The Abaco Flyfishing Guides Association has brought attention to the issue of dying mangrove stands on the west coast of Abaco, i.e., The Marls. Student Zack Jud and I did some research of the causes of mangrove death, and the possibilities are many. We summarize some possibilities after the jump.
We would classify mangrove die-off/decline in three major categories.
1. Direct removal by humans – This is surely the overwhelming cause of the decline of mangrove cover at a global scale (and a core coastal resource management issue in The Bahamas). Extensive areas of mangrove forest are cut down for coastal development and the construction of aquaculture facilities. At a smaller scale, mangroves are removed for the wood, for example, to build houses or making charcoal (see picture from Haiti below). This remains a primary challenge in The Bahamas and beyond, which is why efforts such as this are so critical.
2. Natural mortality – At the other end of the spectrum is death by natural causes – a fact which is common oversight in coastal zone management. Coasts are not static, unchanging, systems. Take for instance an eroding beach, something that managers spend billions of dollars trying to combat (for instance). But those beaches have eroded, accreted and moved for millions of years. For example, some of the favorite dive spots along the east coast of Florida are fossilized remains of beaches that were created when sea levels were much lower, and are now covered by as much as 150’ of water. Conversely, some of the best “mountain” biking in Florida occurs on sandy ridges that are miles inland from the ocean – these are remains of ancient beaches that were formed when sea levels were substantially higher. Humans may play a role in accelerating natural change (see below), but the change itself is not necessarily initiated by humans.
Likewise, mangroves are part of a dynamic environment. Mangroves are adapted to live in very harsh conditions, but within those environments, they need a precise combination of water depth, salinity, and temperature to survive. At times, mangroves trees grow well, and forests can expand. Other times, individual mangroves die and the extent of mangrove coverage may shrink.
Mangrove death may result from myriad factors: salinities that are too low or high, change in nutrient availability, erosion of the substrate, freeze events, and leaf loss following hurricanes are some common examples. Spikes in salinity are often the cause of natural die offs – this NOAA report does a nice job of discussing how salinity can lead to mangrove decline. It is also very common to see massive mangrove die-offs after hurricanes. If leaf loss exceeds a particular threshold following a hurricane, the remaining leaves may not be able to photosynthesize at a level to support the tree, perhaps leading to large-scale death of mangrove stands. Winter freezing constrains the latitudinal distribution of mangroves around the globe. Since mangroves are a tropical species, it is not uncommon to observe extensive die-off following extreme cold events.
As for Abaco, it is reasonable to suggest that one of these causes is leading to changes in the distribution of mangroves. I was on a team that demonstrated extensive damage to mangrove systems in Grenada following Hurricane Ivan (see picture below). Many of the effects were not seen until 1-2 years after the hurricane. In most cases, enough leaves were lost during the hurricane that the mangrove slowly died over a period of months. With the passing of Hurricane Irene over Abaco in August 2011, it is possible that we are only just now seeing effects of storm damage. It is also feasible that some natural variability in weather patterns (rainfall or humidity) or tidal flow may have caused salinities in The Marls to reach lethal levels, as is described in the NOAA report.
Insect pests, like beetles, weevils, and crabs, also can cause considerable harm to mangroves. For example, this parasitic beetle burrows into developing red mangrove propagules and root tips, killing seedlings and stunting root growth (here is a study documenting these effects). Diseases and fungal infections can lead to the death of trees (here is a paper describing a fungal infection that is killing mangrove trees in Puerto Rico). Other insects, a weevil from Asia that has been introduced to Florida (see picture below) feed heavily on mangrove leaves, potentially causing harm to the plant.
We talked with international wetland expert Dr. Brian Silliman about the grazing. He has published the seminal work on how drought and grazing by snails may be leading to declines in salt marsh grasses (summary and paper pdf). He also has experience working with grazers of mangrove leaves. He briefly mentioned that crabs or snails could be the culprit of the grazed leaves in The Marls – but we need more information. Often times fungus or bacteria can infect wounds following grazing, acclerating mangrove death. It also may be possible, much as the salt marsh example, that some environmental factor (e.g., salinity) may be interacting with grazing to cause die off. After getting more information, Brian is going to write a more detailed post about potential grazing impacts soon. Keep the pictures and information coming, and we will continue to follow up. It would be especially useful if someone could visit the sites at night to try and pinpoint what the grazers may be – make sure and get pictures!!
3. Humans Driving Change Indirectly – Coastal systems are areas of dynamic change – not all of which is human-driven. Yet we are increasing rates of change, altering trajectories of change, and initiating new sources of change. For instance, by artificially altering patterns of freshwater inflow to FloridaBay, we have indirectly caused localized salinities to reach levels that were lethal to mangroves. The introduction of invasive species is well documented on this website; there is the possibility that an introduction of a non-native parasitic insect may drive mangrove declines. The herbivorous weevil mentioned above was recently introduced to Florida. Has it spread to The Bahamas? These human-driven impacts are the sources of change that are especially important to identify, as they are ones we can seek minimize or avoid altogether. At this time, it is simply unclear what are some of the indirect, less obvious, manners in which humans may be impacted mangroves onWest Abaco.
Our recommendations – Without long-term monitoring data, it will be very difficult to identify the exact cause of declines of mangroves in The Marls. A first step is to compile observations from fishing guides and other people that frequent The Marls, and try and look for apparent patterns between the onset of mangrove death and major environmental events (hurricanes, freezes, etc.). For example, the observation on declining mangrove health following Hurricane Irene is especially important. That is consistent with the idea that some factor associated with the hurricane may have initiated death of mangroves in certain areas. Do other environmental events correspond with onset of mangrove die off?
Next, it is critical to characterize the areas of decline and compare them with healthy areas with apparently similar characteristics. Important variables to consider include surface salinity, porewater salinity (i.e., salinity around the roots), elevation, position within The Marls matrix, mangrove height, and leaf density. Perhaps most importantly is trying to identify the grazer that seems to be affecting the trees. If someone can visit some sites at night and take pictures of the grazers (or catch them), that would be especially useful.
Mangrove die off is a serious problem, and deserves immediate attention. But it is entirely possible that there isn’t a single, simple, explanation (and thus not one, simple, solution). The more information we can get, the better, and we will continue to keep you updated here on this issue.