Ecology Notes – Oct. 9/12
- Human population growth – a case study: Humans have a large impact on the global environment.
Our population has grown explosively, along with our use of energy and resources. Human
population reached 6.8 billion in 2010, more than double the number of people in 1960. Our use of
energy and resources has grown even more rapidly. From 1860 to 1991, human population
quadrupled in size, and energy consumption increased 93-fold. For thousands of years our
population grew relatively slowly, reaching 1 billion for the first time in 1825. Now we are adding 1
billion people every 13 years. The human population increased relatively slowly until 1825 when the
industrial revolution occurred – then the population increased a lot. The growth rate has slowed
recently to about 1.18% per year, and continues to slow. By 2080, it is predicted there will be roughly
9-10 billion people on earth. Is 10 billion people above the carrying capacity of the human
population? Many people have tried to estimate human carrying capacity. Researchers must make
assumptions about how people would live and how technology would influence our future. Estimates
range from fewer than 1 billion to more than 1000 billion.
- Ecological footprint is the total area of productive ecosystems required to support a population. It
uses data on agricultural productivity, production of goods, resource use, population size, and
pollution. The area required to support these activities is then estimated. In 2006, 11.9 billion
hectares of productive land was available globally. The average ecological footprint of a person is 2.6
hectares. This suggests a carrying capacity of 4.6 billion. With a population of 6.6 billion, we are at a
40% overshoot of carrying capacity. Our resources are being depleted over time. If everyone in the
world uses resources at the same rate as – US citizens in 2006, the carrying capacity would be 1.3
billion people, as Indian citizens in 2006, the carrying capacity would be 14 billion people. Our
ecological footprint changes over time because human use of resources changes from year to year.
- Introduction: One of the ecological maxims is no population can increase in size forever. The limits
imposed by a finite planet restrict a feature of all species – a capacity for rapid population growth.
Ecologists try to understand the factors that limit or promote population growth.
- A life table is a summary of how survival and reproductive rates vary with age. Information about
births and deaths is essential to predict future population size. Life table data for the grass Poa annua
were collected by marking 843 naturally germinating seedlings and then following their fates over
time. Sx = survival rate, which is the chance that an individual of age x will survive to age x+1. For this
grass, the survival rate decreases as they get older. S2 for example equals 0.6, which means an
individual at age x=2 has a 60% chance of surviving to reach age x=3. Ix = survivorship, which is the
proportion of individuals that survive from birth to age x. To calculate this, you take the number alive
(Nx) at the age you’re looking at and divide it by Nx of age 0. I3 for example equals 0.375, which
means that 37.5% of newborns survive to reach age x=3. Fx = fecundity, which is the average number
of offspring a female will have at age x. As individuals die over time, Nx reaches 0.
- Birth and death rates can vary greatly between individuals of different ages. Gambians’ survivorship
depends on the season of birth. Gambians born during the hungry season (July-October when food
stored from the previous year is depleted) had lower survivorship than those born at other times of
the year. The survivorship of US females does not drop below 95% until they are 50 years old.
Gambians born between November and June live longer than Gambians born during the hungry
season, but differences in survivorship between both groups of Gambians and the US are huge.
- In some species, age is not important. In many plants, reproduction is more dependent on size
(related to growth conditions) than age. Life tables can also be based on size or life cycle stage.
- A survivorship curve is a plot of the number of individuals from a hypothetical cohort that will
survive to reach different ages. Survivorship curves can be classified into three general types. Type I – Most individuals survive to old age (dall sheep, humans). Survivorship drops off as they age.
Type II – The chance of surviving remains constant throughout the lifetime (some birds). Type III –
High death rates for young. Those that reach adulthood survive well (species that produce a lot of
offspring). This is the most common type seen in nature. Survivorship curves can vary among
populations of a species, between males and females in a population, and among cohorts of a
population that experience different environmental conditions.
- A population can be characterized by its age structure, which is the proportion of the population in
each age class. Age structure influences how fast a population will grow. If there are many people of
reproductive age (15 to 30), it will grow rapidly. A population with many people older than 55 will
grow more slowly. There can also be zero growth. Life table data can be used to predict age structure
and population size.
- Life table for a hypothetical organism: If the population starts with 100 individuals, age class 0 (n0)
has 20 individuals, age class 1 (n1) has 30, and age class 2 (n2) has 50. This organism dies before it
reaches age 3. To predict population size for the following year, calculate – The number of individuals
that will survive to the next time period and the number of offspring those survivors will produce in
the next time period. To calculate the number of individuals that will survive to the next time period,
multiply the number of individuals in each age class by the survival rate (Sx) for that age class. To
determine the number of newborns in the following year, multiply the fecundity (Fx) by the number
obtained from the first calculation. To get the total population size of the following year, add up all
- Growth rate (λ): Ratio of population size in year t+1 (Nt+1) to population size in year t (Nt).
λ = This provides a measure of the year-to-year population growth rate. Populations grow at
fixed rates when age-specific birth and death rates are constant over time. The proportion