Grasshopper bioindicators of effective large-scale ecological networks

[摘要] ENGLISH ABSTRACT: Land use change is leading to rapid biodiversity loss in terrestrial ecosystems worldwide. Ecologicalnetworks (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 theresulting loss of biodiversity. Conceptually, ENs are similar to habitat corridors but are larger in scaleand more heterogeneous in their design and management. Like corridors, the efficacy of ENs forbiodiversity conservation has been called into question and requires empirical investigation. SouthAfrica boasts a unique system of extensive ENs associated with exotic timber plantations, mostlywithin the highly endangered grassland biome and Indian Ocean Coastal Belt in KwaZulu-NatalProvince, South Africa. In these regions, grasshoppers respond sensitively to management practicessuch as grazing, mowing and burning, justifying their a priori selection as an ecological indicator. Inthis study, I utilize grasshopper assemblages to determine the efficacy of ENs for biodiversityconservation and suggest guidelines for EN optimization. Specifically, I assess grasshopperassemblage sensitivity and robustness to habitat quality within ENs, congruence of grasshoppers withbutterflies, and I identify indicator species which can be utilized by managers for EN assessment infuture. I also perform a case study of isolated fragment utilization by a highly mobile generalistspecies, the bird locust, Ornithacris cyanea (Stoll, 1813) (Acrididae: Cyrtacanthacridinae).Grasshopper assemblages were sampled within ENs in two geographic regions (Zululand andthe KwaZulu-Natal Midlands) and in two years (2007 and 2008) during the peak season ofgrasshopper abundance, late summer (February-April). Local-scale environmental variables relatingto management practices and landscape-scale environmental variables relating to design of the ENswere quantified. Management practices explained, on average, two-thirds of the variability ingrasshopper assemblages that could be explained, while design variables explained one-third. Grassheight 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 someanalyses 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 didnot differ among isolated fragments, connected corridors and reference sites, although they did differamong geographic regions.Not all grasshopper species responded similarly to ENs. There was a strong phylogeneticsignal in species response to environmental variables, with the distribution of highly mobile familiesvarying more among years. Linear, heavily disturbed power line servitudes had higher grasshopperabundance, and generalist species consistent with early successional assemblages. Reference siteshad 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 highhabitat quality. A case study of the bird locust showed that populations of this highly mobile specieswere 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 bioindicationmethod for South Africa's ENs is suggested and has great potential for assessment of a crucial andsensitive trophic layer within the ENs. This method should be field-tested and revised over time asgrasshopper relative abundances and species compositions may change. Increased heterogeneity,simulation of multiple successional stages, and increased connectivity are expected to positivelyimpact biodiversity, particularly of insect primary herbivores. Globally, ENs, if managed anddesigned appropriately, have potential to enhance biodiversity, particularly of smaller residentorganisms which can utilize the ENs for movement and live within them.

[发布机构] Stellenbosch University