🔬General Biology I Unit 45 – 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 consists of multiple populations of different species interacting with each other in a shared environment
• Ecosystem encompasses the biotic (living) and abiotic (non-living) components of an environment and their interactions
• Carrying capacity is the maximum population size an environment can sustain given available resources
• Biodiversity measures the variety of life forms within an ecosystem, including genetic diversity, species diversity, and ecosystem diversity
• Ecological succession describes the gradual process of change in the species composition of a community over time
• Keystone species play a disproportionately large role in maintaining the structure and function of an ecosystem relative to their abundance
• Niche refers to the unique role and resource requirements of a species within its ecosystem

Population Dynamics

• Population dynamics involve changes in population size, density, and distribution over time
• Birth rate and death rate are the primary factors influencing population growth or decline
  • Birth rate is the number of new individuals added to a population through reproduction per unit time
  • Death rate is the number of individuals removed from a population due to mortality per unit time
• Immigration and emigration also affect population size by adding or removing individuals from a population
• Exponential growth occurs when a population increases at a constant rate, resulting in a J-shaped curve (bacteria)
• Logistic growth occurs when a population's growth slows as it approaches the carrying capacity, resulting in an S-shaped curve (most mammals)
• Population density is the number of individuals per unit area or volume (plants in a forest)
• Dispersion patterns describe the spatial arrangement of individuals within a population (clumped, uniform, or random)

Factors Affecting Population Growth

• Density-dependent factors have a greater impact on population growth as population density increases (competition, predation, disease)
  • Competition for limited resources intensifies as population density increases, limiting population growth
  • Predation pressure may increase with higher prey population density, regulating prey numbers
  • Disease transmission is more likely in dense populations, potentially causing population crashes
• Density-independent factors affect population growth regardless of population density (natural disasters, climate change, human activities)
  • Natural disasters such as floods, fires, or hurricanes can drastically reduce population sizes
  • Climate change can alter habitat suitability, leading to population declines or shifts in distribution
  • Human activities like deforestation, pollution, and overharvesting can negatively impact populations
• Resource availability, including food, water, and shelter, is crucial for population growth and survival
• Abiotic factors such as temperature, precipitation, and soil quality can limit population growth by affecting resource availability and habitat suitability
• Reproductive strategies, such as r-selection (many offspring, little parental care) and K-selection (few offspring, high parental investment), influence population growth rates

Community Structure and Interactions

• Interspecific interactions occur between individuals of different species within a community
• Competition is a negative interaction where species compete for limited resources (two bird species competing for nesting sites)
  • Competitive exclusion principle states that two species with identical niches cannot coexist indefinitely
  • Resource partitioning allows species to coexist by utilizing resources differently, reducing competition
• Predation is an interaction where one species (predator) consumes another (prey), affecting both populations
  • Predators can regulate prey populations, preventing overexploitation of resources
  • Prey adaptations such as camouflage, warning coloration, and chemical defenses help avoid predation
• Symbiosis is a close, long-term interaction between two species, which can be mutualistic, commensalistic, or parasitic
  • Mutualism benefits both species involved (pollinators and flowering plants)
  • Commensalism benefits one species while the other is unaffected (barnacles on whales)
  • Parasitism benefits one species (parasite) at the expense of the other (host)
• Trophic levels represent the position of a species in a food chain or food web, with energy transfer between levels
  • Primary producers (autotrophs) convert solar energy into chemical energy through photosynthesis
  • Primary consumers (herbivores) feed on primary producers
  • Secondary consumers (carnivores) feed on primary consumers
  • Tertiary consumers (top predators) feed on secondary consumers
  • Decomposers break down dead organic matter, recycling nutrients back into the ecosystem

Ecological Succession

• Ecological succession is the gradual process of change in the species composition of a community over time
• Primary succession occurs in a previously uninhabited area with no soil (volcanic islands, glacial retreat)
  • Pioneer species colonize the area first, modifying the environment for later successional stages
  • Soil development is a crucial process in primary succession, facilitating the establishment of more complex communities
• Secondary succession occurs in an area where a disturbance has removed the existing community but left the soil intact (abandoned agricultural fields, forest fires)
  • Regeneration of the community is faster than in primary succession due to the presence of soil and remnant organisms
• Successional stages progress from early (pioneer) to intermediate to late (climax) communities
  • Early stages are characterized by fast-growing, opportunistic species with high reproductive rates (grasses, annual plants)
  • Intermediate stages have a mix of early and late successional species, with increasing complexity and diversity
  • Late stages are characterized by slow-growing, long-lived species with low reproductive rates (trees, shrubs)
• Climax community is the final, relatively stable stage of succession, determined by the regional climate and soil conditions
  • Disturbances can reset the successional process, creating a mosaic of different stages within a landscape

Biodiversity and Species Richness

• Biodiversity encompasses the variety of life at all levels, from genes to ecosystems
• Species richness is the number of different species present in a community or ecosystem
• Species evenness refers to the relative abundance of each species within a community
• Biodiversity is important for ecosystem stability, resilience, and the provision of ecosystem services
  • Greater biodiversity can buffer against environmental changes and disturbances
  • Diverse ecosystems are more productive and efficient in resource use
  • Biodiversity provides essential services such as nutrient cycling, pollination, and pest control
• Biodiversity hotspots are areas with exceptionally high species richness and endemism, often threatened by habitat loss (tropical rainforests, coral reefs)
• Habitat heterogeneity (diversity of habitats within an ecosystem) promotes higher biodiversity by providing more niches for species to occupy
• Latitudinal biodiversity gradient describes the increase in species richness from the poles to the equator
  • Factors contributing to this pattern include greater energy availability, longer evolutionary history, and higher speciation rates in the tropics

Human Impact on Ecosystems

• Habitat destruction is the primary cause of biodiversity loss, as it reduces the available space and resources for species (deforestation, urbanization)
• Habitat fragmentation divides contiguous habitats into smaller, isolated patches, reducing connectivity and population viability
• Overexploitation of species through hunting, fishing, and harvesting can lead to population declines and extinctions (whales, sharks, tropical hardwoods)
• Invasive species, introduced intentionally or accidentally by humans, can outcompete native species and disrupt ecosystem balance (kudzu, zebra mussels)
• Pollution, including air, water, and soil contamination, can have detrimental effects on species and ecosystem health (pesticides, plastic waste, oil spills)
• Climate change, driven by human activities such as fossil fuel combustion, alters temperature and precipitation patterns, affecting species distributions and interactions
• Conservation efforts aim to protect and restore biodiversity through habitat preservation, species management, and sustainable resource use (protected areas, captive breeding programs, sustainable forestry)
• Ecosystem restoration involves the active recovery of degraded or destroyed ecosystems to improve biodiversity and ecosystem services (wetland restoration, reforestation)

Real-World Applications and Case Studies

• Yellowstone wolf reintroduction: The reintroduction of gray wolves to Yellowstone National Park in 1995 demonstrated the importance of keystone species in shaping ecosystem structure and function through trophic cascades
• Invasive species management: The control of invasive species such as the brown tree snake in Guam and the European rabbit in Australia highlights the challenges and strategies involved in mitigating their impact on native ecosystems
• Coral reef conservation: Coral reefs, among the most biodiverse ecosystems on Earth, are threatened by multiple stressors, including ocean acidification, warming temperatures, and overfishing, necessitating comprehensive conservation efforts
• Sustainable fisheries management: The implementation of catch limits, no-take zones, and gear restrictions aims to maintain fish populations and ecosystem health while supporting the livelihoods of fishing communities (Marine Stewardship Council certification)
• Urban ecology and green infrastructure: Incorporating green spaces, such as parks, gardens, and green roofs, into urban areas can support biodiversity, improve air and water quality, and provide recreational and educational opportunities for city residents
• Ecological restoration of mining sites: The restoration of decommissioned mining sites involves the reestablishment of native vegetation, soil remediation, and the creation of habitats for wildlife, demonstrating the principles of ecological succession and habitat rehabilitation
• Agroecology and sustainable agriculture: The application of ecological principles to agricultural systems, such as crop diversification, intercropping, and integrated pest management, can enhance biodiversity, soil health, and crop resilience while reducing the reliance on synthetic inputs (organic farming, permaculture)
• Citizen science and community-based conservation: Engaging local communities in biodiversity monitoring, habitat restoration, and conservation decision-making can foster stewardship, generate valuable ecological data, and support the long-term success of conservation initiatives (bird counts, reef check)