Ecology – Population Growth and Regulation

Ecology – the study of interrelationships among living things and their nonliving environment

Environment – 2 components:

•Abiotic – nonliving (soil, water, weather, pH)

•Biotic – living (all forms of life)

Levels of Organization:• Population – all members of a particular

species who live within an ecosystem, interact with one another and can potentially interbreed

• Community – all the interacting populations in an ecosystem

• Ecosystem – all the organisms and their nonliving environment within a defined area (prairie, forest)

• Biosphere – the part of the earth inhabited by living organisms; includes both living and nonliving components

Populations• Population density – number of

individuals/unit area

• Measuring density– In rare cases, population size and density can

be determined by counting the actual number of organisms

– Usually done by random sampling– Common method used is the mark-recapture

method (read in textbook)

• Population distribution – the spatial pattern in which members of a population are dispersed within a given area1. Clumped – members of a population live in groups

(herds, flocks, schools) Advantage includes more individuals to find food, large groups

confuse predators May clump due to resource availability (cotton trees cluster

along streams, animals around water holes)

2. Uniform – organisms maintain a relatively constant distance between individuals Common in animals that are territorial Helps ensure adequate resources for each individual

3. Random – patternless and unpredictable, occurs in the absence of strong attractions or repulsions among individuals, resources equally available (more rare) ex. Trees and plants in rainforest

Dispersion Patterns

Clumpeddispersion

Uniformdispersion

Randomdispersion

Clumped Dispersion

Uniform Dispersion

Random Dispersion

• Population growth – determined by three factors: births, deaths, and migration (immigration – migration into a population; emigration – migration out of a population)

• (births – deaths) + (immigrants – emigrants) = change in population size

• Ultimate size of a population results from a balance between opposing factors: Biotic potential – maximum rate at which the population

could increase, assuming ideal conditions that allow a maximum birth rate and minimum death rate

Environmental resistance – limits set by the environment such as availability of food, space, competition, predation, and parasitism

Biotic Potential• Growth rate (r) – measure of the change in

population size per individual per unit of time

b - d = r

(birth rate) (death rate) (growth rate)

• To determine number of individuals added to a population in a given time period, growth rate (r) is multiplied by original population size (N)

population growth = rN

Biotic Potential• Exponential growth – pattern of continuously accelerating

increase in population size – forms a J-curve– Example: bacteria under ideal lab conditions could produce enough

bacteria to form a foot deep layer over the entire Earth starting with just one cell dividing every 20 min.

• Biotic potential is influenced by:

1. Age at which the organism first reproduces

2. Frequency with which reproduction occurs

3. Average number of offspring produced each time

4. Length of organism’s reproductive life span

5. Death rate of individuals under ideal conditions

• Biotic potential helps ensure that at least one offspring survives to bear it’s own young

• In nature, exponential growth occurs only for a limited time

Regulation of Population Growth• Population size is limited by environmental resistance (such

as available resources)• Boom-and-Bust cycles – populations characterized by rapid

growth followed by a sudden massive die-off

ex. Algae, insects – have seasonal cycles linked to rainfall, temperature, nutrient availability

Insects grow during spring and summer and die with the frost

• S-curves – after a period of growth, populations tend to stabilize at or below the max number the environment can sustain– Reaches a state of equilibrium with growth rate of zero– Carrying capacity – max population size that an ecosystem

can support indefinitely– Typical of long-lived organisms

Limiting Factors – 2 Major Types1. Density independent – limit size of

population regardless of population density

• Weather (causes boom and bust cycles), human activity (pesticides, pollution, habitat destruction), natural catastrophes (flood, fire, hurricane)

2. Density dependent – limit size of population as population density increases

• Major limiting factor for long-lived species• Examples include:

predation and parasitism – control prey/host populations

Competition for limited resources• Intraspecific competition – among individuals of

the same species (very intense, impt in natural selection)

• Interspecific competition – among individuals of different species• Usually, if two species niches overlap too

much, will lead to Competitive Exclusion Overcrowding may lead to physiological and

behavioral changes that increase emigration

Competitive Exclusion Principle• Proposed by G.F. Gause (Gause’s Principle)• If two species niches overlap too much, one species will be

better at getting the resources and the other will be eliminated

Mortality and Survivorship Curves• Populations show characteristics patterns of deaths or

survivorship over time1. Late Loss – show a convex shaped curve, relatively low

infant death rates, most individuals survive to old age

Red curve

Ex. Humans, large animals

Mortality and Survivorship Curves2.Constant Loss – individuals have a fairly constant

death rate, have an equal chance of dying at any time during their life span

Black line

Ex. annual plants, hydra, some invertebrates, some rodents

Mortality and Survivorship Curves3. Early Loss – show a concave curve, produce large numbers

of offspring that receive little parental care, death rate is very high among offspring, those that become adults have a good chance of surviving to old age

Blue curve

Ex. Most invertebrates, plants, many fish

Life History• Traits that affect an organism’s schedule of

reproduction and death• K- selected populations (species or

strategists) and r-selected populations (species or strategists) represent hypothetical models

r-selected populations• Usually do not reach carrying capacity

• Tend to live in unstable, temporary environments

• Generally limited by density independent factors

• Produce large numbers of offspring

• Early loss survivorship curve

K-selected populations• Usually have slow or no population growth

• Usually at or near carrying capacity

• Live in stable predictable environments

• Produce few young and provide parental care

• Competition for limited resources limits population size

• Late loss survivorship curve