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CHAPTER 53 COMMUNITY ECOLOGY Section A: What Is a Community? 1. Contrasting views of communities are rooted in the individualistic and interactive hypotheses 2. The debate continues with the rivet and redundancy models Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Introduction • What is a Community? • A community is defined as an assemblage of species living close enough together for potential interaction. • Communities differ in their species richness, the number of species they contain, and the relative abundance of different species. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 1. Contrasting views of communities are rooted in the individualistic and interactive hypotheses • An individualistic hypothesis depicts a community as a chance assemblage of species found in the same area because they happen to have similar abiotic requirements. Fig. 53.1a Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • An interactive hypothesis depicts a community as an assemblage of closely linked species locked in by mandatory biotic interactions. Fig. 53.1b Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • These two very different hypotheses suggest different priorities in studying biological communities. • In most actual cases, the composition of communities does seem to change continuously. Fig. 53.1c Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 2. The debate continues with the rivet and redundancy models • The rivet model of communities is a reincarnation of the interactive model. • The redundancy model states that most species in a community are not closely associated with one another. • No matter which model is correct, it is important to study species relationships in communities. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings CHAPTER 53 COMMUNITY ECOLOGY Section B1: Interspecific Interactions and Community Structure 1. Populations maybe linked by competition, predation, mutualism, and commensalism Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Introduction • There are different interspecific interactions, relationships between the species of a community. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 1. Populations may be linked by competition, predation, mutualism and commensalism • Possible interspecific interactions are introduced in Table 53.1, and are symbolized by the positive or negative affect of the interaction on the individual populations. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Competition. • Interspecific competition for resources can occur when resources are in short supply. • There is potential for competition between any two species that need the same limited resource. • The competitive exclusion principle: two species with similar needs for same limiting resources cannot coexist in the same place. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • The ecological niche is the sum total of an organism’s use of abiotic/biotic resources in the environment. • An organism’s niche is its role in the environment. • The competitive exclusion principle can be restated to say that two species cannot coexist in a community if their niches are identical. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Classic experiments confirm this. Fig. 53.2 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Resource partitioning is the differentiation of niches that enables two similar species to coexist in a community. Fig. 53.3 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 53.2 • Character displacement is the tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species. • Hereditary changes evolve that bring about resource partitioning. Fig. 53.4 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Predation. • A predator eats prey. • Herbivory, in which animals eat plants. • In parasitism, predators live on/in a host and depend on the host for nutrition. • Predator adaptations: many important feeding adaptations of predators are both obvious and familiar. • Claws, teeth, fangs, poison, heat-sensing organs, speed, and agility. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Plant defenses against herbivores include chemical compounds that are toxic. • Animal defenses against predators. • Behavioral defenses include fleeing, hiding, selfdefense, noises, and mobbing. • Camouflage includes cryptic coloration, deceptive markings. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 53.5 • Mechanical defenses include spines. • Chemical defenses include odors and toxins • Aposematic coloration is indicated by warning colors, and is sometimes associated with other defenses (toxins). Fig. 53.6 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Mimicry is when organisms resemble other species. • Batesian mimicry is where a harmless species mimics a harmful one. Fig. 53.7 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Müllerian mimicry is where two or more unpalatable species resemble each other. Fig. 53.8 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Parasites and pathogens as predators. • A parasite derives nourishment from a host, which is harmed in the process. • Endoparasites live inside the host and ectoparasites live on the surface of the host. • Parasitoidism is a special type of parasitism where the parasite eventually kills the host. • Pathogens are disease-causing organisms that can be considered predators. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Mutualism is where two species benefit from their interaction. • Commensalism is where one species benefits from the interaction, but other is not affected. • An example would be barnacles that attach to a whale. Fig. 53.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Coevolution and interspecific interactions. • Coevolution refers to reciprocal evolutionary adaptations of two interacting species. • When one species evolves, it exerts selective pressure on the other to evolve to continue the interaction. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings CHAPTER 53 COMMUNITY ECOLOGY Section B2: Interspecific Interactions and Community Structure (continued) 2. Trophic structure is a key factor in community dynamics 3. Dominant species and keystone species exert strong controls on community structure 4. The structure of a community may be controlled bottom-up by nutrients or top-down by predators Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 2. Trophic structure is a key factor in community dynamics • The trophic structure of a community is determined by the feeding relationships between organisms. • The transfer of food energy from its source in photosynthetic organisms through herbivores and carnivores is called the food chain. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Charles Elton first pointed out that the length of a food chain is usually four or five links, called trophic levels. • He also recognized that food chains are not isolated units but are hooked together into food webs. Fig. 53.10 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Food webs. • Who eats whom in a community? • Trophic relationships can be diagrammed in a community. • What transforms food chains into food webs? • A given species may weave into the web at more than one trophic level. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 53.11 • What limits the length of a food chain? • The energetic hypothesis suggests that the length of a food chain is limited by the inefficiency of energy transfer along the chain. • The dynamic stability hypothesis states that long food chains are less stable than short chains. Fig. 53.13 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 3. Dominant species and keystone species exert strong controls on community structure • Dominant species are those in a community that have the highest abundance or highest biomass (the sum weight of all individuals in a population). • If we remove a dominant species from a community, it can change the entire community structure. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Keystone species exert an important regulating effect on other species in a community. Fig. 53.14 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • If they are removed, community structure is greatly affected. Fig. 53.15 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 4. The structure of a community may be controlled bottom-up by nutrients or topdown by predators • Simplified models based on relationships between adjacent trophic levels are useful for discussing how communities might be organized. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Consider three possible relationships between plants (V for vegetation) and herbivores (H). • VH VH V H • Arrows indicate that a change in biomass of one trophic level causes a change in the other trophic level. • The bottom-up model postulates V H linkages, where nutrients and vegetation control community organization. • The top-down model postulates that it is mainly predation that controls community organization V H. • Other models go between the bottom-up and topdown extreme models. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings CHAPTER 53 COMMUNITY ECOLOGY Section C1: Disturbance and Community Structure 1. Most communities are in a state of nonequilibrium owing to disturbances 2. Humans are the most widespread agents of disturbance Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Introduction • Disturbances affect community structure and stability. • Stability is the ability of a community to persist in the face of disturbance. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 1. Most communities are in a state of nonequilibrium owing to disturbances • Disturbances are events like fire, weather, or human activities that can alter communities. • Some are routine. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 53.16 • Marine communities are subject to disturbance by tropical storms. Fig. 53.17 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 53.17 • We usually think that disturbances have a negative impact on communities, but in many cases they are necessary for community development and survival. Fig. 53.18 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 2. Humans are the most widespread agents of disturbance • Human activities cause more disturbance than natural events and usually reduce species diversity in communities. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings CHAPTER 53 COMMUNITY ECOLOGY Section C2: Disturbance and Community Structure (continued) 3. Ecological succession is the sequence of community changes after a disturbance Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 3. Ecological succession is the sequence of community changes after a disturbance • Ecological succession is the transition in species composition over ecological time. • Primary succession begins in a lifeless area where soil has not yet formed. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 53.19 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Mosses and lichens colonize first and cause the development of soil. • An example would be after a glacier has retreated. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Secondary succession occurs where an existing community has been cleared by some event, but the soil is left intact. • Grasses grow first, then trees and other organisms. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Soil concentrations of nutrients show changes over time. Fig. 53.20 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings CHAPTER 53 COMMUNITY ECOLOGY Section D: Biogeographic Factors Affecting the Biodiversity of Communities 1. Community biodiversity measures the number of species and their relative abundance 2. Species richness generally declines along an equatorial-polar gradient 3. Species richness is related to a community’s geographic size 4. Species richness on an island depends on island size and distance from the mainland Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Introduction • Two key factors correlated with a community’s biodiversity (species diversity) are its size and biogeography. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 1. Community biodiversity measures the number of species and their relative abundance • The variety of different kinds of organisms that make up a community has two components. • Species richness, the total number of species in the community. • Relative abundance of the different species. • Imagine two small forest communities with 100 individuals distributed among four different tree species. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Species richness may be equal, but relative abundance may be different. Fig. 53.21 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Counting species in a community to determine their abundance is difficult, especially for insects and smaller organisms Fig. 53.22 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 2. Species richness generally declines along an equatorial-polar gradient • Tropical habitats support much larger numbers of species of organisms than do temperate and polar regions. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 53.23 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • What causes these gradients? • The two key factors are probably evolutionary history and climate. • Organisms have a history in an area where they are adapted to the climate. • Energy and water may factor into this phenomenon. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 53.24 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 3. Species richness is related to a community’s geographic size • The species-area curve quantifies what may seem obvious: the larger the geographic area, the greater the number of species. Fig. 23.25 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 4. Species richness on islands depends on island size and distance from the mainland • Because of their size and isolation, islands provide great opportunities for studying some of the biogeographic factors that affect the species diversity of communities. • Imagine a newly formed island some distance from the mainland. • Robert MacArthur and E. O. Wilson developed a hypothesis of island biogeography to identify the determinants of species diversity on an island. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Two factors will determine the number of species that eventually inhabit the island. • The rate at which new species immigrate to the island. • The rate at which species become extinct. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 53.26 • Studies of plants on many island chains confirm their hypothesis. Fig. 53.27 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings