Seasonal dynamics of flock interaction networks across a human‐modified landscape in lowland Amazonian rain forest

Abstract

Although lowland tropical rain forests were once widely believed to be the archetype of stability, seasonal variation exists. In these environments, seasonality is defined by rainfall, leading to a predictable pattern of biotic and abiotic changes. Only the full annual cycle reveals niche breadth, yet most studies of tropical organisms ignore seasonality, thereby underestimating realized conditions. If human‐modified habitats display more seasonal stress than intact habitats, then ignoring seasonality will have particularly important repercussions for conservation. We examined the seasonal dynamics of Amazonian mixed‐species flocks, an important species interaction network, across three habitats with increasing human disturbance. We quantified seasonal space use, species richness and attendance, and four ecological network metrics for flocks in primary forest, small forest fragments, and regenerating secondary forest in central Amazonia. Our results indicate that, even in intact, lowland rain forest, mixed‐species flocks exhibit seasonal differences. During the dry season, flocks included more species, generally ranged over larger areas, and displayed network structures that were less complex and less cohesive. We speculate that—because most flocking species nest during the dry season, a time of reduced arthropod abundance—flocks are simultaneously constrained by these two competing pressures. Moreover, these seasonal differences were most pronounced in forest fragments and secondary forest, habitats that are less buffered from the changing seasons. Our results suggest that seasonality influences the conservation value of human‐modified habitats, raising important questions about how rain forest organisms will cope with an increasingly unstable climate.

A classification scheme for mixed-species bird flocks

“…Similarly, flocking behaviour has been demonstrated to vary seasonally, even in environments that lack strong seasonality, such as the Amazon Rainforest [159,160]. In fact, seasonal differences within flocks can exceed habitat-specific differences between flocks, illustrating that habitat effects could be misleading if seasonality is ignored [160]. Yet, 62% of studies (see Results) were only conducted in one season or did not report whether the studied flocks formed seasonally or year round.…”

“…Thus, assessing the territoriality of mixed flocks becomes a key topic for future research. Similarly, flocking behaviour has been demonstrated to vary seasonally, even in environments that lack strong seasonality, such as the Amazon Rainforest [159,160]. In fact, seasonal differences within flocks can exceed habitat-specific differences between flocks, illustrating that habitat effects could be misleading if seasonality is ignored [160].…”

Amazonian mixed‐species flocks demonstrate flexible preferences for vertical forest structure

“…Our study considered all flock locations equally, but we know that flock space use is influenced by different behaviors, such as defending a territory from neighboring flocks (Munn & Terborgh, 1979) and the constraints of breeding, which is particularly acute for the nuclear flock species (Rutt & Stouffer, 2021). These activities play periodic, but important, roles in flock space use decisions; however, because all of our observations were lumped together as "foraging," we cannot distinguish among behavior-specific habitat preferences.…”

Forest structure predicts species richness and functional diversity in Amazonian mixed‐species bird flocks

“…The core flock obligate species (generally 5–10 species) are frequently joined by other facultative species from the larger forest bird community and can contain as many as 65 species throughout the course of a single day (Martínez & Gomez, 2013). These flocks are known to associate with over 100 species, representing ~36% of the core forest avifauna in central Amazonia (Rutt & Stouffer, 2021). Flock movement is conspicuous, flocks can be easily followed and observed (Mokross et al, 2014), flocks associate with a large percentage of forest avifauna (Rutt & Stouffer, 2021), and they are sensitive to forest disturbance as flock size and diversity significantly decrease in small forest fragments and early successional forest (Maldonado‐Coelho & Marini, 2004; Mokross et al, 2014; Stouffer, 2020; Zou et al, 2018).…”

“…These flocks are known to associate with over 100 species, representing ~36% of the core forest avifauna in central Amazonia (Rutt & Stouffer, 2021). Flock movement is conspicuous, flocks can be easily followed and observed (Mokross et al, 2014), flocks associate with a large percentage of forest avifauna (Rutt & Stouffer, 2021), and they are sensitive to forest disturbance as flock size and diversity significantly decrease in small forest fragments and early successional forest (Maldonado‐Coelho & Marini, 2004; Mokross et al, 2014; Stouffer, 2020; Zou et al, 2018). Mixed‐species flocking species are known to recolonize secondary forests that have regenerated to a certain threshold—at least 4–7 m tall (Rutt et al, 2020)—and even breed in them (Rutt et al, 2021).…”