COS 26-1 - Interacting global changes alter a seed dispersal mutualism

Background/Question/Methods

Urbanization and climate change are two drivers of global change that can alter interactions between species. These drivers often affect communities simultaneously leading to complex changes in biotic interactions, especially when participating organisms are affected asymmetrically by the shared changes. Mutualisms between plants and animals may be particularly susceptible to these asymmetries given the differences in exposure risks associated with mobility. Key Largo woodrats (Neotoma floridana smalli) are an endangered species endemic to hardwood hammocks of northern Key Largo, Florida, threatened by extreme weather events and urbanization. They consume fruits and likely disperse seeds of many native plants. We designed a replicated cafeteria style experiment using 16 species of native fruit, where we simulated an extreme weather event along a gradient of human development. Half of the fruits were exposed to sub-freezing temperatures, as extreme weather is becoming increasingly common and systems in lower latitudes are particularly vulnerable to the effects of unusual cold. Using camera traps, we monitored the patterns and preferences of fruit removal in woodrats and human commensal competitors, hypothesizing that the effects of subfreezing temperatures would interact with urbanization to decrease the probability of woodrats consuming the fruits and removing seeds.

Results/Conclusions

Fruits exposed to sub-freezing temperatures were more likely to be removed overall but that pattern was driven by increased preference by human commensal species (i.e., raccoons - Procyon lotor and opossums - Didelphis virginiana). Woodrats were more likely to remove fresh fruits. Likewise, woodrats removed most fruits when far from human development whereas human commensals removed most fruits close to developed areas. The combination of preference differences for frozen fruits between species and a higher relative abundance of human commensals closer to development resulted in a shift to the relative importance of commensals as seed dispersers. Frozen seeds were predominantly removed by commensal species nearly twice as far from developed areas as fresh seeds, potentially altering seed dispersal pathways as well as reducing the probability of woodrats removing fruit. We demonstrate that asymmetries in the responses of different species to multiple global change drivers and their interaction may alter pathways important to ecosystem services.