Biogeography and macroecology of benthic marine algae

Kerswell, Ailsa Palmer (2006) Biogeography and macroecology of benthic marine algae. PhD thesis, James Cook University.

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Macroecologists strive to understand the distribution and abundance of species over wide spatial scales, long time periods and broad taxonomic categories. The major objective of this thesis was to explored two facets of benthic marine algal diversity, taxonomic richness and community structure, in order to enhance our understanding of the processes that underpin patterns of biodiversity in the marine realm. To do so, I assembled a global database of algal distribution records from the primary literature.

I identified global latitudinal and longitudinal diversity gradients for all genera of benthic marine macroalgae and for species in the Order Bryopsidales. I also quantified the size, location, and overlap of macroalgal geographic ranges to determine how the observed richness patterns are generated. Algal genera exhibit an inverse latitudinal gradient, with biodiversity hotspots in temperate regions, while bryopsidalean species reach peak diversity in the tropics. The geographic distribution of range locations results in distinct clusters of range mid-points. In particular, widespread taxa are centred within tight latitudinal and longitudinal bands in the middle of the Indo-Pacific and Atlantic Oceans while small-ranged taxa are clustered in peripheral locations. I assessed a suite of hypotheses about the causes of marine diversity gradients by comparing algal richness patterns, in combination with the size and location of algal geographic ranges, to the richness and range locations predicted by these hypotheses. The results implicate habitat areas and ocean currents as the most plausible drivers of global marine algae diversity patterns.

Species richness patterns of macroalgae in the order Bryopsidales are strikingly concordant with those of corals and reef fishes throughout the tropical Indo-Pacific Ocean. In order to understand the processes that create and maintain tropical marine diversity gradients, I used the Bryopsidales to test the generality of a model recently developed for coral and reef fish biodiversity. Model selection was used to select the energy-related variables which best predicted species richness. These were then included along with reef area and an estimate of the mid-domain effect in spatial regression models of species richness. The results confirm the role of geographic domain boundaries as a major predictor of marine species richness patterns across a variety of taxa. They also indicate that the relative importance of environmental variables may differ with the taxa in question, with temperature and nitrate being key predictors of algal richness compared to reef area for corals and fishes. Moreover, even though the best models differ for algae versus corals and fishes, the richness patterns predicted by each model deviate from the observed patterns in a consistent manner. This suggests that additional factors, not included in any of the models, are also important in shaping species richness for multiple tropical taxa in the Indo-Pacific Ocean. I propose several candidate factors that may fulfil this role.

A long running controversy in community ecology concerns the extent to which species interactions influence the structure of assemblages. I examined assemblage structure in marine macroalgal communities at a variety of spatial scales in order to test for the existence of Wilson's (1989) guild proportionality assembly rule and to identify the geographical scales at which this rule operates. In order to overcome limitations of the traditional guild-by-guild tests of Wilson (1989), I developed a new guild proportionality test, which examines communities in the aggregate. The functional group composition of algal assemblages was determined for 120 local assemblages using the global database of marine macroalgae distribution records. Using a hierarchy of models and the newly developed guild proportionality test, I examined patterns of assemblage structure at scales ranging from regional to global. Observed communities were compared to null models, which assumed that species occurred in assemblages independently of one another (i.e., "random" assemblage structure). These comparisons revealed highly nonrandom structure in algal assemblages at all scales. Communities were more similar than predicted under a random assembly model within tropical regions and throughout the tropical biome, indicating the existence of guild proportionality within these scales. In contrast, communities were more heterogeneous than predicted in all temperate areas, within oceans and across the globe. These patterns suggest that species interactions homogenize assemblage structure within the tropics, but extrinsic processes such as regional environment and historical contingency play an important role in shaping how assemblages vary within temperate regions and at very broad spatial scales.

Item ID: 2124
Item Type: Thesis (PhD)
Keywords: benthic marine algae; taxonomic richness; population dynamics; Bryopsidales; macroalgae; distribution; abundance
Date Deposited: 23 Mar 2009 22:42
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 0%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 0%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology @ 0%
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