Chapter 53 Community Ecology

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

Populations are linked by interspecific interactions Concept 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

Interspecific interactions Can have differing effects on the populations involved Table 53.1

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 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 When Connell removed Balanus from the lower strata, the Chthamalus population spread into that area. The spread of Chthamalus when Balanus was removed indicates that competitive exclusion makes the realized niche of Chthamalus much smaller than its fundamental niche. RESULTS CONCLUSION Ocean Ecologist Joseph Connell studied two barnacle speciesBalanus balanoides and Chthamalus stellatus that have a stratified distribution on rocks along the coast of Scotland. EXPERIMENT 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. Low tide High tide Chthamalus fundamental niche Chthamalus realized niche Chthamalus Balanus realized niche Balanus Figure 53.2

As a result of competition A species’ fundamental niche may be different from its realized niche

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. distichus perches on fence posts and other sunny surfaces. A. distichus A. ricordii A. insolitus A. christophei A. cybotes A. etheridgei A. alinigar Figure 53.3

Character Displacement 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 G. fortis Beak depth (mm) G. fuliginosa Beak depth Los Hermanos Daphne Santa María, San Cristóbal Sympatric populations G. fuliginosa, allopatric G. fortis, allopatric Percentages of individuals in each size class 40 20 8 10 12 14 16 Figure 53.4

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 (a) Hawkmoth larva (b) Green parrot snake Figure 53.7a, b

In Müllerian mimicry Two or more unpalatable species resemble each other (a) Cuckoo bee (b) Yellow jacket Figure 53.8a, b

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

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

The effects of disease on populations and communities Is similar to that of parasites

Pathogens, disease-causing agents Are typically bacteria, viruses, or protists

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

In general, a small number of species in a community Concept 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

The species diversity of a community Is the variety of different kinds of organisms that make up the community Has two components

Species richness Relative abundance 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 Community 1 A: 25% B: 25% C: 25% D: 25% Community 2 A: 80% B: 5% C: 5% D: 10% D C B A Figure 53.11

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

A terrestrial food chain Food chains Quaternary consumers Tertiary consumers Secondary consumers Primary consumers Primary producers Carnivore Herbivore Plant Zooplankton Phytoplankton A terrestrial food chain A marine food chain Figure 53.12 Link the trophic levels from producers to top carnivores

Smaller toothed whales Food Webs A food web Humans Baleen whales Crab-eater seals Birds Fishes Squids Leopard seals Elephant Smaller toothed whales Sperm whales Carnivorous plankton Euphausids (krill) Copepods Phyto- plankton Figure 53.13 Is a branching food chain with complex trophic interactions

Food webs can be simplified By isolating a portion of a community that interacts very little with the rest of the community Sea nettle Fish larvae Zooplankton Fish eggs Juvenile striped bass Figure 53.14

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 High (control) Medium Low Productivity No. of species No. of trophic links Number of species Number of trophic links 1 2 3 4 5 6 Figure 53.15

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

Number of species present Field studies of sea stars Exhibit their role as a keystone species in intertidal communities (a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates. With Pisaster (control) Without Pisaster (experimental) Number of species present 5 10 15 20 1963 ´64 ´65 ´66 ´67 ´68 ´69 ´70 ´71 ´72 ´73 (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. Figure 53.16a,b

Otter number (% max. count) Observation of sea otter populations and their predation Figure 53.17 Food chain before killer whale involve- ment in chain (a) Sea otter abundance (b) Sea urchin biomass (c) Total kelp density Number per 0.25 m2 1972 1985 1989 1993 1997 2 4 6 8 10 100 200 300 400 Grams per 0.25 m2 Otter number (% max. count) 40 20 60 80 Year Food chain after killer whales started preying on otters Shows the effect the otters have on ocean communities

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 Salt marsh with Juncus (foreground) With Juncus Without Juncus Number of plant species 2 4 6 8 Conditions Figure 53.19

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

Percentage of herbaceous plant cover Long-term experiment studies have shown That communities can shift periodically from bottom-up to top-down Figure 53.20 100 200 300 400 Rainfall (mm) 25 50 75 Percentage of herbaceous plant cover

But through biomanipulation Pollution Can affect community dynamics But through biomanipulation Polluted communities can be restored Fish Zooplankton Algae Abundant Rare Polluted State Restored State

Concept 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 burned for several years has a high propor- tion 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. Figure 53.21a–c

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. (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. Figure 53.22a, b

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 Is the sequence of community and ecosystem changes after a disturbance

Primary succession Secondary 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 McBride glacier retreating 5 10 Miles Glacier Bay Pleasant Is. Johns Hopkins Gl. Reid Gl. Grand Pacific Gl. Canada Alaska 1940 1912 1899 1879 1949 1935 1760 1780 1830 1860 1913 1911 1892 1900 1907 1948 1931 1941 Casement Gl. McBride Gl. Plateau Gl. Muir Gl. Riggs Gl. Figure 53.23

Succession on the moraines in Glacier Bay, Alaska Follows a predictable pattern of change in vegetation and soil characteristics Figure 53.24a–d (b) Dryas stage (c) Spruce stage (d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content. Soil nitrogen (g/m2) Successional stage Pioneer Dryas Alder Spruce 10 20 30 40 50 60 (a) Pioneer stage, with fireweed dominant

Concept 53.4: 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

Vertebrate species richness (log scale) The two main climatic factors correlated with biodiversity Are solar energy input and water availability (b) Vertebrates 500 1,000 1,500 2,000 Potential evapotranspiration (mm/yr) 10 50 100 200 Vertebrate species richness (log scale) 1 300 700 900 1,100 180 160 140 120 80 60 40 20 Tree species richness (a) Trees Actual evapotranspiration (mm/yr) Figure 53.25a, b

The species-area curve quantifies the idea that 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

Number of species (log scale) A species-area curve of North American breeding birds Supports this idea Area (acres) 1 10 100 103 104 105 106 107 108 109 1010 Number of species (log scale) 1,000 Figure 53.26

Island Equilibrium Model Species richness on islands Depends on island size, distance from the mainland, immigration, and extinction

Number of species on island The equilibrium model of island biogeography maintains that Species richness on an ecological island levels off at some dynamic equilibrium point 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. (b) Effect of island size. Large islands may ultimately have a larger equilibrium num- ber of species than small islands because immigration rates tend to be higher and extinction rates lower on large islands. (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. Equilibrium number Small island Large island Far island Near island Immigration Extinction (small island) (large island) (far island) (near island) Rate of immigration or extinction Figure 53.27a–c

Area of island (mi2) (log scale) Studies of species richness on the Galápagos Islands Support the prediction that species richness increases with island size The results of the study showed that plant species richness increased with island size, supporting the species-area theory. FIELD STUDY RESULTS 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. CONCLUSION 200 100 50 25 10 Area of island (mi2) (log scale) Number of plant species (log scale) 0.1 1 1,000 5 400 Figure 53.28

Two different views on community structure Concept 53.5: 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

Population densities of individual species 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) (a) Integrated hypothesis. Communities are discrete groupings of particular species that are closely interdependent and nearly always occur together. Figure 53.29a

Population densities of individual species The individualistic hypothesis Predicts that each species is distributed according to its tolerance ranges for abiotic factors Population densities of individual species 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. Figure 53.29b

Number of plants per hectare In most actual cases the composition of communities Seems to change continuously, with each species more or less independently distributed 600 Number of plants per hectare 400 200 Wet Moisture gradient Dry (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. Figure 53.29c

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