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Chapter 27 Community Interactions Lecture Outlines by Gregory Ahearn, University of North Florida Copyright © 2011 Pearson Education Inc. Chapter 27 At a Glance 27.1 Why Are Community Interactions Important? 27.2 What Is the Relationship Between the Ecological Niche and Competition? 27.3 What Are the Results of Interactions Between Predators and Their Prey? 27.4 What Is Parasitism? 27.5 What is Mutualism? 27.6 How Do Keystone Species Influence Community Structure? 27.7 Succession: How Do Community Interactions Cause Change Over Time? Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.1 Why Are Community Interactions Important? An ecological community consists of all the interacting populations within an ecosystem – A community can encompass the entire biotic, or living, portion of an ecosystem – Interactions between populations in a community help limit their size –Populations maintain a balance between resources and the numbers of individuals consuming them Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.1 Why Are Community Interactions Important? An ecological community consists of all the interacting populations within an ecosystem (continued) – The process by which two interacting species act as agents of natural selection on one another is called coevolution – For example, when killing prey that is easiest to catch, individuals with the best defenses against predation remain – These individuals produce the most offspring and, over time, their characteristics increase within the prey population Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.1 Why Are Community Interactions Important? The most important community interactions are: – Competition, which harms both species – Predation, which benefits the predator but harms the prey – Parasitism, which benefits parasite but harms the host – Mutualism, which benefits both species Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Type of Interaction Effect on Species A Effect on Species B Competition between A and B Harms Harms Predation by A on B Benefits Harms Parasitism by A on B Benefits Harms Mutualism between A and B Benefits Benefits Table 27-1 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.2 What Is the Relationship Between the Ecological Niche and Competition? Each species occupies a unique ecological niche that encompasses all aspects of its way of life – These include: – Its physical home or habitat – The physical and chemical environmental factors necessary for its survival, such as nesting sites, climate, and the type of nutrients it needs – The role that the species performs within an ecosystem, such as what it eats and the other species with which it competes – Although different species share aspects of their niche with others, no two species ever occupy exactly the same ecological niche within a community Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.2 What Is the Relationship Between the Ecological Niche and Competition? Competition occurs whenever two organism attempt to use the same, limited resources – Interspecific competition occurs between members of different species, if they feed on the same things or require similar breeding areas –This type of competition is detrimental to all of the species involved as it reduces their access to resources in limited supply –The greater the overlap of ecological niches, the more intense the interspecific competition Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.2 What Is the Relationship Between the Ecological Niche and Competition? Adaptations reduce the overlap of ecological niches among coexisting species – The competitive exclusion principle states that if two species occupy exactly the same niche with limited resources, one will outcompete the other Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Competitive Exclusion Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.2 What Is the Relationship Between the Ecological Niche and Competition? The competitive exclusion principle was formulated by microbiologist G. F. Gause, who performed laboratory experiments using two species of protists, Paramecium aurelia and P. caudatum – Both species thrived on the same bacteria and fed in the same region of their laboratory flasks – When put into the same flask, P. aurelia always eliminated P. caudatum – Gause repeated the experiment, replacing P. caudatum with P. bursaria, which fed in a different part of the flask – In that case, both species could coexist because they occupied different niches Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Competitive Exclusion P. aurelia P. caudatum (a) Grown in separate flasks (b) Grown in the same flask Biology: Life on Earth, 9e Fig. 27-1 Copyright © 2011 Pearson Education Inc. 27.2 What Is the Relationship Between the Ecological Niche and Competition? Adaptations reduce the overlap of ecological niches among coexisting species (continued) – When species with largely similar ecological niches coexist and compete, each species occupies a smaller niche than it would by itself, a phenomenon called resource partitioning –This reduces interspecific competition, and is the outcome of the coevolution of species with extensive niche overlap Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Resource Partitioning Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.2 What Is the Relationship Between the Ecological Niche and Competition? Ecologist Robert MacArthur explored the competitive exclusion principle by carefully observing five species of North American warbler – These birds all hunt for insects and nest in the same type of eastern spruce tree – MacArthur found that each species concentrates its search for food in specific regions within spruce trees, employs different hunting tactics, and nests at a slightly different time – By dividing up the resources provided by the spruce trees they share, the warblers minimize the overlap of their niches and reduce interspecific competition Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Resource Partitioning Yellow-rumped Bay-breasted warbler warbler Cape May warbler Black-throated green warbler Blackburnian warbler Fig. 27-2 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.2 What Is the Relationship Between the Ecological Niche and Competition? Interspecific competition may reduce the population size and distribution of each species – Although natural selection can reduce niche overlap, interspecific competition may still restrict the size and distribution of competing populations Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.2 What Is the Relationship Between the Ecological Niche and Competition? Interspecific competition may reduce the population size and distribution of each species (continued) – For example, consider Chtlamalus and Balanus barnacles of the Scottish intertidal zone – Chthamalus occupies the upper shore, while Balanus occurs in the lower shore – Ecologist J. Connell scraped off Balanus and the Chthamalus population increased, spreading downward into the area that its competitor had once inhabited – Where the habitat is appropriate for both genera, Balanus conquers because it is larger and grows faster; however, Chthamalus tolerates drier conditions, giving it a competitive advantage on the upper shore Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Resource Partitioning in Barnacles Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.2 What Is the Relationship Between the Ecological Niche and Competition? Competition within a species is a major factor controlling population size – Intraspecific competition, competition between individuals of the same species, is the most intense form of competition –If resources are limited, this is a major factor controlling population size –The evolutionary result of intraspecific competition is that the individuals better suited to survive are more likely to reproduce and pass their traits to the offspring Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Predator–prey interactions shape evolutionary adaptations – Predators eat other organisms; these include herbivores (animals that eat plants) as well as carnivores (animals that eat other animals) –Predators include a grass-eating pika, a bat hunting a moth, and the more familiar example of a hawk eating a bird – Predators tend to be less abundant than their prey Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Ecosystem Roles Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Forms of Predation Fig. 27-3 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Predator–prey interactions shape evolutionary adaptations (continued) – Predator and prey populations exert intense selective pressure on one another, resulting in coevolution –As prey become more difficult to catch, predators must become more adept at hunting Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Some predators and prey have evolved counteracting behaviors – Bat and moth adaptations provide excellent examples of how body structures and behaviors are molded by competition – Bats emit high-pitch sound pulses that bounce off their surroundings, allowing them to navigate and detect prey – Moths (their prey) have evolved ears sensitive to the pitch of sounds the bats emit, and they take evasive actions in response – The bats, in turn, counter by switching the frequency of their sound pulses away from the moth’s sensitivity range Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Camouflage conceals both predators and their prey – Camouflage renders animals inconspicuous even when in plain sight –Predators and prey have evolved colors, patterns, and shapes that resemble their surroundings Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Camouflage by Blending In Fig. 27-4 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Camouflage conceals both predators and their prey (continued) – To avoid detection by predators, some animals have evolved to resemble objects, such as leaves, twigs, seaweed, thorns, or even bird droppings – Some plants have evolved to resemble rocks to avoid detection by herbivores Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Camouflage by Resembling Specific Objects Fig. 27-5a, b Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Camouflage by Resembling Specific Objects Fig. 27-5c, d Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Camouflage conceals both predators and their prey (continued) – Camouflage also helps predators ambush their prey –Examples include the cheetah blending with tall grass and the frogfish resembling a rock Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Camouflage Assists Predators Fig. 27-6 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Bright colors often warn of danger – Some animals have evolved bright warning coloration that attracts the attention of potential predators –Warning coloration advertises that the animal is bad-tasting or poisonous before the predator attacks –Examples include poison arrow frogs, coral snakes, and honey bees Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Warning Coloration Fig. 27-7 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Some prey organisms gain protection through mimicry – Mimicry refers to when members of one species have evolved to resemble another species – Two or more distasteful species may each benefit from a shared warning coloration pattern (Müllerian mimicry) – Predators need only experience one distasteful species to learn to avoid all with that color pattern – For example, toxic monarch and viceroy butterflies have similar wing patterns; if a predator becomes ill from eating one species, it will avoid the other Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Mullerian Mimicry Fig. 27-8 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Some prey organisms gain protection through mimicry (continued) – Some harmless organisms can gain a selective advantage by resembling poisonous species (Batesian mimicry) –For example, the harmless hoverfly avoids predation by resembling a bee –The harmless mountain king snake is protected by a warning coloration that resembles the venomous coral snake Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Batesian Mimicry Fig. 27-9a, b Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Batesian Mimicry Fig. 27-9c, d Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Some prey organisms gain protection through mimicry (continued) – Some animals deter predators by employing startle coloration –These animals may have spots that resemble the eyes of a larger animal –If a predator gets close, the prey will flash its eyespots, startling the predator and allowing the prey to escape –Examples include the peacock moth and the swallowtail caterpillar Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Startle Coloration Fig. 27-10 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? A sophisticated variation on prey mimicking dangerous animals is seen in the snowberry fly – Snowberry flies avoid predation by jumping spiders by mimicking them both visually and behaviorally –When the spider approaches, the fly spreads its wings, jerking them back and forth –This movement resembles the behavior of a jumping spider driving another spider from its territory, and so the attacking spider flees Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. A Prey Mimics Its Predator Fig. 27-11 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Predators may use mimicry to attract prey – In aggressive mimicry, a predator resembles a harmless animal or part of the environment, to lure prey within striking distance –For example, a frogfish dangles a wriggling lure that attracts a curious fish that is then eaten Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Aggressive Mimicry Fig. 27-6b Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Predators and prey may engage in chemical warfare – Predators and prey use toxins for attack and defense –Spiders and poisonous snakes use venom to paralyze their prey and to deter predators –The bombardier beetle sprays boiling-hot chemicals from its abdomen onto its attacker Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Chemical Warfare Fig. 27-12a Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.3 What Are the Results of Interactions Between Predators and Their Prey? Predators and prey may engage in chemical warfare for attack and defense (continued) – Many plants have evolved chemical adaptations that deter their herbivore predators, such as the milkweed –In the case of the milkweed, however, monarch butterfly caterpillars have evolved to tolerate the toxins and store them in their tissues as a defense against predation Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Chemical Warfare Fig. 27-12b Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.4 What Is Parasitism? Parasites live in or on their prey, which are called hosts, usually harming or weakening them but not immediately killing them – Parasites are generally much smaller and more numerous than their hosts –Examples include tapeworms, fleas, ticks, and many types of disease-causing protists, bacteria, and viruses Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.4 What Is Parasitism? Parasites and their hosts act as agents of natural selection on one another – The powerful forces of coevolution between parasitic microorganisms and their hosts can be seen in the variety of infectious bacteria and viruses, and the precision of the immune system that counters their attacks Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.4 What Is Parasitism? Parasites and their hosts act as agents of natural selection on one another (continued) – Nagana, a disease in cattle caused by a parasitic protist, kills cattle imported into areas of Africa, but some African breeds of cattle have evolved an immunity to it and survive – In some regions of Africa, 20-40% of the human population carries a sickle-cell gene that confers protection against the malaria protist parasite Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.5 What Is Mutualism? Mutualism refers to interactions between species in which both benefit – Many mutualistic relationships are symbiotic and involve a close, long-term physical association between the participating species –For example, lichens form a mutualistic relationship between a fungus and an algae –The fungus provides support and protection while obtaining food from the photosynthetic alga Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Mutualism Fig. 27-13a Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.5 What Is Mutualism? Mutualism refers to interactions between species in which both benefit (continued) – Another example of mutualism is the clownfish and sea anemones –The clownfish takes shelter from predators among the venomous tentacles of an anemone, while in turn cleaning it, providing it with scraps of food, and defending it from predators Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Mutualism Fig. 27-13b Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.6 How Do Keystone Species Influence Community Structure? In some communities, a keystone species plays a major role in determining community structure – A keystone species role is out of proportion to its abundance in the community – If a keystone species is removed from the community, normal community interactions are significantly altered and the relative abundance of other species changes dramatically – Keystone species need to be identified and protected so that human activities do not lead to the collapse of entire communities and ecosystems Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.6 How Do Keystone Species Influence Community Structure? In some communities, a keystone species plays a major role in determining community structure (continued) – An example of a keystone species is the predatory sea star Pisaster ochraceous from Washington’s rocky intertidal coast –When removed from their ecosystem, their favored prey, native mussels, became so abundant that they outcompete other invertebrates and algae Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Keystone Species Fig. 27-14a Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.6 How Do Keystone Species Influence Community Structure? In some communities, a keystone species plays a major role in determining community structure (continued) – Another example of a keystone species is the African elephant, which, by grazing on small trees and bushes, prevents the encroachment of forests –This activity helps maintain the grass savanna that supports grazing mammals and their predators Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Keystone Species Fig. 27-14b Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? Most communities do not emerge fully formed from bare rock or naked soil – Instead, they arise through succession, where the community and its nonliving environment change structurally over time – Succession is usually preceded by a disturbance, an event that disrupts the ecosystem either by altering the community, its abiotic (nonliving) structure, or both Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? During succession, most terrestrial communities go through stages – Succession begins with arrival of a few hardy plants, called pioneers –The pioneers alter the ecosystem in ways that favor competing plants, which eventually displace the pioneers Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? During succession, most terrestrial communities go through stages (continued) – Succession often progresses to a relatively stable and diverse climax community – Recurring disturbances can set back the progress of succession –The continuous disturbances maintain communities in earlier, or subclimax, stages of succession Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? There are two major forms of succession – Primary succession – Secondary succession Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? Primary succession occurs “from scratch,” where there is no trace of a previous community – This process may take thousands or even tens of thousands of years – The disturbance that sets the stage for primary succession may be a glacier scouring the landscape to bare rock, or a volcano Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Succession in Progress Fig. 27-15a Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? Secondary succession occurs after a disturbance changes, but does not obliterate, an existing community, leaving remnants such as soil and seeds – This type of succession often takes just hundreds of years – An example is Mount St. Helens, which erupted in 1980 and left a thick layer of nutrient-rich ash that encouraged new growth – Another example is fire, which also produces nutrientrich ash and spares some trees and many healthy roots Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Succession in Progress Fig. 27-15b Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Succession in Progress Fig. 27-15c Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? Primary succession can begin on bare rock – Isle Royal, Michigan, is an example of primary succession – This island in Lake Superior was scraped down to bare rock by glaciers – The bare rock provided a place for pioneer species, such as lichen and mosses Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? Isle Royal, Michigan, is an example of primary succession (continued) – Lichens and mosses were eventually replaced by larger plants, such as bluebell and yarrow, which were in turn replaced by woody shrubs such as blueberry and juniper – Eventually, trees such as jack pine, black spruce, and aspen took root, and the sun-loving shrubs were shaded out – Faster growing trees such as balsam fir, paper birch, and white spruce towered over and replaced the original trees – A tall spruce-fir climax forest was eventually established Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Primary Succession lichens and rock scraped moss on bare rock bare by a glacier 0 Biology: Life on Earth, 9e bluebell, yarrow blueberry, juniper spruce-fir climax forest: jack pine, black spruce, white spruce, balsam fir, aspen paper birch 1,000 Fig. 27-16 Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? An abandoned farm will undergo secondary succession – First to arrive are the fast-growing pioneer species, such as crabgrass, ragweed, and Johnson grass – Perennial plants, such as asters, goldenrod, and perennial grasses, then take over, followed by woody shrubs such as blackberry and smooth sumac – Pine and cedar trees become established, until a climax forest of oak and hickory replace all other species after about a century Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Secondary Succession plowed field ragweed, crabgrass, Johnson grass Virginia pine, eastern red blackberry, smooth sumac cedar aster, goldenrod, Queen Anne's lace, broom sedge grass oak-hickory climax forest: white and black oak, bitternut and shagbark hickory 100 0 Fig. 27-17 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? Succession also occurs in ponds and lakes – This occurs not only through changes within the body of water, but also through an influx of nutrients from outside the ecosystem – Sediments and nutrients carried in by runoff from the surrounding land have a large impact on small freshwater lakes, ponds, and bogs, which gradually undergo succession to dry land – In forests, meadows may be produced by lakes filling in from around the edges; grasses colonize the newly formed soil – As the lake shrinks and the area of the meadow expands, trees will encroach around the meadow’s edges Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Succession in a Small Freshwater Pond Fig. 27-18 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? Succession culminates in a climax community – Succession ends with a relatively stable climax community, which perpetuates itself if not disturbed by outside forces, such as fire –The populations within a climax community have ecological niches that allow them to coexist without replacing one another –Climax communities have more species and more types of community interactions than do earlier stages of succession Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? Succession culminates in a climax community (continued) – Climax species tend to be larger and longer-lived than pioneer species – The exact nature of the climax community at a site reflects the local geological and climatic conditions, such as temperature, rainfall, and elevation Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? Some ecosystems are maintained in a subclimax stage – Frequent disturbances maintain subclimax communities in some ecosystems – A subclimax community example is the tallgrass prairies that once covered northern Missouri and Illinois –Periodic fires maintained the grasses and prevented forests from encroaching Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? Some ecosystems are maintained in a subclimax stage (continued) – Another example of a subclimax community is suburban lawns –Mowing and use of herbicides keep weeds and woody species in check – A further example of a subclimax community is agriculture –Plowing and pesticides keep competing weeds and shrubs from replacing grains Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 27.7 Succession: How Do Community Interactions Cause Change Over Time? Climax communities create Earth’s biomes – The climax communities that form during succession are strongly influenced by climate and geography – Extensive areas of characteristic climax plant communities are called biomes, and include deserts, grasslands, and forests – These biomes dominate broad geographical regions with similar climates Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc.