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What did Thomas Malthus write an essay on?

1798 an essay on the principle of population. Saw the power of population growth and inevitability of suffering. Predicted a famine as crop supply was increasing arithmetically but the population was increasing geometrically. Inspired Darwin's theory.


What is the demographic transition?

Transition from high birth and death rates to lower birth and death rates as a country or region develops from a pre-industrial to an industrialized economic system. Naturally reduces the number of offspring through education.


What is the model for discrete population growth?

N(t) = N(0) λ^t


What is the model for continuous population growth?

N(t) = N(0) e ^ (rt) dN/dt = (b - m) N


Define the intrinsic growth rate?

Rate at which a population increases in size if there are no density-dependent forces regulating the population. Births - Deaths.


What is the Logistic Model?

dN/dt = rN ( [K - N] / K ) Sigmoidal curve, which involves density dependent factors.


Define carrying capacity (K)?

The maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water, and other necessities available in the environment.


What is the Leslie/Transition matrix?

A discrete, age-structured model of population growth that is very popular in population ecology.


What happens for transition matrices that are biologically sensible?

All populations will come to grow/decline at a constant rate. Stable age distribution.


Which type of competition is most important for density dependence?

Intraspecies competition.


What would happen in absence of density dependence?

Populations would grow or decline forever, which is unrealistic.


When is equilibrium reached in a population?

When birth rate equals death rate. Could have more than one equilibrium. Some populations will reach equilibrium at zero if starting at a low density.


When does a population grow?

When r is above zero.


Define an unstable equilibrium?

When there is slight variation, the population doesn't return to the equilibrium.


Define a stable equilibrium?

When there is a slight variation, the population returns to the equilibrium.


Where does a population stabilise on a graph?

Where the growth curve intercepts the y=x line, as this means the population next year equals the population this year.


What is the Allee effect?

Population doesn't necessarily grow exponentially from small sizes. At small population sizes you get a slower growth rate than logistic suggest. Positive density dependence. Low growth rate at low density.


Example of where the Allee effect can occur?

Cooperative behaviours require minimum population size. Mating pairs require a certain population density. Penguins?


What are the types of competition for resources?

Scramble and Contest


What is a contest competiton?

Winner takes all. A situation where available resources, such as food and mates, are utilized only by one or a few individuals, thus preventing development or reproduction of other individuals. The population levels out.


What is a scramble competition?

Share the resources. A situation in which a resource is accessible to all competitors. The population suddenly plummets as there aren't enough resources to sustain any individuals.


What is exploitation competition?

Occurs indirectly through a common limiting resource which acts as an intermediate. For example, use of resources depletes the amount available to others, or they compete for space. Some plant species, for example, are able to extract water and nutrients from the soil faster than surrounding species.


What is interference competition?

Organisms use up resources directly. Once used, the resource is no longer available for other species to use. The two species physically interfere with one another by aggressively attempting to exclude one another from particular habitats.


What is a search image?

The mental image of an object that is apparently possessed by an animal searching for that object, and whose existence is inferred from observation of the behaviour of animals. When aphids are common, birds get a search image and predate more easily. The predator becomes saturated as breeding takes time. This is an unstable equilibrium for the aphids, so the aphid population can spiral out of control.


Do density dependent factors keep populations at a certain size?

No, as other factors are also involved - environmental factors.


What factors are negatively density dependent?

Predation, disease and parasitism.


What factors are positively density dependent?

Cooperative behaviours require minimum population size. Mating pairs require a certain population density. Penguins?


What is cobwebbing?

Plot population density next gen against density this gen. Draw a line from the population now until you meet the curve. Then reflext in y=x axis.


What happens in scramble competition populations?

If population is very large one generation, it will result in a very small population the next generation. 


Boom and bust. 


What happens as you increase the boom & bust of the population?

Takes longer to correct when it overshoots the equilibrium.