Studies of climate change often focus on single species responses to climate drivers; however, climatic changes could lead to phenological mismatches between pairs of interacting species. Mutualisms, such as seed dispersal mutualisms, could be particularly important to understand, because dispersal will critically influence whether range shifts are possible. In Ohio, approximately one third of the herbaceous plants are identified as myrmecochores and Aphaenogaster picea is recognized as the main seed dispersing ants. We examined how ant foraging behavior varied with varying daily temperature and whether ants showed a preference for Trillium erectum or Viola pubescens diaspores. Diaspores of one species were presented at the entrance of marked nests for 4 hours, were replaced as taken and totals recorded every 30 minutes. Next, we used theoretical models to predict how seed preference and increasing temperature variation might influence seed dispersal. We modeled the seed release of two plants and ant activity using either a calendar day based mechanism, which allowed us to focus on the overlap of ant activity and plant seed production, or a degree-day based mechanism, which allowed us to quantify the effects of temperature variation on the total seeds collected.
Results/Conclusions
For our empirical study, we found that Aphaenogaster picea increased foraging efforts during periods of higher temperatures and slightly preferred the larger Trillium erectum diaspores to the smaller Viola pubescens diaspores, though this preference was not statistically significant. These results suggest there is significant variation in daily ant foraging levels due to temperature. For our theoretical study, our model suggested that plants that are less preferred could maintain a competitive advantage by releasing seeds for longer periods of time, thus increasing the likelihood of contact. Temperature variation either had no effect or caused an increase in the number of seeds collected per year, unless the temperature dropped below freezing before seed release of the plants was complete. This system showed a lot of resilience to temperature variations. Plants that are at the highest risk of changes in seed collection due to increase in temperature variation are those with short seed release periods and plants that release seeds early or late in the season. In spite of the high degree of robustness of generalized dispersal mutualisms, there could be a threshold effect (such as the freezing effect seen in our model), beyond which the mutualisms would collapse.