On the centrality and uniqueness of species from the network

Identifyingimportantspeciesformaintainingeco-system functions is a challenge in ecology. Sincespecies are components of food webs, one way toconceptualize and quantify species importance isfrom a network perspective. The importance ofa species can be quantified by measuring the cen-trality of its position in a food web, because acentral node may have greater influence onothersinthenetwork.Aspeciesmayalsobeimpor-tantbecauseithasauniquenetworkposition,suchthat its loss cannot be easily compensated. There-fore, for a food web to be robust, we hypothesizethat central species must be functionally redun-dant in terms of their network position. In thispaper, we test our hypothesis by analysing thePrince William Sound ecosystem. We found thatspecies centrality and uniqueness are negativelycorrelated,and such an observation isalsocarriedover to other food webs.Keywords: species importance; centrality;uniqueness; food web1. INTRODUCTIONOne challenge in ecology is to identify importantspecies [1,2]. Since species are embedded in webs oftrophic interactions, one possible approach to impor-tance is to quantify the network centrality of speciesin a food web [3]. A species with high centrality canaffect many others, maybe also more quickly, viastrong direct and indirect effects [4,5]. These includevertical effects in the top-down and the bottom-updirections [6], and horizontal effects like exploitiveand apparent competitions [7].Different species have different ecological roles andtheir network position may reflect these roles [8,9]. Forinstance, there are producers, top predators and omni-vores. Beyond treating centrality as a proxy to speciesimportance, it is also of emerging interest to knowhow similar the neighbourhoods of species are in afood web [10]. A species can also be importantbecause it has a unique network position with a neigh-bourhood that poorly overlaps with that of others. Asa consequence, species of this kind of importance arenot easily replaceable, and their trophic functionsare not easily taken over by others.If an ecosystem is a functioning unit and has onlyone central species, then intuition suggests that itis prone to failure when such a central species isremoved. Therefore, for an ecosystem to be robust, itshould have several central species, such that the lossof one can be compensated by others. Thus, weargue that species centrality and uniqueness are twoopposing characteristics of existing ecosystems. Inthis paper, we analyse the Prince William Sound(PWS) ecosystem [11] as an illustrative example andask, ‘are central species also unique?’, and test the gen-eralityof our finding by using other food webs.We notethat food webs are dynamic and our analysis onlyfocuses on their topological structure.2. MATERIAL AND METHODS