A global indicator of utilized wildlife populations: Regional trends and the impact of management

Abstract

•Utilized vertebrate populations declined by 50% on average from 1970 to 2016•Body size is an important predictor of trends in utilized populations•Importantly, populations that are managed are more likely to be increasing•Reversal of decline in utilized populations is crucial for biodiversity and people The use of wildlife supports many people for their food, medicine, and livelihoods. Ensuring that this use is sustainable is central to conservation to ensure the persistence of species alongside continued utilization by people. Using more than 11,000 wildlife population trends, we conducted a global analysis of local-scale data to better understand how populations respond to utilization. We found that utilized populations declined on average by 50% between 1970 and 2016 and showed steeper negative trends than populations that were not utilized (−3%). If these trends continue, then this may threaten species survival and be detrimental to people who rely on their use. We also highlight how these trends might be reversed. Encouragingly, populations under targeted management, whether utilized or not, fared better than those that are not managed. This evidence can be used to track progress toward international and national targets on the sustainable use of species. Sustainable use of wildlife is a core aspiration of biodiversity conservation but is the subject of intense debate in the scientific literature, including the extent to which use is impacting species and whether management can mitigate any impact. Although positive and negative outcomes of sustainable use are known for specific taxa or local communities, a global and regional picture of trends in wildlife populations in use is lacking. We use a global dataset of more than 11,000 time series to derive indices of “utilized” and “not utilized” wildlife populations. Our results show that population trends globally are negative on average but that utilized populations tend to decline more rapidly, especially in Africa and the Americas. Crucially, where populations are managed, they are more likely to be increasing. This evidence can inform global biodiversity assessments and provide an operational indicator to track progress toward the Post-2020 Global Biodiversity Framework. Sustainable use of wildlife is a core aspiration of biodiversity conservation but is the subject of intense debate in the scientific literature, including the extent to which use is impacting species and whether management can mitigate any impact. Although positive and negative outcomes of sustainable use are known for specific taxa or local communities, a global and regional picture of trends in wildlife populations in use is lacking. We use a global dataset of more than 11,000 time series to derive indices of “utilized” and “not utilized” wildlife populations. Our results show that population trends globally are negative on average but that utilized populations tend to decline more rapidly, especially in Africa and the Americas. Crucially, where populations are managed, they are more likely to be increasing. This evidence can inform global biodiversity assessments and provide an operational indicator to track progress toward the Post-2020 Global Biodiversity Framework. Direct use of wild species is one of the ways in which biodiversity is fundamental to the subsistence and livelihoods of people.1Hutton J.M. Leader-Williams N. Sustainable use and incentive-driven conservation: realigning human and conservation interests.Oryx. 2003; 37: 215-226https://doi.org/10.1017/s0030605303000395Crossref Scopus (0) Google Scholar, 2Díaz S. Demissew S. Carabias J. Joly C. Lonsdale M. Ash N. Larigauderie A. Adhikari J.R. Arico S. Báldi A. et al.The IPBES Conceptual Framework — connecting nature and people.Curr. Opin. Environ. 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Sci. 2018; 373: 20170017https://doi.org/10.1098/rstb.2017.0017Crossref PubMed Scopus (25) Google Scholar We then use this to contrast trends in utilized populations with those that are not used for the complete set of species in the dataset and only for species with data for utilized and non-utilized populations (“matched”). Finally, using mixed-effect models, we explore the role of targeted management (see “definitions” in experimental procedures) in predicting populations trends in utilized populations. Our results can help to measure progress toward policy targets and identify trends in resources that are important for people. Our results, thus, feed directly into global processes such as the IPBES thematic assessment of sustainable use of wild species15IPBESReport of the Plenary of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services on the Work of its Sixth Session. IPBES, 2018Google Scholar and development of indicators for the Post-2020 Global Biodiversity Framework. Our final dataset comprised 11,123 population time series from 2,944 species, of which 5,811 populations from 1,348 species were coded as utilized, and 5,312 populations from 1,996 species were coded as not utilized (Table S1). For utilized populations, most data were available for fish (n = 3,233), followed by mammals (n = 2,098), birds (n = 331), reptiles (n = 142), and amphibians (n = 7). Fish and mammals had more utilized populations than not, whereas the reverse was true for birds, reptiles, and amphibians (Table S1). Compared with the expected distribution of body mass values for all species, the utilized dataset showed a skewed distribution toward larger-bodied species for birds and mammals but a distribution of body masses to all fish species (Figure S1). Geographically, our sample contained data from all IPBES regions and from 146 countries (Figure 1; Table S2). Utilized and not-utilized populations were found in all regions, but there were noticeable clusters of more utilized populations in parts of Africa, Central Asia, and Canada. The largest regional dataset was for the Americas. Results for Africa are based on the smallest dataset of the regions; data availability throughout the time series dropped after 2012, so the indices were shorter than for the other regions, finishing in 2015 and 2013 for terrestrial/freshwater and marine, respectively. Threat information was available for 3,195 populations—1,694 utilized and 1,501 not utilized (Table S3). There was a difference in the distribution of threats coded between utilized and not utilized populations, with a greater proportion of threats listed as overexploitation for utilized populations (Figure S2). Nearly three-quarters of the overexploitation threats coded for utilized populations were a result of hunting, fishing, and collecting (Figure S3). Of the utilized populations, 46% had information available on targeted management, and 23% were unmanaged (the remainder had no information; Table S4). The index for utilized populations shows a decrease of 69% for terrestrial and freshwater populations (Figure 2; index value in 2016, 0.31; range, 0.21–0.44) and a decrease of 34% for marine populations (Figure 2; index value in 2016, 0.66; range, 0.52–0.85) between 1970 and 2016. Although the overall trend for utilized populations showed a steep decline, there was considerable heterogeneity at the level of individual populations, with 46.3% showing an overall increase, 48.9% showing an overall decrease, and 4.8% were stable in the terrestrial and freshwater index. In the marine index, 53.2% of utilized populations showed an overall decline, 42.6% an overall increase, and 4.2% were stable. We tested the robustness of the indices to time series length. This is important to check when using population trends that vary in sample duration,43Wauchope H.S. Amano T. Sutherland W.J. Johnston A. When can we trust population trends? A method for quantifying the effects of sampling interval and duration.Methods Ecol. Evol. 2019; 10: 2067-2078https://doi.org/10.1111/2041-210x.13302Crossref Google Scholar particularly the effect of short time series that may exhibit more extreme or fluctuating trends and bias in the index.44Leung B. Hargreaves A.L. Greenberg D.A. McGill B. Dornelas M. Freeman R. Clustered versus catastrophic global vertebrate declines.Nature. 2020; 588: 267-271Crossref PubMed Scopus (27) Google Scholar,45Buschke F.T. Hagan J.G. Santini L. Coetzee B.W. Random population fluctuations bias the living Planet index.Nat. Ecol. Evol. 2021; 5: 1145-1152Crossref PubMed Scopus (2) Google Scholar We observed whether similar trends were seen when restricting the dataset to different thresholds for the minimum time series length in numbers of years. When a more stringent minimum threshold for time series length was applied, similar trajectories of decline were observed for indices with a minimum of 5 years, and shallower decline was reported for indices with a minimum of 10 years (Figure S4). The indices for utilized populations trends since 1970 grouped by IPBES regions show disparate trends, with largely tropical regions faring worse than the global indices of utilized populations (Figure 2) and compared with more temperate regions (Figure 3). Africa showed the greatest decline since 1970 in the terrestrial/freshwater and marine subsets; both indices show steeper decline than the global average for utilized species (Figure 3; terrestrial/freshwater index value in 2015, 0.07; range, 0.03–0.16; marine index value in 2013, 0.08; range, 0.04–0.17). The Asia-Pacific index shows a near-continuous decline in the marine index from 1970 to 2016 and an 83% overall decline, which is worse than the global marine index (Figure 3; index value in 2016, 0.17; range, 0.09–0.31); the terrestrial and freshwater index fluctuates from a positive to a negative trend, with high variation in the underlying species trends, and ends at a baseline value similar to 1970, above the global average (Figure 3; index value in 2016, 1.07; range, 0.31–3.76). The terrestrial/freshwater index for the Americas showed a trajectory of decline very similar to the global terrestrial and freshwater index of 67% between 1970 and 2016 (Figure 3; index value in 2016, 0.33; range, 0.19–0.58), whereas the marine index fluctuated throughout the time series and ended at a baseline value similar to 1970, with no significant overall change and a more positive trend than the global marine index (Figure 3; index value in 2016, 1.07; range, 0.78–1.45). The marine indices for Europe and Central Asia showed a slow increase for most of the time series after an initial decline, ending in an overall increase of 41% between 1970 and 2016 (Figure 3; index value in 2016, 1.41; range, 0.95–2.13). The terrestrial/freshwater index had a fluctuating trend for most of the time period but ended with a recent decline (Figure 3; index value in 2016, 0.76; range, 0.43–1.30). Both of these regional indices had trends that were better than the respective global indices. To explore the effect of utilization, we compared trends between utilized and non-utilized populations. For this analysis, we removed all reptile and amphibian data because these two taxa contained low numbers of species and populations in general but particularly those that are in the utilized category, resulting in a large proportional difference when comparing utilized with non-utilized populations. This is likely to make unbalanced comparisons, especially when dividing the dataset into systems (Table S1). This is not to suggest that these two taxa are not important to consider in the context of utilization; indeed, chelonians are one group particularly at risk from use.46Stanford C.B. Iverson J.B. Rhodin A.G.J. Paul van Dijk P. Mittermeier R.A. Kuchling G. Berry K.H. Bertolero A. Bjorndal K.A. Blanck T.E.G. et al.Turtles and tortoises are in trouble.Curr. Biol. 2020; 30: R721-R735https://doi.org/10.1016/j.cub.2020.04.088Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar Comparing the trend for mammals, birds, and fish, populations that are not utilized show a more stable trend, with index values above the 1970 baseline throughout the period, except for a recent decline, resulting in an overall decrease of 3% over the time period (Figure 4A; index value in 2016, 0.97; range, 0.80–1.18). In comparison, the index for utilized populations for the same taxa showed an overall decline of 50% (Figure 4A; index value in 2016, 0.50; range, 0.41–0.62). After 1985, there is no overlap in the confidence intervals of each index, which means they are significantly different. We used mixed-effects models to explore the relationship between utilization, taxonomic class, body size, and time series length with overall population trends as the response variable. Utilization was consistently a useful predictor of overall population trends, with utilized populations more likely to be declining than non-utilized ones (Tables S6–S9). Removing utilization from our models produced significantly worse predictions of population trends (ΔAIC = −10, χ2 = 11.835, p < 0.01). In general, our models did not suggest an interaction between utilization and taxonomic group, highlighting that all taxonomic groups are impacted by utilization. Using our most comprehensive dataset (mammals, birds, and fish in terrestrial, freshwater, and marine systems), body size interacting with class was in our top model, and the coefficients suggest that bird population trends are slightly positive, more so for larger birds; however, the confidence intervals span zero, so these are non-significant (Figures 4B and 4C). Fish trends were significantly positive