How does saltmarsh become mangrove swamp? To find out Aaron Macy and colleagues visited the northern limits of black mangrove Avicennia germinans. The site was Horn Island, a barrier island just off the mainland Mississippi coast. The islands are marshy, dominated by smooth cordgrass Spartina alterniflora currently. However, three A. germinans trees have colonised the island. Macy and his team examined herbivory of the mangrove trees and compared them to herbivory of the cordgrass. What they found was odd.
The authors found the leaves of A. germinans had about 50% more nitrogen in them than S. alterniflora, effectively making them more nutritious per mouthful. It’s not a surprise that herbivores attacked the mangrove trees. However, it was the grass that had the tougher time with herbivores.
Macy and colleagues point out that to make food a plant needs leaf area. The A. germinans leaves have more mass per unit area, so they can lose more mass with less damage than S. alterniflora. Herbivory in cordgrass seems to involve animals causing more damage to get to the bits of the leaf they want to eat.
The authors also found that the cordgrass leaves decomposed faster than the mangrove leaves. This might be due to the shape of the grass leaves, or it might be due to the trees being new arrivals. The microbes on the island will have had plenty of time to optimise for a diet of S. alterniflora. It might be that the microbes best equipped to break down A. germinans aren’t there yet. That suggests there could be big changes to the microscopic environment to come for Horn Island. As the microbial community changes, those shifts will also start to impact higher up the food chain.
While three trees alone aren’t going to convert a sandbar off the Mississippi shore into a tropical island, it might the place to start looking for further changes that will.