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5 POPULATIONS. 5.1 HOW POPULATIONS GROW Describing Populations -researchers study populations' geographic range, density, and distribution, growth rate,

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Presentation on theme: "5 POPULATIONS. 5.1 HOW POPULATIONS GROW Describing Populations -researchers study populations' geographic range, density, and distribution, growth rate,"— Presentation transcript:

1 5 POPULATIONS

2 5.1 HOW POPULATIONS GROW

3 Describing Populations -researchers study populations' geographic range, density, and distribution, growth rate, and age structure. -involve dramatic changes in the size of a population.

4 Geographic Range - area inhabited by a population is called its geographic range -a populations range can vary enormously in size, depending on the species -a hydrillas natural range is Korea, while humans have carried it through every continent

5 Growth rate -A population’s growth rate determines whether the size of the population increases, decreases, or stays the same. -Hydrilla populations in their native habitats have a growth rate of around zero -Cod populations decrease with size

6 Age Structure - age structure is the number of males and females of each age structure. -most plants and animals cannot reproduce until they reach a certain age -among animals, only females can produce offspring.

7 Population Growth -A population will increase or decrease in size depending on how many individuals are added to it or are removed from it. -Factors that can affect population size are birthrate, death rate, and the rate at which individuals enter or leave the population

8 Birthrate and Death Rate -Populations grow if more individuals are born than die in any period of time -A population can grow when its birthrate is higher than death rate -If the death rate is greater than the birth rate the population is likely to shrink

9 Immigration and Emigration -immigration is when a population moves into its range from elsewhere -emigration is when a population may decrease in size when individuals move out - young animals approaching maturity may emigrate from their native areas

10 Exponential Growth -the larger a population gets, the faster it grows -under ideal conditions with unlimited resources, a population will grow exponentially

11 Organisms that reproduce rapidly -I f you the size of a population over time you get a J-shaped curve -It rises slowly at first then faster and faster -If nothing interferes, the populations becomes larger at a faster rate

12 Organisms that reproduce slowly -Many organisms grow faster than bacteria -Newborn elephants take 10 years to mature -If exponential growth continues, the results would be impossible

13 Organisms in new environment -Sometimes when an organism is moved to another environment, a population grows exponentially -Gypsy moths were accidentally released from a laboratory in Boston -They devoured the leaves of many acres

14 Logistic Growth -natural populations don’t grow exponentially for long -bacteria, elephants, hydrilla, and moths don’t cover the earth

15 Phases of Growth -after a short time, the population begins to grow exponentially -in real world populations exponential growth does not continue for long’ - at some point the population growth drops to zero

16 The logistic growth curve -occurs when a populations growth slows and then stops following a period of exponential growth - population growth may slow for several seasons - growth may also slow as death rate increases

17 Carrying Capacity -maximum number of individuals of a particular species that a particular environment can support - when immigration equals emigration, population growth stops - on a graph, the point at which the horizontal line intersects the y-axis represents the carrying capacit y

18 5.2 Limits to Growth

19 Limiting Factors -factor that controls the growth of a population -limiting factors determine the carrying capacity of an environment for a specie s

20 Density-Dependent Limiting Factors - Operate strongly when population density reaches a certain level - Factors include, competition, predation, herbivory, parasitism, disease, and stress

21 Competition - Competition of animals for food, water, and space -Competition is a density-dependent limiting factor -May occur among members of different species who want the same overlapping resources

22 Predation and Herbivory - Populations of predators and prey may cycle up and down over time -From a plants perspective, herbivores are predators -Human activity limits population

23 Parasitism and disease -parasites feed at the expense of their host - the denser the host population, the most parasites - parasitism and disease are density-dependent effects

24 Stress from Overcrowding Some species fight amongst themselves if overcrowded -The fighting may weaken the body’s ability to fight disease do to stress’ -This can lower birthrates and raise the death rate

25 Density-Independent Limiting Factors -affects all populations in similar ways, regardless of size and density - Examples are unusual weather such as hurricanes, drought, or floods

26 True density independence? - Effects can vary with population density -Human activities can place ecological stress in ways that can hamper it -It is hard to say that a limiting factor acts only in a density –independent way

27 Controlling Introduced Species Limiting factors keep the hydrilla population under control -Pests or diseases weaken these populations -This results in runaway population growth

28 5.3 Human Population Growth

29 Historical overview -the rate of population increase has changed dramatically over time - predators and disease were once common and life-threatening

30 Exponential Human Population Growth - As civilizations advances, life became easier, and human pops. Began to grow -Several factors including nutrition and sanitation, reduced deathrates -Lower death rates and higher birthrates, led to exponential growth

31 The Prediction of Malthus -Suggested that only famine and disease can limit population growth -He thought human populations were regulated by competition -His work was important to Charles Darwi n

32 World population growth slows - Exp. Growth continued until the second half of the twentieth century -It reached a peak and then began to drop -It now takes longer for the global population to grow by 1 billion than it did 20 years ago

33 Patterns of Human Population Growth -Demography: study of human populations -The age structure of a population helps predict why some countries have higher birth rate than others

34 The demographic transition -A dramatic change from high birth and death rates to low for both -Divided into 3 stages -US, Japan, and Europe have completed the transition

35 Age structure and Population growth -used to understand the population growth in different countries - In the US, there are almost an equal number of people in each age group - The age structure shows a steady growth

36 Future Population Growth -Demographers use age structures and the effect of diseases to predict the worlds populations -It is suggested that by 2050, the population will reach 9 billion -Global human populations will grow more slowly over than next 50 years than in the last 50


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