Individual-scale variation, species-scale differences: inference needed to understand diversity

Abstract

As ecological data are usually analysed at a scale different from the one at which the process of interest operates, interpretations can be confusing and controversial. For example, hypothesised differences between species do not operate at the species level, but concern individuals responding to environmental variation, including competition with neighbours. Aggregated data from many individuals subject to spatio-temporal variation are used to produce species-level averages, which marginalise away the relevant (process-level) scale. Paradoxically, the higher the dimensionality, the more ways there are to differ, yet the more species appear the same. The aggregate becomes increasingly irrelevant and misleading. Standard analyses can make species look the same, reverse species rankings along niche axes, make the surprising prediction that a species decreases in abundance when a competitor is removed from a model, or simply preclude parameter estimation. Aggregation explains why niche differences hidden at the species level become apparent upon disaggregation to the individual level, why models suggest that individual-level variation has a minor impact on diversity when disaggregation shows it to be important, and why literature-based synthesis can be unfruitful. We show how to identify when aggregation is the problem, where it has caused controversy, and propose three ways to address it.

Macroscale intraspecific variation and environmental heterogeneity: analysis of cold and warm zone abundance, mortality, and regeneration distributions of four eastern US tree species

“…2010; Clark et al. 2011; Dietze and Moorcroft 2011). Also, the outputs of the multivariate analysis via regression trees should be treated as an exploratory analysis to assess useful patterns of demography that need to be probed further, not as definitive results.…”

Biodiversity and the functioning of tropical forests

“…By combining individual traits and species in one statistical model, we were able to separate the effect that individual traits have on individual growth, from the variation caused by evolutionary differences among species (Clark et al 2011). Our focus is on individuals because they are the units that grow and respond to their environment (Clark et al 2011), rather than species.…”

“…By combining individual traits and species in one statistical model, we were able to separate the effect that individual traits have on individual growth, from the variation caused by evolutionary differences among species (Clark et al 2011). Our focus is on individuals because they are the units that grow and respond to their environment (Clark et al 2011), rather than species. While other studies show that differences in growth and other traits among individuals of the same species even exceed the differences in average growth or traits among species (Bolnick et al 2003, Clark 2010, Messier et al 2010), this was not the case in our study.…”

“…Bongers 2006, Wright et al 2010). Yet, as Clark et al (2011) pointed out, "individuals are the objects responding to environmental gradients, not species". Species-specific performance of canopy trees may be partly driven by species-specific life-history traits that allow them to endure in the understory and eventually reach the canopy.…”

“…Still, variation among individuals may be substantial , Thomas et al 2013) and important for their ecological performance (Violle et al 2007) and contributions to population growth (Zuidema et al 2009). Hence, individual-tree level analyses may yield important insights into the drivers of tree growth (Binkley et al 2010, Clark et al 2011.…”