Variation in Population size Flashcards
population
A population is all the organisms of one species in a habitat
Population size
Population size is the total number of organisms of one species in a habitat. This number changes over time because of the effect of varying factors.
The carrying capacity can vary as a result of:
The carrying capacity can vary as a result of:
the effect of abiotic factors
the effect of biotic factors -interspecific and intraspecific competition and predation.
Variation in Population Size
When a species is introduced into a new environment its population grows in a characteristic way.
This growth curve can be seen in experimental evidence, for example bees in a hive or bacteria in culture.
The growth curve is s-shaped curve:
Population Growth Patterns
The growth curve has three phases.
Different factors are responsible for each phase occurring.
These factors will depend on the ecosystem, and this can be illustrated by considering two contrasting examples: bacteria in a liquid culture (reproducing asexually), and hares in a field (reproducing sexually).
Lag Phase
Bacteria in a Liquid Culture
Little growth as bacteria are transcribing genes and synthesising appropriate enzymes for new conditions.
Rapid Growth Phase
Bacteria in a Liquid Culture
Rapid exponential growth. No limiting factors as the bacteria are at relatively low density.
Stable Phase
Bacteria in a Liquid Culture
Slow growth due to accumulation of toxic waste products (e.g. ethanol) or lack of sugar.
Hares in a Field
Lag Phase
Little growth due to small population. Individuals may rarely meet, so there are few matings. Long gestation period so few births.
Rapid Growth Phase
Hares in a Field
Rapid growth, though not exponential. Few limiting factors as hare numbers are at relatively low density.
Stable Phase
Hares in a Field
Slow growth due to intraspecific competition (see later) for food/territory, predation, disease, etc.
carrying capacity.
The maximum stable population size of a species that an ecosystem can support is called its carrying capacity.
Factors Affecting Population Size
During the stable phase the population may reach a maximum number that the ecosystem can support. This is called the carrying capacity.
The carrying capacity may be controlled by many different interacting factors.
It is often difficult to determine which factors are the most important.
The factors fall into two broad categories into abiotic and biotic factors.
Abiotic factors and population size
Temperature – as this becomes lower than optimum, enzymes work more slowly and metabolic rate reduced.
As it becomes higher than optimum, enzymes work less efficiently as they become denatured.
The further away from the optimum, the smaller the population that can be supported.
pH – this affects the action of enzymes. Each enzyme has its optimum pH.
A population of organisms is larger where the appropriate range of pH exists.
Water and humidity – humidity affects transpiration rates in plants and the evaporation of water from the bodies of animals.
In dry conditions, populations are small and consist only of those species adapted to living in these conditions.
Abiotic Factors: Food Supply
A population clearly will depend on the size of its food supply: if there is plenty of food the population increases and vice versa.
An example of this would be red deer introduced to an Alaskan island.
When they are introduced there would be a population increase as there would be lots of vegetation, the large population would then start to graze the vegetation too quickly for the slow growth to replace it, there is a large reduction in food supply and the deer population decreases.
Abiotic Factors: Seasonal Variation
Abiotic factors can vary with the seasons, and this can cause a periodic oscillation in the population size:
This could be explained by the fact that summer tends to be hotter, there may be more reproduction and the population will increase, the converse being true for winter.
This kind of population size difference is normally seen in species with a short life cycle compared to the seasons, such as insects.
Species with long life cycles (i.e. longer than a year) do not change with the seasons like this.
This could be explained by the fact that summer tends to be hotter, there may be more reproduction and the population will increase, the converse being true for winter.
This kind of population size difference is normally seen in species with a short life cycle compared to the seasons, such as insects.
Species with long life cycles (i.e. longer than a year) do not change with the seasons like this.
