Niche breadth, rarity and ecological characteristics within a regional flora spanning large environmental gradients

Aim Species specialization, which plays a fundamental role in niche differentiation and species coexistence, is a key biological trait in relation to the responses of populations to changing environments. Species with a limited niche breadth are considered to experience a higher risk of extinction than generalist species. This work aims to measure the degree of specialization in the regional flora of the French Alps and test whether species specialization is related to species rarity and ecological characteristics. Location This study was conducted in the French Alps region, which encompasses a large elevational gradient over a relatively limited area (26,000 km(2)). Methods Specialization was estimated for approximately 1200 plant species found in the region. Given the inherent difficulty of pinpointing the critical environmental niche axes for each individual species, we used a co-occurrence-based index to estimate species niche breadths (specialization index). This comprehensive measurement included crucial undetermined limiting niche factors, acting on both local and regional scales, and related to both biotic and abiotic interactions. The specialization index for each species was then related to a selection of plant typologies such as Grime strategies and Raunkiaer life-forms, and to two measurements of plant rarity, namely regional area of occupancy and local abundance. Results Specialist species were mainly found in specific and harsh environments such as wetlands, cold alpine habitats and dry heathlands. These species were usually geographically restricted but relatively dominant in their local communities. Although none of the selected traits were sufficient predictors of specialization, pure competitors were over-represented amongst generalist species, whereas stress-tolerant species tended to be more specialized. Main conclusions Our results suggest that co-occurrence-based indices of niche breadth are a satisfactory method for inferring plant specialization using large species samples across very heterogeneous environments. Our results are an empirical validation of the tolerance-dominance trade-off and also provide interesting insights into the long-standing question of which biological properties characterize species with narrow niche breadth that are potentially threatened by global changes in the environment.