There is an urgent need to identify the mechanisms which underpin the impact of climate change impacts on biodiversity, to document observed impacts, to improve future assessments of species’ vulnerability to climate change and to inform adaptive conservation management. This CCI-funded project has produced two key publications (Ockendon et al. 2014 Global Change Biology; Pearce-Higgins et al. 2015 Proceedings of the Royal Society London, Series B) documenting the importance of biotic mechanisms in driving species’ responses to climate change, and showing that the links between climate and species’ populations vary geographically (Fig.1).

We used a systematic review approach to identify 146 published studies of the impacts of variation in temperature and precipitation upon populations of wild species. The majority were from temperate latitudes of Europe and North America, and two thirds of species covered were either birds or mammals. Although we only considered studies that spanned a minimum of 20 years, study duration had a strong positive effect upon the probability of a study detecting a significant relationship between a climatic variable and population change. This emphasises the value of long-term biological monitoring data, and shows how studies spanning decades are required to fully document the impacts of climate change.

The review of mechanisms found significantly greater support for indirect, biotic mechanisms than direct, abiotic mechanisms as mediators of the impact of climate on populations (Ockendon et al. 2014). Thus, it is likely to be changes in the abundance of predators or prey that will cause long-term population changes in other species. These biotic effects were most important in affecting populations of species from higher trophic levels, whilst populations of primary consumers responded equally to biotic and abiotic mechanisms (Fig.2).

The response of populations to climatic variables varied geographically (Pearce-Higgins et al. 2015). Temperature tended to have a greater overall impact on populations than precipitation, although the effects of increased precipitation varied strongly with latitude, being most positive at low latitudes. These findings match those previously found for birds (Pearce-Higgins & Green 2014). Thus, in line with macroecological theory, populations in the tropics appear most affected by changes in rainfall patterns, whereas at higher latitudes, it is likely to be warming that is most important.

This study has two further implications. Firstly, we identified a general lack of data from the tropics whilst also suggesting that the processes linking species’ populations to climate in the tropics are likely to differ from those at higher latitudes, where most studies have been undertaken. Uncertainty is compounded by the fact that precipitation is the key climatic component driving tropical systems, but is much more difficult for climatologists to predict than temperature. Our limited understanding about the impacts of climate change in the tropics highlights an urgent requirement for long-term monitoring of even relatively common and widespread species to be undertaken in these regions. These studies should ideally focus on keystone species and groups, and collect data across a range of trophic levels.

Secondly, we found significant differences in population responses to climatic variables between studies with evidence of a significant climatic trend through time, and those without. Biotic mechanisms were more frequently supported in studies where there was evidence climate change, and that in such studies, the effects of temperature were more negative, and the effects of precipitation, more positive. In other words, climate change is already disrupting ecosystems by affecting interactions between species and altering the response of populations to climatic variables.