  Ecologists and conservation biologists need to identify the relative
importance of species to make sound management decisions and effectively
allocate scarce resources. We introduce a new method, termed environ
centrality, to determine the relative importance of a species in an ecosystem
network with respect to ecosystem energy--matter exchange. We demonstrate the
uniqueness of environ centrality by comparing it to other common centrality
metrics and then show its ecological significance. Specifically, we tested two
hypotheses on a set of 50 empirically-based ecosystem network models. The first
concerned the distribution of centrality in the community. We hypothesized that
the functional importance of species would tend to be concentrated into a few
dominant species followed by a group of species with lower, more even
importance as is often seen in dominance--diversity curves. Second, we tested
the systems ecology hypothesis that indirect relationships homogenize the
functional importance of species in ecosystems. Our results support both
hypotheses and highlight the importance of detritus and nutrient recyclers such
as fungi and bacteria in generating the energy--matter flow in ecosystems. Our
homogenization results suggest that indirect effects are important in part
because they tend to even the importance of species in ecosystems. A core
contribution of this work is that it creates a formal, mathematical method to
quantify the importance species play in generating ecosystem activity by
integrating direct, indirect, and boundary effects in ecological systems.
