Population Interactions and Community Ecology

Population Interactions

• Most organisms have complex interactions with various species.

• Adaptations in one species can cause selection pressure on another, leading to coevolution.

• Coevolution is the reciprocal adaptation of interacting species.

Population Interactions and Their Effects

• Predation:

  • Effects: +/- (predators benefit, prey harmed).

  • Predators gain nutrients, prey are killed or injured.

• Herbivory:

  • Effects: +/- (herbivores benefit, plants harmed).

  • Herbivores gain nutrients, plants are killed or injured.

• Parasitism:

  • Effects: +/- (parasites benefit, hosts harmed).

  • Parasites gain nutrients, hosts are harmed.

• Competition:

  • Effects: -/- (both populations harmed).

  • Both lose access to resources.

• Commensalism:

  • Effects: +/0 (one benefits, other unaffected).

  • One population benefits, the other is unaffected.

• Mutualism:

  • Effects: +/+ (both benefit).

  • Both populations benefit.

Predation and Herbivory

• Predation and herbivory are crucial in ecological communities.

• Predators and herbivores evolve specialized behaviors and structures.

• Specialists: Feed on one or few types of food.

• Generalists: Eat a wide variety of food.

Food Choice

• Optimal foraging theory: An animal’s diet balances costs and benefits.

• Maximizing energy intake involves balancing capture time/energy with food energy.

• Food abundance affects choice: Scarce prey leads to settling for low benefit-to-cost food.

Defenses Against Herbivory

• Plants deter herbivores with:

  • Spines, thorns, irritating hairs.

  • Poisonous chemicals (e.g., cardiac glycosides in milkweed).

  • Compounds mimicking insect hormones.

• Some plants increase toxic compound production when eaten.

• Some herbivores coevolve to recognize and avoid toxic plants.

Passive Defenses Against Predation

• Mimicry: Looking like something unappetizing (e.g., caterpillars like bird droppings).

• Cryptic coloration: Blending with surroundings.

• Startling or intimidating displays to increase apparent size.

• Hiding in protected sites.

Active Defenses Against Predation

• Porcupines release sharp, barbed quills.

• Fighting back (biting, charging, kicking).

• Chemical defenses (skunk spray, neurotoxic skin secretions).

• Insects using poisons from plants (monarch caterpillars and milkweed).

Aposematic Coloration

• Poisonous species advertise with bright patterns (aposematic coloration).

• Examples: Skunks, wasps, monarch butterflies.

• Predators learn to associate color with pain/illness and avoid the animal.

Mimicry

• Mimicry: One species evolves to resemble another as a defense.

• Batesian mimicry: Harmless species (mimic) resembles a dangerous one (model).

• Müllerian mimicry: Multiple unpalatable species share a similar appearance reinforcing predator avoidance.

Interspecific Competition

• Interspecific competition: Competition between different species for the same limiting resources.

• Results in increased mortality and decreased reproduction.

• Reduces population size and growth rate.

Two Forms of Interspecific Competition

• Interference competition: Direct harm between individuals of different species.

  • Animals fight for resources.

  • Plants release toxic chemicals.

• Exploitative competition: Indirect competition by using the same limiting resource.

  • One species reduces resource availability for others.

Competitive Exclusion Principle

• Gause’s competitive exclusion principle: Species relying on the same limiting resources in the same way cannot coexist indefinitely.

• One species will be more successful, efficiently harvesting resources and reproducing.

The Niche Concept

• Ecological niche: Resources a population uses and environmental conditions it requires (food, shelter, nutrients, light, temperature).

• Fundamental niche: All possible conditions and resources a population can use.

• Realized niche: Actual range of conditions and resources used.

• Competition can be visualized by plotting fundamental and realized niches.

Resource Partitioning

• Resource partitioning: Using different resources or using the same resources in different ways to reduce competition.

• Example: Weedy plants collect water/nutrients from different soil depths.

Character Displacement

• Character displacement: Morphological differences in sympatric populations to reduce competition.

• Allopatric populations: (different places) are morphologically similar.

• Sympatric populations: (same place) are morphologically different.

• Example: Finch bill sizes on the Galápagos Islands.

Symbiotic Associations

• Symbiosis: Associations between species with varying effects.

• Commensalism: One species benefits, the other is unaffected (rare).

• Mutualism: Both partners benefit (common).

• Parasitism: One species (parasite) harms the other (host).

Parasites and Parasitoids

• Endoparasites: Live within a host (e.g., tapeworms).

• Ectoparasites: Feed on the exterior of a host (e.g., leeches).

• Parasitoids: Insects lay eggs in other insects, and the young consume the host's tissues.

The Nature of Ecological Communities

• Frederic Clements: Interactive view of communities as “superorganisms”. Mature community composition is at equilibrium.

• Henry A. Gleason: Individualistic view of communities as species assemblages adapted to similar conditions. Composition changes with disturbance.

Two Views of Ecological Communities

• Gradient analyses generally support the individualistic hypothesis.

Species Richness

• Species richness: The number of species in a community.

• Human activities disturb species richness patterns.

• Conservation biologists focus on global patterns to determine regions needing preservation.

Relative Abundance

• Relative abundance: Proportion of individuals within a community.

• Some communities have dominant species and rare species.

• Other communities have more equal distribution of individuals.

Species Diversity

• Species diversity is determined by species richness and relative abundance.

• Example: A forest with more tree species is more diverse (species richness).

• A forest with equal distribution of ten tree species is more diverse (relative abundance).

Trophic Structure: Producers

• Trophic structure: Hierarchy of trophic levels based on feeding relationships.

• First trophic level: Primary producers (autotrophs).

  • Photosynthetic organisms capture sunlight to create chemical energy.

  • Use inorganic molecules to build organic molecules.

  • Chemosynthetic bacteria are primary producers in extreme environments.

Trophic Structure: Consumers

• Animals are consumers (heterotrophs) that acquire energy by eating other organisms.

• Second trophic level: Primary consumers (herbivores).

• Third trophic level: Secondary consumers (carnivores that eat herbivores).

• Fourth trophic level: Tertiary consumers (carnivores that eat other carnivores).

• Omnivores feed at several trophic levels.

Detritivores and Decomposers

• Detritivores (scavengers): Ingest dead organisms, wastes, etc. (e.g., earthworms, vultures).

• Decomposers: Bacteria and fungi that feed on dead organic material. Reduce organic material to inorganic molecules for producers.

Food Chains and Webs

• Trophic structure: Food chain where one organism eats another.

• Straight-line food chains are rare.

• Food web: Interconnected food chains with multiple links.

Food Web Analysis

• Links between trophic levels contribute to community stability when species are eliminated.

• Loss of one or two species has minor effects in species-rich communities.

• Communities have constant proportions of species at high, middle, and low trophic levels.

• There are generally two to three prey species for every predator species.