Spatial mismatch with both herbivores and mutualists during range expansion of Avicennia germinans, the black mangrove
There is increasing appreciation for the need to consider biotic interactions in predictions of species distributions and range shifts. Species undergoing range expansion face the prospects of both enemy release and spatial mismatch with mutualists. Whether this trade-off influences the rate or occurrence of range expansion is largely unknown. We tested for evidence for this trade-off in the black mangrove, Avicennia germinans, which is currently expanding its range northward in Florida. In particular, we examined the interactions between A. germinans and its pollinators and florivores – interactions that directly impact A. germinans’ reproduction and ability to colonize new sites. During the 2013 and 2014 flowering seasons, we monitored rates of floral visitation by insects and the frequency of florivory at six sites spread throughout the eastern Florida A. germinans distribution. We also conducted pollen limitation experiments at two sites – one central to the eastern Florida distribution, and one at the distribution’s northern edge – to detect differences in pollinator effectiveness between sites.
Our results confirm that A. germinans encounters fewer florivores and pollinators toward its expanding range margin, resulting in negatively correlated flower production and fruit set among sites. Floral visitation rates decreased with proximity to the range edge, and while pollen limitation was evident at both sites, it was more pronounced at the northern site. In contrast, trees closer to the range interior had higher proportions of their floral buds damaged by herbivores, and total flower number per tree increased toward the range edge. The identities of florivores and pollinators did not change among the sites. Taken together, these results point to an interaction between spatial location, mutualism, and florivory that causes increased flowering but decreased fruit set for peripheral A. germinans populations. By simultaneously influencing A. germinans reproduction, these spatial patterns may control the rate of A. germinans’ northward spread. Our study illustrates the existence of a trade-off in biotic interactions during range expansion and the importance of considering multiple biotic pressures in predictions of species distributions.