Grasshopper bioindicators of effective large-scale ecological networks

Bazelet, Corinna Sarah (2011-03)

Thesis (PhD)--University of Stellenbosch, 2011.

Thesis

ENGLISH ABSTRACT: Land use change is leading to rapid biodiversity loss in terrestrial ecosystems worldwide. Ecological networks (ENs) are systems of remnant natural habitat which remain intact in a transformed matrix, and have been suggested as a means to mitigate the effects of habitat loss and transformation and the resulting loss of biodiversity. Conceptually, ENs are similar to habitat corridors but are larger in scale and more heterogeneous in their design and management. Like corridors, the efficacy of ENs for biodiversity conservation has been called into question and requires empirical investigation. South Africa boasts a unique system of extensive ENs associated with exotic timber plantations, mostly within the highly endangered grassland biome and Indian Ocean Coastal Belt in KwaZulu-Natal Province, South Africa. In these regions, grasshoppers respond sensitively to management practices such as grazing, mowing and burning, justifying their a priori selection as an ecological indicator. In this study, I utilize grasshopper assemblages to determine the efficacy of ENs for biodiversity conservation and suggest guidelines for EN optimization. Specifically, I assess grasshopper assemblage sensitivity and robustness to habitat quality within ENs, congruence of grasshoppers with butterflies, and I identify indicator species which can be utilized by managers for EN assessment in future. I also perform a case study of isolated fragment utilization by a highly mobile generalist species, the bird locust, Ornithacris cyanea (Stoll, 1813) (Acrididae: Cyrtacanthacridinae). Grasshopper assemblages were sampled within ENs in two geographic regions (Zululand and the KwaZulu-Natal Midlands) and in two years (2007 and 2008) during the peak season of grasshopper abundance, late summer (February-April). Local-scale environmental variables relating to management practices and landscape-scale environmental variables relating to design of the ENs were quantified. Management practices explained, on average, two-thirds of the variability in grasshopper assemblages that could be explained, while design variables explained one-third. Grass height and the time since the last fire event were most consistently influential, while area, context, isolation, proportion of bare ground and proportion of forbs at a site, proved influential in some analyses but not others. This response was robust over time and among geographic regions. Grasshopper species richness and abundance were highly congruent with that of butterflies and did not differ among isolated fragments, connected corridors and reference sites, although they did differ among geographic regions. Not all grasshopper species responded similarly to ENs. There was a strong phylogenetic signal in species response to environmental variables, with the distribution of highly mobile families varying more among years. Linear, heavily disturbed power line servitudes had higher grasshopper abundance, and generalist species consistent with early successional assemblages. Reference sites had more graminivorous species and those with intermediate mobility. Three species of grasshopper were identified with IndVal and validated on an independently collected dataset as indicators of high habitat quality. A case study of the bird locust showed that populations of this highly mobile species were effectively isolated and undergoing anthrovicariance even at short distances. These results indicated that heterogeneous ENs supported diverse grasshopper assemblages, although movement among isolated fragments may have been limited. A grasshopper bioindication method for South Africa’s ENs is suggested and has great potential for assessment of a crucial and sensitive trophic layer within the ENs. This method should be field-tested and revised over time as grasshopper relative abundances and species compositions may change. Increased heterogeneity, simulation of multiple successional stages, and increased connectivity are expected to positively impact biodiversity, particularly of insect primary herbivores. Globally, ENs, if managed and designed appropriately, have potential to enhance biodiversity, particularly of smaller resident organisms which can utilize the ENs for movement and live within them.

AFRIKAANSE OPSOMMING: Veranderinge in grondgebruik lei tot ‘n vinnige verlies van biodiversiteit in terrestriële ekosysteme wêreldwyd. Ekologiese netwerke (ENs) is sisteme van oorblywende natuurlike habitat wat ongeskonde bly in getransformeerde matrikse, en is voorgestel as ‘n manier om die verlies aan biodiversiteit as gevolg van habitatverlies en transformasie te verminder. Konseptueel, is ENs soortgelyk aan habitat korridors, maar is groter in omvang en meer heterogeen in hulle ontwerp en bestuur. Soos korridors, is die effektiwiteit van ENs om biodiversiteit te bewaar bevraagteken, en vereis dit empiriese ondersoek. Suid-Afrika spog met ‘n unieke stelsel van uitgebreide ENs wat geassossieer is met uitheemse plantasies, meestal in die hoogs bedreigde grasveld bioom en Indiese Oseaan kusstrook in KwaZulu- Natal Provinsie, Suid Afrika. In hierdie streek reageer sprinkane sensitief op bestuurspraktyke soos beweiding, sny en brand. In hierdie studie het ek sprinkaan gemeenskappe gebruik om die effektiwiteit van ENs vir bewaring van biodiversiteit te bepaal en om riglyne te stel vir EN optimering. Spesifiek het ek sprinkaan gemeenskap sensitiwiteit en robustness tot habitat kwaliteit bepaal, oreenstemming tussen sprinkane en skoenlappers getoets, en het ek indikator species geïdentifiseer wat in die toekoms deur bestuurders gebruik kan word vir EN assessering. Ek het ook ‘n gevallestudie gedoen van geïsoleerde fragment benutting deur ‘n hoogs mobiele, algemene spesie, Ornithacris cyanea (Stoll, 1813) (Acrididae: Cyrtacanthacridinae). Sprinkaan gemeenskappe was versamel in twee geografiese gebiede (Zululand en die KwaZulu-Natal Midlands) en in twee jare (2007 en 2008) gedurende die seisoen van hoogste sprinkaan aktiwiteit, laat somer (Februarie – April). Plaaslike-skaal omgewingsveranderlikes met betrekking tot bestuurspraktyke en landskap-skaal omgewingsveranderlikes met betrekking tot die ontwerp van ENs is gekwantifiseer. Bestuurspraktyke verduidelik, gemiddeld, twee derdes van die variasie in die sprinkaan gemeenskappe, terwyl ontwerpveranderlikes een derde verduidelik. Grashoogte en tyd sedert die laaste brand was konsekwent invloedryk, terwyl area, konteks, isolasie, hoeveelheid kaal grond en hoeveelheid kruidagtige plantegroei invloedryk was in sekere analyses, maar in ander nie. Hierdie reaksie was konstant oor tyd en tussen geografiese gebiede. Sprinkaan spesies rykheid en hoeveelheid was hoogs kongruent met die van skoenlappers, en het nie verskil tussen geïsoleerde fragmente, verbinde korridors en verwysingsareas nie, maar hulle het verskil tussen geografiese streke. Nie alle sprinkaan spesies het soortgelyk gereageer tot ENs nie. Daar was ‘n sterk filogenetiese sein in spesies reaksies op die omgewingsveranderlikes, met meer variasie tussen jare in die verspeiding van hoogs mobiele families. Liniêre, hoogs versteurde kraglyn dienspaaie het hoer sprinkaan hoeveelhede gehad en algemene spesies wat geassosieer is met gemeenskappe in vroeë stadiums van suksessie. Verwysingsareas het meer graminivorous spesies gehad en spesies met intermediêre mobiliteit. Drie sprinkaan spesies is geïdentifiseer met IndVal en bevestig met ‘n onafhanklik versamelde dataset as indikators van hoë kwaliteit habitat. ‘n Gevallestudie op O. cyanea het aangedui dat populasies van hierdie hoogs mobiele spesie geïsoleerd is en dat hulle anthrovicariance ondergaan, selfs op kort afstande. Hierdie resultate dui aan dat heterogene ENs diverse sprinkaan gemeenskappe ondersteun, hoewel beweging tussen geïsoleerde fragmente dalk beperk is. Verhoogde heterogeneïteit, simulasie van verskeie suksessiewe fases en meer konnektiwiteit sal moontlik ‘n positiewe invloed op biodiversiteit hê, aangesien sprinkane verteenwoordigend was van insek primêre herbivore. Wêreldwyd, as ENs op ‘n gepaste manier bestuur en ontwerp word, het dit die potensiaal om biodiversiteit te bevorder. Dit is veral belangrik vir kleiner, plaaslike organisms wat ENs kan benut vir beweging en as leefhabitat. ‘n Sprinkaan bioindikasie metode vir Suid-Afrika se ENs is voorgestel en het groot potensiaal vir die assessering van ‘n belangrike en sensitiewe trofiese laag binne die ENs. Hierdie metode moet in die praktyk getoets word en hersien word oor tyd om veranderinge in sprinkaan relatiewe hoeveelhede en spesies komposisies in ag te neem.

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