Effects of crowding due to habitat loss on species assemblage patterns

Abstract

Terrestrial animals are negatively affected by habitat loss, which is assessed on a landscape scale, whereas secondary effects of habitat loss, such as crowding, are usually disregarded. Such impacts are inherently hard to address and poorly understood, and there is a growing concern that they could have dire consequences. We sampled birds throughout a deforestation process to assess crowding stress in an adjacent habitat remnant in the southern Brazilian Atlantic Forest. Crowding is expected of highly mobile taxa, especially given the microhabitat heterogeneity of Neotropical forests, and we hypothesized that the arrival of new individuals or species in refuges shifts assemblage patterns. We used point counts to obtain bird abundances in a before‐after‐control‐impact design sampling of a deforestation event. Temporal changes in taxonomic and functional diversity were examined with metrics used to assess alpha and beta diversity, turnover of taxonomic and functional similarity, and taxonomic and functional composition. Over time increased abundance of some species altered the Simpson index and affected the abundance‐distribution of traits in the habitat remnant. Taxonomic composition and functional composition changed in the remnant, and thus bird assemblages changed over time. Taxonomic and functional metrics indicated that fugitives affected resident assemblages in refuges, and effects endured >2 years after the deforestation processes had ceased. Dissimilarity of taxonomic composition between pre‐ and postdeforestation assemblages increased, whereas functional composition reverted to preimpact conditions. We found that ecological disruptions resulted from crowding and escalated into disruptions of species’ assemblages and potentially compromising ecosystem functioning. It is important to consider crowding effects of highly mobile taxa during impact assessments, especially in large‐scale infrastructure projects that may affect larger areas than is assumed.

Crowding after sudden habitat loss affects demography and social structure in a bat population

“…This study, using a quasi‐experimental design and focusing on a bat population affected by a new hydroelectric dam, confirmed our initial expectations that after sudden and irreversible habitat loss due to anthropogenic disturbance, mobile terrestrial animals converge in nearby refuges, causing marked, albeit transient increases in population size (Vallejos et al., 2020). Our results also provide evidence that this crowding can have population‐level effects, as suggested by the decrease in apparent survival after disturbance, and the apparent disruption of social structure as revealed by changes in local genetic relatedness.…”

“…Together with recent studies (Vallejos et al., 2020), our research highlights crowding as one of the potential impacts of habitat loss from anthropogenic disturbances on terrestrial vertebrates. We show for the first time that individuals escaping habitat loss increase population size at nearby refuges, and that crowding can negatively affect some demographic parameters and disrupt social structure.…”

“…This study, using a quasi-experimental design and focusing on a bat population affected by a new hydroelectric dam, confirmed our initial expectations that after sudden and irreversible habitat loss due to anthropogenic disturbance, mobile terrestrial animals converge in nearby refuges, causing marked, albeit transient increases in population size (Vallejos et al, 2020). Our results also provide evidence that this crowding can have population-level effects, as suggested by the decrease in apparent survival after disturbance, and the apparent disruption of social structure as revealed by changes in local genetic F I G U R E 2 Estimates (point) and 95% CI (whiskers) of real parameters of S (apparent survival excluding transients), pent (probability that an individual is a new arrival), Ψ (state transitionsprobability that an individual in a bridge at location near or far in primary period t moves elsewhere in primary period t + 1, given it survives to period t + 1), Φ (residency -probability that an individual in the study area associated with a bridge at location near or far at capture occasion j of primary period t, and who first arrived in the study area during previous capture occasions, is still in the study area at capture occasion j + 1), and N (population size) per bridge, according to the best model identified through multistage model building and selection.…”

“…The construction of hydroelectric dams represents a typical example of sudden, and irreversible, habitat loss due to human activities (Gracey & Verones, 2016), providing a convenient setting to evaluate the occurrence and consequences of crowding in animal populations (Vallejos et al., 2020). During the construction phase, habitat loss generally occurs when woody vegetation is removed upstream of the dam to prevent water quality degradation through the decay of plant biomass in the new reservoir (Li et al., 2017).…”

“…In the following years, there is further habitat loss because of the flooding of the river valley (Nilsson & Berggren, 2000). These disturbances cause animals to flee and converge in nearby refuges, where their fate and population impacts are virtually unknown (but see Vallejos et al., 2020). However, it is possible that significant crowding may occur, particularly in species for which habitat refuges are scarce in the surrounding landscape.…”