Ecology-Interspecific InteractionsIntroductionThe niche is one of the most important concepts in ecology. Paradoxically,it is also one of the hardest to define (Ecology is still a young science). In essence,an organism’s niche is how it makes a living: the environmental conditions ittolerates, the important resources it needs to survive, and its ways of obtainingthose resources.In obtaining energy, nutrients, etc., populations of one species frequentlyinteract with populations of other species.Types of Interspecific InteractionsInteraction Effect on Species 1 Effect on Species 2Neutralism 0 0Competition - -Commensalism + 0Amensalism 0 -Mutualism + +Predation, Parasitsm,Hervivory+ -Neutralism is the most common type of interspecific interaction. Neitherpopulation affects the other. Any interactions that do occur are indirect orincidental.Amensalism is when one species suffers and the other interacting speciesexperiences no effect. This is sometimes viewed as a very asymmetric form ofcompetition.Mutualism in an interspecific interaction between two species, that benefits bothmembers. Populations of each species grow, survive and/or reproduce at a higherrate in the presence of the other species. Mutualisms are widespread in nature,and occur among many different types of organisms.Commensalism is an interspecific interaction where one species benefits and theother is unaffected. They are ubiquitous in nature. For example, birds nesting intrees are commensal. Commensal organisms frequently live in the nests, or on thebodies, of the other species.Predators, parasites, parasitoids, and herbivores obtain food at the expense oftheir hosts or prey.Predators tend to be larger than their prey, and consume many preyduring their lifetimes.Parasites and pathogens are smaller than their host. Parasites may haveone or many hosts during their lifetime. Pathogens are parasitic microbes-many generations may live within the same host. Parasites consume theirhost either from the inside (endoparasites) or from the outside(ectoparasites).Parasitoids hunt their prey like predators, but lay their eggs within thebody of a host, where they develop like parasites.Herbivores are animals that eat plants. This interaction may resemblepredation, or parasitism.Competition occurs when organisms in the same community seek the samelimiting resource. This resource may be prey, water, light, nutrients, nest sites,etc. Competition among members of the same species is intraspecific.Competition among individuals of different species is interspecific. Individualsexperience both types of competition, but the relative importance of the twotypes of competition varies from population to population and species to speciesTypes of CompetitionExploitation competition occurs when individuals use the same limitingresource or resources, thus depleting the amount available to others.Interference competition occurs when individuals interfere with theforaging, survival, or reproduction of others, or directly prevent theirphysical establishment in a portion of a habitat.Outcomes of CompetitionExploitation competition may cause the exclusion of one species. Forthis to occur, one organism must require less of the limiting resource tosurvive. The dominant species must also reduce the quantity of theresource below some critical level where the other species is unable toreplace its numbers by reproduction.Exploitation does not always cause the exclusion of one species. They maycoexist, with a decrease in their potential for growth. For this to occur,they must partition the resource.Interference competition generally results in the exclusion of one of thetwo competitors.The Competitive Exclusion PrincipleEarly in the twentieth century, two mathematical biologists, A.J. Lotka andV. Volterra developed a model of population growth to predict the outcome ofcompetition. Their models suggest that two species cannot compete for the samelimiting resource for long. Even a minute reproductive advantage leads to thereplacement of one species by the other. This is called the competitive exclusionprincipal.A famous experiment by the Russian ecologist, G.F. Gausse demonstratedthat Paramecium aurellia outcompetes and displaces Paramecium caudatum inmixed laboratory cultures, apparently confirming the principle.Interestingly, recent studies by other authors showed coexistence. Thedifference in outcomes may be due to the existence of something called a killerparticle in Gausse’s strain of P. aurellia.) Subsequent laboratory studies on otherorganisms have generally resulted in competitive exclusion, provided that theenvironment was simple enough. Thomas Park showed that, via interferencecompetition, the confused flour beetle and the red flower beetle would notcoexist. One species always excluded the other.Resource PartitioningSpecies that share the same habitat and have similar needs frequently useresources in somewhat different ways - so that they do not come into directcompetitionfor at least part of the limiting resource. This is called resource partitioning.Resource partitioning obviates competitive exclusion, allowing the coexistenceof several speciesusing the same limiting resource.In lab today, you begin an experimental study of the interaction betweentwo species of unicellular algae. "Algae" is a general term for photosynthetic,aquatic organisms that are not vascular plants. Algae are predominantlyunicellular or colonial organisms, although there are multicellular algae (seaweeds).They are predominantly eukaryotes, although cyanobacteria are sometimesconsidered to be algae. Many algae are true autotrophs, but others aremixotrophs or facultative heterotrophs (when there is no light, they revert to apredatory habit.)Algae are tremendously diverse organisms. There are 10,000 species ofdiatoms (one group of algae) alone. A typical pond might harbor a hundredspecies. The existence of so many species of algae is considered to be a bit of amystery in ecology. Algae have very similar metabolic and physiologicalrequirements-light, water, and small amounts of nitrogen, phosphorus, iron,calcium, and silica (for diatoms). Some algae also require other nutrients, such as Bvitamins. According to the principal of competitive exclusion, one might expect agiven habitat to harbor a single species of algae, and yet this is not usually thecase.Some algae that you might see in your experiment include:Euglena gracilis- This is a