Replenishment of Coral Reefs The Role of Reproduction and Recruitment in the Ecology of Reefs

Most reeforganisms are sessile (corals, sponges, seaweeds) or use only a small portion of a much larger reef habitat (most reef fishes). Thus, colonization of new habitats is achieved through recruitment of juvenile organisms that may drift for long distances in the plankton before settling onto, and using a small area of reef.Consequently, the production and recruitment of juvenile organisms is a key factor in the ecology of reefs as the replenishment of plant and animal populations is integral to the resilience and recovery of reefs in the face of natural and anthropogenic disturbances.

Coral species differ considerably in their modes of reproduction and in the ability of their larvae to disperse to new reefs. Many corals reproduce both asexually through fragmentation and sexually by the production of gametes. Important reef-building corals such as acroporids are extremely successful at reproducing asexually and are dispersed when storms break apart parent colonies and spread the fragments to new portions of a reef where they can reattach and grow. Sexual reproduction in corals is also variable in that corals are typically either brooders or spawners. Brooders release fertilized larvae into the water column while spawners release sperm and eggs into the water column, where they fertilize and disperse with the ocean currents. These fertilized larvae will eventually exit the plankton and return to reefs as newly recruited juvenile corals. Research from the Great Barrier Reef, Australia has shown that there is large variation in the abundance of coral recruits across both large and small spatial scales. The best predictor of differences in recruitment rates among reefs was the fecundity, not abundance, of adult corals and explained 72% of the variation in recruitment for acroporid corals. Recruitment rates decreased dramatically as the fecundity of adults decreased, but this decrease was not linear; a small decrease in the fecundity of adults resulted in a dramatic decrease in juvenile recruitment (Figure 6). These results suggest that processes such as sedimentation, eutrophication, and competition with seaweeds, all ofwhich reduce the fecundity of adult corals, could dramatically affect the replenishment of coral populations.

Recruitment of juveniles is also important to the replenishment of fish populations and considerable research has gone into determining how recruitment processes affect the assembly of reef fish communities. Most reef fishes, like corals, have planktonic larvae that can disperse wide distances from their point of origin. One of the key questions in the ecology of reef fishes is how the recruitment of juvenile fishes is related to the density of fishes already on the reef (i.e., whether local patterns of recruitment are density dependent or density independent). Despite considerable research on the subject, little consensus has been reached and studies have shown that recruitment rates can be either positively or negatively correlated with adult abundance (positively or negatively density dependent), or show no correlation at all (density independent). These relationships may vary with the species being studied, with location, or with the currents and physical processes prevailing at the time of the test. Continued research is needed to generalize how recruit and adult densities are related and how environmental and biological variables change these relationships.

A key component to the replenishment of populations of coral reef organisms is the extent to which reefs are connected to other reefs (i.e., whether juveniles recruit to

250 200

0 20 40 60 80 100 Percentage gravid

Figure 6 Relationship between the percentage of coral colonies (Acropora hyacinthus) that were gravid and the number of coral recruits. Each point is a separate reef on the Great Barrier Reef, Australia. From Hughes TP, Baird AH, Dinsdale EA, etal. (2000) Supply-side ecology works both ways: The link between benthic adults, fecundity, and larval recruits. Ecology 81: 2241-2249.

0 20 40 60 80 100 Percentage gravid

Figure 6 Relationship between the percentage of coral colonies (Acropora hyacinthus) that were gravid and the number of coral recruits. Each point is a separate reef on the Great Barrier Reef, Australia. From Hughes TP, Baird AH, Dinsdale EA, etal. (2000) Supply-side ecology works both ways: The link between benthic adults, fecundity, and larval recruits. Ecology 81: 2241-2249.

reefs from local or distant sources). Coral reefs, and marine ecosystems in general, differ from many terrestrial systems in that juvenile organisms have the potential to ride ocean currents and be dispersed over wide distances potentially connecting geographically distant populations. However, the actual extent that marine populations are connected to each other is still a topic of vigorous debate. This knowledge is crucial to the protection and management of reefs as the connectivity of populations of coral reef organisms will determine whether local populations can be managed with efforts based close to the target population (if the system is fairly closed and recruitment from local populations is frequent) or if management of local populations will necessitate international cooperation (if reefs are fairly open and recruitment is driven by larvae from distant reefs). Thus, solving the question of connectivity among reefs is critical to the preservation of reef health.

Initial models of connectivity for fish populations in the Caribbean suggested that many of the populations were very open and well connected to other populations hundreds of kilometers away. However, these models were based on passive dispersal of fish larvae and simple models of surface currents and did not account for the effect of larval behavior on dispersal or for the effects of fine-scale oceanographic processes. Thus, viewing larvae as passive dispersal agents may overestimate the actual dispersal of larvae and the connections among reefs. Recent models of connectivity in the Caribbean that account for larval behavior suggest that fish populations are less connected than assumed under passive dispersal models and that different regions of the Caribbean are essentially isolated from each other, at least on an ecological timescale. However, there are considerable differences among regions of the Caribbean as to the extent that reefs are connected to distant areas as some regions appear to import a large portion of recruits while other regions are primarily self-seeding. The differences in the relative importance of local and long-distance recruitment of juveniles among regions of the Caribbean underscores the role that careful planning will play in the implementation of marine reserves for protection of coral reefs as understanding how reefs are connected to one another will influence how large reserves should be and where they should be located.

Was this article helpful?

0 0
10 Ways To Fight Off Cancer

10 Ways To Fight Off Cancer

Learning About 10 Ways Fight Off Cancer Can Have Amazing Benefits For Your Life The Best Tips On How To Keep This Killer At Bay Discovering that you or a loved one has cancer can be utterly terrifying. All the same, once you comprehend the causes of cancer and learn how to reverse those causes, you or your loved one may have more than a fighting chance of beating out cancer.

Get My Free Ebook


Post a comment