Modelling effects of seed dispersal patterns and natural enemy attack on seedling spatial patterns

Noelle Beckman and company have an early-view paper in the Journal titled “The interacting effects of clumped seed dispersal and distance- and density-dependent mortality on seedling recruitment patterns”.  Read the abstract here.

The authors have provided a short synopsis of the paper and a photo of spider monkeys on Barro Colorado Island.

Spider monkeys on Barro Colorado Island. Photo credit: Noelle Beckman.

Plant communities can harbor hundreds of species. Biologists have long debated the contribution of different mechanisms to the maintenance of this high biodiversity. The seedling patterns resulting from a combination of seed dispersal and plant mortality due to natural enemies of seeds, including predators and pathogens, are hypothesized to play a key role in maintaining plant diversity. In models, seed dispersal is typically modeled as declining evenly with distance from the tree, but vertebrates disperse seeds in clumps to areas they prefer. For example, spider monkeys disperse 92% of seeds from Virola calophylla, a relative of nutmeg, and deposit approximately half of these in clumps under areas they prefer to sleep. Although not typically incorporated in models, this clumped dispersal may be common, as approximately 80% of tropical woody plants are primarily dispersed by vertebrates and 56% of flowering plants are dispersed by animals. This clumped seed deposition alters plant interactions with natural enemies with consequences for plant spatial patterns and diversity.

There have been a handful of modeling studies investigating how natural enemies shape spatial patterns of seedlings; the little work on this subject focuses on vertebrate and invertebrate seed predators. Although pathogens are expected to be a major driver in seedling survival and have very different movement patterns and life histories compared to animals, pathogens have not been incorporated in models. Pathogens are known to have fast generation times, short dispersal distances, high fecundity, and the ability to remain dormant in the soil for several years, while insect seed predators are able to search larger areas and have longer generation times. Furthermore, because larvae develop inside seeds, consuming and consequently killing seeds, seed mortality depends on the ratio of insect eggs to seeds dispersed rather than the density of seeds.

We used  simulation models to examine how clumped versus even seed deposition, insect seed predation, and pathogen attack structure seedling spatial patterns around an isolated tree. Simulation models are useful to examine processes that occur over large spatial areas and long time periods that would be difficult to manipulate in the field. When seeds dispersed greater distances than natural enemies, we found that insect seed predators and pathogens resulted in peaked spatial patterns, suggesting the role of natural enemies in maintaining plant diversity. Although pathogens dispersed much shorter distances than seeds, pathogens were able to track seeds because of their fast generation times and spore dormancy. In contrast to pathogens, insect seed predators became satiated under the tree when insects dispersed longer distances than seeds. Clumping increased the probability of seedling survival under both insect seed predation and pathogen attack as it led to local satiation of insect seed predators and made it harder for pathogens to track seeds. Our study suggests that the differing assumptions of movement and life history between pathogens and insects, the degree of clumping among seeds, and the relative dispersal distances of seeds and natural enemies are crucial to determining establishment rates and spatial patterns of seedlings.

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