TITLE:
Comparisons and Contrasts between Asymmetry and Nestedness in Interacting Ecological Networks
AUTHORS:
Gilberto Corso, N. F. Britton
KEYWORDS:
Bipartite Interaction Networks, Nestedness, Asymmetry, Ecology of Communities
JOURNAL NAME:
Open Journal of Ecology,
Vol.4 No.11,
August
14,
2014
ABSTRACT:
We compare and contrast asymmetry and
nestedness, two concepts used in the characterisation of the
specialist-generalist balance in bipartite ecological interaction networks. Our
analysis is relevant to mutualistic networks such as those consisting of
flowering plants and pollinators, or fruiting plants and frugivores, or antagonistic
networks such as those consisting of plants and herbivores, in an ecological
community. We shall refer to the two sets of species in the bipartite network
as plants and animals, the usual but not the only ecological situation. By
asymmetry we mean either connectivity asymmetry or dependence asymmetry, which
are essentially equivalent. Asymmetry expresses two attributes: generalists
interact preferentially with specialists, and specialists avoid interacting
with each other. Nested patterns, in principle, should express these same two
features and one more: the presence of a core of interactions among
generalists. We compute the full set of perfectly nested patterns that are
possible in an L × L matrix with N interactions representing an ecological network
of L plants and L animals, and point out that the number of nested arrangements
grows exponentially with N. In addition, we analyse asymmetry for the full set
of perfectly nested patterns, and identify extremes of asymmetry inside the
universe of nested patterns. The minimal asymmetry is marked by a modular core
of interactions between species that are neither specialists nor generalists.
On the other hand, the case of maximal asymmetry is formed by a set of few
generalists and many specialists with equal connectivity. The stereotypic case
of nestedness with a core of interactions among generalists has intermediate
asymmetry.