Ch 53 Introduction, community ecology

1. Ch 53 Introduction, community ecology • 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.

2. 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 (survival, reproduction)

3. Species Interactions • There are four general types of interactions among species in a community: • Competition occurs when individuals use the same resources—resulting in lower fitness for both (/). • Consumption occurs when one organism eats or absorbs nutrients from another, increasing the consumer’s fitness but decreasing the victim’s fitness (+/). • Mutualism occurs when two species interact in a way that confers fitness benefits to both (+/+). • Commensalism occurs when one species benefits but the other species is unaffected (+/0).

5. Three Themes • As you analyze each type of species interaction, watch for three key themes: • Species interactions may affect the distribution and abundance of a particular species. • 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. • The outcome of interactions among species is dynamic

6. 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 density-dependent growth. • Interspecific competition occurs when members of different species use the same limiting resources.

7. 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.

9. 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.

11. 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.

13. When One Species Is a Better Competitor • Gause’s experiments illuminated an important distinction: • A species’ fundamental niche is the resources it uses or conditions it tolerates in the absence of competitors. • 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.

15. 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.

16. 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.

18. 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.

19. Consumption • Consumption is a +/– interaction that occurs when one organism eats another. • There are three major types of consumption: • Herbivory is the consumption of plant tissues by herbivores. • Parasitism is the consumption of small amounts of tissues from another organism, or host, by a parasite. • Predation is the killing and consumption of most or all of another individual (the prey) by a predator.

20. Constitutive Defenses • 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.

21. 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: • Müllerian mimicry is the resemblance of two harmful prey species. • Batesian mimicry is the resemblance of an innocuous prey species to a dangerous prey species.

23. 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.

24. Why Don’t Herbivores Eat Everything? • Biologists recently conducted a meta-analysis—theycompiled 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: • The top-down control hypothesis suggests that predation or disease limits herbivores. • The bottom-up limitation hypothesis suggests that plants provide poor nutrition or are well-defended against herbivory.

25. 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.

26. 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.

27. 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.

29. 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.

30. 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. • 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.

31. 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.

33. 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.

34. Disturbance and Change in Ecological Communities • The impact of disturbance is a function of three factors: • Type of disturbance. • Frequency of disturbance. • 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.

35. 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.

38. 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.

39. 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.

42. Succession • Three factors determine the pattern and rate of species replacement during succession at a particular time and place: • The particular traits of the species involved. • How the species interact. • Historical and environmental circumstances, such as the size of the area involved and weather conditions.

43. 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.

44. 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: • Facilitation occurs when early-arriving species make conditions more favorable for the arrival of certain later species. • Tolerance happens when existing species do not affect the probability that subsequent species will become established. • Inhibition occurs when the presence of one species inhibits the establishment of another.

45. 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.

47. 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.

48. 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.

49. 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.

50. 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: • Larger islands than smaller islands. • Nearshore islands versus remote islands.