POPULATION GROWTH

POPULATION GROWTH

Natality • Increases population size • Each species will have its own maximum birth rate • Maximum birth rates are seen when conditions are ideal • This can lead to exponential growth © 2010 Paul Billiet ODWS

Phases of population growth Phase 1: Log or exponential phase • Unlimited population growth • The intrinsic rate of increase (r) • Abundant food, no disease, no predators etc Phase 2: Decline or transitional phase • Limiting factors slowing population growth © 2010 Paul Billiet ODWS

Phase 3 Plateau or stationary phase • No growth • The limiting factors balance the population’s capacity to increase • The population reaches the Carrying Capacity (K) of the environment • Added limiting factors will lower K • Removing a limiting factor will raise K © 2010 Paul Billiet ODWS

Modelling population growth, the maths • Population growth follows the numbers of individuals in a population through time. The models try to trace what will happen little by little as time passes by • A small change in time is given by ∆t This is usually reduced to dt • Time may be measured in regular units such as years or even days or it may be measured in units such as generations • A small change in numbers is given by ∆NThis is usually reduced to dN • A change in numbers as time passes by is given by: dN/dt © 2010 Paul Billiet ODWS

Real examples of exponential growth • Pest species show exponential growthhumans provide them with a perfect environment • Alien speciesWhen a new species is introduced accidentally or deliberately into a new environment It has no natural predators or diseases to keep it under control © 2010 Paul Billiet ODWS

European starling (Sturnus vulgaris) • Between 1890 and 1891, 160 of these birds were released in Central Park New York. • By 1942 they had spread as far as California. • An estimate population of between 140 and 200 million starlings now exist in North America • One of the commonest species of bird on Earth Image Credit: http://www.columbia.edu/ © 2010 Paul Billiet ODWS

Current distribution CJKrebs (1978) Ecology European starling (Sturnus vulgaris)

Begon, Townsend & Harper (1990) Ecology The Colorado Beetle (Leptinotarsa decemlineata)

r-strategists boom and bust! • Maximum reproductive potential when the opportunity arrives • Periodic population explosions • Pests and pathogens (disease causing organisms) are often r-species © 2010 Paul Billiet ODWS

The Carrying Capacity • Darwin observed that a population never continues to grow exponentially for ever • There is a resistance from the environment • The food supply nesting sites decrease • Competition increases • Predators and pathogens increase • This resistance results from negative feedback © 2010 Paul Billiet ODWS

The Carrying Capacity • This too can be modelled • It needs a component in it that will slow down the population growth as it reaches a certain point, the carrying capacity of the environment (K) • The equation is called the logistic equation • dN/dt = rN[(K-N)/N] • When N<K then dN/dt will be positivethe population will increase in size • When N=K then dN/dt will be zerothe population growth will stop • Should N>K then dN/dt will become negativethe population will decrease © 2010 Paul Billiet ODWS

K-strategists long term investment • These species are good competitors • They are adapted to environments where all the niches are filled • They have long life spans • Lower reproductive rates but … • High degree of parental care thus … • Low infant mortality • K-strategist flowering plants produce fewer seeds with a large amount of food reserve © 2010 Paul Billiet ODWS

POPULATION GROWTH