Identifying Keystone Species In Food Webs Based On An Energy-budget Model

Currently our Earth's ecosystems are suffering several serious problems,such as the accelerating rate of species extinctions and the biodiversity loss.Prior studies point out that we have entered what has been described as the Sixth Great Extinction.Species are not isolated,but connected with each other into complex food webs.The effects of the extinction of a species could spread along the food web and trigger the cascading extionctions,i.e.secondary extioncions.This would undoubtedly worsen the already severe problem of biodiversity loss.Therefore,identifying the keystone species in food webs correctly would be helpful in understanding the underly mechanisms of the response of ecosystems to species loss.It can further help determining the priority of species to be conserved,and increase the efficient allocation of limited resources.This study firstly gathered and analyzed the food web data of carbon flux,with 20 food webs selected as the objects of the study.The classical topological approach was applied to simulate the secondary extinctions in different deletion sequences and investigate the structure importance of different species.Then,a dynamic model based on energy budget was built up to depict the biomass change of species in the food webs.The secondary extioncions in different deletion sequences were recorded based on the dynamical simulations,in order to investigate the functional importance of different species.These deletion sequences reflect several importance indices at four different scales,including the biomass which is based on species traits,the indices considering trophic link weight and indirect effects which is based on the interactions between species,the contribution of species to trophic modules,and the distance of species to other compartments.These four groups of indices investigate the importance of species importance and detect the keystone species from small scale to large scale and from simple to complex.The findings of these four groups of indices are listed as following:(1)In both topological and dynamical analyses,the species removal based on maximum biomass caused more secondary extinctions and more damage to food web stability than the removal based on minimum biomass.This indicates that the species with large biomass can play important role in maintaining the structures and functions of food webs.Degree centrality,one of the classical indicators,performed well in topological analyses,but it could not indicate the species importance in dynamical analyses.(2)The species removal considering link weight and indirect effects triggered more secondary extinctions and caused more damage to food web stability than the traditional indicator based on number of links.This indicates that the indices considering link weight or indirect effects performed more effective than the traditional indicator in measuring the species importance.This conclusion is consistent in both topological and dynamical analyses.(3)In both topological and dynamical analyses,species remo vals based on the contribution of species to the IGP resource generated more secondary extinctions than random deletion,while removals based on the contribution to the IGPrey or IGPredator showed no significant difference with random deletion in the numbe r of secondary extinctions.This indicates the species which contribute much to the IGP resource are crutial in maintaining the structures and functions of food webs.The species which contribute much to the IGPrey or IGPredator do no show any importance.(4)In both topological and dynamical analyses,species removals based on the shortest distance of a species to the compartments caused more secondary extincitons and more damage to food web stability than removals based on the longest distance and random removals.This indicates the species connecting different compartments would be more important in maintaining the whole structures and functions of food webs.(5)Comparing the resistance of food webs to different deletion sequences,the following findings have been achieved: in topological analyses,food webs with high directed connectance,i.e.well connected food webs,showed high resistance to species loss;in dynamical analyses,food webs with high richness showed low resistance to species loss,while food webs with high directed connectance or weighted connectance showed high resistance to species extinctions.This study proposed new indices measuring species importance at four different scales.Topological and dynamical approaches were used to test th e efficience of these indices.These new indices provide reference for the development of the strategy of biodiversity protection.This study also proved that the traditional degree centrality could be used in measuring the structure importance of species,but should not be used in measuring the functional importance.This finding suggest that the prior studies have overestimated the relative importance of degree centrality.Furthermore,a food web dynamic model has been built up based on the carbon flux between species.The parameter values of this model are calculated from the empirical data and thus should be of great application in theoretical studies in the future.