topic 15 - population growth and dynamics

Life history strategy

Overall pattern of how a species allocated its energy and resources to growth, reproduction, and survival over its lifetime

Exponential growth

Growth will eventually slow, cannot grow exponentially forever because limited by resources

r>0

Population increases indefinitely

r<0

Population decreases, possibly toward extinction

r=0

Population stays constant over time

Logistic growth

• Population will not grow exponentially forever

• Exponential growth multiplied by term that includes carrying capacity

• Population growth will remain at a constant size when N zero equals K

• No further population growth at this time

Population dynamics

• Factors affecting individual survival

• Population dynamics could be environmental, biological

Density-dependent factors

Change population growth depending on the number of individuals in the population

• Competition for resources: the higher the population, more competition for resources

• Predators: maybe attracted to areas with high densities of prey, allowing them to capture a larger proportion of individuals and causing death rate of prey population to rise

• Disease: may spread more easily in dense population than in population with few individuals, causing rise in death rate

Density-independent factors

Regardless number of individuals, these factors will impact the population

• Severe heat waves: harm population as a whole

• Storm events

• Pollution (eg. oil spill)

Environmental stochasticity

Unpredictable fluctuation in environmental conditions

• Causes variability in resources such as good

• Cause variability in predators and other environemtnal conditions

• Change in birth and death rates

Metapopulations

• Group of geographically isolated populations linked together by dispersal

• Source and sink populations

Eg.

Glanville fritillary butterfly (Melitaea cinxia) in the Aland islands, Finland

• Each island has small, isolated butterfly populations

• Some die out each year, but other recolonise them

Spotted owl (Strix occidentalis) in fragmented forests of North America

• Lives in patches of old-growth forest separated by clearings

• Movement between forest patches keeps the overall population connected

Source population (metapopulation)

A high quality habitat where births exceed deaths.

This means the population can produce surplus individuals that disperse to other patches.

Birth rate > Death rate → Net exporters of individuals

Eg. A lush meadow that provides plenty of food and nesting sites for butterflies — producing more than enough individuals to sustain itself and send migrants to other areas

Support local population growth and can be net exporters of individuals (emigration) to other patches

Sink population (metapopulation)

A low quality habitat where deaths exceed births.

The population cannot sustain itself without immigrants from source areas

Birth rate < Death rate → Net importer of individuals

Eg. A dry or shaded meadow patch where butterflies can survive for a while but not reproduce enough — it only persists because individuals immigrate from nearby source patches

Mortality exceeds births and populations are reliant on immigration to persist

Colonisation rate

Proportion of unoccupied sites that become occupied per unit time

Extinction rate

Proportion of occupied sites that go extinct per unit time