Mammal defaunation negatively affects the regeneration of the critically endangered Araucaria angustifolia

• Sites with higher defaunation of medium- to large-bodied mammals had lower density of araucaria regenerants. • Sites with lower small mammal richness had higher density of araucaria regenerants. • The density of araucaria regenerants increased with the density of araucaria adults. • There was no significant relationship between canopy cover and density of araucaria regenerants. Overexploitation of forest systems by human actions can lead to the local and selective extinction of animal species, a phenomenon known as defaunation. Several studies showed increased abundance of small mammals under high-defaunation of medium- and large-bodied animals. The absence of seed dispersers can preclude the regeneration of several plant species. Araucaria angustifolia is classified as a keystone and a critically endangered species, so understanding the factors limiting its regeneration is essential. The main objective of this study was to quantify the effects of small mammal richness and mammal defaunation under the regeneration of araucaria across a regional scale in southern Brazil. We expected that sites with greater defaunation and larger assemblages of small mammals would demonstrate less regeneration – since this could lead to reduced seed dispersal and increased seed predation. Our best-fitting model considered adult density, defaunation, richness of small-bodied mammals, canopy openness, and the presence/absence of cattle as predictors of araucaria regenerant density. We found a negative relationship between mammal defaunation and araucaria regeneration. Our results also revealed that the top-10 richest sites in terms of small mammals had a defaunation rate of 0.60 (18% larger than the mean defaunation) and embraced only 100 araucaria regenerants (1.7% of total). Therefore, the mammal defaunation is an important threat to Araucaria Mixed Forest (AMF) functioning and conservation.