Absence of light is a fundamental characteristic of subterranean ecosystems; thus, productivity must be supported indirectly by influx of detritus from effulgent environments. I examined how this influx impacts the carrying capacity of a cave predator: the whip spider Phrynus longipes (Pocock, 1894). Although solitary, territorial and cannibalistic, this species occurs at extremely high densities in caves. To test the hypothesis that this is an effect of nutrient flow and not cave structure, I examined whether guano deposition at cave entrances predicted estimated population sizes of whip spiders. I found a strong correlation, suggesting that whip spider carrying capacities are at least partly determined by nutrient influx to the cave ecosystem. Larger guano deposits support a larger community of arthropod detritivores, which act as prey to this top predator in a bottom-up effect. This highlights the importance of considering surface environmental and population health along with commercial guano harvesting when studying and conserving caves and the species therein.