Population Ecology
Population ecology examines how populations of organisms change in size over time and the factors that influence these changes. Understanding population dynamics is essential for ecology, conservation, and understanding the impact of human activity on ecosystems. This lesson covers population growth, limiting factors, competition, predator-prey relationships, and methods for estimating population size.
Key Definition: A population is a group of organisms of the same species living in a particular area at a particular time, whose members can interbreed.
Population Growth Curves
When a population colonises a new habitat with abundant resources, it typically shows a characteristic pattern of growth:
The Sigmoid (S-Shaped) Growth Curve
A population growing in a limited environment typically follows an S-shaped (sigmoid) curve with four phases:
1. Lag Phase
- The population is small and growing slowly.
- Few individuals are reproducing; they are acclimatising to the new environment.
- Birth rate is low because there are few reproductive individuals.
2. Exponential (Log) Phase
- Resources are abundant, and there are few limiting factors.
- Birth rate greatly exceeds death rate.
- The population grows exponentially (doubles at regular intervals).
- The rate of growth accelerates as more individuals reproduce.
3. Stationary (Plateau) Phase
- The population reaches the carrying capacity (K) of the environment.
- Birth rate equals death rate; the population size stabilises (with fluctuations).
- Limiting factors prevent further growth.
4. (Possible) Decline Phase
- If conditions deteriorate (e.g., disease, resource depletion, environmental change), the death rate may exceed the birth rate, and the population declines.
Carrying Capacity (K)
- The carrying capacity is the maximum population size that a given environment can sustain indefinitely, given the available resources.
- It is not fixed — it can change if environmental conditions change (e.g., food supply increases or decreases, new predators arrive).
Exponential Growth (J-Shaped Curve)
- In an environment with unlimited resources and no limiting factors, a population would grow exponentially without levelling off.
- This rarely occurs in nature for extended periods, but may be seen briefly when a species colonises a new, resource-rich habitat (e.g., bacteria in a nutrient broth, an invasive species with no predators).
Factors Affecting Population Size
Factors that influence population size can be classified as biotic (living) or abiotic (non-living):
Abiotic Factors
- Temperature — affects enzyme activity, metabolic rate, and the distribution of ectotherms.
- Light intensity — affects photosynthesis in plants, which determines primary productivity and the food available for consumers.
- Water availability — essential for all life; limits populations in arid environments.
- Oxygen concentration — particularly important in aquatic environments.
- Soil pH and mineral ion availability — affects plant growth.
- Pollution — toxic substances can directly kill organisms or reduce reproductive success.
Biotic Factors
- Competition — for resources (food, water, territory, mates).
- Predation — predators reduce prey populations.
- Disease and parasitism — reduce survival and reproductive rates.
- Food availability — limits the population of consumers.
- Mutualism — benefits both species, potentially increasing population sizes.
Competition
Competition occurs when two or more organisms require the same limited resource. It is a major factor controlling population size.
Intraspecific Competition
- Competition within the same species.
- All individuals have the same resource requirements, so competition is intense.
- As population density increases, intraspecific competition increases, reducing the per capita growth rate.
- This is a density-dependent factor — its effect increases as the population grows, acting as a negative feedback mechanism that stabilises the population near carrying capacity.
- Examples: robins competing for territories; oak seedlings competing for light in a woodland.
Interspecific Competition
- Competition between different species.
- Occurs when two or more species occupy a similar ecological niche (have overlapping resource requirements).
- Can lead to competitive exclusion — the better-adapted species outcompetes and eliminates the other (the competitive exclusion principle).
- Alternatively, species may undergo niche differentiation (resource partitioning), evolving to exploit slightly different resources and coexisting.
- Example: red squirrels and grey squirrels in the UK. Grey squirrels outcompete red squirrels for food and carry squirrelpox virus, to which they are immune but red squirrels are not.
Predator-Prey Relationships