Ecology: Population and Community Dynamics – Study Notes

Ecology Basics

Introduction to Ecology

Ecology is the scientific study of interactions between organisms and their environment, encompassing both abiotic (non-living) and biotic (living) factors. Ecologists analyze how these interactions influence the distribution and abundance of organisms across various levels of biological organization.

• Abiotic factors: Non-living environmental components such as temperature, light, water, nutrients, soil, and wind.

• Biotic factors: Living components including bacteria, protists, fungi, plants, animals, and interactions like competition and symbiosis.

Ecology is studied at five hierarchical levels:

• Organism: Adaptations of individual organisms to their environment.

• Population: Groups of individuals of the same species in a given area.

• Community: Interactions among different populations in a shared environment.

• Ecosystem: Interactions between communities and their abiotic environment, focusing on energy flow and nutrient cycling.

• Biosphere: Global interactions among ecosystems affecting the entire Earth.

Population Ecology

Population Size and Density

A population consists of all individuals of the same species living in a specific area at the same time. Demography is the statistical study of populations, focusing on changes in size and structure over time. Key measures include:

• Population size (N): Total number of individuals.

• Population density: Number of individuals per unit area or volume.

Measuring Population Size

Direct counting is often impractical, so ecologists use sampling methods:

• Quadrat Method: Used for immobile or slow-moving organisms. Counts in small plots are extrapolated to estimate total population.

• Mark-Recapture Method: Used for mobile organisms. Involves marking a sample, releasing them, and later recapturing a second sample to estimate population size using the formula:

$N = \frac{Mn}{m}$

• M: Number marked in first sample

• n: Total in second sample

• m: Number of marked recaptures

Patterns of Dispersion

Dispersion patterns describe how individuals are distributed in space:

• Clumped: Individuals aggregate in patches (e.g., herds, seed dispersal near parent plants).

• Uniform: Even spacing, often due to territoriality.

• Random: Unpredictable spacing, often seen in wind-dispersed plants.

Life Tables and Survivorship Curves

Life tables summarize birth and death rates at different life stages, helping predict population trends. Survivorship curves graphically represent the proportion of individuals surviving at each age:

• Type I: High survival early, steep decline at old age (e.g., humans, large mammals).

• Type II: Constant mortality rate (e.g., birds, some reptiles).

• Type III: High mortality early, survivors live long (e.g., fish, insects).

Age interval (years)	Number surviving at beginning	Number dying in interval	Age-specific mortality rate
0–0.5	1000	54	0.054
0.5–1	946	145	0.1533
1–2	801	12	0.015
2–3	789	13	0.0165
3–4	776	12	0.0155
4–5	764	30	0.0393
5–6	734	46	0.0627
6–7	688	48	0.0698
7–8	640	69	0.1078

Age-Sex Structures

Age-sex pyramids display the distribution of a population by age and gender, providing insights into growth trends and demographic transitions.

Life History Strategies: r- and K-selection

Life history strategies describe patterns of growth, reproduction, and survival shaped by natural selection:

• r-selection: High reproductive rate, low parental investment, high mortality (e.g., insects, fish).

• K-selection: Low reproductive rate, high parental investment, low mortality (e.g., mammals, birds).

Trait	r Selection	K Selection
Age of maturation	Young	Older
Number of offspring	Many	Few
Frequency of breeding	Frequent	Infrequent
Size of offspring	Small	Larger
Mortality rates	High	Low
Examples	Mice, insects	Whales, humans

Population Dynamics and Growth Models

Population dynamics studies changes in population size and composition over time. Two primary mathematical models describe population growth:

• Exponential Growth: Occurs when resources are unlimited. Population grows at a constant rate, producing a J-shaped curve.

$\frac{dN}{dT} = r_{max}N$

• Logistic Growth: Occurs when resources are limited. Growth slows as population approaches carrying capacity (K), producing an S-shaped curve.

$\frac{dN}{dT} = r_{max}N \left( \frac{K-N}{K} \right)$

Regulation of Population Size

Population size is regulated by limiting factors:

• Density-dependent factors: Effects increase with population density (e.g., competition, predation, disease).

• Density-independent factors: Effects are unrelated to population density (e.g., natural disasters, climate).

Community Ecology

Interspecific Interactions

Species interact in various ways, influencing community structure and population dynamics:

Interaction	Effect on Species 1	Effect on Species 2	Description
Competition	-	-	Both species are harmed
Predation	+	-	One benefits, one is harmed
Mutualism	+	+	Both benefit
Commensalism	+	0	One benefits, other unaffected

Resource Partitioning

Resource partitioning allows similar species to coexist by utilizing different resources or occupying different niches, reducing direct competition.

Community Structure and Diversity

Community structure is described by species richness (number of species) and species diversity (richness and relative abundance). High diversity often confers greater stability and resilience to disturbance.

Simpson’s Diversity Index

Simpson’s Diversity Index quantifies biodiversity by considering both species richness and evenness:

$D = 1 - \sum \left( \frac{n}{N} \right)^2$

• n: Number of individuals of a particular species

• N: Total number of individuals of all species

Summary Table: Key Concepts in Population and Community Ecology

Concept	Definition	Example/Application
Population	Group of same species in an area	Bison herd
Community	All populations in an area	Prairie ecosystem
Ecosystem	Community + abiotic factors	Grassland with soil, water, air
Biosphere	All ecosystems on Earth	Global scale

Additional info: These notes provide foundational concepts in ecology, focusing on population and community dynamics, which are essential for understanding broader biological and environmental processes.