Community Ecology II

Discussion Session #8

  • Date: Wednesday, Apr 22

Analysis of Spectacular Study

  • Experiment to analyze the mimetic relationships and predation effectiveness:   - Artificial Snakes Experiment:     - Two types of artificial snakes used:       - Artificial kingsnakes (mimicking nonvenomous counterparts).       - Brown artificial snakes (control).     - Findings:       - The presence of coral snakes significantly affected the attack rates on the artificial snakes:         - 83% attacks on artificial kingsnakes when coral snakes were absent.         - 17% attacks on artificial kingsnakes when coral snakes were present.       - For brown artificial snakes:         - 84% attacks when coral snakes were absent.         - 16% attacks when coral snakes were present.

Community Composition Determinants

  • Key factors that determine the species composition of a community include:   - Number of Species: A measure of the total variety of species present.   - Relative Abundances of Species: The proportions of each species in relation to the total number of individuals in the community.   - Species Identity: The specific types of species present, which may have varying impacts on the community dynamics.   - This concept is the focus of ongoing research in community ecology, termed “Community Assembly.”

Diversity versus Richness

  • Comparison of two communities:   - Community 1: Equal distribution among four species (A, B, C, D).     - A: 25%, B: 25%, C: 25%, D: 25%.   - Community 2: Unequal distribution, with species A being dominant.     - A: 80%, B: 5%, C: 5%, D: 10%.

Factors that Affect Species Diversity

  1. Disturbance: Events that can alter community structure and species diversity.    - Examples of disturbances include:      - A) Fire burning a meadow.      - B) Glaciers covering a valley.      - C) Fallen trees in a forest.      - Answering which of these is a disturbance:        - Option E) All of the above.

  2. Area: The size of an area can impact the number of species it can support (island biogeography).    - Graphical Representation:      - Areas of islands (hectares) plotted against the number of plant species (log scale).      - The graph shows a logarithmic relationship indicating that larger areas tend to support more species.

  3. Latitude:    - Latitudinal Diversity Gradients: The principle that biodiversity is typically higher closer to the equator.    - Example data presentation: Species richness of swallowtail butterflies and trees across different latitudes.

  4. Isolation:    - Implications from Robert MacArthur and E. O. Wilson's theories:      - Larger islands generally have higher species diversity due to greater immigration rates and lower extinction rates.

Island Biogeography Theory

  • MacArthur and Wilson (1967): Proposed the equilibrium theory of island biogeography, which includes:   - Rate of change in number of species on islands determined by:     - Immigration of new species.     - Extinction of existing species.

  • Theories state that:   - Near islands have higher immigration rates than far islands, leading to greater species diversity on near islands.   - Large islands support more species due to larger area and resources compared to small islands.   - This results in a balance where species immigration and extinction rates define community diversity at equilibrium.

Empirical Implications

  • The study emphasizes the critical interactions between species and their environments that shape biodiversity within ecosystems. Understanding these principles is essential for conservation efforts and to predict ecological changes due to environmental alterations.


  • Prey species employ various adaptations to avoid predation, including:
      - Crypsis:
        - Definition: A form of camouflage that allows an organism to avoid detection by predators through blending into their surroundings.
        - Example: Creatures like the leaf-tailed gecko or cuttlefish display remarkable ability to change color and texture, enabling them to seamlessly integrate with their environments such as trees or rocky substrates.
        - Types: Active crypsis, where the organism changes according to the environment, and passive crypsis, where the organism's physical characteristics remain static but still provide camouflage.
      - Aposematism:
        - Definition: A biological warning coloration that signals to potential predators that an organism is toxic or unpalatable, thereby avoiding attacks based on learned avoidance.
        - Example: Brightly colored frogs like the poison dart frog or the iconic monarch butterfly showcase vivid patterns that serve as warning signals to birds and other predators.
        - Evolutionary Significance: This type of adaptation is crucial as it enhances the survival of the species, making them recognizable as dangerous or unpalatable.
      - Batesian Mimicry:
        - Definition: A form of mimicry where a harmless species imitates the coloration, shape, or behavior of a toxic or harmful species to deter predators from attacking.
        - Example: The scarlet kingsnake mimics the color patterns of the venomous eastern coral snake, helping it evade predators that would avoid the more toxic coral snake.
        - Limitations: The success of Batesian mimicry depends on the ratio of mimics to models; too many mimics can dilute the warning signal and render this adaptation ineffective.
      - Müllerian Mimicry:
        - Definition: A type of mimicry where two or more harmful species evolve to look similar to one another, thereby reinforcing the avoidance behavior in predators.
        - Example: Cuckoo bees and yellow jackets exhibit this type of mimicry, as both species are unpalatable to predators, which learn to avoid them more efficiently when they share similar warning traits.
        - Mutual Benefit: Benefits both species as they reinforce the predator's learned aversion to harmful traits, enhancing survival for all species involved.