Biodiversity Gains? The Debate on Changes in Local- vs Global-Scale Species Richness

Do changes in biodiversity at local scales reflect the declines seen at global scales? This debate dates back at least 15 years (Sax and Gaines, 2003), but has recently been revived by several authors (e.g., Gonzalez et al., 2016; Vellend et al., 2017). The heated exchanges on this issue are reminiscent of earlier debates on the nature of the field, and if “conservation biology,” which originated as a crisis discipline focused on protecting biodiversity (Soulé, 1985), should transition to “conservation science,” which attempts to maximize the preservation of biodiversity together with human well-being (Kareiva and Marvier, 2012). Why would a debate that largely centers on data interpretation—as the debate about changes in local-scale biodiversity does—become so intense? It is probably because this debate involves the primary rationale for the modern conservation movement—that human activities are causing major declines in biodiversity around the world (Ripple et al., 2017), and that these declines are bad from the perspectives of science, ethics and values, ecosystem services, and human well-being (Pascual et al., 2017). This particular debate, though, addresses primarily one aspect of recent trends in biodiversity: changes in species richness at local scales in relatively intact ecological communities. These trends do not necessarily reflect changes in other components of biodiversity (e.g., functional, phylogenetic, genetic, or phenotypic), changes in biodiversity in heavily altered landscapes (e.g., agricultural fields or urbanized areas), or changes in global species richness—at which scale biodiversity is clearly declining (Ceballos et al., 2015; Ceballos et al., 2017; Pimm et al., 2014). Assessing local changes in species richness provides insights that are ecologically important and conservation-relevant, but these studies are often hampered by a shortage of baseline data and the absence of long-term monitoring. Researchers therefore must often rely on data from studies that were designed to address other research questions, such as how plant communities recover from disturbance (Cardinale et al., 2017-in this issue). These studies and data are also not distributed around the world in an ideal way to assess global patterns of local changes in species richness (Gonzalez et al., 2016). Given these constraints, research groups have taken different approaches to explore the topic: some groups have compared species richness in human-disturbed and reference sites (e.g., Moreno-Mateos et al., 2017), and some have analyzed data from sites that had been resurveyed over time (Dornelas et al., 2014; Vellend et al., 2013). Together results suggest that dramatic human disturbances, such as agricultural expansion and urbanization, result in significant declines in local species richness, but that in areas where ecosystems are still relatively intact, such as old fields and second growth forests, species richness tends to remain relatively constant or even increase over time (Vellend et al., 2017). Thus, if you go to a specific place—such as New Zealand, Hawaii, or Massachusetts—the forests, wetlands, and grasslands might now have fewer native species, but the same number or more species overall than they did in the past. How is this possible? This stability in local-scale biodiversity usually results from the presence of non-native species numerically offsetting losses of native species (Vellend et al., 2017)—i.e., 30 native species might disappear from a location, but 30, 40, or more non-native species might arrive and become established—although the evidence for the generality of this pattern is questioned (Cardinale et al., 2017-in this issue). This debate even spilled over into public attention when Pyron (2017) asserted in theWashington Post that human-caused extinction of species is not a problem, because if the 50 native reptiles of South Florida went extinct they would be replaced by the 90 non-native reptiles are now living there. Some conservation researchers have reacted strongly to these findings. They have argued that some of the meta-analyses (e.g., Dornelas et al., 2014; Vellend et al., 2013) are flawed because the underlying data: (1) are not spatially representative of biodiversity around the globe (they are biased toward North America and Europe), (2) are not representative of the primary drivers of biodiversity change (they omit sites that had been converted to agriculture or urban development), (3) combine data from sites that had recently been perturbed and are likely losing species with data from sites that were recovering from perturbations, and (4) rely on time series that lack historical baselines, making it difficult to accurately characterize biodiversity change (summarized in Cardinale et al., 2017-in this issue). These points are sound, but the criticisms do not discount the value of the analyses that Vellend et al. (2013), Dornelas et al. (2014), and others have done. Their analyses are reasonable and convincing given the available data, and the weaknesses, which the authors acknowledge, are difficult or impossible to avoid. Moreover, several subsequent studies in different ecological communities have found similar patterns of high species turnover and relatively steady or increasing species richness at local scales (Barnagaud et al., 2017; Dunic et al., 2017; Elahi et al., 2015; Gotelli et al., 2017; Jones et al., 2017). Conservation researchers, practitioners, and policymakers should recognize the limitations of the results, but should consider their implications as the research continues, and adjust their goals and actions accordingly (admittedly easier said than done) (Fleischman and Briske, 2016; Knight et al., 2008). In reading papers on both sides of this controversy, we at Biological Conservation want to highlight three key points