Community Ecology Reading: Freeman, Chapter 52 • Students should be able to identify the emergent properties of a community • Students should be able to identify and understand the key interactions that occur among species in a community. • Students should be able to distinguish between biological communities, populations, and ecosystems. • Students should be able to identify the major assumptions of the theory of island biogeography, and its predictions What is a Community? • A community is an assemblage of plant animal, fungal, and microbe populations that live in a particular area or habitat. – Interaction characterizes communities. Populations of the various species in a community utilize, decompose, compete with, and alter the fates of each other. Together they form a system with its own emergent properties. • Community ecology seeks to explain the underlying mechanisms that create, maintain, and determine the fate of biological communities. Typically, patterns are documented by observation, and used to generate hypotheses about processes, which are not always easy to observe directly. – Patterns-vegetation zonation, species lists, seasonal distribution of activity, and association of certain species. – Processes-herbivory, competition, predation risk, nutrient availability, patterns of disturbance, energy flow, history, and evolution Emergent Properties of a Community • Spatial and Temporal Structure • Species Richness • Species Diversity (Even-ness) • Trophic Structure • Succession and Disturbance • Spatial Structure is the way species are distributed relative to each other. – Some species provide a framework that creates habitats for other species. These species, in turn create habitats for others, etc. • Example: – Trees in a rainforest are stratified into several different levels, including a canopy, several understories, a ground level, and roots. – Each level is the habitat of a distinct collection of species. Some places, such as the pools of water that collect at the base of tree branches, may harbor entire communities of their own. • Temporal structure is the timing of the appearance and activity of species. • Some communities, i.e., arctic tundra, seasonal ponds, have pronounced temporal species, other communities have less. – Example: Many desert plants and animals are dormant most of the year. They emerge, or germinate, in response to seasonal rains. Other plants stick around year round, having evolved adaptations to resist drought. • Species Richness - is the number of species in a community. – Clearly, the number of species we can observe is function of the area of the sample. It also is a function of who is looking. Thus, species richness is sensitive to sampling procedure • Diversity is the number of species in the community, and their relative abundances. • Species are not equally abundant, some species occur in large percentage of samples, others are poorly represented. • Some communities, such as tropical rainforests, are much more diverse than others, such as the great basin desert. • Species Diversity is often expressed using an index of diversity. The Shannon Wiener index of diversity (or just the Shannon index) is an example: • H’= -S pi Ln pi Ecological Succession •How do communities come to be? •Ecological communities are dynamic in terms of species turnover, species richness, species diversity and structure Ecological Succession •Definition: The more or less regular colonization, recovery and turnover (replacement) of species within a community following a disturbance • Examples; – A sphagnum bog community may persist for only a few decades before the process of ecological succession changes transform it into the surrounding Black Spruce Forest. – A forest fire may destroy a large area of trees, clearing the way for a meadow. Eventually, the trees take over and the meadow is replaced. – Lava flows in eventually weather, crack, and allow the establishment of vegetation. Over time this vegetation allows a soil to form, and ultimately, forest. Succession begins with a Disturbance • Disturbances are events such as floods, fire, droughts, overgrazing, and human activity that alter communities by eliminating species, reducing population sizes, and altering resource availability, climate and physical properties. The nature of the disturbance • Primary Succession: The disturbance results in a sterile, novel environment. Succession can only proceed through the colonization of species from elsewhere. Involves major temporal dynamics in the physical. • Secondary Succession: The disturbance may extirpate some species but others survive at low population sizes. Involves both recovery and colonization of species. Less dramatic temporal dynamics in the physical environment. • Lichens, Earlysuccessional lava cacti and swordferns. • Students standing on 200 year old lava flows in Nicaragua. Lichens kick-off primary succession on lava Agents of Disturbance for Primary Succession • Volcanoes • Asteroids • Retreating glaciers • Sand dune formation (shores of Lake Michigan) • Completely paved by development Agents of Disturbance for Secondary Succession • Fire • Hurricanes & Tornadoes • Floods • Abandoned farmland or other human landscapes (Detroit!) • Clear-cuts in forests Yellowstone Fire 1988 Trends in Succession • Disturbance creates opportunities for new species to invade an area and establish themselves. • With time community goes from being physically harsh to more physically benign • With time community goes from biotically benign to biotically harsh Trends in species characteristics • Early successional: High intrinsic growth rates, good dispersal and establishers, tolerate physical and climatic stress, poor competitors and/or susceptible to herbivores of predators. • Late successional: High carrying capacities, often poorer dispersers, less tolerant of physical stress, strong competitors and predators. Disturbance, Invasion, Succession: Key processes • Facilitation: early successional species modify the environment, and create opportunities for subsequent species to invade or establish. This tends to speed succession and make succession regular and predictable • Inhibition: early species, via priority effects, gain an advantage and slow the establishment of later species. This slows succession and makes it less predictable – alternate states. Endpoint of succession depends on disturbance characteristics • Amplitude and frequency • Climax communities more likely when disturbances are high amplitude and low frequency • Disturbance communities are those with high frequency of low amplitude disturbances • Intermediate amplitudes and frequencies may maximize species richness and diversity A climax community is a more or less permanent and final stage of a particular succession, often characteristic of a restricted area. Climax communities are characterized by slow rates of change, compared with more dynamic, earlier stages. They are dominated by species tolerant of competition for resources. •Trophic Structure • A trophic interaction is a transfer of energy: i.e., eating, decomposing, obtaining energy via photosynthesis. • Trophic structure is the hierarchy of feeding in a community. • For every community, a diagram of trophic interactions called a food web. – Energy flows from the bottom to the top. A Simple Marine Food Web Killer Whales Sharks Harbor Seals Yellowfin Tuna Mackerel Cod Halibut Zooplankton Unicellular Algae and Diatoms Killer Whales Harbor Seals Mackerel Zooplankton Phytoplankton One path through a food web is a food chain. The Niche •A key concept in community ecology whose definition has changed over time (and varies among community ecologists). •Elton: Where does it live? What does it eat? Who eats it? •Hutchinson: Fundamental niche describes the range of climatic and physical conditions that can support the species in the absence of negative biotic interactions. •Realized Niche: The actual range of conditions that a species can succeed including competition and predation from other species. •Species often create niches for other species •Keystone Species • These species are disproportionately important in communities. • Generally, keystone species act to maintain species diversity. • The extinction of a keystone species eliminates the niches of many other species. • Frequently, a keystone species modifies the environment in such a way that other organisms are able to live, in other cases, the keystone species is a predator that maintains diversity at a certain trophic level. Sometimes, they are mutualists, or “engineers”. Examples of Keystone Species • California Sea Otters: This species preys upon sea urchins, allowing kelp forests to become established. • Pisaster Starfish: Grazing by Pisaster prevents the establishment of dense mussel beds, allowing other species to colonize rocks on the Pacific Coast. • “Mangrove” Trees: Mangrove seeds disperse in salt water. They take root and form a dense forest in saltwater shallows, allowing other species to thrive • The Acorn Banksia: At certain times of year, Banksia prionotes is the sole source of food for honeyeaters, which in turn, are the pollinators for many other species of plants in Western Australia. • Students should be able to identify and understand the key interactions that occur among species in a community. • • Types of Interspecific Interactions Effect on Species 1 • Competition • Commensalism • Amensalism • Mutualism • Predation, + + - • Parasitism, Herbivory Effect on Species 2 0 0 + + Competition • Competition occurs when individuals of the same or different species restrict each others’ access to limiting resources. 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. • Individuals experience both types of competition, but the relative importance of the two types of competition varies from population to population and species to species “Styles” of Competition • Exploitation competition occurs when individuals permanently or temporally use up the limiting resource or resources, thus depleting the amount available to others. • Interference competition occurs when individuals via antagonistic behaviors or caching behaviors prevent others from accessing the limiting resource (territoriality, dominance hierarchies, etc.). Example of Interference Competition • The confused flour beetle, Triboleum confusum, and the red flour beetle, Triboleum castaneum cannibalize the eggs of their own species as well as the other, thus interfering with the survival of potential competitors. • In mixed species cultures, one species always excludes the other. Which species prevails depends upon environmental conditions, chance, and the relative numbers of each species at the start of the experiment. Outcomes of Competition • Exploitation competition may cause the exclusion of one species. This occurs if the more efficient species reduces the quantity of the resource below some critical level where the other species is unable to survive and replace its numbers by reproduction. • Exploitation does not always cause the exclusion of one species. They may coexist, with a decrease in their potential for growth. For this to occur, they must partition the resource. • In the absence of other trade-offs, interference competition results in the exclusion of one of the two competitors. The Competitive Exclusion Principle • Early in the twentieth century, two mathematical biologists, A.J. Lotka and V. Volterra developed a model of population growth to predict the outcome of competition. • Their models suggest that two species cannot compete for the same limiting resource for long. Even a minute reproductive advantage leads to the replacement of one species by the other. • This is called the competitive exclusion principal. Evidence for Competitive Exclusion. • A famous experiment by the Russian ecologist, G.F. Gausse demonstrated that Paramecium aurellia outcompetes and displaces Paramecium caudatum in mixed laboratory cultures, apparently confirming the principle. • (Interestingly, this is not always the case. Later studies suggest that the particular strains involved affect the outcome of this interaction). Resource Partitioning • Species that share the same habitat and have similar needs frequently use resources in somewhat different ways - so that they do not come into direct competition for at least part of the limiting resource. This is called resource partitioning. • Resource partitioning is though to be an evolutionary response to interspecific competition, • One of the best known cases of resource partitioning occurs among Caribbean anoles. – As many as five different species of anoles may exist in the same forest, but each stays restricted to a particular space: some occupy tree canopies, some occupy trunks, some forage close to the ground. – When the brown anole was introduced to Florida from Cuba, it excluded the green anole from the trunks of trees and areas near the ground: the green anole is now restricted to the canopies of trees:the resource (space, insects) has been partitioned among the two species – (for now at least, this interaction may not be stable in the long run because the species eat each other’s young). Two Organisms Cannot Occupy Exactly the Same Niche. This is sometimes called Gausse’s rule(although Gausse never put it exactly that way). -Experiments by Gausse (Paramecium), Peter Frank (Daphnia), and Thomas Park (Triboleum) have confirmed it for simple laboratory scenarios. -This creates a bit of a paradox, because so many species exist in nature using the same resources. -The more complex environments found in nature may enable more resource partitioning. A Classic Experiment • In the early 1960’s, Joseph Connell, a scientist at the University of California, studied the effects of competition on the distribution of two species of barnacles, Cthalamus stellatus, and its competitor, Semibalanus balanoides. • The two species live in a stratified distribution along the Scottish Coast. • His objective was to determine the extent to which competition imposed this relationship between the two species. Right- C. stellatus. • Observations: – Semibalanus is most concentrated in the lower intertidal area. – Chthamalus is most concentrated in the upper intertidal area. – The free-swimming larvae of each species can settle anywhere on the rocky shoreline, and presumably be able to grow to be an adult. • Question: why don't we see Semibalanus and Chthamalus gro wing together? – Right-Semibalanus balanoides • Experiment 1: – Connell removed Chthamalus from the upper area, and no Semibalanus replaced it. – Inference: Semibalanus could not survive in an area that experienced so much desiccation (due to low tides). – Conclusion: Semibalanus's realized niche was the same as its fundamental niche. • Experiment 2: – Connell removed Semibalanus from the lower area and Chthamalus replaced it. – Inference: Semibalanus was a more successful competitor in the lower intertidal zone. – Conclusion: the fundamental niche and realized niche for Chthamalus were not the same—its realized niche was smaller due to interspecific competition. • The original paper is currently online at – http://www.life.illinois.edu/ib/453/connell.pdf Commensalism • Commensalism is an interspecific interaction where one species benefits and the other is unaffected. • Commensalisms are ubiquitous in nature: birds nesting in trees are commensal. • Commensal organisms frequently live in the nests, or on the bodies, of the other species-these are called inquilines • Examples of Commensalism: • Ant colonies harbor rove beetles as commensals. These beetles mimic the ants behavior, and pass as ants. They eat detritus and dead ants. • Anemonefish live within the tentacles of anemones. They have specialized mucus membranes that render them immune to the anemone’s stings. They gain protection by living in this way. A termite and a ”termitophilic” inquiline rove beetle. Mutualism • Mutualism in an interspecific interaction between two species that benefits both members. • Populations of each species grow, survive and/or reproduce at a higher rate in the presence of the other species. • Mutualisms are widespread in nature, and occur among many different types of organisms. Examples of Mutualism • Most rooting plants have mutualistic associations with fungal mychorrhizae. Mychorrhizae increase the capability of plant roots to absorb nutrients. In return, the host provides support and a supply of carbohydrates. • Many corals have endosymbiotic organisms called zooxanthellae (usually a dinoflagellate). These mutualists provide the corals with carbohydrates via photosynthesis. In return, they receive a relatively protected habitat from the body of the coral. Mutualistic Symbiosis • Mutualistic Symbiosis is a type of mutualism in which individuals interact physically, or even live within the body of the other mutualist. Frequently, the relationship is essential for the survival of at least one member. – Example: Lichens are a fungal-algal symbiosis (that frequently includes a third member, a cyanobacterium.) The mass of fungal hyphae provides a protected habitat for the algae, and takes up water and nutrients for the algae. In return, the algae (and cynaobacteria) provide carbohydrates as a source of energy for the fungus. Facultative vs. Obligate Mutualisms • Facultative Mutualisms are not essential for the survival of either species. Individuals of each species engage in mutualism when the other species is present. • Obligate mutualisms are essential for the survival of one or both species. Other Examples of Mutualisms • Flowering plants and pollinators. (both facultative and obligate) • Parasitoid wasps and polydna viruses. (obligate) • Ants and aphids. (facultative) • Termites and endosymbiotic protozoa. (obligate) • Humans and domestic animals. (mostly facultative, some obligate) Parasitoid wasps and polydna viruses Predation, Parasitism, Herbivory • Predators, parasites, parasitoids, and herbivores obtain food at the expense of their hosts or prey. • The predator consumes some or all of the prey as its limiting resource. • The prey are either killed or harmed in ways that reduce their survival or reproduction • Predators tend to be larger than their prey, and consume many prey during their lifetimes. • Parasites and pathogens are smaller than their host. • Parasites may have one or many hosts during their lifetime. – Some even steal parental care or other resources • Parasites consume their host either from the inside (endoparasites) or from the outside (ectoparasites). – Pathogens are parasitic disease-causing microbes. • Many generations may live within the same host. Bombus hyperboreus, a social parasite of arctic bumblebees, takes over their colonies and enslaves their workers. • Parasitoids hunt their prey like predators, but lay their eggs within the body of a host, where they develop like parasites. • Herbivores are animals that eat plants. This interaction may resemble predation, or parasitism. Class Discussion: What is the “purpose” of a Barracuda, an Aspen Tree, Asian tiger mosquito? What about a mosquito that only bites lizards? Does any organism need a purpose to justify its existence?
