Population Growth

Population Growth Chapter 11

Outline • Geometric Growth • Exponential Growth • Logistic Population Growth • Limits to Population Growth • Density Dependent • Density Independent • Intrinsic Rates of Increase

Population Growth • Populations are dynamic. • Increasing, decreasing, and responding to changes in the biotic and abiotic environments.

Population Growth • Phytoplankton population increases in spring in response to increasing light and nutrients. • Zooplankton populations respond by increasing their numbers. • Later, the numbers decrease in response to lower levels of sunlight and nutrients and increased competition and predation.

Geometric & Exponential Population Growth • In the presence of abundant resources, populations can grow rapidly – at geometric or exponential rates. • A population growing at its maximum rate will grow slowly at first and then faster and faster.

Geometric Growth • In a population where generations do not overlap, growth occurs in discrete annual pulses. • Phlox, other annual plants, many insects • Geometric growth • J-shaped curve

Exponential Growth • Populations with overlapping generations and unlimited resources show exponential growth. • rmax = the intrinsic rate of increase. • Maximum pop growth rate.

Exponential Growth • Natural populations may grow exponentially for a limited period of time – as long as all resources are abundant. • J-shaped curve

Conditions for Exponential Growth • Exponential growth begins with favorable conditions and low population densities.

Conditions for Exponential Growth • Hunting & habitat destruction reduced population size of the whooping crane to 22 individuals by 1940. • Protected in both the overwintering area and the breeding area.

Exponential Population Growth • The Eurasian collared dove expanded its range into western Europe. • Its populations grew at exponential rates for a decade or more. • Starting to slow around 1970.

Logistic Population Growth • As resources are depleted, population growth rate slows and eventually stops: logistic population growth. • S-shapedpopulation growth curve. • Carrying capacity (K) is the number of individuals the environment can support. • Finite amount of resources can only support a finite number of individuals.

Logistic Population Growth • S-shaped growth curves have been observed in laboratory studies of yeast and protozoa.

S-shaped growth curves have also been observed in some natural populations. Logistic Population Growth

Logistic Population Growth • The logistic growth equation adds in the carrying capacity component.

Logistic Population Growth • As the population size (N) approaches the carrying capacity (K), growth slows. • When N = K the right side of the equation equals zero and population growth stops.

Logistic Population Growth • The idea behind the concept of carrying capacity is that a given environment can only support so many individuals of a particular species. • At some point, one or more resources will become limiting.

Logistic Population Growth • For the barnacles studied by Connell, K is largely determined by the amount of space available on rocks for attachment by new barnacles.

Logistic Population Growth • For African buffalo, K appears largely determined by disease and the amount of grass available for food.

Limits to Population Growth • Environment limits population growth by altering birth and death rates. • Density-dependent factors • Biotic • Disease, resource (food, water, shelter, mates) competition • Density-independent factors • Abiotic • Natural disasters, extreme temperatures

Galapagos Finch Population Growth • Galapagos finches have been well studied by Peter & Rosemary Grant, along with their students & colleagues.

Galapagos Finch Population Growth • Boag and Grant - Geospiza fortis was the numerically dominant finch (1,200) on Daphne Major. • After drought of 1977, population fell to 180.

Galapagos Finch Population Growth • Food plants failed to produce seed crop after 1977 drought. • 1983 - 10x normal rainfall caused population to grow (1,100).

Galapagos Finch Population Growth • This increase in population size was due to an increase in seeds that the adults eat and an increase in caterpillars, which are fed to the young.

Cactus Finches • On Genovesa, the population of Geospiza conirostris showed a positive correlation between the number of clutches laid and total annual rainfall.

Cactus Finches and Cactus Reproduction • Two species of finches studied by the Grants specialize on cacti. • Geospiza conirostris • Geospiza scandens

Cactus Finches and Cactus Reproduction • They documented several ways finches utilize cacti: • Open flower buds in dry season to eat pollen • Consume nectar and pollen from mature flowers • Eat seed coating (aril) • Eat seeds • Eat insects from rotting cactus pads • In return, the finches disperse some seeds and pollinate some cactus flowers.

Cactus Finches and Cactus Reproduction • Finches tend to destroy stigmas, thus flowers cannot be fertilized. • Wet season activity may reduce seeds available to finches during the dry season.

Cactus Finches and Cactus Reproduction • Opuntia helleri main source for cactus finches. • Negatively impacted by El Nino (1983). • Stigma snapping delayed recovery. • Interplay of biotic and abiotic factors.

Cactus Finches and Cactus Reproduction • Populations of Galapagos finches and their food plants are a good model of how the environment can affect birth and death rates. • Sometimes purely abiotic. • Opuntia swelling with water & falling over. • Sometimes the effects of the physical environment are mediated by a biological source. • Drought causes fewer seeds to be produced resulting in starvation. • Sometimes there is a complex mixture of biotic and abiotic factors. • Drought and damage to flowers by finches both result in fewer seeds during the dry season.

Population Growth