Step back! Niche dynamics in cave-dwelling predators

The geometry of the Hutchinson’s hypervolume derives from multiple selective pressures defined, on one hand, by the physiological tolerance of the species, and on the other, by intra- and interspecific competition. The quantification of these evolutionary forces is essential for the understanding of the coexistence of predators in light of competitive exclusion dynamics. We address this topic by investigating the ecological niche of two medium-sized troglophile spiders (Meta menardi and Pimoa graphitica). Over one year, we surveyed several populations in four subterranean sites in the Western Italian Alps, monitoring monthly their spatial and temporal dynamics and the associated physical and ecological variables. We assessed competition between the two species by means of multi regression techniques and by evaluating the intersection between their multidimensional hypervolumes. We detected a remarkable overlap between the microclimatic and trophic niche of M. menardi and P. graphitica, however, the former —being larger in size— resulted the best competitor in proximity of the cave entrance, causing the latter to readjust its spatial niche towards the inner part, where prey availability is scarcer (“step back effect”). In parallel to the slight variations in the subterranean microclimatic condition, the niche of the two species was also found to be seasonal dependent, varying over the year. With this work, we aim at providing new insights about the relationships among predators, demonstrating that energy-poor environments such as caves maintain the potential for diversification of predators via niche differentiation and serve as useful models for theoretical ecological studies.