Population Ecology

Populations A population is a group of individuals of the same species that live in the same area

Population Ecology: Vocabulary Density: The number of individuals per unit area/volume Example: 47 elephants/km2 Dispersion: The pattern of spacing among individuals in a population Clumped Uniform Random

Uniform Environmental conditions are uniform Causes COMPETITION or antagonism between organisms

Clumping Most common Reproductive patterns favor clumping Social behaviors lead to clumping Optimal density is usually intermediate (medium)

Random No competition No tendency to group/clump Conditions are uniform Rarely happens!

Factors That Influence Population Size There are 3 major factors that influence population size: 1. the number of births 2. the number of deaths 3. the number of individuals that enter or leave a population - immigration: individuals entering an existing population - emigration: individuals leaving an existing population

Reproductive Episodes Clutch size: Number of offspring produced at each reproductive episode Semelparity A life history in which an organism spends most of its energy in growth and development, expend their energy in one large reproductive effort, and then die Many insects, annual plants, salmon, etc.

Reproductive Episodes Iteroparity A life history pattern in which organisms produce fewer offspring at a time over a span of many seasons Example: humans, panda bears, etc.

Estimating Population Size The mark-recapture method can be used to estimate the size of a population Capture, mark, release Recapture and count Equation: N = Number marked x Total catch 2nd time Number of marked recaptures

Patterns of Population Growth Exponential Growth: Occurs in ideal conditions with unlimited resources J shaped curve Book example: 1 bacterium (reproducing every 20 minutes) could produce enough bacteria to form a 1-foot layer over the entire surface of the Earth in a day

Patterns of Population Growth Exponential growth cannot continue indefinitely It is characteristic of populations who are entering a new environment OR those whose numbers are rebounding from a catastrophic events

r- strategists Grow exponentially when environmental conditions allow; when conditions worsen, population size plummets. Short life span Reproduce early in life Many offspring/large clutch size Usually small in size Little or no parental care Bacteria, some plants, insects

Patterns of Population Growth Logistic Growth: Pattern of population growth which takes into account the effect of population density on population growth Occurs when resources become more scarce Characterized by an S-shaped curve

Patterns of Population Growth Carrying capacity (K): The maximum number of individuals that a particular environment can support over a long period of time Determined by such limiting factors as crowding and food resources Graph levels off at carrying capacity K-selected populations (equilibrial populations) live near or at the carrying capacity

K-strategists Density stays near carrying capacity. Large, slow growing organisms Small population sizes Long life span; slow maturation Few young/small clutch size Reproduce late in life Parental care Most large mammals; endangered species

Carrying Capacity

Boom-and-Bust Cycles

Limiting Factors There are a number of factors that limit the size of populations: Density-dependent limiting factors Density-independent limiting factors

Density-Dependent Limiting Factors The effect of density-dependent limiting factors intensifies as the population increases Intraspecific competition Food, space, etc. Predation Disease (if caused by pathogen/contagious)

Density-Independent Limiting Factors The occurrence and severity of density-independent limiting factors are unrelated to population size Climate Disease (if not caused by pathogen/not contagious) Pollution

The Interaction of Limiting Factors Density-dependent and density-independent limiting factors often work together to regulate the size of a population Deer in snowy winter Starve from lack of food (DDLF) Severity of winter/depth of snow determines access to food (DILF)

Survivorship Curves Type I- live to old age & die (most large mammals) Type II- constant mortality rate (rodents, lizards, hydra) Type III- high mortality at young age, but if they survive they live a long life.

Age-Structure Diagrams

Human Population Growth

Human Population Growth The human population has been increasing exponentially since approximately 1650 http://www.pbs.org/wgbh/nova/worldbalance/numbers.html

Human Population Growth Implications of exponential human population growth: Lack of food supplies Lack of space Lack of natural resources (metals, fossil fuels, etc) Lack of sites for waste disposal Ecologists cannot agree on a carrying capacity for Earth Are we going to reach carrying capacity through individual choices and/or government programs? OR Is Earth’s population going to “level off” as a result of mass deaths?