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Chapter 53 (pgs. 1174 – 1196) Community Ecology AP minknow •The difference between a fundamental niche and a realized niche •The role of competitive exclusion in interspecific competition. •The symbiotic relationships of parasitism, mutualism, and commensalism. •The impact of keystone species on community structure. •The difference between primary and secondary succession. • • • • • • • • • • • What Is a Community? 1.Explain the relationship between species richness and relative abundance. 2.Define and compare the individualistic hypothesis of H.A. Gleason and the interactive hypothesis of F.E. Clements with respect to communities. Interspecific Interactions and Community Structure 3.List four possible specific interactions and explain how the relationships affect the population densities of the two species. 4.Explain how interspecific competition may affect community structure. 5.Describe the competitive exclusion principle and explain how competitive exclusion may affect community structure. 6.Define an ecological niche and restate the competitive exclusion principle using the niche concept. 7.Explain how resource partitioning can affect species diversity. 8.Define and compare predation, herbivory, and parasitism. 9.Relate some specific predatory adaptations to the properties of the prey. 10.Describe the defense mechanisms that evolved in plants to reduce predation by herbivores. • 11.Explain how cryptic coloration and warning coloration aid an animal in avoiding predators. • 12.Distinguish between Batesian mimicry and Müllerian mimicry. • 13.Describe how predators use mimicry to obtain prey. • 14.Distinguish among endoparasites, ectoparasites, and pathogens. • 15.Distinguish among parasitism, mutualism, and commensalism. • 16.Distinguish between a food chain and a food web. Describe the factors that transform food chains into food webs. • 17.Describe two ways to simplify food webs. • 18.Summarize two hypotheses that explain why food chains are relatively short. • 19.Explain how dominant and keystone species exert strong control on community structure. Give several examples of each. • 20.Describe and distinguish between the bottom-up and topdown models of community organization. Also describe some models that are intermediate between those two extremes. • Disturbance and Community Structure • 21.Describe how disturbances affect community structure and composition. Illustrate this point with several well-studied examples. • 22.Give examples of humans as widespread agents of disturbance. • 23.Describe and distinguish between primary and secondary succession. • 24.Describe and distinguish among facilitation, inhibition, and toleration. • 25.Describe the process and pattern of succession on moraines in Glacier Bay. • Biogeographic Factors Affecting the Biodiversity of Communities • 26.Describe and distinguish between species richness and relative abundance. • 27.Describe the data necessary to measure biodiversity. • 28.Describe and explain how species richness varies along the equatorial-polar gradient. • 29.Define the species-area curve. • 30.Explain how species richness on islands varies according to island size and distance from the mainland. What Is a Community? • A biological community – Is an assemblage of populations of various species living close enough for potential •The various animals and plants surrounding interaction this watering hole •Are all members of a savanna community in southern Africa 53.1: A community’s interactions include competition, predation, herbivory, symbiosis, and disease • Populations are linked by interspecific interactions – That affect the survival and reproduction of the species engaged in the interaction – These interaction can have differing effects on the populations involved Competition • Interspecific competition Click on picture to watch movie – Occurs when species compete for a particular resource that is in short supply • Strong competition can lead to competitive exclusion – The local elimination of one of the two competing species The Competitive Exclusion Principle The competitive exclusion principle States that two species competing for the same limiting resources cannot coexist in the same place Ecological Niches • Habitat - the area where an organism lives, including the biotic and abiotic factors that affect the organism. • Resources - any necessity of life, such as water, nutrients, light, food, or space. Habitat + Resources = ????? • The ecological niche – Is the total of an organism’s use of the biotic and abiotic resources in its environment Warbler Niches • Can you have two separate organisms occupying the same exact niche??? NO •The niche concept allows restatement of the competitive exclusion principle •Two species cannot coexist in a community if their niches are identical However, ecologically similar species can coexist in a community – If there are one or more significant difference in their niches As a result of competition A species’ fundamental niche may be different from its realized niche Ecologist Joseph Connell studied EXPERIMENT two barnacle speciesBalanus balanoides and Chthamalus stellatus that have a stratified distribution on rocks along the coast of Scotland. When Connell removed Balanus from the lower strata, the Chthamalus population spread into that area. RESULTS High tide High tide Chthamalus Balanus Chthamalus realized niche Chthamalus fundamental niche Balanus realized niche Ocean Ocean Low tide Low tide In nature, Balanus fails to survive high on the rocks because it is unable to resist desiccation (drying out) during low tides. Its realized niche is therefore similar to its fundamental niche. In contrast, Chthamalus is usually concentrated on the upper strata of rocks. To determine the fundamental of niche of Chthamalus, Connell removed Balanus from the lower strata. The spread of Chthamalus when Balanus was removed indicates that competitive exclusion makes the realized niche of Chthamalus much smaller than its fundamental niche. CONCLUSION Resource Partitioning • Resource partitioning is the differentiation of niches – That enables similar species to coexist in a A. insolitus community usually perches on shady branches. A. ricordii A. distichus perches on fence posts and other sunny surfaces. A. insolitus A. alinigar A. distichus A. christophei A. cybotes A. etheridgei Character Displacement G. fortis G. fuliginosa • In character displacement – – There is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species Sympatric population – geographically overlapping Allopatric population – geographically isolated Figure 53.4 Santa María, San Cristóbal 40 Percentages of individuals in each size class – Beak depth Sympatric populations 20 0 Los Hermanos 40 G. fuliginosa, allopatric 20 0 Daphne 40 G. fortis, allopatric 20 0 8 10 12 Beak depth (mm) 14 16 Predation • Predation refers to an interaction – Where one species, the predator, kills and eats the other, the prey •Feeding adaptations of predators include •Claws, teeth, fangs, stingers, and poison •Animals also display •A great variety of defensive adaptations Defensive Adaptations • • • • Cryptic coloration Aposematic coloration Batesian mimicry Mullerian mimicry Cryptic coloration, or camouflage Makes prey difficult to spot Figure 53.5 Aposematic coloration – Warns predators to stay away from prey – Poison arrow frog Figure 53.6 In Batesian mimicry – A palatable or harmless species mimics an unpalatable or harmful model (b) Green parrot snake Figure 53.7a, b (a) Hawkmoth larva In Müllerian mimicry Two or more unpalatable species resemble each other (a) Cuckoo bee Figure 53.8a, b (b) Yellow jacket Herbivory • Herbivory, the process in which an herbivore eats parts of a plant – Has led to the evolution of plant mechanical and chemical defenses and consequent adaptations by herbivores Community Interactions • Symbiosis – any relationship in which two species live closely together. – There are three types of Symbiosis • Parasitism – Disease • Mutualism • Commensalism Parasitism • In parasitism, one organism, the parasite – Derives its nourishment from another organism, its host, which is harmed in the process Parasitism • Parasitism exerts substantial influence on populations – And the structure of communities – Parasite – Host • Endoparasites • Ectoparasites • Parasitoidism Disease • The effects of disease on populations and communities – Is similar to that of parasites Pathogens, disease-causing agents Are typically bacteria, viruses, or protists Mutualism • Mutualistic symbiosis, or mutualism – Is an interspecific interaction that benefits both species Figure 53.9 Commensalism • In commensalism – One species benefits and the other is not affected • Commensal interactions have been difficult to document in nature – Because any close association between species likely affects both species Figure 53.10 Interspecific Interactions and Adaptation • Evidence for coevolution – Which involves reciprocal genetic change by interacting populations, is scarce •However, generalized adaptation of organisms to other organisms in their environment •Is a fundamental feature of life 53.2: Dominant and keystone species exert strong controls on community structure • In general, a small number of species in a community – Exert strong control on that community’s structure • Dominant Species – Those species in a community that have the highest abundance or highest biomass. These species exert a powerful control over the occurrence and distribution of other species. • Keystone Species – A species that is not necessarily abundant in a community yet exerts strong control on community structure by the nature of its ecological role or niche Species Diversity • The species diversity of a community – Is the variety of different kinds of organisms that make up the community – Has two components • Species richness – Is the total number of different species in the community • Relative abundance – Is the proportion each species represents of the total individuals in the community Species Diversity • Two different communities – Can have the same species richness, but a different relative abundance A B A community with an even species abundance Is more diverse than one in which one or two species are abundant and the remainder rare Figure 53.11 C D A: 25% Community 1 B: 25% C: 25% D: 25% A: 80% Community 2 B: 5% C: 5% D: 10% Trophic Structure • Trophic structure – Is the feeding relationships between organisms in a community – Is a key factor in community dynamics • We can look at trophic structure through – Food Chains – Food Webs Food chains – Link the trophic levels from producers to top carnivores – Help to show the flow of energy through an ecosystem Quaternary consumers Carnivore Carnivore Tertiary consumers Carnivore Carnivore Secondary consumers Carnivore Carnivore Primary consumers Zooplankton Herbivore Primary producers Plant Figure 53.12 A terrestrial food chain Phytoplankton A marine food chain Humans Food Webs – Is a branching food chain with complex trophic interactions Smaller toothed whales Baleen whales Crab-eater seals Birds Sperm whales Elephant seals Leopard seals Fishes Squids Carnivorous plankton Copepods Euphausids (krill) Phytoplankton Figure 53.13 Food Webs • Food webs can be simplified – By isolating a portion of a community that interacts very little with the rest of the community Juvenile striped bass Sea nettle Fish larvae Figure 53.14 Fish eggs Zooplankton Limits on Food Chain Length • Each food chain in a food web – Is usually only a few links long • There are two hypotheses – That attempt to explain food chain length •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 •Proposes that long food chains are less stable than short ones Limits on Food Chain Lengths • Most of the available data – Support the energetic hypothesis 6 Figure 53.15 No. of species 5 No. of trophic links 4 5 4 3 3 2 2 1 1 0 0 High (control) Medium Productivity Low Number of trophic links Reduction of energy input in a tree-hole community (Queensland, Australia) had a direct affect on the length of the food chain Number of species 6 Species with a Large Impact • Certain species have an especially large impact on the structure of entire communities – Either because they are highly abundant r because they play a pivotal role in community dynamics • Dominant Species • Keystone Species Dominant Species • One hypothesis suggests that dominant species – Are most competitive in exploiting limited resources • Another hypothesis for dominant species success – Is that they are most successful at avoiding predators Keystone Species • Field studies of sea stars Number of species present – Exhibit their role as a keystone species in intertidal communities With Pisaster (control) 20 15 10 Without Pisaster (experimental) 5 0 1963 ´64 ´65 ´66 ´67 ´68 ´69 ´70 ´71 ´72 ´73 (a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates. (b) When Pisaster was removed from an intertidal zone, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which Pisaster was not removed, there was little change in species diversity. Keystone Species • Observation of sea otter populations and their predation Otter number (% max. count) 100 – Shows the effect the otters have on ocean communities 80 60 40 20 0 (a) Sea otter abundance Grams per 0.25 m2 400 300 200 100 0 Number per 0.25 m2 (b) Sea urchin biomass 10 8 6 4 2 0 1972 1985 1989 Year Figure 53.17 Food chain before killer whale involvement in chain (c) Total kelp density 1993 1997 Food chain after killer whales started preying on otters Ecosystem “Engineers” (Foundation Species) • Some organisms exert their influence – By causing physical changes in the environment that affect community structure Foundation Species • Some foundation species act as facilitators Number of plant species – That have positive effects on the survival and reproduction of some of the other species in the community 8 6 4 2 0 With Juncus Salt marsh with Juncus (foreground) Without Juncus Conditions Bottom-Up and Top-Down Controls • The bottom-up model of community organization – Proposes a unidirectional influence from lower to higher trophic levels • In this case, the presence or absence of abiotic nutrients – Determines community structure, including the abundance of primary producers Bottom-Up and Top-Down Controls • The top-down model of community organization – Proposes that control comes from the trophic level above • In this case, predators control herbivores – Which in turn control primary producers Long-term experiment studies have shown That communities can shift periodically from bottom-up to top-down 100 Rainfall determines community controls in this Chilean desert comm. (Non-El Nino) Dry (El Nino) Wet Percentage of herbaceous plant cover Bottom-Up 75 50 25 Top-Down 0 0 100 200 Rainfall (mm) 300 400 Pollution – Can affect community dynamics • But through biomanipulation – Polluted communities can be restored Polluted State Fish Restored State Abundant Rare Zooplankton Rare Abundant Algae Abundant Rare 53.3: Disturbance influences species diversity and composition • Decades ago, most ecologists favored the traditional view – That communities are in a state of equilibrium • However, a recent emphasis on change has led to a nonequilibrium model – Which describes communities as constantly changing after being buffeted by disturbances What Is Disturbance? • A disturbance – Is an event that changes a community – Removes organisms from a community – Alters resource availability Fire - Is a significant disturbance in most terrestrial ecosystems – Is often a necessity in some communities (a) Before a controlled burn. A prairie that has not (b) During the burn. The burned for detritus several years has a high serves as fuel for fires. proportion of detritus (dead grass). •The intermediate (c) After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living. disturbance hypothesis •Suggests that moderate levels of disturbance can foster higher species diversity than low levels of disturbance The large-scale fire in Yellowstone National Park in 1988 – Demonstrated that communities can often respond very rapidly to a massive disturbance (a) Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance. Figure 53.22a, b ( b) One year after fire. This photo of the same general area taken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground. Human Disturbance • Humans – Are the most widespread agents of disturbance • Human disturbance to communities – Usually reduces species diversity • Humans also prevent some naturally occurring disturbances – Which can be important to community structure Ecological Succession • Ecosystems are constantly in flux. Ecological Succession – is the series of predictable changes that occurs in an ecosystem over time. • Primary succession • Secondary succession Primary Succession • Occurs on surfaces where no soil exists, usually after a volcanic eruption. (receding glaciers) – 1. Bare rock community is populated by an pioneer species (first species to populate an area). Usually lichens (fungus and alga). – 2. Pioneer species help to form soil and puts nutrients into soil. – 3. Plants begin to grow then off to the races Secondary Succession • Occurs in an community where everything has been removed but the soil. • What could cause the process of primary succession to begin? Succession on the moraines in Glacier Bay, Alaska – Follows a predictable pattern of change in vegetation and soil characteristics (a) Pioneer stage, with fireweed dominant (b) Dryas stage 60 Soil nitrogen (g/m2) 50 40 30 20 10 0 Pioneer Dryas Alder Spruce Successional stage (d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content. (c) Spruce stage Further Your Information • Read 53.4 and 53.5 Page 1175 Page 1180