🦉Intro to Ecology Unit 4 – Population 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
• Density measures the number of individuals per unit area or volume (plants per square meter, fish per liter)
• Dispersion patterns describe the spatial arrangement of individuals in a population (clumped, uniform, random)
• Demographics involve the study of population characteristics such as age structure, sex ratio, and birth and death rates
• Carrying capacity ($K$) represents the maximum population size an environment can sustain given available resources
• Exponential growth occurs when a population increases at a constant rate, resulting in a J-shaped curve
  • Characterized by a constant per capita rate of increase ($r$)
• Logistic growth involves a population increasing until it reaches carrying capacity, resulting in an S-shaped curve
  • Incorporates density-dependent factors that limit growth as population size approaches $K$

Population Characteristics and Dynamics

• Population size measures the number of individuals in a population at a given time
• Population density calculates the number of individuals per unit area or volume (plants per hectare, insects per cubic meter)
• Age structure refers to the proportion of individuals in different age classes within a population (pre-reproductive, reproductive, post-reproductive)
  • Influences population growth and future size
• Sex ratio compares the number of males to females in a population, affecting reproductive potential
• Birth rate quantifies the number of new individuals added to a population through reproduction per unit time
• Death rate measures the number of individuals removed from a population due to mortality per unit time
• Immigration involves individuals moving into a population from other areas, increasing population size
• Emigration occurs when individuals leave a population to inhabit other areas, decreasing population size

Growth Models and Patterns

• Exponential growth model assumes a constant per capita rate of increase ($r$), resulting in a J-shaped curve
  • Described by the equation $\frac{dN}{dt} = rN$, where $N$ is population size and $t$ is time
• Logistic growth model incorporates density-dependent factors and carrying capacity ($K$), resulting in an S-shaped curve
  • Represented by the equation $\frac{dN}{dt} = rN\left(1-\frac{N}{K}\right)$
• Geometric growth occurs when a population increases by a constant factor per unit time (doubling, tripling)
• Boom-and-bust cycles involve rapid population growth followed by a sharp decline due to resource depletion or other factors (insect outbreaks, algal blooms)
• Stable age distribution refers to the proportions of individuals in each age class remaining constant over time
  • Achieved when birth and death rates are balanced
• Survivorship curves depict the proportion of individuals surviving to each age class (Type I, II, III)

Factors Affecting Population Size

• Density-dependent factors have a greater impact on population growth as density increases (competition, predation, disease)
  • Regulate population size and prevent indefinite growth
• Density-independent factors affect populations regardless of their density (natural disasters, climate change, human disturbance)
• Resource availability, such as food, water, and habitat, limits population growth and determines carrying capacity
• Predation can control prey populations and shape their behaviors and adaptations
• Competition for limited resources among individuals of the same species (intraspecific) or different species (interspecific) affects survival and reproduction
• Disease outbreaks can significantly reduce population size, particularly in dense or stressed populations
• Human activities, such as habitat destruction, overharvesting, and pollution, can negatively impact population size and growth

Life History Strategies

• Life history strategies represent the allocation of resources to growth, reproduction, and survival
• r-selected species prioritize rapid growth, early reproduction, and high offspring quantity (bacteria, annual plants, insects)
  • Adapted to unstable or unpredictable environments
• K-selected species invest in slower growth, delayed reproduction, and higher offspring quality (elephants, whales, primates)
  • Adapted to stable or predictable environments
• Trade-offs exist between traits such as offspring quantity and quality, early and late reproduction, and growth and survival
• Semelparity refers to a life history strategy where organisms reproduce once and then die (annual plants, salmon, mayflies)
• Iteroparity involves organisms reproducing multiple times throughout their lifetime (perennial plants, most mammals and birds)
• Parental investment varies among species, with some providing extensive care to offspring and others providing minimal care

Population Interactions

• Competition occurs when individuals or populations compete for limited resources, leading to reduced growth and survival
  • Intraspecific competition involves individuals of the same species
  • Interspecific competition involves individuals of different species
• Predation describes the interaction where one organism (predator) consumes another (prey), affecting population dynamics
  • Predators can regulate prey populations and influence their behaviors and adaptations
• Herbivory involves animals consuming plants, which can shape plant communities and ecosystem processes
• Mutualism is an interaction where both species benefit (pollination, seed dispersal, coral-algae symbiosis)
• Commensalism occurs when one species benefits while the other is unaffected (barnacles on whales, epiphytes on trees)
• Parasitism involves one species (parasite) living on or within another (host), causing harm to the host
  • Can regulate host populations and drive coevolutionary arms races

Human Impact on Populations

• Habitat loss and fragmentation due to human activities (deforestation, urbanization, agriculture) reduce available space and resources for populations
• Overexploitation involves the unsustainable harvest of populations for human use (overfishing, poaching, logging)
  • Can lead to population declines and extinctions
• Invasive species, often introduced by humans, can outcompete native species and disrupt ecosystems
• Pollution, such as chemical contaminants and plastic waste, can harm populations and their habitats
• Climate change, driven by human activities, alters temperature, precipitation, and sea levels, affecting population distributions and survival
• Conservation efforts aim to protect and restore populations and their habitats (protected areas, captive breeding, habitat restoration)
  • Requires understanding of population ecology and collaboration among scientists, policymakers, and communities

Real-World Applications and Case Studies

• Fisheries management applies population ecology principles to ensure sustainable harvest of fish populations
  • Involves monitoring population size, setting catch limits, and regulating fishing practices
• Invasive species control programs aim to prevent the spread and mitigate the impacts of non-native species (zebra mussels, kudzu, cane toads)
  • Utilizes knowledge of population growth, dispersal, and interactions
• Endangered species recovery plans focus on increasing population size and genetic diversity of threatened species (California condors, black-footed ferrets)
  • Incorporates captive breeding, reintroduction, and habitat protection
• Pest management in agriculture and forestry applies understanding of population dynamics to control crop pests and forest insects
  • Integrates biological, chemical, and cultural control methods
• Vaccination programs rely on population ecology concepts to prevent the spread of infectious diseases (measles, polio, COVID-19)
  • Aims to achieve herd immunity by vaccinating a sufficient proportion of the population
• Ecological restoration projects use knowledge of population characteristics and interactions to restore degraded ecosystems (prairies, wetlands, coral reefs)
  • Involves reintroducing key species, managing invasive species, and monitoring population recovery