Population and Community Ecology: Study Guide for Exam #3

Population Ecology

Population Growth

Population ecology examines how and why the number of individuals in a population changes over time and space. Understanding population growth is fundamental for conservation, management, and predicting ecological outcomes.

• Population Density: Defined as the number of individuals per unit area or volume. Differences in density between sites can indicate habitat quality, resource availability, or recent disturbances.

• Dispersion Patterns: Three main types:

  • Clumped: Individuals aggregate in patches; most common in nature due to resource distribution or social behavior.

  • Uniform: Evenly spaced individuals, often due to territoriality or competition.

  • Random: Position of each individual is independent of others; rare in nature.

• Mark-Recapture Method (Lincoln-Petersen): Used to estimate population size. Assumes marked and unmarked individuals are equally likely to be captured, and that the population is closed (no immigration/emigration).

  • Formula: $N = \frac{sn}{x}$, where N = estimated population size, s = number of individuals marked and released, n = total captured in second sample, x = marked recaptures.

• Demography: The study of vital statistics of populations and how they change over time. Cohort: A group of individuals of the same age, followed from birth to death.

• Survivorship Curves: Three idealized types:

  • Type I: Low death rates early and middle life (e.g., humans, large mammals).

  • Type II: Constant death rate over lifespan (e.g., birds, some reptiles).

  • Type III: High death rates for young, lower for survivors (e.g., oysters, many fish).

• Exponential Growth Model: Describes idealized, unlimited population growth.

  • Equation: $\frac{dN}{dt} = r_{max} N$

  • Produces a J-shaped curve; occurs when resources are abundant.

• Logistic Growth Model: Incorporates carrying capacity (K), the maximum population size the environment can support.

  • Equation: $\frac{dN}{dt} = r_{max} N \frac{K-N}{K}$

  • Produces an S-shaped (sigmoid) curve with three phases: lag, exponential, and plateau.

• Carrying Capacity (K): The maximum population size that a particular environment can sustain.

• Applications: Logistic models are used in conservation biology to manage endangered species and predict population recovery.

Density Dependence

Population size is regulated by factors that may depend on or be independent of population density.

• Density-Dependent Factors: Effects increase as population density increases (e.g., competition, predation, disease, territoriality, waste accumulation).

• Density-Independent Factors: Affect populations regardless of density (e.g., weather, natural disasters).

• Five Mechanisms of Density-Dependent Regulation:

  1. Competition for resources

  2. Predation

  3. Toxic wastes

  4. Intrinsic factors (e.g., stress, hormonal changes)

  5. Territoriality

• Population Dynamics: The study of how complex interactions between biotic and abiotic factors cause variation in population size over time.

• Case Studies: Examples include wolf population cycles and the classic hare/lynx predator-prey cycles.

• Metapopulations: Groups of populations linked by immigration and emigration; important for species persistence in fragmented habitats.

• Age-Structure Pyramids: Graphical representations of the age distribution within a population; used to predict future growth trends.

• Demographic Transition: The shift from high birth and death rates to lower birth and death rates, often associated with economic development; global human growth rate is slowing as more countries undergo this transition.

Community Ecology

Species Interactions

Communities are shaped by interactions among species, which can be competitive, exploitative, or facilitative.

• Interspecific Competition: Occurs when species compete for a resource in short supply.

• Competitive Exclusion Principle: Two species competing for the same limiting resource cannot coexist indefinitely.

• Fundamental vs. Realized Niche:

  • Fundamental Niche: The full range of environmental conditions a species can occupy.

  • Realized Niche: The actual conditions occupied due to competition and other biotic factors.

• Resource Partitioning: Differentiation of niches that enables similar species to coexist (e.g., different warbler species feeding in different parts of a tree).

• Character Displacement: Tendency for characteristics to diverge more in sympatric than allopatric populations (e.g., finch beak size differences on Galápagos Islands).

• Forms of Exploitation:

  1. Predation

  2. Herbivory

  3. Parasitism

• Prey Defense Strategies: Includes cryptic coloration, aposematic coloration, mechanical and chemical defenses, and mimicry.

• Batesian vs. Müllerian Mimicry:

  • Batesian: A harmless species mimics a harmful one.

  • Müllerian: Two or more harmful species resemble each other, reinforcing avoidance by predators.

• Mutualism: Both species benefit (e.g., pollinators and flowers).

• Commensalism: One species benefits, the other is unaffected (e.g., barnacles on whales).

• Facilitation: One species has positive effects on another without direct contact (e.g., plants that improve soil for others).

• Foundation Species: Species that have a strong role in structuring a community by creating or maintaining habitat (e.g., corals, trees).

Diversity and Trophic Structure

Community diversity and the flow of energy through trophic levels are central to ecosystem function and stability.

• Species Diversity: Has two components:

  • Species Richness: Number of different species in a community.

  • Relative Abundance: Proportion each species represents of all individuals.

• Shannon Diversity Index: Quantifies species diversity, accounting for both richness and evenness.

  • Formula: $H' = -\sum_{i=1}^{S} p_i \ln p_i$

  • Where pi is the proportion of individuals in the ith species, and S is the total number of species.

• Diversity and Stability: Greater species diversity often leads to greater community stability, as shown in experiments such as those at Cedar Creek.

• Trophic Levels: The hierarchical levels in a food chain or web, including primary producers, primary consumers, secondary consumers, etc.

• Energy Flow: Energy decreases at each trophic level due to inefficiency of transfer (typically ~10% per level).

• Food Chains vs. Food Webs: Food chains are linear, while food webs are complex networks of feeding relationships.

• Limits to Food Chain Length: Typically 3–5 levels, limited by energy loss (energetic hypothesis) and body size constraints.

• Keystone vs. Foundation Species:

  • Keystone Species: Exert strong control on community structure by their ecological roles, not necessarily by abundance (e.g., sea stars).

  • Foundation Species: Have large effects due to high abundance or biomass (e.g., trees in a forest).

• Community Control:

  • Bottom-Up: Community structure controlled by nutrient supply and primary producers.

  • Top-Down: Predators control community structure (trophic cascade).

Disturbance, Succession, and Biogeography

Disturbances and spatial patterns influence community composition and diversity over time and space.

• Disturbance: An event that changes a community by removing organisms or altering resource availability (e.g., fire, storms, human activity).

• Characterization of Disturbances: By frequency, intensity, and scale.

• Intermediate Disturbance Hypothesis: Moderate levels of disturbance foster greater species diversity than low or high levels.

• Succession:

  • Primary Succession: Occurs in lifeless areas (e.g., after volcanic eruption, glacial retreat).

  • Secondary Succession: Occurs where a disturbance has cleared an existing community but left the soil intact (e.g., after fire).

• Latitudinal Gradient in Species Richness: Species richness generally increases toward the equator. Hypotheses include:

  • Evolutionary History Hypothesis: Tropical communities are older and have had more time to diversify.

  • Climate Hypothesis: More stable, productive, and predictable climates in the tropics support more species.

• Species-Area Curve: Larger geographic areas tend to have more species.

  • Equation: $S = cA^z$

  • Where S = number of species, A = area, c and z are fitted constants.

• Island Equilibrium Model (MacArthur & Wilson): The number of species on an island represents a balance between immigration and extinction rates, which are affected by island size and distance from the mainland.

Dispersion Pattern	Description	Example
Clumped	Individuals aggregate in patches	Starfish on a rocky shore
Uniform	Evenly spaced individuals	Nesting penguins
Random	Unpredictable spacing	Dandelions in a field

Survivorship Curve Type	Description	Example
Type I	Low mortality early, high late	Humans
Type II	Constant mortality rate	Songbirds
Type III	High mortality early, low late	Oysters

Species Interaction	Effect on Species 1	Effect on Species 2	Example
Competition	-	-	Plants competing for light
Predation	+	-	Wolf and deer
Mutualism	+	+	Bees and flowers
Commensalism	+	0	Barnacles on whales
Parasitism	+	-	Tapeworm in mammals

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard biology textbooks.