Community Ecology: Interactions and Adaptations

Community Ecology

Definition of a Community

A community in ecology refers to a group of populations of different species living close enough for interspecific interactions to occur. These interactions shape the structure and dynamics of the community.

• Ecological niche: The sum of a species' use of the biotic and abiotic resources in its environment.

• Types of interactions: Interactions can be classified by their effects on the interacting species: -/-, +/-, +/+, or +/0.

Competition

Nature of Competition

Competition occurs when two or more species compete for the same limited resource. This interaction is typically negative for both species (-/-) because the presence of competitors reduces the availability of resources for each.

• Competitive exclusion principle: Two species competing for the same limiting resource cannot coexist permanently in the same place.

• Resource partitioning: The differentiation of niches that enables similar species to coexist in a community.

Predation

Predator-Prey Interactions

Predation is an interaction in which one species, the predator, kills and eats the other, the prey. This is a +/- interaction: the predator benefits, while the prey is harmed.

• Predator adaptations: Predators have evolved various adaptations to capture prey, such as acute senses, claws, fangs, speed, and camouflage.

• Prey adaptations: Prey species have evolved defenses such as hiding, fleeing, forming groups, and defensive coloration.

Predator Adaptations

• Acute senses: Many predators have highly developed senses for detecting prey (e.g., infrared receptors in snakes).

• Speed and agility: Some predators, such as cheetahs, rely on speed to catch prey.

• Ambush strategies: Some predators use camouflage and stealth to ambush prey.

Prey Adaptations

• Behavioral defenses: Hiding, fleeing, forming herds or schools, and fighting back are common strategies.

• Distraction and confusion: Prey may scatter or use sudden movements to disorient predators.

Examples of Prey Behavioral Adaptations

• Zebras: Encircle attacked family members and may bite if attacks continue.

• Prairie dogs: Bob heads and scream to alert others, with different calls for different predators.

• Mobbing: Prey species may mob predators to drive them away.

Prey Coloration Adaptations

• Cryptic coloration: Camouflage that makes prey difficult to spot.

• Aposematic coloration: Warning coloration that signals toxicity or unpalatability.

• Batesian mimicry: Harmless species mimics a harmful one.

• Müllerian mimicry: Two unpalatable species mimic each other.

Herbivory

Herbivore-Plant Interactions

Herbivory is an interaction in which an organism eats parts of a plant or alga. This is a +/- interaction: the herbivore benefits, while the plant is harmed.

• Land herbivores: Grazing mammals (cattle, sheep, hippos), grasshoppers, beetles.

• Oceanic herbivores: Snails, sea urchins, tropical fish, manatees.

Herbivore Adaptations

• Selective feeding: Some herbivores are very picky and can detect toxins in plants.

• Specialized digestion: Adaptations in teeth and digestive systems to process plant material.

Plant Adaptations Against Herbivory

• Chemical defenses: Production of toxins (e.g., swainsonine in locoweeds, strychnine in Strychnos toxifera).

• Physical defenses: Thorns, spines, and tough leaves.

• Unpalatable compounds: Substances that taste bad but are not poisonous (e.g., cinnamon, peppermint, cloves).

Symbiosis

Types of Symbiotic Relationships

Symbiosis is a close and long-term biological interaction between two different biological organisms. The effects can be negative, positive, or neutral for each participant.

• Parasitism (+/-): One organism (parasite) benefits at the expense of the host. Includes endoparasites (inside host) and ectoparasites (on host surface).

• Mutualism (+/+): Both species benefit. Can be facultative (not required for survival) or obligate (required for at least one species).

• Commensalism (+/0): One species benefits, the other is neither helped nor harmed.

Examples of Parasitism

• Tapeworms: Endoparasites living inside the host's digestive tract.

• Ticks and lice: Ectoparasites living on the external surface of the host.

• Aphids: Feed on plant sap, sometimes with mutualistic relationships with ants.

Examples of Mutualism

• Clownfish and sea anemones: Clownfish gain protection, anemones receive food particles.

• Acacia and ants: Ants live in acacia trees and defend them from herbivores.

• Humans and honeyguides: Birds lead humans to beehives; both benefit from honey and wax.

Examples of Commensalism

• Hitch-hiking species: Organisms that use others for transportation or access to resources without affecting them.

• Scavengers: Animals that feed on leftovers from larger predators.