population dynamics chapter 14: population ecology

Population Dynamics

Chapter 14: Population Ecology

Characteristics of Populations

• Habitat– Place where an organism or species normally

lives

• Species– Organisms that are able to interbreed with

one another to produce fertile offspring– Resemble one another in appearance,

behaviour, chemistry and genetic make-up– Mule-not a species but a hybrid (donkey and

horse)

Characteristics of Populations-cont’d

• Population size– Number of individuals of a species occupying a

given area/volume at a given time

• Population density (D)D=N N: Total number of individuals

S S: Space occupied by the population

• Small organisms usually have higher population densities than larger organisms


• Crude density– Population density measured in terms of the

number of organisms of the same species within a habitat’s total area

• Ecological density– Population density measured in terms of the

number of organisms of the same species within the area actually used by those organisms (ex. land animals do not live in lakes)


• Population dispersion patterns vary by species within a habitat (fig. 4, page 652)– Clumped dispersion (ex. schools of fish)– Uniform dispersion (ex. King penguins)– Random dispersion (ex. rainforest trees)

• Rarely found in nature


• It is often impractical to count exact numbers of individuals in a population– Biologists resort to counting a sample of the

population at a particular time and then use this number to estimate total size

– There are three main sampling techniques:• Quadrat sampling• Mark-recapture method• Technological tracking devices


• Quadrat sampling– Used for stationary or small organisms (ex.

plants, insects)– A quadrat (a defined area/frame; ex. 1m2) is

used and samples of one or more populations is counted within that area

– Population size and density can be determined based on that quadrat’s data, which can be extrapolated for the entire area


• Mark-Recapture Method– Used for mobile wildlife populations (ex. fish,

bears)– A sample of animals is captured, marked in

some way and then released– Those organisms are allowed to mix randomly

with unmarked animals and after a period of time, a second sample of animals is captured

• The proportion of marked to unmarked animals in this sample can be used to estimate the size of the entire population


• Mark-Recapture Method-cont’dM = m

N n

M = Total # marked

N = Total population

m = # of recaptures

n = Size of 2nd sample


• Technological Tracking Devices– When capturing animals for estimating

population size, researchers may also attach radio collars, satellite-linked devices or other technological equipment

• Used to track migration and/or behaviour patterns• Information can be mapped in geographic

information systems (GIS)• Must ensure these devices do not harm the

animals or restrict their activities

Measuring and Modeling Population Change

• Resources available to individuals in a population are finite and can be biotic (living) or abiotic (non-living)

• Carrying capacity– The maximum number of organisms that can

be sustained by available resources over a given period of time (aka biocapacity)


• Population dynamics– Changes in population characteristics determined by

natality, mortality, immigration and emigration

• Fecundity– The potential for a species to produce offspring in one

lifetime (ex. high fecundity-starfish; low fecundity-humans)

– Fertility is often significantly less than this due to food availability, mating success, disease, migration


• Biotic potential– The maximum rate a population can increase

under ideal conditions (ex. E. coli can double every 12 minutes)

• Population change =[(births + immigration)-(deaths + emigration)] x 100

initial population size (n)


• Open population– Size and density influenced by births, deaths, immigration

and emigration– Most wild populations are open

• Closed population– Size and density Influenced by births and deaths only– No migration occurs– Rarely found in nature (ex. isolated animals on an island)– Laboratory settings are closed – Mark-recapture sampling (due to short time frame

between samples)


• Three general survivorship patterns of species:

Type I: High survivorship until fairly late in life, then high mortality

Typically produce small numbers of offspring

Type II: A fairly constant death rate

Type III: Low survivorship early in life (ex. green sea turtles)

Typically produce large numbers of offspring


• Geometric growth– For many species, while deaths can occur

throughout the year, births are restricted to a particular time of the year (breeding season)

• Population grows rapidly during breeding season then declines throughout the rest of the year until the next breeding season

– Growth rate is a constant from year to year but not during the year


• Geometric growth-cont’d


• Exponential Growth– A wide variety of species are able to reproduce at a

fixed rate on a continuous basis– Doubling time is a constant value– Populations exhibiting this growth increase in

numbers rapidly– Results in a J-shaped growth curve– Although the overall graph appears similar to

geometric growth (J-shape), in this case, there are no fluctuations (jumps) as a result of breeding times


• Exponential growth-cont’d


• Logistic Growth– Most common growth pattern seen in nature– Geometric and exponential models of

population growth assume that a population will grow indefinitely at the same rate

• Realistically, food, water, light and space in the ecosystem become factors that limit population growth as resources are consumed and the population nears the ecosystem’s carrying capacity

• At the carrying capacity, the number of births and the number of deaths become equal


Logistic Growth-cont’d


• Logistic Growth-cont’d– S-shaped (sigmoidal) curve– Three distinct phases:

• Lag phase: Population is small and increasing slowly

• Log phase: Population undergoes very rapid growth

• Stationary phase: Population growth slows as the population experiences environmental resistance. Occurs at or close to the carrying capacity. Population is at dynamic equilibrium (births=deaths) and no net increase in numbers occurs.

Factors Affecting Population Change: Density-Dependent Factors

• Density-Dependent Factors– Play a greater role in limiting population growth

as the population increases in size

– Ex. competition, predation, disease

• Intraspecific competition– When the individuals of a population of the

same species compete for the same resources• Growth rate slows as the population increases• Can reduce the number of offspring born


• Predation– A carnivorous predator (of one species)

catches, kills and consumes prey (of another species)

– Some predators prefer one type of prey over another if that prey has a larger population and is easier to catch

• Helps to regulate the number of preferred prey


• Disease– In dense or overcrowded populations,

pathogens are able to pass more easily from host to host, since there are more hosts close together

• Population deceases in size due to increased mortality


• Allee Effect– When population numbers become so low

that the species cannot reproduce fast enough to offset mortality

• Mates are difficult to find since there are so few individuals

• Population usually does not survive; especially harmful to threatened species


• Minimum viable population size– The smallest number of individuals needed to

ensure a population can continue for a given period of time

• Population can cope with variations in natality and mortality, as well as environmental changes or disasters

• Population maintains enough genetic variation amongst its members

Factors Affecting Population Change: Density-Independent Factors

• Density-Independent Factors– Play a role in limiting population growth

regardless of population size– Ex. extreme weather, human intervention

• Ex. Thrips (insect) have lower reproductive success at lower temperatures

• Insecticide application (can result in death directly or through biomagnification)

Factors Affecting Population Change: Density-Independent Factors

• Limiting Factor– The essential resource that is in shortest

supply or unavailable– Determines how much the individual or

population can grow (affects the biotic potential)

– Ex. light, space, water, nutrients

Interactions Within Communities

• Community– Made up of all populations of different species

within an ecosystem

• Interspecific competition– When the individuals of different species

compete for the same resources– Restricts population growth– A driving force for populations of species to

evolve adaptations to continue to use resources


• Ecological Niche (organism’s “occupation”)– An organism’s biological characteristics, including the

use of, and interaction with abiotic and biotic resources in its environment

• Fundamental Niche– An organism’s biological characteristics and the set of

resources individuals in the population are theoretically capable of using under ideal conditions

• Ex. If resources were abundant and no competition existed


• Realized Niche– An organism’s biological characteristics and the set of

resources individuals in the population actually use under existing environmental conditions

• Symbiosis– Two species maintain a close, usually physical

association– At least one of the species benefits– Includes mutualism, commensalism and parasitism


• Types of Interspecific Competition– I. Interference competition: Involves fighting

over shared resources by individuals of different species

– II. Exploitative competition: Involves consumption of shared resources by individuals of different species

• May limit resource availability to another species


• Interspecific Competition– Strongest competition occurs between

populations of species that experience niche overlap

– Competition declines due to 3 possible outcomes:• Population size of the weaker competitor declines• One species may change its behaviour so that it

survives on different resources• Individuals of one population may migrate to another

habitat where resources are more plentiful


• Resource partitioning– Avoidance of, or reduction in, competition for

similar resources by individuals of different species occupying different non-overlapping ecological niches

– Ex. Anolis lizards partition their tree habitats by occupying different perching sites

– Ex. plants differing in their root systems to allow them to acquire water and mineral ions from the same environment

Interactions Within Communities• Predation

– Type of interspecific interaction– Population density of one species (predator)

increases while the population density of the other species (prey) decreases

– A cyclical relationship: population of prey increases; more food for predators and its population increases; food (prey) starts to decrease resulting in decrease in predator population; less predators allows the prey population to increase once again

• If other prey is available, it alters this relationship


• Canadian lynx-snowshoe hare cycle (10 years)


• Defence Mechanisms– Evolved due to repeated encounters with

predators over time• Have initiated the evolution of counter-adaptations in

some predators

– Plants: morphological defences (thorns, hooks, spines, needles) and chemical defences (distasteful, toxic)

– Insects: some use chemicals produced by their food as protection from their predators (ex. monarch butterfly)


• Passive Defence Mechanisms– Ex. Hiding– Ex. Camouflage– Ex. Visual warning (ex. rings) to predators of

chemical defences (poisons)– Ex. Mimicry

• Batesian: a harmless species mimics a harmful species (ex. Viceroy butterflies mimic monarch butterflies)

• Mullerian: several unrelated species resemble one another and are all harmful (minimizes predation)


• Active Defence Mechanisms– Ex. Fleeing from predators– Ex. Alarm calls (prey mobs the predator)– More costly in terms of energy required


• Types of Symbiotic Relationships:– Mutualism (+/+)

• Both organisms benefit• Neither organism is harmed • Ex. bacteria in our intestinal tract (they get

nutrients from the food we consume and produce vitamins that we need)

• Ex. pollination (insects and animals ingest pollen/nectar, and pollen stuck to bodies of insects and animals gets spread to other flowers)

• Obligatory mutualism: neither species can survive without the other

Interactions Within Communities– Commensalism (+/0)

• One organism benefits and the other organism is unaffected

• Ex. remora (small fish) attach themselves to sharks and feed on small pieces of the shark’s prey and get free transportation

– Parasitism (+/-)• One organism (parasite) benefits at the expense of the

other organism (host)• Host is harmed but is usually not killed• ~1/4 animal species is thought to be a parasite• Ex. Plasmodium (malaria), tapeworms, fleas, lice• Social parasite: manipulates the social behaviour of

their hosts to complete their life cycle (ex. cowbirds use nests of smaller birds and these smaller bird newborns are usually killed)


• Introduction of Exotic (non-indigenous) Species– Can disrupt ecosystems’ dynamic equilibrium and

displace indigenous species to such a degree that they impact on the biodiversity in that ecosystem

– Since non-indigenous species often have few predators in that area, they can reduce or eliminate indigenous species by outcompeting them for food and habitat, or by preying on them

– Ex. European rabbit in Australia (page 686)– Ex. West Nile virus (from Uganda) is believed to have

been introduced into North America accidently via an exotic frog species

population dynamics chapter 14: population ecology

Documents

unmarked animals

mobile wildlife populations

estimating population

population dynamicschapter

populationsmall organisms

bearsa sample of animals

releasedthose organisms

land animals