Introduction
Whittaker’s use of precipitation
and temperature to predict the world’s biomes (Whittaker, 1975) is
fundamental to our understanding of how climate organizes ecosystems.
Precipitation and temperature have
also been used to predict habitat productivity, hypothesized to be a
significant driver of terrestrial biotic diversity at large scales
(Currie, 1991; Hawkins et al., 2003). These theories inform our
understanding of global patterns in diversity and help define the
fundamental niche of species based on abiotic factors but do not explain
the processes at local scales where species interact, i.e. the biotic
factors that define a species’ realized niche and drive community
assembly (Hutchinson, 1957; Griesemer, 1994).
At broad scales, climate, habitat productivity, and habitat
heterogeneity are important predictors of spatial patterns in
biodiversity (Currie, 1991; Hawkins et al., 2003; Stein et al., 2014).
The habitat productivity and habitat heterogeneity hypotheses are
potentially the best predictors of species diversity (Tews et al., 2004;
Storch, Bohdalková, and Okie, 2018). Of these two, the habitat
productivity hypothesis posits that energy in the form of resources such
as vegetation productivity is known to limit the number of species able
to co-occur in one space (Currie, 1991). The strength and importance of
productivity-diversity relationships outperform other measures like
habitat heterogeneity when investigating diversity trends at broad
scales (Storch et al., 2006). However, when scaled down, the strength of
productivity-diversity relationships is weaker and less consistent
across different habitat types (Mittelbach et al., 2001), and the
relative importance of factors explaining diversity is not well known
(Field et al., 2009; Zellweger et al., 2016) nor consistent across
biomes and ecoregions (He and Zhang, 2009). Thus, essential links
between local and broad scale processes are needed for a better
understanding of the associations between climate, habitat productivity,
heterogeneity, and biodiversity.
Alternatively, at local scales the
habitat heterogeneity hypothesis
posits that habitat structure, the three-dimensional arrangement of
vegetation across the landscape, is elemental to the composition of
species within a community; predicting that increased structural
heterogeneity supports a greater number of species by providing a higher
density of niches with unique microhabitats compared to more homogenous
habitats (MacArthur and MacArthur, 1961; Vierling et al., 2008). Higher
niche density in more structurally complex habitats, such as forests, is
associated with greater partitioning of resources (Goetz et al., 2010;
Carrasco et al., 2018), and
accounting for differences in
three-dimensional vegetation structure improves estimates of animal
diversity ( Huang et al., 2014; Zellweger et al., 2016).
Three-dimensional structural vegetation heterogeneity also creates
varied microclimates that allow species to persist in otherwise
inhospitable environments (Huey et al., 2012), which will be
increasingly important as climate continues to change. Habitat
heterogeneity does predict avian richness at a local scale based on
three-dimensional structure (Cooper et al., 2020a) and can scale up to a
national level using satellite-based metrics (Huang et al., 2014;
Farwell et al., 2020). However, little is known about the degree of
similarity in habitat heterogeneity within or between biomes.
While species richness and species diversity are important indicators of
the number of species and individuals in a community, functional
diversity also informs us of the functional breadth, similarities, and
distinctiveness of species in a community (Violle et al., 2012; Cadotte
et al., 2011; Gagic et al., 2015). Thus, in addition to improving our
knowledge of the relationships among habitat productivity, structure,
and species richness, functional diversity can provide important
linkages to the broader ecosystem. The decline and loss of species which
are functionally connected to their ecosystems through seed dispersal,
herbivory, predation, and pollination could lead to the decline of
ecosystem resilience. Thus, understanding the functional diversity of
species assemblages is important for understanding the stability and
resilience of ecosystems.
Here we attempt to harmonize classic ecological theories over a broad
range of ecosystems and scales to create a framework that can predict
the extent to which climatic and vegetation structural characteristics
drive variation in species and functional diversity. Our study focuses
on forest ecosystems, the most biodiverse of terrestrial ecosystems, at
National Ecological Observation Network (NEON) sites across North
America. We use data from high resolution airborne sensors (light
detection and ranging (lidar), and hyperspectral cameras), flux towers,
and avian point count surveys to examine avian richness and functional
diversity of North American forest ecosystems. The replicated nature of
NEON samples and sites allows us to investigate ecological relationships
within sites at local scales, between sites at regional scales, and
between all sites at a national scale. We hypothesize that the
combination of habitat heterogeneity and productivity within forests
paired with precipitation and temperature (i.e. Whittaker’s prediction
of biomes) will more accurately predict avian richness and functional
diversity across multiple spatial scales and forest ecosystems compared
to temperature and precipitation alone, which will ultimately advance
our understanding of links between broad and local scale drivers of
species and functional diversity. We aim to answer the following
questions:
Question 1: How are precipitation, temperature and
three-dimensional forest structure related?
Question 2: Does each forest ecosystem have a unique
three-dimensional structural signature?
Question 3: What is the relative strength of temperature,
precipitation and forest structure in predicting differences in avian
richness and functional diversity?