🐇Honors Biology Unit 18 – Population and Community Ecology

Key Concepts and Definitions

• Population refers to a group of individuals of the same species living in a specific area at a given time
• Community encompasses all the populations of different species interacting in a particular ecosystem
• Carrying capacity ($K$) represents the maximum population size an environment can sustain given its resources and conditions
• Density-dependent factors (competition, predation) have a greater impact on population growth as the population size increases
• Density-independent factors (natural disasters, climate) affect populations regardless of their size or density
• Ecological niche describes the role and position of a species within its ecosystem, including its interactions and resource use
  • Fundamental niche includes all possible resources and conditions a species could potentially utilize
  • Realized niche is the actual niche a species occupies due to competition and other limiting factors
• Biodiversity measures the variety of life at different levels, including genetic diversity, species diversity, and ecosystem diversity

Population Dynamics

• Population growth is influenced by birth rates, death rates, immigration, and emigration
• Exponential growth occurs when a population increases at a constant rate, resulting in a J-shaped curve
  • Characterized by abundant resources and minimal competition or predation
• Logistic growth occurs when a population's growth slows as it approaches the carrying capacity, forming an S-shaped curve
  • Density-dependent factors become more influential as the population size increases
• Age structure diagrams (pyramids) illustrate the distribution of individuals across different age groups within a population
• Survivorship curves depict the proportion of individuals surviving to each age class
  • Type I: high survival until old age (humans, elephants)
  • Type II: constant mortality rate throughout life (birds, lizards)
  • Type III: high mortality in early life stages, followed by high survival (fish, insects)
• Population cycles result from predator-prey interactions or resource availability fluctuations (lynx-hare, lemmings)

Community Interactions

• Competition occurs when species vie for limited resources, leading to reduced growth, survival, or reproduction
  • Intraspecific competition happens within the same species
  • Interspecific competition occurs between different species
• Predation is an interaction where one species (predator) hunts and consumes another (prey)
  • Predators can regulate prey populations and maintain ecosystem balance
• Symbiosis involves close and long-term interactions between two species
  • Mutualism benefits both species (clownfish and sea anemones)
  • Commensalism benefits one species while the other is unaffected (barnacles on whales)
  • Parasitism benefits one species (parasite) at the expense of the other (host)
• Herbivory is a plant-animal interaction where animals consume plants or algae
• Facilitation occurs when one species enhances the survival, growth, or reproduction of another (nurse plants in deserts)

Ecological Niches

• Niche partitioning allows species to coexist by utilizing different resources or habitats within an ecosystem
  • Resource partitioning involves species using different food sources, foraging times, or feeding strategies
  • Spatial partitioning occurs when species occupy different microhabitats or vertical strata (canopy, understory)
• Competitive exclusion principle states that two species with identical niches cannot coexist indefinitely
• Character displacement is the divergence of traits (morphological, behavioral) in sympatric species to minimize competition
• Niche breadth refers to the range of resources a species can utilize or tolerate
  • Generalist species have a wide niche breadth and can adapt to various conditions (raccoons, coyotes)
  • Specialist species have a narrow niche breadth and are adapted to specific resources or habitats (koalas, pandas)
• Keystone species have a disproportionately large impact on their ecosystem relative to their abundance (sea otters, beavers)

Biodiversity and Species Richness

• Species richness is the number of different species present in a community or ecosystem
• Species evenness measures the relative abundance of each species within a community
• Alpha diversity refers to the diversity within a particular habitat or ecosystem
• Beta diversity compares the diversity between different habitats or ecosystems
• Gamma diversity encompasses the total diversity across a region or landscape
• Latitudinal biodiversity gradient shows a general increase in species richness from the poles to the equator
  • Factors contributing to this pattern include greater productivity, stability, and niche specialization in tropical regions
• Biodiversity hotspots are areas with exceptionally high species richness and endemism, often threatened by habitat loss (Amazon rainforest, Madagascar)

Ecosystem Stability and Disturbance

• Resistance is the ability of an ecosystem to withstand disturbances without significant changes in structure or function
• Resilience refers to an ecosystem's capacity to recover and return to its original state after a disturbance
• Succession is the gradual process of community change over time following a disturbance
  • Primary succession occurs on newly exposed or formed substrates (volcanic islands, glacial moraines)
  • Secondary succession takes place after a disturbance in an area with pre-existing soil (abandoned farmland, forest fires)
• Climax community represents the final, relatively stable stage of succession, determined by the regional climate and soil conditions
• Intermediate disturbance hypothesis suggests that moderate levels of disturbance can promote higher species diversity
  • Prevents competitive exclusion and allows for the coexistence of species with different life histories and competitive abilities
• Ecological redundancy occurs when multiple species perform similar functional roles within an ecosystem, providing resilience to disturbances

Human Impact on Populations and Communities

• Habitat loss and fragmentation are major threats to biodiversity, reducing available space and connectivity for species
  • Deforestation, urbanization, and agricultural expansion are primary drivers
• Overexploitation involves the unsustainable harvest of species for human use (overfishing, poaching)
• Invasive species are non-native organisms that can disrupt ecosystems and outcompete native species
  • Introduced intentionally (ornamental plants, pets) or accidentally (ballast water, cargo)
• Pollution (air, water, soil) can have detrimental effects on populations and communities
  • Bioaccumulation and biomagnification concentrate pollutants up the food chain
• Climate change, driven by anthropogenic greenhouse gas emissions, alters temperature, precipitation, and ocean chemistry
  • Shifts in species ranges, phenology, and interspecific interactions can lead to community reorganization and potential extinctions
• Conservation efforts aim to protect and restore populations, communities, and ecosystems
  • Protected areas (national parks, wildlife reserves) safeguard habitats and species
  • Captive breeding and reintroduction programs help recover endangered populations
  • Ecosystem restoration projects focus on rehabilitating degraded habitats

Real-World Applications and Case Studies

• Yellowstone wolf reintroduction (1995) demonstrated the cascading effects of predators on ecosystem structure and function
  • Reduced elk populations, allowing for the recovery of riparian vegetation and associated species
• Invasive lionfish in the Caribbean and Atlantic have disrupted coral reef communities
  • Voracious predators that outcompete native fish and reduce recruitment
  • Control efforts include targeted removals and promoting consumption as a food source
• Coral bleaching events, caused by rising ocean temperatures and acidification, have devastating impacts on coral reef ecosystems
  • Symbiotic algae (zooxanthellae) are expelled, leading to coral starvation and mortality
  • Affects the diversity and abundance of reef-associated species and the livelihoods of coastal communities
• Monarch butterfly populations have declined due to habitat loss and reduced milkweed availability
  • Milkweed is the exclusive host plant for monarch larvae
  • Conservation initiatives involve planting milkweed gardens and protecting overwintering sites in Mexico
• Marine protected areas (MPAs) have been established to conserve biodiversity and manage fisheries
  • No-take reserves prohibit fishing and other extractive activities, allowing for the recovery of fish populations and habitats
  • Great Barrier Reef Marine Park in Australia is a successful example of a large-scale MPA network