Mangroves are salt-tolerant trees and shrubs that form dense forests in the intertidal zone of tropical and subtropical coastlines. These woody plants thrive in a harsh environment where most other flora cannot survive the combination of tidal submersion and high salinity. The resulting ecosystem, known as a mangal, is a highly productive habitat that acts as a natural buffer between marine and terrestrial environments. Understanding their global distribution requires examining the environmental prerequisites that dictate where they can grow.
Specific Environmental Needs
Mangroves are classified as halophytes, meaning they have evolved specialized physiological mechanisms to manage the osmotic stress of a saline environment. Some species, like the red mangrove, possess an exclusion mechanism in their roots that effectively filters out up to 90% of the salt before water enters the vascular system. Other species, such as the black mangrove, take in the salt and then actively excrete the excess through specialized salt glands located on their leaves.
Mangroves root in fine-grain, waterlogged coastal sediments that are poorly aerated and severely lacking in oxygen. To overcome this limitation, many species have developed aerial roots, such as pneumatophores or prop roots, which project upward to facilitate gas exchange. Mangroves rely on the regular flooding and ebb of the tide, which distributes nutrients, oxygen, and helps flush out accumulating salts. They are found in sheltered areas, such as estuaries, lagoons, and river deltas, because their root systems are vulnerable to the erosive force of high-energy waves.
Global Geographical Zones
The global distribution of mangroves is divided into two biogeographic realms that contrast sharply in species richness. The most biodiverse region is the Indo-West Pacific (IWP) realm, spanning the coastlines of East Africa, South Asia, Southeast Asia, and Australia. This region is considered the center of mangrove diversity, hosting over 54 species, with the highest concentration found in the Indonesian archipelago around the Makassar Strait.
The Atlantic East Pacific (AEP) realm encompasses the coasts of the Americas, the Caribbean, and West Africa. This region features significantly lower diversity, supporting only about 10 to 12 species. The difference in species numbers is attributed to historical geological barriers, including the wide expanse of the East Pacific Ocean and the African continent. The relatively recent closure of the Central American Isthmus also created a land bridge that isolated the Atlantic and East Pacific populations, limiting species dispersal and evolution in the AEP realm.
Northern and Southern Range Limits
The range limits of mangroves are governed by their intolerance to low temperatures. While they thrive in tropical conditions, the limiting factor for their poleward expansion is frost events. Mangroves cannot survive in areas where the mean air temperature of the coldest month consistently drops below 68°F (20°C).
The plants’ tissues are not adapted to freezing, and a single extreme cold event can cause widespread mortality, setting the northern and southern latitudinal limits for the ecosystem. This temperature constraint restricts mangroves to a belt between 32° North and 38° South latitude. The continued presence of a mean minimum temperature above 20°C allows for the long-term establishment and growth of these coastal forests.