Community and Ecosystem Ecology

Organism Interactions in Ecosystems

Organisms interact with each other and their environment in various ways, forming complex networks within ecosystems. These interactions shape community structure and ecosystem function.

• Competition: Occurs when organisms vie for the same resources (e.g., food, space).

• Predation: One organism (predator) feeds on another (prey).

• Mutualism: Both organisms benefit from the interaction (e.g., pollinators and flowering plants).

• Commensalism: One organism benefits, the other is unaffected.

• Parasitism: One organism benefits at the expense of another.

• Example: Bees pollinating flowers (mutualism); wolves hunting deer (predation).

Biodiversity: Definition, Measurement, Importance, and Aspects

Biodiversity refers to the variety of life in an ecosystem. It is crucial for ecosystem resilience and function.

• Measurement: Biodiversity is measured by species richness (number of species) and species evenness (relative abundance).

• Importance: High biodiversity increases ecosystem stability, productivity, and resistance to disturbances.

• Two Aspects:

  • Genetic Diversity: Variation within species.

  • Species Diversity: Variety of species in a community.

• Example: Tropical rainforests have high species richness and genetic diversity.

Succession: Primary vs. Secondary

Succession is the process by which ecosystems change and develop over time following a disturbance.

• Primary Succession: Occurs in areas where no soil exists (e.g., after volcanic eruption).

• Secondary Succession: Occurs in areas where soil remains after a disturbance (e.g., after a forest fire).

• Why Succession Occurs: Driven by changes in environmental conditions and species interactions.

• Example: Mosses and lichens colonizing bare rock (primary); grasses and shrubs regrowing after fire (secondary).

Energy Flow and Trophic Levels

Energy moves through ecosystems via food chains and food webs, passing from one trophic level to another.

• Trophic Levels:

  • Primary Producers: Autotrophs (plants, algae) that convert solar energy to chemical energy.

  • Primary Consumers: Herbivores that eat producers.

  • Secondary Consumers: Carnivores that eat herbivores.

  • Tertiary Consumers: Carnivores that eat other carnivores.

  • Decomposers: Break down dead organisms, recycling nutrients.

• Energy Replacement: Energy must be replaced because it is lost as heat at each trophic level (Second Law of Thermodynamics).

• Formula:

• Example: Only about 10% of energy is transferred from one trophic level to the next.

Nutrient Cycling

Nutrients move within ecosystems through biogeochemical cycles (e.g., carbon, nitrogen cycles). Unlike energy, nutrients are recycled.

• Why Nutrients Do Not Need Replacement: Nutrients are cycled and reused by organisms and decomposers.

• Example: Nitrogen is fixed by bacteria, used by plants, and returned to the soil by decomposers.

Evolution and Cladograms

Cladograms: Interpretation and Use

Cladograms are diagrams that show evolutionary relationships among organisms based on shared characteristics.

• How to Read: Branch points represent common ancestors; closer branches indicate closer relationships.

• Example: A cladogram showing mammals, reptiles, and birds based on shared traits.

Three Domains of Life and Viruses

Life is classified into three domains: Bacteria, Archaea, and Eukarya. Viruses are not classified as living organisms.

• Bacteria: Prokaryotic, diverse metabolic pathways.

• Archaea: Prokaryotic, often found in extreme environments.

• Eukarya: Eukaryotic, includes plants, animals, fungi, protists.

• Viruses: Non-living, lack cellular structure, require host for replication.

• Example: Escherichia coli (Bacteria), Halobacterium (Archaea), Homo sapiens (Eukarya).

Prokaryotes vs. Eukaryotes

Prokaryotes (Bacteria and Archaea) lack a nucleus and membrane-bound organelles. Eukaryotes have a nucleus and organelles.

• Prokaryotes: Small, simple cells, reproduce by binary fission.

• Eukaryotes: Larger, complex cells, reproduce by mitosis/meiosis.

• Example: Streptococcus (prokaryote); Amoeba (eukaryote).

Endosymbiotic Theory

The Endosymbiotic Theory explains the origin of eukaryotic cells from prokaryotic ancestors.

• Evidence:

  • Mitochondria and chloroplasts have their own DNA.

  • Double membranes around organelles.

  • Similarities to prokaryotes in size and reproduction.

• Explanation: Eukaryotes arose when a prokaryote engulfed another, forming a symbiotic relationship.

• Example: Mitochondria originated from ancestral aerobic bacteria.

Prokaryotes

Bacteria vs. Archaea

Bacteria and Archaea are both prokaryotes but differ in cell structure, metabolism, and habitats.

• Bacteria: Peptidoglycan cell walls, diverse environments.

• Archaea: Unique cell membrane lipids, often extremophiles.

• Example: Thermophiles (Archaea) vs. Staphylococcus (Bacteria).

Basic Groups of Bacteria and Archaea

Bacteria and Archaea are classified based on shape, metabolism, and ecological roles.

• Bacteria: Cocci (spherical), Bacilli (rod-shaped), Spirilla (spiral).

• Archaea: Methanogens, Halophiles, Thermophiles.

• Example: Halobacterium (Halophile), Escherichia coli (Bacillus).

Roles in Ecosystem

Bacteria and Archaea play essential roles in nutrient cycling, decomposition, and symbiosis.

• Decomposers: Break down organic matter.

• Nitrogen Fixers: Convert atmospheric nitrogen to usable forms.

• Symbionts: Live in association with other organisms (e.g., gut bacteria).

• Example: Rhizobium bacteria fix nitrogen in plant roots.

Horizontal Gene Transfer

Horizontal Gene Transfer (HGT) is the movement of genetic material between organisms other than by descent.

• Methods:

  • Transformation: Uptake of DNA from environment.

  • Transduction: Transfer by viruses (bacteriophages).

  • Conjugation: Direct transfer via cell-to-cell contact.

• Example: Antibiotic resistance genes spreading among bacteria.

Protists

Differences from Other Eukaryotes and Prokaryotes

Protists are a diverse group of eukaryotes, often unicellular, that differ from other eukaryotes and prokaryotes.

• From Prokaryotes: Have nucleus and organelles.

• From Other Eukaryotes: Usually unicellular, lack specialized tissues.

• Example: Paramecium (protist) vs. Streptococcus (prokaryote).

Basic Groups of Protists

Protists are classified into groups based on their mode of nutrition and movement.

• Algae: Photosynthetic protists (e.g., Chlamydomonas).

• Protozoa: Animal-like, heterotrophic (e.g., Amoeba).

• Slime Molds: Fungus-like, decomposers (e.g., Physarum).

• Example: Euglena (mixotrophic protist).

Summary Table: Domains of Life and Key Differences

Additional info: Academic context was added to clarify definitions, examples, and explanations for each topic, as the original notes were brief and exam-focused.