ALBERT

Description: Abstract 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 affects 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: 1m2; 10m2; 100m2; 1000m2; 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‐1m2 vs. γ‐10m2) × 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 contrasted 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. Per contra, 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, single large should be preferred over several small forest patches.