Spatial patterns of symbionts and hosts arising from propagule redirection

Background/Question/Methods

The addition of habitat can influence migration and population dynamics in two ways: 1) by increasing regional density in proportion to the increase of habitat (field of dreams), and 2) by redirecting potential colonists away from existing habitat, yielding no increase in regional density (propagule redirection). In marine systems, colonization arises through the settlement of larvae into the habitat. When the habitat and settlers are represented by hosts and symbionts (e.g., corals and fish), spatial settlement processes will not only affect the total number of settlers in the system, but also the spatial patterning and size distributions of the host (e.g., coral). We use a combination of simulations and probabilistic models to test the effects of propagule redirection on the production and spatial patterning of hosts and symbionts. We further explore how these effects are modified by changes in host growth rate, mutualism strength, abundance of settlers, and symbiont life-span.

Results/Conclusions

Different spatial distributions and abundances arise out of the field of dreams and propagule redirection hypotheses. The spatial distribution of symbionts does not differ from CSR (complete spatial randomness) under the field of dreams hypothesis, but is clustered under propagule redirection: more isolated habitat has greater density of symbionts. Additionally, because nearby hosts compete for settlers, hosts become evenly dispersed throughout space as the degree of redirection increases. The differences in production and spatial patterning between the two settlement hypotheses are less pronounced when habitat growth rate is low, settlers are more abundant, and symbionts are long-lived. Because the redirection of settlers affects not only overall production, but also the spatial patterning of hosts and symbionts, these results have implications for habitat restoration.