5 POPULATIONS

5.1 HOW POPULATIONS GROW

Describing Populations

-researchers study populations' geographic range, density, and distribution, growth rate,

and age structure. -involve dramatic changes in the size of a

population.

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

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

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.

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

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

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

Exponential Growth

-the larger a population gets, the faster it grows

-under ideal conditions with unlimited resources, a population will grow exponentially

Organisms that reproduce rapidly

- If 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

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

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

Logistic Growth

-natural populations don’t grow exponentially for long

-bacteria, elephants, hydrilla, and moths don’t cover the earth

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

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

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 capacity

5.2 Limits to Growth

Limiting Factors

-factor that controls the growth of a population

-limiting factors determine the carrying capacity of an environment for a species

Density-Dependent Limiting Factors

- Operate strongly when population density reaches a certain level

- Factors include, competition, predation, herbivory, parasitism, disease, and stress

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

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

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

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

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

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

Controlling Introduced Species

Limiting factors keep the hydrilla population under control

- Pests or diseases weaken these populations

- This results in runaway population growth

5.3 Human Population Growth

Historical overview

-the rate of population increase has changed dramatically over time

- predators and disease were once common and life-threatening

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

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 Darwin

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

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

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

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

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