Population Ecology
Increased food production
population growth increased substantially when agriculture became mechanized (17th century Europe). Selection of higher yields and more fertilizer allowed food production to grow, increasing the amount of additional humans we could feed.
Current lifestyle
A second reason for this change is due to changes in our current lifestyle. Early 20th century: more of the world worked on farms and ate their own food. Kids were an asset: important workforce, which compensated for the extra mouths to feed. Today, there is an increasingly urban lifestyle. Now, kids are costly, they don't bring money to the household, and we now have birth control - less "oops" babies.
population
A group of individuals in a single species that occupy the same general area. They tend to rely on the same resources, are influenced by the same environmental factors, and are likely to interact and breed with one another.
Logistic growth
An idealized population growth is slowed by limiting factors as the population size increase. This is the case for most species, as exponential growth eventually comes to a halt. dN/dT = rN((K - N)/K) K is the carrying capacity (the maximum population size that a particular environment can sustain ("carry"). Resources are finite. This number is not fixed, as abiotic and biotic factors can influence it at any given time, causing it to fluctuate.
Ecological foot print
Calculations of the human carrying capacity is, many times, based on the ecological footprint. The question it answers is how much land and how many resources each person on the planet requires to live. When the total area of ecologically protective land on Earth is divided by the global population, we each have a share of about 1.8 global hectares. (ability to produce resources and absorb waste) The average ecological footprint of the world's population is 2.7 gha per person. We currently have overshot the planet's carrying capacity to sustain us (+0.9 gha). Many countries like the United Arab Emirates and the USA over consume much more with respect to other countries like India, which consume less on average than the 2.7 gha per person on average.
Limiting factors
Environmental factors that restrict population growth are called limiting factors (most of the time they are density-dependent ones).
Medical advances
Germ theory of disease was first proposed in 17th century. Then 1.5 centuries later, human health was revolutionized with vaccinations (Ex. Hepatitis A, Hep. B, Influenza, Measles, Diphtheria, etc). People stopped dying of easily treatable diseases. This led to longer life spans and higher childhood survival rates, so more children could grew and had children of their own.
Lowering our carrying capacity
Humans have figured out how to raise their carrying capacities, so far, indefinitely. This was possible by eliminating limiting factors that would make our numbers level off centuries ago.
Are we reaching our carrying capacity?
In 1798, Thomas Malthus predicted that the Earth could not indefinitely support an ever-increasing human population. For 2 centuries, scientists have dismissed Malthus' hypothesis due to technological advancement. Now, with dispute for limiting resources, scholars are starting to reconsider Malthus' prediction. This is called the "Neo-Malthus theory". Malthus predicted that the limiting factor for the human population would be food. This is because food production grows linearly (at a constant rate of change) while populations tend to grow exponentially (at an exponential rate of change). Hence, at some point in time, the population would exceed the amount of food needed to support that population, and as a result, war, famine and other disasters would flourish. Even now, we can see countries fight over natural resources and the human population growth rate as whole declining.
Exponential growth
In an ideal scenario, a population will grow exponentially. This is because the increase of reproducing individuals lead to an exponential increase of the number of offspring, leading another exponential increase, etc.
Human population
Nowadays in the developed world, few children die before reaching reproductive maturity. In 1998 US: 1% of all children born alive die before the age of 5. In 17th century city of York: 60% had died and only 30% made it to the threshold of reproduction (15 years) and only 20% remained alive by the age of 20. Since the industrial revolution, we have been able to change our survivorship curve be more like a type 1 and not a type 3. Since around the year 1650, the human population has been through the longest period of exponential growth of any large animal in history. Putting things into perspective, there are 80 year olds today who have watched the human population triple. This all is extremely strange for the human population as we reproduce like K strategists. For the past few centuries, however, our population has been behaving more like that of an r strategist. Additionally, exponential growth, even for r strategists, does not usually go on for 350 years.
K strategists
Populations with k-selected traits (k strategists) tend to be large-bodied, love lived animals. They develop slowly and raise fewer offspring. They allocate energy to their own survival and to the survival of their descendants. They also tend to stay at carrying capacity (hence the "k"). Ex. humans, elephants, tigers, pandas
R strategists
Populations with r-selected life history traits (r strategists) tend to be small-bodied, short lived animals (and plants). They develop and reach sexual maturity rapidly. They produce large number of offspring (for higher probability of survival), and they tend to offer little or no parental care. Most have advantage in habitats with unpredictable disturbances (fire, flood, hurricane, drought, cold weather). Hence the name "r" strategists, they tend to follow follow exponential growth and optimize their value of r. Even if a high % of their population is killed from a disaster, relatively large number of them will live on and reproduce. Natural selection acts fast for them. Ex. Bacteria, rodents, mice, etc
Hygienic advances
Sewage systems were implanted starting in the 1500s and were widely used by the 1800s. This stopped the amount of diseases and pathogens from spreading throughout the human population. Hence, more people lived.
Survivorship curves
Survivorship curves are used to keep track of the chance of an individual in a given population surviving the various age groups.
Current changes in human population
The human population continues to increase, but the growth rate is slowing. Reduced family size was a result of the demographic transition many countries in the world have gone through. After which, women delay reproduction and choose to have fewer children as their status and education increase. This has been a worldwide phenomenon for developed countries who have had an industrial transition.
Our limiting factors
The things we need to survive are food, water, housing, non-renewable resources (metals, and fossil fuels). Everything we need also needs spaces (to grow, mine, dispose waste, etc). Space is more or less the overall necessity for all of these things. If we run out of space, we can't obtain these other important necessities. Calculations for our limiting factors are often based on ecological footprints, which calculate the use of resources each human uses.
Life history
The traits that affect an organism's schedule of reproduction and death make up its life history. These key life traits include age of first reproduction, the frequency of reproduction, number of offspring, and the amount of parental care given. It's important to recognize that natural selection cannot optimize all of these traits simultaneously. Some choose some in place of others. Increase in the area of one trait could adversely decrease in the area of another.
Living comfortably inhospitable places
Through the means of heating, air conditioning, warmer clothes, airplanes and trucks (for food transportation), humans could live and reproduce in locations that would have otherwise been inhabitable. This predictably, gave humans more space to colonize and as a result, have more offspring.
Type 1 curves
Type 1s have high survivorship until old age, then they have rapidly decreasing survivorship. Ex. Humans today, tortoises
Type 2 curves
Type 2s survivorship decreases at a steady, regular pace. At any given age interval, the decrease in the number f surviving individuals remains constant from year to year. Ex. Most birds
Type 3 curves
Type 3s have high mortality early in life, but those that survive the early years live long lives. Ex. Plants, rodents, insects, etc
Exponential growth model
dN/dT = rN where... dN/dT is the growth rate of the population (number of new individuals added) r is the per capita rate of increase (average contribution of each individual to population growth) N is the population size (number of individuals in the population at a particular time) Ex. calculating r - If a population of 100 rabbits has 50 births and 20 deaths in a month, the net increase of rabbits is 30. 30 divided by the original population (100) is = 0.3 or r.
density dependent factors
limiting factors that depend on the population's size and density. These can include food, space, water, resources, predation, disease, etc.
density independent factors
limiting factors that don't depend on the population's size. These include droughts, temperature, day length, climate, environmental disasters, etc.
