Abstract
We quantify the relative importance of multi‐scale drivers of reef fish assemblage structure on isolated coral reefs at the intersection of the Indian and Indo‐Pacific biogeographical provinces. Large (>30 cm), functionally‐important and commonly targeted species of fish, were surveyed on the outer reef crest/front at 38 coral reef sites spread across three oceanic coral reef systems (i.e. Christmas Island, Cocos (Keeling) Islands and the Rowley Shoals), in the tropical Indian Ocean (c. 1.126 x 106 km2). The effects of coral cover, exposure, fishing pressure, lagoon size and geographical context, on observed patterns of fish assemblage structure were modelled using Multivariate Regression Trees. Reef fish assemblages were clearly separated in space with geographical location explaining ~53 % of the observed variation. Lagoon size, within each isolated reef system was an equally effective proxy for explaining fish assemblage structure. Among local‐scale variables, ‘distance from port’, a proxy for the influence of fishing, explained 5.2% of total variation and separated the four most isolated reefs from Cocos (Keeling) Island, from reefs with closer boating access. Other factors were not significant. Major divisions in assemblage structure were driven by sister taxa that displayed little geographical overlap between reef systems and low abundances of several species on Christmas Island corresponding to small lagoon habitats. Exclusion of geographical context from the analysis resulted in local processes explaining 47.3% of the variation, highlighting the importance of controlling for spatial correlation to understand the drivers of fish assemblage structure. Our results suggest reef fish assemblage structure on remote coral reef systems in the tropical eastern Indian Ocean reflects a biogeographical legacy of isolation between Indian and Pacific fish faunas and geomorphological variation within the region, more than local fishing pressure or reef condition. Our findings re‐emphasise the importance that historical processes play in structuring contemporary biotic communities.
Highlights
Patterns of species diversity in tropical marine taxa are highly heterogeneous across longitudinal gradients (Tittensor et al, 2010)
Biological transport through the Indo-Pacific barrier is complicated by two factors: (i) the degree of porosity and other environmental conditions of the barrier, which have been regulated by fluctuations in sea level, primarily during the Pleistocene; and (ii) taxonomic variation in species ability to move through the barrier, thereby regulating the geographical distribution and genetic structure of coral reef fishes between the two oceans
We compared the role of geography, reef geomorphology, wave exposure, coral cover, and fishing pressure on the structure of large and functionally important coral reef fishes. (ii) We identify the key species driving taxonomic patterns in reef fish assemblage structure among and within reef systems and (iii) evaluate how taxonomic patterns affect the abundance of key functional groups of fish among reef systems
Summary
Patterns of species diversity in tropical marine taxa are highly heterogeneous across longitudinal gradients (Tittensor et al, 2010). The location of the biogeographical break between Indian and Pacific reef fish populations generally lies to the west of Australia in the vicinity of east longitude 72° (Briggs & Bowen, 2012; Iacchei, Gaither, Bowen, & Toonen, 2016) This biogeographical area, extending from the reef systems of the southern Indonesian island chains and western Australia at approximately 118°E out to the archipelagos of the Central Indian Ocean at 71°E, represents a dynamic region in ecological and evolutionary terms. The purpose of this study was to evaluate the evolutionary and ecological drivers of fish assemblage structure across isolated coral reef systems at the intersection of the Indian and Pacific Ocean biogeographical provinces These reef systems are important because they represent the western range edge for many Pacific Ocean species and the eastern range edge for many Indian Ocean species as well as overlapping distributions of Pacific and Indian Ocean sister taxa (Choat, klanten, Van Herwerden, Robertson, & Clements, 2012; Hobbs et al, 2014; Sorenson, Santini, Carnevale, & Alfaro, 2013). We discuss the extent to which patterns in reef fish assemblage structure are likely to reflect legacies of contemporary and evolutionary processes, based on phylogenetic and ecological characteristics of species observed across this dynamic biogeographical region
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