ECOLOGY LEC 4

Community

A group of interacting species in an area

Community structure

The composition and relative abundance of species

Community function

Energy flow and nutrient cycling within a community

Community dynamics

Changes in community structure or function over time

Species richness

The number of species in a community

Species composition

The relative abundance of different species

Trophic interactions

Feeding relationships between organisms

Food web

A network of interconnected food chains

Trophic level

The position of an organism in a food web

Primary producers

Organisms that produce energy (plants, algae)

Energy flow direction

From producers to higher trophic levels

Dominant species

Species with high biomass that strongly influence a community

Ecosystem engineers

Species that modify the physical environment

Keystone species

Species with a large impact despite low abundance

Example of keystone species

Sea otters

Top-down control

Higher trophic levels control lower trophic levels

Bottom-up control

Lower trophic levels control higher trophic levels

Trophic cascade

Effects of predators cascade down to lower trophic levels

Sea otter trophic cascade

Sea otters ↓ urchins → ↑ kelp forests

Effect of removing sea otters

More urchins → less kelp (urchin barrens)

Role of kelp forests

Provide habitat and increase biodiversity

Example of top-down disruption

Orcas reducing sea otters → urchin increase

Atlantic coast trophic cascade

Overfishing reduced predators → urchins increased → kelp declined

Urchin barrens

Areas dominated by sea urchins with little kelp

Regime shift

A sudden and lasting change in ecosystem structure

Example of regime shift

Kelp forest → urchin barrens

Causes of regime shifts

Removal of keystone species, disease, climate change, nutrient input

Top-down control importance

Predators regulate ecosystems and prevent overgrazing

Applications of top-down control

Wildlife management, pest control, resource management

Zoonotic disease

Disease transmitted from animals to humans

Lyme disease

A bacterial disease caused by Borrelia burgdorferi

Symptoms of Lyme disease

Fever, joint pain, arthritis, bullseye rash

Vector

Organism that transmits a pathogen

Lyme disease vector

Black-legged tick (Ixodes scapularis)

Direct transmission

Pathogen moves directly between hosts

Indirect transmission

Pathogen uses a vector

Tick life cycle

Larva → nymph → adult → eggs

How ticks get Lyme pathogen

By feeding on infected hosts

Accidental host

Humans are not required for pathogen lifecycle

Historical Lyme disease pattern

Common → declined → increased again

Role of deer in Lyme disease

Primary host for adult ticks

Effect of deer increase

More ticks → more disease spread

Role of small mammals (mice)

Increase infected tick populations

Effect of predators on Lyme disease

Foxes reduce mice → fewer infected ticks

Effects of Coyotes on Lyme disease

Coyotes reduce foxes indirectly → more mice → more Lyme disease

Role of migratory birds

Spread ticks to new regions

Forest recovery effect

Increased deer → increased ticks → more Lyme disease

Key ecological interactions in Lyme disease

Deer, mice, predators, birds, and environment all interact

Climate effect on Lyme disease

Warming temperatures allow ticks to expand range

Main idea of disease ecology

Disease spread depends on ecological interactions

Redundancy in ecosystems

Multiple species perform similar roles

Resilience

Ability of ecosystem to recover after disturbance

Importance of biodiversity

Increases ecosystem stability and resilience

Summary of trophic cascades

Predators indirectly affect multiple trophic levels

Summary of zoonotic disease

Ecological interactions strongly influence disease spread