Chapter 57- Community ecology

Understand the definition of and characteristics of communities; 

• community is defined as all the populations of different species that live and interact within a particular area at the same time. 

characteristics:

• Species Richness – the number of different species in the community.

• Species Abundance – the number of individuals of each species.

• Species Diversity – a combination of richness and abundance; greater diversity often means greater resilience.

• Trophic Structure – the feeding relationships and energy flow (e.g., producers, consumers, decomposers).

• Dominant Species – species that have the most biomass or influence over environmental conditions.

• Keystone Species – species that have a disproportionately large effect on their community relative to their abundance.

• Succession and Stability – communities change over time through ecological succession and may reach a stable climax state.

 how does species composition dictate how a community works?

• Species composition affects how a community functions by influencing energy flow, food web interactions, and ecosystem stability. 

• Some species play critical roles (like keystone or foundation species), and their presence or absence can significantly impact the whole system. 

• Diverse communities with species that perform similar roles are usually more stable and resilient. In contrast, the loss or introduction of certain species (like invasive ones) can disrupt the balance and function of the community.

Describe competition and the different types of interactions between species; recognize whether the interactions are positive or negative for both organisms or positive or negative for one organism but not the other

Competition occurs when two or more organisms use the same limited resource (like food, water, space)

Intraspecifically – between members of the same species.

Interspecifically – between different species.

Know what a niche is and how species niches can overlap; recognize symmetric and asymmetric competition in a niche model; describe fundamental vs. realized niches; describe niche partitioning, competitive exclusion, and how it could lead to evolution of a species (character displacement)

Recognize and interpret trophic structures (food webs): producers, primary, secondary, tertiary, quaternary consumers)

Producers (Autotrophs)

• 🌱 Examples: Plants, algae, phytoplankton

• Function: Use sunlight (photosynthesis) to make energy-rich compounds—base of the food web.

Primary Consumers (Herbivores)
🐛 Examples: Caterpillars, rabbits, zooplankton

• Function: Eat producers.

Secondary Consumers (Carnivores or Omnivores)

• 🐍 Examples: Frogs, spiders, small fish

• Function: Eat primary consumers.

Tertiary Consumers

• 🦅 Examples: Snakes, hawks, bigger fish

• Function: Eat secondary consumers.

Quaternary Consumers (Top Predators)

• 🐳 Examples: Orcas, polar bears

• Function: Apex predators—few or no predators themselves.

Decomposers (Not a Trophic Level but Super Important)

• 🍄 Examples: Fungi, bacteria

• Function: Break down dead organisms, recycle nutrients back to the environment.

Be able to interpret a “top-down” or “bottom-up” effect in a food web

top-down: 

Definition: When changes at the top of the food web (like predators) affect lower levels. Driven by consumers (usually predators).

bottom up: 

Definition: When changes at the bottom of the food web (like producers or nutrients) affect higher levels. Driven by resources (like sunlight, water, nutrients).

Describe species diversity and richness (what constitutes both); recognize the most diverse communities and communities that are lacking in richness and/or diversity

High Richness, Low Diversity: A community might have many species, but if one species dominates, diversity is low.
Example: A zoo has 100 species, but 90% are ants = low diversity despite high richness.

Low Richness, High Diversity (rare): Few species, but in equal numbers. Still considered moderate diversity at best.

High Richness + High Evenness = High Diversity

Low Richness + Low Evenness = Low Diversity

Understand what a keystone species is and what it contributes to a community; study the examples given

Explain how pathogens and parasites affect species diversity

Reduce Dominance of One Species

• Pathogens and parasites often target the most abundant or dominant species.

• This reduces competition, allowing less common species to survive and thrive.

• ✅ Result: Increases overall species diversity.

Regulate Population Size

• They help keep populations in check (similar to predators).

• Prevents overpopulation and resource depletion by any single species.

Alter Competitive Relationships

• If one species is more vulnerable to a disease or parasite, it may be outcompeted.

• This can shift community structure and lead to niche changes.

Cause Local Extinctions (Sometimes)

• In extreme cases, pathogens can wipe out species, especially if they’re new or invasive.

• This can decrease diversity temporarily or permanently.

Explain primary and secondary succession after a community disturbance

• Primary succession is the start from nothing—bare rock or other lifeless environments.

• Secondary succession is faster, starting from soil that still contains nutrients and life.

• Both processes go through stages that lead to a more stable, mature community over time.

Understand the nature and effects of an invasive species

• Invasive species are non-native organisms that can disrupt ecosystems by outcompeting, predating, or spreading diseases to native species.

• Their rapid growth and lack of natural predators often allow them to dominate ecosystems, causing harm to local biodiversity.

• The economic and environmental costs of invasive species can be significant, making their management and control a major concern.