chapter 16

Population ecology

A population can grow until it approaches the carrying capacity of the environment.
- A group of individual organisms of the same species in one area comprise a population, such as a pod of lobsters.
  A species is a collection of similar organisms that can interbreed, such as lobsters.
  Populations of several species interacting in one area comprise a community; a marine community may consist of lobsters, anemones, clams, fish.
  The biological community and the non-living elements in their environment (habitat) make up an ecosystem, such as an ocean.
- Under ideal conditions, a population can grow at an exponential growth rate, (r) assuming birth rate is higher than death rate.
  Eventually the growth is limited by the carrying capacity of the environment and becomes logistic.
  - This population starts with 500 individuals, 125 are born and 25 die the first year. What is the growth rate?
    - Net gain is 100 / 500, or 20% per year. This would yield 600 after 1 year, then 720, 864, and 1036.8 after 4 years.
- German cockroaches at Smithsonian Museum of Natural History can reproduce quickly by exponential growth if resources are plentiful, reaching the carrying capacity of the room.
  Bacteria exponential growth:
- Carrying capacity is the maximum population size that an environment can sustain a species indefinitely.
  Desert locusts can reproduce rapidly in favorable years, exhibiting exponential growth.
  When the population exceeds the carrying capacity of the environment, the locusts become migratory, forming huge swarms to seek food elsewhere.
- Populations cannot sustain exponential growth for too long; eventually limited environmental resources cause growth to slow down.
  Logistic growth describes population growth that is gradually reduced as the population nears the environment's carrying capacity.
Populations sometimes exhibit predator-prey cycles.
- The lynx and snowshoe hare populations of Canada show regular cycles every 10 years or so.
  These cycles seem to be driven by inter-dependent predator-prey relationships.
  - The hare population size grows,
  - providing more food for the lynx,
  - which then reproduce at a higher rate,
  - causing them to eat great numbers of the hares,
  - thereby reducing the hare population (and the lynx's food source),
  - causing the lynx population to crash,
  - enabling the hare population to grow again.
  Note each cycle endures a bottleneck, possibly leading to extinction.
Commercial harvesting aims to obtain maximal sustainable yield.
- Maximum sustainable yield
  Commercial exploitation of target organisms can sustain good yields by harvesting below midpoint on the logistic growth curve, where the population can recover.
  Harvesting near the carrying capacity can lead to reduced yields in the future.
The root of environmental problems is the rapid growth of the human population.
- Human population growth
  The human population has been growing almost exponentially over the last 300 years.
  This growth cannot continue indefinitely before the carrying capacity of the environment is reached.

Future population growth rates can be assessed by an age pyramid .

A population can be divided into the percentages of individuals that are in specific age groupings (cohorts), such as 5-year bands, and can be graphed in an age pyramid.
The U.S. age pyramid shows a "baby boom" generation born between 1945 and 1964.
The baby boomers are nearing retirement age, with fewer younger cohorts to support them.

Age pyramids

Developing countries have a triangular pyramid due to high birth rate and high death rate of older individuals. The large proportion of people below reproductive age will lead to rapid growth as they begin to bear children.	Industrialized countries show a more rectangular pyramid. Low birth rates and low death rates of older individuals yield a more uniform distribution of cohorts.