Population Dynamics: The Numbers of Organisms

1
Chapter 52

• Population Dynamics The Numbers of Organisms

2
Introduction

• Ecology is the study of the interactions of
  organisms with their physical environment and
  with each other.
• A population can be defined as a group of
  interbreeding organisms, members of the same
  species found in the same locality

3
Properties of Population

• A population has very different properties from
  the individuals it comprises.
• The individual is transitory, but the population
  endures, year after year.

• Among the properties of populations that are not
  properties of the individual organisms are growth
  patterns, mortality patterns, age structure,
  density, and dispersion

4
Patterns of Population Growth

• In the absence of net immigration the increase is
  equal to the birth rate minus the death rate.
• Thus, the rate of increase can be equal to zero,
  or it can be a positive or a negative figure.
  This property of population is called its per
  capita rate of increase, and it is symbolized by
  the letter r.
• The simplest model of population growth, in which
  the number of individuals increases at a constant
  rate , is known as exponential growth. It is
  described by an equation dN/dtrN
• Population cannot long continue to increase
  exponentially without reaching some environmental
  limits imposed by shortages of food, space,
  oxygen, nesting, accumulation of waste,
  competition with other species, or increased
  predation on population.

5
The Effect of the Carrying Capacity

• Carrying capacity-it is the average number of
  individuals of the population that the
  environment can support under a particular set of
  conditions.
• For animals the carrying capacity is determined
  by food supply or access to sheltered sites. For
  plants the factor maybe access to sunlight or
  availability of water.
• S-shaped, or sigmoid curve, on the right is
  called logistic.

• http//dieoff.org/p74f07.gif

6
Mortality Patterns and Age structure

• Another important property affecting both the
  size and composition of a population is the
  mortality pattern.
• Maximum longevity of the individual does not
  affect mortality patterns for the population as a
  whole.
• Graph to the right is an example of age
  structure.
• Mortality patterns affect another property, age
  structure.
• The age structure of a population is the
  proportions of individuals of different ages in
  the population.

http//www.iom.ipko.org/Projects/kdserhs/images/im
age002.gif
http//www.worc.ac.uk/departs/envman/courses/hort/
teach.hor/PHENV_handouts/pher65.jpg
7
Density and Dispersion

• Population density is the number of individual
  organisms per unit area or volume.
• The three basic patterns are
• Random
• Uniform
• Clumped
• Dispersion factors are often dependant on
  distribution of essential resources.

-Dispersion patterns are not necessarily fixed
and may vary seasonally or at different stages in
the life cycle.
http//www.uwyo.edu/dbmcd/popecol/feblects/Fig6.3D
ispersion.jpg
8
The Regulation of Population Size

• It is difficult to understand why fluctuations in
  population size occur which is very important
  because population fluctuations that effect one
  species usually effect many other species as
  well.
• Limiting Factors
• -Of critical importance is the organisms range
  of tolerance for such factors as light,
  temperature, available water, salinity, nesting
  space, and shortages (or excesses) of required
  nutrients.
• -If any of these factors are in demand or in some
  cases too extreme then population will not grow.

9
Density-Dependent and Density-Independent Factors

• Factors that cause changes in either the birth
  rate or mortality rate as the density of the
  population changes are said to be
  density-dependent.

• Environmental disturbances that effect the
  population are said to be density-independent
  factors

10
Population Cycles

• Population cycles is a phenomenon that has not
  yet been fully understood but there are several
  hypotheses that try and justify this strange
  phenomenon.
• Many species of voles which is shown on the
  picture to the right is an example of population
  cycles. Voles undergo a population fluctuations
  that peak every three to four years scientists
  have tried to come up with a viable explanation
  but it is still a mystery.

http//www.wildlifetrust.org.uk/herts/images/water
vole.jpg
11
Life-History Patterns

• Life-history patterns-are groups of coadapted
  traits affecting reproductive survival.
• The Alternatives
• -Alternative life-patterns have been given a
  variety of names for example prodigal and
  prudent, meaning prodigal successful under some
  circumstances where prudent is not.

12
Early and Late Reproduction

• Whether breeding is early or late can greatly
  influence the rate of population growth.
• Among larger mammals, juvenile survival depends
  on mothers size, experience and social position.
• If you have a short life expectancy for example
  an annual plant then it will put all its energy
  into one reproductive effort, this will increase
  probability of surviving offspring.
• On the other hand an organism that lives for a
  much longer period of time tends not to put as
  much energy in producing offspring thus lowering
  chances of survival.

13
The Asexual Advantage Parthenogenesis

• Parthenogenesis is a type of asexual
  reproduction, it is the development of an
  organism from an unfertilized egg.
• Parthenogenesis in plants has many advantages but
  one thing that it does lack is a parental support
  system.
• Completely asexual species are also found among
  invertebrates as well as plants, but many
  organisms alternate between asexual and sexual.
• Typically the asexual phase occurs when
  conditions are favorable for rapid local growth.

-Dandelions are known to reproduce
parthenogenetically
Curtis, Helenaand Barnes N.Sue. Biology worth
publishes,INC.
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