The following post is a reflection on a field trip I took to Belize as part of my tropical ecological class.
In the marine environments, there were three main ecosystems that we explored; seagrass and sand flats, coral reefs, and mangrove forests. Of all of these, the coral reefs had the highest species richness due to their high primary productivity. This is due to the dual nature of coral, that of a producer and a predator. Coral itself is a tiny animal that hunts plankton with tiny barbs. Most coral tissue also contains an algae called zooxanthellae which provides energy to the coral via photosynthesis. This creates available energy for others, like parrotfish that eat corals. It also allows the coral to create a structurally complex area. The mangroves have the next highest level of primary production and are almost as structurally complex as the coral reefs. On the other hand, the sandy and grassy flats have little structure to them. When snorkeling there very few species besides 3 species of ray were seen.
Mangroves are a group of woody plants that grow in areas that are exposed to salt water. The species we saw the most of were the red mangrove (Rhizophora mangle). These trees often have prop roots. The served as great habitats for a large variety of sponges. The roots were also the ideal habitats for baby fish. We saw smaller versions of many of the species we saw at the reef. The water was also slower in this area. This is important because it allows for silt to settle out, and the roots of mangroves need soft sediments to grow well. The area inside of the barrier reef is ideal for these types of environments to develop because the reef crest was able to decrease the wave action in the area. The roots were also very lightweight. This was a result of the aerenchyma tissue in the roots which are sponge-like root tissues that allow for the transportation of oxygen. The roots also provide habitats for young fish, increase friction between the wave and the tree, and trap sediments. The decrease water speed due to increased friction was very noticeable, as we traveled deeper into the mangroves the waves lessened until it was almost still.
The species richness of the coral reef appeared to be heavily affected by niche speciation in spatial terms. We saw a variety of fish that used the different spatially regions of a coral reef. The nurse sharks and rays feed on the bottom. Some fish like black hamlets (Hypoplectrus nigricans) preferred to live on the top of the coral mounds. Other larger fish like the queen angelfish (Holacanthus ciliaris) live near corals in isolated pockets. Lastly, some fish such as the squirrelfish (Holocentrus sp.) prefer to live underneath the coral mounds. Species evenness did change because the availability of certain structures was greater than others. For example, there was lots of space for black hamlets, but there were fewer isolated pockets for French angelfish (Pomacanthus paru). These are also examples of specialization which happens when a species becomes uniquely adapted to a narrow resource. This reduces competition between species in the reef which allows more species to live there.
The coral reef in some areas was clearly more subject to top-down forces than others. Often corals are in competition with algae, in the last reef we snorkeled this was partially noticeable with the brown algae growing in some areas. This is kept in check by the fish that prey on the algae in these areas. If the fish are removed then the coral will not be able to compete with the algae. The reef where the algae were seen growing was also where the local fishermen like to fish. The increase in fishing for these species has been linked to an increase in the number of algae in an area. It is possible that overfishing is or could affect the reef in this area. Many of the fish caught were served to tourist on the island so it is possible that eco-tourism could harm the reef if more fishing regulations are not put in place.
There was bleaching in some areas of the reef. Bleaching usually occurs when the temperature of the water is too great and the algae in a symbiotic relationship with the coral start to produce toxic compounds to the coral. Because of this, the coral ejects these cells, and since the cells coral the coral the coral then appears white. It is possible for a coral to survive this event but this depends on its resilience. Some species of coral are more resilient than others so the biodiversity of species is highly important. The species that appeared to be maintained the most by bleaching were the branching corals. These also were the corals that were washed up on shore the most.
The area effect was also witnessed in the reef. This says that there is increased species richness with increased area. The larger the patches of reefs the great the size of the fish, and the species richness. I would also argue that the area effect could explain why some habitats contained fewer species, particularly if you compared the available space to organisms in the reefs, mangroves and sea flats. The more surface area available the more species our class found. As the more of a type of ecosystem, we explored the more species we found. In smaller areas, there were fewer species than in larger areas.