A paleontological perspective on ecosystem assembly rules in the Paleozoic terrestrial realm

Abstract

The principles of ecosystem and community assembly developed by modern ecologists should be, in principle, applicable to the evolutionary assembly of terrestrial ecosystems during the Paleozoic. There are three broad, general, not time-specific Assembly Rules that have been described by ecologists: dispersal constraints (i.e., can a species reach a given location?), environmental constraints (i.e., if it can reach the location, can a species survive under the prevailing physical conditions there?), and biotic constraints (i.e, once on site, can a species co-exist with or compete successfully against occupants, if any?). These three constraints are, in fact, filters, and function to mediate the process of evolution, selection acting only as a passive arbiter of variation. A paleontological perspective adds consideration of irreducible historical contingency that invisibly, unless explicitly considered, affects the detailed manifestation of the other three; this also can be and has been accessed to some degree via considerations of phylogeny. An explicitly ecological perspective provides a framework to conceptualize the development of early ecosystems via the evolutionary addition of plant-based architectural complexity and the addition of the fungal, arthropod, and vertebrate components. For long-term patterns, such as the increase in structural complexity of vegetation through the Devonian and Carboniferous, assembly rules help to explain long lag times between the origin of innovations and their rise to widespread prominence. For individual paleocommunities, they help to resolve questions of biodiversity - whether the taxonomic record of an assemblage is oversplit or overlumped, for example. That evolution takes place within the framework of ecology is undisputed. But what exactly is that framework? At the most basic level, it is assembly rules.