C.PopulationSu11

C.PopulationSu11 - Population Ecology POPULATION ECOLOGY THE REALM OF ECOLOGY • Biosphere • Ecosystem • Community • Population

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Unformatted text preview: Population Ecology POPULATION ECOLOGY THE REALM OF ECOLOGY • Biosphere • Ecosystem • Community • Population Ecology: Biosphere Ecosystems Communities Interactions among members of the same species in a given habitat. Populations Organisms SPECIES and POPULATION •  Species – Interbreed – Fertile offspring •  Population – Interacting group – Share resources – Geographical range Factors that Limit Population Size POPULATION DYNAMICS 1.  2.  3.  4.  Size (N): # of individuals Density: # of individuals per unit area Distribution: dispersal within an area Age structure: proportion in each age category •  Often gender-specific 5.  Growth patterns: changes in population size and/or density over time 6.  Life history strategies: cost/benefit in stable vs. unstable environments Factors that Limit Population Size •  Abiotic (nonliving) Limiting Factors –  Temperature –  Water –  Soil type –  Sunlight –  Salinity –  Wind stress –  Altitude, depth •  Density Dependent Limiting Factors –  Limited resources •  Biotic (living) Limiting Factors –  Food source –  Competition –  Predators –  Social factors, mates –  Pathogens, parasites –  Vegetation •  Density Independent Limiting Factors –  Natural disasters •  •  •  •  •  •  Food Water Safe refuge Predation Competition Living space –  Disease, Pollution •  Hurricanes •  Floods, landslides, volcanoes •  Drought, frost –  Environmental insult •  Deforestation •  Pesticide •  Fire –  Climatic change Heyer 1 Population Ecology Density, Dispersal, & Distribution (a) 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. (b) 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. (c) Random. Dandelions grow from windblown seeds that land at random and later germinate. POPULATION AGE STRUCTURE •  Demography & Life Tables •  Survivorship Curves Figure 52.3 POPULATION AGE STRUCTURE Vital Statistics of Populations •  Age structure is relative number of individuals of each age. Sex ratio is % of females to males. •  Study of human populations = demography Life Tables •  Created in one of two ways: POPULATION AGE STRUCTURE Vital Statistics of Populations •  Average births per individual = fecundity. •  Population birth rate = natality. •  Population death rate = mortality. •  Generation time = age at first reproduction. POPULATION AGE STRUCTURE Cohort Survivorship Curve •  Number of a cohort surviving to subsequent years 1.  Follow a cohort. or 2.  Snapshot of a population at a specific time point. Heyer 2 Population Ecology POPULATION AGE STRUCTURE Cohort Survivorship Curve •  Number of a cohort surviving to subsequent years Survivorship Curves • Type I: low juvenile mortality • Type II: constant mortality • Type III: high juvenile mortality •  Constructed from Life History Tables Beldings Ground Squirrels Fig. 53.5 Fecundity Influences Mortality •  Survivorship curves reflect life tables. •  Tradeoffs exist between survivorship & reproductive traits. •  There is a balancing allocation of resources. •  Survivorship curves reflect life tables. •  Tradeoffs exist between survivorship & reproductive traits. •  There is a balancing allocation of resources. Figure 52.7 Population growth patterns: changes over time Births and immigration add individuals to a population. • Population size (N) depends on: Births Immigration PopuIation size Emigration Deaths Deaths and –  Natality = birth rate (b) emigration remove individuals from a – Mortality = death rate (d) population. –  Immigration = migration into the population (i) –  Emigration = migration out of the population (e) –  Growth rate (r) = (b-d) + (i-e) EXPERIMENT Researchers in the Netherlands studied the effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.) 100 Male Parents surviving the following winter (%) Fecundity Influences Mortality Female 80 60 40 20 0 Reduced brood size Normal brood size Enlarged brood size CONCLUSION The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents. Population Growth Rate •  N = # individuals •  ∆N/∆t = change in population size over time ♦ b = birth rate ♦ d = death rate •  ∆N/∆t = (N*b)–(N*d) •  r = b–d •  ∆N/∆t = rN •  In Sri Lanka, overpopulation continues to escalate despite success in decreasing per capita birth rate. •  ↓↓d→↑r, despite ↓b. ↑r →↑ ∆N/∆t Heyer 3 Population Ecology Exponential Growth •  r : population growth rate •  rmax : biotic potential – potential growth rate under ideal conditions •  K : carrying capacity •  Population multiplies by a constant factor. •  Growth rate not limited by resources. •  “J”-shaped growth curve. – maximum population that the environment can sustain over long periods of time. – determined by biotic and abiotic limiting factors. Exponential Growth Curves •  Growth = ∆N/∆t = rN   {r=b-d} •  Rate of population growth only limited by rmax. Logistic growth •  Growth is limited by density-dependent resources or other factors •  Decrease growth rate produces “S”-shaped (sigmoidal) curve •  “K-limited” •  “r-limited” Fur seal population" Laboratory populations with defined resources exhibit density dependence Growth Equations: Exponential vs. Logistic •  Exponential •  Growth rate (G) = dN/dt = rN •  This growth is always increasing. •  Logistic •  Growth rate (G) = dN/dt = rN([K-N]/K)  When N <<< K (pop is v. low), [K-N] = K and dN/dt = rN(K/K) = rN (growth is exponential). “K-selected” Heyer  When N approaches K, [K-N] approaches zero and dN/dt = rN(0/K) = 0 (growth stops). 4 Population Ecology Growth Equations: Exponential vs. Logistic A population reaches carrying capacity when growth rate is zero 2,000 dN dt Population size (N) 1,500 = 1.0N Exponential growth K = 1,500 Logistic growth 1,000 dN dt = 1.0N 1,500 - N 1,500 500 0 0 5 10 Number of generations •  Exponential   dN/dt = rN •  Logistic   dN/dt = rN([K-N]/K) 15 Figure 52.12 Carrying Capacity •  Population size that can be sustained by a habitat •  Requires renewable resources •  Carrying capacity (K) changes as resources flux with size of population •  If a population does not limit its size to the carrying capacity, it will deplete its resources and suffer a sharp crash in numbers due to starvation and/or disease — “boom & bust” pattern. Outcome of Exponential Growth •  Exceed carrying capacity (K) & crash. –  cyclic exponential (“J-shaped) growth curves punctuated by crashes. –  typical of species who make tons of tiny kids –  “r -selected species” K “Boom and Bust” Population Cycles LOG SCALE Fort Bragg, CA “Boom and Bust” Population Cycles •  “r-selected”: J-type growth rate limited by r, but cannot be sustained indefinitely beyond K. •  “K-selected”: S-type growth rate limited by K •  “r-selected” Heyer •  “r-selected” •  Population cycles between a rapid increase and then a sharp decline. •  Population cycles between a rapid increase and then a sharp decline. 5 Population Ecology Trophic (food resources) limiting factors 160 120 Figure 52.21 Snowshoe hare Lynx population size (thousands) –  Original hypothesis Hare population size (thousands) •  Top-down regulation (populations regulated by higher levels of the food chain): increase in predator (lynx) population causes a decrease in the prey (hare) population. 9 Lynx 80 6 40 3 0 1850 1875 1900 Year 1925 0 •  Bottom-up regulation (populations regulated by lower levels of the food chain): increase in hare population causes an overconsumption of the vegetation; decrease in vegetation causes a decrease in hare population; decrease in hare population causes a decrease in predator (lynx) population Life History Traits Trade-offs, game theory and the allocation of resources For species inhabiting unstable, unpredictable environments; or species with very high juvenile mortality: •  The odds of suitable habitat for the next generation are low. •  Therefore, natural selection favors the generalist populations that opportunistically harvest any available resource to grow as fast as possible when they can, and quickly produce many offspring distributed over a wide area to increase chance of hitting someplace good. (“weeds”) •  “r-selected” — select for high reproductive potential   For species inhabiting stable environments: •  Long-term strategy is most successful. •  Natural selection favors the specialist populations that excel at harnessing the particular available resources to displace competitors. Spend resources on becoming dominant species and increasing the odds of a few offspring to succeed with you. •  “K-selected” — select for intrinsic growth limitations for sustainable population over time.   Reproductive Strategies Figure 52.8 •  Semelparity –  Produce one huge batch of offspring and then die Life History Traits –  Revised hypothesis. Hare populations oscillate even in the absence of lynxes. r-selected Type: K-selected Major source of Juvenile predation / Competition mortality Sporadic catastrophes Generation time (age) Short (young) Long (old) –  Produce several smaller batches of offspring distributed over time Iteroparous Fecundity Very high Low Newborn size Small Large Dispersal of young High Low Parental care Low/none High Precocial Altricial Very high Low Survivorship curve Pop. growth curve (b) Some plants, such as this coconut palm, produce a moderate number of very large seeds. Large Semelparous Juvenile mortality • Iteroparity Small Reproduction Newborn behavior (a) Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds. Adult size Type III Cyclic Type I Sigmoidal K-selected populations Life History Plasticity Daphnia ostracod in culture •  Equilibrium population density (b=d) at or below carrying capacity. •  Must either ↑d or ↓b or both. Density-dependent birth rate Birth or death rate per capita Density-dependent birth rate •  Switch from r-limited growth to K-limited, before environmental degradation is irreversible. –  At low population densities, short generation time, high fecundity. –  At high densities, change physiology to longer generation time, more body growth, lower fecundity. Heyer Densitydependent death rate Equilibrium density Population density (a) Both birth rate and death rate change with population density. Densityindependent death rate Equilibrium density Population density (b) Birth rate changes with population density while death rate is constant. Densityindependent birth rate Density-dependent death rate Equilibrium density Population density (c) Death rate changes with population density while birth rate is constant. Figure 52.14 6 Population Ecology Even K-limited populations may fluctuate over time K-selected populations •  “Good” K-selected species achieve equilibrium density by decreasing birth rate as population approaches K. •  Variations in limiting factors cause variations in K FIELD STUDY Average clutch size 3.8 1,000 100 3.6 3.2 3.0 2.8 0 0 10 0 100 10 Seeds planted per m2 (a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases. Steady decline probably caused largely by wolf predation 2,500 3.4 Moose population size 10,000 Average number of seeds per reproducing individual (log scale) Researchers regularly surveyed the population of moose on Isle Royale, Michigan, from 1960 to 2003. During that time, the lake never froze over, and so the moose population was isolated from the effects of immigration and emigration. Over 43 years, this population experienced two significant increases and collapses, as well RESULTS as several less severe fluctuations in size. 4.0 20 30 40 50 60 70 80 2,000 1,500 1,000 500 Density of females 0 (b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply. Dramatic collapse caused by severe winter weather and food shortage, leading to starvation of more than 75% of the population 1960 1970 1980 Year 1990 2000 CONCLUSION The pattern of population dynamics observed in this isolated population indicates that various biotic and abiotic factors can result in dramatic fluctuations over time in a moose population. Figure 52.18 Figure 52.15 Human Population Growth 1987 5 1974 4 1961 3 1928 2 1802 1 Industrial Revolution begins Agricultural based urban societies 8000 B.C. 6 The Plague 4000 B.C. 3000 B.C. 2000 B.C. 1000 B.C. 0 1000 A.D. Humans can artificially increase carrying capacity Human population (billions) 1999 •  Technological advances avoid natural growth constraints – Hunting and gathering – Agricultural revolution – Industrial revolution – Scientific revolution 0 2000 A.D. • Paradox or time bomb??? The history of human population growth • Homo sapiens life history traits show Type I survivorship that should correlate with a K-selected sigmoidalmillion/yr. •  Human pop now increases by 80 growth curve. • But, –  That’s a new LA every two weeks !! our actual growth curve is exponential!!! • What happens to a population that exceeds its carrying capacity? Age structure pyramids Demographic Transition •  Zero population growth = High birth rates – High death rates •  Zero population growth = Low birth rates – Low death rates 50 Birth or death rate per 1,000 people 40 30 20 10 1750 Heyer Mexico Birth rate 0 Fig. 53.25 Sweden Birth rate Death rate 1800 Death rate 1850 1900 1950 2000 2050 Year 7 Population Ecology Human carrying capacity is not infinite •  Resources will eventually be depleted •  Economic resources allow exploitation of natural resources •  Industrialized nations consume more resources per capita Earth’s Human Carrying Capacity •  Ecological Footprint = land per person needed to support resource demands •  US footprint is 10X the India footprint •  Countries above the mid-line are in ecological deficit (above carrying capacity) Ecological footprint vs. ecological capacity Ecological Footprint Your Personal Footprint! •  Countries above the mid-line are in ecological deficit (above carrying capacity) •  United States   4.7% of the world population   Produces 21% of all goods and services   Uses 25% available processed minerals and nonrenewable energy resources   Generates at least 25% of world’s pollution and trash •  India   17% of the world population   Produces 1% goods and services   Uses 3% available processed minerals and nonrenewable energy resources   Generates 3% world’s pollution and trash •  U.S. consumes 50 times more resources than India (per person) •  US footprint is 10X the India footprint Ecological Heyer •  The overpopulation and overconsumption by the human population are triggering an enormous array of problems, ranging from food sources (agriculture, fisheries), waste, air and water pollution, energy and mineral use, habitat destruction, and species extinction. You can calculate your own ecological footprint by going to the following URL: •  http://www.myfootprint.org/ footprint vs. ecological capacity 8 ...
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This note was uploaded on 09/02/2011 for the course BIOL 6C taught by Professor Sundram during the Spring '09 term at DeAnza College.

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