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.

- 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

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

-factor that controls the growth of a population 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 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