Mangroves: The Survivors
Think of a swamp. You’re probably imagining a dark and somewhat ominous place with trees emerging from murky water, coated in layers of sticky algae with scary creatures like alligators swimming around. Well, to be perfectly honest, you’re not far off...
However something you probably didn’t think of immediately, is salt water. Most of our familiar swampy ecosystems in the US are the iconic red maple (Acer rubrum) and bald cypress (Taxodium distichum) swamps found throughout North America. These are freshwater systems, forested wetlands that deal with the incredible stress of being inundated with water for most of the year. Flooded soils create anaerobic conditions and seedlings have to survive despite often being fully submerged in water.
So where does salt water come in you ask? Mangrove swamps! These forested wetlands reside in coastal environments, often estuaries, where tides bring saline water in from the ocean each day. Not only do mangrove trees have to deal with the normal wetland conditions of suffocating, flooded soils, but they also have to live without freshwater. Mangroves have developed specialized mechanisms and adaptations to survive in these extremely harsh conditions. But what’s even more amazing about these plants, even more than the fact that they have developed ways to live in un-livable acidic soils, teeming with hydrogen sulfide gas, or water with salt content more than double the average ocean salinity (for example in the Siné-Saloum Delta, in Senegal), is that these characteristics arose independently around 16 times throughout evolutionary history. Convergent evolution (where unrelated or distantly related organisms evolve similar adaptations) led to different plant families developing similar ways of coping in these harsh coastal environments.
Mangroves are a group of plants linked through common characteristics and abilities. Scientists have discovered fossilized mangrove pollen, the first physical evidence of mangroves, from 69 million years ago during the Late Cretaceous, a time when mosasaurs roamed the seas and triceratops traversed the Earth. So mangroves have been around, flourishing despite their stressful environments, for a long time.
But when you consider the existence of over 60,065 species of trees in the world*, only about 70 species of these plants are actually mangroves. It can be easy to neglect these trees, especially when you also realize that mangrove ecosystems are globally rare, and only comprise 0.39% of Earth’s forest area. So then why are we even talking about these murky saltwater swamps?
Well, because not only do mangroves provide countless ecosystem services such as sequestering carbon, fostering faunal biodiversity and protecting shorelines, but mangrove trees’ ability to live in such harsh conditions represent a remarkable feat of evolutionary innovation. (*It is important to note that there are still countless species that scientists have yet to identify.)
Within this small, unique group of plants exists incredible diversity. Not only are there trees and shrubs that are mangrove species, but also ferns (Acrostichum spp.) and even one species of palm tree (Nypa fruticans)! There is also a drastic divide between what’s known as the Atlantic East Pacific (AEP) and the Indo-West Pacific (IWP) floral realms. The AEP, which includes mangrove habitats in places such as Florida, Mexico, Costa Rica, Brazil, Nigeria and Senegal has completely different species than mangroves in the IWP, in Indonesia, Thailand, Malaysia, Bangladesh and Australia. The AEP region only has 11 species of mangroves, while the IWP has 62 species, and there is only one mangrove species that is found in both regions (Acrostichum aureum).
These diverse species occupy individual niches within their communities, and a huge variety of other organisms, especially humans, depend on the valuable services that diverse mangrove ecosystems provide. This dynamic composition of floral diversity is critical to the health and stability of mangrove ecosystems, and these systems are in dire need of global attention as they struggle with the devastating effects of widespread and rapidly accelerating deforestation and changing climates.