23.0 Introduction

1. Ecology
1. The study of relationships of organisms with one another and the environment
2. Includes properties of populations, emphasizing population dynamics fig. 23.1

23.1 Populations are individuals of the same species that live together

1. Population Ecology
1. Population
1. Individuals of a species that live together
2. Huge cities of human populations scattered all over the world fig 23.2

2. The Science of Ecology
1. How organisms interact with their environment and each other
2. Understand why certain organisms live in one place, not another
3. Physical and biological variables for distribution and numbers of organisms
4. Determine principles to predict future interactions of organisms and their environment

3. Levels of Ecological Organization
1. Populations
1. Individuals of same species
2. Interbreed with one another
3. Share habitat, use same pool of resources

2. Communities
1. Populations of different species living together in same place
2. Utilize different resources

3. Ecosystems
1. Community plus its nonliving factors
2. Regulates flow of energy from sun
3. Cycling of essential elements

4. Biome
1. Major collections of land plants, animals and microorganisms
2. Occur over wide geographical areas, have distinct characteristics

4. Population Structure
1. Population size
1. Direct bearing on the ability of a given population to survive
2. Very small populations are more likely to become extinct
3. Inbreeding can be a negative factor
1. Lowers vigor by direct genetic effects
2. Produces reduced levels of variability

4. Extinction is more likely to occur in areas that change radically

2. Population density
1. With wide spacing, individuals may only rarely interact
2. Limit reproductive capabilities even if absolute numbers are high

3. Population dispersion
1. Way in which individuals are arranged fig 23.3
2. Randomly spaced
1. Do not interact strongly with each other or nonuniform microenvironment
2. Not common in nature

3. Uniformly spaced
1. Interact directly with one another
2. In animals may result from social interactions
3. in plants may reflect competition for sunlight or water

4. Clumped
1. Respond to uneven distribution of resources
2. Common in nature

5. Metapopulations
1. Clumped populations that undergo periodic extinction and recolonization
2. Fragmented habitat, suitable sites scattered in large stretches of unsuitable sites
3. Persists as long as local recolonization equals or exceeds local extinction rate

23.2 Population dynamics depend critically upon age distribution

1. Demography
1. Statistical Study of Populations
1. Measurement of people, therefore the characteristics of populations
1. Helps predict ways in which sizes of populations will alter the future
2. Accounts for age distribution and changing population size over time

2. Future population size depends on present age structure and sex ratio

2. Age Structure
1. Annual versus perennial organisms
1. Annual plants and insects time reproduction to particular season
1. Reproduce, then die
2. All individuals are same age

2. Perennial plants, longer-lived animals contain members of more than one generation
1. Individuals of different ages reproducing within the population
2. Cohort: Group of individuals of same age

2. Cohort exhibits certain characteristic changes
1. Fecundity: Characteristic birth rate = offspring produced in a standard time
2. Mortality: Number of individuals that die over same period
3. Rate of population growth depends on difference between the two

3. Population's age structure
1. Relative number of individuals in each cohort
2. Different ages have different fecundity and death rates
3. Has critical impact on populations growth rate
4. When proportion of young individuals is large, population grows rapidly

3. Sex Ratio
1. Proportion of males to females
2. Number of births generally related to number of females
3. May not be closely related to number of males, especially in "harem" conditions
1. Reduce number of males, changes only identity of reproductive males, not births
2. Reducing males in monogamous pairs decreases births

4. Mortality and Survivorship Curves
1. Intrinsic rate of increase depends on age and reproductive performance
2. Age distribution: Proportion of individuals in different age categories
1. Constant environment stabilizes a population's age distribution
2. Distribution varies by species

3. Sex ratio and generation time can also affect population growth
4. Stable population
1. Population with constant size through time
2. Birth + immigration = death + emigration

5. Survivorship curves express characteristics of populations fig 23.4
6. Survivorship: Percentage of original population that survives to a given age
7. Types of survivorship curves
1. Type II
1. Example: Hydra
2. Straight curve
3. Individuals are likely to die at any age

2. Type III
1. Example: Oysters
2. Produce vast numbers of offspring, few survive to reproduce
3. Once established mortality is low

3. Type I
1. Example: Humans
2. Relatively low mortality when young
3. High mortality in postreproductive years

5. Life Tables
1. Mortality tables indicate chance of survival at any age
2. Constructed by following fate of a cohort from birth to death
1. Cohort identified and followed for many years
2. Difficult task, individuals may mingle with individuals of other cohorts

3. Example: Loew's study of red deer in Scotland
1. Deer live for up to 16 years, females can breed when four
2. Counted all deer in 1957, included number of calves (less than one year old)
3. Chose female calves as cohort to be followed through subsequent years
4. Every dead deer from 1957 to 1966 was examined to determine if in this cohort
5. Constructed life table tbl 23.1
6. Constructed survivorship curve, type I curve fig 23.5
1. Low mortality when young
2. Greater mortality after four, when entering reproductive age

4. Alternate approach to cohort life tables
1. Estimate mortality from age structure at one point in time
2. Called a static or cross-sectional life table

23.3 Life histories often reflect trade-offs between reproduction and survival

1. The Cost of Reproduction
1. Complete Life Cycle of an Organism Is Its Life History
1. Very diverse, dependent on kind of organism
2. Organisms cannot always reproduce at early age, produce large families
3. Limited by available resources
4. Natural selection favors maximizing total reproductive value (TRV)

2. Reproductive Trade-Offs
1. Increasing reproduction may decrease survival, reduce future reproduction fig 23.6
2. Example: Douglas fir tree
1. Produces more cones increases current reproductive value (CRV)
2. Grows more slowly decreasing future residual reproductive value (RRV)

3. Comparison: Birds producing more offspring each year to delaying reproduction
1. More offspring, increasing CRV
1. More likely to die that year, decrease RRV
2. Or likely to produce smaller clutch size next year, decrease RRV

2. Delaying reproduction
1. Individual may grow faster, larger
2. Enhances future reproduction

4. Cost of reproduction (CR): Reduction in future reproductive potential
5. Natural selection favors life history with greatest CRV = RRV
1. CRV tends to go up as RRV goes down
2. Cost of contemporary reproduction contributes to decrease in RRV
3. Trade-off influences evolution of organism's life history

6. Additional trade-off in number versus. size of individuals
1. Larger offspring, fewer produced
2. Example: Poultry production negative correlation between egg size and number

7. Related trade-off number of offspring versus degree of parental care

3. The Role of Habitat
1. High CR habitat
1. Reduced growth from present reproduction negatively impacts RRV
2. Habitat with intense competition between individuals
1. Reducing present reproduction may allow increased growth
2. Followed by improved competitive ability and subsequent increased RRV

2. Low CR habitat
1. RRV not greatly affected by present reproduction
2. When mortality unavoidable, increased size worthless in future
3. Total reproductive value is same for any level of growth

2. Life History Adaptations
1. Optimum Clutch Size
1. Clutch size: Greatest number of offspring produced in a reproductive event
2. Cost-of-reproduction trade-off: Greater number and small versus fewer and large
3. Lack proposed that natural selection favors compromise clutch size
1. Allows the maximum number to come to maturity
2. Known as "Lack clutch size"
3. Experiments suggest proposal is wrong
4. Observed natural clutch size is not most productive
5. Proposal ignores cost of reproduction
6. Large clutch may cost to much in RRV fig 23.7
7. Favored clutch size less than what appears to be most productive

2. Reproductive Events per Lifetime
1. Trade-off between age and fecundity
2. Semmelparity:focus all resources on single reproductive event followed by death
3. Iteroparity: Produce offspring many times over life history
4. Species that reproduce yearly must not over tax reproduction in any one year fig 23.8
5. Semelparity characteristics
1. Short-lived species, great cost of surviving from brood to brood
2. Example: Plants growing in harsh habitats
3. Also favored when fecunditity entails large reproductive cost
4. Example: Pacific salmon migrating to spawn

3. Age at First Reproduction
1. Animals that live longer generally reproduce later fig 23.9
1. Birds gain experience as juveniles before expending high costs of reproduction
2. Advantage of experience outweighs energy investment in survival and growth

2. In short-lived animals quick reproduction is more critical than juvenile training

23.4 Population growth is limited by the environment

1. Biotic Potential
1. Population Size Remains Constant Regardless of Offspring Produced
1. Unchecked, most populations would increase dramatically
2. Must consider circumstances and factors that limit population growth

2. The Exponential Growth Model
1. Biotic potential (r): Population growing without limits at maximal rate
2. dN/dt = riN
1. N = number of individuals within a population
2. dN/dt = rate of change of population number over time
3. ri = intrinsic rate of growth for that population

3. Actual rate of population increase (r)
1. Difference between birth rate and death rate per given number of individuals
2. Corrected for net emigration (movement out) or immigration (movement in)
3. r = (b - d) + (i - e)
4. Movement of individuals can have great impact on population growth rate

4. Innate capacity for growth is exponential, represented by growth curve
1. Rate of growth remains constant
2. Actual increase in numbers accelerates as population increases
3. Analogous to compounding interest on an investment

5. Such patterns of growth occur for only short periods fig 23.10
1. New organism reaches new habitat with abundant resources

3. Carrying Capacity
1. Populations always reach a limit imposed by environmental shortages
2. Size for such stabilization is the carrying capacity (K)
1. A dynamic rather than static value
2. Changes as characteristics of place change

4. The Logistic Growth Model
1. Rate of growth slows as population reaches carrying capacity
1. Fewer resources for individuals to use
2. Growth curve limited by one or more factors

2. Logistic growth equation
1. dN/dt = rN(K-N/K)
2. dN/dt = growth rate of the population
3. r = rate of increase
4. N = number of individuals present at any one time
5. K = carrying capacity

3. As a population grows in size, the rate of increase declines until N=K fig 23.11
1. Competition among individuals for resources increases
2. Build up of wastes
3. Increased ratio of predation

4. Relationship is an S-shaped sigmoid growth curve fig 23.12
5. As the population stabilizes its rate of growth slows down

2. Applying Growth Models to Real Populations
1. Many Species Have Fast Rates of Population Growth
1. Growth not effectively controlled by reductions in population size
1. Small populations quickly enter an exponential pattern of growth
2. Growth best described by exponential growth model

2. Most populations have slower growth, described by logistic growth model
1. Populations with sigmoid growth curves limited as resources limited
2. Number of individuals supported is the carrying capacity (K)

2. r and K Selected Adaptations
1. Life history adaptations characterized by rapid growth and sudden crashes
1. Reproduce early, produce many small offspring that mature quickly
2. Have high rate of increase (r)
3. Called r selected adaptations
4. Examples: Dandelions, aphids, mice, cockroaches fig 23.13

2. Life history adaptations favor survival
1. Individuals competing for limited resources
2. Reproduce late, have small broods
3. Offspring are large, mature slowly, receive intensive parental care
4. Favor reproduction near carrying capacity
5. Called K selected adaptations
6. Examples: Coconut palms, whooping cranes, whales

3. Most populations show life history adaptations in a continuum
1. Range form completely r selected traits to completely s selected traits
2. Certain adaptations exist at the extremes of the continuum tbl 23.2

3. The Influence of Population Density
1. Density-Independent Effects
1. Operate regardless of the population size
2. Include factors such as weather and physical disruption of habitat

2. Density-Dependent Effects
1. Depend on size of population, regulate its growth
2. Accompanied by hormonal changes that alter animal behavior
3. Example: Migratory locusts
1. When crowded, produce hormones causing a migratory phase
2. Take off as swarm, travel long distances to new habitats fig 23.14

4. In general have an increasing effect as population increases

3. Maximizing Population Productivity
1. Agriculture and fisheries depend on characteristics of a sigmoid growth curve
1. Maximize productivity by exploiting population early in rising part of curve
2. Populations grow rapidly, net productivity is highest

2. Commercial fisheries harvest at the steep, rapidly growing part of the curve
1. Produces optimal yield, maximum sustainable yield from population
2. Harvested at this point results in best sustainable yield
3. Over harvesting smaller population can destroy its productiveness
4. Can also lead to extinction
5. Example: Peruvian anchovy fisheries in 1972 fig 23.15
6. Difficult to determine population levels of commercially valuable species
7. Hard to determine yield best suiting continued productive harvesting

4. Human Populations
1. Humans Exhibit Many K Selected Life History Traits
1. Small brood size, late reproduction, high degree of parental care
2. Traits evolved during early life history of hominids
1. Limited resources of environment controlled population size
2. Populations regulated by food, disease, predators
3. Unusual disturbances affected population size
4. Population grew only slowly during early human history

2. The Advent of Exponential Growth
1. Humans have expanded populations by technical innovations
1. Control over food supply
2. Weapons ward off predators
3. Cures for many diseases
4. Improvements in shelter and storage capacities

2. Humans have expanded carrying capacity of their habitats
1. Escape confines of logistic growth
2. Reenter exponential phase of sigmoid growth curve

3. Human population has grown explosively in last 300 years
1. Birth rate has remained relatively constant (30 per 1000 per year)
2. Death rate has decreased dramatically (from 29 to 13 per 1000 per year)
3. Difference (17) produces 1.7% increase per year

4. Such increase has produced human population of almost 6 billion fig 23.16
1. 100 million individuals add to world population each year
2. Human population will double in 40 years

3. Population Pyramid
1. Growth of human population not occurring at constant rate over whole planet
2. Some countries, Mexico, growing rapidly,birth rate exceeds death rate fig 23.17
3. Assessed graphically by population pyramid
1. Bar graph displaying numbers in each age category
2. Males to left, females to right or vertical age axis
3. Number of older females usually greater than older males

4. Can predict demographic trends in births and deaths
1. Rectangular shape indicates stable population
2. Triangular shape shows country with rapid future growth
3. Inverted triangle shape represents population that is shrinking

5. Examples of population pyramids for U.S. And Kenya fig 23.18
1. U.S. has nearly rectangular shape, not expanding rapidly
2. Cohort of 55 to 59 year olds are Depression babies, smaller than adjoining cohorts
3. Cohort 25 to 44 years old is "baby boom"
4. Triangular shape of Kenya shows explosive future growth
5. Population expected to double in less than 20 years

4. An Uncertain Future
1. Rapid human population growth is challenge to future of biosphere
1. Adding one million people every three days
2. Staggering growth rate in certain countries tbl 23.3

2. Key element is uneven distribution of growth
1. 90% in developing countries fig 23.19
2. Reducing fraction of those living in industrialized countries
3. Growth centered in areas least able to deal with pressures of rapid growth

3. Harsh consequence of increasing gap between rich and poor
1. 23% live in industrial countries, $17,900 per capita income
2. 77% in developing countries, $810 per capita income
3. 85% of capital wealth in industrialized, 15% in developing countries
4. 80% of energy used by industrialized, 20% by developing countries
5. 94% of scientists in industrialized, 6% in developing countries

4. Unknown whether world can sustain population of 6 billion
1. Cannot expect to increase carrying capacity indefinitely
2. Unavoidable need to equalize birth and death rates
3. Must lower birth rates fig 23.17