Despite a relatively rich scavenger community and a positive relationship between carcass removal and species richness, overall carcass removal was dominated by a single species, the Carrion crow. Our results support recent findings suggesting that biodiversity, in terms of species richness, may not be necessary to maintain certain components of ecosystem functioning and the subsequent ecosystem services they provide. In this study we have quantified the first step in this process, the consumption of carcasses by scavengers. Clearly this can be considered an ecosystem service as removal of carcasses from the environment is beneficial to humans. What remains unclear is how this component of ecosystem functioning impacts other ecosystem processes and ultimately overall ecosystem function.

Nine species were identified as active scavengers, which is within the range (6–29 species, mean = 14) found in a wide variety of largely undisturbed habitats10. That the number of scavengers is lower than the global average is unsurprising given the highly modified, agricultural nature of our study system.

In natural habitats the scavenger community is generally highly structured and carrion resources are distributed fairly evenly throughout the community10,18,19. In more altered, large scale agricultural habitats, the competitive balance for carrion favours fewer, more generalist consumers18. Our study system consisting of small scale agriculture, sparse woodland fragments and interspersed rural human habitation will be even more fragmented, which may act to suppress the activity of some scavengers. This may help the highly mobile species, particularly those able to transmit information on carcass location socially, such as corvids20, to monopolise carcass resources. Hence habitat fragmentation may be a driving force behind the dominance of crows as scavengers. Indeed, this represents a clear opportunity for future research.

Of course species richness is only one aspect of biodiversity, of equal importance is the abundance of individuals within a population and the levels of activities which contribute to ecosystem functioning21. Whilst the relationship between species richness and ecosystem function has received considerable attention, the role of abundance and the shape of the abundance versus functional delivery relationship remains largely unknown. Here we find no relationship between the abundance of crows and rates of carcass removal. However, carcass removal was directly and asymptotically related to the activity of crows, with a very sharp rise in rates of removal for a small increase in activity, suggesting that relatively low levels of activity are needed to maintain the ecosystem function, with little or no gain made after the asymptotic level of activity. Mechanistically this is likely due to competition over a relatively small carcass, with only a limited number of crows able to feed at any one time. Of course the shape of this relationship will likely be influenced by the density and size spectra of the carcass resource, both of which have rarely been quantified. Whilst our results demonstrate that abundance is not related to carcass removal rates in our system, we suspect that abundance will be important at a wider landscape level. For scavengers to be effective in their removal they must be able to search vast areas for carcasses (as is the case with vultures), or be abundant and widespread enough to locate carcasses.

Although we attempted to ensure our study replicated natural scavenging behaviour there are a number of limitation in the experimental design that could be addressed in the future. First, we only used a single carcass species and size, whereas different scavenger species may have preferences for different kinds of carcasses. Second. we only placed the carcasses in the field around midday, which may potentially be advantageous for diurnal scavengers.

Our findings build on recent work demonstrating the functional importance of the most abundant species in maintaining ecosystem function5,6,7,18. This is not to say that species richness is not important, particularly at wider spatial scales encompassing different environments. In this particular study system crows are clearly the primary scavengers, although this may not be the case in other habitats. Greater species richness should also provide resilience within ecosystems to habitat alteration22,23 or to the decline or loss of abundant species. Being abundant does not necessarily protect species from decline. For example, across Europe the most common bird species seem recently to have undergone the most rapid declines24 and historically there have been numerous cases of once common species being driven to extinction5. Whist the main ecosystem function provider in our study, the Carrion crow, is not in decline24, it is one of the most disliked bird species in the UK25 and can be legally killed as a pest species.