2.4Ecology
Population ecology — EST 2 Biology Revision Notes & Topical Questions
5 min read 9 parts Lesson 4 of 8 2 quizzes
Lesson summary
SAT Biology population ecology: analyse how births, deaths, migration, limiting factors and carrying capacity change population size. A population contains members of one species living in the same area and capable of producing fertile offspring.
Section 1 of 9Population change
Population change
A population contains members of one species living in the same area and capable of producing fertile offspring. Population size rises through births and immigration and falls through deaths and emigration.
Food, water, shelter, predators, disease and competition influence birth, death and migration rates. Both biotic and abiotic changes can therefore alter population density and distribution.
Section 2 of 9Exponential and logistic growth
Exponential and logistic growth
With abundant resources, a population can grow exponentially, producing a J-shaped curve. The increase becomes progressively faster because more reproducing individuals are present.
In nature resources become limiting. Growth slows as competition increases, producing logistic growth and an S-shaped curve that levels near the environment's carrying capacity.
Section 3 of 9Carrying capacity
Carrying capacity
Carrying capacity is the maximum population an environment can sustain over time with its available resources and conditions. It can change if the habitat or resource supply changes.
When a population exceeds carrying capacity, resource shortage can reduce birth rate and increase death or emigration. Negative feedback tends to return the population toward a sustainable level.
Section 4 of 9Density-dependent limits
Density-dependent limits
Competition, disease, parasitism and predation usually become stronger as population density rises. Their effects are therefore density dependent.
Weather extremes, fires and geological events can affect populations regardless of density. These density-independent factors may cause sudden changes in population size.
Section 5 of 9Predator-prey cycles
Predator-prey cycles
Abundant prey can support an increase in predator numbers. Greater predation then reduces the prey population, leaving less food for predators.
Predator numbers subsequently fall, allowing prey to recover. The two populations may therefore oscillate, with the predator peak usually following the prey peak.
Section 6 of 9Food webs and trophic cascades
Top predator
Species in a community are connected through food webs. Removing or introducing a top predator can change the abundance of prey, which can then affect organisms at lower trophic levels. A chain of these indirect effects is called a trophic cascade.
Energy enters a food web through producers and is passed to consumers. The size of each trophic level depends partly on energy available from the one below it. A change to one population can therefore affect several interdependent species, not only the species it directly eats.
Section 7 of 9Competition and niches
Competition and niches
Competition
Competition occurs when organisms need the same limited resource, such as food, space or light. Individuals that obtain the resource more successfully are more likely to survive and reproduce, so competition can contribute to natural selection.
Niche partitioning
Niche partitioning reduces competition when species use a resource in different places, at different times or in different ways. For example, two birds may eat similar insects but forage at different heights in a tree.
Section 8 of 9Three forms of symbiosis
Parasitism:
Parasitism one organism benefits while its host is harmed; a tick feeding on a mammal is an example. Commensalism: one benefits while the other has no clear benefit or harm; a cattle egret may catch insects disturbed by grazing cattle.
Mutualism:
Mutualism both partners benefit. A bird that eats ticks gains food while the large mammal has parasites removed. In each example, state the effect on both organisms to identify the interaction correctly.
Section 9 of 9Trace an ecological change
To predict the effect of removing a predator, first identify its prey. The prey may increase; if it feeds heavily on producers, producer biomass may then decrease. Real food webs contain several feeding links, so the size of each effect depends on other available food and predators.
Check:
Check Why might two similar species coexist in one habitat? Answer: They may partition their niches by using resources at different times or in different locations, reducing direct competition.
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