The of complex ecological interactions are operating during

The physiological response of an organism to increasing temperature follows a sigmoid curve, in which an initial rapid rise in functional processes (e.g., respiration, growth rate) slows, plateaus, and then declines as a critical lethal threshold is reached and then exceeded 58. Mangrove plants and animals presumably respond so, but the critical temperatures at which functionality plateaus and organisms begin to die are uncertain. Rates of leaf photosynthesis for most species peak at temperatures at or below 30 °C 58, and leaf CO2 assimilation rates of many species decline, either sharply or gradually, as temperature increases from 33 to 35 °C 58. Photosynthesis in exposed leaves is often depressed due to photoinhibition; mid-day declines of assimilation have been observed ensuring survival for the photochemical machinery.What has been the response of mangroves in the field to the ongoing rise in temperature? Temperature increases alone are likely to result in faster growth, reproduction, photosynthesis, and respiration, changes in community composition, diversity, and an expansion of latitudinal limits 59. Field data indicate that mangroves are indeed currently expanding into higher latitudes in North America 60, 61, 62, 63•, New Zealand 64, Australia 65, 66, southern Africa 67, and southern China 68. This global expansion polewards is most likely in response to the global rise in sea surface temperatures 69.As these changes are occurring in the subtropics and tropics, mangrove expansion may also be coupled to changes in precipitation 70. In an analysis of mangrove latitudinal changes, Quisthoudt et al. 71 found that temperature alone does not delimit the latitudinal range of Rhizophora and Avicennia due partly to large regional differences in monthly temperature change, for instance, warmest month temperatures are higher at the latitudinal limits in the northern, than in the southern, hemisphere. While mangrove expansion and salt marsh contraction are consistent with the poleward increase in temperature 72•• and the reduction in the frequency of extreme cold events 73•, other variables such as changes in precipitation cannot be ruled out as co-factors 74.The expansion of mangroves at the expense of salt marshes suggests that a number of complex ecological interactions are operating during the transition 63•, 75, 76. Proffitt and Travis 76 propose that this migration may be facilitated by increasing propagule abundance from greater reproductive rates and greater genetic variation caused by outcrossing. From field surveys conducted along the Atlantic and Gulf coasts of Florida, they found that reproductive frequencies varied significantly, but increased with latitude and more strongly along the Gulf coast, with a concomitant increase in outcrossing. The migration of mangroves is self-re-enforcing; more colonizers lead to more propagules and outcrossing leads to enhanced genetic variation, thus perpetuating and promoting adaptation to a new environment.What effect has the rise in temperature had and/or will have on mangrove-associated fauna? No studies have yet demonstrated a change in mangrove fauna associated with global warming, but the results from a few studies 77, 78, 79 of macro- and megafauna from adjacent habitats have implications for mangrove organisms. An experimental study 79 has shown that juvenile mullet (Liza vaigiensis) and crescent terapon (Terapon jarbua) frequenting tropical seagrass beds can be acclimated to higher water temperatures, approaching the critical limits for marine vertebrates. Other organisms such as tropical gastropods 78 may respond actively by seeking cooler sites to survive when temperatures exceed 33 °C. However, tropical organisms are closer to their upper thermal thresholds than boreal and temperate organisms, and are thus more vulnerable to rising temperature 80, 81•.Mangrove responses to increasing or decreasing precipitation are more straightforward, but such changes are likely to co-occur with rises in sea level, temperature, and atmospheric CO2concentration. Compared to arid-zone stands, mangrove forests in the wet tropics have greater biomass and productivity, consist of less dense but taller trees, and tend to inhabit finer sediment deposits, but there are no clear species richness or diversity patterns between high and low precipitation areas 82; low species richness may be attributable to high variability in annual rainfall. But mangroves clearly thrive in wet environments where they can likely deal less stressfully with lower salinity and more available fresh water.Global PredictionsWhat then are we to predict about the global future of mangroves in the face of climate change? There have been a number of general and local prognostications 1, 28, 33, 83, especially in regard to sea level rise 4••, 8•, 31•, but there have been few attempts at global prediction 2, 84••. There has been only one sophisticated attempt to forecast mangrove distributions under climate change 84••. Using several mangrove databases for 30 species across 8 genera, Record et al. 84•• used the BIOMOD model to make predictions of mangrove species and community distributions under a range of sea level rise and global climate scenarios up to the year 2080. The model runs came up with two clear predictions: (1) some species will continue migrating polewards but experience a decline in available space; and (2) Central America and the Caribbean will lose more species than other parts of the world. The latter prediction is in agreement with the work of Polidoro et al. 85•• in which extinction risk of threatened species was assessed and the main geographical area of concern was found to be the Atlantic and Pacific coasts of Central America.The recent climatological forecasts by the Intergovernmental Panel on Climate Change (IPCC) 3, 86 for until the end of this century predict that globally (1) sea surface temperatures will rise by 1–3 °C, (2) oceanic pH will decline by 0.07–0.31, and (3) mean atmospheric CO2concentrations will increase to 441 ppm (from 391 ppm in 2011). Regional differences (Table 1) will occur for some parameters such as (1) sea level, which will continue to rise globally at an average rate between 1.8 and 2.4 mm year?1; (2) precipitation will increase and decrease in some regions such that arid areas will become more arid and the wet tropics will become wetter; and (3) salinity will change in tandem with changes in precipitation. Considering these climatic predictions and the known and likely responses of mangroves to changes in temperature, salinity, sea level rise, etc., I offer some predictions (Fig. 3):Prediction 1 (red lines): Mangrove forests along arid coasts will decline as salinities increase, freshwater becomes most scarce, and critical temperature thresholds are reached more frequently (e.g., NW Australia, Pakistan, Arabian Peninsula, both Mexico coasts).Prediction 2 (orange lines): Mangrove forests will decline as sediment yield declines, salinity increases, and sea level rises in tropical river deltas subject to subsidence intervals (e.g., the Sundarbans; the Mekong, Zaire, Fly Rivers).Prediction 3 (purple lines): Mangrove forests will decline as sea level rises and there is little or no upland space to colonize (e.g., low islands of Oceania, many Caribbean islands).Prediction 4 (blue lines): Mangroves forests will continue to expand their latitudinal range as temperature and atmospheric CO2 concentrations increase (New Zealand, USA, Australia, China).


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