Chapter 37: Communities and Ecosystems – Study Guide and Key Concepts

Chapter 37: Communities and Ecosystems

Key Vocabulary and Concepts

This section covers essential terms and concepts related to ecological communities, species interactions, and ecosystem processes. Understanding these terms is foundational for interpreting ecological relationships and environmental impacts.

• Biomagnification: The process by which the concentration of toxic substances increases in organisms at each successive trophic level of a food chain. For example, mercury accumulates in fish-eating birds at higher concentrations than in the water or small fish.

• Biodiversity: The variety of life in a particular habitat or ecosystem, including species diversity, genetic diversity, and ecosystem diversity.

• Carrying Capacity: The maximum population size of a species that an environment can sustain indefinitely, given the available resources.

• Camouflage: An adaptation that allows organisms to blend in with their surroundings to avoid predation. Example: The green coloration of a praying mantis among leaves.

• Coevolution: The reciprocal evolutionary adaptations of two interacting species. Example: Flowers and their pollinators evolving together.

• Community: All the populations of different species that live and interact in a particular area.

• Competition: The interaction between organisms or species that vie for the same resources in an ecosystem.

• Consumer: An organism that obtains energy by feeding on other organisms. Consumers are classified as primary, secondary, or tertiary based on their position in the food chain.

• Ecosystem: A community of living organisms and their physical environment interacting as a system.

• Ecological Niche: The role and position a species has in its environment, including all its interactions with the biotic and abiotic factors.

• Food Chain / Food Web: A food chain is a linear sequence of organisms through which nutrients and energy pass. A food web is a complex network of interconnected food chains in an ecosystem.

• Herbivory: The consumption of plant material by animals.

• Invasive Species: Non-native species that spread widely in a new environment, often causing harm to native species and ecosystems.

• Interspecific Competition: Competition between individuals of different species for the same resources.

• Keystone Species: A species that has a disproportionately large effect on its ecosystem relative to its abundance. Example: Sea otters in kelp forest ecosystems.

• Mutualism: An interaction between two species in which both benefit. Example: Bees and flowering plants.

• Parasitism: A relationship where one organism (the parasite) benefits at the expense of another (the host).

• Pathogens: Disease-causing organisms, such as bacteria, viruses, or fungi.

• Predation: An interaction in which one organism (the predator) kills and eats another (the prey).

• Producer: An organism, such as a plant or algae, that produces organic molecules from inorganic substances through photosynthesis or chemosynthesis.

• Relative Species Abundance: The proportion of each species in a community relative to the total number of individuals.

• Species: A group of organisms that can interbreed and produce fertile offspring.

• Species Richness: The number of different species present in a community.

Major Concepts and Applications

Interspecific Interactions

Interspecific interactions are relationships between individuals of different species within a community. These interactions shape community structure and influence species evolution.

• Types of Interspecific Interactions:

  • Competition: Both species are harmed by the interaction (−/−).

  • Predation: One species benefits, the other is harmed (+/−).

  • Herbivory: Animal eats plant; plant is harmed (+/−).

  • Parasitism: Parasite benefits, host is harmed (+/−).

  • Mutualism: Both species benefit (+/+).

  • Commensalism: One species benefits, the other is unaffected (+/0).

• Describing Interactions: From a description, identify the type of interaction by considering the effects on each species.

Niche Overlap and Competition

When two species have overlapping niches, they compete for the same resources. This can lead to competitive exclusion or resource partitioning.

• Competitive Exclusion Principle: Two species competing for the same limiting resource cannot coexist at constant population values.

• Resource Partitioning: Species evolve to use different resources or occupy different niches to reduce competition.

Predator Avoidance Mechanisms

Both plants and animals have evolved adaptations to avoid predation.

• Plants:

  • Thorns, spines, and tough leaves

  • Chemical defenses (toxins, bitter compounds)

• Animals:

  • Camouflage (cryptic coloration)

  • Mimicry (resembling another species)

  • Warning coloration (aposematism)

  • Behavioral adaptations (e.g., fleeing, playing dead)

Biogeochemical Cycles: Phosphorus and Nitrogen

These cycles describe the movement of essential elements through ecosystems.

• Phosphorus Cycle:

  1. Weathering of rocks releases phosphate into soil.

  2. Plants absorb phosphate; animals obtain it by eating plants.

  3. Decomposition returns phosphate to soil.

  4. Phosphate may be lost to oceans and incorporated into new rocks.

• Nitrogen Cycle:

  1. Nitrogen fixation: Bacteria convert atmospheric N2 to ammonia (NH3).

  2. Nitrification: Ammonia is converted to nitrites (NO2−) and then nitrates (NO3−).

  3. Assimilation: Plants absorb nitrates; animals eat plants.

  4. Ammonification: Decomposers convert organic nitrogen back to ammonia.

  5. Denitrification: Bacteria convert nitrates back to N2 gas, returning it to the atmosphere.

Biomagnification in Food Chains

Biomagnification refers to the increasing concentration of toxic substances in organisms at higher trophic levels.

• Process: Toxins such as DDT or mercury are not easily broken down and accumulate in organisms. As each trophic level consumes the previous one, the concentration of toxins increases.

• Example: Small fish accumulate mercury from water; larger fish eat many small fish, concentrating mercury further; birds or humans eating large fish receive the highest concentrations.

Interpreting Biodiversity Graphs

Biodiversity graphs may show species richness, relative abundance, or changes in diversity over time or across habitats.

• Species Richness: The number of different species present.

• Relative Abundance: The proportion of each species in the community.

• Application: Use graphs to compare diversity between ecosystems or to assess the impact of disturbances or invasive species.

Summary Table: Types of Species Interactions

Interaction Type	Effect on Species 1	Effect on Species 2	Example
Competition	−	−	Two bird species competing for the same nesting site
Predation	+	−	Lion and zebra
Herbivory	+	−	Caterpillar eating a leaf
Parasitism	+	−	Tapeworm in a dog
Mutualism	+	+	Bees and flowers
Commensalism	+	0	Barnacles on a whale

Key Equations

• Carrying Capacity (K) in Logistic Growth: $\frac{dN}{dt} = rN \left(1 - \frac{N}{K}\right)$ Where:

  • N = population size

  • r = intrinsic rate of increase

  • K = carrying capacity

Additional info: Academic context and examples have been added to clarify and expand upon the brief study guide points, ensuring the notes are self-contained and suitable for exam preparation.