Theory: Impact of disperser loss on forest communities, plant spatial patterns, and plant diversity

Background/Question/Methods

Forests are experiencing global declines in vertebrate abundances, many of which are important seed dispersers of plants. Declines in seed disperser abundances will reduce dispersal of vertebrate-dispersed plants compared to abiotically dispersed plants with consequences for seedling spatial patterns and diversity.  Seed dispersal establishes the spatial template that determines future local interactions; natural enemies alter this initial pattern. Seed dispersal is typically modeled as monotonically decreasing with distance from the tree, but vertebrates disperse seeds in clumps to areas they prefer. This clumped seed deposition alters plant interactions with natural enemies with consequences for plant diversity. Seedling patterns resulting from plant mortality due to specialized seed predators and pathogens are hypothesized to play a key role in maintaining plant diversity. I investigate the influence of seed dispersal by wind and vertebrates and patterns of plant mortality due to insects and pathogens on seedling spatial patterns and coexistence and examine how hunting modifies these patterns using spatially explicit stochastic models. 

Results/Conclusions

Seedling recruitment patterns observed in the model reproduce the range of patterns observed empirically. I find that recruitment patterns and plant diversity are sensitive to the type of natural enemy attack, the dispersal mode, and the relative dispersal distances of seeds and natural enemies. Reducing dispersal distances of seeds increases seed mortality due to pathogens, and when seed dispersal distances are lower than insect dispersal, increases satiation of insect predators under low to intermediate insect fecundity. I discuss the importance of accurately describing seed dispersal and natural enemy movement and natural histories to improve our  understanding of seedling distributions and diversity. A basic understanding of how animals and pathogens affect plant diversity is essential as seed dispersal, predation, and diseases are expected to change in the face of increasing anthropogenic pressures on ecosystems, such as hunting.