Measuring ecological specialization along a natural stress gradient using a set of complementary niche breadth indices

Aims: Ecological specialization refers to a restricted ecological niche breadth for a species, resulting from the trade-off between range of the resources it can exploit and efficacy in exploiting a specific resource. One hypothesis predicts that specialist species should be dominant in stressful environments, whereas generalist species should be dominant at less extreme environmental gradients. An alternative hypothesis states that specialization should be favoured in stable systems, and that ecological disturbances should affect specialist species negatively. These hypotheses have been poorly tested. Mediterranean coastal dune systems are characterized by strong sea-inland environmental gradients, ideal for tackling this challenge. We ask: (1) is distribution of specialist and generalist plant species related to this gradient; and (2) do different specialization indices indicate congruent or complementary patterns? Location: Coastal dune systems of central Italy. Methods: We used data on plant species cover in 570 community plots at varying distance from the sea. We quantified species specialization, following three approaches related to resource use or to ecological impact, based on: (1) variability of measured environmental values (or a proxy); (2) variability of co-occurring species; (3) variability of community-weighted trait means. We first compared specialization indices across different habitats of the coastal dune zonation. We then analysed the patterns of variation of communities' mean specialization values along the sea-inland stress gradient. Finally, we studied congruencies and mismatches among different indices. Results: We found significant variation in the degree of specialization along the gradient, across several indices. Habitats closer to the sea and under greater stress held higher proportions of specialized species and had higher average specialization levels. Sheltered backdune habitats harboured both generalist and specialist species, but were dominated by generalists. Conclusions: We conclude that specialist species tend to dominate in the most extreme environmental conditions, but are subordinate in milder conditions. We found a certain degree of congruence among different indices, indicating that most species that are specialists in their resource use also tend to have specialized impacts in the community. Nevertheless, the indices also provided complementary information, suggesting it is important to consider multiple niche dimensions in order to assess more robust patterns of specialism and generalism.