Population connectivity and species diversity of Pacific coral reefs

Open Access
- Author:
- Boulay, Jennifer Nicole
- Graduate Program:
- Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 17, 2014
- Committee Members:
- Iliana Brigitta Baums, Dissertation Advisor/Co-Advisor
Katriona Shea, Committee Chair/Co-Chair
Todd C Lajeunesse, Committee Member
George H Perry, Committee Member
Dr Jorge Cortés, Special Member - Keywords:
- dispersal
connectivity
niche
Porites
Eastern Tropical Pacific
Guam
Acropora
cryptic species - Abstract:
- Unrecognized genetic and species diversity in biotic interactions involving foundation fauna can impede our understanding of ecological drivers of community structuring. In my thesis, I show that what has been considered a single species in the Eastern Tropical Pacific (ETP), Porites lobata, includes a morphologically similar yet ecologically distinct species, Porites evermanni. Porites lobata is replaced by P. evermanni at continental sites and towards higher latitudes along the continental coast, indicating that the two species occupy different ecological niches. Further, rates of asexual fragmentation differed significantly (p=0.01) between the two species as ascertained by genotyping at 11 microsatellite loci from sampling using a spatially explicit random method. Although P. evermanni reproduces extensively via fragmentation, P. lobata does so rarely. This crucial difference in population maintenance may be a driver of population structuring especially in the face of extreme El Niño-Southern Oscillation (ENSO) disturbance events which are frequent in the ETP. At some coastal sites, larger, asexually-produced fragments rather than smaller, sexually-produced larvae appear to have the advantage. Asexual fragmentation in these corals is thought to be facilitated by presence of endolithic Lithophaga mussels. Where both Porites species occur, the number of mussels/cm2 is significantly higher in P. evermanni based on field (p<0.001) and photographic (p=0.011) evidence. Additionally, we show that both corals associate primarily with Symbiodinium clade C15 (identified by DGGE of the ITS2 region); however, P. lobata was found to bleach more readily then P. evermanni when growing in close proximity (p<0.001). Differential bleaching frequency of the coral holobionts thus could be explained by host physiology differences, or by as yet undetected, host-specific Symbiodinium lineages below the C15 resolution level, or a combination of both. To explore the importance of abiotic factors in determining the geographical distribution differences between P. lobata and P. evermanni holobionts, a maximum entropy model was constructed, and indicated that temperature was indeed important in determining Porites/Symbiodinium species distribution. Thus, niche differentiation in these two closely related coral hosts and their symbionts is likely driven by preferences of the holobionts to abiotic factors explaining the divergent ecological response to thermal stress. Hidden diversity within the coral-Symbiodinium association has until now obscured differences in trophic interactions, reproductive dynamics and bleaching susceptibility, related to differential response to local temperature regimes. Including biotic factors that facilitate asexual reproduction with environmental abiotic factors in a multiple linear regression model to predict the distribution of Porites species revealed that biotic interactions are valuable for explaining differences in species distributions. Therefore, the variation in biotic interactions crucial to population maintenance across species’ ranges should be considered in light of future climate change. When confronted with large-scale disturbances brought on by climate change, population maintenance is often dependent on supply of larval recruits from unimpacted locations. In the case of Eastern Pacific Porites, reefs were well connected across the region, likely due to the long-lived, symbiont containing larval stage of the Porites species. In contrast, reef-building Acropora species have shorter lived larvae and their fragile, branching growth form make them susceptible to physical fragmentation sometimes resulting in large clonal patches. By applying high resolution genetic markers, gene flow and the importance of asexual versus sexual reproduction in population maintenance were investigated in dominant staghorn corals, Acropora cf. pulchra of Guam. High levels of asexual fragmentation were observed at considerable spatial scales and a possible barrier to larval dispersal was identified where strong eddies serve to return larvae to their natal reef. Thus maintenance of coral communities in Guam depends on asexual recruitment by fragmentation possibly perpetuating locally well adapted genotypes while reducing diversity. Management efforts should thus operate on small scales and focus on preserving clonal diversity in this system. Overall the work presented in this thesis shows how genetic markers allow for investigations of coral population dynamics operating at multiple spatial scales and reveals how population processes impact coral communities locally and regionally.