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Transcript
Chapter 52 Population Ecology Density and Dispersion • Density is the number of individuals per unit area or volume • Dispersion is the pattern of spacing among individuals within the boundaries of the population LE 52-2 Births Immigration Population size Emigration Deaths LE 52-3a Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory. LE 52-3b Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors. LE 52-3c Random. Dandelions grow from windblown seeds that land at random and later germinate. Demography • Demography is the study of the vital statistics of a population and how they change over time • Death rates and birth rates are of particular interest to demographers LE 52-4 Number of survivors (log scale) 1,000 100 Females 10 Males 1 0 2 4 6 Age (years) 8 10 Number of survivors (log scale) LE 52-5 1,000 I 100 II 10 III 1 0 50 Percentage of maximum life span 100 Life History Diversity • Life histories are very diverse • Species that exhibit semelparity, or “big-bang” reproduction, reproduce once and die • Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly “Trade-offs” and Life Histories • Organisms have finite resources, which may lead to trade-offs between survival and reproduction LE 52-8a Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least some will grow into plants and eventually produce seeds themselves. LE 52-8b Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring. • In animals, parental care of smaller broods may facilitate survival of offspring • Carrying capacity (K) is the maximum population size the environment can support The Logistic Model and Life Histories • Life history traits favored by natural selection may vary with population density and environmental conditions • K-selection, or density-dependent selection, selects for life history traits that are sensitive to population density • r-selection, or density-independent selection, selects for life history traits that maximize reproduction Territoriality • In many vertebrates and some invertebrates, territoriality may limit density Health • Population density can influence the health and survival of organisms • In dense populations, pathogens can spread more rapidly Predation • As a prey population builds up, predators may feed preferentially on that species Toxic Wastes • Accumulation of toxic wastes can contribute to density-dependent regulation of population size LE 52-22 Human Population Growth 5 4 3 2 The Plague 1 8000 B.C. 4000 B.C. 3000 B.C. 2000 B.C. 1000 B.C. 0 1000 A.D. 0 2000 A.D. Human population (billions) 6 Regional Patterns of Population Change • To maintain population stability, a regional human population can exist in one of two configurations: – Zero population growth = High birth rate – High death rate – Zero population growth = Low birth rate – Low death rate • The demographic transition is the move from the first state toward the second state LE 52-24 Birth or death rate per 1,000 people 50 40 30 20 10 Sweden Birth rate Mexico Birth rate Death rate 0 1750 1800 Death rate 1850 1900 Year 1950 2000 2050 Age Structure • One important demographic factor in present and future growth trends is a country’s age structure • Age structure is the relative number of individuals at each age • It is commonly represented in pyramids LE 52-25 Rapid growth Afghanistan Male Female Slow growth United States Male Female Age Age 85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 8 6 4 2 0 2 4 Percent of population 6 8 Decrease Italy Male Female 85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 8 6 4 2 0 2 4 Percent of population 6 8 8 6 4 2 0 2 4 Percent of population 6 8 • Age structure diagrams can predict a population’s growth trends • They can illuminate social conditions and help us plan for the future Infant Mortality and Life Expectancy • Infant mortality and life expectancy at birth vary greatly among developed and developing countries but do not capture the wide range of the human condition LE 52-26 80 50 Life expectancy (years) Infant mortality (deaths per 1,000 births) 60 40 30 20 60 40 20 10 0 0 Developed countries Developing countries Developed countries Developing countries LE 52-27 Ecological footprint (ha per person) 16 14 12 New Zealand 10 USA Germany Netherlands Japan Norway 8 6 Australia Canada Sweden UK Spain 4 World China India 2 0 0 2 4 6 10 12 8 Available ecological capacity (ha per person) 14 16 Competition • Interspecific competition occurs when species compete for a resource in short supply • Strong competition can lead to competitive exclusion, local elimination of a 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 LE 53-2 Chthamalus Balanus High tide High tide Chthamalus realized niche Chthamalus fundamental niche Balanus realized niche Ocean Low tide Ocean Low tide Resource Partitioning • Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community LE 53-3 A. insolitus usually perches on shady branches. A. ricordii A. distichus perches on fence posts and other sunny surfaces. A. insolitus A. aliniger A. distichus A. christophei A. cybotes A. etheridgei Predation • Predation refers to interaction where one species, the predator, kills and eats the other, the prey • Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison • Prey display various defensive adaptations • Behavioral defenses include hiding, fleeing, selfdefense, and alarm calls • Animals also have morphological and physiological defense adaptations • Cryptic coloration, or camouflage, makes prey difficult to spot Video: Seahorse Camouflage • Animals with effective chemical defense often exhibit bright warning coloration, called aposematic coloration • Predators are particularly cautious in dealing with prey that display such coloration • In Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful model LE 53-7 Green parrot snake Hawkmoth larva • In Müllerian mimicry, two or more unpalatable species resemble each other LE 53-8 Cuckoo bee Yellow jacket Herbivory • Herbivory refers to an interaction in which an herbivore eats parts of a plant or alga • It has led to evolution of plant mechanical and chemical defenses and adaptations by herbivores Parasitism • In parasitism, one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process • Parasitism exerts substantial influence on populations and the structure of communities Disease • Effects of disease on populations and communities are similar to those 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 Video: Clownfish and Anemone Commensalism • In commensalism, one species benefits and the other is apparently unaffected • Commensal interactions are hard to document in nature because any close association of two species likely affects both LE 53-12 Quaternary consumers Carnivore Carnivore Tertiary consumers Carnivore Carnivore Secondary consumers Carnivore Carnivore Primary consumers Herbivore Zooplankton Primary producers Plant A terrestrial food chain Phytoplankton A marine food chain LE 53-13 Humans Smaller toothed whales Baleen whales Crab-eater seals Birds Leopard seals Fishes Sperm whales Elephant seals Squids Carnivorous plankton Copepods Euphausids (krill) Phytoplankton LE 53-14 Sea nettle Juvenile striped bass Fish larvae Fish eggs Zooplankton Keystone Species • In contrast to dominant species, keystone species are not necessarily abundant in a community • They exert strong control on a community by their ecological roles, or niches Number of species present LE 53-16 20 With Pisaster (control) 15 10 Without Pisaster (experimental) 5 0 1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 LE 53-21 Before a controlled burn. A prairie that has not burned for several years has a high propor-tion of detritus (dead grass). During the burn. The detritus serves as fuel for fires. After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living. LE 53-22 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. One year after fire. This photo of the same general area taken the following year indicates how rapidly the com-munity 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 and later-arriving species may be linked in one of three processes: – Early arrivals may facilitate appearance of later species by making the environment favorable – They may inhibit establishment of later species – They may tolerate later species but have no impact on their establishment LE 53-24 Pioneer stage, with fireweed dominant Dryas stage 60 Soil nitrogen (g/m2) 50 40 30 20 10 0 Pioneer Dryas Alder Successional stage Spruce Nitrogen fixation by Dryas and alder increases the soil nitrogen content. Spruce stage LE 54-2 Tertiary consumers Microorganisms and other detritivores Detritus Secondary consumers Primary consumers Primary producers Heat Key Chemical cycling Energy flow Sun 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 Pyramids of Production • A pyramid of net production represents the loss of energy with each transfer in a food chain LE 54-11 Tertiary consumers Secondary consumers Primary consumers Primary producers 10 J 100 J 1,000 J 10,000 J 1,000,000 J of sunlight Pyramids of Biomass • In a biomass pyramid, each tier represents the dry weight of all organisms in one trophic level • Most biomass pyramids show a sharp decrease at successively higher trophic levels LE 54-12a Trophic level Dry weight (g/m2) Tertiary consumers 1.5 Secondary consumers 11 Primary consumers 37 Primary producers 809 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. • Certain aquatic ecosystems have inverted biomass pyramids: Primary consumers outweigh the producers LE 54-12b Trophic level Primary consumers (zooplankton) Primary producers (phytoplankton) Dry weight (g/m2) 21 4 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). Pyramids of Numbers • A pyramid of numbers represents the number of individual organisms in each trophic level LE 54-13 Trophic level Tertiary consumers Number of individual organisms 3 Secondary consumers 354,904 Primary consumers 708,624 Primary producers 5,842,424 LE 54-17a Transport over land Solar energy Net movement of water vapor by wind Precipitation over ocean Evaporation from ocean Precipitation over land Evapotranspiration from land Percolation through soil Runoff and groundwater LE 54-17b CO2 in atmosphere Photosynthesis Cellular respiration Burning of fossil fuels and wood Carbon compounds in water Higher-level Primary consumers consumers Detritus Decomposition LE 54-17c N2 in atmosphere Assimilation NO3– Nitrogen-fixing bacteria in root nodules of legumes Decomposers Ammonification NH3 Nitrogen-fixing soil bacteria Nitrifying bacteria Nitrification NH4+ NO2– Nitrifying bacteria Denitrifying bacteria LE 54-17d Rain Geologic uplift Weathering of rocks Plants Runoff Consumption Sedimentation Soil Plant uptake of PO43– Leaching Decomposition Agriculture and Nitrogen Cycling • Agriculture removes nutrients from ecosystems that would ordinarily be cycled back into the soil • Nitrogen is the main nutrient lost through agriculture; thus, agriculture greatly impacts the nitrogen cycle • Industrially produced fertilizer is typically used to replace lost nitrogen, but effects on an ecosystem can be harmful Toxins in the Environment • Humans release many toxic chemicals, including synthetics previously unknown to nature • In some cases, harmful substances persist for long periods in an ecosystem • One reason toxins are harmful is that they become more concentrated in successive trophic levels • In biological magnification, toxins concentrate at higher trophic levels, where biomass is lower LE 54-23 Concentration of PCBs Herring gull eggs 124 ppm Lake trout 4.83 ppm Smelt 1.04 ppm Zooplankton 0.123 ppm Phytoplankton 0.025 ppm Depletion of Atmospheric Ozone • Life on Earth is protected from damaging effects of UV radiation by a protective layer or ozone molecules in the atmosphere • Satellite studies suggest that the ozone layer has been gradually thinning since 1975 LE 54-26 350 Ozone layer thickness (Dobson units) 300 250 200 150 100 50 0 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Year (Average for the month of October) • Destruction of atmospheric ozone probably results from chlorine-releasing pollutants produced by human activity LE 54-27 Chlorine from CFCs interacts with ozone (O3), forming chlorine monoxide (CIO) and oxygen (O2). Chlorine atoms O2 O3 Chlorine CIO O2 Sunlight causes Cl2O2 to break down into O2 and free chlorine atoms. The chlorine atoms can begin the cycle again. CIO Cl2O2 Sunlight Two CIO molecules react, forming chlorine peroxide (Cl2O2). LE 54-28 October 1979 October 2000