Species loss dramatically represents the mass extinction
currently underway [1–4]. Yet, species extinctions are an
inadequate measure of biodiversity loss and do not provide
information about changes in the capacity of particular
species to contribute to the functioning of ecosystems.
Balmford et al. (this issue) provide persuasive arguments
for focusing on changes in population size and habitat
extent when measuring the state of nature. We also
emphasize that the relationship between biodiversity and
human well being is primarily a function of populations of
species. Whether the benefits of biodiversity are conveyed
directly (e.g. food) or indirectly (e.g. pollination), their
supply is generally determined by the diversity of
populations producing them [5]. Hence, population change
can have a substantial impact on an ecosystem that is
independent of changes in species diversity.