A gaping hole in boreal conservation research: Effects of size and aggregation of conservation areas on species diversity at the landscape scale

Area-based conservation measures are the main approach to preserve forest biodiversity. However, there is no common view on the best strategy in relation to spatial aggregation of conservation areas, for a given total area preserved. We conducted a systematic literature review to evaluate the effect of mean patch size and aggregation of conservation areas on landscape-scale biodiversity in boreal forest. Our main objectives were to find empirical evidence regarding whether few large or several small conservation areas protect more biodiversity and investigate how the spatial aggregation of conservation areas affects biodiversity. We searched specifically for studies comparing biodiversity across many small vs. few large and dispersed vs. aggregated conservation areas, controlling for total area protected. Although our initial search resulted in a large number of articles, not a single study assessed landscape-scale biodiversity in many small vs. few large, or dispersed vs. aggregated conservation areas, of a spatial scale relevant to average-sized or even small nature reserves (i.e. ≥ 10 ha). We did find 5 studies comparing many small vs. few large conservation patches within clear-cuts, and one study of forest patches within a national park (ca 7 ha). The conservation areas on clear-cuts were patches of retained trees, with the smallest patches being single trees. The effect of patch size on biodiversity varied among studies, mostly indicating neutral effects of patch size. While the results of these studies are relevant to clear-cuts, their relatively small spatial extent (the largest retained patch being 1.2 ha) precludes extrapolation to scales relevant to reserves in boreal forest. Our review exposes an extensive knowledge gap regarding consequences of the sizes of conservation areas on landscape-scale boreal forest biodiversity. Until such information is available, we recommend a combined approach involving both small and large conservation areas in boreal forest.