Population Ecology Definitions Population growth Population regulation Environmental

Population Ecology • Definitions • Population growth • Population regulation • Environmental factors that regulate growth • Human population growth and regulation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Population ecology is the study of populations in relation to environment – Including environmental influences on population density and distribution, age structure, and variations in population size Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Dynamic biological processes influence population density, dispersion, and demography • A population – Is a group of individuals of a single species living in the same general area Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Density is the result of a dynamic interplay – Between processes that add individuals to a population and those that remove individuals from it Births and immigration add individuals to a population. Births Immigration Popu. Iation size Emigration Deaths Figure 52. 2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Deaths and emigration remove individuals from a population.

Potential for population increase • It is quite high • A single bacterium can reproduce by fission every 20 min, in 36 hours there will be enough bacteria to form a layer foot deep over the entire world • A pair of elephants could produce a population of 19 million in 750 years Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Geometric/Exponential Models • Populations in which reproduction is restricted to a particular season of the year have non-overlapping generations. Their growth is modeled using geometric equations (time interval is discrete) • In populations in which reproduction happens continuously, their growth can be modeled using exponential equations (time interval is continuous) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

How do populations grow? We can envision a population consisting of few individuals living in an ideal, unlimited environment; the only restrictions: inherent physiological limitations due to life history – The population will increase in size with: Change in population = Births + Immigration − Deaths − Emigration size during time interval Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Verbal equation of population growth For simplicity: Change in population size during time interval = Births during − Deaths during time interval Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Exponential model Change in population size during time interval = Births during − Deaths during time interval Let N = population size; t = time, ΔN = change in population size; Δt = change in time Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Then now we have: ΔN/Δt= B−D Where: B= No. of births in the population during the time interval D= No. of deaths in the population during the time interval Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

We can now express births and deaths as the average number of births and deaths per individual during a specified time interval: b = per capita birth rate d = per capita death rate We can obtain the numbers of births and deaths in a population by multiplying the per capita birth rate times the population size and the per capita death rate times the population size. Hence, we can revise the population growth equation using the per capita rates: ΔN/Δt = b. N − d. N Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

We combine the per capita birth and death rates into the per capita growth rate: r=b−d A population grows when r is positive, declines when r is negative, or stays the same when r = 0 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

We can now rewrite the equation using the per capita growth rate: ΔN/Δt = r. N Assuming reproduction happens continuously, we can use differential calculus notation to express the equation as follows: d. N/dt = r. N Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

In a population increasing under ideal environmental conditions, the per capita growth rate may assume the maximum growth rate for the species called intrinsic rate of increase, rmax. The equation for exponential population growth is then: d. N/dt = rmax. N Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Exponential population growth – Results in a J-shaped curve 2, 000 Population size (N) d. N 1. 0 N dt 1, 500 1, 000 500 0 Figure 52. 9 d. N 0. 5 N dt 0 10 5 Number of generations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 15

• The J-shaped curve of exponential growth – Is characteristic of some populations that are rebounding Elephant population 8, 000 6, 000 4, 000 2, 000 0 1900 1920 Figure 52. 10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1940 Year 1960 1980

• The logistic growth model includes the concept of carrying capacity • Exponential growth – Cannot be sustained for long in any population • A more realistic population model – Limits growth by incorporating carrying capacity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Carrying capacity (K) – Is the maximum population size the environment can support Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Logistic Growth Model • In the logistic population growth model – The per capita rate of increase declines as carrying capacity is reached Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• We construct the logistic model by starting with the exponential model – And adding an expression that reduces the per capita rate of increase as N increases Per capita rate of increase (r) Maximum Positive N K 0 Negative Figure 52. 11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Population size (N)

Logistic equation • Maximum sustainable population size is K • K − N tells us how many additional individuals can the environment sustain • (K − N)/K tells us what fraction of K is still available for population growth • Hence: d. N/dt = rmax. N (K−N) K Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• A hypothetical example of logistic growth Table 52. 3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The logistic model of population growth – Produces a sigmoid (S-shaped) curve 2, 000 Population size (N) d. N 1. 0 N dt 1, 500 Exponential growth K 1, 500 Logistic growth 1, 000 d. N 1. 0 N dt 1, 500 N 1, 500 0 Figure 52. 12 0 5 10 Number of generations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 15

How well do these populations fit the logistic population growth model? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Populations are regulated by a complex interaction of biotic and abiotic influences • There are two general questions we can ask – About regulation of population growth Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• What environmental factors stop a population from growing? • Why do some populations show radical fluctuations in size over time, while others remain stable? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Population Change and Population Density • In density-independent populations – Birth rate and death rate do not change with population density • In density-dependent populations – Birth rates fall and death rates rise with population density Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Density-Dependent Population Regulation • Density-dependent birth and death rates – Are an example of negative feedback that regulates population growth – Are affected by many different mechanisms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Competition for Resources • In crowded populations, increasing population density – Intensifies intraspecific competition for resources 4. 0 3. 8 Average clutch size Average number of seeds per reproducing individual (log scale) 10, 000 100 3. 6 3. 4 3. 2 3. 0 2. 8 0 0 100 Seeds planted per m 2 (a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases. 10 20 30 40 50 60 70 Density of females (b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply. Figure 52. 15 a, b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 80

Territoriality • In many vertebrates and some invertebrates – Territoriality may limit density Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Cheetahs are highly territorial – Using chemical communication to warn other cheetahs of their boundaries Figure 52. 16 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Oceanic birds – Exhibit territoriality in nesting behavior Figure 52. 17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Health • Population density – Can influence the health and survival of organisms • In dense populations – Pathogens can spread more rapidly Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Predation • As a prey population builds up – Predators may feed preferentially on that species Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Toxic Wastes • The accumulation of toxic wastes – Can contribute to density-dependent regulation of population size Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Human population growth has slowed after centuries of exponential increase • No population can grow indefinitely – And humans are no exception Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Global Human Population • The human population – Increased relatively slowly until about 1650 and then began to grow exponentially 5 4 3 2 The Plague 1 Figure 52. 22 8000 B. C. 4000 B. C. 3000 B. C. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2000 B. C. 1000 B. C. 0 1000 A. D. 0 2000 A. D. Human population (billions) 6

• Though the global population is still growing – The rate of growth began to slow approximately 40 years ago 2. 2 2 Percent increase 1. 8 1. 6 2003 1. 4 1. 2 1 0. 8 0. 6 0. 4 0. 2 0 1950 Figure 52. 23 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1975 2000 Year 2025 2050

Regional Patterns of Population Change • To maintain population stability – A regional human population can exist in one of two configurations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Zero population growth = High birth rates – High death rates • Zero population growth = Low birth rates – Low death rates Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The demographic transition – Is the move from the first toward the second state Birth or death rate per 1, 000 people 50 40 30 20 10 Sweden Mexico Birth rate Death rate 0 1750 1800 1850 Figure 52. 24 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1900 Year 1950 2000 2050

Age Structure • One important demographic factor in present and future growth trends – Is a country’s age structure, the relative number of individuals at each age Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Age structure – Is commonly represented in pyramids Rapid growth Afghanistan Male Female 8 6 4 2 0 2 4 6 8 Percent of population Age 85 80– 84 75– 79 70– 74 65– 69 60– 64 55– 59 50– 54 45– 49 40– 44 35– 39 30– 34 25– 29 20– 24 15– 19 10– 14 5– 9 0– 4 Slow growth United States Female Male 8 6 4 2 0 2 4 6 8 Percent of population Figure 52. 25 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Age 85 80– 84 75– 79 70– 74 65– 69 60– 64 55– 59 50– 54 45– 49 40– 44 35– 39 30– 34 25– 29 20– 24 15– 19 10– 14 5– 9 0– 4 Decrease Italy Female Male 8 6 4 2 0 2 4 6 8 Percent of population

• Age structure diagrams – Can predict a population’s growth trends – Can illuminate social conditions and help us plan for the future Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Global Carrying Capacity • Just how many humans can the biosphere support? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Estimates of Carrying Capacity • The carrying capacity of Earth for humans is uncertain Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Ecological Footprint • The ecological footprint concept – Summarizes the aggregate land water area appropriated by each nation to produce all resources it consumes and to absorb all wastes it generates – Is one measure of how close we are to the carrying capacity of Earth Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Ecological footprints for 13 countries Ecological footprint (ha person) – Show that the countries vary greatly in their footprint size and their available ecological capacity 16 14 12 New Zealand 10 USA Germany Japan Netherlands Norway 8 6 UK Spain 4 World China India 2 0 Australia Canada Sweden 0 2 4 6 8 10 12 Available ecological capacity (ha person) Figure 52. 27 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 14 16