Community diversity of marine fishes in complete and habitat islands

Insular aquatic habitats have the potential to expand upon current theory by providing alterna/tive perspectives on island biogeographical processes. Here, we investigate whether marine lakes—seawater surrounded by land—show patterns expected for ‘complete’ (e.g., ‘true’ oceanic islands) or ‘habitat’ islands using species, phylogenetic, and functional diversity of fishes. We surveyed 22 locations across three location types: stratified lakes and mixed lakes (i.e., islands) and ocean (i.e., mainland). Stratified lake communities were depauperate in species, and distinct in the proportion of traits related to colonization, environmental persistence, and niche competition; they are akin to ‘complete’ islands. Mixed lake communities had higher species richness and redundancy in traits when compared to stratified lake communities, and substantial turnover in beta diversity of species, lineages, and traits when compared to the mainland ocean communities; they are akin to ‘habitat’ islands. While two categories of marine lakes—stratified and mixed—were defined a priori based on physical structure and roughly correspond to ‘complete’ and ‘habitat’ islands, communities indicate variation within, and possibly a gradation between location types, including overlapping confidence intervals and outlier cases along primary axes of variation. Such gradation in marine lakes may be attributable to the youth and small size of these marine islands (<20 ky and < 0.1 km 2 ) relative to the ‘complete’ islands common to island biogeography theory (predominantly > 1 my and > 1 km 2 ). Marine islands, therefore, may be defined from a biophysical perspective of communities as providing an emerging perspective with the potential to elucidate the community assembly of young islands generally. Fish communities in stratified lakes are distinguished from mixed lakes and ocean communities by their species, phylogenetic, and functional diversity. Stratified lake communities share attributes of ‘true’ oceanic islands, such as being geographically isolated, species-poor, and potential hotspots of endemism, in contrast to mixed lake communities that exhibit similarities with ‘habitat’ islands, having high connectivity, being relatively species-rich, lacking endemicity, and having trait redundancy. Stratified lakes do not share relationships with area and age as in mature and large islands; instead, they may illustrate the dynamics of small and young islands. Island biogeography theory is relevant beyond classical terrestrial island types; by applying the theory to marine systems, we can better explore the processes shaping young island communities, which are historically understudied. An island may be identified best by how organisms experience and respond to the geographical and environmental conditions.