A Productivity Paradox In Very Wet Mangrove Forests Of The Neotropics?

Mangrove forests are found at the interface between oceans and land in tropical and subtropical coasts. Under-appreciated and severely degraded for centuries, these coastal ecosystems are now considered valuable assets that protect people against tsunamis, store high quantities of carbon and sustain fisheries productivity in coastal waters. Provisioning of food is one of this ecosystem services that we usually associate with mangroves, yet we still struggle to understand what makes a mangrove ecosystem productive.

Ecosystem models help scientists to understand the complexities of natural systems. These models may prove useful when making decisions about management and conservation. One of such models is Ecopath with Ecosim (EwE), which was initially developed in the 1980’s, subsequently refined and improved and widely used by aquatic scientists all over the world (> 500 EwE models developed ever since).

EwE allows to define different components of an ecosystem represented by biological species or group of species and to link them via trophic interactions. Human activities (i.e. fishing) taking place within this ecosystem can also be included. EwE, therefore, can help mangrove ecologists and conservation practitioners in understanding the processes within a food web of an ecosystem and how this is ultimately affected by fishing and vice-versa.

The Panama Bight eco-region in the west coast of central and South America is considered to have one of the most preserved mangrove ecosystems in the Neotropics and is thus a widely recognized conservation hotspot. Highly developed mangrove forests are found along the southern Colombian and northern Ecuadorian Pacific coasts. The remoteness of this area contributes to the relatively pristine nature of some of these places. Humans living in scattered small towns have lived for centuries along these coasts deriving their livelihoods from the resources that mangroves provide, e.g. collecting mangrove cockles and crabs, and targeting estuarine fishes.

A group of researchers from Colombia (Universidad del Valle) and Germany (Leibniz Centre for Tropical Marine Research-ZMT) joined efforts to produce an ecosystem model of a mangrove area in the Panama Bight eco-region synthesizing for the first time knowledge from a particular system – Bahía Málaga in the Colombian Pacific coast – collected in the last ~10 years.

The mangroves here are probably the wettest in the world, – rainfall can easily reach 7 m every year -, and represent an especially interesting system to investigate which factors determine the productivity of a mangrove ecosystem. The resulting EwE model of Bahía Málaga indicated that mangrove trees contributed most to the biomass of the whole system (96%) and that secondary production in the form of macrobenthos (crabs, cockles) and fish biomass was extremely low compared to that of other mangrove systems.

The transfer of energy through trophic levels seems to be very low in this system being the reason for an impoverished biomass of primary and secondary order consumers. The characteristics of this system indicate that despite being little affected by anthropogenic activities, any increase in such activities (i.e. fishing) may severely affect the capacity of the mangrove to deliver food to human populations. The astounding nature of this mangrove forest contrasts with the apparent natural scarcity of other biotic living forms in this system in an unexpected productivity paradox.

The group of researchers is now interested in comparing the results of this ecosystem model with others from the region in Panama and Ecuador. At the same time, they are trying to integrate other functional groups and other socio-economic aspects into their model that may be relevant as to obtain a holistic understanding of how mangrove ecosystems in this eco-region function.

This study, Modeling trophic flows in the wettest mangroves of the world: the case of Bahía Málaga in the Colombian Pacific coast, was authored by members of the Ecomanglares (Universidad del Valle, led by Professor Jaime Cantera) and Resource Management (ZMT, led by Prof. Matthias Wolff) working groups, and was recently published in special issue on mangrove ecosystems in the journal Hydrobiologia.