Climate change, human activities and landscape changes influence biodiversity in multiple ways, leading to species loss and dramatic ecosystem changes1. Maintaining biodiversity is essential for human well-being, both physically and mentally2. Not only do diverse ecosystems provide important services such as clean drinking water, they might also be the source of novel drugs, biological pest-control agents and ecosystem engineers3. Yet, our understanding of many fundamental aspects of biodiversity remains insufficient. Numerous basic research questions remain unanswered. Why do individuals of a species live here but not there? How many of each species can coexist locally? Do rules for the local assembly of biodiversity exist and, if so, how flexible are they? How can high biodiversity and its associated roles, such as seed and pollen dispersal, be restored?
Species, as part of ecosystems, form intricate webs of interdependence. To investigate such ecological and environmental dependencies, it is useful to start where most knowledge is already available. The study of birds — of which around 9,800 species are known — offers a superb opportunity to gain insight into the evolution and maintenance of biodiversity on a global scale4. Knowledge about the evolutionary history of birds and their interactions with other species is unparalleled5. It is now possible to map the entire, global diversity of birds (Fig. 1) as well as, for example, population densities, dynamics and life-history traits6, and to link projected patterns of anthropogenic change to single species distributions on a global scale, allowing projections of future bio-diversity7 (Fig. 2).
Addressing the numerous unanswered questions might shed light on the reasons for the relative successes and failures of species in invading new habitats, and in coping with changes in their surroundings. Scaling up from individual species, the nature and strength of interactions between organisms provides insights into how individuals share and partition their ecological niches — in other words, how they survive given the resources in their environment and the community of which they are a part.
Environmental change offers an unprecedented opportunity to analyse and to better understand the basic rules of biodiversity — the conventions that govern communities, such as whether certain species can co live in proximity. This will allow better predictions of whether bio-diversity responses to global change will be gradual or discontinuous, and how they will affect ecosystem and, ultimately, human health. Once an ecosystem is degraded, biodiversity is usually vastly diminished, typically through a cascade effect that involves all taxonomic groups. By under-standing the species-assembly rules that support biodiversity, we will be in a better situation to bioengineer the rapid recovery of habitats — if not to their original state, then at least as close to their original functionality in terms of providing essential ecosystem services8.