7.2 Community Ecology: Species Interactions and Trophic Dynamics

Community Ecology: Species Interactions and Trophic Dynamics

Species Interactions in Communities

Communities are shaped by the interactions among species, which can have direct or indirect effects on community structure and function. Some species exert particularly strong influences, leading to the concepts of dominant and keystone species.

• Direct interactions: Occur when one species directly affects another (e.g., predation, competition).

• Indirect interactions: Effects are mediated through one or more intermediary species (e.g., trophic cascades).

Dominant Species

Dominant species are those that are the most abundant or have the highest biomass in a community. Their presence often defines the structure and resources available in the ecosystem.

• Characteristics:

  • Large size and/or high abundance.

  • Provide habitat or food for other species.

  • Often strong competitors for resources such as space, nutrients, or light.

• Competitive Exclusion: Dominant species may exclude others from resources, as seen with barnacles in intertidal zones.

• Examples:

  • Douglas fir (Pacific Northwest) and sugar maple (Eastern US) are current dominant tree species in their respective regions.

  • Historical shifts: Western hemlock was once dominant in the Pacific Northwest but was replaced by Douglas fir due to logging and changes in light availability. In the Eastern US, American chestnut was replaced by sugar maple after chestnut blight.

Keystone Species

Keystone species have a disproportionately large effect on community structure relative to their abundance or biomass. Their removal can cause dramatic changes in the ecosystem.

• Characteristics:

  • Low abundance or biomass.

  • High interaction strength with other species.

• Types of Keystone Species:

  • Predators: e.g., sea stars (Pisaster), sea otters.

  • Key food resources: e.g., fig trees that provide fruit during scarce periods.

  • Habitat modifiers: e.g., beavers that build dams and alter landscapes.

  • Ecosystem processors: e.g., decomposers.

• Example: Sea Star (Pisaster)

  • Preys on mussels, preventing their competitive exclusion of other species.

  • Increases species diversity by creating open space for other organisms.

  • Term "keystone species" was first applied to Pisaster by Bob Paine (1966).

Trophic Interactions: Bottom-Up and Top-Down Control

Trophic interactions describe how energy and nutrients move through food webs, influencing community structure through bottom-up or top-down processes.

Bottom-Up Control

In bottom-up control, the abundance of organisms at each trophic level is determined by the productivity and availability of resources at lower levels (e.g., primary producers).

• Example: Aquatic Ecosystems

  • As primary productivity increases, biomass of primary producers rises.

  • Herbivore biomass increases in response to more food.

  • Carnivore biomass increases as herbivores become more abundant.

  • Cycles may occur as changes at one level affect others.

Trophic Level	Effect of Increased Productivity
Primary Producers	Increase in biomass
Herbivores	Increase follows producers
Carnivores	Increase follows herbivores

Top-Down Control and Trophic Cascades

In top-down control, higher trophic levels (predators) regulate the abundance of organisms at lower levels, often resulting in trophic cascades.

• Trophic Cascade: The effects of predators on prey can indirectly affect the abundance or diversity of species at lower trophic levels.

• Example: Egret-Rhino-Tick System

  • Rhino (host) supports ticks (ectoparasite).

  • Egret (cleaner) eats ticks, benefiting both itself and the rhino (by reducing tick load and disease transmission).

  • Removal of egrets leads to more ticks and potentially declining rhino populations.

• Example: Sea Otters in Kelp Forests

  • Sea otters prey on sea urchins, which graze on kelp.

  • Removal of otters leads to urchin population explosions and destruction of kelp forests ("urchin barrens").

  • Reintroduction of otters restores kelp forests by controlling urchin numbers.

Predator Present	Predator Absent
Low urchin numbers, healthy kelp forests	High urchin numbers, kelp forests decline

Interaction Strength and Community Structure

Species with high interaction strength can dramatically alter community composition and ecosystem function. Ecologists study these interactions to understand and predict the consequences of species additions or removals, including those caused by human activities.

• High-impact species (dominant or keystone) can cause cascading effects throughout the community.

• Human impacts, such as species introductions or removals, can have far-reaching consequences.

Case Studies: Human Impacts on Communities

Introduced Predators: The Brown Tree Snake in Guam

The accidental introduction of the brown tree snake to Guam after WWII had devastating effects on the island's community structure.

• Before introduction: Diverse community with birds, bats, reptiles, and insects; no native nocturnal arboreal carnivore.

• After introduction: Brown tree snake preyed on many native species, especially birds and bats.

• Consequences:

  • 9 of 11 native bird species extinct; 2 of 3 native bat species extinct or nearly so.

  • Loss of pollinators led to expected changes in vegetation.

  • Increase in insect populations, causing agricultural damage and higher incidence of insect-borne diseases (e.g., dengue fever).

Community Component	Before Snake	After Snake
Birds	11 native species	2 remaining
Bats	3 native species	1 remaining, declining
Insects	Controlled by birds	Population explosion

Species Interactions in Yellowstone National Park

The removal and later reintroduction of wolves in Yellowstone National Park illustrates the importance of top predators in regulating community dynamics.

• Early 1900s: Wolves extirpated, leading to unchecked elk populations.

• Effects:

  • Elk overgrazed aspen, willow, and cottonwood saplings, preventing forest regeneration.

  • Declines in beaver populations (due to loss of building materials) and songbirds (loss of nesting habitat).

• Wolf reintroduction (1995):

  • Elk populations reduced and altered grazing behavior.

  • Recovery of aspen, willow, and cottonwood.

  • Increase in beaver and songbird populations.

  • Demonstrates the cascading effects of top predators on ecosystem structure and function.

Summary Table: Dominant vs. Keystone Species

Feature	Dominant Species	Keystone Species
Abundance/Biomass	High	Low
Effect on Community	Proportional to abundance	Disproportionately large
Examples	Douglas fir, barnacles	Sea otter, sea star, beaver

Key Terms

• Dominant species: Species that are most abundant or have the highest biomass in a community.

• Keystone species: Species with a disproportionately large effect on community structure relative to their abundance.

• Trophic cascade: Indirect effects of predators on lower trophic levels.

• Competitive exclusion: The process by which one species outcompetes and eliminates another from a habitat.

• Bottom-up control: Community structure regulated by resource availability at lower trophic levels.

• Top-down control: Community structure regulated by consumers at higher trophic levels.

Equations and Concepts

• Interaction Strength (IS): Quantifies the effect of one species on the abundance of another. A general formula: $IS = \frac{(N_{with} - N_{without})}{N_{control}}$ Where $N_{with}$ is the abundance of the target species with the interacting species present, $N_{without}$ is the abundance without the interacting species, and $N_{control}$ is the control abundance.

Additional info: Academic context and definitions have been added to clarify and expand upon the lecture content for self-contained study notes.