Population Ecology

Populations as Interacting Groups of Organisms

Populations are groups of organisms of the same species living in a given area. Members of a population interact with each other, and reproductive isolation is used to distinguish one population of a species from another.

• Population: A group of organisms of the same species living in a specific area.

• Reproductive isolation: The inability of members of different populations to interbreed, distinguishing populations.

• Example: Biological species concept uses reproductive isolation to define species boundaries.

Estimation of Population Size by Random Sampling

Population size is often estimated using random sampling rather than counting every individual. This method is practical and can be accurate if sampling is done correctly.

• Random sampling: Every individual in a population has an equal and independent chance of being chosen. This reduces bias.

• Systematic sampling: Individuals are chosen at fixed intervals, which may introduce bias.

• Key point: Random sampling does not guarantee the sample is representative of the entire population, but it helps estimate population size.

• Difference: The estimate from sampling may differ from the true population size.

Standard Deviation in Population Sampling

Standard deviation measures the amount of variation in the number of individuals per sample unit (e.g., quadrat). It helps assess how evenly a population is spread.

• High standard deviation: Indicates high variation between sample units; species distribution is patchy or uneven.

• Low standard deviation: Indicates little variation; species are evenly distributed.

Estimating Population Size for Mobile Organisms: Mark-Release-Recapture

Mobile organisms require special methods for population estimation, such as mark-release-recapture and the Lincoln index.

• Mark-release-recapture: Individuals are captured, marked, released, and then recaptured. The proportion of marked individuals in the second sample is used to estimate population size.

• Lincoln index formula:

• = estimated population size

• = number of individuals caught and marked initially

• = number of individuals recaptured

• = number of marked individuals recaptured

• Assumptions: The population is closed (no immigration/emigration), all individuals have equal chance of being caught, and marked individuals are not more or less attractive to predators.

Population Regulation and Growth

Negative Feedback Control of Population Size

Population size can be regulated by density-dependent and density-independent factors. These factors influence population growth and stability.

• Density-dependent factors: Effects depend on population size (e.g., competition, predation, disease).

• Density-independent factors: Effects do not depend on population size (e.g., weather, natural disasters).

• Negative feedback: Most density-dependent factors stabilize population size by reducing growth as population increases.

• Examples: Predation, competition for resources, disease transmission.

Logistic (Sigmoidal) Model of Population Growth

The logistic model describes population growth with an initial exponential phase, a transitional phase, and a plateau phase as the population reaches carrying capacity.

• Exponential phase: Rapid population increase due to abundant resources.

• Transitional phase: Growth slows as limiting factors increase.

• Plateau phase: Growth rate is zero; population stabilizes at carrying capacity ().

• Carrying capacity (): The maximum population size that an environment can support.

Logistic growth equation:

• = population size

• = intrinsic rate of increase

• = carrying capacity

Graphical representation: S-shaped curve with three phases.

Community Interactions

Intraspecific Relationships

Intraspecific relationships are interactions between individuals within a population, including cooperation and competition.

• Intraspecific cooperation: Individuals of the same species help each other, improving survival (e.g., parental care, hunting in groups).

• Intraspecific competition: Individuals of the same species compete for resources (e.g., food, mates, territory).

Communities and Ecosystems

A community consists of all the populations of different species living and interacting in a given area. Communities include plants, animals, fungi, and bacteria.

• Community: Association of populations of different species in a given area.

• Ecosystem: Community plus the physical environment.

Interspecific Relationships: Herbivory, Predation, Competition, Mutualism, Parasitism, Pathogenicity

Interspecific relationships are interactions between populations of different species. They include various ecological interactions:

• Herbivory: Consumption of plant material by animals (e.g., giraffes eating acacia leaves).

• Predation: One species attacks and kills another (e.g., lynx eating snowshoe hares).

• Interspecific competition: Different species compete for the same limited resource (e.g., soil minerals for plants).

• Mutualism: Both species benefit from the interaction (e.g., pollination by insects).

• Parasitism: One species benefits at the expense of another (e.g., tapeworms in mammals).

• Pathogenicity: One species causes disease in another (e.g., Plasmodium causing malaria in humans).

Symbiosis and Resource Competition

Examples of Symbiotic Relationships

Symbiosis refers to close and long-term biological interactions between two different biological organisms.

• Root nodules in Fabaceae: Rhizobium bacteria fix nitrogen in legume roots, benefiting both plant and bacteria.

• Mycorrhizae in Orchidaceae: Fungi form symbiotic relationships with orchid roots, aiding nutrient absorption.

• Zooxanthellae in hard corals: Photosynthetic algae live in coral polyps, providing carbon-based energy.

Resource Competition Between Endemic and Invasive Species

Competition for resources can determine the success of endemic versus invasive species.

• Endemic species: Native to a specific location; adapted to local conditions.

• Invasive species: Non-native species introduced to new environments; often outcompete endemic species due to lack of natural predators or controls.

• Example: Invasive species may spread rapidly and cause harm to native ecosystems.

Summary Table: Types of Ecological Interactions

Additional info:

• Population ecology is a foundational topic in General Biology, covering how populations are measured, regulated, and interact within communities.

• Understanding these concepts is essential for studying ecosystems, conservation biology, and environmental science.