Subtle physiological and morphological differences explain ecological success of sympatric congeners

Sympatric congeners with similar physiological and morphological characteristics may appear to overlap in niche space but respond to environmental change in different ways leading to population decline of one species while the other remains stable. Understanding why sympatric congeners vary in their ecological success can be challenging, but is particularly necessary given the magnitude of human-induced environmental change among ecosystems. We propose that identifying a complex of subtle, interacting characters among congeners may be more effective in elucidating both historical coexistence and divergent ecological success in contemporary habitats compared to identifying just one apparent limiting similarity between species. Using this subtle difference hypothesis, we examined how metabolic rate associated with habitat use and internal and external morphology collectively influenced the ecological success of a common and a rare sturgeon species that differ dramatically in their conservation status due to environmental change. Multivariate analyses of gut morphology (e.g., intestine length) combined with respirometry on sand and gravel habitats were incorporated into a bioenergetics model to compare how the fishes responded to habitat change and food quality. Energetic tradeoffs induced by habitat type and underlying morphological differences led to different predicted growth rates. Compared with the more prevalent species, the rare and endangered fish needed to seek different habitats with less energetic costs and switch to foraging at a higher trophic level to persist. Our results corresponded to observed differences in ecological success between these species in the wild. Thus, subtle physiological and morphological differences may lead to dramatic differences in ecological success in contemporary habitats for species that are very similar ecologically.