Towards evidence-based conservation of subterranean ecosystems

Abstract

Subterranean ecosystems are among the most widespread environments on Earth, yet we still have poor knowledge of their biodiversity. To raise awareness of subterranean ecosystems, the essential services they provide, and their unique conservation challenges, 2021 and 2022 were designated International Years of Caves and Karst. As these ecosystems have traditionally been overlooked in global conservation agendas and multilateral agreements, a quantitative assessment of solution-based approaches to safeguard subterranean biota and associated habitats is timely. This assessment allows researchers and practitioners to understand the progress made and research needs in subterranean ecology and management. We conducted a systematic review of peer-reviewed and grey literature focused on subterranean ecosystems globally (terrestrial, freshwater, and saltwater systems), to quantify the available evidence-base for the effectiveness of conservation interventions. We selected 708 publications from the years 1964 to 2021 that discussed, recommended, or implemented 1,954 conservation interventions in subterranean ecosystems. We noted a steep increase in the number of studies from the 2000s while, surprisingly, the proportion of studies quantifying the impact of conservation interventions has steadily and significantly decreased in recent years. The effectiveness of 31% of conservation interventions has been tested statistically. We further highlight that 64% of the reported research occurred in the Palearctic and Nearctic biogeographic regions. Assessments of the effectiveness of conservation interventions were heavily biased towards indirect measures (monitoring and risk assessment), a limited sample of organisms (mostly arthropods and bats), and more accessible systems (terrestrial caves). Our results indicate that most conservation science in the field of subterranean biology does not apply a rigorous quantitative approach, resulting in sparse evidence for the effectiveness of interventions. This raises the important question of how to make conservation efforts more feasible to implement, cost-effective, and long-lasting. Although there is no single remedy, we propose a suite of potential solutions to focus our efforts better towards increasing statistical testing and stress the importance of standardising study reporting to facilitate meta-analytical exercises. We also provide a database summarising the available literature, which will help to build quantitative knowledge about interventions likely to yield the greatest impacts depending upon the subterranean species and habitats of interest. We view this as a starting point to shift away from the widespread tendency of recommending conservation interventions based on anecdotal and expert-based information rather than scientific evidence, without quantitatively testing their effectiveness.

Keywords: biospeleology; cave; climate change; conservation biology; ecosystem management; extinction risk; groundwater; legislation; pollution; subterranean biology.

Fig. 1

Summary of the sampled literature and extracted metadata. (A) PRISMA diagram depicting the flow of information through the different phases of the systematic literature review. For the list of studies extracted from the Web of Science, including excluded studies with reasons for exclusion, see Appendix S1. (B) Summary of the metadata collected for the database. For the link to the data repository see Section VII. Original silhouettes by Irene Frigo.

Fig. 2

Summary of the surveyed literature. Proportion of conservation interventions tested across our data set by biogeographic region (A), habitat (B), taxon (C), threat (D), and conservation intervention (E). Size of the circle in A indicates the number of conservation interventions. For definitions of subterranean habitat types used in B see Section I.2. In B and C, ‘Not specific’ means that the study did not refer to a specific subterranean species/system. In D, ‘Multiple’ means that three or more threat groups were considered. Note that total numbers in each panel within the figure may differ slightly from the overall total number of interventions (1,954), because: (i) data were missing for some entries in the database (i.e. we could not derive some information); (ii) some studies focused on multiple biogeographic regions, taxa, or habitats.

Fig. 3

Chord diagram showing interrelationships among conservation interventions and threats across our data set. Threats are listed in the upper portion of the diagram and conservation interventions in the lower portion. Original silhouettes by Irene Frigo.

Fig. 4

Temporal trends in research on conservation measures and threats in subterranean ecosystems. (A) Proportion of conservation interventions tested across our data set by year. Inset scatterplot is the proportion of conservation interventions tested per year between 2000 and 2021 (partial data up to October for 2021), with the line fitted using a binomial generalised linear model. (B, C) Annual changes in the relative proportions of studies reporting different threats (B) and conservation interventions (C), with lines fitted using individual binomial generalised linear models. Solid lines are fitted values (slope) and shaded surfaces indicate the associated 95% confidence intervals. Bright colours highlight significant trends. Estimated regression parameters are given in Table 2.