Population dynamics explores how and why populations change over time. It's all about birth rates, death rates, and migration patterns. These factors determine whether a population grows, shrinks, or stays stable.
Growth models help predict population changes. The exponential model shows rapid growth in ideal conditions, while the logistic model accounts for real-world limits. Understanding these models is key to managing wildlife, controlling pests, and planning for human population needs.
Population Growth Models
Exponential Growth Model
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- Populations can grow exponentially when resources are abundant and there are no limiting factors
- Exponential growth occurs when the per capita rate of increase remains constant, resulting in a J-shaped curve
- The exponential growth model is described by the equation $\frac{dN}{dt} = rN$, where $N$ is the population size, $r$ is the intrinsic growth rate, and $t$ is time
- Exponential growth is unsustainable in the long term because resources eventually become limiting (nutrients, space)
Logistic Growth Model and Carrying Capacity
- The logistic growth model accounts for the limiting factors that restrict population growth
- As population size increases, the per capita rate of growth decreases due to competition for resources, resulting in an S-shaped curve
- The logistic growth model is described by the equation $\frac{dN}{dt} = rN(\frac{K-N}{K})$, where $K$ is the carrying capacity
- Carrying capacity is the maximum population size that an environment can sustain indefinitely, given the available resources (food, water, habitat)
- When a population reaches its carrying capacity, the birth rate equals the death rate, resulting in a stable population size
Factors Affecting Population Growth
Density-Dependent Factors
- Density-dependent factors are biological or environmental factors that have a greater impact on population growth as population density increases
- Examples of density-dependent factors include competition for resources (food, water, space), predation, disease, and parasitism
- As population density increases, these factors become more intense, slowing down population growth and potentially leading to population decline
- Density-dependent factors are the primary regulators of population size and help maintain populations near their carrying capacity
Density-Independent Factors and Life History Strategies
- Density-independent factors affect population growth regardless of population density
- Examples of density-independent factors include natural disasters (earthquakes, hurricanes), severe weather conditions (droughts, floods), and human activities (habitat destruction, pollution)
- r-selected species have a life history strategy characterized by rapid growth, early maturity, high fecundity, and short lifespan (bacteria, annual plants, insects)
- K-selected species have a life history strategy characterized by slow growth, late maturity, low fecundity, and long lifespan (elephants, whales, humans)
- r-selected species are better adapted to unstable environments and can quickly colonize new habitats, while K-selected species are better adapted to stable environments and invest more resources in fewer offspring
Population Characteristics
Age Structure and Survivorship Curves
- Age structure refers to the proportion of individuals in different age groups within a population
- Age structure diagrams (pyramids) can be used to visualize the distribution of individuals across age groups and predict future population growth
- Three types of age structure diagrams: expanding (broad base, rapid growth), stationary (stable), and contracting (narrow base, declining growth)
- Survivorship curves depict the proportion of individuals in a cohort (born at the same time) that survive to each age
- Three types of survivorship curves: Type I (low mortality until old age, humans), Type II (constant mortality throughout life, birds), and Type III (high early mortality followed by low mortality, oysters)
- Survivorship curves provide insights into the life history strategies and environmental pressures faced by different species