The dynamics of community assembly has a rich history in ecological theory. Recently, there has been much interest in assessing how changes in diversity within assembling communities impact the structure of interactions and vice versa. Here we examine a novel theoretical framework that seeks to generate communities by distilling many types of ecological interactions into a small number of unique pairwise directed links between species, including, but not limited to, 'assimilate' interactions (e.g. resource dependencies) and 'need' interactions (e.g. reproductive or habitat dependencies). Different pairwise combinations of directed link types between species give rise to the larger diversity of species interactions observed in nature, such as consumer-resource, and both service-resource and service-service mutualisms. Moreover, our framework permits the explicit inclusion of interactions that create or modify abiotic elements, which other species may depend on for survival, such as nutrients or habitat. Inclusion of both biotic and abiotic agents permits more complex and indirect interdependencies between species, effectively incorporating the concept of ecosystem engineering into interaction networks, where the environment can be altered by a species or groups of species, thereby facilitating or inhibiting the colonization of others.
Results/Conclusions
Our framework makes specific predictions that are borne out by observations in the field. First, we find that communities initially exhibit higher connectance (link density), which is quickly eroded to empirically observed values as competition for resources increases exclusion. Niche overlap among species follows similar trends: there is initially a greater degree of overlap between species, and as the system settles to a steady state, this overlap becomes minimized, mirroring observations of assembly in natural grasslands. Importantly, we find that an increase in the number of engineering species, by creating a greater number of direct and indirect interdependencies, constrains the assembly of communities initially, yet promotes assembly as the system matures. This leads to communities that exhibit greater diversity, however the increased species richness facilitated by engineers comes at a cost: as the number of engineers grows early in the assembly process, extinction cascades become larger. Our framework shows that despite the complexity of real communities, some of the most remarkable processes and patterns such as competitive exclusion, resource complementarity and extinction cascades, can be generated by simple interaction rules. Moreover our findings indicate that ecosystem engineering might be an important component that is overlooked in ecological network theory.