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Community Ecology 53 BIOLOGICAL SCIENCE FOURTH EDITION SCOTT FREEMAN Lectures by Stephanie Scher Pandolfi © 2011 Pearson Education, Inc. Key Concepts • Interactions among species, such as competition, consumption, and mutualism have two main outcomes: (1) They affect the distribution and abundance of the interacting species, and (2) they are agents of natural selection and thus affect the evolution of the interacting species. The nature of interactions between species frequently changes over time. © 2011 Pearson Education, Inc. Key Concepts • The assemblage of species found in a biological community changes over time and is primarily a function of climate and chance historical events. • Species richness is higher in large islands near continents than in small, isolated islands, due to differences in immigration and extinction. Species richness is also higher in the tropics and lower toward the poles, but the mechanism responsible for this pattern is still controversial. © 2011 Pearson Education, Inc. Introduction • A biological community consists of interacting species, usually living within a defined area. • Biologists want to know how communities work, and how to manage them in a way that will preserve species and create an environment that people want to live in. © 2011 Pearson Education, Inc. Species Interactions • Because the species in a community interact almost constantly, the fate of a particular population may be tightly linked to the other species that share its habitat. • Biologists analyze interactions among species by considering the effects on the fitness—the ability to survive and produce offspring—of the individuals involved. © 2011 Pearson Education, Inc. Species Interactions • A relationship between two species that provides a fitness benefit to members of one of the species is a + interaction. Such a relationship that hurts members of one of the species is a – interaction. A relationship that has no effect on the members of either species is a 0 interaction. © 2011 Pearson Education, Inc. Species Interactions • There are four general types of interactions among species in a community: 1. Competition occurs when individuals use the same resources—resulting in lower fitness for both (/). 2. Consumption occurs when one organism eats or absorbs nutrients from another, increasing the consumer’s fitness but decreasing the victim’s fitness (+/). 3. Mutualism occurs when two species interact in a way that confers fitness benefits to both (+/+). 4. Commensalism occurs when one species benefits but the other species is unaffected (+/0). © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Three Themes • As you analyze each type of species interaction, watch for three key themes: 1. Species interactions may affect the distribution and abundance of a particular species. 2. Species act as agents of natural selection when they interact. In biology, a coevolutionary arms race occurs between predators and prey, between parasites and hosts, and between other types of interacting species. 3. The outcome of interactions among species is dynamic and conditional. © 2011 Pearson Education, Inc. Competition • Competition is a –/– interaction that lowers the fitness of the individuals involved. When competitors use resources, those resources are not available to help individuals survive better and produce more offspring. • Intraspecific competition occurs between members of the same species. – Because intraspecific competition for resources intensifies as a population’s density increases, it is a major cause of densitydependent growth. • Interspecific competition occurs when members of different species use the same limiting resources. © 2011 Pearson Education, Inc. Using the Niche Concept to Analyze Competition • Early work on interspecific competition focused on the concept of the niche—the range of resources that the species is able to use or the range of conditions it can tolerate. • Interspecific competition occurs when the niches of two species overlap. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. When One Species Is a Better Competitor • The competitive exclusion principle, formulated by G. F. Gause, states that it is not possible for species within the same niche to coexist. • The hypothesis was inspired by a series of experiments Gause did with similar species of the unicellular pond-dweller Paramecium. – Grown in separate cultures, both species exhibited logistic growth. – When the two species grew in the same culture together, only one species exhibited logistic growth; the other species was driven to extinction. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. When One Species Is a Better Competitor • Asymmetric competition occurs when one species suffers a much greater fitness decline than the other. • In symmetric competition, each species experiences a roughly equal decrease in fitness. • If asymmetric competition occurs and the two species have completely overlapping niches, the stronger competitor is likely to drive the weaker competitor to extinction. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. When One Species Is a Better Competitor • Gause’s experiments illuminated an important distinction: 1. A species’ fundamental niche is the resources it uses or conditions it tolerates in the absence of competitors. 2. A species’ realized niche is the resources it uses or conditions it tolerates when competition occurs. • If asymmetric competition occurs and the niches of the two species do not overlap completely, the weaker competitor will move from its fundamental niche to a realized niche, ceding some resources to the stronger competitor. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Experimental Studies of Competition • Joseph Connell performed a series of experiments to test the competitive exclusion principle. • Connell used a common experimental strategy in competition studies—removing one of the competitors and observing the response by the remaining species. • Experimental evidence supports competitive exclusion of Chthamalus barnacles from the lower intertidal zone by Balanus barnacles. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Fitness Trade-Offs in Competition • The ability to compete for a particular resource is only one aspect of an organism’s niche. • If individuals are extremely good at competing for a particular resource, they are probably less good at enduring drought conditions, warding off disease, or preventing predation―there is a fitness trade-off. © 2011 Pearson Education, Inc. Mechanisms of Coexistence: Niche Differentiation • Because competition is a –/– interaction, there is strong natural selection on both species to avoid it. • The predicted eventual outcome is an evolutionary change in traits that reduces the amount of niche overlap and the amount of competition. • This change in resource use is called niche differentiation or resource partitioning. • The change in species’ traits is called character displacement. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Mechanisms of Coexistence: Niche Differentiation • Peter and Rosemary Grant recently documented character displacement in Galápagos finches. • After a severe drought in 1977, selection favored larger beak size in the medium ground finch, Geospiza fortis. – Only those individuals with larger beaks were able to crack open the fruits of their major food source, Tribulus cistoides. © 2011 Pearson Education, Inc. Mechanisms of Coexistence: Niche Differentiation • A second severe drought occurred in 2003; by this time the large ground finch, Geospiza magnirostris, had become established on the island. • The Grants’ measurements revealed that this time, only the smallest-beaked G. fortis individuals survived. • Data on feeding behavior indicated that G. magnirostris were outcompeting G. fortis for Tribulus cistoides; only G. fortis that could eat extremely small seeds efficiently could survive. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Competition and Conservation • One of the goals of conservation biology is to keep biological communities intact. • One of the major threats to communities is invasive species. • Recent experiments have shown that communities that contain a large number of different species are more resistant to invasion than communities with a smaller number of species. • In other words, competition can help communities resist invasion. © 2011 Pearson Education, Inc. Consumption • Consumption is a +/– interaction that occurs when one organism eats another. • There are three major types of consumption: 1. Herbivory is the consumption of plant tissues by herbivores. 2. Parasitism is the consumption of small amounts of tissues from another organism, or host, by a parasite. 3. Predation is the killing and consumption of most or all of another individual (the prey) by a predator. © 2011 Pearson Education, Inc. Constitutive Defenses • Natural selection strongly favors traits that allow individuals to avoid being eaten. • Constitutive or standing defenses are defenses that are always present and include: – Avoidance (hiding, with or without camouflage, or running, flying or swimming away). – Poison (many plants lace their tissues with compounds that are toxic to consumers). – Schooling and flocking behaviors that confuse predators. – Fighting back, with the use of weaponry or toxins. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Constitutive Defenses • Some of the best-studied constitutive defenses involve mimicry— the close resemblance of one species to another. • There are two forms of mimicry: 1. Müllerian mimicry is the resemblance of two harmful prey species. 2. Batesian mimicry is the resemblance of an innocuous prey species to a dangerous prey species. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Inducible Defenses • Although constitutive defenses can be extremely effective, they are expensive in terms of the energy and resources that must be devoted to producing and maintaining them. • Many prey species have inducible defenses—defensive traits produced only in response to the presence of a predator. • Inducible defenses are efficient energetically, but they are slow—it takes time to produce them. • For example, mussels have thicker shells and attach more strongly to a substrate only in the presence of crabs. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Inducible Defenses • The data on the mussels and crabs are correlational in nature, however, so they are open to interpretation and criticism by critics of the induced defense hypothesis. • Biologists tested the hypothesis more rigorously by conducting experiments on mussel thickness in and out of the presence of crabs. • Results support the hypothesis that mussels do in fact increase their investment in defense in the presence of crabs. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Can Animal Predators Reduce Prey Populations? • Species interactions have a strong impact on the evolution of predator and prey populations. • Experiments have supported the hypothesis that predators play an important role in the density-dependent growth of prey populations. • The data available to date indicate that in many instances, predators are efficient enough to reduce prey populations below carrying capacity. © 2011 Pearson Education, Inc. Why Don’t Herbivores Eat Everything? • Biologists recently conducted a meta-analysis—they compiled the results of more than 100 studies—and raised the question of why herbivores don’t eat more of the available plant food. • Biologists routinely consider two hypotheses to help answer the question of why herbivores do not eat more of the food available: 1. The top-down control hypothesis suggests that predation or disease limits herbivores. 2. The bottom-up limitation hypothesis suggests that plants provide poor nutrition or are well-defended against herbivory. © 2011 Pearson Education, Inc. Why Don’t Herbivores Eat Everything? • Cottonwood trees and two of their herbivores, beavers and leaf beetles, provide an example of both top-down and bottom-up controls on herbivory. • Top-down control, nitrogen limitation, and effective defense are all important factors in limiting the impact of herbivory. • The mix of factors that keeps the world green varies from species to species and habitat to habitat. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Adaptation and Arms Races • When consumers and prey interact over time, a coevolutionary arms race begins. – Consumers evolve traits that increase their efficiency. – In response, prey evolve traits that make them unpalatable or elusive. – This leads to selection on consumers for traits that counter the prey adaptation, and so on. • An example is the interaction between the parasite Plasmodium (the consumer) and their host humans (the prey). © 2011 Pearson Education, Inc. Adaptation and Arms Races • Plasmodium are unicellular protists that cause malaria, which kills at least a million people a year. • Recent data suggest that humans and Plasmodium are locked in a coevolutionary arms race. – In West Africa, the HLA-B53 allele confers protection against malaria by displaying a signal that induces an immune response. – Individuals with at least one HLA-B53 allele are better able to fight malarial infections. – However, some Plasmodium populations appear to have evolved resistance to these defenses. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Life Cycle of A Malaria Parasite Web Activity: Life Cycle of A Malaria Parasite © 2011 Pearson Education, Inc. Can Parasites Manipulate Their Hosts? • In some instances, parasites do manipulate their hosts. • For example, nematodes (roundworms) parasitize a species of treedwelling ants and lay eggs in the ant’s posterior-most body region, causing it to appear red instead of the normal black color. • Infected ants also hold the region up in a “flagging” posture, making them look like berries; as a result birds are more likely to feed on infected ants than uninfected ants. © 2011 Pearson Education, Inc. Can Parasites Manipulate Their Hosts? • The nematodes can only complete their life cycle inside the birds, before being shed in bird feces that are subsequently eaten by ants. • Biologists suggest that these nematodes not only change the appearance of the ants, they also manipulate their behavior, greatly increasing the likelihood that the parasite will be transmitted to a new host. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Using Consumers as Biocontrol Agents • Research on the dynamics of predator-prey interactions has given biologists the ability to control pests by introducing predators or parasites. • In agriculture and forestry, the use of predators and parasites as biocontrol agents is a key part of integrated pest management: strategies to maximize crop and forest productivity while using a minimum of insecticides or other types of potentially harmful compounds. © 2011 Pearson Education, Inc. Mutualisms • Mutualisms are +/+ interactions that involve a wide variety of organisms and rewards. Examples of mutualisms can be found between: – Flowering plants and their pollinators. – Mycorrhizal fungi and plant roots. – Bacteria that fix nitrogen and certain species of plants. – Rancher ants and aphids. – Farmer ants and fungi. – Crematogaster ants and acacia trees. – Cleaner shrimp and fish. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. The Role of Natural Selection in Mutualism • Even though mutualisms benefit both species, the interaction does not involve individuals from different species being altruistic. • The benefits received in a mutualism are a by-product of each individual pursuing its own self-interest by maximizing its ability to survive and reproduce. © 2011 Pearson Education, Inc. Mutualisms Are Dynamic • An experiment explored whether the relationship between ants and treehoppers is mutually beneficial. • The study found that the ants benefited by receiving food from the treehoppers, and the treehoppers benefited because the ants kept their predator, the jumping spiders, away. – However, when jumping spider populations were low, the treehoppers did not benefit. • Because the costs and benefits of species interactions are fluid, an interaction between the same two species may vary from parasitism to mutualism to competition. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Community Structure • Research on species interactions usually focuses on just two species at a time, but biological communities contain many thousands of species. • To understand how communities work, biologists explore how combinations of many species interact. © 2011 Pearson Education, Inc. How Predictable Are Communities? • Frederick Clements hypothesized that biological communities are stable, integrated, and orderly entities with a highly predictable composition. • Clements argued that communities develop by passing through a series of predictable stages dictated by extensive interactions among species, and that this development culminates in a stable final stage called a climax community. © 2011 Pearson Education, Inc. How Predictable Are Communities? • Henry Gleason, in contrast, contended that the community found in a particular area is neither stable nor predictable. • According to Gleason, it is largely a matter of chance whether a similar community develops in the same area after a disturbance occurs. © 2011 Pearson Education, Inc. Experimental Tests • A study of planktonic communities in experimental ponds showed that identical communities do not develop in identical habitats. Each pond had a unique species assemblage. • The overall message of research on community structure suggests that Clements’ position was too extreme; Gleason’s view is closer to accurate. • Although both biotic interactions and climate are important in determining which species exist at a certain site, chance and history also play a large role. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Mapping Current and Past Species’ Distributions • If communities are predictable assemblages, the same group of species should almost always be found growing together. • Historical data on plant communities showed that groups of species change their ranges independently of one another; fossil pollen studies suggest that plant community composition has always been dynamic, rather than static. Although both biotic interactions and climate are important in determining which species exist at a certain site, chance and history also play a large role. © 2011 Pearson Education, Inc. How Do Keystone Species Structure Communities? • Even though species are not predictable assemblages, the structure of a community can change dramatically if a single species of predator or herbivore is removed from or added to a community. • A keystone species is a species that has a much greater impact on the surrounding species than its abundance would suggest. • For example, the sea star Pisaster is a keystone species in some intertidal areas. When Pisaster was removed from experimental areas, the number of species present and the complexity of the habitat changed radically. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Community Dynamics • Like cells, individuals, and species, communities can be described in one word: dynamic. © 2011 Pearson Education, Inc. Disturbance and Change in Ecological Communities • Community composition and structure may change radically in response to changes in abiotic and biotic conditions. • A disturbance is any event that removes some individuals or biomass from a community. • The important feature of a disturbance is that it alters some aspect of resource availability. © 2011 Pearson Education, Inc. Disturbance and Change in Ecological Communities • The impact of disturbance is a function of three factors: 1. Type of disturbance. 2. Frequency of disturbance. 3. Severity of disturbance • Most communities experience a characteristic type of disturbance, and in most cases, disturbances occur with a predictable frequency and severity. – This is called a community's disturbance regime. © 2011 Pearson Education, Inc. Determining a Community’s Disturbance Regime • Ecologists use two approaches to determine the pattern of disturbance in a community: 1. Inference of long-term patterns from data obtained in shortterm analysis. 2. Reconstruction of the history of a particular site. © 2011 Pearson Education, Inc. The Importance of Understanding Disturbance Regimes • Biologists determined the history of disturbance in a fire-prone community by studying tree rings. • The results of this study established that fires are quite frequent in the community examined. • Biologists are now better able to manage these forests by allowing, monitoring, and controlling burns in them. • To maintain communities in good condition, biologists have to ensure that the normal disturbance regime occurs. Otherwise, community composition changes dramatically. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Succession • Succession is the recovery, the development of communities, that follows a severe disturbance. • Primary succession occurs when a disturbance removes the soil and its organisms, as well as organisms that live above the surface. • Secondary succession occurs when a disturbance removes some or all of the organisms from an area but leaves the soil intact. © 2011 Pearson Education, Inc. Succession • Early successional communities are dominated by species that are short lived and small in stature, and that disperse their seeds over long distances. • Late successional communities are dominated by species that tend to be long lived, large, and good competitors for resources such as light and nutrients. • The specific sequence of species that appears over time is called the successional pathway. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Succession • Three factors determine the pattern and rate of species replacement during succession at a particular time and place: 1. The particular traits of the species involved. 2. How the species interact. 3. Historical and environmental circumstances, such as the size of the area involved and weather conditions. © 2011 Pearson Education, Inc. The Role of Species Traits • Dispersal capability and the ability to withstand harsh conditions are particularly important early in succession. • Pioneering species, the first organisms to arrive at a newly disturbed site, tend to be “weedy”; weeds are plants adapted for growth in disturbed soils. • Early successional species devote most of their energy to reproduction and little to competitive ability. • These species have good dispersal ability, being able to tolerate severe abiotic conditions, and high reproductive rates. © 2011 Pearson Education, Inc. The Role of Species Interactions • Once colonization has begun, succession depends more on how species interact with each other. • During succession, existing species can have one of three effects on subsequent species: 1. Facilitation occurs when early-arriving species make conditions more favorable for the arrival of certain later species. 2. Tolerance happens when existing species do not affect the probability that subsequent species will become established. 3. Inhibition occurs when the presence of one species inhibits the establishment of another. © 2011 Pearson Education, Inc. The Role of Chance and History • In addition to species traits and species interactions, the pattern and rate of succession depend on the historical and environmental context in which they occur. • Succession is also affected by the particular weather or climate conditions that occur during the process. Variation in weather and climate causes different successional pathways to occur in the same place at different times. © 2011 Pearson Education, Inc. A Case History: Glacier Bay, Alaska • An extraordinarily rapid and extensive glacial recession is occurring at Glacier Bay, and it has thus become an important site for studying succession. • Originally researchers found one successional pathway, but more recent research has suggested that three successional pathways have occurred in this area. • Species traits and species interactions tend to make succession predictable, whereas history and chance events contribute a degree of unpredictability. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Succession Web Activity: Succession, Part 1 © 2011 Pearson Education, Inc. Species Richness in Ecological Communities • Species richness is the number of species present in a given community. • Species diversity is a weighted measure that incorporates a species’ relative abundance, as well as its presence or absence. © 2011 Pearson Education, Inc. Predicting Species Richness • The number of species is usually positively correlated with habitat size. However, islands in the ocean have smaller numbers of species than do areas of the same size on continents. • The number of species present on an island is a product of just two events: immigration and extinction. • Robert MacArthur and Edward O. Wilson contended that the rates of both of these processes should vary with the number of species present on an island. © 2011 Pearson Education, Inc. Predicting Species Richness • Immigration rates should decline as the number of species on the island increases because: – Individuals that arrive are more likely to represent a species that is already present. – Competition should prevent new species from becoming established when many species are already present on an island. • Extinction rates should increase as species richness increases, because niche overlap and competition for resources will be more intense. © 2011 Pearson Education, Inc. The Role of Island Size and Isolation • MacArthur and Wilson formulated the model called the theory of island biogeography. • Their theory makes two predictions—species richness should be higher on: 1. Larger islands than smaller islands. 2. Nearshore islands versus remote islands. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Applying the Theory • The theory of island biogeography is important because: 1. It is relevant to a wide variety of island-like habitats such as alpine meadows, lakes and ponds, caves, and habitats isolated by human development. 2. It is relevant to species that have metapopulation structure. 3. It made specific predictions that could be tested. 4. It can help inform decisions about the design of natural preserves. © 2011 Pearson Education, Inc. Global Patterns in Species Richness • Biologists have long understood that large habitat areas tend to be species rich, and the theory of island biogeography has been successful in framing thinking about how species richness should vary among island-like habitats. • Researchers have had a much more difficult time explaining what may be the most striking pattern in species richness: • In the mid-1800s, biologists recognized that communities in the tropics have more species than communities in temperate or subarctic environments. © 2011 Pearson Education, Inc. The Latitudinal Gradient Data compiled in the intervening years have confirmed the existence of a strong latitudinal gradient in species diversity—for communities as a whole as well as for many taxonomic groups. • To explain this pattern, biologists have had to consider two principles: 1. The causal mechanism must be abiotic. 2. The species diversity of a particular area is the sum of four processes: speciation, extinction, immigration, and emigration. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. The Latitudinal Gradient • Over 30 hypotheses have been suggested to explain the latitudinal gradient. Among them: 1. The high-productivity hypothesis proposes that high productivity promotes high diversity. 2. The energy hypothesis contends that high temperature increases productivity and the likelihood that organisms can tolerate the physical conditions in a region. 3. The area and age hypothesis argues that the tropical regions have had more time and space for speciation than other regions. © 2011 Pearson Education, Inc. The Latitudinal Gradient 4. The intermediate disturbance hypothesis states that regions with a moderate type, frequency, and severity of disturbance should have high species richness and diversity. • Each of these factors may influence diversity, but no single hypothesis offers a convincing explanation for the global diversity gradient. © 2011 Pearson Education, Inc.