Growth Limitations Human Population POPULATIONS Growth Limitations Human Population

Stable Populations Stable ecosystems are established by keystone species. Keystone species are the population of organisms that keep the ecosystem in balance; without this population the ecosystem would disappear. Sea otters are a keystone species in the kelp forest aquatic ecosystem. Sea otters eat sea urchins; sea urchins eat kelp. If the sea urchins grew too large numbers then they would completely destroy the kelp forest which is home to hundreds of other animals. If the sea otter disappears then the kelp forest will be destroyed.

Characteristics of all Populations Geographic distribution- Where? Density- How many per unit of area? Growth Rate- How fast do they reproduce and grow? Population Age Structure- How old are the individuals in the population?

Density (Population Spacing) Dispersal patterns within a population Provides insight into the environmental associations & social interactions of individuals in population clumped Within a population’s geographic range, local densities may vary substantially. Variations in local density are among the most important characteristics that a population ecologist might study, since they provide insight into the environmental associations and social interactions of individuals in the population. Environmental differences—even at a local level—contribute to variation in population density; some habitat patches are simply more suitable for a species than are others. Social interactions between members of the population, which may maintain patterns of spacing between individuals, can also contribute to variation in population density. random uniform

Population Growth Factors that affect size: Number of births Number of deaths Number of individuals that enter or leave the population (immigration, emigration)

How fast can a population grow? Populations usually stay about the same size from year to year because various factors kill many individuals before they can reproduce. Predation Disease Natural disasters Competition

Exponential Growth Population grows fast When graphed it forms a J-shaped curve Occurs in nature under conditions where plenty of food, water and space are available and there is minimal competition and predation

Logistic Growth Population growth slows or stops after exponential growth Population growth slows when it reaches carrying capacity Carrying capacity (K) is determined by limited resources 500 400 300 200 100 20 10 30 50 40 60 Time (days) Number of cladocerans (per 200 ml)

Resources have limits Resource limits: A species reaches its carrying capacity when it consumes a resource at the same rate that the resource is produced. This resource is then called the limiting factor for the species.

Limiting Factors DENSITY DEPENDENT: death occurs more quickly in crowded populations rather than in a sparse population. Limited resources, predation, and disease often result in higher death rates in a high density population DENSITY INDEPENDENT: a certain proportion of a population dies regardless of the density. This affects all members of a population in a uniform way. Severe weather and natural disasters are often density independent causes of death

Methods for measuring population size Random Sampling- rather than count all individuals in an area a grip can be used to identify the number per unit of area and then multiplied to the complete area Capture and Release-each captured organism is tagged and released. Then a recapture is done and the population size can be estimated by comparing the number captured that are tagged vs not tagged.

The Human Population The human population grew more in 20th century than ever before (Exponential growth) Why? Increased food production Improved hygiene with industrial and scientific revolutions Can it continue?

Human Population Growth Significant advances in medicine through science and technology Industrial Revolution

Demography 2 Categories: Developed Countries: have higher average incomes, slower population growth, and diverse industrial economics Developing Countries: lower average incomes, simple and agricultural based economics, and rapid population growth.

Distribution of population growth uneven distribution of population: 90% of births are in developing countries 11 10 high fertility uneven distribution of resources: wealthiest 20% consumes ~90% of resources increasing gap between rich & poor 9 8 medium fertility 7 low fertility 6 World total World population in billions 5 4 Developing countries 3 2 1 Developed countries 1900 1950 2000 2050 Time

Age structure Diagram Relative number of individuals of each age What do these data imply about population growth in these countries?

Women and Fertility What leads to decreases in birth rates? Increased education Economic independence for women Large families are valuable in communities in which children work or take care of older family members Movement of people seeing children as a financial burden rather than an asset Fertility rate Developed countries: 1.6 children/woman; Developing countries: 3.1 children/woman