Ants disperse seeds further in habitat patches with corridors

Melissa A. Burt and Susan R. Whitehead, Department of Biological Sciences, Virginia Tech, Blacksburg, VA, Nick Haddad, W.K. Kellogg Biological Station Long-Term Ecological Research, Michigan State University, Nick Haddad, Department of Integrative Biology, Michigan State University, Julian Resasco, Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO

Background/Question/Methods
Habitat loss and fragmentation are two of the biggest threats to global biodiversity and ecosystem services. The breaking up of habitat into smaller, isolated patches results in a landscape in which it is more difficult to move from patch to patch and, furthermore, remaining habitat patches are susceptible to edge effects. While much work has been done to understand the effects of habitat loss and fragmentation on seed dispersal of vertebrate-dispersed plants, less is known about how habitat loss and fragmentation affect plants relying on other mechanisms of seed dispersal, such as myrmecochory (seed dispersal by ants). Changes to the landscape may impact seed dispersal by ants through effects on ant community composition or effects on the seed dispersal behavior of ants. To understand how habitat connectivity and edge effects impact ant seed dispersal, we conducted ant community sampling with pitfall traps and direct observations of seed removal by ants from depots in a landscape-scale experiment that manipulated connectivity (via corridors) and amount of edge across habitat patches. We asked: (1) How is seed dispersal by ants affected by connectivity and edge effects?, and (2) Are these effects mediated by changes in (a) ant community composition or (b) ant behavior?
Results/Conclusions
While we found no evidence that connectivity, edge amount, or edge proximity affected the time it took ants to discover seeds, the number of seeds dispersed, or ant species richness visiting depots, we did find that habitat connectivity had edge proximity dependent effects on seed dispersal distance. Ants dispersed seeds almost five times further in connected patch centers than in isolated patch centers, a pattern that does not appear to be the result of any one ant species. In isolated patches, but not in connected patches seeds were dispersed about two times further from depots at patch edges than in patch centers. With our pitfall trap ant community sampling, we found that neither the ant community as a whole nor the subset of seed-dispersing species were affected by connectivity or edge amount, but both were significantly affected by proximity to a patch edge. Differences in community composition between patch centers and edges seem to be driven by differences in the absolute abundance of ants, not in relative species abundances. Overall, these results suggest that connectivity impacts an important aspect of seed dispersal and that these impacts may be driven by changes in behavior as well as community level changes.