Community Ecology Notes

Factors Influencing Community Structure

• Foundation Species: Large or abundant species that significantly impact community structure by providing habitat and food for other organisms.

• Interspecific Interactions: Predation and other interactions affect the number and types of species present.

• Disturbances: Events like marine heat waves, storms, and human activities remove organisms or alter resource availability.

Interspecific Interactions

• Interactions between species can be helpful, harmful, or neutral for the individuals involved.

• Community structure is affected by the number, composition, and relative abundance of species.

• Interspecific interactions: Interactions between different species, including:

  • Competition

  • Predation

  • Herbivory

  • Parasitism

  • Mutualism

  • Commensalism

Competition

• Occurs when different species use a limiting resource, negatively affecting the survival and reproduction of both.

• Species do not compete for abundant resources.

• Competitive Exclusion: Local elimination of the inferior competitor when two species use the same limited resources.

Ecological Niches and Natural Selection

• Ecological Niche: The specific set of biotic and abiotic resources a species uses.

  • Example: A tropical tree lizard's niche includes temperature range, branch size, activity time, and insect prey.

Resource Partitioning

• Resource Partitioning: Differentiation of niches that allows similar species to coexist.

• Two species cannot coexist permanently if their niches are identical.

• Ecologically similar species can coexist with significant differences in their niches.

Fundamental vs. Realized Niche

• Fundamental Niche: The niche potentially occupied by a species.

• Realized Niche: The portion of the fundamental niche actually occupied by that species.

• Competition may cause a species' realized niche to differ from its fundamental niche.

Predation

• Interaction where one species (predator) kills and eats another (prey).

• Predators often have:

  • Acute senses to find and identify prey.

  • Adaptations like claws, fangs, or poison.

  • Speed and agility.

  • Camouflage.

Prey Defenses

• Prey species may have:

  • Behavioral defenses (hiding, fleeing, herds, schools).

  • Morphological and physiological adaptations.

  • Mechanical or chemical defenses.

Chemical Defenses and Aposematic Coloration

• Some animals synthesize or accumulate toxins from their diet.

• Aposematic Coloration: Bright warning coloration in animals with chemical defenses.

• Predators avoid brightly colored prey.

Cryptic Coloration

• Cryptic Coloration: Camouflage that makes prey difficult to see in their environment.

Mimicry

• Batesian Mimicry: A palatable species mimics an unpalatable one.

• Müllerian Mimicry: Two or more unpalatable species resemble each other, enhancing predator avoidance learning.

Predator Mimicry

• Mimicry can also evolve in predators to help them approach prey.

• Example: Mimic octopus mimicking dangerous marine animals.

Herbivory

• Interaction where an herbivore eats parts of a plant or alga.

• Herbivores typically harm plants but do not kill them.

• Most herbivores are invertebrates.

• Herbivores have specialized adaptations like:

  • Chemical sensors to detect toxicity or nutritional value.

  • Specialized teeth or digestive systems.

Plant Defenses

• Plants have mechanical defenses (spines, thorns) and chemical defenses (toxins).

• Non-toxic chemical defenses can cause abnormal development or be distasteful to herbivores.

Parasitism

• One organism (parasite) derives nourishment from another (host), harming the host.

• Endoparasites: Live inside the host.

• Ectoparasites: Live on the external surface of the host.

• Many parasites have complex life cycles involving multiple hosts.

Mutualism

• Interspecific interaction benefiting both species.

• Sometimes species depend on each other for survival and reproduction.

• In other cases, both species can survive alone.

Commensalism

• Interaction where one species benefits and the other is neither harmed nor helped.

• Example: Shade-tolerant wildflowers benefiting from the shade provided by trees, without affecting the trees.

• Some commensal interactions can become mutualistic.

Species with a Large Impact

• Certain species have a large impact on community structure due to their abundance or role.

• Foundation Species: Strong effects due to large size or high abundance; provide habitat or food; competitively dominant in exploiting key resources.

Keystone Species

• Keystone Species: Exert strong control by their pivotal ecological roles; not usually abundant.

Ecosystem Engineers

• Ecosystem Engineers: Create or alter the physical environment.

• Some foundation species (e.g., trees) are ecosystem engineers because they create habitat for other species.

Disturbance and Community Composition

• Ecologists initially viewed communities as being at equilibrium, with competition as a key factor.

• Climax Community: Species in the community function as an integrated unit.

Alternative Views on Community Structure

• Other ecologists challenged the single climax community concept.

• Differences in soils, topography, and other factors create many possible stable communities.

• Communities viewed as chance assemblages of species with similar abiotic requirements.

Nonequilibrium Model

• Nonequilibrium Model: Describes communities as constantly changing after disturbance.

Characterizing Disturbance

• Disturbance types, frequency, and severity vary among communities.

• High disturbance levels result from frequent and intense disturbances.

• Low disturbance levels result from low frequency or intensity.

Intermediate Disturbance Hypothesis

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

• High disturbance excludes slow-growing species.

• Low disturbance allows competitively dominant species to exclude less competitive ones.

Ecological Succession

• Ecological Succession: Pattern of colonization and species replacement after a severe disturbance.

• Primary Succession: Begins in a virtually lifeless area (e.g., volcanic island).

  • Prokaryotes and protists are initially present.

  • Lichens and mosses arrive first; soil gradually develops.

  • Organic matter accumulates as early colonizers decompose.

  • Plant community establishes after soil develops.

Secondary Succession

• Secondary Succession: Recolonization after a disturbance removes most, but not all, organisms.

• Example: Abandoned agricultural land returning to its original state.

Glacial Retreat and Primary Succession Example

• Example of primary succession near Glacier Bay, Alaska, showing pioneer, Dryas, alder, and spruce stages.