BBP in Brief, Issue 6, May 2006

BBP Publication Highlights
Dan Brumbaugh, Kate Holmes (AMNH-CBC), and Claire Paris (University of Miami-RSMAS)

Several BBP peer-reviewed research articles have been published in books and journals in the last few months, bringing the total to date to more than 15 – including recent publications in Proceedings of the National Academy of Sciences of the U.S.A., Ecological Applications, Biological Conservation, and Advances in Marine Biology. Many of these are available via the BBP website’s publication page. Several more articles are in press and many more are in various stages of preparation.

In January of this year, two BBP articles were published in Science. The first, “Fishing, trophic cascades, and the process of grazing on coral reefs”, reported on the potential effect on corals of protecting fishes inside Caribbean marine reserves. Since reserves are marine protected areas that ban fishing, discussions about tropical marine reserves generally focus on their effects on reef fishes and fisheries while the effects of reserves on coral communities are much less clear. In fact, some scientists have speculated that by allowing predatory fish such as Nassau grouper to flourish, a marine reserve might inadvertently harm reefs by depressing the numbers of herbivorous fishes, such as parrotfish, which could then allow seaweed to flourish and outcompete corals.

BBP scientists, however, found that parrotfish, far from being wiped out, are prospering because the reserve that allows predator populations to thrive also allows for the recovery of large parrotfish. Although smaller species of parrotfish are negatively affected by the accumulation of predators inside a Bahamian reserve, larger parrotfish species survive long enough to outgrow the groupers’ mouth size while also benefiting from the reserve’s fishing prohibition. Since larger parrotfish eat much more seaweed than smaller individuals, the net effect inside the reserve is a doubling of the grazing intensity, leading to a four-fold reduction in the amount of seaweed on the reef. In addition to demonstrating some of the surprising complexity of ecological interactions on coral reefs, this research provides the first demonstration of how the establishment of marine reserves in the Caribbean can help reduce seaweed, thereby facilitating the potential recovery of corals on reefs.

The article was accompanied by a perspective, “Complexities of coral reef recovery” (by coral ecologist Ove Hoegh-Guldberg) in the same issue of Science, and generated a great deal of press – including coverage in The New York Times, The Times (U.K.), The Economist, The Independent, Nature, Scientific American, and The Bahamas Naturalist and Journal of Science; on National Public Radio’s Talk of the Nation’s Science Fridays; and in online reports of the Discovery Channel, National Geographic, and the BBC.

In the other recent BBP Science article, “Scaling of connectivity in marine populations” (originally published electronically in Science Express on December 15, 2005; abstract available via the URL above), BBP scientists estimated, for the first time, typical dispersal distances for multiple species of coral-reef fishes, mapped out networks of ecologically relevant larval exchange (i.e., levels that are needed to sustain a population) between coral-reef patches, and distinguished hypothetical populations across the wider Caribbean.

They did this using a new high throughput biophysical modeling system that allows highly efficient simulations of dispersing larvae. Such virtual larvae – representing spawning productions of trillions of eggs – were released from 260 50-km local population patches and tracked as they dispersed according to realistic hydrodynamics, species-specific early life-history traits, and larval demographic factors. Because these simulations were run with equal numbers of larvae from each reef, without taking into account the current health of the coral reef ecosystem (i.e., areas that have been heavily overfished or have experienced the loss or degradation of coral reef habitat), the researchers essentially modeled pristine populations. By integrating physical and biological processes, such as realistic renditions of ocean circulation, adult spawning, and larval behavior for a variety of species, they explored how larval exchange of reef fishes would operate in the absence of pervasive human impacts. An improved understanding of these “undisturbed” connectivity patterns should help in designing effective conservation measures to maintain or restore natural levels of biodiversity and ecosystem function.

Results indicate that most larvae settle at 10-100 km from the spawning site depending on seascape structure (the geomorphology, oceanographic regimes, and natural patchiness of habitats) and certain larval traits such as time in the plankton, patterns of larval vertical migration, and the degree of active settlement. The study illustrates that the maintenance of locally breeding populations depends both on larvae self-recruiting to their patches of origin and subsidies of larvae from outside the local area, emphasizing the critical aspect of small-scale ecological networks. Further, the results reveal distinct regions of demographic isolation. For example, The Bahamas region, including the Turks and Caicos islands, stands out as an isolated enclave, with relatively high levels of internal recruitment and strong interconnectedness, limited larval exchange with the northern central Cuban shelf, and virtually no exchange with other regions of the Caribbean. This finding is important for Bahamian resource management as it suggests the opportunity for managers to influence and capture the potential ecological benefits from marine reserves and other forms of spatial management across the country.

In addition, the researchers found that regional patterns in demographic connectivity matched published genetic data for neon gobies and elkhorn coral. At the wider spatial scale of the Caribbean region, the conservation of such genetic diversity may also require taking connectivity into account during conservation planning, including the use of networks of marine protected areas (MPAs) across gradients of genetic variability. Beyond helping with MPA network design, other applications of the study’s modeling approach include simulations of the spread of invasive species, and tools for dealing with ongoing impacts from climate change.

The printed article made the cover of Science, with a composite picture of diverse larvae, and was backed up by an additional perspective, “Staying connected in a turbulent world” (by marine ecologist Bob Steneck) in the same issue.