Reduced competition may allow generalist species to benefit from habitat homogenization

Complex environments support high biodiversity and diverse microhabitat availability, which may reduce the intensity of competition among species. Both natural and anthropogenic disturbances reduce the structural complexity of habitats, leading to homogenization. High abundances of common, generalist species in disturbed habitats may be driven by reduced competition from specialists in similar habitats. We quantified habitat availability for and utilization of three co-occurring arboreal geckos (Australian native house geckos [Gehyra dubia], northern velvet geckos [Oedura castelnaui] and eastern spiny-tailed geckos [Strophurus williamsi]) in four replicated grazing regimes in an experimental grazing trial in north-east Queensland, Australia. Native house geckos were most abundant in heavily grazed habitats, whereas the two other species rarely co-occurred (either with each other or with native house geckos). Geckos displayed resource partitioning of habitat features, such as tree species and structural characteristics. We found evidence of interspecific competition among gecko species, in which native house geckos shifted their habitat selection in the presence of velvet geckos. In the absence of other geckos, native house geckos preferred rough, peeling bark and dead trees, yet in the presence of velvet geckos, native house geckos shifted away from dead trees, and used more structurally complex trees, probably due to high niche overlap with velvet geckos. Native house geckos were more resistant to the negative effects of livestock grazing than either velvet or spiny-tailed geckos. In the absence of other species, native house geckos used a wider range of microhabitats. Synthesis and applications. Species assemblages are often the results of multiple or complex factors, including predation pressure, habitat availability or competitive interactions. The homogenizing effects on habitat structure caused by livestock grazing reduce diversity and suitability for microhabitat specialists. Reduced competition can therefore promote the abundance of microhabitat generalist species, such as Australian native house geckos, suggesting that livestock grazing leads to homogenization and simplification of habitat structure, which ultimately leads to changes in species composition through reduced competition. Understanding species' responses to disturbance, and more broadly, habitat complexity, is crucial for maintaining or increasing biological diversity in anthropogenically modified landscapes.