Ecology Lecture No. 9: Population Growth & Reproduction
Tuesday October 9 , 2012
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.
-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 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 1,000 billion.
-An 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 was 2.6 hectares. This suggests a
carrying capacity of 4.6 billion and as we are a population with over 6.6 billion, a 40% overshoot of
carrying capacity is thereby estimate for humanity.
Introduction To Population Growth:
-Recall that 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 – the 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
(relatively simple with immobile organisms).
-As the generations progressed, four key patterns were noted in the population: the species declined in
number (N), a decrease in survival rate S (xhance that an individual of age x will survive to age x + 1), a
decrease in survivorship I xproportion of individuals that survive from birth to age x), and an eventual
decrease in fecundity F (average number of offspring a female will have at age x).
x -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” (when food is stored from
the previous year is depleted) had lower survivorship than those born at other times of the year (when
this drought situation was not present). In some species, age is not important. For example, 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 (when age is a weak indicator of actual growth).
-A survivorship curve is the 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 reach old age), Type II (the chance of surviving remains constant throughout a
lifetime and Type III (young have high death rates, but those that reach adulthood survive well).
-A population can be characterized by its age structure — 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 (around 15 to 30), it will grow rapidly. For example, a population with many people older than 55
will grow more slowly.
-There are three primary population pyramids present when examining age structures: Rapid growth (a
population with youths in excess and a lack of elderly individuals), zero growth (a population where
youths and the elderly have relatively the same proportion) and negative growth (a population where
there is an excess of elder individuals and a lack of youths).
-Life table data can be used to predict age structure and population size for future generations. To
predict population size for the following year, calculate: the number of individuals that will survive to
the next time period and the # of offspring those survivors will produce in the next time period.
Growth Rate (λ):
-Growth rate (λ) is the ratio of population size in year t + 1 (N t+1o population size in year t (N ).tThe
equation is listed as follows: λ = N t+1/ Nt
-If survival or fecundity rates change, the population growth rate will change. Fo