Chapter 54 Community Ecology PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero
Overview: What Is a Community? A biological community Is an assemblage of populations of various species living close enough for potential interaction
The various animals and plants surrounding this watering hole Are all members of a savanna community in southern Africa Figure 53.1
Concept 54.1: 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
Interspecific interactions Can have differing effects on the populations involved Table 53.1
Competition Interspecific competition 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 The ecological niche Is the total of an organism s use of the biotic and abiotic resources in its environment
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 EXPERIMENT Ecologist Joseph Connell studied two barnacle species Balanus balanoides and Chthamalus stellatus that have a stratified distribution on rocks along the coast of Scotland. RESULTS When Connell removed Balanus from the lower strata, the Chthamalus population spread into that area. Chthamalus Balanus Chthamalus realized niche Balanus realized niche High tide High tide Chthamalus fundamental niche Ocean Low tide Ocean Low tide Figure 53.2 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. CONCLUSION The spread of Chthamalus when Balanus was removed indicates that competitive exclusion makes the realized niche of Chthamalus much smaller than its fundamental niche.
As a result of competition A species fundamental niche (could occupy) may be different from its realized niche (actually occupies)
Resource Partitioning Resource partitioning is the differentiation of niches That enables similar species to coexist in a community A. insolitus usually perches on shady branches. A. ricordii A. distichus perches on fence posts and other sunny surfaces. A. alinigar A. distichus A. insolitus A. christophei A. cybotes A. etheridgei Figure 53.3
Character Displacement In character displacement There is a tendency for characteristics to be more divergent in sympatric populations (same area) of two species than in allopatric populations (separated by some geographic barrier) of the same two species G. fuliginosa G. fortis Beak depth Figure 53.4 Percentages of individuals in each size class 40 20 0 40 20 0 40 20 0 Santa María, San Cristóbal Los Hermanos Daphne Sympatric populations G. fuliginosa, allopatric G. fortis, allopatric 8 10 12 14 16 Beak depth (mm)
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
Cryptic coloration, or camouflage Makes prey difficult to spot Figure 53.5
Aposematic coloration Warns predators to stay away from prey Figure 53.6
In some cases, one prey species May gain significant protection by mimicking the appearance of another
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
Parasitism In parasitism, one organism, the parasite Derives its nourishment from another organism, its host, which is harmed in the process
Parasitism exerts substantial influence on populations And the structure of communities
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 Figure 53.10
Commensal interactions have been difficult to document in nature Because any close association between species likely affects both species
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
Concept 54.2: 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
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
Two different communities Can have the same species richness, but a different relative abundance B A C D Community 1 A: 25% B: 25% C: 25% D: 25% Figure 53.11 Community 2 A: 80% B: 5% C: 5% D: 10%
A community with an even species abundance Is more diverse than one in which one or two species are abundant and the remainder rare
Trophic Structure Trophic structure Is the feeding relationships between organisms in a community Is a key factor in community dynamics
Food chains Link the trophic levels from producers to top carnivores Carnivore Carnivore Quaternary consumers Tertiary consumers Carnivore Carnivore Secondary consumers Carnivore Carnivore Primary consumers Herbivore Zooplankton Primary producers Figure 53.12 Plant A terrestrial food chain Phytoplankton A marine food chain
Food Webs A food web Humans Is a branching food chain with complex trophic interactions Baleen whales Crab-eater seals Smaller toothed whales Leopard seals Elephant seals Sperm whales Birds Fishes Squids Carnivorous plankton Euphausids (krill) Copepods Figure 53.13 Phytoplankton
Food webs can be simplified By isolating a portion of a community that interacts very little with the rest of the community Sea nettle Juvenile striped bass 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.
Most of the available data Support the energetic hypothesis 6 No. of species 6 Number of species 5 4 3 2 1 No. of trophic links 5 4 3 2 1 Number of trophic links 0 High (control) Medium Low 0 Figure 53.15 Productivity
Species with a Large Impact Certain species have an especially large impact on the structure of entire communities Either because they are highly abundant or because they play a pivotal role in community dynamics
Dominant Species Dominant species Are those species in a community that are most abundant or have the highest biomass Exert powerful control over the occurrence and distribution of other 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 Keystone species Are not necessarily abundant in a community Exert strong control on a community by their ecological roles, or niches
Field studies of sea stars Exhibit their role as a keystone species in intertidal communities Number of species present 20 15 10 5 0 With Pisaster (control) Without Pisaster (experimental) 1963 64 65 66 67 68 69 70 71 72 73 Figure 53.16a,b (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.
Observation of sea otter populations and their predation Shows the effect the otters have on ocean communities Grams per Otter number 0.25 m 2 (% max. count) 100 80 60 40 20 0 (a) Sea otter abundance 400 300 200 100 0 (b) Sea urchin biomass Number per 0.25 m 2 10 8 6 4 2 0 1972 1985 1989 1993 1997 Figure 53.17 Food chain before killer whale involvement in chain (c) Total kelp density Year 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
Beaver dams Can transform landscapes on a very large scale Figure 53.18
Some foundation species act as facilitators That have positive effects on the survival and reproduction of some of the other species in the community 8 Number of plant species 6 4 2 Figure 53.19 Salt marsh with Juncus (foreground) 0 With Juncus Conditions Without Juncus
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
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 Percentage of herbaceous plant cover 75 50 25 0 0 100 200 300 400 Figure 53.20 Rainfall (mm)
Pollution Can affect community dynamics But through biomanipulation Polluted communities can be restored Fish Polluted State Abundant Restored State Rare Zooplankton Rare Abundant Algae Abundant Rare
Concept 54.3: 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 Figure 53.21a c (a) Before a controlled burn. A prairie that has not burned for several years has a high proportion of detritus (dead grass). (b) During the burn. The detritus serves as fuel for fires. (c) After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.
The intermediate 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 Ecological succession Is the sequence of community and ecosystem changes after a disturbance
Primary succession Occurs where no soil exists when succession begins Secondary succession Begins in an area where soil remains after a disturbance
Early-arriving species May facilitate the appearance of later species by making the environment more favorable May inhibit establishment of later species May tolerate later species but have no impact on their establishment
Retreating glaciers Provide a valuable field-research opportunity on succession Grand Pacific Gl. Canada Alaska 0 5 10 1940 1912 1948 Miles 1899 1941 1907 1879 1948 1931 1911 1935 1879 1949 1879 1900 1892 1913 Reid Gl. 1879 1860 Johns Hopkins Gl. Glacier Bay 1830 1780 1760 Pleasant Is. Figure 53.23 McBride glacier retreating
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 50 Soil nitrogen (g/m 2 ) 40 30 20 10 Figure 53.24a d 0 Pioneer Dryas Alder Spruce Successional stage (d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content. (c) Spruce stage
Concept 54.4: Biogeographic factors affect community diversity Two key factors correlated with a community s species diversity Are its geographic location and its size
Equatorial-Polar Gradients The two key factors in equatorial-polar gradients of species richness Are probably evolutionary history and climate
Species richness generally declines along an equatorial-polar gradient And is especially great in the tropics The greater age of tropical environments May account for the greater species richness
Climate Is likely the primary cause of the latitudinal gradient in biodiversity
The two main climatic factors correlated with biodiversity Are solar energy input and water availability Tree species richness 180 160 140 120 100 80 60 40 20 0 100 300 500 700 900 1,100 Actual evapotranspiration (mm/yr) (a) Trees Vertebrate species richness (log scale) 200 100 50 10 1 (b) Vertebrates 500 1,000 1,500 2,000 Potential evapotranspiration (mm/yr) Figure 53.25a, b
Area Effects The species-area curve quantifies the idea that All other factors being equal, the larger the geographic area of a community, the greater the number of species
A species-area curve of North American breeding birds Supports this idea 1,000 Number of species (log scale) 100 10 Figure 53.26 1 1 10 100 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 Area (acres)
Island Equilibrium Model Species richness on islands Depends on island size, distance from the mainland, immigration, and extinction
The equilibrium model of island biogeography maintains that Species richness on an ecological island levels off at some dynamic equilibrium point Rate of immigration or extinction Rate of immigration or extinction Rate of immigration or extinction Equilibrium number Small island Large island Far island Near island Figure 53.27a c Number of species on island (a) Immigration and extinction rates. The equilibrium number of species on an island represents a balance between the immigration of new species and the extinction of species already there. Number of species on island (b) Effect of island size. Large islands may ultimately have a larger equilibrium number of species than small islands because immigration rates tend to be higher and extinction rates lower on large islands. Number of species on island (c) Effect of distance from mainland. Near islands tend to have larger equilibrium numbers of species than far islands because immigration rates to near islands are higher and extinction rates lower.
Studies of species richness on the Galápagos Islands Support the prediction that species richness increases with island size FIELD STUDY Ecologists Robert MacArthur and E. O. Wilson studied the number of plant species on the Galápagos Islands, which vary greatly in size, in relation to the area of each island. RESULTS 400 200 Number of plant species (log scale) 100 50 25 10 5 0 0.1 1 10 100 1,000 Area of island (mi 2 ) (log scale) Figure 53.28 CONCLUSION The results of the study showed that plant species richness increased with island size, supporting the species-area theory.
Concept 54.5: Contrasting views of community structure are the subject of continuing debate Two different views on community structure Emerged among ecologists in the 1920s and 1930s
Integrated and Individualistic Hypotheses The integrated hypothesis of community structure Describes a community as an assemblage of closely linked species, locked into association by mandatory biotic interactions
The individualistic hypothesis of community structure Proposes that communities are loosely organized associations of independently distributed species with the same abiotic requirements
The integrated hypothesis Predicts that the presence or absence of particular species depends on the presence or absence of other species Population densities of individual species Environmental gradient (such as temperature or moisture) Figure 53.29a (a) Integrated hypothesis. Communities are discrete groupings of particular species that are closely interdependent and nearly always occur together.
The individualistic hypothesis Predicts that each species is distributed according to its tolerance ranges for abiotic factors Population densities of individual species Figure 53.29b Environmental gradient (such as temperature or moisture) (b) Individualistic hypothesis. Species are independently distributed along gradients and a community is simply the assemblage of species that occupy the same area because of similar abiotic needs.
In most actual cases the composition of communities Seems to change continuously, with each species more or less independently distributed Number of plants per hectare 600 400 200 0 Wet Moisture gradient Dry Figure 53.29c (c) Trees in the Santa Catalina Mountains. The distribution of tree species at one elevation in the Santa Catalina Mountains of Arizona supports the individualistic hypothesis. Each tree species has an independent distribution along the gradient, apparently conforming to its tolerance for moisture, and the species that live together at any point along the gradient have similar physical requirements. Because the vegetation changes continuously along the gradient, it is impossible to delimit sharp boundaries for the communities.
Rivet and Redundancy Models The rivet model of communities Suggests that all species in a community are linked together in a tight web of interactions Also states that the loss of even a single species has strong repercussions for the community
The redundancy model of communities Proposes that if a species is lost from a community, other species will fill the gap
It is important to keep in mind that community hypotheses and models Represent extremes, and that most communities probably lie somewhere in the middle