Regional diversity can increase owing to either the packing of species within regional niche space or the expansion of regional niche space. Yet, the primary factors dictating these dynamics remain poorly understood. The question of whether species pack or expand niche space along environmental gradients is strongly related to bounded versus unbounded hypotheses. Moreover, a related set of hypotheses about niche packing versus expansion takes into account temporal environmental dynamics and especially historical environmental stability. Prior examinations on the mode of filling a niche space along gradients in species richness have ignored plausible inter‐clade interactions in niche filling dynamics even though processes that generated contemporary patterns in species richness can vary across taxonomic groups and phylogenetic scales.
We assess the relative influence of net primary productivity (NPP) and historical environmental stability over the Last Glacial Maximum (LGM, ~21 kyr BP) on the packing and expansion of niche space. We focus our study in the Australian Wet Tropics (AWT) as the region has undergone significant habitat shifts with rain forests expanding and contracting over the LGM period, thus providing an ideal opportunity to evaluate the relative contribution of current and historical factors in niche filling dynamics. We measured patterns of niche filling (niche packing vs. niche expansion) as the standardized departure of observed functional diversity (FD) from its null expectation. We fitted spatial models for vertebrates, and for each constituent class (mammals, birds, amphibians and reptiles) separately, to evaluate the relative effects of NPP and environmental stability on species richness and niche filling patterns.
Results/Conclusions
Historical environmental stability had a greater effect than NPP on species richness and niche filling patterns. However, the directionality of this effect depended on phylogenetic scale, with vertebrates exhibiting niche packing while each constituent class (except reptiles) exhibited niche expansion with increasing environmental stability.
Intra‐class competition presumably leads to niche differentiation and expansion, whereas the overlap of functional traits among species from different classes leads to niche packing. That environmental stability over millennia is associated with an expanding niche space across multiple vertebrate classes suggests that the accumulation of FD within communities requires long recovery times.