Recolonization of secondary forests by locally extinct fauna through the lens of range expansion: Four open questions

Recolonization of secondary forests happens when individuals disperse from a nearby source old-growth forest populations. This pattern of recolonization could be (a) the result of a random subset of individuals dispersing and colonizing nearby secondary habitats. Instead, the set of recolonizing individuals may not be random but have a particular set of characteristics. (b) Old-growth source populations could show spatial sorting where highly dispersive individuals (those with larger limbs, or exploratory and aggressive behavior) are overrepresented in the forest patch edges and more likely to colonize nearby patches. These are often known as “pull” expansions because highly dispersive individuals living at the edge of the source population are the ones “pulling” the expansion. Alternatively, (c) because old-growth populations are expected to be at carrying capacity recolonization may be driven by subordinate individuals that cannot outcompete dominant conspecifics and disperse looking for alternative territories. This is the case of “push” expansions when dispersal is driven by these subordinate individuals that are pushed away due to density dependence. Even though deforestation is ongoing particularly in the tropics (Roberts et al., 2021), recovering tropical forests are becoming a prevalent component of the Anthropocene landscape. This recovery is in part due to socioeconomic changes leading to the abandonment of agricultural fields (Aide et al., 2013; Aide & Grau, 2004). These abandoned fields undergo ecological succession developing complex forest structures that, given the appropriate conditions, eventually provide the minimum requirements for the recolonization of locally extinct fauna. The resulting biodiversity patterns following this recolonization process have been well described in the empirical literature and synthesized in at least nine reviews or meta-analyses (Acevedo-Charry & Aide, 2019; Bowen et al., 2007; Chazdon, 2014; Dent & Wright, 2009; Dunn, 2004; Hughes et al., 2020; Lennox et al., 2018; Lindenmayer & Hobbs, 2004; Thompson & Donnelly, 2018). The general patterns are clear. Animal species richness can recover quickly after abandonment depending on geographical region, landscape attributes, and degree of disturbance (Dunn, 2004). However, animal species composition takes longer, with some forest specialists and functional groups missing from secondary forests (Acevedo-Charry & Aide, 2019). After the synthesis of these general fauna recolonization patterns still much remain unknown about the eco-evolutionary mechanisms that drive these patterns. Range expansion theory is commonly invoked to describe the eco-evolutionary mechanisms that drive multiple types of population expansions. This theoretical framework has the potential to contribute to our understanding of fauna recolonization of secondary forests. Here we describe four open questions about population-level processes driving secondary forest recolonization of locally extinct fauna through the lens of range expansion. We synthesize these ideas with the aim of providing a framework to guide future research.