Single large or several small? Applying biogeographic principles to tree-level conservation and biodiversity offsets

Land development contributes to the clearance of large trees that are sometimes offset with many smaller trees as compensatory wildlife habitat. But are many smaller trees a valid biodiversity offset for the loss of a single large tree? To answer this question, we tested predictions underpinned by island biogeography theory. Targeting birds, we investigated size and landscape context effects at 72 trees of three sizes (small, medium, and large) located in four landscape contexts (reserves, pasture, urban parklands, and urban built-up areas). Significant positive relationships occurred between tree basal area and bird abundance and species richness in all landscape contexts. SLOSS (single large or several small) analysis revealed that in modified landscapes, several small and medium trees supported an equivalent number of individuals and species as a single large tree, but the same pattern was weaker in reserves. Extrapolated rarefaction curves revealed that in reserves and urban built-up areas, many small or medium trees accumulated the same number or more species than large trees. However, in pasture and urban parklands, many small or medium trees accumulated fewer species than large trees. Overall, 29% of bird species were recorded only at large trees, highlighting that many smaller trees will not be suitable habitat compensation for all species. Complementary approaches to biodiversity offsets are needed, balancing large tree preservation and revegetation. Response patterns for birds at trees conformed to some biogeographic predictions (species-area relationship), but not others (habitat-isolation relationship), underscoring the need for novel conceptual frameworks for habitat structures in modified landscapes.