Analysis On Stability Of Food Webs Based On Modules And Keystone Species

In recent years,due to huge impact of human activity and other global events,the Earth's ecosystems are suffering serious problems such as decline of biodiversity and accelerated extinction of species.As species in food webs are interrelated and interdependent,the impact of abundance reduction or extinction of a species will inevitably be transmitted through food web,which in turn affects other species and even triggers to a series of secondary extinctions,which bring great difficulties to the studies on ecosystem diversity and stability.Based on this point,ecologist proposed several indicators to detect the influential species so that the priority of species to be conserved can be determined.On the other hand,ecologist introduced the concept of motifs or modules from complex network theory to trace the global change of ecosystems at the perspective of local organization.Therefore,it is necessary to study the influence of modules and key species on the structure and stability of food web at the view of local structures,analyze the influence of the change of environment factors on the number of modules and the role of keystone species,in order to better understand the structure of food web and changes in stability.This is conducive in determining the protection priority of species,allocating limited resources in the protection of more important modules or species.The data of this study were obtained through a two-year experiment in four blocks of outdoor stream channel at the Freshwater Biological Association River Laboratory by Guy Woodward et al.Eight food webs are divided into two groups,one for drought treatment and the other for control.The object of this study is to uncover how drought affects the structure and stability of the food webs from two perspectives.From the perspective of modules,we introduce Exponential Random Graph Models(ERGMs),the commonly used method in studying social networks,into the research of food webs,and study four basic modules:tri-trophic food chain,omnivory,exploitation competition and apparent competition.We explore the roles of these modules in the overall structure of the food webs and how drought alters their roles.From the point of keystone species,we use numerical simulations to determine the secondary extinction of species loss following different removal sequences based on three kinds of eigenvectors(left,right and average eigenvectors).The eigenvectors are used to identify the key species,and then analyze the influences of drought on the sequence of keystone species.We got four major results:(1)The range of number of tri-trophic food chain,omnivory,exploitation competition and apparent competition in control group were 59-219,10-83,1377-2654 and 1821-3033.The range of number of the four basic modules were 10-20,1-9,526-1280 and 1487-2170.Due to the impact of drought,the number of all four basic modules tends to decrease.(2)The four basic modules(tri-trophic food chain,omnivory,exploitative competition and apparent competition)play major roles in explaining the structure of the entire food web.The significant parameters of ERGMs were different for control and drought block across all the four pairs,indicating the regime shift of the food web structure caused by drought.(3)The sequential removal of species following three eigenvectors caused more secondary extinctions(smaller R50)than the random removal,indicating that all three eigenvectors are effective in identifying the key species,of which the left eigenvector is more efficient because it has the least R50.(4)By analyzing the effect of drought on the keystone species by the left eigenvector,drought changes the order of the keystone species greatly.Among the top ten species,only the order of two species dose not change.The stability of the food web has changed correspondingly.The food webs of drought group have higher R50 and lower stability than control group.This study analyzed the effects of drought on the structure and stability of food webs from two different scales.The effects of drought on the number of modules and their functions were analyzed by ERGMs.The topological and dynamical methods were used to verify the effective of eigenvectors to recognize the keystone species.It was found that the removal of sequence according to the left eigenvector leads to more secondary extinction.So,right eigenvectors were not the most accurate indicator of the keystone species,and the left eigenvector performed better.We used the left eigenvector to analysis the influence of the drought and found that the sequence of keystone species were changed greatly.Among the top ten species,only the order of two species dose not change.Drought have an irreversible effect on ecosystems,changing the structure and function of food webs.The stability of food webs has also undergone some changes.Previous studies mostly focused on the impact of reduced species diversity on food webs.Here we analyzed the effects of drought from the view of module and keystone species.Modules and keystone species,as an important part of the food web,deserve more attention,which is of great practical importance for the conservation of ecosystem diversity and stability.