Carrying Capacity How do limiting factors effect a population?
Limiting Factors • Those factors which are in low supply and therefore have the effect of keeping down – limiting – the growth of a population • Ex – availability of food, water, shelter, and space; competition for resources, predation, and disease
Carrying Capacity • The maximum number of individuals of a population that can be supported in a given environment sustainably.
Density – Independent Factors • Drought • Freezes • Hurricanes • Floods • Forest Fires
Darwin’s Finches • This graph (from P. T. Boag and P.R. Grant in Science, 214:82, 1981) shows the decline in the population of one of Darwin's finches brought on by severe drought that reduced the quantity of seeds on which this species feeds. As drought ended in 1978, the population recovered but not to the pre 1975 numbers due to change in the carrying capacity.
Checks on populations growth • Island populations are particularly sensitive to population decline brought about by catastrophic events
Density – Dependent Factors • Intraspecific Competition • Between members of the same species • Ex – Gypsy Moth • 1980 infestation in New England • Resulted in larger infestation in 1981 • Offspring of 1980 parents • Caterpillars defoliated 50 foot trees in 72 hours • Mass die out
Density – Dependent Factors • Interspecific Competition • When two species share overlapping niches, they may be forced into competition for the resources of that niche • Ex - Paramecia
Paramecia • This graph (based on the work of G. F. Gause) shows the effect of interspecific competition on the population size of two species of paramecia, Paramecium aurelia and Paramecium caudatum. • When the two species were cultured together, P. aurelia is the better competitor of the two species. • Notice initial growth followed by decline in the P.caudatum. • The population of P. aurelia is always greater than that of P.caudatum.
Density – Dependent Factors • Reproductive Competition • Declining birth rates lead to reduced population • Ex – Fruit Flies, Lab rats, Honeybees • Living under crowded conditions have fewer offspring
Density – Dependent Factors • Migration • As population increases, many of its members emigrate
Density – Dependent Factors • Predation • As population increases, its predators are able to harvest more easily • Ex – Flour Beetle
Flour Beetle • In graph1, 2 species of flour beetle were started with equal numbers. Tribolium population increases while Oryzaephilus is driven to extinction. • Graph 2 shows both species again but glass tubes have been added to the medium. Notice how the difference in population in population growth of Oryzaephilus.
Density – Dependent Factors • Parasitism • Parasite are able to pass from host to host more easily as the population density of the hosts increases • Ex – Black Death • Epidemics are more severe in cities than rural areas
Population Cycles • Some populations go through regular periods of boom and bust • Driven by predation, or infestation of parasites • Ex – voles, red grouse, and lemmings