Question 3: What is the relative strength of temperature,
precipitation and forest structure in predicting differences in avian
richness and functional diversity?
Harmonizing Whittaker’s theory of ecosystem organization alongside the
habitat productivity and heterogeneity hypotheses led to increased
predictive power of species richness and functional diversity (comprised
of functional richness and functional redundancy) both within and
between forest biomes, thus connecting local and broad spatial scales.
Temperature and precipitation are relatively robust predictors of
average species richness and functional diversity gradients across all
NEON sites but fail to describe the large variation of species richness
and functional diversity within any given site or region. Forest
structure metrics improved the ability for our models to predict
variation in diversity, especially at local scales, which is consistent
with studies that show vegetation structure to be secondary to
vegetation productivity and climate variables when describing large
scale diversity gradients (Roll et al., 2015). However, at extreme
temperatures and amounts of precipitation, where energy becomes a
limiting factor, the effect sizes of climatic variables were much larger
than that of productivity and heterogeneity, which is consistent with
patterns of species richness reported in other parts of the world (Coops
et al., 2018).
The effect of climatic energy, in the form of temperature and
precipitation, and habitat structure on bird diversity result in
non-linear patterns (Carrasco et al., 2018; Bae et al., 2018). For
example, increases in canopy heterogeneity, as indicated by coefficient
of variation in LAD horizontally, increase both species and functional
richness, and decrease functional redundancy, but only up to a point,
before creating a bell-shaped curve (Figure S1). Beyond that point,
canopy heterogeneity have a negative impact on species richness and
functional richness but increase functional redundancy potentially in
part to patchy habitats with elements such as edge effects or canopy
gaps which create high structural heterogeneity that may not be
conducive to many forest-dependent species. For example, increased edge
density is known to decrease forest dependent avian richness and
functional richness in Andean forests compared to larger continuous
forests (Jones et al., 2021). Similarly, functional redundancy and
species richness increase with increasing temperature up to roughly 3 ºC
and 12 ºC respectively, after which they both decline with increasing
temperature. On the other hand, species richness declines with
increasing precipitation up to roughly 1200 mm/year. However, functional
redundancy decreases with low levels of precipitation and increased with
high amounts of precipitation. This is not the same as the biodiversity
patterns of other organisms such as plants, where plant and tree
diversity tend to be positive with increasing temperatures and
precipitation totals (Chu et al., 2019). Thus, climate and structure
simultaneously affect species richness and functional diversity
relationships, independently driving broadscale patterns in the
diversity of different taxonomic groups (Field et al., 2009) and forest
structure conditions (Fahey et al., 2019), while interacting uniquely at
local scales to form microclimates (Von Arx et al., 2013; Davis et al.,
2019).
Studies of bird functional diversity provide a window into the causes
and consequences of forest ecosystem resilience or degradation. While
85% of the current forests in the Americas are potentially threatened,
with 40% of threatened forest ecosystems at risk of ecological collapse
(Ferrer-Paris et al., 2019), understanding the drivers of functional
diversity can provide insight into how we can maintain or potentially
restore ecosystem integrity to these forests. Based on our models, avian
functional diversity and species richness is highest in forests with
relatively warm temperatures with moderate amounts of rainfall as well
as sites with high levels of habitat heterogeneity and structural
complexity. Although we do not include tropical forests, we hypothesize
that extreme temperatures would diminish species richness and functional
diversity without a relatively large amount of heterogeneity in canopy
structure or productivity. This would add support to the hypothesis that
forest complexity is essential for stabilizing temperatures under the
canopy (Davies-Colley et al., 2000; Vanwalleghem and Meentemeyer, 2009),
which is an indicator of long-term temperature variability within a
region and supports both avian richness and phylogenetic diversity
(Voskamp et al., 2017). Therefore, a healthy and structurally complex
forest is necessary to support avian species richness and functional
diversity which in turn promote healthy forest ecosystems.
Here we present a new method for predicting species and functional
diversity by harmonizing existing ecological theories to assess and
predict diversity at local and macrosystem scales. We also conclude that
climatic controls, such as a forest’s ability to buffer and stabilize
extreme temperatures (De Frenne et al., 2019), plays a substantial role
in how forest structure relates to avian richness and functional
diversity. A critical next step will be to expand the scope of these
data to tropical forests and eventually to other types of ecosystems to
assess the universality of this paradigm in driving and predicting
species and functional diversity. The better we understand the
foundation of these relationships and how they might vary across biomes,
the clearer the picture will be for how local and regional factors
combine to influence the biogeography of species diversity. We hope that
this study will be a step towards building more accurate maps of
biodiversity, microrefugia, and forest integrity across the continent,
and eventually the globe, using remote sensing data. In addition,
understanding how biodiversity is changing has urgent implications for
conservation, land management decisions, and public policy at local,
national, and international scales. The importance of forest structure,
precipitation, and temperature in our models can be used to predict the
response of species and functional diversity to shifting climatic
regimes, which could be accounted for in future models of biodiversity
responses to climate change.