What comes after seed dispersal? Factors controlling seed retrieval by rodents

Background/Question/Methods

Assessing the relative importance of extrinsic and intrinsic factors affecting seed ecology is a key challenge to fully understand the role of plants and animals in seed dispersal. Seed retrieval is a crucial phase within the seed dispersal process since the probability of a seed escaping recovery will determine the probability that it will survive and develop into a new seedling. Intrinsic seed factors (e.g. size, content) may have evolved the ability to manipulate scatter-hoarder behavior to decrease the probability of seed recovery. However, it is far from clear whether seed recovery is mostly a plant-driven process or incidental to the animal activity (mostly an animal-driven process). Following this, we compare the relative importance of intrinsic (seed weight, seed quality, seed-drop timing) and extrinsic seed factors (i.e. mostly animal-driven) in the probability of seed recovery. We considered the next animal-driven factors: microsite of deposition, burial depth, cache size, frequency of seed handling and distance to shelter. We used acorns of Quercus pyrenaica Willd. in a temperate oak forest in Southern Europe where scatter-hoarding rodents are the main acorn dispersers. We used model averaging techniques to ascertain the relative importance (value from 0 to 1) of each variable.

Results/Conclusions

Overall, intrinsic seed factors (Mean Importance = 0.29; range 0.09-0.44) were less important than extrinsic (Mean Importance = 0.80; range 0.27-1.00). Frequency of seed handling (i.e. number of times each acorn was handled) and burial depth were the main factors affecting the probability of seed recovery. Thus, seeds that were dispersed repeatedly or buried at deeper sites had lower probability of being finally recovered by rodents. Likewise, microsite of seed deposition was also an important factor. Acorns that were hoarded in burrows showed significantly lower probability of being recovered in comparison to those that were cached under the ground or within the litter. Seed quality was the most important intrinsic seed factor in explaining seed recovery but ranked 5^th out of 8 in the relative importance of each variable. Interestingly, seed weight and seed-drop timing were variables of low importance, with relative values of 0.35 and 0.09, respectively. This study shows the importance of extrinsic seed factors (i.e. animal-driven factors) over intrinsic factors (i.e. plant-driven) within the seed recovery phase. By quantitatively analyzing the behavioral components of the seed recovery process, we can better understand the role of secondary dispersers as a selective force for seeds.