A new species-based metric for island biogeography
Above: The response of the new metric ‘ecorichness’ to island area for terrestrial isopods of central Aegean islands. Size of dots is proportional to the number of habitats of each island.
Habitat diversity, as an estimate of environmental heterogeneity, has been considered among the main factors shaping patterns of diversity in insular communities but its definition and measurement has not yet found a consensus among researchers. Area is the most commonly used predictor of species richness, in the absence of good information on habitat diversity. Despite controversies on how to best describe and measure habitats, their diversity should be a critical component of any robust theory of island ecology and biogeography, crucial for any synthesis of the factors shaping species diversity for a wide range of study systems. Thus far, most theories consider all species as ecologically equivalent. In real communities, though, each species covers a certain range of habitat specialization, from narrow specialists to wide generalists. The few island biogeography studies that incorporated ecological specialization, mostly used a crude binary classification of species into generalists and specialists. A step forward, therefore, will be to use a finer characterization of species’ position within the generalist-specialist continuum but this assumes a detailed knowledge of species’ habitat preferences and a meaningful classification of habitats in the study area.
In this study, we explored the effects of area and environmental heterogeneity on a metric that substitutes species richness with the standardized sum of species’ specialization range. We follow an approach that corresponds to the ‘fundamental Grinnelian specialization’ by replacing the occurrence of each species in the presence/absence matrix with the number of habitat types it exploits in the study system. Then, for a given island, we estimated the ‘ecorichness’ index by summing specialization range for the species occurring on it and standardizing for species richness.
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Sfenthourakis, S, Triantis, KA, Proios, K, Rigal, F. The role of ecological specialization in shaping patterns of insular communities. J Biogeogr. 2020; 00: 1– 10. https://doi.org/10.1111/jbi.14012
We applied the metric to a real data set with terrestrial isopods from the Aegean islands (Greece), for which we had earlier made a detailed description of species’ habitat ranges. Based on our previous work and theoretical considerations, we expected small island communities to consist mainly of generalists while specialists should be added as area increases. Such an assumption would lead to a decreasing or a unimodal ‘ecorichness’ – area curve. No matter the pattern at the left part of the curve, the right part, corresponding to larger islands, should always show a decreasing trend. Furthermore, according to our predictions, the response of ‘ecorichness’ to area should be affected by the occurrence and strength of a ‘Small Island Effect’ (SIE), and should peak at an area threshold where the contribution of generalists is maximized, decreasing afterwards due to the increasing contribution of specialists with a small additive effect on ‘ecorichness’ values. It should be noted that the predictions of our approach are not based on actual ‘ecorichness’ values but on the pattern of its response to area (or other factors). Therefore, it should work with any measurement of habitat diversity and could be used in comparisons among different systems and taxa.
In agreement with our predictions, the relationship of ‘ecorichness’ with area (on a logarithmic scale) is expressed as a hump-shaped curve, ascending for small values of area and descending for areas larger than the Small Island Effect threshold. The ‘heightening’ of the left part of the ecorichness–area curve (i.e. increase of ‘ecorichness’ values for smaller islands) after the exclusion of the narrow specialist halophilous species, is also in accordance with original predictions that communities of small islands consist mostly, but not exclusively, of generalist species, while the contribution of specialists becomes important above the SIE threshold. Implications of our approach may shed light also on the debate about the presumed possibility of negative effects of very high habitat diversity on species richness. The ‘ecorichness’ approach supports a continuous increase or, at least, a stationary richness with increasing habitat diversity, given that real communities consist of a variable mixture of specialist and generalist species.
The habitat range distribution of terrestrial isopods on central Aegean islands. Porcellio flavomarginatus is a, not very narrow, specialist that exploits limestone-rich areas and rocky outcrops in dry Mediterranean habitats, while Armadillo officinalis is a generalist exploiting all Mediterranean-type ecosystems.
We believe that the ‘ecorichness’ approach can offer useful insights into processes shaping species−area, and species-habitat diversity relationships, through a quantification of the ‘ecological resource space’ used by species. Our approach falls within the general framework of a species-based theory of (island) biogeography and addresses aspects of community structure implicit in ‘assembly rules’ theory. It is important to explore more case studies since the scarcity of detailed data on species’ habitat ranges does not allow for a generalization of the present results, but we hope that our study will trigger the production of more such data in the near future. Also, we expect that a synthesis of ‘ecorichness’ with functional diversity metrics should offer important insights into processes shaping island communities. Finally, an even finer quantification of species’ habitat ranges should render an ‘ecorichness’ approach even more robust.
Written by:
Spyros Sfenthourakis (1), Kostas A. Triantis (2), Konstantinos Proios (3), and Francois Rigal (4)
(1) Professor, Department of Biological Sciences, University of Cyprus, Lefkosia (Cyprus).
(2) Associate Professor, Department of Biology, University of Athens, Athens (Greece).
(3) PhD Student, Department of Biology, University of Athens, Athens (Greece).
(4) Assistant Professor, Université de Pau et des Pays de l’Adour, Pau (France).