🪺Environmental Biology Unit 3 – Population Ecology: Dynamics and Trends

Key Concepts in Population Ecology

• Population ecology focuses on the study of populations within a species and how they change over time
• Populations are groups of individuals of the same species that live in the same area and interact with each other
• Population size refers to the total number of individuals in a population at a given time
• Population density measures the number of individuals per unit area or volume (e.g., 100 trees per hectare)
• Carrying capacity is the maximum population size that an environment can sustain given the available resources
• Birthrate and death rate are the primary factors that influence population growth or decline
• Immigration and emigration also affect population size by adding or removing individuals from a population

Population Growth Models

• Exponential growth occurs when a population increases at a constant rate, resulting in a J-shaped curve
  • Exponential growth is described by the equation: $N_t = N_0 e^{rt}$, where $N_t$ is the population size at time $t$, $N_0$ is the initial population size, $r$ is the growth rate, and $t$ is time
• Logistic growth occurs when a population's growth slows down as it approaches the carrying capacity, resulting in an S-shaped curve
  • Logistic growth is described by the equation: $\frac{dN}{dt} = rN\left(1-\frac{N}{K}\right)$, where $\frac{dN}{dt}$ is the rate of change of the population size, $r$ is the intrinsic growth rate, $N$ is the population size, and $K$ is the carrying capacity
• The logistic growth model accounts for the limited resources available in an environment, which constrains population growth
• Exponential and logistic growth models help predict population trends and understand the factors influencing population dynamics

Factors Affecting Population Size

• Biotic factors are living components of the environment that affect population size, such as predation, competition, and parasitism
• Abiotic factors are non-living components of the environment that influence population size, such as temperature, precipitation, and nutrient availability
• Resource availability, including food, water, and shelter, plays a crucial role in determining population size
• Predator-prey interactions can regulate population sizes through predation pressure and prey availability
• Interspecific competition occurs when individuals of different species compete for the same limited resources, affecting population sizes
• Intraspecific competition occurs when individuals of the same species compete for resources, which can limit population growth
• Disease outbreaks and parasitism can reduce population size by increasing mortality rates

Density-Dependent vs. Density-Independent Factors

• Density-dependent factors are those whose effects on a population change as the population density changes
  • Examples of density-dependent factors include competition, predation, and disease
• Density-independent factors affect populations regardless of their density
  • Examples of density-independent factors include natural disasters (earthquakes, hurricanes) and human activities (habitat destruction, pollution)
• Density-dependent factors often have a regulatory effect on population growth, preventing populations from growing indefinitely
• Density-independent factors can cause sudden and drastic changes in population size, regardless of the population's density
• Understanding the interplay between density-dependent and density-independent factors is crucial for predicting population dynamics and developing conservation strategies

Life History Strategies

• Life history strategies describe the patterns of growth, reproduction, and survival that a species exhibits throughout its life cycle
• r-selected species have high reproductive rates, short lifespans, and invest little energy in parental care (bacteria, insects)
  • r-selected species are adapted to unstable or unpredictable environments where rapid population growth is advantageous
• K-selected species have low reproductive rates, long lifespans, and invest significant energy in parental care (elephants, whales)
  • K-selected species are adapted to stable environments where competition for resources is high, and long-term survival is prioritized
• Life history strategies exist on a continuum, with many species exhibiting intermediate strategies between r-selection and K-selection
• Trade-offs between reproduction and survival are common in life history strategies, as resources allocated to one function cannot be used for the other

Population Dynamics in Ecosystems

• Population dynamics describe the changes in population size and structure over time within an ecosystem
• Trophic cascades occur when changes in the population size of one species affect multiple trophic levels in an ecosystem
  • Example: the reintroduction of wolves in Yellowstone National Park led to a decrease in elk populations, which allowed for the recovery of riparian vegetation
• Keystone species have a disproportionately large impact on the ecosystem relative to their abundance (sea otters, beavers)
  • The removal or decline of a keystone species can lead to significant changes in the ecosystem's structure and function
• Invasive species can disrupt population dynamics by outcompeting native species and altering ecosystem processes (kudzu, zebra mussels)
• Population cycles, such as predator-prey cycles, demonstrate the complex interactions between species and their effects on population dynamics

Human Impact on Population Trends

• Habitat loss and fragmentation due to human activities (deforestation, urbanization) can lead to population declines and local extinctions
• Overexploitation of resources, such as overfishing and overhunting, can cause rapid population declines and alter ecosystem dynamics
• Climate change, driven by human activities, can affect population distributions, phenology, and survival rates
  • Example: rising temperatures can cause shifts in the timing of bird migrations and plant flowering, leading to mismatches between species interactions
• Pollution, including air, water, and soil contamination, can reduce population sizes by increasing mortality rates and reducing reproductive success
• Conservation efforts, such as habitat protection and species reintroduction programs, can help mitigate human impacts on population trends

Real-World Applications and Case Studies

• The management of fisheries relies on understanding population dynamics and implementing sustainable harvest strategies (quotas, size limits)
• Invasive species control programs aim to reduce the impact of non-native species on native populations and ecosystems (eradication, biocontrol)
• Endangered species recovery plans use knowledge of population ecology to develop strategies for increasing population sizes and ensuring long-term survival (captive breeding, habitat restoration)
• Disease outbreak management, such as the COVID-19 pandemic, relies on understanding population dynamics and implementing control measures (vaccination, social distancing)
• Agricultural pest management employs principles of population ecology to develop integrated pest management strategies that minimize crop damage while reducing reliance on pesticides (biological control, crop rotation)
• Urban wildlife management requires understanding the population dynamics of species living in human-dominated landscapes (deer, coyotes) to minimize human-wildlife conflicts and ensure coexistence