Marine and freshwater mammals are increasingly threatened due to human activity. To improve conservation practice, decisions should be informed by the available evidence on the effectiveness of conservation actions. Using a systematically collated database of studies that test the effectiveness of actions to conserve marine and freshwater mammals, we investigated the gaps and biases in the available scientific evidence base. While there is a growing evidence base covering actions to address key threats (e.g. fisheries and bycatch) to marine and freshwater mammal populations, we identified large geographic and taxonomic biases. There was no relationship between the number of studies and marine mammal species per ecoregion and we found biases towards coastal areas of the Global North, with many regions and species having little or no evidence available. The number of studies per species did not correlate with (1) the threat level, (2) evolutionary distinctiveness or (3) the public 'popularity' of the study species. We also found a mismatch between actions tested and the actions suggested as needed in the International Union for the Conservation of Nature (IUCN) Red List. Several of these gaps and biases likely reflect the feasibility of researching marine mammal populations; many species can be difficult to access, with limited baseline information on populations and threats, and testing actions can require costly long-term monitoring. Prioritizing the most cost-effective conservation strategies for marine and freshwater mammal species will require a comprehensive evidence base on the effects of actions. Continuing to build the necessary baseline data, and focusing future research and funding towards the priority gaps identified in this study will be important to deliver this target.

The widely held assumption that any important scientific information would be available in English underlies the underuse of non-English-language science across disciplines. However, non-English-language science is expected to bring unique and valuable scientific information, especially in disciplines where the evidence is patchy, and for emergent issues where synthesising available evidence is an urgent challenge. Yet such contribution of non-English-language science to scientific communities and the application of science is rarely quantified. Here, we show that non-English-language studies provide crucial evidence for informing global biodiversity conservation. By screening 419,679 peer-reviewed papers in 16 languages, we identified 1,234 non-English-language studies providing evidence on the effectiveness of biodiversity conservation interventions, compared to 4,412 English-language studies identified with the same criteria. Relevant non-English-language studies are being published at an increasing rate in 6 out of the 12 languages where there were a sufficient number of relevant studies. Incorporating non-English-language studies can expand the geographical coverage (i.e., the number of 2° × 2° grid cells with relevant studies) of English-language evidence by 12% to 25%, especially in biodiverse regions, and taxonomic coverage (i.e., the number of species covered by the relevant studies) by 5% to 32%, although they do tend to be based on less robust study designs. Our results show that synthesising non-English-language studies is key to overcoming the widespread lack of local, context-dependent evidence and facilitating evidence-based conservation globally. We urge wider disciplines to rigorously reassess the untapped potential of non-English-language science in informing decisions to address other global challenges. Please see the Supporting information files for Alternative Language Abstracts.

Generating, collating and using scientific evidence is key to effective conservation. We illustrate this with examples from medicine and conservation, and in more depth with an example from our own studies, in which we assess the methods (overpasses and underpasses) used to reduce habitat fragmentation and mortality of bats caused by roads. Evidence is defined as the results of quantitative tests of interventions that directly address conservation effectiveness against clearly stated goals. The results show that methods used in the past were rarely tested quantitatively and when tested were rarely effective. We discuss why it can be difficult to get scientific evidence accepted and used routinely in conservation policy and practice. Finally, we discuss how evidence gathering and use can be improved in mitigation projects.

Bats are long-lived mammals with low reproductive rates, making them susceptible to developments that reduce reproductive output or increase mortality. Roads destroy. degrade and fragment habitats, reducing the ability of bats to roost, feed and reproduce. Current mitigation techniques have not been proven to be effective at conserving bats at the population level.34.1 Road effects vary among species depending on flight style and habitat use, but many species are affected.34.2 Roads can act as barriers to the movement of bats, and many species suffer traffic mortality attempting to cross roads.34.3 Artificial light deters some bat species while attracting others, and a policy of no lighting' is recommended for bats.34.4 Traffic noise may reduce the flight activity and foraging efficiency of bats.34.5 Protection of roosts and foraging sites during and after roadworks is critical to bat survival.34.6 Underpasses can effectively reduce the barrier effect and reduce the number of roadkills for some bat species.34.7 Other attempts to reduce the barrier and mortality effects for bats are unproven, and further research is required before widespread implementation.It is important to conduct thorough pre-construction bat surveys and bring objectivity and rigour into the design and testing of mitigation features.