The role of ecological specialization in shaping patterns of insular communities

Abstract

Aim Research on the response of species richness to area and environmental heterogeneity so far has not addressed possible effects of species’ differences in ecological specialization. Herein we provide a new metric, ‘ecorichness’, in an attempt to fill this gap. Location Aegean islands (Greece). Taxon Terrestrial isopods. Methods ‘Ecorichness’ estimates an island's biodiversity by integrating species richness and the specialists‐generalists spectrum. We calculated ‘ecorichness’ for terrestrial isopods from 43 Aegean islands based on the habitats they exploit. ‘Ecorichness’ then was regressed on area, habitat diversity and the K‐parameter of the Choros model using linear and quadratic models, compared based on AICc. A reduced data set, without halophilous species and coastal habitats, as well as an alternative description of habitat diversity, also was explored. The small island effect (SIE) thresholds identified using both a path analysis approach and piecewise continuous linear models were compared to the area of maximum ‘ecorichness’. Results ‘Ecorichness’ response to area and habitat heterogeneity was best fitted by quadratic models with peaks located at an area similar to the SIE threshold identified by path analysis. Different measures of habitat diversity produced similar patterns. Exclusion of coastal species and habitats shows that the response of ‘ecorichness’ to area is mostly shaped by the increasing contribution of specialists in the assemblages of larger islands. Main conclusions ‘Ecorichness’ facilitates exploration of the role of ecological specialization in shaping community patterns. It can be applied to different community data sets, whenever habitat range exploitation can be quantified. Results from the case study accord with previous suggestions that the relative contribution of generalists and specialists differs between small and large island communities.

Area, environmental heterogeneity, scale and the conservation of alpine diversity

“…Our preliminary analyses corroborate the reasoning of Sfenthourakis et al (2021) on the influence of the representation of species niches on environmental gradients on the phenomenon of HEAT. Our simulation also illustrated that this phenomenon can be a function of the discreteness of the representation of the environmental gradient.…”

“…In preliminary analyses with all species having identical niches in the middle of the environmental gradient, regional richness decreased with increased environmental heterogeneity at all simulated areas, and the unimodal pattern presented by Allouche et al (2012) is evident with 2 and 3 clusters (Appendix S1, Appendix 3: Figure A3.1). As noted by Sfenthourakis et al (2021), these are model‐selected instances of species being specialists relative to the environmental gradient.…”

“…First, the modes were evenly distributed along the gradient for maximum niche differentiation and σ was constant for all species. Second, all species had an identical niche with the mode centered on the gradient; rather than demonstrating only effects of area (for which its usefulness is limited by extreme competitive exclusion), illustrates HEAT and how species parameterization can account for it in models as noted by Sfenthourakis et al (2021). Third, modes were clustered at 10 evenly distributed points along the gradient and then minor random variation was added (limited runs with 2 and 3 clusters further illustrated HEAT in preliminary analyses).…”

“…We did not simulate specialist versus generalist species which would require further parameterization (e.g. Malanson, 1997); Sfenthourakis et al (2021) noted that this lack can affect modelling of the HEAT phenomenon.…”

“…Although evidence to the contrary exists (Hortal et al, 2009(Hortal et al, , 2013, it appears that HEAT has a sound theoretical basis (Allouche et al, 2012;Ben-Hur & Kadmon, 2020a). Yet, this trade-off could also be an artefact of model resolution or spatially discrete habitat patterns (Durrett & Levin, 1994) in addition to the niche characteristics of species (Sfenthourakis et al, 2021).…”