Beta diversity peaks at regional scales across ants, birds, diatoms and trees

For nearly a century, ecologists have argued about whether ecological communities are real, discrete entities in nature or merely convenient abstractions. A new study published in Frontiers of Biogeography offers what may be the most direct empirical answer yet: communities appear to be regional in scale, with the sharpest differences between neighboring assemblages emerging not at local patches but at spatial grains of hundreds to tens of thousands of square kilometers. The finding, drawn from four wildly different groups of organisms across the contiguous United States, could reshape both community ecology and the practice of biodiversity conservation.

The research, led by David Jenkins and colleagues, tackles the question through beta diversity, the measure of how dissimilar two communities are in their species composition. The logic is elegant: if the clearest possible separation between adjacent communities occurs at a particular spatial resolution, then that resolution represents the empirical size of a community. Maximal beta diversity, in other words, marks the scale at which nature draws its sharpest boundaries between assemblages of species.

To test this, the team assembled point data for ants, birds, diatoms, and trees, four taxa chosen for their radically different life histories, dispersal abilities, and data origins. Ant records spanned two centuries of literature synthesis, totaling nearly 25,000 samples and 699 species. Bird data came from the eBird community science platform, with over 810,000 cleaned reports from January and July between 2010 and 2015, standardized as counts per person-hour. Diatoms were sampled from the top five centimeters of lake sediment across 1,070 lakes in a single year, yielding 1,957 species. Tree data, covering stems wider than five centimeters at breast height, came from the US Forest Service’s Forest Inventory Analysis across four years spaced five years apart, totaling nearly 40,000 samples and 403 species.

The analytical framework organized all georeferenced observations into hexagonal polygons, with cell size defining the spatial grain. At each grain, ranging from one square kilometer to one million square kilometers, the researchers calculated mean pairwise beta diversity between each central hexagon and its immediate neighbors. This neighborhood-based approach, borrowed conceptually from remote sensing research on optimized spatial resolution, controls for sample size across grains because hexagonal grids are constrained to fewer than six neighbors.

A critical methodological decision involved how beta diversity was calculated. The team used a relatively new deviation from a null model based on spatial heterogeneity, called beta NBD for its underlying negative binomial distribution. This measure, developed by Youhua Chen and Fangliang He, represents clustered species patterns more accurately than the older deviation based on spatial homogeneity. The newer measure made only modest adjustments to observed beta diversity and showed no vulnerability to sample size effects, whereas the older measure substantially altered observed values and increased with greater sampling effort.

The results were strikingly consistent. For all four taxa, beta diversity peaked at intermediate, regional spatial grains. Trees showed peak values at roughly 300 square kilometers, remarkably stable across four evaluated years spanning two decades. Winter birds peaked at approximately 500 square kilometers, while summer birds and ants both peaked near 2,000 square kilometers. Diatoms, the microscopic algae inhabiting lake bottoms, showed the largest peak at about 11,000 square kilometers. Hump-shaped quadratic curves fit the data far better than linear models, with Akaike Information Criterion weights exceeding 0.95 in all cases.

These rankings defied the team’s initial expectations. They had predicted that diatoms, confined to discrete lake habitats, would show the smallest community scales, followed by ants, trees, and birds. Instead, diatoms exhibited the largest peak, and trees the smallest. The researchers suggest that diatom community size roughly matched the average density of sampled lakes across the contiguous US, meaning the apparent regional scale may actually reflect the average dispersion of discrete diatom habitats rather than long-range dispersal. For terrestrial taxa inhabiting intergrading habitats, the regional peaks may reflect genuinely regional processes.

The repeatability of the patterns adds considerable weight to the findings. Bird beta diversity scaling was consistent across six consecutive years, though sensitive to seasonality, with winter communities concentrating at smaller spatial scales than summer ones. Tree patterns showed very little within- or among-annual variation at their peak grain. Given that birds and trees differ so enormously in life history and mobility, this commonality suggests that community assembly processes provide a strong and general basis for the observed regional scaling.

The results broadly support the regional community concept advanced by Robert Ricklefs, who argued in 2008 that regional scales should be preferred to local ones to capture the full suite of selective conditions driving diversity. The relatively low beta diversity at fine spatial grains in this study supports Ricklefs’ argument that local communities are one end of spatiotemporal continua rather than empirically distinct entities. The authors suggest that metacommunity ecology may be wise to treat local communities as sample points rather than bounded units, steering the field toward a species-centric approach.

Beyond its conceptual implications, the work bears fundamental consequences for conservation biogeography in the Anthropocene. If ecological communities are regional in scale, then local conservation efforts can best ensure success when coordinated regionally, through corridors and networks among conserved sites. The findings may also help researchers reconsider the appropriate spatial grains to use when studying diversity responses to land use and climate change, suggesting that rasterized study units matching peak beta diversity would obtain maximal resolution. The authors note that similar peaked spatial scaling may exist for other biogeographical patterns, potentially revealing maximal signal amidst noise and better representing natural patterns than maps designed for visual appeal.

Subject of Research: Peak beta diversity occurs at regional spatial grains

Article Title: Peak beta diversity occurs at regional spatial grains

Article References: Ohyama, L., Bogota-Gregory, J. D., & Jenkins, D. G. (2024). Peak beta diversity occurs at regional spatial grains. Frontiers of Biogeography, 17, Article e132675. https://doi.org/10.21425/fob.17.132675

Image Credits: AI Generated

DOI: 10.21425/fob.17.132675

Keywords: Peak, beta, diversity, occurs, regional, spatial, grains, scientific research