Intraspecific variation in herbivory, density-dependence, and growth at the edge of a mangrove’s expanding range
Recent climate change and human-mediated dispersal are causing widespread shifts in spatial patterns of species occurrence and abundance. These range shifts can have significant impacts on the structure and function of local communities. Intraspecific variation, density-dependence, and altered interactions with consumers are generally important for regulating the pattern and tempo of range expansions. I studied relative and interactive effects of these factors on the black mangrove, Avicennia germinans on the Atlantic coast of Florida, USA. Mangroves are expanding their spatial extent poleward (northward) in coastal ecosystems in this region in recent decades due to milder winters, thereby colonizing and displacing salt marsh habitats. I created field plots with varying densities of black mangrove seedlings at the extreme northern edge of the species’ range and monitored survival and levels of herbivore damage over time. The seedlings were derived from parents growing at varying latitudes along the Florida coast and family group traits were assessed using seedlings grown in a climate chamber. I assessed the preferences of salt marsh insect herbivores for mangrove leaves from different sources using experimental enclosures.
1. All measures of seedling characteristics, including growth rate, resource allocation, and tolerance to damage, significantly varied among seedlings from different parents and often demonstrated latitudinal trends. The signal of inter-family variation was greater when maternal effects of resource provisioning (seed size) were taken into account.
2. Marsh insect herbivores readily consumed mangrove leaves even in the presence of marsh vegetation, and preferred leaves from higher latitudes and younger plants.
3. Patterns of herbivore damage on seedlings in experimental plots were explained by grass height (an indicator of herbivore abundance) and by an interaction between seedling density and parental origin: greater seedling density increased herbivory to a greater extent for seedlings from higher latitudes.
4. Patterns of individual mortality were associated with plot-level rates of herbivory, consistent with the idea that herbivores had lethal as well as sub-lethal impacts.
5. In conclusion: the extent to which herbivores are slowing the colonization and domination of suitable habitat by black mangroves at their current distributional limit is unknown. However our results suggest that that edge-associated consumer species may impact mangrove population dynamics and may be important for understanding the past and future distribution of this species.