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Community Ecology Reading: Freeman, Chapter 50, 53.

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1 Community Ecology Reading: Freeman, Chapter 50, 53

2 What is a community? A community is an assemblage of plant and animal populations that live in a particular area or habitat. –Populations of the various species in a community interact and form a system with its own emergent properties.

3 Pattern vs. Process Pattern is what we can easily observe directly - vegetation zonation, species lists, seasonal distribution of activity, and association of certain species. Process gives rise to the pattern- herbivory, competition, predation risk, nutrient availability, patterns of disturbance, energy flow, history, and evolution.

4 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 tested. –Not all science is experimental. Hypotheses tests can involve special observations, or experiments.

5 Emergent Properties of a Community Scale Spatial and Temporal Structure Species Richness Species Diversity Trophic structure Succession and Disturbance

6 Scale is the size of a community. Provided that the area or habitat is well defined, a community can be a system of almost any size, from a drop of water, to a rotting log, to a forest, to the surface of the Pacific Ocean.

7 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.

8 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.


10 Temporal structure is the timing of the appearance and activity of species. Some communities, i.e., arctic tundra and the decay of a corpse, 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.


12 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

13 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 Simpson’s diversity index: D=1-  p i  

14 Example Problem A community contains the following species: Number of Individuals Species A 104 Species B 71 Species C 19 Species D 5 Species E 3 What is the Simpson index value for this community?

15 Answer: Total Individuals= ( )=202 P A =104/202=.51 P B =19/202=.09 P C =71/202=.35 P D =5/202=.03 P E =3/202=.02 D=1- {(.51) 2 +(.09) 2 +(.35) 2 +(.03) 2 +(.02) 2 } D=1-.40=.60

16 Clicker Question In the example above, what was the species richness? A..60 B. 202 individuals C. 5 species D..40 E. None of the above

17 Succession, Disturbance and Change In terms of species and physical structure, communities change with time. –Ecological succession, the predictable change in species over time, as each new set of species modifies the environment to enable the establishment of other species, is virtually ubiquitous.

18 Example; 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.

19 Disturbances are events such as floods, fire, droughts, overgrazing, and human activity that damage communities, remove organisms from them, and alter resource availability.

20 Some Agents of Disturbance Fire Floods Drought Large Herbivores Storms Volcanoes Human Activity


22 Disturbance creates opportunities for new species to invade an area and establish themselves. These species modify the environment, and create opportunities for other species to invade. The new species eventually displace the original ones. Eventually, they modify the environment enough to allow a new series of invaders, which ultimately replace them, etc. Disturbance, Invasion, Succession

23 Invasion: Disturbance creates an ecological vacuum that can be filled from within, from outside, or both. For example, forest fires clear away old brush and open up the canopy, releasing nutrients into the soil at the same time. Seeds that survive the fire germinate and rapidly grow to take advantage of this opportunity. At the same time, wind-borne and animal- dispersed seeds germinate and seek to do the same thing. The best invaders have good dispersal powers and many offspring, but they are often not the best competitors in the long run.

24 Succession Disturbance of a community is usually followed by recovery, called ecological succession. The sequence of succession is driven by the interactions among dispersal, ecological tolerances, and competitive ability. –Primary succession-the sequence of species on newly exposed landforms that have not previously been influenced by a community, e.g., areas exposed by glacial retreat. –Secondary succession occurs in cases which vegetation of an area has been partially or completely removed, but where soil, seeds, and spores remain.

25 Early in succession, species are generally excellent dispersers and good at tolerating harsh environments, but not the best interspecific competitors. As ecological succession progresses, they are replaced with species which are superior competitors, (but not as good at dispersing and more specialized to deal with the microenvironments created by other species likely to be present with them). Early species modify their environment in such a way as to make it possible for the next round of species. These, in turn, make their own replacement by superior competitors possible.

26  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.

27 An Influential ecologist named F.E. Clements argued that communities work like an integrated machine. These “closed” communities had a predictable composition. According to Clements, there was only one true climax in any given climatic region, which was the endpoint of all successions. Other influential ecologists, including Gleason, hypothesized that random events determined the composition of communities. He recognized that a single climatic area could contain a variety of specific climax types.

28 Evidence suggests that for many habitats, Gleason was right, many habitats never return to their original state after being disturbed beyond a certain point. –For example; very severe forest fires have reduced spruce woodlands to a terrain of rocks, shrubs and forbs.

29 An incredibly rapid glacial retreat is occurring in Glacier Bay, Alaska. In just 200 years, a glacier that once filled the entire bay has retreated over 100km, exposing new landforms to primary succession. –Clements would have predicted that succession today would follow the sequence of ecological succession that has occurred in the past for other parts of Alaska. –In fact, three different successional patterns seem to be occurring at once, depending upon local conditions. Thus, Clements’ view of succession is somewhat of an oversimplification.

30 Are Climax Communities Real?  Succession can take a long time.  For example, old-field succession may require years to reach climax community. But in this time frame, the probability that a physical disturbance (fire, hurricane, flood) will occur becomes so high, the process of succession may never reach completion.

31 Increasing evidence suggests that some amount of disturbance and nonequilibrium resulting from disturbance is the norm for most communities. One popular hypothesis is that communities are usually in a state of recovery from disturbance. –An area of habitat may form a patchwork of communities, each at different stages of ecological succession. Thus, disturbance and recovery potentially enable much greater biodiversity than is possible without disturbance.

32 Are biological communities real functional units? Do communities have a tightly prescribed organization and composition, or are they merely a loose assemblage of species? This is an unsolved problem in ecology. Clements argued that communities are stable, functional units with a fixed composition-each integrated part needs the others. Every area should ultimately have the same species, given time. Gleason argued that their composition is unstable and variable-they are more like assemblages of everything that can live together in one place

33 The Kiddie Pool Experiment Jenkins and Buikema conducted an experiment to see whether artificial ponds would develop predictable assemblages of freshwater microorganisms. -if this were the case, it would support the notion that communities are real, integrated units. -They set up 12 identical “ponds” and filled them with sterile water. Came back in year to study the composition of the resulting communities.

34 Result-the ponds had very different compositions of species. Accidents of dispersal, and different dispersal capabilities affected which species ended up in each pond. The early arrival of certain competitors, and predators greatly affected the ability of later species to colonize later. -Gleason’s view was supported. Composition of communities is dictated largely by chance and history.

35 Trophic structure is the hierarchy of feeding. It describes who eats whom (a trophic interaction is a transfer of energy: i.e., eating, decomposing, obtaining energy via photosynthesis). For every community, a diagram of trophic interactions called a food web. Energy flows from the bottom to the top.

36 A Simple Food Web Killer Whales Sharks Harbor Seals Yellowfin Tuna Mackerel Cod Halibut Zooplankton Unicellular Algae and Diatoms

37 One path through a food web is a food chain. Killer Whales Harbor Seals Mackerel Zooplankton Phytoplankton

38 The niche concept is very important in community ecology. A niche is an organism’s habitat and its way of making a living. An organism’s niche is reflected by its place in a food web: i.e, what it eats, what it competes with, what eats it. Each organism has the potential to create niches for others.

39 Keystone 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.

40 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: Actually, many species of trees are called mangrove trees. Their seeds disperse in salt water. They take root and form a dense forest in saltwater shallows, allowing other species to thrive

41 Trophic Cascades Species at one trophic level influence species at other levels; the addition or subtraction of species affects the entire food web. –This causes positive effects for some species, and negative effects for others. This is called a trophic cascade. For instance, removing a secondary consumer might positively affect the primary consumers they feed upon, and negatively affect the producers that are food for primary consumers.

42 Top down vs. Bottom up Most biological communities have both top-down and bottom-up effects on their structure and composition. –In a well known study of ponds by Matthew Leibold, it was demonstrated that the biomass of herbivores (zooplankton) was positively correlated to the biomass of producers (algae), indicating a top down effect. –He intentionally introduced fish to some ponds, The result was a decrease in zooplankton and increase in producers, indicating a top down effect.

43 Badly scanned from Rose and Mueller (2006)

44 Types of Interspecific Interactions Effect on Effect on Species 1 Species 2 Neutralism 0 0 Competition - - Commensalism + 0 Amensalism - 0 Mutualism + + Predation, - + Parasitism, Herbivory

45 Neutralism Neutralism the most common type of interspecific interaction. Neither population affects the other. Any interactions that do occur are indirect or incidental. Example: the tarantulas living in a desert and the cacti living in a desert

46 Competition Competition occurs when organisms in the same community seek the same limiting 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. 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

47 “Styles” of Competition Exploitation competition occurs when individuals use the same limiting resource or resources, thus depleting the amount available to others. Interference competition occurs when individuals interfere with the foraging, survival, or reproduction of others, or directly prevent their physical establishment in a portion of a habitat.

48 Some specific types of competition Consumptive competition Preemptive competition Overgrowth competition Chemical composition Territorial competition Encounter competition

49 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.

50 Outcomes of Competition Exploitation competition may cause the exclusion of one species. For this to occur, one organism must require less of the limiting resource to survive. The dominant species must also reduce the quantity of the resource below some critical level where the other species is unable to 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. Interference competition generally results in the exclusion of one of the two competitors.

51 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.

52 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).


54 Other experiments... Subsequent laboratory studies on other organisms, have generally resulted in competitive exclusion, provided that the environment was simple enough. Example: Thomas Park showed that, via interference competition, the confused flour beetle and the red flower beetle would not coexist. One species always excluded the other.

55 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.

56 Resource partitioning obviates competitive exclusion, allowing the coexistence of several species using the same limiting resource. Resource partitioning could be an evolutionary response to interspecific competition, or it could simply be that competitive exclusion eliminates all situations where resource partitioning does not occur.

57 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).

58 Character Displacement Sympatric populations of similar species frequently have differences in body structure relative to allopatric populations of the same species. This tendency is called character displacement. Character displacement is thought to be an evolutionary response to interspecific competition.

59 Example of Character Displacement The best known case of character displacement occurs between the finches, Geospiza fuliginosa and Geospiza fortis, on the Galapagos islands. When the two species occur together, G. fuliginosa has a much narrower beak that G fortis. Sympatric populations of G fuliginosa eats smaller seeds than G fortis: they partition the resource. When found on separate islands, both species have beaks of intermediate size, and exploit a wider variety of seeds. These inter-population differences might have evolved in response to interspecific competition.

60 Competition and the Niche An ecological niche can be thought of in terms of competition. The fundamental niche is the set of resources and habitats an organism could theoretically use under ideal conditions. The realized niche is the set of resources and habitats an organism actually used: it is generally much more restricted due to interspecific competition (or predation.)

61 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.

62 Amensalism Amensalism is when one species suffers and the other interacting species experiences no effect. Example: Redwood trees falling into the ocean become floating battering- rams during storms, killing large numbers of mussels and other inter- tidal organisms.

63 Allelopathy involves the production and release of chemical substances by one species that inhibit the growth of another. These secondary substances are chemicals produced by plants that seen to have no direct use in metabolism. This same interaction can be seen as both amensalism, and extremely one- sided interference competition-in fact it is both.

64 Example: Allelopathy in the California Chaparral Black Walnut (Juglans nigra) trees excrete an antibiotic called juglone. Juglone is known to inhibit the growth of trees, shrubs, grasses, and herbs found growing near black walnut trees. Certain species of shrubs, notably Salvia leucophylla (mint) and Artemisia californica (sagebrush) are known to produce allelopathic substances that accumulate in the soil during the dry season. These substances inhibit the germination and growth of grasses and herbs in an area up to 1 to 2 meters from the secreting plants.

65 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. 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.


67 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.

68 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.

69 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.

70 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.

71 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)

72 Predation, Parasitism, Herbivory Predators, parasites, parasitoids, and herbivores obtain food at the expense of their hosts or prey.

73 Predators tend to be larger than their prey, and consume many prey during their lifetimes.


75 Parasites and pathogens are smaller than their host. Parasites may have one or many hosts during their lifetime. Pathogens are parasitic microbes-many generations may live within the same host. Parasites consume their host either from the inside (endoparasites) or from the outside (ectoparasites).

76 Parasitoids hunt their prey like predators, but lay their eggs within the body of a host, where they develop like parasites.

77 Herbibores are animals that eat plants. This interaction may resemble predation, or parasitism.


79 Predator-Prey and Parasite-Host Coevolution The relationships between predator and prey, and parasites and hosts, have coevolved over long periods of time.

80 About 50 years ago, an evolutionary biologist named J.B.S. Haldane suggested that the interaction between parasite and host (or predator and prey) should resemble an evolutionary arms race: First a parasite (or predator) evolves a trait that allows it to attack its host (or prey). Next, natural selection favors host individuals that are able to defend themselves against the new trait. As the frequency of resistant host individuals increases, there is natural selection for parasites with novel traits to subvert the host defenses. This process continues as long as both species survive. Recent data on Plasmodium, the cause of malaria, support this model.

81 Example of Parasite-Host Coevolution The common milkweed, Asclepias syriaca has leaves that contain cardiac glycosides: they are very poisonous to most herbivores. This renders them virtually immune to herbivory by most species. Monarch butterfly larvae have evolved the ability to tolerate these toxins, and sequester them within their bodies. They are important specialist hervivores of milkweeds. These sequestered compounds serve the additional purpose of making monarch larvae virtually inedible to vertebrate predators.

82 Predator-Prey Population Dynamics Predation may be a density-dependent mortality factor to the host population-and prey may represent a limiting resource to predators. The degree of prey mortality is a function of the density of the predator population. The density of the prey population, in turn, affects the birth and death rates of the predator population. i.e, when prey become particularly common, predators increase in numbers until prey die back due to increased predation, this, in turn, inhibits the growth of prey. Typically, there is a time lag effect.

83 There is often a dynamic balance between predators and prey that is necessary for the stability of both populations. Feedback mechanisms may control the densities of both species.

84 Example of Regulation of Host Population by a Herbivore In the 19th century, prickly pear cactus, Opuntia sp. was introduced into Australia from South America. Because no Australian predator species existed to control the population size of this cactus, it quickly expanded throughout millions of acres of grazing land. The presence of the prickly pear cactus excluded cattle and sheep from grazing vegetation and caused a substantial economic hardship to farmers. A method of control of the prickly pear cactus was initiated with the introduction of Cactoblastis cactorum, a cactus eating moth from Argentina, in By 1930, densities of the prickly pear cactus were significantly reduced.

85 Sometimes predator species can drive their prey to localized extinction. If there are no alternate prey, the predator then goes extinct. If the environment is coarse grained, this makes the habitat available for recolonization by the prey species. Example: The parasitic wasp Dieratiella rapae is a very efficient parasitoid. One female can oviposit into several hundred aphids during its lifetime. Frequently, aphids are driven locally extinct and the adults must search for new patches when they emerge. Once the aphid and the host are gone, the host plants may become re-infested with aphids.

86 In other cases, there are alternate prey to support the predator and the prey is permanently excluded. Example: Freshwater fish such as bluegills and yellow perch frequently exclude small invertebrates such as Daphnia pulex from ponds. The fish then switch to other prey such as insects larvae.

87 The time-lag effect may lead to predator- prey oscillations. Most predators do not respond instantaneously to the availability of prey and adjust their reproduction accordingly. If predator populations grow faster than prey populations, they may overshoot the number of prey that are able to support them This leads to a rapid decline in the prey, followed by a rapid decline in the predator. Once the predator becomes rare, the prey population may begin growing again. This pattern is called a predator-prey oscillation.

88 Cycles in the population dynamics of the snowshoe hare and its predator the Canadian lynx (redrawn from MacLulich 1937). Note that percent mortality is an elusive measure, it may, or may not, be useful since mortality varies with environment and time.

89 In the 1920s, A. J. Lotka (1925) and V. Volterra (1926) devised mathematical models representing host/prey interaction. The Lotka-Volterra curve assumes that prey destruction is a function not only of natural enemy numbers, but also of prey density, i.e., related to the chance of encounter. This model predicts the predator-prey oscillations sometimes seen in nature. Populations of prey and predator were predicted to flucuate in a regular manner (Volterra termed this "the law of periodic cycle"). Lotka-Volterra model is an oversimplification of reality. In nature, many different factors affect the densities of predators and their prey.


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