Chapter 5: Populations

Section 5.1: How Populations Grow *population – a group of organisms of a single species living in the same area at the same time. A. Describing Populations *researchers study population’s geographic range, density and distribution, growth rate, and age structure. 1. Geographic Range a) geographic range (definition) – the area inhabited by a population.

b) Geographic range can vary in size ex) bacteria in a rotting pumpkin – does not have a large geographic range, but hydrilla that was tossed into a canal in Florida have run rampant

2. Density and Distribution a) population density – the number of individuals per unit area. b) distribution – how individuals in populations are spread out in their area. 1) random – individuals are distributed randomly throughout their area. Ex) purple lupines

2) uniform – individuals are spaced fairly evenly throughout their environment ex) King penguins 3) clumped – individuals are in tight packs or clusters ex) striped catfish

3. Growth Rate a) a population’s growth rate determines whether the size of a population: 1) increases 2) decreases 3) stays the same b) GR = Birth Rate – Death Rate 1) GR > 0 = more birth than death 2) GR < 0 = less birth than death

4. Age Structure a) age structure – the # of males and females of EACH AGE in a population. b) some animals cannot reproduce until they are a certain age, therefore it is good to know how many reproducing organisms there are in a population to determine if the population will grow.

B. Population Growth *a population will increase or decrease in size depending on how many individuals are added to it or removed from it. *4 key factors affect population size: 1) birth rate 2) death rate 3) immigration 4) emigration

1. Birth rate and Death rate a) growth – populations will grow if Birth > Death b) 0 growth – Birth rate = Death rate c) decline – populations will decrease if Death > Birth 2. Immigration & Emigration a) immigration – causes populations to grow; occurs when new individuals move into an area from elsewhere.

b) emigration – causes populations to decrease; occurs when organisms leave an area. C. Exponential Growth *if you provide a population with all the food and space it needs, protect it from predators and disease, and remove its waste products, the population will grow indefinitely. 1. exponential growth – the larger a population gets, the faster it grows. *under ideal conditions w/ unlimited resources a population will grow exponentially, with a graph that is shaped like a “J.”

Exponential Growth

a) Organisms that reproduce rapidly tend to: 1) be small in body size 2) start to reproduce at a young age 3) have many offspring, but do not usually care for them 4) have a short lifespan 5) examples – flies, mosquitoes, numerous insects

Exponential growth, the bacteria example: *bacteria are capable of reproducing every 20 minutes after 20 minutes – 2 cells after 60 minutes – 8 cells (2 3 ) after 2 hours - 64 cells (2 6 ) after 3 hours cells (2 9 ) after 24 hours -4, 720, 000, 000, 000, 000, 000, 000

b) Organisms that reproduce slowly tend to: 1) be larger in body size 2) reproduce at a later age 3) have few offspring at a time and tend to care for their young 4) have a relatively long life span 5) examples – humans, dogs, elephants

Exponential Growth, the elephant example: *produce only 1 offspring every 2-4 yrs, takes about 10 years for a baby elephant to mature and be able to have babies of their own: If they had unlimited resources, their population size would increase dramatically over time.

c) Organisms in new environments 1. sometimes when an organism is moved to a new environment it may grow exponentially for a time. 2. example – Hydrilla, Gypsy month, European rabbit

D. Logistic Growth *natural populations do not grow exponentially for long; sooner or later something inhibits exponential growth from continuing. 1. Phases of Growth a) Phase 1: Exponential Growth 1) initial growth period 2) resources are unlimited so individuals can grow and reproduce rapidly 3) few individuals die and many offspring are produced

b) Phase 2 : Growth Slows 1) population is still growing, but at a much slower rate 2) resources are becoming limited c) Phase 3: Growth Stops “leveling off stage” 1) occurs when growth rate = 0 2) birth rate = death rate

2. The logistic growth curve (S shaped curve) a) logistic growth – occurs when a population’s growth slows, then stops after a period of exponential growth. b) S-shaped curve – graph has the shape of an “S.” c) may occur due to: 1) decrease in birth rate 2) increase in death rate 3) decrease in immigration 4) increase in emigration

3. Carrying Capacity – the maximum number of individuals of a particular species that an environment can support *once a population reaches the Carrying Capacity of it’s environment a variety of factors stabilize it at that size.

a) If populations are under the carrying capacity, the birth rate will exceed the death rate until carrying capacity is reached. b) If populations are OVER the carrying capacity, then death rate will exceed the birth rate until the population drops back down to the carrying capacity line.

Section 5.2 Limits to Growth

*why does a species that is “well-behaved” in one environment grow out of control in other environments? A. Limiting Factors 1. Limiting Factor – a factor that controls the growth of a population. Ex) competition predation parasitism/ disease weather natural disaster

2. Acting separately or together, limiting factors determine the carrying capacity of a species. 3. Limiting factors keep most natural populations somewhere between extinction and overpopulation.

B. Density-dependent Limiting Factors 1. density-dependent limiting factors – depend on the “density” (# of organisms in an area). As a population’s density goes up, these factors will begin to play a role in limiting population growth. Includes: a) competition b) predation c) herbivory d) parasitism/disease e) stress from overcrowding

2. Competition a) when populations become overcrowded organisms fight for food, water, space, and sunlight. b) can lower birthrates and increase death rates. c) the more individuals living in an area, the sooner the resources get used up, and thus competition occurs.

3. Predation and Herbivory ex) wolves, moose, plants on Isle Royale a) Predator-prey relationships 1) moose increases; wolves increase 2) wolves increase; eventually moose decreases 3) moose decreases; wolves begin to starve 4) as wolves decrease; moose can rebuild

b) Herbivore Effects *on Island Royale, if moose population is high they will eat many of the balsam firs. 1) increase in moose; decrease in firs 2) decrease in firs then results in starving moose 3) as moose decrease, firs replenish

c) Humans as Predators *in some situations human activity limits populations ex) overfishing for codfish in New England has caused codfish populations to decline

4. Parasitism and Disease a) Parasites and disease causing organisms feed at the expense of their hosts; weakening them and causing disease or death. b) The more dense or “overcrowded” an area, the easier it is to spread disease and parasites. Ex) CPV outbreak on Island Royale killed all but 13 wolves, this lead to an over population of moose which easily passed winter ticks to one another causing hair loss and weakness

5. Stress from overcrowding a) some species fight amongst themselves if overcrowding occurs b) fighting leads to stress c) stress weakens the body’s immune system making it easier to get sick d) stress can: 1) increase death 2) decrease birth 3) increase emigration

C. Density – Independent Living Factors 1. density-independent factors – affect ALL populations regardless of population size and density. 2. examples: a) hurricanes b) drought c) floods d) fires

3. True density independence? *sometimes these factors can vary with population density ex) Island Royale harsh winter killed many plants, therefore moose population decreased, but if moose population was smaller they may have been ok *hard to say if a limiting factor acts ONLY in an independent way.

4. Controlling Introduced Species It is often hard to control introduced species b/c they often have no natural predators or diseases in that area, thus they run rampant and can actually cause native species to decline or even become extinct. Ex) European rabbits introduced to Australia

Section 5.3 Human Population Growth

*How quickly is the human population growing? *Worldwide there are more than 400 births/second I. Historical Overview *The human population, like populations of other other organisms, tends to increase. The rate of that increase has changed dramatically over time. *For most of human existence the population grew slowly because life was harsh – food was hard to find, predators and disease were common; therefore human death rates were high.

A. Exponential Human Population Growth 1. As civilization advanced, life became easier, and the human population began to grow more rapidly. 2. Industrial Revolution – 1800’s; food became more reliable and goods could be shipped around the world. 3. reduction in death rates – death rates went down b/c of better nutrition, sanitation, medicine, and improved healthcare. 4. birth rates remained high

5. Increase in birth rates + decrease in death rates = POPULATION EXPLOSION

B. The predictions of Malthus 1. Thomas Malthus – English economist; suggested only war, famine, and disease could limit population growth. *all were “density-dependent” factors 2. Malthus’ work was crucial to Charles Darwin

C. World Population Growth Slows *What is happening to human population growth today? – human population growth reached a peak, then began to drop. 2. The size of global human population is still growing rapidly, but the rate of growth is slowing down. 3. “Doubling” a) it took 123 billion years for human population growth to double from 1 billion to 2 billion in 1927

b) Then it took just 33 years to grow another billion c) But today it takes longer for the population to grow by a billion than it did 20 years ago. WHY?

II. Patterns of Human Population Growth * Why do population growth rates differ among countries? 1. demography – the scientific study of human populations; attempts to explain how these populations change over time. 2. Birth rates, death rates, and the age structure of a population help predict why some countries have high growth rates while others grow more slowly.

A. The Demographic Transition 1. demographic transition – a dramatic change from high birth rates and death rates to low birth rates and death rates stages: a) stage I – BR and DR high b) stage II – BR high, DR low c) stage III – BR down, DR down

3. US, Japan, and Europe have completed the demographic transition. 4. Parts of S. America, Africa, and Asia are in stage II 5. India and China are still in stage I

*J shaped human growth curve may become more like an S

B. Age Structure & Population Growth 1. age structure – graphs that depict the number of males and females in particular age groups. 2. United States vs. Guatemala a) In US there are equal numbers of people in each age group. b) Guatemala has many more young children and teenagers than adults.

United States vs Guatemala Age Structure Pyramids

C. Future Population Growth 1. To predict how the world population may grow, demographers consider many factors such as: a) age structure of each country b) effects of disease on death rates ex) AIDS 2. 9 billion – current predictions suggest that by 2050 world population will = 9 billion. *are we at, below, or above our carrying capacity? What do you think??????????