Abstract
The outcome of species interactions may manifest differently at different spatial scales; therefore, our interpretation of observed interactions will depend on the scale at which observations are made. For example, in ladybeetle–aphid systems, the results from small‐scale cage experiments usually cannot be extrapolated to landscape‐scale field observations. To understand how ladybeetle–aphid interactions change across spatial scales, we evaluated predator–prey interactions in an experimental system. The experimental habitat consisted of 81 potted plants and was manipulated to facilitate analysis across four spatial scales. We also simulated a spatially explicit metacommunity model parallel to the experiment. In the experiment, we found that the negative effect of ladybeetles on aphids decreased with increasing spatial scales. This pattern can be explained by ladybeetles strongly suppressing aphids at small scales, but not colonizing distant patches fast enough to suppress aphids at larger scales. In the experiment, the positive effects of aphids on ladybeetles were strongest at three‐plant scale. In a model scenario where predators did not have demographic dynamics, we found, consistent with the experiment, that both the effects of ladybeetles on aphids and the effects of aphids on ladybeetles decreased with increasing spatial scales. These patterns suggest that dispersal was the primary cause of ladybeetle population dynamics in our experiment: aphids increased ladybeetle numbers at smaller scales because ladybeetles stayed in a patch longer and performed area‐restricted searches after encountering aphids; these behaviors did not affect ladybeetle numbers at larger spatial scales. The parallel experimental and model results illustrate how predator–prey interactions can change across spatial scales, suggesting that our interpretation of observed predator–prey dynamics would differ if observations were made at different scales. This study demonstrates how studying ecological interactions at a range of scales can help link the results of small‐scale ecological experiments to landscape‐scale ecological problems.
Highlights
Patterns of population dynamics are usually spatial scale dependent: Study outcomes might depend on the scale at which observations and measurements are made (Levin, 1992; Wiens, 1989)
We found that even though ladybeetles severely reduced aphid populations when both species were present within a patch, aphids had a much higher colonization rate and occupied many more patches than did ladybeetles
This result implies that ladybeetles had less of an effect on the overall aphid population at the metacommunity scale, likely because the higher population level colonization rate of aphids promoted the persistence of their metapopulation
Summary
Patterns of population dynamics are usually spatial scale dependent: Study outcomes might depend on the scale at which observations and measurements are made (Levin, 1992; Wiens, 1989). We consider the issue of spatial scale using an aphid- ladybeetle system Aphids and their coccinellid predators are a model system for studying biological control and predator–prey interaction dynamics (Dixon, 2000). Despite the strong impact of individual ladybeetles on individual aphid colonies (Minoretti & Weisser, 2000), the effectiveness of ladybeetles in suppressing aphid populations at large spatial scales is variable (Kindlmann, Yasuda, Sato, Kajita, & Dixon, 2015) These different lines of evidence suggest that some ladybeetle–aphid systems might persist at large spatial scales as metacommunities consisting of patches with and without predators, and that results from individual patches cannot be scaled up to the landscape without understanding the metacommunity dynamics. We simulated an additional model scenario that included predator demographic processes to explore how this would change model outcomes
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