What can the parameters of the species–area relationship (SAR) tell us? Insights from Mediterranean islands

Abstract

AbstractAimThe species–area relationship (SAR) is often modelled by the linearized power function logS = logc + zlogA, whereSis species richness,Ais area, logcis the intercept andzis the slope. Although investigating howcandzvalues vary across taxa and archipelagos can provide insights into the biology of theSAR, this approach has many caveats. In this study, we aim to clarify how and whySARs should be properly compared for the same taxon among different areas, or among different taxa in the same area.LocationMediterranean. We considered 18–46 Tyrrhenian islands (0.000024–223 km2) and 32–65 Aegean islands (0.0058–8261 km2).MethodsWe usedOLSregressions to estimatecandzvalues for various taxonomic groups: land snails, isopods, centipedes, tenebrionids and reptiles. We usedANCOVAs to test (1) if different taxa have differentzandcvalues within the same island group (possibly due to their dispersal ability and ecological characteristics), and (2) if the same taxon has differentzandcvalues in different island groups (possibly due to differences in historical processes and isolation).Resultszvaried between 0.141 and 0.309, whilecvaried between 2.717 and 12.286 species per unit area (1 km2). For tenebrionids, centipedes and land snails, we found highercvalues in the Tyrrhenian islands than in the Aegean islands. Overall,cvalues were highest for land snails.Main conclusionsOur results demonstrate the importance of comparingSARs either of different groups within the same area, or of the same group in different areas. Furthermore, we identify the intercept, rather than the slope, as being dependent on the biogeographical dynamics (relict versus equilibrium faunas) and species ecology (dispersal capabilities and population abundance).