Population

• Species


  Unique type of organisms or an individual organism that can interbreed with its own kind
• Population


  Group of the same species that live in a particular area at a particular time
• Population size


  • Number of organisms in a population
• Population density


  Amount or number of individuals in a population per unit area or volume
• Patterns of population dispersion


  • Clumped
  • Uniform
  • Random
• Clumped dispersion


  Individuals are clustered in groups, reflecting a patchy distribution of resources
• Uniform dispersion


  Individuals are evenly spaced over the area they occupy, reflecting territoriality and intense competition
• Random dispersion


  Individuals have unpredictable spacing, reflecting a lack of strong interactions
• Quadrat method


  1. Identify population size within quadrat boundaries

  2. Measure size (area) of quadrat

  3. Calculate population density
• Mark-recapture method


  1. Take sample from population

  2. Mark individuals in sample

  3. Release marked individuals back into population

  4. Take new sample and identify marked individuals

  5. Calculate population size
• Population density formula: D = N/A
• Population size formula using mark-recapture: N = (M*n)/m
• It is important to note that the term "species" always ends with an "s" even if you are referring to only one organism
• A species with a very large population is more likely to be at risk of extinction
• At the end of this module, you should be able to:

  • Explain typical patterns of change in population size (e.g., Exponential vs. Logistic growth; Boom-and-Bust curve)
  • Discuss the concept of carrying capacity and its relation to logistic population growth
  • Graph and interpret logistic and exponential growth of a population
  • Use simulations to understand logistic growth and estimate population size

  
• The population of humans as of the end of 2020 was 7,794,798,739
• The environment has its carrying capacity that will determine the maximum number of the individuals in the population that can be comfortably supported. The population is being affected by several limiting factors such as competition, predation, diseases and natural calamities and therefore, no known species have increased its population indefinitely.
• Depending on the reproductive capacity of the species and the limiting factors in the environment, different populations may be following one of the following patterns: the exponential growth pattern (also called the J- curve), the logistic growth pattern (also called the S- curve) and the boom-and-bust pattern.
• Exponential growth


  If one (1) bacterium, that can divide by binary fission in 20 minutes, is placed in a petri dish full of nutrients, it will reach one million in less than 7 hours. If the nutrients in the petri dish are replenished, then, the bacterial population will continuously grow.
• The number of individuals added to the human population is doubling through the years, taking a J-shape pattern in the graph and the carrying capacity has not been fully reached yet.

  
• Logistic growth


  Since resources such as food, spaces and shelter are not in infinite supply, therefore the population has to fluctuate depending on the availability of these materials. The graph shows an S shape pattern which starts at an increasing rate from time 0-2. However, from time 2-3, the increase slowed down, and started to be tapered off at time 3. The carrying capacity (the maximum population size that a particular environment can sustain) has been reached when the population reached 100 percent at time 3.
• If the population has been stable and established in its environment, there might be fluctuations in the number from time to time but the average is maintained through the years.
• Boom-and-bust curve


  Some insect populations, such as the mosquitoes, rapidly increase during summer. However, it also rapidly decreases, falling way below the carrying capacity and leaving few surviving individuals, during the wintertime.
• Exponential growth can only exist if there are infinite natural resources available which is quite impossible since the environment has limitations in relation to the population that it can support.
• In exponential growth, the population rate is accelerating, that is increasing at a greater and greater rate. When population size is plotted over time, a J-shaped growth curve is produced.
• With the abundance of the limiting resources, the carrying capacity varies over place and time. Factors like refuge from predators, shelter, availability of nutrients, energy, suitable nesting, water, and energy can all be limiting factors.
• Populations exhibit logistic growth when resources are limited, when resources become scarce, population expansion decreases. It levels off when the carrying capacity of the environment is reached. This results in an S-shaped curve.
• In the boom and bust curve, the population rapidly increases, but due to limiting factors, it rapidly decreases.
• At the end of this module, you should be able to: Differentiate between density-dependent and density-independent factors that limit population growth, and give examples of each
• As of mid-2017, the world population reached 7.6 billion
• Over the last twelve years, approximately 1 billion inhabitants were added to the world population
• Yearly, the world population is growing by 1.10 percent, or approximately 83 million people
• By 2030, the global population is expected to reach 8.6 billion. In 2050, the global population is expected to reach 9.8 billion and 11.2 billion in 2100
• The world's population is 49.6% female and 50.4% male
• Carrying capacity


  Maximum size of the population that can be supported by the resources found in the environment
• Density-dependent limiting factors


  Factors that affect the population growth rate differently depending on the density of the population
• Density-dependent limiting factors


  • As the population increases, some of these limiting factors make the population growth rate to decrease
  • This is an example of negative feedback that limits population growth
• Density-dependent limiting factors


  • Competition within the population
  • Predation
  • Disease and parasites
  • Waste accumulation
• Density-independent limiting factors


  Factors that affect the growth rate but are independent of the density of the population
• Density-independent limiting factors


  • Severe weather conditions
  • Pollution
  • Natural disasters