Integrating biotic and abiotic controls of reproduction in a range expanding species, the black mangrove
For sessile organisms facing changing environmental conditions, a major determinant of their ability to shift their distribution – and the velocity at which they do so – is their reproductive rate. Reproduction, or lack thereof, is known to determine exotic plant species’ success or failure in introduced environments. However, if and how reproduction shapes range limits of naturally occurring species is less well known. Furthermore, while reproduction itself can be easy to measure, the factors (both biotic and abiotic) that influence reproduction vary in space and time, making predicting reproductive output across large scales difficult. We tested our ability to make such predictions using 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, variation in A. germinans density, and variation in abiotic conditions. During the 2014 flowering season, 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 recorded tree density, size, and physical variables at each site. We sought to integrate these reproductive controls and compare them to actual A. germinans fecundity.
Our results indicate that despite substantial spatial variation in the biotic and abiotic factors influencing A. germinans reproduction, A. germinans maintains high reproductive output across large spatial scales. Visitation rates by pollinators decreased with proximity to the A. germinans range edge, and the effectiveness of the pollinator assemblage declined with increasing latitude, as well. Florivory showed no distinct latitudinal trend, but varied considerably site to site. Crowding increased significantly toward the range edge, while tree size decreased. Apart from large canopy trees in centrally located populations, light availability was much greater for trees growing at the range edge compared to those growing in taller, more mixed mangrove stands in the range interior. Despite decreased pollinator quantity and quality, occasional high rates of florivory, and high conspecific density, edge populations maintained high fecundity by increasing their allocation to reproduction, in the form of abundant flowers. We hypothesize that favorable abiotic conditions, especially with regard to light availability, allow for this increased allocation to flowers. Altogether, A. germinans faces a trade-off between biotic and abiotic conditions that influence its reproduction, resulting in consistently high rates of reproduction among populations, and contributing to this species’ rapid range expansion.