Demography of the demosponge Amphimedon compressa: Evaluation of the importance of sexual versus asexual recruitment to its population dynamics

Abstract

We used size-based population matrix models to describe the demography of the demosponge Amphimedon compressa. The relative importance of growth, survivorship, and recruitment to population growth (λ) was assessed by performing elasticity and life table response experiment (LTRE) analyses. We also evaluated the relative contribution of sexual and asexual recruitment to λ by analyzing four different scenarios: (1) the combined impact of sexual and asexual recruitment, (2) the impact of only sexual recruits, (3) the impact of only asexual fragmentation, and (4) the impact of no recruitment. Size-based transition matrices were parameterized with field data collected at two sites in the southwest coast of Puerto Rico: Media Luna West (MLW), a reef exposed to high water movement and at Las Pelotas (LP), a reef experiencing low water motion. Estimated λ of 0.8940 and 0.7973 at MLW and LP respectively suggest that both populations are declining. Elasticity analysis indicated that survivorship may be the most important contributor to λ at both sites. However, λ at MLW was influenced more by the survivorship of small individuals whereas survivorship of the large size-class contributed the most to λ at LP. LTRE analysis indicates that the difference in λ between sites was mostly due to difference in survivorship of small sponges. At both sites, λ decreased considerably when sexual recruitment was excluded from the model whereas the absence of asexual recruitment barely changed λ. Therefore, it is suggested that sexual recruitment plays a major role (in comparison to asexual fragmentation) in the population dynamics of this sponge at the studied sites. We conclude that spatial variability in water motion plays an important role in population dynamics of A. compressa by influencing survivorship patterns, including the relative contribution of asexual and sexual recruitment to population growth.