Written by Ian Hendy
Photos Courtesy of Tom Peschak
Current research, in the Mangrove forests of the Yucatan Peninsula, Mexico. Mangroves are salt-tolerant trees that fringe intertidal zones populating marine, freshwater and terrestrial habitats. They provide a range of ecosystem services including coastal protection, carbon sequestration and environmental buffering for fauna, including biodiversity maintenance mechanisms and nurseries. Maintenance of the biodiversity reflects the mechanisms that underpin species-habitat relationships and may provide insights into the responses of a community in relation to climate change. Ecosystem services are being lost due to declines in mangrove forests, magnifying ecological and economic consequences for the fauna, especially with threats from rising sea temperatures.
Mangrove forests deliver many ecosystem services, particularly the nursery function enhanced by the complexity of specially adapted tree roots, called prop roots that sit above the substrata. A less understood ecosystem service is how mangrove trees may benefit biodiversity through environmental buffering. In Mexico, we study, Rhizophora mangle trees for their biodiversity function of reducing thermal stress for the mosquitofish, Gambusia affinis. During cooler hours in the morning vast numbers of mosquitofish were found in open channels. At the same time, few mosquitofish utilised shaded areas within the R. mangle prop roots. In the morning, mosquitofish were feeding in the channels, but when water temperatures reached 38˚C, mosquitofish migrated to cooler areas of the channel to maximise their feeding potential before temperatures reached lethal limits. During the afternoon, when all areas of the channels exceeded temperatures above 38˚C all mosquitofish migrated into the shaded areas provided by the R. mangle trees. Channel water reached a daytime maximum temperature of 46˚C. But, within shaded R. mangle areas daytime thermal maxima for mosquitofish were reduced by 6.2°C. We found that the majority of mosquitofish were immature and the thermal stresses caused by excessive channel temperatures may impose serious developmental impacts that would be reduced by occupying the shaded R. mangle areas. Thus, R. mangle trees provide a little known refuge of environmental buffering for juvenile and adult fish, which would otherwise be exposed to lethal thermal stress. Future increasing sea surface temperatures may reveal a greater use of this little known ecosystem service. This study provides an example of a mechanism whereby mangrove forests support intertidal biodiversity through environmental buffering.