Introduction
Declines of Amazonian bird species in remote locations have recently
attracted a great deal of attention to the potential impacts of global
change in the most extensively intact forest landscape on earth (Blake
& Loiselle 2015; Stouffer et al. 2020). However, our
understanding of these trends in Amazonian bird communities is based on
only two studies (Blake & Loiselle 2015; Stouffer et al. 2020).
Blake and Loiselle (2015) reported declines over an 8 year period from a
remote lowland rainforest in Ecuador that may be explained by
regional-wide precipitation patterns, although they hypothesize that
these changes might also be explained by a series of strong La NiƱa
events. More recently, Stouffer et al (2020) reported declines of
terrestrial insectivores over >35 years in an isolated
study site, near Manaus, Brazil. To broaden inference of trends across
the region and across species, we evaluated the entire bird community at
a site in southeastern Peru over a36-year interval. Our results contrast
with previous finding and highlight the urgent need for long term
surveys to better understand whether global change is driving widespread
species declines in the lowland tropics.
Studies of ecological communities in remote locales absent other
large-scale human impacts presents a unique opportunity to evaluate how
biodiversity is affected by global change. Several long-term studies of
neotropical tree communities have made theoretical and empirical
advances in understanding long-term community dynamics (Rees et
al. 2001; Swenson et al. 2012) . With regards to animal studies,
several authors have documented long-term community change in the
tropics - albeit for subsets of the community (Blake & Loiselle 2015;
Brawn et al. 2017; Stouffer et al. 2020). What do these
studies conclude in aggregate-across plant and animal groups, about how
lowland tropical species are faring at locations with little direct
human disturbance?
A major impediment to studying changes in tropical bird communities is
deriving precise population estimates for the majority of species in the
community; species-specific challenges in dealing with suites of species
that require a variety of sampling techniques across different spatial
scales, which complicates accurate abundance estimation (Robinsonet al. 2018). In 1982, Terborgh et al. (1990) undertook
what became a seminal censusing effort and fully described for the first
time, in great detail and precision, the structure and organization of
the entire bird community present in a 97-ha plot of intact and mature
Amazonian floodplain forest at Cocha Cashu Biological Station (Silmanet al. 2003). At the time, the study was monumental: the
scientific community had barely learned to identify many bird species in
South America, let alone how to quantify their abundances using sampling
techniques designed for temperate or nearctic forest bird communities.
Here, after a 36 year interval, we re-censused the entire bird community
on the 97-ha plot at Cocha Cashu originally surveyed by Terborghet al . (1990) , using the same methodology, in order to evaluate
changes to a bird community in a remote Amazonian site that remains
little-influenced by human activity. As in the original study, our
approach allowed us to estimate population size for the entire bird
community, providing a rich window into changes in the structure and
organization of the community. We tested key hypotheses about which the
local and regional processes are likely to drive patterns of community
structure and organization. Previous studies such as those mentioned
above have mostly attributed large changes in insectivores and decreased
recruitment of understory species to regional changes in precipitation
patterns. If these regional processes had also influenced the bird
community at Cocha Cashu, we would expect to find changes, across entire
suites of species (e.g. declines across all terrestrial insectivores as
has been reported elsewhere (Van Klink et al. 2020; Wagner 2020).
Alternatively, if local processes were the main determinants for
community composition over the two time periods, we posited that changes
in some species would be evident due to changes in specific habitats.
For example, some changes may be attributed to a natural mass-mortality
event of Guadua sp. bamboo patches found on or near the plot
(Socolar et al. 2013). Thus, changes in abundance and persistence
in the community would be evident mostly in species associated with
bamboo and species associated with primary successional habitat
influenced by the Manu river, with declines expected in both.
The focus here is to document patterns of change in a hyper-diverse
community over time. Our results show that, by-and-large, the structure
of the community has persisted virtually unchanged. This large-scale
observational comparative study is a necessary first step to propose
hypotheses regarding the role of fluctuating resources and biotic
interactions such as competition and mutualism that may play a role in
explaining the few changes we recorded. We end by stressing the
importance of developing new analytical techniques and combining these
with traditional census methods in order to generate more consistent
long-term datasets for tropical bird communities.