Restoration of trophic structure in an assemblage of omnivores, considering a revegetation chronosequence.
Abstract
Habitat loss is considered the single greatest driver of species extinctions and amelioration of this threat through habitat restoration is increasingly important in maintaining biodiversity and ecosystem functions. Success of restoration depends largely on the ability of species of a diversity of functional types to recolonize a restored site. We used a restoration chronosequence to investigate the success of revegetation efforts (direct-seeding and tube stock methods) in returning species and trophic functions of trophically diverse ant assemblages to land formerly used for livestock grazing in south-eastern Australia. We examined assemblages based on: (i) species composition; (ii) trophic position and diversity, measured using stable isotopes of nitrogen; and (iii) species richness of trophic guilds based on δ15N isotopes ('δ15N guilds') and prior knowledge of diet ('traditional trophic groups'). Responses for δ15N trophic guilds were more distinct than for those based on traditional trophic groups. Assemblage trophic position (mean species δ15N enrichment) decreased and trophic diversity (variance among species in δ15N enrichment) increased along the chronosequence from pastures to remnants. These patterns resulted from an increase in the richness of species with low δ15N enrichment (i.e. more 'herbivorous' species) along the chronosequence, which was also reflected in increases in the species richness of sugar feeders. Recovery of direct-seeded sites lagged behind tube stock sites of the same age, suggesting a role for habitat structure. Species composition followed a trajectory suggestive of partial recovery, with revegetated sites supporting assemblages intermediate to pastures and remnants, but distinct from both. Synthesis and applications. Tree planting promotes recovery of species composition and trophic function. Planting method affects the speed of functional recovery, although minimally. Species on the herbivorous end of the spectrum of omnivory were slower to recover than more predaceous species and may require deliberate restoration of features associated with mature trees. Trophic guilds based on objective measures of diet respond more clearly to restoration than do those based on subjective observations and offer a more insightful method.