Biological traits and interaction patterns in a seed dispersal network: Inferring the consequences of defaunation

Background/Question/Methods

Understanding how the morpho-physiological traits of species affect the structure of natural communities they form a part of, can be of major importance to understand the functioning of species interactions networks. Furthermore, detailed information about species traits can be highly valuable to understand the effect of conservation-oriented problems such as defaunation—the loss of particular animals from the network. Here we identify the interaction rules shaping seed dispersal networks and examine how species traits determine the interaction pattern. We also simulate the loss of particular animal species to illustrate how interaction rules change under different defaunation scenarios. We use data from a complete (i.e. non-defaunated) seed-dispersal network from Brazilian Pantanal including both birds and mammals. We compare three possible theoretical models describing species interactions derived from food-web analyses: i) Cascade model (all the species have the same probability to interact until a threshold is reached where the interaction is no longer possible); ii) Niche model (the interaction probability follows a Gaussian distribution) and iii) Truncated Niche model (a combination of the two previous ones). As the interaction rules may be different for birds and mammals, we performed the analyses both for the entire community and separate for each group.

Results/Conclusions

We found that, in general, the community was structured under the Cascade model, but when separating birds and mammals, the latter fit better under the Niche model. The main processes underlying the structure of the seed-dispersal assemblage were related to size of the interacting species (seed and fruit size, and body size for the animals). Moreover, large fruits presented a higher probability of being consumed by mammals than small fruits, but this trend was opposite for birds. Surprisingly, the loss of species (e.g., overhunting, loss of threatened species, elimination of exotic species) did not significantly change the interaction rules of the system. The model that better defined the interaction pattern did not change under different defaunation scenarios, except for the mammals, where the differences between the Niche and the Cascade models were small, indicating that both models may be appropriate in describing the pattern in which mammals interact with plants. We conclude that the biological correlates shaping the structure of seed-dispersal networks are related to size restrictions of the interacting species, and this pattern seems to be robust to different defaunation scenarios.