How species interactions are modified by habitat fragmentation remains poorly understood. We experimentally evaluated how habitat fragmentation impacts a tritrophic interaction between plants, mutualistic fungivorous mites, and leaf fungus. We focused on three consequences of habitat fragmentation: reduced patch connectivity, increased patch edge:area, and increased edge proximity. Our work took place within seven experimentally-fragmented landscapes, where patches differ in isolation (connected by a corridor or not) and edge:area (high vs low). We selected focal plant individuals (n=109 Quercus nigra) either near patch edges or in patch centers. The majority of domatia-dwelling mites in our system are likely to be fungivorous, and so to examine the effects of the mites on leaf fungi, we eliminated mites from half of our focal plants (n=54). We did this by sealing domatia with pruning tar (preventing mites from living on those leaves) and later counting fungal hyphae on leaves (n=327 leaves) of both treatment and control oaks (domatia covered vs open, respectively). We also counted the number of mites present on each sampled leaf. We asked how fungal abundance is affected by: (1) tar treatment (mite abundance) (2) edge proximity (3) patch connectivity and (4) patch edge:area. We predicted that fungal abundance would be higher on oaks from which mites were excluded, at edges of patches (better microclimate for fungi), and in connected and edgier patches.
Results/Conclusions
(1) Fungal hyphal counts were qualitatively lower on control trees (high mite abundance), but this effect was not statistically significant. (2) Counts of fungal hyphae were higher at edges of patches (p=0.04), as predicted. (3) We did not find an effect of habitat connectivity, (4) or patch edge:area on fungal abundance, indicating that large, landscape-scale factors may be less important than within-patch factors, such as edge effects, in regulating fungal abundance. Additionally, the disconnect between the number of mites and the abundance of their expected food source, the fungal hyphae may be due to a low population of mites in our system generally, as this species interaction is known to be most beneficial to the plant when mite populations are >20 mites/leaf, while we observed mite populations with an average of 0.2 mites/leaf. These findings demonstrate that this tritrophic interaction is influenced by habitat fragmentation, but largely through local-level factors, in this case proximity to the edge of a patch, which may have stronger impact on this species interaction than landscape-level factors that typically operate between patches.