19.4 community ecology

Populations of one species never live in isolation; they form an ecological community in a given habitat.
Diversity of the Community: Refers to the number of species and their relative abundance within a habitat.
- Example: Glaciers of Antarctica have low species diversity but contain various organisms, whereas tropical rainforests exhibit high diversity.
Study of Ecology at the Community Level: Focuses on species interactions and competition for resources.

Predator-Prey Relationship: Involves individuals of one population killing and consuming individuals from another population.
Population Cycles: Predator and prey population sizes fluctuate in cycles related to one another.
- Example: Lynx (predator) and snowshoe hare (prey) exhibit cycles based on data spanning 100 years in North America.
- Period: Approximately 10 years, with predator population lagging 1-2 years after prey population.
- Explanation: Increased hare numbers provide food for lynx, leading to an increase in their population. Overhunting leads to a decrease in hare numbers, causing a subsequent decline in lynx.

Predation drives adaptation via strong selective pressures.
Heritable traits: Traits allowing prey to evade predators will increase in prevalence over generations.
Defenses: Can be mechanical, chemical, physical, or behavioral.
- Mechanical Defenses: Armor in animals or thorns in plants discourages attacks.
- Chemical Defenses: Many plants produce toxic secondary compounds, e.g., foxglove (produces digitalis, a heart medication that is toxic when consumed).
- Physical Appearance: Some species don adaptations like coloration and shape for camouflage (e.g., the tropical walking stick mimics twigs).
- Behavioral Adaptations: Strategies include playing dead or forming schools/flocks.
- Warning Coloration: Distinct coloration signals toxicity (e.g., monarch caterpillar, fire-bellied toads).
- Mimicry: Non-toxic species imitating the coloration of toxic ones (e.g., hoverflies mimicking wasps).

Definition: Close, long-term interactions between individuals of different species, categorized as follows:
- Commensalism: One species benefits, the other is neither harmed nor helped (Example: Birds nesting in trees).
- Mutualism: Both species benefit from the relationship (Example: Termites and protozoa in their guts, or lichens as a partnership between fungi and photosynthetic organisms).
- Parasitism: One species benefits while harming the other (Example: Tapeworms in humans, Plasmodium causing malaria).

Resources: Often limited, leading to competition among species in a habitat.
Niche: The unique set of resources a species uses, including interactions with others.
Principle Statement: Two species cannot occupy the same niche, leading to extinction of one if competition occurs for the same resources.
- Experimental Example: Paramecium aurelia outcompetes Paramecium caudatum when grown together due to resource competition.

Community Dynamics: Changes over time often due to disturbances such as natural disasters and human impact.
Biodiversity: Refers to the variety of different species in a given area and their relative abundance.
- Species Richness: The number of different species in a habitat, with more richness near the equator and less near the poles.
- Relative Species Abundance: Number of individuals in a species compared to all individuals across species.

Foundation Species: Organisms that provide basic structure to the community, typically abundant primary producers (e.g., kelp in kelp forests, tree species in forests).
Keystone Species: Have a significant impact on the ecological community’s structure and biodiversity (e.g., Pisaster ochraceus, the sea star, affects mussel populations and biodiversity).

Invasive Species: Non-native organisms altering communities they invade (e.g., purple loosestrife, zebra mussel, Asian carp).
Example: Asian carp introduced in the 1970s in U.S. water systems, compete with native fish for resources, impacting ecosystems and human economies.

Community Equilibrium: Dynamic state with relatively constant species numbers, but species identities and relationships change.
Succession: Sequential changes in species composition over time following environmental disturbances:
- Primary Succession: Occurs on newly formed land (e.g., post-volcanic eruptions) where pioneer species like lichens establish.
- Secondary Succession: Follows disturbance in an existing ecosystem but with remnants of the previous community (e.g., post-wildfire forest recovery) leading to a climax community over time.

Primary Succession: Involves colonization by pioneer species (e.g., lichens, hardy plants) on new substrates like lava.
Secondary Succession: Follows disturbances with existing soil (e.g., wildfires returning nutrients to the soil for rapid recolonization) leading back to a climax community.
- Example: Oak and hickory forests recovering after wildfires, progressing through stages of succession until reaching equilibrium.