53 communities text

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

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

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 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 Low tide High tide Chthamalus Balanus realized niche Balanus Ocean 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 0 40 20 0 40 20 0 8 10 12 14 16 Figure 53.4

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 (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 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 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

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 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

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

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

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 0 1 2 3 4 5 6 0 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

Field studies of sea stars Exhibit their role as a keystone species in intertidal communities Figure 53.16a,b (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 0 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.

Observation of sea otter populations and their predation Shows the effect the otters have on ocean communities 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 m 2 1972 1985 1989 1993 1997 0 2 4 6 8 10 0 100 200 300 400 Grams per 0.25 m 2 Otter number (% max. count) 0 40 20 60 80 100 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 Figure 53.19 Salt marsh with Juncus (foreground) With Juncus Without Juncus Number of plant species 0 2 4 6 8 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

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 Figure 53.20 0 100 200 300 400 Rainfall (mm) 0 25 50 75 100 Percentage of herbaceous plant cover

Pollution Can affect community dynamics But through biomanipulation Polluted communities can be restored Fish Zooplankton Algae Abundant Rare Rare Abundant 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 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. 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 Figure 53.22a, b (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.

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 Figure 53.23 McBride glacier retreating 0 5 10 Miles Glacier Bay Pleasant Is. Johns Hopkins Gl. Reid Gl. Grand Pacific Gl. Canada Alaska 1940 1912 1899 1879 1879 1949 1879 1935 1760 1780 1830 1860 1913 1911 1892 1900 1879 1907 1948 1931 1941 1948 Casement Gl. McBride Gl. Plateau Gl. Muir Gl. Riggs Gl.

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/m 2 ) Successional stage Pioneer Dryas Alder Spruce 0 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

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) 1 100 300 500 700 900 1,100 180 160 140 120 100 80 60 40 20 0 Tree species richness (a) Trees Actual 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 Area (acres) 1 10 100 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 Number of species (log scale) 1 10 100 1,000 Figure 53.26

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 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. (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 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. Equilibrium number Small island Large island Far island Near island Immigration Extinction Extinction Immigration Extinction Immigration (small island) (large island) (large island) (small island) Immigration Extinction Immigration (far island) (near island) (near island) (far island) Extinction Rate of immigration or extinction Rate of immigration or extinction Rate of immigration or extinction

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 0 Area of island (mi 2 ) (log scale) Number of plant species (log scale) 0.1 1 10 100 1,000 5 400 Figure 53.28

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

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

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

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 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. 0 200 400 600 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

Overview: Ecosystems, Energy, and Matter An ecosystem consists of all the organisms living in a community As well as all the abiotic factors with which they interact

Ecosystems can range from a microcosm, such as an aquarium To a large area such as a lake or forest Figure 54.1

Regardless of an ecosystem’s size Its dynamics involve two main processes: energy flow and chemical cycling Energy flows through ecosystems While matter cycles within them

Concept 54.1: Ecosystem ecology emphasizes energy flow and chemical cycling Ecosystem ecologists view ecosystems As transformers of energy and processors of matter

Ecosystems and Physical Laws The laws of physics and chemistry apply to ecosystems Particularly in regard to the flow of energy Energy is conserved But degraded to heat during ecosystem processes

Trophic Relationships Energy and nutrients pass from primary producers (autotrophs) To primary consumers (herbivores) and then to secondary consumers (carnivores)

Energy flows through an ecosystem Entering as light and exiting as heat Figure 54.2 Microorganisms and other detritivores Detritus Primary producers Primary consumers Secondary consumers Tertiary consumers Heat Sun Key Chemical cycling Energy flow

Nutrients cycle within an ecosystem

Decomposition Decomposition Connects all trophic levels

Detritivores, mainly bacteria and fungi, recycle essential chemical elements By decomposing organic material and returning elements to inorganic reservoirs Figure 54.3

Concept 54.2: Physical and chemical factors limit primary production in ecosystems Primary production in an ecosystem Is the amount of light energy converted to chemical energy by autotrophs during a given time period

Ecosystem Energy Budgets The extent of photosynthetic production Sets the spending limit for the energy budget of the entire ecosystem

The Global Energy Budget The amount of solar radiation reaching the surface of the Earth Limits the photosynthetic output of ecosystems Only a small fraction of solar energy Actually strikes photosynthetic organisms

Gross and Net Primary Production Total primary production in an ecosystem Is known as that ecosystem’s gross primary production (GPP) Not all of this production Is stored as organic material in the growing plants

Net primary production (NPP) Is equal to GPP minus the energy used by the primary producers for respiration Only NPP Is available to consumers

Different ecosystems vary considerably in their net primary production And in their contribution to the total NPP on Earth (c) Lake and stream Open ocean Continental shelf Estuary Algal beds and reefs Upwelling zones Extreme desert, rock, sand, ice Desert and semidesert scrub Tropical rain forest Savanna Cultivated land Boreal forest (taiga) Temperate grassland Tundra Tropical seasonal forest Temperate deciduous forest Temperate evergreen forest Swamp and marsh Woodland and shrubland 0 10 20 30 40 50 60 0 500 1,000 1,500 2,000 2,500 0 5 10 15 20 25 Percentage of Earth’s net primary production Key Marine Freshwater (on continents) Terrestrial 5.2 0.3 0.1 0.1 4.7 3.5 3.3 2.9 2.7 2.4 1.8 1.7 1.6 1.5 1.3 1.0 0.4 0.4 125 360 1,500 2,500 500 3.0 90 2,200 900 600 800 600 700 140 1,600 1,200 1,300 2,000 250 5.6 1.2 0.9 0.1 0.04 0.9 22 7.9 9.1 9.6 5.4 3.5 0.6 7.1 4.9 3.8 2.3 0.3 65.0 24.4 Figure 54.4a–c Percentage of Earth’s surface area (a) Average net primary production (g/m 2 /yr) (b)

Overall, terrestrial ecosystems Contribute about two-thirds of global NPP and marine ecosystems about one-third Figure 54.5 180 120W 60W 0 60E 120E 180 North Pole 60N 30N Equator 30S 60S South Pole

Primary Production in Marine and Freshwater Ecosystems In marine and freshwater ecosystems Both light and nutrients are important in controlling primary production

Light Limitation The depth of light penetration Affects primary production throughout the photic zone of an ocean or lake

Nutrient Limitation More than light, nutrients limit primary production Both in different geographic regions of the ocean and in lakes

A limiting nutrient is the element that must be added In order for production to increase in a particular area Nitrogen and phosphorous Are typically the nutrients that most often limit marine production

Nutrient enrichment experiments Confirmed that nitrogen was limiting phytoplankton growth in an area of the ocean EXPERIMENT Pollution from duck farms concentrated near Moriches Bay adds both nitrogen and phosphorus to the coastal water off Long Island. Researchers cultured the phytoplankton Nannochloris atomus with water collected from several bays. Figure 54.6 Coast of Long Island, New York. The numbers on the map indicate the data collection stations. Long Island Great South Bay Shinnecock Bay Moriches Bay Atlantic Ocean 30 21 19 15 11 5 4 2

Figure 54.6 (a) Phytoplankton biomass and phosphorus concentration (b) Phytoplankton response to nutrient enrichment Great South Bay Moriches Bay Shinnecock Bay Starting algal density 2 4 5 11 30 15 19 21 30 24 18 12 6 0 Unenriched control Ammonium enriched Phosphate enriched Station number Phytoplankton (millions of cells per mL) 8 7 6 5 4 3 2 1 0 2 4 5 11 30 15 19 21 8 7 6 5 4 3 2 1 0 Inorganic phosphorus (g atoms/L) Phytoplankton (millions of cells/mL) Station number CONCLUSION Since adding phosphorus, which was already in rich supply, had no effect on Nannochloris growth, whereas adding nitrogen increased algal density dramatically, researchers concluded that nitrogen was the nutrient limiting phytoplankton growth in this ecosystem. Phytoplankton Inorganic phosphorus RESULTS Phytoplankton abundance parallels the abundance of phosphorus in the water (a). Nitrogen, however, is immediately taken up by algae, and no free nitrogen is measured in the coastal waters. The addition of ammonium (NH 4  ) caused heavy phytoplankton growth in bay water, but the addition of phosphate (PO 4 3 ) did not induce algal growth (b).

Experiments in another ocean region Showed that iron limited primary production Table 54.1

The addition of large amounts of nutrients to lakes Has a wide range of ecological impacts

In some areas, sewage runoff Has caused eutrophication of lakes, which can lead to the eventual loss of most fish species from the lakes Figure 54.7

Primary Production in Terrestrial and Wetland Ecosystems In terrestrial and wetland ecosystems climatic factors Such as temperature and moisture, affect primary production on a large geographic scale

The contrast between wet and dry climates Can be represented by a measure called actual evapotranspiration

Actual evapotranspiration Is the amount of water annually transpired by plants and evaporated from a landscape Is related to net primary production Figure 54.8 Actual evapotranspiration (mm H 2 O/yr) Tropical forest Temperate forest Mountain coniferous forest Temperate grassland Arctic tundra Desert shrubland Net primary production (g/m 2 /yr) 1,000 2,000 3,000 0 500 1,000 1,500 0

On a more local scale A soil nutrient is often the limiting factor in primary production Figure 54.9 EXPERIMENT Over the summer of 1980, researchers added phosphorus to some experimental plots in the salt marsh, nitrogen to other plots, and both phosphorus and nitrogen to others. Some plots were left unfertilized as controls. RESULTS Experimental plots receiving just phosphorus (P) do not outproduce the unfertilized control plots. CONCLUSION Live, above-ground biomass (g dry wt/m 2 ) Adding nitrogen (N) boosts net primary production. 300 250 200 150 100 50 0 June July August 1980 N  P N only Control P only These nutrient enrichment experiments confirmed that nitrogen was the nutrient limiting plant growth in this salt marsh.

Concept 54.3: Energy transfer between trophic levels is usually less than 20% efficient The secondary production of an ecosystem Is the amount of chemical energy in consumers’ food that is converted to their own new biomass during a given period of time

Production Efficiency When a caterpillar feeds on a plant leaf Only about one-sixth of the energy in the leaf is used for secondary production Figure 54.10 Plant material eaten by caterpillar Cellular respiration Growth (new biomass) Feces 100 J 33 J 200 J 67 J

The production efficiency of an organism Is the fraction of energy stored in food that is not used for respiration

Trophic Efficiency and Ecological Pyramids Trophic efficiency Is the percentage of production transferred from one trophic level to the next Usually ranges from 5% to 20%

Pyramids of Production This loss of energy with each transfer in a food chain Can be represented by a pyramid of net production Figure 54.11 Tertiary consumers Secondary consumers Primary consumers Primary producers 1,000,000 J of sunlight 10 J 100 J 1,000 J 10,000 J

Pyramids of Biomass One important ecological consequence of low trophic efficiencies Can be represented in a biomass pyramid

Most biomass pyramids Show a sharp decrease at successively higher trophic levels Figure 54.12a (a) Most biomass pyramids show a sharp decrease in biomass at successively higher trophic levels, as illustrated by data from a bog at Silver Springs, Florida. Trophic level Dry weight (g/m 2 ) Primary producers Tertiary consumers Secondary consumers Primary consumers 1.5 11 37 809

Certain aquatic ecosystems Have inverted biomass pyramids Figire 54.12b Trophic level Primary producers (phytoplankton) Primary consumers (zooplankton) (b) In some aquatic ecosystems, such as the English Channel, a small standing crop of primary producers (phytoplankton) supports a larger standing crop of primary consumers (zooplankton). Dry weight (g/m 2 ) 21 4

Pyramids of Numbers A pyramid of numbers Represents the number of individual organisms in each trophic level Figure 54.13 Trophic level Number of individual organisms Primary producers Tertiary consumers Secondary consumers Primary consumers 3 354,904 708,624 5,842,424

The dynamics of energy flow through ecosystems Have important implications for the human population Eating meat Is a relatively inefficient way of tapping photosynthetic production

Worldwide agriculture could successfully feed many more people If humans all fed more efficiently, eating only plant material Figure 54.14 Trophic level Secondary consumers Primary consumers Primary producers

The Green World Hypothesis According to the green world hypothesis Terrestrial herbivores consume relatively little plant biomass because they are held in check by a variety of factors

Most terrestrial ecosystems Have large standing crops despite the large numbers of herbivores Figure 54.15

The green world hypothesis proposes several factors that keep herbivores in check Plants have defenses against herbivores Nutrients, not energy supply, usually limit herbivores Abiotic factors limit herbivores Intraspecific competition can limit herbivore numbers Interspecific interactions check herbivore densities

Concept 54.4: Biological and geochemical processes move nutrients between organic and inorganic parts of the ecosystem Life on Earth Depends on the recycling of essential chemical elements Nutrient circuits that cycle matter through an ecosystem Involve both biotic and abiotic components and are often called biogeochemical cycles

A General Model of Chemical Cycling Gaseous forms of carbon, oxygen, sulfur, and nitrogen Occur in the atmosphere and cycle globally Less mobile elements, including phosphorous, potassium, and calcium Cycle on a more local level

A general model of nutrient cycling Includes the main reservoirs of elements and the processes that transfer elements between reservoirs Figure 54.16 Organic materials available as nutrients Living organisms, detritus Organic materials unavailable as nutrients Coal, oil, peat Inorganic materials available as nutrients Inorganic materials unavailable as nutrients Atmosphere, soil, water Minerals in rocks Formation of sedimentary rock Weathering, erosion Respiration, decomposition, excretion Burning of fossil fuels Fossilization Reservoir a Reservoir b Reservoir c Reservoir d Assimilation, photosynthesis

All elements Cycle between organic and inorganic reservoirs

Biogeochemical Cycles The water cycle and the carbon cycle Figure 54.17 Transport over land Solar energy Net movement of water vapor by wind Precipitation over ocean Evaporation from ocean Evapotranspiration from land Precipitation over land Percolation through soil Runoff and groundwater CO 2 in atmosphere Photosynthesis Cellular respiration Burning of fossil fuels and wood Higher-level consumers Primary consumers Detritus Carbon compounds in water Decomposition THE WATER CYCLE THE CARBON CYCLE

Water moves in a global cycle Driven by solar energy The carbon cycle Reflects the reciprocal processes of photosynthesis and cellular respiration

The nitrogen cycle and the phosphorous cycle Figure 54.17 N 2 in atmosphere Denitrifying bacteria Nitrifying bacteria Nitrifying bacteria Nitrification Nitrogen-fixing soil bacteria Nitrogen-fixing bacteria in root nodules of legumes Decomposers Ammonification Assimilation NH 3 NH 4 + NO 3  NO 2  Rain Plants Consumption Decomposition Geologic uplift Weathering of rocks Runoff Sedimentation Plant uptake of PO 4 3 Soil Leaching THE NITROGEN CYCLE THE PHOSPHORUS CYCLE

Most of the nitrogen cycling in natural ecosystems Involves local cycles between organisms and soil or water The phosphorus cycle Is relatively localized

Decomposition and Nutrient Cycling Rates Decomposers (detritivores) play a key role In the general pattern of chemical cycling Figure 54.18 Consumers Producers Nutrients available to producers Abiotic reservoir Geologic processes Decomposers

The rates at which nutrients cycle in different ecosystems Are extremely variable, mostly as a result of differences in rates of decomposition

Vegetation and Nutrient Cycling: The Hubbard Brook Experimental Forest Nutrient cycling Is strongly regulated by vegetation

Long-term ecological research projects Monitor ecosystem dynamics over relatively long periods of time The Hubbard Brook Experimental Forest Has been used to study nutrient cycling in a forest ecosystem since 1963

The research team constructed a dam on the site To monitor water and mineral loss Figure 54.19a (a) Concrete dams and weirs built across streams at the bottom of watersheds enabled researchers to monitor the outflow of water and nutrients from the ecosystem.

In one experiment, the trees in one valley were cut down And the valley was sprayed with herbicides Figure 54.19b (b) One watershed was clear cut to study the effects of the loss of vegetation on drainage and nutrient cycling.

Net losses of water and minerals were studied And found to be greater than in an undisturbed area These results showed how human activity Can affect ecosystems Figure 54.19c (c) The concentration of nitrate in runoff from the deforested watershed was 60 times greater than in a control (unlogged) watershed. Nitrate concentration in runoff (mg/L) Deforested Control Completion of tree cutting 1965 1966 1967 1968 80.0 60.0 40.0 20.0 4.0 3.0 2.0 1.0 0

Concept 54.5: The human population is disrupting chemical cycles throughout the biosphere As the human population has grown in size Our activities have disrupted the trophic structure, energy flow, and chemical cycling of ecosystems in most parts of the world

Nutrient Enrichment In addition to transporting nutrients from one location to another Humans have added entirely new materials, some of them toxins, to ecosystems

Agriculture and Nitrogen Cycling Agriculture constantly removes nutrients from ecosystems That would ordinarily be cycled back into the soil Figure 54.20

Nitrogen is the main nutrient lost through agriculture Thus, agriculture has a great impact on the nitrogen cycle Industrially produced fertilizer is typically used to replace lost nitrogen But the effects on an ecosystem can be harmful

Contamination of Aquatic Ecosystems The critical load for a nutrient Is the amount of that nutrient that can be absorbed by plants in an ecosystem without damaging it

When excess nutrients are added to an ecosystem, the critical load is exceeded And the remaining nutrients can contaminate groundwater and freshwater and marine ecosystems

Sewage runoff contaminates freshwater ecosystems Causing cultural eutrophication, excessive algal growth, which can cause significant harm to these ecosystems

Acid Precipitation Combustion of fossil fuels Is the main cause of acid precipitation

North American and European ecosystems downwind from industrial regions Have been damaged by rain and snow containing nitric and sulfuric acid Figure 54.21 4.6 4.6 4.3 4.1 4.3 4.6 4.6 4.3 Europe North America

By the year 2000 The entire contiguous United States was affected by acid precipitation Figure 54.22 Field pH  5.3 5.2–5.3 5.1–5.2 5.0–5.1 4.9–5.0 4.8–4.9 4.7–4.8 4.6–4.7 4.5–4.6 4.4–4.5 4.3–4.4  4.3

Environmental regulations and new industrial technologies Have allowed many developed countries to reduce sulfur dioxide emissions in the past 30 years

Toxins in the Environment Humans release an immense variety of toxic chemicals Including thousands of synthetics previously unknown to nature One of the reasons such toxins are so harmful Is that they become more concentrated in successive trophic levels of a food web

In biological magnification Toxins concentrate at higher trophic levels because at these levels biomass tends to be lower Figure 54.23 Concentration of PCBs Herring gull eggs 124 ppm Zooplankton 0.123 ppm Phytoplankton 0.025 ppm Lake trout 4.83 ppm Smelt 1.04 ppm

In some cases, harmful substances Persist for long periods of time in an ecosystem and continue to cause harm

Atmospheric Carbon Dioxide One pressing problem caused by human activities Is the rising level of atmospheric carbon dioxide

Rising Atmospheric CO 2 Due to the increased burning of fossil fuels and other human activities The concentration of atmospheric CO 2 has been steadily increasing Figure 54.24 CO 2 concentration (ppm) 390 380 370 360 350 340 330 320 310 300 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 1.05 0.90 0.75 0.60 0.45 0.30 0.15 0  0.15  0.30  0.45 Temperature variation (C) Temperature CO 2 Year

How Elevated CO 2 Affects Forest Ecology: The FACTS-I Experiment The FACTS-I experiment is testing how elevated CO 2 Influences tree growth, carbon concentration in soils, and other factors over a ten-year period Figure 54.25

The Greenhouse Effect and Global Warming The greenhouse effect is caused by atmospheric CO 2 But is necessary to keep the surface of the Earth at a habitable temperature

Increased levels of atmospheric CO 2 are magnifying the greenhouse effect Which could cause global warming and significant climatic change

Depletion of Atmospheric Ozone Life on Earth is protected from the damaging effects of UV radiation By a protective layer or ozone molecules present in the atmosphere

Satellite studies of the atmosphere Suggest that the ozone layer has been gradually thinning since 1975 Figure 54.26 Ozone layer thickness (Dobson units) Year (Average for the month of October) 350 300 250 200 150 100 50 0 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

The destruction of atmospheric ozone Probably results from chlorine-releasing pollutants produced by human activity Figure 54.27 1 2 3 Chlorine from CFCs interacts with ozone (O 3 ), forming chlorine monoxide (ClO) and oxygen (O 2 ). Two ClO molecules react, forming chlorine peroxide (Cl 2 O 2 ). Sunlight causes Cl 2 O 2 to break down into O 2 and free chlorine atoms. The chlorine atoms can begin the cycle again. Sunlight Chlorine O 3 O 2 ClO ClO Cl 2 O 2 O 2 Chlorine atoms

Scientists first described an “ozone hole” Over Antarctica in 1985; it has increased in size as ozone depletion has increased Figure 54.28a, b (a) October 1979 (b) October 2000

Chapter 55 Conservation Biology and Restoration Ecology

Overview: The Biodiversity Crisis Conservation biology integrates the following fields to conserve biological diversity at all levels Ecology Evolutionary biology Physiology Molecular biology Genetics Behavioral ecology

Restoration ecology applies ecological principles In an effort to return degraded ecosystems to conditions as similar as possible to their natural state

Tropical forests Contain some of the greatest concentrations of species Are being destroyed at an alarming rate Figure 55.1

Throughout the biosphere, human activities Are altering ecosystem processes on which we and other species depend

Concept 55.1: Human activities threaten Earth’s biodiversity Rates of species extinction Are difficult to determine under natural conditions The current rate of species extinction is high And is largely a result of ecosystem degradation by humans Humans are threatening Earth’s biodiversity

The Three Levels of Biodiversity Biodiversity has three main components Genetic diversity Species diversity Ecosystem diversity Genetic diversity in a vole population Species diversity in a coastal redwood ecosystem Community and ecosystem diversity across the landscape of an entire region Figure 55.2

Genetic Diversity Genetic diversity comprises The genetic variation within a population The genetic variation between populations

Species Diversity Species diversity Is the variety of species in an ecosystem or throughout the biosphere

An endangered species Is one that is in danger of becoming extinct throughout its range Threatened species Are those that are considered likely to become endangered in the foreseeable future

Conservation biologists are concerned about species loss Because of a number of alarming statistics regarding extinction and biodiversity

Harvard biologist E. O. Wilson has identified the Hundred Heartbeat Club Species that number fewer than 100 individuals and are only that many heartbeats from extinction (a) Philippine eagle (b) Chinese river dolphin (c) Javan rhinoceros Figure 55.3a–c

Ecosystem Diversity Ecosystem diversity Identifies the variety of ecosystems in the biosphere Is being affected by human activity

Biodiversity and Human Welfare Human biophilia Allows us to recognize the value of biodiversity for its own sake Species diversity Brings humans many practical benefits

Benefits of Species and Genetic Diversity Many pharmaceuticals Contain substances originally derived from plants Figure 55.4

The loss of species Also means the loss of genes and genetic diversity The enormous genetic diversity of organisms on Earth Has the potential for great human benefit

Ecosystem Services Ecosystem services encompass all the processes Through which natural ecosystems and the species they contain help sustain human life on Earth

Ecosystem services include Purification of air and water Detoxification and decomposition of wastes Cycling of nutrients Moderation of weather extremes And many others

Four Major Threats to Biodiversity Most species loss can be traced to four major threats Habitat destruction Introduced species Overexploitation Disruption of “interaction networks”

Habitat Destruction Human alteration of habitat Is the single greatest threat to biodiversity throughout the biosphere Massive destruction of habitat Has been brought about by many types of human activity

Many natural landscapes have been broken up Fragmenting habitat into small patches Figure 55.5

In almost all cases Habitat fragmentation and destruction leads to loss of biodiversity

Introduced Species Introduced species Are those that humans move from the species’ native locations to new geographic regions

Introduced species that gain a foothold in a new habitat Usually disrupt their adopted community (a) Brown tree snake, introduced to Guam in cargo (b) Introduced kudzu thriving in South Carolina Figure 55.6a, b

Overexploitation Overexploitation refers generally to the human harvesting of wild plants or animals At rates exceeding the ability of populations of those species to rebound

The fishing industry Has caused significant reduction in populations of certain game fish Figure 55.7

Disruption of Interaction Networks The extermination of keystone species by humans Can lead to major changes in the structure of communities Figure 55.8

Concept 55.2: Population conservation focuses on population size, genetic diversity, and critical habitat Biologists focusing on conservation at the population and species levels Follow two main approaches

Small-Population Approach Conservation biologists who adopt the small-population approach Study the processes that can cause very small populations finally to become extinct

The Extinction Vortex A small population is prone to positive-feedback loops That draw the population down an extinction vortex Small population Inbreeding Genetic drift Lower reproduction Higher mortality Loss of genetic variability Reduction in individual fitness and population adaptability Smaller population Figure 55.9

The key factor driving the extinction vortex Is the loss of the genetic variation necessary to enable evolutionary responses to environmental change

Case Study: The Greater Prairie Chicken and the Extinction Vortex Populations of the greater prairie chicken Were fragmented by agriculture and later found to exhibit decreased fertility

As a test of the extinction vortex hypothesis Scientists imported genetic variation by transplanting birds from larger populations

The declining population rebounded Confirming that it had been on its way down an extinction vortex EXPRIMENT Researchers observed that the population collapse of the greater prairie chicken was mirrored in a reduction in fertility, as measured by the hatching rate of eggs. Comparison of DNA samples from the Jasper County, Illinois, population with DNA from feathers in museum specimens showed that genetic variation had declined in the study population. In 1992, researchers began experimental translocations of prairie chickens from Minnesota, Kansas, and Nebraska in an attempt to increase genetic variation. RESULTS After translocation (blue arrow), the viability of eggs rapidly improved, and the population rebounded. CONCLUSION The researchers concluded that lack of genetic variation had started the Jasper County population of prairie chickens down the extinction vortex. Number of male birds (a) Population dynamics (b) Hatching rate 200 150 100 50 0 1970 1975 1980 1985 1990 1995 2000 Year Eggs hatched (%) 100 90 80 70 60 50 40 30 1970-74 1975-79 1980-84 1985-89 1990 1993-97 Years Figure 55.10

Minimum Viable Population Size The minimum viable population (MVP) Is the minimum population size at which a species is able to sustain its numbers and survive

A population viability analysis (PVA) Predicts a population’s chances for survival over a particular time Factors in the MVP of a population

Effective Population Size A meaningful estimate of MVP Requires a researcher to determine the effective population size, which is based on the breeding size of a population

Case Study: Analysis of Grizzly Bear Populations One of the first population viability analyses Was conducted as part of a long-term study of grizzly bears in Yellowstone National Park Figure 55.11

This study has shown that the grizzly bear population Has grown substantially in the past 20 years Number of individuals 150 100 50 0 1973 1982 1991 2000 Females with cubs Cubs Year Figure 55.12

Declining-Population Approach The declining-population approach Focuses on threatened and endangered populations that show a downward trend, regardless of population size Emphasizes the environmental factors that caused a population to decline in the first place

Steps for Analysis and Intervention The declining-population approach Requires that population declines be evaluated on a case-by-case basis Involves a step-by-step proactive conservation strategy

Case Study: Decline of the Red-Cockaded Woodpecker Red-cockaded woodpeckers Require specific habitat factors for survival Had been forced into decline by habitat destruction (a) A red-cockaded woodpecker perches at the entrance to its nest site in a longleaf pine. (b) Forest that can sustain red-cockaded woodpeckers has low undergrowth. (c) Forest that cannot sustain red-cockaded woodpeckers has high, dense undergrowth that impacts the woodpeckers’ access to feeding grounds. Figure 55.13a–c

In a study where breeding cavities were constructed New breeding groups formed only in these sites On the basis of this experiment A combination of habitat maintenance and excavation of new breeding cavities has enabled a once-endangered species to rebound

Weighing Conflicting Demands Conserving species often requires resolving conflicts Between the habitat needs of endangered species and human demands

Concept 55.3: Landscape and regional conservation aim to sustain entire biotas In recent years, conservation biology Has attempted to sustain the biodiversity of entire communities, ecosystems, and landscapes

One goal of landscape ecology, of which ecosystem management is part Is to understand past, present, and future patterns of landscape use and to make biodiversity conservation part of land-use planning

Landscape Structure and Biodiversity The structure of a landscape Can strongly influence biodiversity

Fragmentation and Edges The boundaries, or edges, between ecosystems Are defining features of landscapes (a) Natural edges. Grasslands give way to forest ecosystems in Yellowstone National Park. (b) Edges created by human activity. Pronounced edges (roads) surround clear-cuts in this photograph of a heavily logged rain forest in Malaysia. Figure 55.14a, b

As habitat fragmentation increases And edges become more extensive, biodiversity tends to decrease

Research on fragmented forests has led to the discovery of two groups of species Those that live in forest edge habitats and those that live in the forest interior Figure 55.15

Corridors That Connect Habitat Fragments A movement corridor Is a narrow strip of quality habitat connecting otherwise isolated patches

In areas of heavy human use Artificial corridors are sometimes constructed Figure 55.16

Movement corridors Promote dispersal and help sustain populations

Establishing Protected Areas Conservation biologists are applying their understanding of ecological dynamics In establishing protected areas to slow the loss of biodiversity

Much of the focus on establishing protected areas Has been on hot spots of biological diversity

Finding Biodiversity Hot Spots A biodiversity hot spot is a relatively small area With an exceptional concentration of endemic species and a large number of endangered and threatened species Terrestrial biodiversity hot spots Equator Figure 55.17

Biodiversity hot spots are obviously good choices for nature reserves But identifying them is not always easy

Philosophy of Nature Reserves Nature reserves are biodiversity islands In a sea of habitat degraded to varying degrees by human activity One argument for extensive reserves Is that large, far-ranging animals with low-density populations require extensive habitats

In some cases The size of reserves is smaller than the actual area needed to sustain a population Biotic boundary for short-term survival; MVP is 50 individuals. Biotic boundary for long-term survival; MVP is 500 individuals. Grand Teton National Park Wyoming Idaho 43 42 41 40 0 50 100 Kilometers Snake R. Yellowstone National Park Shoshone R. Montana Wyoming Montana Idaho Madison R. Gallatin R. Yellowstone R. Figure 55.18

Zoned Reserves The zoned reserve model recognizes that conservation efforts Often involve working in landscapes that are largely human dominated

Zoned reserves Are often established as “conservation areas” (a) Boundaries of the zoned reserves are indicated by black outlines. (b) Local schoolchildren marvel at the diversity of life in one of Costa Rica’s reserves. Nicaragua Costa Rica Panama National park land Buffer zone PACIFIC OCEAN CARIBBEAN SEA Figure 55.19a, b

Some zoned reserves in the Fiji islands are closed to fishing Which actually helps to improve fishing success in nearby areas Figure 55.20

Concept 55.4: Restoration ecology attempts to restore degraded ecosystems to a more natural state The larger the area disturbed The longer the time that is required for recovery

Whether a disturbance is natural or caused by humans Seems to make little difference in this size-time relationship Recovery time (years) (log scale) 10 4 1,000 100 10 1 10 3 10 2 10 1 1 10 100 1,000 10 4 Natural disasters Human-caused disasters Natural OR human- caused disasters Meteor strike Groundwater exploitation Industrial pollution Urbanization Salination Modern agriculture Flood Volcanic eruption Acid rain Forest fire Nuclear bomb Tsunami Oil spill Slash & burn Land- slide Tree fall Lightning strike Spatial scale (km 2 ) (log scale) Figure 55.21

One of the basic assumptions of restoration ecology Is that most environmental damage is reversible Two key strategies in restoration ecology Are bioremediation and augmentation of ecosystem processes

Bioremediation Bioremediation Is the use of living organisms to detoxify ecosystems

Biological Augmentation Biological augmentation Uses organisms to add essential materials to a degraded ecosystem

Exploring Restoration The newness and complexity of restoration ecology Require scientists to consider alternative solutions and adjust approaches based on experience

Exploring restoration worldwide Truckee River, Nevada. Kissimmee River, Florida. Equator Figure 55.22

Tropical dry forest, Costa Rica. Succulent Karoo, South Africa. Rhine River, Europe. Coastal Japan. Figure 55.22

Concept 55.5: Sustainable development seeks to improve the human condition while conserving biodiversity Facing increasing loss and fragmentation of habitats How can we best manage Earth’s resources?

Sustainable Biosphere Initiative The goal of this initiative is to define and acquire the basic ecological information necessary For the intelligent and responsible development, management, and conservation of Earth’s resources

Case Study: Sustainable Development in Costa Rica Costa Rica’s success in conserving tropical biodiversity Has involved partnerships between the government, other organizations, and private citizens

Human living conditions in Costa Rica Have improved along with ecological conservation Infant mortality (per 1,000 live births) 200 150 100 50 0 1900 1950 2000 80 70 60 50 40 30 Year Life expectancy Infant mortality Life expectancy (years) Figure 55.23

Biophilia and the Future of the Biosphere Our modern lives Are very different from those of early humans who hunted and gathered and painted on cave walls (a) Detail of animals in a Paleolithic mural, Lascaux, France Figure 55.24a

But our behavior Reflects remnants of our ancestral attachment to nature and the diversity of life, the concept of biophilia (b) Biologist Carlos Rivera Gonzales examining a tiny tree frog in Peru Figure 55.24b

Our innate sense of connection to nature May eventually motivate a realignment of our environmental priorities

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