To determine whether one species blocks another out of an area, one approach is to infer assembly rules, which reconstruct the sequence in which species were added to an evolving community. For example, the presence of a plant species might support the establishment of a beetle that feeds on the plant, and a wasp that in turn parasitizes the beetle. Each of these species, like a puzzle piece, might block the entry of some competing species into the community. But whether a species holds an exclusive functional place cannot easily be identified by studying a community as an isolated unit; local communities are not isolated assemblages and are better thought of as members of a metacommunity of linked smaller ecosystems. Consequently, observing the existence of two functionally similar species in a particular community could reflect that there is room for both species in the assembly or that they really belong to what are mostly distinct, neighboring communities. For example, in a particular brackish coastal lagoon, the species scophtalmus rhombus and solea solea are not only both fish, but have comparable functional traits such as eye diameters, caudal fin aspect ratios, and length-to-body-depth ratios. This functional similarity could imply mutual exclusivity, but another possibility is that scophtalmus rhombus and solea solea occupy positions in the same community within the lagoon, perhaps because food is abundant or because they are less functionally similar than they appear; another is that they occupy exclusive positions in neighboring communities within that lagoon or the mouth of that lagoon to the coastal seas, and they fact that they have been found near each other reflects an exception rather than the rule.