Corridors increase food web subsidies for isolated predatory spiders

Background/Question/Methods

Corridors are a conservation method to facilitate species movement by increasing connectivity between fragmented habitats. Ecological processes such as predator-prey dynamics should be maintained in otherwise isolated populations by providing food web subsidies. Without maintaining these ecological processes, corridors are destined to reduce diet breadth and trophic stability. Yet, corridor research has focused on single species movement and not on the factors that affect trophic dynamics. Our project had 2 aims: 1. to test if stable isotope marking is an effective method to trace food web connections in a terrestrial field setting; 2. to test if corridors facilitate food web subsidies. We tested the effect of connectivity on food web subsidies in experimental landscapes at the Savannah River Site in Aiken, SC, designed as a corridor movement study. We have 8 landscapes designed as a movement experiment by introducing an element into a central habitat patch and measure movement to peripheral connected and unconnected patches. To test for trophic subsidies, we enriched flowering plants in the center patch with a 15N stable isotope to trace through the food web and collected green lynx spiders (Peucetia viridans), an ambush predator that hunts on flowering plants, in peripheral patches. We then tested for 15N enrichment between spiders in corridor-connected and unconnected habitat patches 150m away from the sprayed patch. If corridors facilitate food web subsidies, then spider 15N in connected patches should be higher than those found in isolated habitats. We collected green lynx spiders within connected and unconnected patches to test the effect of connectivity on body condition. 

Results/Conclusions

We found that enriching 15N into plant communities did enrich 15N of spiders as compared to negative control spiders (t₇= 2.631; P = 0.0339). The enrichment in spiders was consistent across individuals and four times the 15N of negative control spiders.  We conclude that using 15N is an appropriate mark for food web studies.

For aim 2, we found that spiders in connected patches had a 40% increase compared to their pre-enrichment, whereas unconnected patches had no significant increase in 15N (F_2,11 = 3.056; P = 0.032). Our results demonstrate that corridors are responsible for increasing prey movement through a mosaic of habitats and that we can successfully trace prey movement via predator consumption of 15N. This is one of the few studies to test for community-level effects of corridors and shows that they support food web dynamics at the landscape scale.