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Home , Food chain, Population density, Population dynamics

Population Structure & Dynamics

Population : POPULATION Interactions among members of the same species in a given . DYNAMICS 1. Size (N): # of individuals • Species 2. Density: # of individuals per unit area – Interbreed 3. Distribution: dispersal within an area – Fertile offspring 4. Age structure: proportion in each age category • Population • Often gender-specific – Interacting group 5. Growth patterns: changes in – Share resources and/or density over time – Geographical range 6. history strategies: cost/benefit in stable vs. unstable environments

Factors that Limit Population Size Factors that Limit Population Size • Abiotic (nonliving) Limiting Factors • Density Dependent Limiting Factors – Temperature – Limited resources – • – Soil type • Water • Safe refuge – Sunlight • – Salinity • – Wind stress • Living space – Altitude, depth – Disease,

• Biotic (living) Limiting Factors • Density Independent Limiting Factors – Food source – Natural disasters – Competition • Hurricanes – Predators • Floods, landslides, volcanoes – Social factors, mates • Drought, frost – Pathogens, parasites – Environmental insult • – Vegetation • • Fire – Climatic change

Density, Dispersal, & POPULATION AGE STRUCTURE Distribution (a) Clumped. For many animals, such as these wolves, living in groups • & Life Tables increases the effectiveness of , spreads the work of • Survivorship Curves protecting and caring for young, and helps exclude other individuals from their . (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. Figure 53.4

Heyer 1 Population Structure & Dynamics

POPULATION AGE STRUCTURE POPULATION AGE STRUCTURE Vital Statistics of Vital Statistics of Populations

• Age structure is • Average births per relative number of individual = fecundity. individuals of each age. • Population birth rate Sex ratio is % of = natality. females to males. • Population rate • Study of = mortality. populations = • Generation time = demography age at first reproduction.

Life POPULATION AGE STRUCTURE Tables Cohort Survivorship Curve • Number of a cohort surviving to subsequent years • Created in one of two ways: 1 Follow a cohort or 2 Snapshot of a population at a specific time point

• Type I: low juvenile mortality POPULATION AGE STRUCTURE Survivorship Curves • Type II: constant mortality Cohort Survivorship Curve • Type III: high juvenile mortality • Number of a cohort surviving to subsequent years • Constructed from Life History Tables Beldings Ground Squirrels

Fig. 53.5 Fig. 53.6

Heyer 2 Population Structure & Dynamics

Fecundity Influences Mortality Fecundity Influences Mortality EXPERIMENT Researchers in he studied the effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced • Survivorship curves • Survivorship curves 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 reflect life tables. reflect life tables. following winter. (Both males and females provide care for chicks.) • Tradeoffs exist • Tradeoffs exist 100 Male between survivorship between survivorship Female 80 & reproductive traits. & reproductive traits. 60

• There is a balancing • There is a balancing 40 allocation of resources. allocation of resources. 20

0 Reduced Normal Enlarged Parents surviving the following winter (%) brood size brood size brood size

CONCLUSION The lower survival rates of kestrels w th larger broods indicate that caring for more offspring negatively affects Figure 52.7 survival of the parents.

Births and add individuals to a population. Rate Population Births Immigration growth patterns: PopuIation changes over time size • N = # individuals • ∆N/∆t = change in population size over • Population size (N) depends on: Emigration time Deaths and ♦ b = birth rate – Natality = birth rate (b) emigration remove individuals from a ♦ d = death rate – Mortality = death rate (d) population. • ∆N/∆t = (N*b)–(N*d) – Immigration = migration into the population (i) • r = b–d – Emigration = migration out of the population (e) • ∆N/∆t = rN • In Sri Lanka, continues to escalate – Growth rate (r) = (b-d) + (i-e) despite success in decreasing per capita birth rate • ↓↓d→↑r, despite ↓b ↑r →↑ ∆N/∆t

Exponential Growth

• r : population growth rate

• rmax : biotic potential – potential growth rate under ideal conditions • K : – maximum population that the environment can sustain over long periods of time. – determined by biotic and abiotic • Population multiplies by a constant factor. • Growth rate not limited by resources. limiting factors. • “J”-shaped growth curve.

Heyer 3 Population Structure & Dynamics

Carrying Capacity determined by Density-Dependent Limiting Factors Curves • Growth = ∆N/∆t = rN . {r=b-d}

Competition for resources Disease Predation • Rate of population growth only limited by

rmax. • “r-limited”

Territoriality Intrinsic factors Toxic 5 µm

Figure 53.18

Logistic growth Laboratory populations with defined resources exhibit • Growth is limited by density-dependent resources or other factors • Decrease growth rate produces “S”-shaped (sigmoidal) curve • “K-limited”

Fur seal population “K-limited”

Growth Equations: Growth Equations: Exponential vs. Logistic Exponential vs. Logistic • Exponential 2,000

• Growth rate (G) = dN/dt = rN dN = 1.0N Exponential dt • This growth is always increasing. growth 1,500 K = 1,500 • Logistic Logistic growth

• Growth rate (G) = dN/dt = rN([K-N]/K) 1,000 dN 1,500 N = 1.0N dt 1,500 • Exponential

 When N <<< K (pop is v. low), [K-N] = K and Population size ( N )  dN/dt = rN dN/dt = rN(K/K) = rN (growth is exponential). 500 • Logistic

 When N approaches K, [K-N] approaches zero  dN/dt = rN([K-N]/K) 0 and dN/dt = rN(0/K) = 0 (growth stops). 0 5 10 15 Number of generations Figure 52.12

Heyer 4 Population Structure & Dynamics

A population reaches carrying capacity when growth rate is zero 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 • “r-limited”: J-type growth rate limited by r, and/or disease — “boom & bust” pattern. but cannot be sustained indefinitely beyond K. • “K-limited”: S-type growth rate limited by K

Outcome of Exponential Growth “Boom and Bust” Population Cycles • 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” Fort Bragg, CA Bragg, Fort SCALE LOG

K

• “r-selected” • Population cycles between a rapid increase and then a sharp decline.

“Boom and Bust” Population Cycles Trophic (food resources) limiting factors • Top-down regulation (populations regulated by higher levels of the ): increase in predator (lynx) population causes a decrease in the prey (hare) population. – Original hypothesis 160 Snowshoe hare

120 Lynx 9 80 6 (thousands) (thousands) (thousands) (thousands) 40 3 Lynx population size Lynx Hare population size Harepopulation size 0 0 1850 1875 1900 1925 Figure 52.21 Year • Bottom-up regulation (populations regulated by lower levels of the food chain): increase in hare population causes an over- consumption of the vegetation; decrease in vegetation causes a decrease in hare population; decrease in hare population causes a • “r-selected” decrease in predator (lynx) population • Population cycles between a rapid increase and then a sharp decline. – Revised hypothesis. Hare populations oscillate even in the absence of lynxes.

Heyer 5 Populations & Life History Strategies

Life History Traits Trade-offs, and the allocation of resources Life History Diversity  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, favors the generalist populations that • A life history entails three main variables opportunistically harvest any available to grow as fast as possible when they can, and quickly produce many offspring distributed over a wide 1. The age at which reproduction begins area to increase chance of hitting someplace good. (“weeds”) • “r-selected” — select for high reproductive potential 2. How often the reproduces  For species inhabiting stable environments: • Long-term strategy is most successful. 3. How many offspring are produced per • Natural selection favors the specialist populations that excel at harnessing the particular available resources to displace competitors. Spend resources reproductive episode 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.

Type: r-selected K-selected Reproductive Strategies Figure 52.8 Major source of Juvenile predation / Competition • Semelparity mortality Sporadic catastrophes – Produce one huge batch Generation time (age) Short (young) Long (old) of offspring and then die Adult size Small Large Reproduction Semelparous Iteroparous

(a) Most weedy plants, such as this dandelion, grow Fecundity Very high Low quickly and produce a large number of seeds. Newborn size Small Large • Iteroparity Dispersal of young High Low Parental care Low/none High – Produce several smaller Traits History Life batches of offspring Newborn behavior Precocial Altricial distributed over time Juvenile mortality Very high Low Survivorship curve Type III Type I (b) Some plants, such as this coconut palm, produce a Pop. growth curve Cyclic Sigmoidal moderate number of very large seeds.

Life History Plasticity K-selected populations Daphnia ostracod • Equilibrium (b=d) in culture at or below carrying capacity. • Must either ↑d or ↓b or both.

Density-dependent Density-dependent Density-dependent birth rate birth rate Density- Density- death rate independent independent Density- dependent death rate birth rate death rate

Equilibrium Equilibrium Equilibrium Birth or death rate per capita rate per capita density density density • Switch from r-limited growth to K-limited, before Population density Population density Population density (a) Both birth rate and death rate (b) Birth rate changes with (c) Death rate changes with environmental degradation is irreversible. change with population density. population density while death population density while birth rate – At low population densities, short generation time, high fecundity. rate is constant. is constant. – At high densities, change physiology to longer generation time, more body growth, lower fecundity. Figure 52.14

Heyer 6 Populations & Life History Strategies

Density-dependent mortality K-selected populations Predator selectivity Kelp bass • “Good” K-selected species achieve equilibrium density Kelp perch (predator) (prey) by decreasing birth rate as population approaches K. 1.0 4.0 10,000 0.8 3.8

3.6 0.6 1,000 3.4

0.4 (log scale) 3.2 3.0 Average clutch size clutch size Average

Average number of seeds Average 100 per reproducing individual

Proportional mortality 0.2 0 2.8 0 10 100 0 10 20 30 40 50 60 70 80 0 Seeds planted per m2 Density of females 0 10 20 30 40 50 60 Kelp perch density (number/plot) (a) Plantain. The number of seeds (b) Song sparrow. Clutch size in the song sparrow on produced by plantain (Plantago major) Mandarte Island, British Columbia, decreases as Figure 52.17 decreases as density increases. density increases and food is in short supply. Figure 52.15

Even K-limited populations may 2011- 7 Human fluctuate over time 1999- 6 • Variations in limiting factors cause variations in K Population 1987- 5

FIELD STUDY Researchers regularly surveyed the population of moose on Isle Royale, Michigan, from 1974- 4 1960 to 2003. During that time, the lake never froze over, and so the moose population was isolated from Growth the effects of immigration and emigration. RESULTS Over 43 years, this population experienced two significant increases and collapses, as well 1960- 3 as several less severe fluctuations in size. Dramatic collapse caused 1927- 2,500 Steady decline by severe winter weather 2 probably caused and food shortage, 2,000 largely by wolf Agricultural-based leading to starvation of 1804- Human population (billions) urban societies Industrial Revolution 1 predation more than 75% of the Black Plague 1,500 population 1,000 0 5000 4000 3000 2000 1000 0 1000 2000 500 BCE BCE BCE BCE BCE CE CE Figure 53.22 Moose population size Moose population size 0 The history of human population growth 1960 1970 1980 1990 2000 Year • Human pop now increases by 80 million/yr. CONCLUSION The pattern of observed in this isolated population indicates that – That’s a new LA every two weeks !! various biotic and abiotic factors can result in dramatic fluctuations over time in a moose population. Figure 52.18 • Projected 8 billion in 2024. 10 billion by 2050.

2011- 7 can artificially increase Human 1999- 6 carrying capacity Population 1987- 5 Growth 1974- 4 1960- 3 • Technological advances avoid

1927- 2 natural growth constraints

Agricultural-based

1804- Human population (billions) urban societies Industrial Revolution 1 – Hunting and gathering Black Plague 0 – Agricultural revolution 5000 4000 3000 2000 1000 0 1000 2000 • ParadoxBCE or timeBCE bomb??? BCE BCE BCE CE CE Figure 53.22 – Industrial revolution • Homo sapiens life history traits show Type I survivorship that should – Scientific revolution correlate with a K-selected sigmoidal growth curve. • But, our actual growth curve is exponential!!! • What happens to a population that exceeds its carrying capacity?

Heyer 7 Populations & Life History Strategies

Age structure pyramids • = High birth rates – High death rates 50 • Zero population growth = Low birth rates – Low death rates

40

30

20

10 Sweden Mexico Birth rate Birth rate

Birth or death rate per 1,000 people Death rate Death rate 0 1750 1800 1850 1900 1950 2000 2050 Year Fig. 53.25

Human carrying capacity ’s Human Carrying Capacity is not infinite • = land per person needed to support resource • Resources will eventually be depleted demands • Economic resources allow exploitation • US footprint is of natural resources 10X the footprint • above the • Industrialized nations consume more mid-line are in resources per capita 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 • The overpopulation and by the human . Produces 21% of all goods and services population are triggering an enormous array of problems, . Uses 25% available processed minerals and nonrenewable resources ranging from food sources (, fisheries), ,

. Generates at least 25% of world’s air and , energy and use, habitat pollution and trash destruction, and species . You can calculate • India

. 17% of the world population your own ecological footprint by going to the following

. Produces 1% goods and services URL:

. Uses 3% available processed minerals and nonrenewable energy resources . Generates 3% world’s pollution and • http://www.myfootprint.org/ trash • U.S. consumes 50 times more resources than India (per person) • US footprint is 10X the India footprint Ecological footprint vs. ecological capacity

Heyer 8