  In abstract terms, ecosystem ecology is about determining when two
ecosystems, superficially different, are alike in some deeper way. An external
observer can choose any ecosystem property as being important. In contrast, two
ecosystems are equivalent from the point of view of the organisms they contain
if and only if for each species, the proportional population growth rate does
not differ between the ecosystems. Comparative studies of ecosystems should
therefore focus on patterns in proportional population growth rates, rather
than patterns in other properties such as relative abundances. Popular
activities such as measuring dissimilarity, and representing dissimilarity via
ordination, can then be done from the point of view of the organisms in
ecosystems. Summarizing the state of an ecosystem under this approach remains
challenging. In general, the dynamics on equivalence classes of ecosystems
defined in this way are structurally different from the dynamics of ecosystems
as seen by an external observer. This may limit the extent to which natural
selection can act on ecosystem structure.
