Understanding species-environment associations is a fundamental goal of basic and applied ecology. Environmental niche modeling is a valuable tool for evaluating these associations; however, commonly-used correlative models may not elucidate the causal link between environmental gradients and individual performance that drives variation in population dynamics and consequent distribution and abundance across the landscape. In this study, we present niche models that predict the abundance of two congeneric stream fishes (Teleostei: Cyprinidae), Pimephales notatus and P. vigilax, in the central Great Plains, USA. To test potential mechanisms for spatial variation in abundance, we conducted in-stream experiments to quantify variation in four individual performance currencies—juvenile survival, condition, and growth and adult fecundity—along a stream size gradient.
Results/Conclusions
Niche models indicated that stream size explains the most variation in abundance, with P. notatus decreasing and P. vigilax increasing in abundance from upstream to downstream. Experimental results indicated that juvenile condition and growth and adult fecundity increased from upstream to downstream for both species. These findings suggest that these performance traits may limit the abundance of P. vigilax in headwaters thus corroborating niche model predictions, but other performance traits drive the observed upstream to downstream decrease in abundance of P. notatus. Proximal environmental variables that vary along the river continuum such as temperature, resource availability, and/or biotic interactions may drive this variation in individual performance traits. This study demonstrates the complementary utility of (1) environmental niche models as tools for the identification of relevant environmental gradients along which abundance varies and (2) experimental tests of individual performance currencies along these relevant environmental gradients. This complementary approach aids in the development of mechanistic niche models that are transferable in space and time.