Population dynamics

For marine reserves to function as effective harvest refuges for exploited species, the reserve must protect a substantial proportion of the population for an indefinite period of time. Because most marine reserves are space-limited, the buildup and equilibrium population sizes of mobile species will be influenced by the size and boundary conditions of the refuge. A logistic rate model was used to predict equilibrium population sizes in a marine harvest refuge, based on species-specific dispersal dynamics and the spatial configuration of the refuge. The model parameters were derived for Caribbean spiny lobsters and queen conch in an isolated marine reserve at Glover’s Reef, Belize, and were compared to observed population change over a 5-yr period. Spiny lobsters and queen conch, the two most heavily exploited species in the Caribbean, differ in larval recruitment rates (immigration) and mobility of adults (emigration). The expected increase in the population size of spiny lobsters in this refuge was 250% and queen conch was 420% over that of the initial fished population. The observed densities of lobsters and conch in the refuge approached the predicted estimates within three years. To further explore the impact of alternative spatial configurations on refuge populations, the model was run on the same populations in two hypothetical refuges. In a refuge of the same area but 50% less absorbing boundary (adjacent to intensively fished areas), the spiny lobster population was expected to be 30% larger than the equilibrium population size in the original refuge, whereas the queen conch population was not expected to change from that in the original refuge. In a refuge that was 50% larger and with 50% less absorbing boundary, the spiny lobster population was expected to increase 110% and the queen conch population was expected to increase 50% over the equilibrium population size in the original refuge. Relatively minor changes in refuge area and boundary conditions may thus result in major population-level responses by exploited species, depending on dispersal dynamics and habitat availability. This simple model may be applicable for rapid assessment of the potential efficacy of proposed harvest refuges. 

Mimetic behavior signifies organisms evolving to share behaviors and a common resemblance despite different phylogeny. Relationships in mimicry rely on the characteristics of mimics and models: appearance and size, vertical and geographical distribution, mimic to model abundance, behavioral shifts of the mimic and observed benefits. Using these criteria, species from the Hypoplectrus genus (hamlets) were analyzed as potential aggressive mimics. Using a visual census, the distribution of each mimic and model were surveyed and behaviors of individuals within the mimetic pairs were video recorded. There was 80-94% difference between the population densities of two potential mimetic pairs: Hypoplectrus nigricans (black hamlet) and Stegastes adustus, (dusky damselfish) and Hypoplectrus unicolor (butter hamlet) and Chaetodon capistratus (foureye butterflyfish). Data collected for the potential mimetic pair, Hypoplectrus chlorurus (yellowtail hamlet) and Microspathodon chyrsurus (yellowtail damselfish) does not support the hypothesis because the population density of the supposed mimic was higher than that of the potential model. In addition, for all studied pairs, no notable behavioral shifts were observed, and therefore whether the studied pairs are cases of mimicry is still a question.

This student research was retrieved from Physis: Journal of Marine Science XVII (Spring 2015)19: 43-50 from CIEE Bonaire.

Coral reef environments exhibit numerous ecological interactions between different organisms. The habitat structure of a healthy coral reef is composed of many different coral species, with various fish species inhabiting the reef. Coral reef studies often focus on a large spatial scale rather than smaller local scale environments within the reef. The objective of this study was to compare fish populations associated with the microhabitat surrounding individual coral heads of two different species. The purpose of this study was to determine if there were differences in fish abundance, fish species richness, and fish diversity between two massive stony corals, Diploria strigosa and Orbicella annularis. These two corals are common on many Caribbean reefs but are morphologically different; therefore, it was hypothesized that they would show differences between their associated fish assemblages. By conducting fish count observations on both D. strigosa and O. annularis, I was able to compare means between the coral associated fish populations using statistical tests. No statistically significant differences were found between these two coral species for mean fish abundance, species richness, or diversity. One possible explanation is that the larger scale reef environment and processes may have a significant effect on local fish populations found on individual coral heads. By studying the microhabitats of coral species and the associated fish assemblages, we can gain a better understanding of fish population dynamics of coral reefs across larger ecological scales—both regionally and globally

This student research was retrieved from Physis: Journal of Marine Science XVIII (Fall 2015)19: 61-69 from CIEE Bonaire.

Abstract:

Marine spatial population dynamics are often addressed with a focus on larval dispersal, without taking into account movement behavior of individuals in later life stages. Processes occurring during demersal life stages may also drive spatial population dynamics if habitat quality is perceived differently by animals belonging to different life stages. In this study, we used a dual approach to understand how stage-structured habitat use and dispersal ability of adults shape the population of a marine fish species. Our study area and focal species provided us with the unique opportunity to study a closed island population. A spatial simulation model was used to estimate dispersal distances along a coral reef that surrounds the island, while contributions of different nursery bays were determined based on otolith stable isotope signatures of adult reef fish. The model showed that adult dispersal away from reef areas near nursery bays is limited. The results further show that different bays contributed unequally to the adult population on the coral reef, with productivity of juveniles in bay nursery habitat determining the degree of mixing among local populations on the reef and with one highly productive area contributing most to the island’s reef fish population. The contribution of the coral reef as a nursery habitat was minimal, even though it had a much larger surface area. These findings indicate that the geographic distribution of nursery areas and their productivity are important drivers for the spatial distribution patterns of adults on coral reefs. We suggest that limited dispersal of adults on reefs can lead to a source–sink structure in the adult stage, where reefs close to nurseries replenish more isolated reef areas. Understanding these spatial population dynamics of the demersal phase of marine animals is of major importance for the design and placement of marine reserves, as nursery areas contribute differently to maintain adult populations.

In December 2009, we sampled 62 6-minute random-systematic counting points (k) to estimate the density and population size of yellow-shouldered parrots and brown-throated parakeets in a survey region (A) of 7,873 hectares, which covered the WashingtonSlaagbai National Park and forest, suburban, and agricultural areas between Brasil, Karpata, Dos Pos, Rincón, and Fontein. In March 2010, we sampled 104 points, covering a survey region of 17,000 hectares that included forest, urban, suburban, and agricultural areas in northern, central, and southern Bonaire....