Population Ecology

Populations Population ecology is the study of populations in relation to the environment A population is a group of individuals of the same species that live in the same area

Population Ecology: Vocabulary To describe populations, we have to talk about how they are situated Density: The number of individuals per unit area/volume Example: 47 elephants/km2 Dispersion: The pattern of spacing among individuals in a population Clumped Uniform Random

Uniform Environmental conditions are uniform Causes COMPETITION or antagonism between organisms (territoriality)

Clumping Most common Reproductive patterns favor clumping Social behaviors lead to clumping Optimal density is usually intermediate (medium)

Random No competition No tendency to group/clump Conditions are uniform Rarely happens!

Estimating Population Size Rarely, it is possible to count the number of species in a population Usually, a statistical method is needed to determine population size The mark-recapture method can be used to estimate the size of a population Capture, mark, release Recapture and count Equation: N = Number marked x Total catch 2nd time Number of marked recaptures

Factors That Influence Population Size There are 3 major factors that influence population size: 1. the number of births 2. the number of deaths 3. the number of individuals that enter or leave a population - immigration: individuals entering an existing population - emigration: individuals leaving an existing population

DEMOGRAPHICS In looking at populations, biologists must also look at demographics (the vital statistics of a population and how they change over time) Birth rates Death rates Life tables Survivorship curves Reproduction rates Growth rates Age structures

DEMOGRAPHICS One tool in demographics is to estimate the life span of organisms To do this they often create a life table, age summary of a population A cohort (group of individuals of the same age) are studied to see what percentage of the population dies This can determine life expectancy, survivorship at each age category, or male vs. female survivorship

Survivorship Curves A way to represent a life table is a survivorship curve, a plot of the proportion of individuals that survive at each age group When discussing survivorship curves, there are 3 general categories

Survivorship Curves Type I- live to old age & die (most large mammals) Type II- constant mortality rate (rodents, lizards, hydra) Type III- high mortality at young age, but if they survive they live a long life (sea turtles).

DEMOGRAPHICS Another important aspect of demographics is reproductive rates For obvious reasons, biologists who study reproductive rates only focus on females Age of fertility Number of offspring for each age group Time of year Spawning cycles Reproductive tables summarize this data

Life Histories Life history are traits that affect an organism’s schedule of reproduction and survival Clutch size: Number of offspring produced at each reproductive episode Semelparity A life history in which an organism spends most of its energy in growth and development, expend their energy in one large reproductive effort, and then die Many insects, annual plants, salmon, etc.

Life Histories Iteroparity A life history pattern in which organisms produce fewer offspring at a time over a span of many seasons Example: humans, panda bears, etc.

Life Histories (Semelparity vs. Iteroparity) Many factors contribute to the life history of an organism Finite resources Reproduction vs. survival Number and size of offspring Paternal investment in offspring

POPULATION GROWTH MODELS Different models of how populations grow

Formulas off your Cheat Sheet Rate: dY/dt Generic expression for change of some variable (Y) over time (t) Population Growth: dN/dt = (B – D) The change in population size (N) over time (t) is the same as the birth rate (B) minus the death rate (D) Exponential Growth: dN/dt =rmax •N The change in population size over time is equal to the growth rate (rmax) multiplied with the current population size

Formulas off your Cheat Sheet Logistic Growth: dN/dt =rmax •N • [(K-N)/K] The growth rate over time is the same as exponential growth adjusted for carrying capacity (K)

Types of Population Growth (start here) One of the biological imperatives is to reproduce and pass on genetic material to succeeding generations. Yet population growth is controlled by the environment and limited resources This causes different patterns of population growth

Patterns of Population Growth Exponential Growth: Occurs in ideal conditions with unlimited resources J shaped curve Example: 1 bacterium (reproducing every 20 minutes) could produce enough bacteria to form a 1-foot layer over the entire surface of the Earth in 36 hours

Exponential Growth Exponential growth is a useful model when studying populations that are introduced into a new, unfilled, environment Recovery after a catastrophe Exponential Growth: dN/dt =rmax •N rmax is the maximum rate of population growth for the species

Patterns of Population Growth Exponential growth cannot continue indefinitely It is characteristic of populations who are entering a new environment OR those whose numbers are rebounding from a catastrophic events

Patterns of Population Growth Logistic Growth: Pattern of population growth which takes into account the effect of population density on population growth Occurs when resources become more scarce Characterized by an S-shaped curve

Patterns of Population Growth dN/dt =rmax •N • [(K-N)/K] Carrying capacity (K): The maximum number of individuals that a particular environment can support over a long period of time Determined by such limiting factors as crowding and food resources Graph levels off at carrying capacity K-selected populations (equilibrial populations) live near or at the carrying capacity

Carrying Capacity

K-strategists (Life history) Density stays near carrying capacity. Large, slow growing organisms Small population sizes Long life span; slow maturation Few young/small clutch size Reproduce late in life Parental care Most large mammals; endangered species

r- strategists (Life history) Grow exponentially when environmental conditions allow; when conditions worsen, population size plummets. Short life span Reproduce early in life Many offspring/large clutch size Usually small in size Little or no parental care Bacteria, some plants, insects

Environmental Factors Abiotic and biotic influences on population size

Limiting Factors There are a number of factors that limit the size of populations: Density-dependent limiting factors (depends on the size of the population) Density-independent limiting factors (does not matter the size of the population)

Density-Dependent Limiting Factors The effect of density-dependent limiting factors intensifies as the population increases Intraspecific competition Food, space, etc. Territoriality Predation Waste build up Disease (if caused by pathogen/contagious)

Density-Independent Limiting Factors The occurrence and severity of density-independent limiting factors are unrelated to population size Climate Disease (if not caused by pathogen/not contagious) Pollution

The Interaction of Limiting Factors Density-dependent and density-independent limiting factors often work together to regulate the size of a population Deer in snowy winter Starve from lack of food (density-dependent) Severity of winter/depth of snow determines access to food (density-independent)

Population Dynamics Population dynamics is the study of the environmental factors that cause variations in the population size Looks at fluctuations in population over time to examine stability Immigration and emigration also affect population Metapopulations are when you have several interconnected populations

Boom-and-bust cycle Another phenomena that affects population are predator-prey relationships. Each population is interdependent and causes a boom-and-bust cycle The prey population increases which causes the predator population to increase The prey are over hunted and their population crashes This causes the predator population to crash Now, with fewer predators, the prey population can again increase (recovery gives a geometric increase)

Boom-and-Bust Cycles

Human Population Growth

Increase in Human Population Agricultural Revolution - Major period of population growth began when humans started to cultivate crops and domesticate animals Industrial Revolution – Improved food production and distribution Health Care – germ theory lead to improved hygiene, better waste removal and water treatment

Decrease in Human Population Plague – disease that greatly reduces the size of population (Black Plague in 1300’s reduced the population in England by 50%) Famine –a severe food shortage causing starvation and death (Potato Famine of 1840’s/China 1870-1890) War – death by combat, disease, cut off from food supply (Germany 1618-1648/WWI/WWII)

Human Population Growth The human population is unlike any other organism Since about 1650, we have remained in an exponential population increase Population increases by about 201,000 people/day worldwide

Human Population Growth

Human Population Growth Even though there is a tremendous increase in human population, it is not evenly distributed around the globe Regional areas have different population trends Some regions have stable regional human populations (birth rate is the same as death rate) Other regions show incredible growth rates

Industrialized Nations An emerging nation usually has a very high birth rate, but also a high death rate (disease, lack of modern medical treatment, famine) An industrialized nation usually has a low death rate, but also a low death rate Moving from an emerging nation to an industrialized nation is known as a demographic transition

Human Population Growth In the 1950s, mortality rates began to rapidly drop (advances in medicine and sanitation) Yet, the birth rates have not always dropped Has caused a huge increase in population in some nations About 80% of the world’s population lives in emerging nations

Age Structure One way to determine human population growth is to look at the nations’ age structure, relative number of individuals at each age By looking at the age structure of a population, you can determine the population growth

Age-Structure Diagrams

Human Population Growth Implications of exponential human population growth: Lack of food supplies Lack of space Lack of natural resources (metals, fossil fuels, etc) Lack of sites for waste disposal Ecologists cannot agree on a carrying capacity for Earth (2 – 40 billion) Are we going to reach carrying capacity through individual choices and/or government programs? OR Is Earth’s population going to “level off” as a result of mass deaths?