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.