Using plant invasions to compare occurrence- and abundance-based calculations of biotic homogenisation: Are results complementary or contradictory?

Authors: BUONAIUTO, DANIEL - University Of Massachusetts; BARNETT, DAVID - National Ecological Observatory Network (NEON); Blumenthal, Dana; NEBHUT, ANDREA - Stanford University; PEARSE, IAN - Us Geological Survey; SOFAER, HELEN - Us Geological Survey; SORTE, CASCADE - University Of California Irvine; CORBIN, JEFFREY - Union College; EARLY, REGAN - University Of Exeter; GARBOWSKI, MAGDA - New Mexico State University; IBANEZ, INES - University Of Michigan; LAUGHLIN, DANIEL - University Of Wyoming; PETRI, LAÍS - Michigan State University; VILÀ, MONTSERRAT - University Of Sevilla; BRADLEY, BETHANY - University Of Massachusetts

Submitted to: Global Ecology and Biogeography
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/16/2025
Publication Date: 3/6/2025
Citation: Buonaiuto, D.M., Barnett, D.T., Blumenthal, D.M., Nebhut, A.N., Pearse, I., Sofaer, H.R., Sorte, C.J., Corbin, J.C., Early, R., Garbowski, M., Ibanez, I., Laughlin, D.C., Petri, L., Vilà, M., Bradley, B.A. 2025. Using plant invasions to compare occurrence- and abundance-based calculations of biotic homogenisation: Are results complementary or contradictory? Global Ecology and Biogeography. 34(3). Article e70022. https://doi.org/10.1111/geb.70022.
DOI: https://doi.org/10.1111/geb.70022

Interpretive Summary: Biotic homogenization, a reduction in the distinctiveness of ecological communities across landscape, is one consequence biological invasions. Quantifying homogenization is therefore critical for biodiversity conservation and management, but the relative merits of different approaches to quantifying homogenization are poorly understood. In this study, we used a dataset of more than 70,000 vegetation survey plots across the contiguous United States to assess differences in biotic homogenization when calculated with occurrence- vs. abundance-based metrics of beta diversity. In 78% of our plot comparisons, occurrence- and abundance-based calculations agreed in direction, and the two metrics were generally well-correlated but not interchangeable. When data permits, combining the two approaches can offer insights into the role of invasions and local extirpations in driving biotic homogenization. These results will help scientists and land managers better understand the conditions under which biological invasions reduce diversity among ecological communities.

Technical Abstract: Aim: Beta diversity quantifies the distinctiveness of ecological communities across the landscape. Its decline, known as biotic homogenization, is an outcome of anthropogenic global change and thought to be one of many consequences of biological invasions. Quantifying patterns of biotic homogenization is a critical task for promoting biodiversity conservation and management. Species occurrence- (presence/absence) and abundance-based analyses can produce contradictory assessments of the magnitude and direction of changes in beta diversity. Previous work in this area suggests these contradictions should be less frequently observed in nature than in theory, but a growing number of empirical studies report discrepancies between occurrence- and abundance-based approaches. Understanding if these discrepancies are driven by a few isolated cases or are systematic across a diversity of ecosystems is important because it would allow us to better understand the general patterns, mechanisms, and impacts of biotic homogenization. Innovation: In this study, we used a dataset of more than 70,000 vegetation survey plots across the contiguous United States to assess differences in biotic homogenization when calculated with both occurrence- and abundance-based metrics of beta diversity. We estimated biotic homogenization by comparing beta diversity of invaded plot pairs to environmentally matched uninvaded ones with both classes of metrics. Main conclusions: In 78% of our plot comparisons, occurrence- and abundance-based calculations agreed in direction, and the two metrics were generally well-correlated. However, 1 in 5 cases we evaluated disagreed in direction, with one metric showing homogenization while the other showed differentiation, indicating that the metrics are not interchangeable. Contrary to previous expectations, we found that these proportions were well aligned with theory. When data permits, combining the two approaches can offer insights into the role of invasions and local extirpations in driving biotic homogenization or its opposite, differentiation.