Introduction
Identifying general principles that govern the distribution and abundance of species across Earth’s ecosystems is a fundamental pursuit in ecology. Over the past three decades, ‘trait-based’ ecology—focused on the role that measurable characteristics of organisms play in mediating geographic distribution and abundance—has emerged as a major conceptual lens through which to describe general processes that might drive patterns of biodiversity across the biosphere (Hansen and Urban1992; McGill et al. 2006; Mindel et al. 2016). Traits-based theory stemmed from descriptive correlations between the frequency of phenotypic traits hypothesized to affect individual and population level success across taxa and environmental contexts (e.g. Grime 1988; Diaz and Cabido 1997). But the ultimate potential of traits-based approaches might allow scientists to predict ecological outcomes in new contexts (i.e. functional traits; McGill et al. 2006).
The need for predictive trait-based approaches is increasingly urgent due to mounting evidence that stresses like climate change, biological invasion, and over-exploitation are having profound effects on ecosystems and the socioeconomic benefits they provide. While ecological communities are inherently dynamic, with species membership and relative abundance varying over time and space (Cottenie 2005, Dornelas et al. 2014), unprecedented levels of anthropogenic stress are now driving species range and density changes that far exceed historical levels (Chapin et al. 2000, IUCN 2008). Effects are proving to be unequal across species (Zavaleta et al. 2009, Sunday et al. 2012, Bates et al. 2014), so ecological communities are essentially being pulled apart, and reassembled with new member combinations (Hobbs et al. 2009, García Molinos et al. 2015). What will future species assemblages look like, and how will they function, in the face of these major disturbances? Identifying species’ characteristics that recur across unrelated taxa and confer information about species performance under a range of environmental and biotic interactions offers the potential to predict ecological dynamics as novel ecosystem configurations form under global change. For example, as abiotic conditions defining the fundamental niche shift under climate change, biotic traits that confer species’ dispersal and survival abilities help define the shape of realized niches they are likely to occupy (McGill 2006; Early and Sax 2011; Estrada et al. 2018).
Here we present a quantitative review of trait-based biodiversity research and its application to global change ecology to address the following questions: (1) Which traits offer promising insights into the outcomes of environmental and biotic filtering across ecosystem types and taxonomic groups? (2) To what extent (and in what contexts) are traits-based insights being applied to predict the ecological outcomes of global change? (3) What are the potential barriers to predicting the outcome of environmental and biotic interactions using traits? (4) What research techniques and conceptual frameworks can help us move beyond describing ecological patterns and towards prediction? Our review is framed from the perspective that species’ behavioural, morphological, physiological, and life history attributes mediate scale-dependent environmental and biotic filters on distribution and abundance across land and seascapes (Figure 1). Species’ characteristics that confer success under abiotic environmental conditions such as temperature, light, acidity, moisture (in terrestrial systems) and dissolved oxygen (in aquatic systems) provide the coarsest filter on species distribution (i.e. Filter 1 in Figure 1; the boundaries of the fundamental niche). Traits that influence the outcome of biotic interactions within local environments further mediate species persistence and co-existence (i.e. including trophic interactions; Filter 3 in Figure 1). A host of traits that confer information on species’ dispersal ability mediate feedbacks between the effects of abiotic and biotic interactions on species’ range and relative abundances (i.e. dispersal-limited controls on redistribution; Filter 2 in Figure 1). Knowledge of the role that traits play in filtering across these scales facilitates prediction about species persistence and relative abundance in new contexts such as altered environmental conditions (e.g. climate-mediated shifts in thermal regimes; Figure 1A vs B).