Evaluating the correlation between area, environmental heterogeneity, and species richness using terrestrial isopods (Oniscidea) from the Pontine Islands (West Mediterranean)

Gabriele Gentile,Stefano Taiti,Roberto Argano

doi:10.1007/s13127-021-00523-x

Abstract

Area and environmental heterogeneity influence species richness in islands. Whether area or environmental heterogeneity is more relevant in determining species richness is a central issue in island biogeography. Several models have been proposed, addressing the issue, and they can be reconducted to three main hypotheses developed to explain the species-area relationship: (1) the area-per se hypothesis (known also as the extinction-colonisation equilibrium), (2) the random placement (passive sampling), and the (3) environmental heterogeneity (habitat diversity). In this paper, considering also the possible influence of geographic distance on island species richness, we explore the correlation between area, environmental heterogeneity, and species richness by using faunistic data of Oniscidea inhabiting the Pontine Islands, a group of five small volcanic islands and several islets in the Tyrrhenian Sea, located about 60 km from the Italian mainland. We found that the colonisation of large Pontine Islands may occur via processes independent of geographic distance which could instead be an important factor at a much smaller scale. Such processes may be driven by a combination of anthropogenic influences and natural events. Even in very small-size island systems, environmental heterogeneity mostly contributes to species richness. Environmental heterogeneity could influence the taxocenosis structure and, ultimately, the number of species of Oniscidea via direct and indirect effects, these last mediated by area which may or may not have a direct effect on species richness.

Highlights

The positive relationship between species diversity and area is certainly one of the most solid paradigms in island biogeography
Whereas we found no correlation between geographic distance and species richness in the first set, we found a strong and significant negative correlation (r = − 0.826; p < 0.01) in the second set
No evidence of a correlation between β-diversity and geographic distance was found for the Sicilian archipelagos (Cazzolla Gatti et al, 2018), whereas an inverse relationship between geographic distance and faunal similarity has been observed (Sfenthourakis, 1996b)

Summary

Introduction

The positive relationship between species diversity and area is certainly one of the most solid paradigms in island biogeography. Several models have been proposed, addressing the issue, and they can be reconducted to three main hypotheses developed to explain the species-area relationship: (1) the areaper se hypothesis (known as the extinction-colonisation equilibrium), (2) the random placement (passive sampling), and the (3) environmental heterogeneity (habitat diversity). The third hypothesis is based on the observation that larger islands are generally more structured, with the occurrence of different types of habitat that would determine higher diversity. In this regard, it has been theorised that area and environmental heterogeneity may multiplicatively interact to predict species richness better than by area alone (Triantis et al, 2003). Studies based on path analysis, a method that implies causal relationships, have shown that the influence of area on species richness has a direct component and an indirect component through the effect of area on environmental heterogeneity (Kohn & Walsh, 1994; Power, 1972; Triantis et al, 2006)

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