Top predator introduction changes the effects of spatial isolation on freshwater community structure

Abstract

1Spatial isolation can differentially affect the distribution of predators and thus affect lower 2 trophic levels by resulting in trophic cascades. Similarly, the introduction of top predators into 3 isolated ecosystems can cause the same cascading effects because they mostly prey upon larger 4 frequently predatory taxa, indirectly benefiting consumers. Here we experimentally tested 5 whether spatial isolation can affect the outcome and strength of the cascading effects caused by 6 fish on macroinvertebrate community structure. We found that fish did reduce the abundance of 7 predators but had no effect on consumers. Spatial isolation, however, did cause trophic cascades, 8 but only in the absence of fish. We believe this happened because fish also preyed upon 9 consumers when they increase in abundance. Additionally, and in contrast with simple 10 theoretical expectations for metacommunities, we found that the difference between ponds with 11 and without fish increased with isolation, probably because fish dampened the cascading effects 12 of spatial isolation. 13

Spatial community variability: Interactive effects of predators and isolation on stochastic community assembly

“…We conducted three sampling surveys of freshwater macroinvertebrates after ~3 weeks (18 to 23-Feb-2017), ~8 weeks (23 to 27-Mar-2017), and ~12 weeks (20 to 24-Apr-2017) of the experiment. We have previously used data from this experiment to address other questions about community structure and more detailed information on the experimental design and data collection is available in the Methods and Supplementary Material of Pelinson et al (2020).…”

“…In freshwater ponds, fish tend to be the top predators of the community (Wellborn et al 1996). Because they are usually visually oriented predators, they preferentially prey upon more conspicuous taxa which frequently are large-bodied predatory insects (Diehl 1992, Goyke and Hershey 1992, Wellborn et al 1996, Batzer et al 2000, Pelinson et al 2020. Therefore, fish can exclude more vulnerable species from the metacommunity, decreasing gamma, and consequently, beta-diversity .…”

“…CC-BY-ND 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint this version posted December 22, 2020. ; https://doi.org/10.1101/2020.12.22.423949 doi: bioRxiv preprint (Pelinson et al 2020). Thus, if species interactions are important and the most abundant taxa (i.e.…”

“…Freshwater insects are known to vary in their ability to colonize differently isolated habitats (Shulman and Chase 2007, Chase and Shulman 2009, Hein and Gillooly 2011, Pelinson et al 2020. Predatory insects generally have smaller population sizes and greater generation times if compared to their prey, thus they have fewer events of dispersal than herbivores and detritivores (Shulman and Chase 2007, Chase and Shulman 2009, Hein and Gillooly 2011, Pelinson et al 2020). The consequence is that some of those predatory insects may be absent from highly isolated ponds, decreasing gamma (Hendrickx et al 2009) and consequently, beta-diversity (Chase and Myers 2011).…”

“…1d and 1f). (H4) Finally, even though fish preferentially prey upon large-bodied predatory insects, they might also substantially prey upon whatever taxa are most abundant (Pelinson et al 2020), thus if species interactions are important, the presence of fish should decrease their importance by reducing the abundance of any taxa who successfully colonize ponds first ( Fig. 1e and 1f).…”