Stronger predation in a subtropical community dampens an invasive species-induced trophic cascade

Abstract

Invasive predators can disrupt important predator–prey dynamics thereby facilitating invasions into prey communities. Lower latitude food webs can have strong consumer pressure and top-down control due to trophic complexity, however, and thus could be more robust to alterations by introduced predators than higher latitude food webs. We compared the strength of a marine invasive species-induced trophic cascade in a temperate and subtropical system, hypothesizing that a non-native tertiary consumer could facilitate non-native basal prey establishment through the consumption of a native secondary consumer. We further predicted that the ecological importance of this cascade may be reduced in the subtropical community relative to the temperate community due to stronger consumer pressure from top predators. Using marine invertebrates, we conducted experiments in temperate Long Island Sound, Connecticut, USA and subtropical Indian River Lagoon, Florida, USA to test our hypothesis. We further examined predation pressure on tertiary consumers in order to assess any top-down control of the invasive species-induced cascade from higher trophic levels. We found evidence of a trophic cascade in both regions, but it was only maintained under ambient predation pressure in the temperate region. Our results also suggest that strong predation pressure on the non-native portunid crabs in the subtropical region may help explain the weakened cascade under ambient conditions. Studying predator invasions in the context of full communities and from a macroecological perspective can be used to predict both local and large scale patterns of invasion, and lead to identifying locations where high-impact invasions are more likely to occur.