The role of biotic relationships in biological invasions as exemplified by zooplanktonic communities

Abstract

Zooplanktonic communities are known to be amongthe most vulnerable ones with respect to the invasionsof alien species [6]. The invasion of new species intoplanktonic communities often causes changes in thestructure and functions of the entire ecosystem of therecipient body of water. Prediction of such changes is,and has always been, one of the main goals of ecology[5, 13, 14].There are grounds to believe that exploitative com-petition is a potent barrier against the invasion of alienspecies into zooplankton. However, competition as afactor preventing the invasion of new species into com-munities may be expressed differently, depending onthe trophic conditions and predator pressure in thegiven body of water. In this study, we attempted to usesimulation modeling to predict the changes in the spe-cies structure of zooplankton after introduction of apredator under different conditions of food supply.EXPERIMENTALCladocera were the prototype of the model objects.To attain the objectives set in this study, we developeda model of the population dynamics of cladoceransunder the conditions of food depletion. The depen-dences of the species population parameters (fecundity,mortality, and the duration of postembryonic develop-ment) on the food concentration were specified on thebasis of our own observations in natural ecosystemsand laboratory experiments [4], as well as data pub-lished by other researchers [1, 2]. The food concentra-tion was determined as a result of its reproduction andconsumption. The numbers of species were calculatedfor each stage, taking into account the numbers of newindividuals entering the given stage and individualsleaving it by transiting to the next stage or dying. Themodel included a delay of the response of populationparameters to changes in food concentration, whichimitated the storage of nutrients in individuals. We tookinto account that the functions of the dependence ofpopulation parameters on food concentration changedwith the body size and age of individual animals. Theequations used in the model are published elsewhere[3, 4]. Preliminarily, we created a database on morethan 2000 species whose population parameters variedwithin their actual ranges.The effect of a predator (plankton-eating fish) wassimulated by varying the mortality function. Thetrophic states of waters (oligotrophic, mesotrophic,eutrophic, or hypertrophic) were determined by the rateof food resource reproduction in the body of water. Weperformed two series of simulation experiments: with-out predator pressure and a series where the minimumdeath rate varied depending on both the body size andthe numbers of cladocerans.RESULTS AND DISCUSSIONWe simulated 2000 combinations of five crustaceanspecies randomly selected from the database. In mostof these random combinations or “invasions,” only onespecies (rarely, two species) were eventually left in thecommunity (Fig. 1). In other words, the principle ofcompetitive exclusion was complied with. Thus, thezooplanktonic community should be expected to havepoor species diversity in the absence of a predator.If we introduced a predator into the system, thenumber of coexisting species competing for the sameresource increased. In eutrophic and hypertrophic bod-ies of water, the number of coexisting competitorsincreased to four. Under oligotrophic conditions, apredator proved to be incapable of affecting the numberof coexisting species. Apparently, the competition lead-ing to the exclusion of weaker species is more impor-tant for the formation of the community species compo-sition if food is deficient. Under mesotrophic condi-tions, biodiversity also increased, but less than at higherrates of food reproduction.The average concentration of food in mesotrophicand, especially, eutrophic waters increased after theinvasion of a predator compared to this value in theabsence of a predator (Fig. 2). Apparently, the foodconcentration increased upon the invasion of a predator