Demographic and genetic consequences of forest fragmentation
Our results go in line with studies addressing the impact of patch size and isolation on the genetic diversity of cryptogamic organisms in temporal similar historically fragmented landscapes to our study (~30–50 years). For example, reduced genetic diversity has been observed related to small population size, patch size isolation, and habitat degradation (Holá et al., 2015; Patiño et al., 2010; Wang et al., 2013; Otálora et al., 2011). As well our results in an experimental Amazonian fragmented landscape support theoretical predictions of island biogeography theory and metapopulation theory on the genetic diversity (Whittaker et al., 2008; Vellend, 2003; Vellend & Geber, 2005) observed for oceanic islands (Costanzi & Steifetten, 2019; Hill et al., 2017; Whittaker et al., 2017; Hamabata et al., 2019), and other insular landscape (Aycrigg & Garton, 2014; Hepenstrick et al., 2022). However, climate and habitat fragmentation synergistic effects on biodiversity might take a long time to be apparent (Chase et al., 2020; Scott et al., 2021). For example, some species might become extinct immediately after habitat disturbance, whereas others persist to eventually become locally extinct (extinction debts) years after deforestation isolates local populations (Tilman et al., 1994; Kuussaari et al., 2009). During this time lag, species fitness decreases due to low reproduction and local recruitment which will further exacerbate under future climate extremes, such as droughts (Scott et al., 2021); therefore, affecting species’ long-term survival probability (Figueiredo et al., 2019). Species experiencing time-delayed extinctions (extinction debts) are not necessarily doomed to extinction, as new colonization events could pay for such debts (Kuussaari et al., 2009).
Under future climate scenarios, the ability of bryophyte species to escape local extinction by tracking suitable habitats is of high concern (Zanatta et al., 2020) despite their high dispersal capacities (Vanderpoorten et al., 2019). Even though a diverse genetic pool of individuals can reach a disturbed local population, effective dispersal would be mediated by the local environmental conditions (Hedenäs et al., 2021). Amazon lowland bryophyte communities were thought to be composed of highly dispersive species (Schuster, 1983) with low floristic dissimilarity mainly driven by niche process (Mota de Oliveira & ter Steege, 2015). Yet spore traps on the 325 m height Amazon Tall Tower Observatory (ATTO) capture few bryophyte diaspores, suggesting a low frequency of airborne propagules in the atmosphere (Mota de Oliveira et al., 2022). The high spatial genetic structure of Amazonian plant species across a regional spatial scale (Nazareno et al., 2019; Ledent et al., 2020; Campos et al., 2022), combined with non-random deforestation events in the Amazon (Taubert et al., 2018; Matricardi et al., 2020) will deplete important source populations for the genetic diversity maintenance (Lowe et al., 2005). Herein, our results offer evidence in support of our H1 for genetic drift over the 40 years that populations have been isolated in the present fragmented landscape. This establishes a premise that dispersal limitation in bryophyte metapopulations (Ledent et al., 2020) will be susceptible to stochastic demographic and genetic processes, which may accelerate even more the pace of extinction as ecological barriers rapidly increase with non-random deforestation in the Amazon (Taubert et al., 2018; Fischer et al., 2021).