Spatial competition

Corals have built reefs on the benthos for millennia, becoming an essential element in marine ecosystems. Climate change and human impact, however, are favoring the invasion of non-calcifying benthic algae and reducing coral coverage. Corals rely on energy derived from photosynthesis and heterotrophic feeding, which depends on their surface area, to defend their outer perimeter. But the relation between geometric properties of corals and the outcome of competitive coral-algal interactions is not well known. To address this, 50 coral colonies interacting with algae were sampled in the Caribbean island of Curaçao. 3D and 2D digital models of corals were reconstructed to measure their surface area, perimeter, and polyp sizes. A box counting algorithm was applied to calculate their fractal dimension. The perimeter and surface dimensions were statistically non-fractal, but differences in the mean surface fractal dimension captured relevant features in the structure of corals. The mean fractal dimension and surface area were negatively correlated with the percentage of losing perimeter and positively correlated with the percentage of winning perimeter. The combination of coral perimeter, mean surface fractal dimension, and coral species explained 19% of the variability of losing regions, while the surface area, perimeter, and perimeter-to-surface area ratio explained 27% of the variability of winning regions. Corals with surface fractal dimensions smaller than two and small perimeters displayed the highest percentage of losing perimeter, while corals with large surface areas and low perimeter-to-surface ratios displayed the largest percentage of winning perimeter. This study confirms the importance of fractal surface dimension, surface area, and perimeter of corals in coral-algal interactions. In combination with non-geometrical measurements such as microbial composition, this approach could facilitate environmental conservation and restoration efforts on coral reefs.

Mutualistic symbiosis is a finely tuned relationship between two species in which each receives a service that increases its own fitness in exchange for providing service to another. The evolutionary stability of such a relationship is dependent on all species performing in an honest manner. However, many species that participate in mutualistic symbiosis have been observed cheating, or taking benefits beyond those evolutionarily agreed upon. This study attempted to identify factors that contribute to the frequency of cheating at cleaning stations on coral reefs. In these relationships, small fish and crustaceans clean parasites from larger host organisms. Client abundance and proximity of cleaning stations were examined as indicators for competition between cleaners and client choice. These factors put pressure on cleaners to cooperate by creating competition for clients. It was found that there was a greater abundance of clients at stations where cheating occurred less frequently, suggesting that clients may have chosen those stations for the higher quality service demonstrated. Proximity of cleaning stations did not seem to influence the frequency of cheating. Finally, obligate cleaners spent more time cleaning individual clients and cheated less frequently than facultative cleaners, demonstrating their higher dependence on the relationship. Understanding the factors that motivate cleaners and clients to cooperate at cleaning stations is an important component to comprehending community dynamics on reefs, but it is not as clear of a relationship as is commonly described.

This student research was retrieved from Physis: Journal of Marine Science XIV (Fall 2013)19: 33-40 from CIEE Bonaire.

Populations of Diadema antillarum have had low densities ever since its mass mortality event in 1983. A slow population density increase has resulted from fertilization complications due to extensive distance between individuals. The relationships between D. antillarum and their competitors and predators as a cause for the lack of population recovery has not been directly studied. The correlation of D. antillarum density with the abundance of predators, competitors, and microalgae, was studied to determine additional possible explanations for the low density of individuals. There were three dives during the day at six sites. The day dives included observational fish counts and transects and quadrates to assess percent algae cover in a 10 m2 . While the night dives include observation counts of all of the urchins in the 10 m2 . No increase was found in Diadema antillarum density compared with a study in 2009 (0.005 individuals per m 2 ). No significant correlation was determined between D. antillarum density and predator density. A weak, positive correlation between competitor density and D. antillarum density was determined. In contrast, a strong, positive correlation between percent algae cover and D. antillarum density was found This study revealed additional pressures on D. antillarum population (e.g. competitors, percent algae cover), which could account for the slow recovery of local D. antillarum population in Bonaire.

This student research was retrieved from Physis: Journal of Marine Science XIV (Fall 2013)19: 25-32 from CIEE Bonaire.

Processes affecting reef ecosystems have three levels of organization: macroscale, mesoscale and microscale. These processes are conducive to interspecific competition amongst various coral and aggressive invertebrate species. Surveys of these organisms’ distribution, abundance and ecological description of their intra/interspecific competition have been conducted throughout the Caribbean. Previous research has found that scleractinian corals in reef slope ecosystems are frequently outcompeted by sessile aggressive invertebrates, such as Clionid sponges, encrusting bryozoans, encrusting gorgonians and overgrowing mat tunicates. Furthermore, interspecific spatial competition between corals and aggressive invertebrates has been observed to increase in frequency with depth. This project analyzed the distribution and abundance of coral-aggressive invertebrate spatial competition along a fringing reef ecosystem on the west coast of Bonaire. Belt transects were laid out between 200 m north and south of the GPS coordinates N 12°09.6 12’ W 068° 16.9 15’, at two depths (10 and 15 m) along the reef slope. Instances of spatial competition involving individuals at least 10 cm in length were photographed for further analysis. Five coral species and 25 aggressive invertebrate species were encountered in a total of 216 coral-aggressive invertebrate interactions. Quantitative data analysis showed that the orange lumpy encrusting sponge (Scopalina ruetzleri) was the most abundant aggressor at 15 and 10 m, although mean total area covered by coralaggressive invertebrate interactions and their frequency did not increase with depth. Findings could be used as a baseline for future scientific marine research, potentially on growth rate of competing species and the underlying mechanisms responsible for their interspecific spatial competition.

This student research was retrieved from Physis: Journal of Marine Science XVII (Spring 2015)19: 17-25 from CIEE Bonaire.

Consumer-mediated indirect effects at the community level are difficult to demonstrate empirically. Here, we show an explicit indirect effect of overfishing on competition between sponges and reef-building corals from surveys of 69 sites across the Caribbean. Leveraging the large-scale, long-term removal of sponge predators, we selected overfished sites where intensive methods, primarily fish-trapping, have been employed for decades or more, and compared them to sites in remote or marine protected areas (MPAs) with variable levels of enforcement. Sponge-eating fishes (angelfishes and parrotfishes) were counted at each site, and the benthos surveyed, with coral colonies scored for interaction with sponges. Overfished sites had >3 fold more overgrowth of corals by sponges, and mean coral contact with sponges was 25.6%, compared with 12.0% at less-fished sites. Greater contact with corals by sponges at overfished sites was mostly by sponge species palatable to sponge preda- tors. Palatable species have faster rates of growth or reproduction than defended sponge species, which instead make metabolically expensive chemical defenses. These results validate the top-down conceptual model of sponge community ecology for Caribbean reefs, as well as provide an unambiguous justification for MPAs to protect threatened reef-building corals.

An unanticipated outcome of the benthic survey component of this study
was that overfished sites had lower mean macroalgal cover (23.1% vs. 38.1% for less-fished sites), a result that is contrary to prevailing assumptions about seaweed control by herbivorous fishes. Because we did not quantify herbivores for this study, we interpret this result with caution, but suggest that additional large-scale studies comparing intensively overfished and MPA sites are warranted to examine the relative impacts of herbivorous fishes and urchins on Caribbean reefs.