Population Ecology

A Population is a group of interbreeding individuals living in the same space at the same time.

It's good to stick together. Some of the benefits of herds are:

  • Help with finding resources
  • Protection from predation
  • Hunting in packs is highly effective
The Wildebeast herd in Africa is the largest herd in the world.

Population Dispersion Patterns

Population dispersion patterns are the patterns that appear with the distribution of certain plants or animals in their habitat. The three patterns are Clumped, Uniform, and Random. The most common dispersion pattern is clumped.

Humans-clumped, Creosote Bushes and Emperor Penguins-uniform, Dandelions-Random

What Limits Population Growth?

  • Competition
  • Immigration/Emigration
  • Catastrophic Events or Natural Disasters
  • Seasonal Fluctuations in Resources
  • Tolerances of climate variations

Density Dependent Factors

  • Competition
  • Disease
  • Famine

All of these things are effected by how closely a population lives with each other, so they are Density Dependent.

Density Independent Factors

Natural Disasters such as drought, fires, floods, etc. These are all things you have no control over, so they are called Density Independent.

Two types of Population curves: Exponential Vs. Logistic Growth (Malthusian Growth)

Exponential Growth is shown in Red below and has a J-shape due to doubling each unit of time. Logistic Growth has an S-shape due to the addition of Carrying Capacity (K). Carrying Capacity is the maximum size a population can sustainably become.

Thomas Malthus was an English Priest that looked at available food and population growth and determined there was not enough left to feed the English people. He determined a carrying capacity and then proclaimed that they should stop feeding the poor, old, and sick in order to survive. Thus the alternative name of a logistic growth curve is Malthusian Growth. He did not take into account human ingenuity and innovations which led to us being able to grow more food per acre.

What happens after a population hits carrying capacity? There are four different major possibilities. Stable curves might occur in an undisturbed section of the Rainforest with a Primate or other high trophic level organism. Cyclic curves are similarly shaped, but go farther above and below K due to a predator/prey relationship. Irruptive curves occur when a predator is removed and the prey eats too many of its resources causing an overshoot of growth and a dieback once the resources are gone. Irregular curves usually are the result of human activity interrupting the natural cycles.

K-Selected Vs r-selected Species

How would you describe the following organisms? What would their strategy for survival be?

Dandelions scatter their seeds to the wind. Cockroaches have three neural networks that allow them to respond to light, pressure, and temperature immediately. A single mating pair of rats can cause the reproduction of 10,000 rats in a year.

The r-selected species are also known as opportunists. The r is a variable for biotic potential which is the maximum size a population could reach if the conditions were ideal. The r-strategists have a lot of babies and then leave them, so if they all by some chance were able to survive they would be at their biotic potential. Traits of r-selected species include:

  • High rate of reproduction
  • Many small offspring
  • Little to no parental care
  • High population growth rate
  • Generalists
  • Early successional species
  • Early reproductive age
  • Short lives (<1 year)
  • Opportunists: insects, weeds

How would you describe these animals? What would their strategy for survival be?

Humpback Whales have very specific migratory patterns and eat only plankton, krill, or small fish. They have no teeth, but instead have baleen to filter their food. Elephants live in small groups and migrate together towards resources. The matron of the group must have a strong memory or the herd will die. Pandas eat mostly bamboo, but will supplement their diet with other plants and even meat on occasion. They are incredibly selective of their mates, and have long gestational periods. Saguaro cacti young are nurtured by a "nurse tree," usually an ironwood or mesquite tree, that shelters the cactus and provides shade. The cactus will eventually grow and take over the space of the nurse tree.

K-selected species are known as Competitors. The K variable represents Carrying Capacity, which is the maximum size a population can grow and still remain sustainable. K-strategists only have a few babies and nurture them to adulthood, so the overall population size fluctuates closely around the carrying capacity. Traits of K-selected species include:

  • Fewer, larger offspring
  • High parental care/internal development
  • Later reproductive age
  • Lower population growth rate
  • Population size stable around K
  • Specialists
  • Late successional species
  • Competitors: elephants, large plants

Survivorship Curves

Type I curves represent K-selected species and are known as Late Loss curves. The organism only has a few young, nurtures them to adulthood, and usually die of old age. (Red Curve)

Type II curves are known as constant loss curves. Birds, reptiles, and small mammals face mortality challenges throughout their lives. Developing countries can

Type III curves represent r-selected species and are known as Early Loss curves. These organisms have lots of young and provide less parental care. Most die off, with the few that survive primarily focusing on reproduction into adulthood.

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