  A central question in ecology is to understand the ecological processes that
shape community structure. Niche-based theories have emphasized the important
role played by competition for maintaining species diversity. Many of these
insights have been derived using MacArthur's consumer resource model (MCRM) or
its generalizations. Most theoretical work on the MCRM has focused on small
ecosystems with a few species and resources. However theoretical insights
derived from small ecosystems many not scale up large ecosystems with many
resources and species because large systems with many interacting components
often display new emergent behaviors that cannot be understood or deduced from
analyzing smaller systems. To address this shortcoming, we develop a
sophisticated statistical physics inspired cavity method to analyze MCRM when
both the number of species and the number of resources is large. We find that
in this limit, species generically and consistently perturb their environments
and significantly modify available ecological niches. We show how our cavity
approach naturally generalizes niche theory to large ecosystems by accounting
for the effect of this emergent environmental engineering on species invasion
and ecological stability. Our work suggests that environmental engineering is a
generic feature of large, natural ecosystems and must be taken into account
when analyzing and interpreting community structure. It also highlights the
important role that statistical-physics inspired approaches can play in
furthering our understanding of ecology.
