DEMOGRAPHIC-MODELS FOR LEPTOGORGIA-VIRGULATA, A SHALLOW-WATER GORGONIAN

I used time-invariant and time-varying matrix models to analyze the demography of Leptogorgia virgulata, a shallow-water gorgonian. For a local population in the northeastern Gulf of Mexico, I estimated monthly rates of recruitment, colony growth, and mortality in a mapped 24-m2 plot for 2 yr. In a time-invariant model, average mortality and recruitment rates were nearly balanced, so the population growth rate, 1n(lambda), was close to 0.0. An elasticity analysis showed recruitment contributed < 5% to the measured rate of population growth. The most important component of population growth rate was survivorship, particularly of the large size classes. Results were similar for a patch model that incorporated spatial variation in recruitment and colony growth rates. Several published transition matrices of forest trees, which have a similar life history, were also characterized by low elasticities for recruitment. Fluctuations in population size of L. virgulata were analyzed with a time-varying matrix model. I randomized certain elements in the 23 monthly projection matrices and simulated the population track. For models with random temporal variation in survivorship, standard deviations (and coefficients of variation) of population size were consistently larger than observed. This result suggests that temporal variation in mortality rates tended to damp population fluctuations. The damping occurred at low population sizes: models with random variation in survivorship generated significantly smaller minimum population sizes than observed. In contrast, population tracks with random temporal variation in recruitment were not consistently different from observed. Although recruitment is widely regarded as an important factor structuring marine communities, its contribution to the temporal (but not spatial) dynamics of L. virgulata was minimal. This finding may be typical of long-lived organisms with delayed reproduction and indeterminate growth, such as forest trees and many sessile marine invertebrates.