Ecology 2 PDF

## Population Growth ### Factors Limiting Population Growth * **Rainfall:** Too low for true forests. * **Bush:** Low trees + shrubs leading to lots of bush. * **Herbaceous Plants:** Few. * **Spekboom:** Grows in a dense thicket biome. * **Elephants:** Clear spekboom + other shrubs for mammals to...

## Population Growth ### Factors Limiting Population Growth * **Rainfall:** Too low for true forests. * **Bush:** Low trees + shrubs leading to lots of bush. * **Herbaceous Plants:** Few. * **Spekboom:** Grows in a dense thicket biome. * **Elephants:** Clear spekboom + other shrubs for mammals to move through, allowing animals to move from biome to biome. ### Population Growth * **Population:** All individuals of the same species that share the same habitat and can potentially interbreed. ### Population Dispersion * **Spatial distribution of individuals:** within geographical range. * **Random:** Individuals are distributed unpredictably within uniform habitats. * **Clumped:** Individuals group together due to patchy habitats, social groups, and reproductive patterns. * **Uniform:** Individuals repel each other and tend to be evenly spaced because resources are in short supply. ### Demography * Statistical study of changes in population size, age structure, and sex ratios. * Predict probability of population to die. * Important for the management and conservation of species. * For example, COVID-19 cases in 2020, saw exponential growth. ### Exponential Growth (J Curve) * Change in population over time. ### Formula: $(ΔΝ/Δt) = dN/dt = (b − d)N$ * **b:** Per capita birth rate - number of births in population, during a specified time period, divided by population size. $b = B/N$ * **d:** Per capita death rate - number of deaths divided by populations size during the same time period. $d = D/N$ * **(b-d):** Per capita growth rate (r) of population expressed per individual per unit time. ### Exponential Growth Equation $dN/dt = (b-d)N$ $dN/dt = rN$ * **r > 0:** Population is growing. * **r < 0:** Population is getting smaller. * **r = 0:** Size is not changing (zero population growth). ### Example: RSA Rhinos * 1900: 50 white rhinos. * 2014: 18000 white rhinos + 1700 black rhinos. Total: 19700. * Birth rate: 0.17. * In one year, 19700 animals can have up to 3349 babies. * Death rate: 0.028 * 19700 rhinos = about 552 deaths. * 3349 - 552 = 2797 (without poaching) * % per capita growth = (b-d)/total population size x 100. * Negative no. = more deaths. ### Do Populations Ever Grow Exponentially? * **Exponential growth model:** Occurs only in certain forms. * **At the beginning:** Populations tend to follow an exponential pattern. * When resources are unlimited, a population will have more births than deaths and **r** remains constant (exponential growth). * **Logistic growth model:** Resources are never unlimited. As populations encroach on carrying capacity of the resource, their growth rate decreases * **Carrying capacity (K):** Maximum number of individuals that a specific resource in a given area can maintain. ### Population Growth Under Ideal Conditions * Per capita birth rate is high + per capita death rate is low. * **r:** As high as it can be. * Max. per capita growth rate ($r_{max}$) -> populations intrinsic rate of increase. ### Formula: $dN/dt = r_{max}N$ ### Carrying Capacity * Population sizes are limited by a number of factors (e.g. shortage of resources). * Maximum number of individuals an environment can support. * Varies from one habitat to another. * Population will never exceed carrying capacity. ### Logistic Model (S shaped) * Per capita growth rate decreases as population size approaches carrying capacity. * Combines an exponential growth model with a section to account for the decrease in per capita growth as it approaches the **K**. * Influenced by environmental factors and carrying capacity. ### Formula: $dN/dt = rN (K - N)/K$ * **Very small population:** Population growth is close to exponential $(K-N)/K$ ≈ 1. * **Large population:** Rate of increase is very low $(K-N)/K$ ≈ 0 ### Intraspecific Competition * Dependence of two or more individuals of the same species on the same limiting resource. * **Animals:** Food, water, nesting site, and space. * **Plants:** Sunlight, water, inorganic nutrients, and growth space. * Leads to uniform distribution. * Rate of predation + disease increase with density. ### Exponential vs Logistic Growth * **Density-dependent mortality (logistic):** * Limited space, food, etc. * More diseases, predators, etc. * $(K-N)/K$ decreases as carrying capacity is approached. * **Density-independent effects (exponential):** * Catastrophes which kill at the same rate, not dependent on population size. * Example: asteroid. ### Population Cycles * Depending on the exact combination of density dependent and density independent factors, populations can experience different cycles. * **Stable:** Relatively constant. Population doesn't change much over a year. * **Cyclic:** Most animals. Exists between 2 organisms (predator + prey). When one population increases, the other decreases. * **Chaotic:** Density independent effects are high, common in insects. ### Metapopulation Dynamics * Study of networks of separate populations that interact with each other through the exchange of individuals. * **Source population:** Birth > deaths. Emigration to other habitats. * **Sub-optimal populations:** Births < deaths. Immigration from suitable habitats. * **Unsuitable:** No population, almost immediate death. ### No. Of Breeding Seasons * **Iteroparous:** Organisms produce offspring several times over the course of several breeding seasons. Survive longer than semelparous. * **Semelparous:** Organisms focus all their reproductive energy into a single breeding season. * **Usually lots of offspring:** In environments where survival isn't guaranteed. * Can be: Long lived + overlapping, or short + discrete. * Example: Agave. Releases seeds in a single season to increase probability that some seeds will germinate. * Time to maturity: In ephemeral habitats, reach maturity quickly so the next generation can start. ### r-K Scheme Of Life History Strategies * **Species vary along habitats.** * **Intrinsic rate of natural increase (r):** Rapid, density independent mortality. * **Mortality is density dependent (K):** Carrying capacity. * **r:** Lots of offspring early in life, grow quickly; minimal parental scare investment. * **K:** Little offspring late in life, grow over a long period of time, large parental care investment. * Every organism has some features from both groups. ### Population Growth + Age Structure * Growth rate for humans: 1.2% between 200-2009. * South Korea and Japan = negative birth rates. ### Age Distribution for Different Growth Rates * **Negative growth:** Older people are being removed from the economy and are not replaced by new births. * **Rapid growth:** Many young people and less old people. Most of the population is above reproductive age, so the population will decrease as a whole. ### Demographic Transition Model * Stages: * **Stage 1:** Preindustrial. * **Stage 2:** Transitional. * **Stage 3:** Industrial. * **Stage 4:** Postindustrial.