Community Ecology and Species Interactions

Ecological Community

A group of interacting populations of different species occurring together in the same place and time.

Competition

Both species are negatively affected (-/-) due to shared resource use (e.g., water, space).

Amensalism

One species is negatively affected, while the other is unaffected (-/0).

Commensalism

One species benefits, while the other is unaffected (+/0).

Mutualism

Both species benefit (+/+).

Facilitation

One species benefits without harming the other (+/0), often prominent in stressful environments (e.g., vegetation reducing evaporation to increase soil water availability).

Predation

The predator benefits by killing and eating the prey (+/-). Predators consume many prey in their lifetime.

Herbivory

The herbivore benefits by eating plants (+/-).

Parasitism

The parasite benefits by using the host without immediately killing it (+/-). Parasites may attack multiple hosts.

Food Chain

Represents the transfer of matter and energy from one organism to another (e.g., producer to consumer).

Food Web

A complex network of food chains, encompassing all consumer-resource relationships within a community (e.g., Northwest Atlantic food web).

Trophic Levels

Groups of functionally similar organisms defined by how they obtain food.

Primary Producers

Autotrophs (e.g., plants) that produce energy via photosynthesis.

Primary Consumers

Heterotrophs that eat primary producers (e.g., herbivores).

Secondary Consumers

Heterotrophs that eat primary consumers.

Tertiary Consumers

Heterotrophs that eat secondary consumers.

Consumer Impact

Consumers can limit lower trophic levels (e.g., grazers reduce plant diversity and vegetation structure).

Plant Defenses

Plants invest in traits to reduce palatability (e.g., spines in Acacia karroo deter herbivory by bushbuck and boer goats).

Environmental Variability

Limits herbivore populations (e.g., migration or diet shifts in dry seasons allow higher densities; Illius & O'Connor, 2000).

Predator Control

Predators regulate herbivore populations (e.g., sea otters reduce urchins, increasing kelp).

Trophic Cascades

Predator-prey interactions that alter species abundances across multiple trophic levels in a food web.

Sea Otters

More otters → fewer urchins → more kelp.

Yellowstone Wolves

Wolf reintroduction → fewer, skittish elk → more trees → more beavers → altered river flow; also suppressed coyotes, boosting other species (Ripple & Beschta, 2003).

Mechanism of Trophic Cascades

Changes at one trophic level (e.g., predator abundance) cascade to affect other levels, influencing community structure.