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?