COMMUNITY Ecology

Population Growth Dynamics

Growth Rates (r) Categorization

• Population Size Categories:

  • High r

  • Moderate r

  • Low r

  • Very low r

• Influence on Generations: Each category affects population dynamics differently.

• Exponential Growth: Occurs under conditions where growth rate (r) is constant and density independent.

Exponential Growth Equation

• Population growth can be modeled by the equation:

  • $rac{dN}{dt} = rN$

  • Where:

  • $N<em>t = N</em>0 e^{rt}$

    • $N_t$ = number of individuals after time $t$

    • $N_0$ = initial population size at time $t=0$

    • $e$ = base of natural logarithms

    • $r$ = exponential growth rate

• The rate of population growth $r$ remains constant, while the number of individuals added increases as $N$ increases.

Logistic Growth Dynamics

Characteristics of Logistic Growth

• Density dependence: The growth rate slows at high population densities.

• Carrying Capacity (K):

  • This is the maximum number of individuals of a particular species that an environment can support.

  • Growth begins exponentially when the population is low but decreases as it approaches carrying capacity.

Logistic Growth Equation

• The equation that models the effects of density is:

  • $rac{dN}{dt} = r_{max}N\bigg(1 - rac{N}{K}\bigg)$

  • Where:

    • $r_{max}$ = intrinsic growth rate (maximum possible growth rate)

    • $K$ = carrying capacity of the environment

    • $N$ = population size

Factors Influencing Population Size Changes

Density-Independent Factors

• Examples include:

  • Colonization of a new habitat

  • Recovery after a disaster

Density-Dependent Factors

• Factors resulting from biotic interactions such as:

  • Competition

  • Disease

  • Predation

  • Waste

  • Stress-mediated behavior

Community Ecology Learning Objectives

• Define a biological community.

• Identify how species interactions affect individual fitness and community composition.

• Predict evolutionary outcomes from competitive interactions.

• Compare types of consumption and resistance mechanisms.

• Describe examples of mutualism.

• Identify attributes of a keystone species.

• Predict succession trajectories after disturbances (primary and secondary).

• Contrast species richness with species diversity.

• Calculate species diversity within communities.

Definitions of Biological Units

• Atoms

• Molecules

• Cells

• Tissues

• Organ

• Body systems

• Organism

• Population

• Community

• Ecosystem

• Biosphere

Biological Community Defined

• A biological community is a group of interacting species living within a defined area.

Species Interactions

Importance of Species Interactions

• The fate of a species is greatly influenced by the species sharing its habitat.

• Species interactions significantly affect individual relative fitness.

Consequences of Species Interactions

• The three primary outcomes of species interactions are:

  • Positive (+): fitness benefit to individuals

  • Negative (-): fitness disadvantage to individuals

  • Neutral (0): no effect on individual fitness.

Themes for Understanding Species Interactions

1. Species interactions can determine the distribution and abundance of species.

2. Species interactions can act as agents of natural selection (e.g., coevolutionary arms race).

3. The outcomes of species interactions are dynamic and conditional.

Types of Species Interactions

General Types

1. Commensalism (+/0):

   • One species benefits while the other is unaffected.

2. Competition (-/-):

   • Both individuals use the same resources, lowering fitness for both.

3. Consumption (+/-):

   • One organism consumes another, benefiting the consumer and harming the victim.

4. Mutualism (+/+):

   • Both species benefit from the interaction.

Commensalism Details

• Challenges in Measurement: Difficult to demonstrate due to the conditional nature of “no response.”

• Examples:

  • Ants stirring up insects while hunting, benefiting antbirds that tag along.

  • House sparrows as human commensals.

  • Hermit crabs using discarded shells for protection.

Competition Details

• Intraspecific Competition:

  • Competition within the same species; major cause of density-dependent growth and natural selection.

• Interspecific Competition:

  • Competition among different species for the same limited resources.

Niche Concept in Competition

• Niche: The range of resources that a species can use or the conditions it can tolerate.

• Competition Dynamics:

  • Occurs when niches of two species overlap, potentially leading to resource allocation conflicts.

Competitive Dynamics

Asymmetric vs. Symmetric Competition

• Asymmetric Competition: One species significantly declines in fitness compared to another, potentially leading to extinction.

• Symmetric Competition: Both species experience roughly equal fitness declines.

Competitive Exclusion Principle

• Proposed by G.F. Gause:

  • It posits that two species competing for the same niche cannot coexist indefinitely.

• Experimental Evidence: Illustrated with two species of Paramecium (P. caudatum and P. aurelia), where P. aurelia exhibited logistic growth while P. caudatum was driven to extinction.

Mechanisms of Coexistence

• Niche Differentiation/Resource Partitioning: Evolutionary adaptation allowing species to utilize different resources.

• Character Displacement: Evolutionary changes in traits to facilitate resource use differentiation; observable in Darwin's finches.

Mutualism Details

Overview of Mutualistic Interactions

• Involves diverse organisms, benefiting both participants. Notable examples include:

  • Pollinators and flowering plants

  • Nitrogen-fixing bacteria and plants

  • Ants farming aphids

Keystone Species

Definition and Importance

• A keystone species: Has a disproportionately large impact on the community relative to its abundance.

• Example: Pisaster sea star's removal drastically alters species richness and ecosystem structure.

Succession in Ecosystems

Definitions

• Succession: Development of biological communities following disturbances.

  • Primary Succession: Begins after a complete removal of organisms and soil, e.g., volcanic eruptions.

  • Secondary Succession: Occurs after partial disturbances (e.g., fires) that leave soil intact.

Successional Dynamics

Early vs Late Successional Communities

• Early Successional Communities:

  • Characterized by short-lived, small stature species with long-distance seed dispersal.

• Late Successional Communities:

  • Comprised of large, long-lived species that are strong competitors for light and nutrients.

Species Richness and Diversity

Definitions

• Species Richness: The number of different species within a community.

• Species Diversity: A measure incorporating species richness and evenness of species abundance.

Diversity Calculation: Shannon Diversity Index

• Defined as:

  • $H = - ext{sum}(p<em>i imes ext{ln}(p</em>i))$

  • Where:

  • $p_i$ = proportion of individuals of species $i$

  • $S$ = number of species (richness).

Example of Species Diversity Calculation

• If a community consists of species A to F with counts, proportions (p), and contributions (p*ln(p)), the diversity can be computed systematically.

Conclusion on Species Diversity Assessment

• Communities are evaluated on species presence and abundance, leading to assessments of diversity metrics such as the Shannon index, and comparisons between different communities can highlight ecological health and richness.