Forest fragmentation shapes the alpha–gamma relationship in plant diversity

Questions: Forest fragmentation affects biodiversity locally (α diversity) and beyond — at relatively larger scales (γ diversity) — by increasing dispersal and recruitment limitations. Yet, does an increase in fragmentation affect the relationship between α and γ diversity and what can we learn from it?. Location: Northern France. Methods: We surveyed 116 forest patches across three fragmentation levels: none (continuous forest); intermediate (forest patches connected by hedgerows); and high (isolated forest patches). Plant species richness of both forest specialists and generalists was surveyed at five nested spatial resolutions across each forest patch: 1 m²; 10 m²; 100 m²; 1,000 m²; and total forest patch area. First, we ran log-ratio models to quantify the α–γ relationship. We did that separately for all possible combinations of fragmentation level (none vs intermediate vs high) × spatial scale (e.g., α-1 m² vs γ-10 m²) × species type (e.g., α-specialists vs γ-specialists). We then used linear mixed-effects models to analyze the effect of fragmentation level, spatial scale, species type and all two-way interaction terms on the slope coefficient extracted from all log-ratio models. Results: We found an interaction effect between fragmentation level and species type, such that forest specialists shifted from a linear (i.e., proportional sampling) to a curvilinear plateau (i.e., community saturation) relationship at low and high fragmentation, respectively, while generalists shifted from a curvilinear to a linear pattern. Conclusions: The impact of forest fragmentation on the α–γ relationship supports generalist species persistence over forest specialists, with contrasting mechanisms for these two guilds. As fragmentation increases, forest specialists shift from proportional sampling towards community saturation, thus reducing α diversity likely due to dispersal limitation. Contrariwise, generalists shift from community saturation towards proportional sampling, thus increasing α diversity likely due to an increase in the edge:core ratio. To ensure long-term conservation of forest specialists, one single large forest patch should be preferred over several small ones.