Spillover: A specific movement process

Background/Question/Methods

Spillover is the uni-directional movement from one donor habitat to a different, adjacent recipient habitat. This second location is subsequently influenced by the propagule, for example, through the introduction of beneficial ecological functions. Although, as defined, spillover is a distinct movement type, often the term is generalized to include secondary movement patterns in which the propagules leave the two-habitat system, return to the donor habitat, or maintain a presence at the edge. This movement is derived from processes such as dispersal, foray loops, and range movement, respectively, not spillover. We performed a systematic literature review to synthesize how the term is classified as well as the methods used to quantify it. We hypothesized that, if the definition lacks specificity, the methods used to quantify spillover would be insufficient to identify propagules moving through this pattern alone. Additionally, to address some of our perceived weaknesses of this generalization, we supplemented the review with a spatially explicit model that assesses changes in population dynamics and donor/recipient habitat quality. This model includes four movement patterns commonly used to define spillover. We hypothesized that population structure and habitat quality would diverge over generational time among different foraging and movement behaviors.

Results/Conclusions

In the review, 201 articles met our search criteria and were further subdivided into 317 empirical studies to account for multiple experiments within one article. Of studies with clear definitions (55%), 45% described spillover as a movement process (twice as likely as an effect) and 17% specifically used our movement pattern. Contrastingly, only 4% of studies quantified the movement of propagules. Instead, the potential consequences of propagules spilling-over, such as changes in richness (26%), organism presence/absence (24%), and abundance (20%) were assessed and spillover movement was inferred. Expectedly, the methods used in all but two of the studies in our review could not differentiate propagules that moved through spillover from the propagules present due to other movement patterns. As we hypothesized with our model, different movement patterns may change population dynamics (e.g., abundance, persistence), individual fitness (e.g., growth rate, fecundity), and individual behavior (e.g., competitive, dispersive). This in turn can affect the landscape as resources are exploited differently (e.g., foraging, transience) among movement behaviors. We propose that the addition of mark-recapture techniques (used by only 3% of studies) in both habitats would help identify propagules using different movement patterns and should be given more consideration in future studies.