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ZOL 3702 LEARNING UNIT 4LIFE AND DEATH IN UNITARY AND MODULAR ORGANISMS
Textbook reference: pg 89-131
Learning outcomes:
Use the basic comparison of population growth and make deductions.
Give an overview of the approaches to population growth of unitary and modular organisms.
Discuss the quantification of demographic processes.
Discuss the different kinds of life cycle using examples.
Apply life tables, mortality and survivorship curves and comparisons for annual and perennial organisms.
Apply reproductive rate, length of generations, rate of increase in calculating population increase.
Describe and explain the demography of perennial and continuous semelparous organisms.
ECOLOGICAL FACTS OF LIFE pg 89
The number of individuals at a particular point in time can be represented as follows:
Nnow = Nthen + B – D + I – E or Nt + 1 = Nt + B – D + I – E
This is a fundamental ecological fact of life.
This says but the number of a particular species currently occupying a side of interest (Nnow) is equal to
the numbers previously there (Nthen), plus the number of births between then and now (B), minus the
number of deaths (D), plus the number of immigrants (I), minus the number of emigrants (E).
The main aim of ecology: describe, explain and understand the distribution and abundance of organisms.
In the application of ecology, there is an attempt to comprehend Nnow and predict Nf.
INDIVIDUALS - UNITARY AND MODULAR ORGANISMS pg 89-94
The individuals in a population cannot be regarded as identical.
Almost all species pass through a number of stages in their lifecycle . The different stages are likely to be
influenced by different factors and to have different rates of migration, death, and reproduction.
Even within a stage, individuals can differ in quality or condition. The most obvious aspect of this is size,
but it is also common for individuals to differ in the number of stored reserves they possess.
Individuals can be unitary or modular.
Uniformity amongst individuals is unlikely, especially when organisms are modular rather than unitary.
In unitary organisms, form is highly determinate: barring aberrations, all dogs have 4 legs, two eyes.
Humans are unitary organisms.
A life begins when a sperm fertilizes an egg to form a zygote. This implants in the wall of the uterus, and
the complex process is of embryonic development commence.
By 6 weeks the fetus has a recognizable nose, eyes, ears and limbs with digits. The fetus continues to grow
until birth and then the infant grows until the 18th year of life; but the only changes in form as opposed to
size or the relatively minor ones associated with sexual maturity.
Death can intervene at any time but for surviving individuals the succession of phases is likely predictable.
In modular organisms, timing and form are both unpredictable.
The zygote develops into a unit of construction (a module) which then produces further similar modules.
Individuals are composed of a highly variable number of these modules, and their program of
development is dependent on their interaction with their environment.
The product is almost always branched , and except for a juvenile phase, effectively immobile.
Most plants are modular however, there are important groups of modular animals, protists, and fungi.
Therefore, the potential for individual differences is greater in modular than in unitary organisms.
In growth of a higher plant, the fundamental module of construction above ground is the leaf with its
axillary bud and internode of the stem. As the bud develops, it produces further leaves, each bearing buds
, in their axils. The plant grows by accumulating these modules. At some stage in development, a new sort
of module appears, associated with reproduction (flowers), giving rise to new zygotes.
Modules that are specialized for reproduction usually cease to give rise to new modules.
The roots of a plant are also modular, although the modules are different.
The program of development in modular organisms is determined by the proportion of modules that are
allocated to different roles.
GROWTH FORMS OF MODULAR ORGANISMS
Modular organisms may broadly be divided into those that concentrate on vertical growth and those that
spread their modules laterally, over or in a substrate.
Many plants produce new root systems associated with a laterally extending stem: these are the
rhizomatous and stoloniferous plants.
The connections between the parts of such plants may die and rot away, so that the product of the
original zygote becomes represented by physiologically separated parts.
Modules with the potential for separate existence are known as ramets.
The most extreme examples of these plants separating as they grow are species of floating aquatics like
duckweeds. Entire ponds may be filled with separate and independent parts produced by a single zygote.
Trees are an example of plants whose growth is concentrated vertically.
The feature distinguishing trees and shrubs is the connecting system linking modules together and
connecting them to the root system. This does not rot away, but thickens with wood, conferring
perenniality. Most of the structure of a woody tree is dead, with a thin layer of living material lying
immediately below the bark. The living layer continually regenerates new tissue.
We can often recognize two or more levels of modular construction.
The strawberry is an example: leaves are repeatedly developed from a bud, but these leaves are arranged
in rosettes. It grows by adding new leaves to a rosette and producing new rosettes on stolons.
Trees also exhibit modularity at different levels: the leaf with its axillary bud, the whole shoot on which
the leaves are arranged, and the whole branch systems that repeat a characteristic pattern of shoots.
Many animals, despite variations in their method of growth and reproduction, are modular.
WHAT IS THE SIZE OF A MODULAR POPULATION?
In modular organisms, the number of surviving zygotes can give only a partial and misleading impression
of the size of the population.
In modular organisms the distribution and abundance of individuals is important, but it is often more
useful to study the distribution and abundance of modules.
SENESCENCE - OR THE LACK OF IT - IN MODULAR ORGANISMS
There is often no programmed senescence of whole modular organisms - they appear to have perpetual
somatic youth.
Even in trees that accumulate their dead stem tissues, death often results from becoming too big or
succumbing to disease rather than from programmed senescence.
Roots, buds, flowers, and modules of animals all go through phases of youth, middle age, senescence, and
death.
COUNTING INDIVIDUALS pg 94-95
To determine ecological factors such as birth, death and growth, individuals must be counted.
This means counting individuals and modules.
Most studies concentrate on effects of demographic processes, rather than on the processes themselves.
The term population is used to describe a group of individuals of one species.
What actually constitutes population will vary from species to species and from study to study.
In some cases, the boundaries of a population are apparent. In other cases, boundaries are determined
more by investigators purpose or convenience.
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