

2.3 Experimental design and set-up

A greenhouse experiment was established at the Netherlands Institute of Ecology (NIOO-KNAW 51°59′15.3″N and 5°40′14.8″E) during the summer of 2018 (July–October). The experiment was set up as a full factorial randomized block design, with 3 × 3 × 2 treatment combinations of monocultures of three native submerged plant species (C. demersum, M. spicatum en P. perfoliatus), three levels of competition (no native plants, low density and high density) and absence (no snails) or presence of herbivory (with snails). The 18 treatments were replicated six times using a block design, yielding a total of 108 mesocosms (Figure S1). The greenhouse controlled conditions consisted of a 16/8 hr light/dark cycle at a mean temperature of 21 ± 3°C during the day and 16 ± 3°C during the night.

The mesocosms consisted of 13 L glass cylinder aquaria (18.5 cm diameter and 48 cm height) filled with a bottom layer of artificial plant pond sediment (150 g resulting in a layer of

1 cm depth) with a top layer of washed sand (2 kg resulting in a layer of

5 cm). Each aquarium was filled with 8 L lake water (resulting in 27 cm depth), leaving the upper 15 cm free to prevent snails from escaping. The water level was maintained constant during the whole experiment by refilling once a week with lake water to compensate for evapotranspiration. Abiotic parameters were monitored throughout the experiment and the growing conditions were found to be suitable for the plants (M ± SD, n = 1,166, water temperature 23.3 ± 1.0°C, dissolved oxygen 12.9 ± 1.9 mg/L, conductivity 283 ± 30 µS/cm, pH 9.7 ± 0.7 and alkalinity 2.12 ± 0.47 mEq/L).

To establish native plant communities for the competition treatment, we cut 99 non-rooted apical shoots without lateral shoots from the cultivation tanks from each of the native species C. demersum, M. spicatum en P. perfoliatus. We cut 15 cm long apical shoots and washed them in running tap water to remove any material attached. We randomly selected 15 of the 99 shoots of each species, dried these individually to a constant mass at 60°C for at least 48 hr, and weighed them for initial biomass measurements (dry weight, DW). We established the competition levels by pairing the invader E. densa with a single native plant species at different native shoot planting densities. The planting densities of each native plant species versus E. densa were manipulated to be 0:2 shoots (no competition, invader growing alone), 1:2 (low competition) and 6:2 (high competition), corresponding to

37 plants/m 2 (low competition) and

222 plants/m 2 (high competition) respectively before the invader introduction. These shoot densities are within the range observed in natural conditions (Li et al., 2015 ). The plant shoots of the rooted species were planted 5 cm deep in the sediment while the shoots of the non-rooted submerged species C. demersum were dropped in the water.

The native plants were left to establish for 2 weeks (24 July to 6 August) to allow the growth of at least one new shoot. Then, we introduced the invader by planting two E. densa non-rooted apical shoots per aquarium (7 August), which is considered to represent medium propagule pressure (Li et al., 2015 ). We chose shoots with an apical tip because these have a higher ability to regenerate, colonize and grow than shoots without apical tips (Riis, Madsen, & Sennels, 2009 ). To determine the introduced biomass in DW, we randomly selected 15 E. densa shoots, dried these to a constant mass at 60°C for at least 48 hr, and weighed them individually. Egeria densa was allowed to root for 2 days before we added the herbivore treatment, to simulate an early stage of establishment of E. densa in the new temperate native aquatic community.

In the herbivory treatment, we added two L. stagnalis snails per aquarium to half of our experimental units (10 August), representing intermediate snail densities observed in the field (Elger et al., 2007 ). We selected snails of the same size (shell length 30 ± 1 mm, wet weight 2.19 ± 0.27 g, M ± SD, n = 108) and starved the snails for 48 hr before adding them to standardize their appetite as is common practice in feeding trials (following Grutters et al., 2017 ).

2.4 Harvest and data collection

At the end of the experiment (after 8 weeks, on 8 October), we removed the herbivores, harvested the alien and native plants and, as we observed the growth of filamentous green algae Spirogyra sp. in our mesocosms, we harvested its biomass present on the plants and in the water column (Figure S2). We washed all the plants from each aquarium in an individual container to ensure that all the filamentous algae were kept. Then, this remaining water together with the water left in the aquarium after plant removal was filtered over a sieve of 0.106 mm mesh size. The filamentous algae biomass on the sieve was washed and dried to a constant mass at 60° for at least 48 hr, and weighed to determine DW. We measured invader E. densa performance in terms of the following growth parameters: total root and shoot DW, summing values from both introduced propagules and total biomass summing total root and shoot DW. We also determined native plant biomasses. All plants were dried to a constant mass at 60° for at least 48 hr, and weighed to determine DW.

2.5 Feeding trials

Herbivory consumption rates and preferences depend on plant palatability (Grutters et al., 2017 ). To determine plant palatability for the snails, we performed 24 hr no-choice feeding trials following established protocols (Elger & Barrat-Segretain, 2002 , 2004 Grutters et al., 2017 ). Plant material for the trials was collected from the same cultivation tanks that provided plants for the greenhouse experiment, and washed to remove any attached material. Snails of similar size (shell length 28.9 ± 1.8 mm, M ± SD, n = 48) were selected for the feeding trials.

Ninety-six plastic cups (volume of 500 ml) were filled with 375 ml groundwater (20°C, pH 8, conductivity 212 µS/cm). Twenty-four cups were used per plant species, of which each received approximately 0.2 g (wet weight) of non-apical shoots of either C. demersum, E. densa, M. spicatum or newly grown leaves of P. perfoliatus (one species per cup). Half of the cups received one individual of L. stagnalis whereas the other half was kept snail free, to be used as control to correct for autonomous changes in plant biomass due to growth. Snails were starved for 48 hr prior to the trial to standardize their appetite. All cups were covered with a mesh of size 1 mm to prevent snails from escaping. All cups were randomly placed on a rack in laboratory conditions at 20°C and exposed to a 16:8 hr day:night cycle (Figure S3). All snails were removed from their respective cup after 24 hr and euthanized by freezing at −20°C. Their soft body tissue was separated from their shells and dried in the oven at 60°C for at least 48 hr. The dry weights of plant fragments remaining in each cup were determined as described previously (see Section 2.4).

2.6 Data analyses

To disentangle the direct and indirect effects of native plants and herbivores on biotic resistance, we used piecewise Structural Equation Modeling (piecewiseSEM, Lefcheck, 2016 ). SEM has been shown to be an important tool to describe complex natural systems (Grace, Michael Anderson, Han, & Scheiner, 2010 ). For each of the three native plant species (C. demersum, M. spicatum en P. perfoliatus), we fitted models to investigate whether the native plants, herbivores, filamentous algae and their possible second-order interactions affected the invader E. densa performance (measured as total biomass at the end of the experiment). We fitted GLMM with block (the six replicates) as a random factor in all models (Pinheiro, Bates, DebRoy, Sarkar, & Team, 2018 ). We included these models in the SEM and performed model selection based on AICc criteria starting with the full model that included all second-order interactions among herbivory, filamentous algae biomass and native biomass. The best fitting models (lowest AICc) included only all main effects. Normality of model residuals, homoscedasticity and the influence of possible outliers were checked by visually inspecting plots of residual versus fitted values and quantile-quantile plots of model residuals. Native plant competition was evaluated using native plant species biomass as a continuous independent variable. PiecewiseSEM was performed in the software r (R Core Team, 2017 ) using the packages nlme and piecewiseSEM (Lefcheck, 2016 ).


Science and Products

Western Waters Invasive Species and Disease Research Program

Researchers at the Northern Rocky Mountain Science Center's Western Waters Invasive Species and Disease Research Program work extensively with federal, state, tribal, regional, and local partners to deliver science to improve early detection and prevention of invasive species and disease understand complex interactions that promote invasive species and disease, and their impacts (and.


The reproductive biology of male cottonmouths (Agkistrodon piscivorus): Do plasma steroid hormones predict the mating season?

To better understand the proximate causation of the two major types of mating seasons described for North American pitvipers, we conducted a field study of the cottonmouth (Agkistrodon piscivorus) in Georgia from September 2003 to May 2005 that included an extensive observational regime and collection of tissues for behavioral, anatomical, histological, and hormone analysis. Enzyme immunoassays (EIA) of plasma samples and standard histological procedures were conducted on reproductive tissues. Evidence from the annual testosterone (T) and sexual segment of the kidney (SSK) cycle and their relationship to the spermatogenic cycle provide correlative evidence of a unimodal mating pattern in this species of pitviper, as these variables consistently predict the mating season in all snake species previously examined under natural conditions. In most reptiles studied to date, high plasma levels of T and corticosterone (CORT) coincide during the mating period, making the cottonmouth an exception to this trend we suggest two possible explanations for increased CORT during spring (regulation of a spring basking period), and decreased CORT during summer (avoiding reproductive behavioral inhibition), in this species.


Organic Carbon Stocks in all Pools Following Land Cover Change in the Rainforest of Madagascar

Mieja Razafindrakoto , . Herintsitohaina Razakamanarivo , in Soil Management and Climate Change , 2018

Abstrak

Land use change , along with the release of carbon (C) as carbon dioxide, constitutes a major source of emissions that contribute to climate change. Consequently, accurate carbon stock estimation is required to both inform and mitigate climate change. This study determined the importance of five C pools, including above-ground biomass (AGB), below-ground biomass (BGB), soil organic C (SOC), deadwood (DW), and litter, as well as the effect of land use change on these five pools for a region in eastern Madagascar. We assessed the importance of each pool, as well as the effect of land use change, on a closed-canopy forest (CC), tree fallow (TF), shrub fallow (SF), and degraded land (DL). Our results show that more C was stored in below-ground pools than in above-ground pools, and that SOC represented the largest (76.49%) contributor to the total C stock (186.64 Mg C ha − 1 ), followed by AGB (13.54%) and BGB (6.64%). DW represented an important pool in CC, representing 6.64% of the total C stock in this land use type. Conversely, the litter pool represented the lowest contribution to total C stock. Among the five pools, only the SOC showed little variation following land use change, while AGB, DW, and BGB were the most affected after deforestation and subsequent land degradation, most notably from CC to TF. The litter showed significant decreases of C stock from CC to TF and SF. These results highlighted the importance of considering all five pools in an accurate estimate of C stock for a better implementation of initiatives, such as “Reducing C Emissions from Deforestation and forest Degradation” (REDD +).


(21) The 4G Ranch Wetlands: Operating for Our Future

Presenter: Allison Lewis, Jacobs, [email protected]

Co-Authors: Rafael Vazquez-Burney, Jacobs Engineering, [email protected]

Abstract: In 2017, the largest groundwater recharge wetland in the world, known as the 4G Ranch Wetlands, was constructed in Pasco County. Groundwater recharge wetlands are constructed wetlands that do not have a surface water outflow and water is applied at the rate of infiltration to the underlying aquifer. The 4G Ranch Wetlands serve as a wet-weather management option for Pasco County’s reuse system and recharge 5 mgd on annual average to the surficial and Upper Floridan Aquifer. Located in an area suffering prolonged drawdown by regional wellfields , the 4G Ranch Wetlands also restore nearby hydrologically-altered lakes and wetlands.

Through a public-private partnership, the 3,000-acre 4G Ranch was identified as a suitable site for the infiltration wetland system. In 2015, the 176-acre groundwater recharge wetland was designed, and construction followed in 2016 and 2017. The 4G Ranch Wetlands are comprised of 15 individual cells that are each operated via water level measurements and flow control valves. Driven by the 4G Ranch’s desire to use the system for recreation, the wetland system includes several ecological design features and a mosaic of wetland habitats with transitional, shallow, and deep-water areas.

The wetlands have been in operation since 2017 and water levels of each wetland cell are adjusted seasonally to achieve healthy wetland hydroperiods and encourage the growth of desirable wetland species. Since operation, the 4G Ranch wetlands have been monitored for the success of the planted wetland vegetation establishment, the rate of infiltration, nitrate reduction, and presence and diversity of wildlife.

This presentation will describe the project, construction methods and lessons learned, and an update on the success of the overall wetland system following approximately two years of operation.

Biography: Allison joined CH2M now Jacobs as a water engineer after receiving her Masters in Environmental Engineering from the University of Florida in 2014. Her studies there focused on ecological engineering and wetlands. While attending UF, she had the opportunity to work with Dr. Bob Knight at Wetland Solutions where she gained experience in the ecological assessment of springs and the permitting and design of treatment wetlands in North Florida. Since joining Jacobs, Allison has supported various natural treatment systems projects including treatment wetland designs and ecological assessments, groundwater recharge wetland model updates, and biochemical reactor pilot studies and designs.


Spatial distribution of road-kills and factors influencing road mortality for mammals in Northern New York State

One of the most obvious impacts of roads on wildlife is vehicle-induced mortality. The aims of this study were to examine the spatial pattern of mammal–vehicle collisions (MVCs), identify and examine factors that contribute to MVCs, and determine whether the factors that increase the odds of MVCs are similar between species. On 103 road surveys that covered 7,094 total km I recorded the location of each MVC along the survey route. I measured landscape and roadway features associated with each MVC and used kernel density and network analysis tools to identify road mortality hotspots and measure spatial clustering of MVCs. I used logistic regression to model the likelihood of MVCs for all mammal data and separately for Porcupine (Erethizon dorsatum), Raccoon (Procyon lotor), Skunk (Mephitis mephitis), Muskrat (Ondatra zibethicus) and Cottontail (Sylvilagus floridanus) data sets. I identified 51 MVC hotspots and found spatial clustering of MVCs for Porcupines, Raccoons and Skunks. Two landscape variables, distance to cover and the presence of an ecotone, as well as one road variable, road width, appeared as broadly important predictors of mammalian road mortality, though there was also species-specific variation in factors that increased the risk of MVCs. Field-measured variables were more important than remotely-measured variables in predicting the odds of MVCs. Conservation implications are that mitigation of landscape features associated with higher risk of vehicle-collisions may reduce the number of MVCs in general, but species-specific research is required to more carefully tailor mitigation efforts for particular species.

Dit is 'n voorskou van intekeninginhoud, toegang via jou instelling.


3 RESULTS

3.1 Compositional differences between wetlands

Estimated sample coverage was generally high (mean = 94%) indicating effective sampling of each taxonomic group per wetland type (Table 1). Within each site, a sufficient number of samples were taken to capture the majority of plant species, and however, sampling more beetles would have resulted in a greater number of species being found (Appendix S2). Half the total species pool of plants (156 species) and 45% of the species pool for beetles (66 species) was shared by both wetland types. For both taxonomic groups, a higher proportion of the total species pool was found only within BP (31% and 30% for plants and beetles respectively), compared to OW (19% and 20% respectively). These general differences can be visualized in the unconstrained ordination (Figure 1a,c). Despite the large overlaps in the hulls for each taxon group between wetland types, the mean species composition (as represented by the centre of each ordispider) differed significantly between wetlands for both plants (bl < .001) and beetles (bl = .034). Total beta diversity was strongly dependent on turnover for both plants and beetles (96% and 94% respectively), rather than nestedness (4% and 6%).

Group Wetland type (no. of plots surveyed) Total species observed Unique to wetland (% of overall total) Estimated sample coverage (%) No of significant indicators Rarity score
Aquatic plants BP (n = 250) 126 48 (30.8%) 98 27 1.46 ± 0.03 (1.00–4.16)
OW (n = 250) 108 30 (19.2%) 99 10 1.40 ± 0.03 (1.00–3.85)
Beetles BP (n = 50) 54 18 (30.0%) 88 2 1.91 ± 0.07 (1.00–3.50)
OW (n = 50) 47 12 (20.0%) 89 0 1.89 ± 0.09 (1.00–3.00)

A total of 37 plant species were significant indicators (bl < .05) of a wetland type (Figure 1b), the majority being associated with BP. Only two beetle species were significantly associated with BP (Ilybius ater en Haliplus heydeni), and no indicator beetles were found for OW (Figure 1d).

Rarity scores of plants and beetles did not differ significantly between wetland types (plants: bl = .496 beetles: bl = .625), and none of the species found were listed as endangered or threatened on the Swedish red list (The Red List, 2015 ). Two non-native plant species were found in OW (Mimulus guttatus en Acorus calamus) and none in BP. However, both species were uncommon where present and occurred in <1% of plots sampled. No non-native beetle species were found.

3.2 Environmental basis for differences between wetlands

When both species assemblages were constrained by local environmental variables (see Appendix S1), the separation of the two wetland types was more distinct (Figure 2). In both cases, the overall constrained models were significant (bl < .001 (plants) bl = .018 (beetles)). For plants, plots from BP were associated with more woody debris, open and bare ground, while those in OW had greater leaf litter, plant height and plant coverage (Figure 2a). Water depth was the only significant environmental variable that explained beetle assemblages, though was driven by one outlying site (Figure 2b). When this outlier was removed, the overall model was not significant (bl = .186). Wetland type accounted for a significant proportion of the compositional differences for plants (bl < .001), over and above the effect of other variables, but not for beetles (bl = .136). However, only 11% of variance in composition was explained in either model.

3.3 Differences in growth strategies between wetlands

No significant differences were found between wetland types in the representation of the competitor growth strategy in the quadrat-level vegetation (bl = .16) (Figure 3a). However, in BP the representation of stress tolerators was significantly lower (bl = .01), while ruderals were more common (bl = .002) in comparison with OW (Figure 3b,c). Specifically among the subset of indicator species, the mean representation of growth strategies in BP indicator plant species (23.2 ± 3.7, 29.9 ± 5.3, 46.8 ± 5.4% for CSR respectively) contrasted strongly with the OW indicators (51.5 ± 11.1, 39.3 ± 10.8, 9.1 ± 3.5% for CSR respectively), highlighting a strong characterization of BP vegetation by ruderals and OW vegetation by competitors and stress tolerators.


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