The biogeography of invasion in tropical and temperate seagrass beds: Testing interactive effects of predation and propagule pressure

Aim Recent work has documented latitudinal gradients of biotic resistance, revealing diminished invasion success in the tropics as compared to the temperate zone. However, no studies have explored the biogeography of biotic resistance simultaneously with propagule pressure, which can greatly influence invasion dynamics and covary with latitude. Location 9-41 degrees latitude, north-western Atlantic seagrass beds. Methods We conducted field experiments to test the interactive effects of propagule pressure (experimentally placed recruits) and biotic resistance (predation) on invader performance in temperate and tropical seagrass beds. For these experiments, we used marine invertebrate propagules from bryozoans (Bugula neritina) and tunicates (Didemnum spp.). We also quantified natural recruitment with and without exposure to predators. Results Surprisingly, predation substantially reduced invader survival at almost all latitudes. Overall, invaders experienced 15%-27% survival with predation as opposed to 75%-87% survival without predation. These patterns did not change when we increased local scale propagule pressure of Bugula by over 2-fold. However, predation had no effect on invader survival in Florida, where natural recruitment was up to 500-fold greater than other sites. We also measured substantial in situ recruitment of Bugula onto bare experimental surfaces that was not diminished with exposure to predators at mid-latitudes, suggesting a regional scale predator swamping effect. Conclusions Contrary to recent findings of latitudinal variation in biotic resistance, we found that predation strongly reduced invader success in both temperate and tropical seagrass beds. However, our results also indicate that propagule pressure (natural recruitment) can influence invasion at the regional scale to overwhelm native communities. Our data suggest that predation and propagule pressure act at varying spatial scales to affect biogeographic patterns of invasion. The importance of latitudinal variation in these interactions is largely untested but deserves attention given that globalization will continue to facilitate opportunities for invasion.