BIOS 6150: Ecology
Dr. Stephen Malcolm, Department of Biological Sciences
•  Week 2: Defining Populations: Birth,
Death & Movement.
•  Lecture summary:
•
•
•
•
•
Increases & decreases
Modularity
Life cycles
Life tables
Movement
Gypsy moth, Lymantria dispar,
First instar larvae on egg masses
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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2. Increases and decreases:
•  Populations increase in size through births (B)
and immigration (I)
•  Populations decrease in size through deaths (D)
and emigration (E). Thus,
•  Nnow = Nthen + B - D + I - E
•  To describe the distribution and abundance of
individuals in populations now and in the future.
•  But what is an individual?
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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3. Unitary and Modular organisms:
•  Individuals vary according to age, stage,
size, weight, maturity, sex, etc.
•  Unitary organisms are highly determinate in
growth form with only a single "module”
(e.g. a mouse).
•  Modular organisms are made up of repeated
modules, such as leaves or branches of a plant
(Fig. 4.2).
•  Modules with the potential for independent existence
are called ramets and the genetically distinct entity
made up of the ramet modules is called a genet.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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4. Genets and ramets:
•  Genet:
•  Is the organism that has been produced by a single
zygote even if it is highly 'ramified' because it has
the same genotype.
•  Ramet:
•  Is a module produced asexually that has the
potential for independent growth and reproduction.
•  Clone:
•  Is a population of ramets with the same 'maternal'
parentage (they may or may not be joined).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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5. Life cycles:
•  Semelparous:
•  Sometimes called monocarpic in plants.
•  Single bout of reproductive activity per
individual.
•  Iteroparous:
•  Sometimes called polycarpic in plants.
•  Multiple bouts of reproductive activity per
individual (see Fig. 4.10).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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6. Life Tables:
•  In order to follow these life cycles in
detail we construct life tables with
fecundity schedules from which we
can determine:
•  Survivorship curves
•  Age-specific mortality
•  Patterns of birth among different age
classes
•  Described in fecundity schedules.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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7. The Cohort Life Table (Table 4.1):
•  Describes the fate of a single cohort of
individuals:
•  A group of individuals born within the same short
interval of time as in annual plants and animals.
•  Actual numbers (data = ax values) are
transformed to proportional data for
comparison (lx) which are then used to
calculate the proportion dying in each stage
(dx) and the mortality rate (qx) as dx/lx.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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8. Life Tables and k-values:
•  dx values can be summed but do not
represent stage specific intensity of
mortality, whereas qx values do, but they
can't be summed.
•  These two useful properties are combined in
k-values where:
•  kx = log10ax - log10ax+1
•  same as log10(ax/ax+1)
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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9. Fecundity Schedules:
•  The last 3 columns of Table 4.1 represent the
fecundity schedule, or age-specific egg
production for the common field
grasshopper.
•  The raw data for egg production are listed
under Fx and this gives mx eggs per
surviving individual in each stage
(22,617/1,300 = 17) and lxmx eggs as a
proportion of the original cohort in the
stage.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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10. The basic reproductive rate Ro:
•  Cohort life table data can be reduced to a
single value, the basic reproductive
rate Ro which is the mean number of
offspring produced per original individual
by the end of the cohort.
•  This can be calculated in two ways:
•  Ro = ΣFx/ao
or,
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
Ro = Σlxmx
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11. Population increase or decrease:
•  Compare Table 4.1 where Ro = 0.51
(population declined) with Table 4.2
where Ro = 2.41 (population increased).
•  This means that when:
•  Ro <1 the population will decline.
•  Ro >1 the population will increase.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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12. Mortality and survivorship:
•  The life table
data of Table 4.2
are plotted in
Fig. 4.7 as
mortality (qx &
kx) and
survivorship
(log10lx) curves to
show how both
change with time
in the life cycle
of Phlox
drummondii.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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13. Survivorship curves:
•  Survivorship curves classified by Pearl & Deevey as:
•  Type I (late loss):
•  like humans and elephants or well-defended species where
mortality occurs mostly at the end of the maximum lifespan (mostly K-selected species).
•  Type II (constant loss):
•  constant probability of death with age, perhaps like plant seed
banks.
•  Type III (early loss):
•  marine fish or planktonic species with high initial mortality,
followed by high survivorship of the survivors (mostly rselected species) (Fig. 4.8).
•  Age-specific mortality is shown in Fig. 4.20 where most mortality
occurs in post-reproductive plants.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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14. Rate of increase for overlapping
generations:
•  For overlapping generations, generation time
is represented as time T and so the
population reproductive rate is represented
by a “continuous” rate of increase:
•  r = intrinsic rate of natural increase
•  which is related to the “discrete” basic
reproductive rate (Ro) by:
•  r = lnRo/T
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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15. Human populations and the
“demographic transition”
•  Human populations are subject to the same
processes of birth, death, immigration and
emigration in both more developed and less
developed countries (Figs 4.23 & 4.24).
•  The “demographic transition” occurs
when declines in birth rates associated with
increased per capita wealth lag behind
declines in death rates.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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16. Distribution and Movement
•  Patterns of dispersion:
•  Dispersion is the distribution pattern produced as a result of
movement: either dispersal or migration.
•  There are three basic patterns of dispersion:
•  Random
•  equal probability of occupying any point in space
•  Regular (regular, even or overdispersed)
•  evenly spaced because individuals tend to avoid each other
•  Aggregated (contagious, clumped or underdispersed)
•  individuals are closer together than expected by chance because of a
tendency to be attracted
•  At different scales the same observation (e.g. aphids on a leaf) can appear
aggregated at large scales (landscape or woodland ecosystem), random at
intermediate scales (leaves of a host tree), or regular (single leaf) (Fig. 5.1).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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17. Dispersal:
•  Movement of individuals from the homesite, or,
•  Spreading of individuals away from each other
•  e.g. of offspring from their parents or from regions of high
density to regions of lower density
(Begon et al. glossary p 958).
•  Basically it is the movement of organisms in response to
resource distribution and habitat conditions (both abiotic temperature etc., and biotic - competitors, predators etc.).
•  Patterns of dispersal are commonly shown by a
negatively asymptotic curve with most dispersing
units falling near their origin (Figs. 5.5 & 5.12).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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18. Dispersal can be increased in 2 ways:
•  (1) More dispersing units
•  (2) Greater vagility
•  i.e. increased dispersal ability by increased
abaptations for dispersal on wind, water etc., or
•  better abapted movement behaviors.
•  Reasons for dispersal include:
•  (1) avoiding crowding
•  proximate: ultimate = avoidance of resource depletion.
•  (2) avoiding competition with relatives.
•  (3) avoiding inbreeding and negative genetic
consequences.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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19. Dispersal in time or space
(passive & active):
•  Periodical cicadas (Fig. 6.5) are essentially
dispersing in time to avoid predation through
synchronous emergence every 13 years (like
plant dormancy).
•  Other species disperse actively in space in
ways that facilitate their immigration to islands
of habitat (real islands or habitat patches):
•  as in Fig. 5.3 for birds, or Fig. 6.13 for the
Colorado potato beetle.
•  Most individuals disperse away from the birth site!
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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20. Dispersal polymorphisms:
•  Dispersal variation among life
history stages (bet-hedging) as in
Fig. 5.10.
•  Passive dispersal of animals in water:
• Marine versus freshwater organisms
(Fig. 5.9); or,
• Passive dispersal of seeds (Fig. 5.5).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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21. Range Expansion:
•  “Range Expansion is the product of successful
dispersal into an area not formerly occupied by
the species”
•  Range expansion usually follows events such as:
•  (1) Removal of an ecological, physiological or
behavioral barrier.
•  (2) Formerly unsuitable habitat becomes suitable.
•  (3) An evolutionary shift (could be components of 1
and 2).
•  (see Figs 2-19, 2-20 & 2-21 from Brewer, 1994)
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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22. Migration and life cycles:
•  Begon et al. define migration as:
•  “The movement of individuals, and
commonly whole populations, from one
region to another,” or,
•  “the mass directional movements of
large numbers of a species from one
location to another.”
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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23. Baker’s definition of migration:
•  Baker (1978) uses:
•  “the act of moving from one spatial unit to
another”
•  Baker uses this broad definition so that life
histories of variable temporal and spatial scale
can be included as in Fig. 5.14:
•  multiple journeys, single return journeys and oneway journeys.
•  Or as in Fig. 5.16 can be a response to
environmental stochasticity causing outbreaks.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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24. My definition of migration:
•  I prefer the following definition:
•  predictable and directional movement
of populations (or individuals and
their genes) between spatially
separated resources.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 4.2, 3rd ed.,
see Fig. 4.1, 4th ed.):
Different kinds
of modularity in
plants (left) and
animals (right)
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 4.10
(3rd ed.):
Range of
semelparous and
iteroparous life
cycles for shortlived and longlived species
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Table 4.1 (3rd ed): Cohort life table for the common
field grasshopper, Chorthippus brunneus
Fecundity schedule
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Table 4.1 (4th ed.): Cohort life table for the
plant Phlox drummondii
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 4.8: Classification of survivorship curves
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 4.20 (3rd ed.): Survivorship curves for 2 cohorts of
biennial white sweet clover, Melilotus alba.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 4.23 (3rd ed.): Age distribution for
France on 1 January 1992
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 4.24
(3rd ed.):
(a) 1980 age
distribution in
developing (left)
and developed
(right) countries;
(b) Changing
birth, death and
net increase
rates;
(c) Population
projections
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 5.1 (3rd ed.
see Fig 6.3, 4th ed.):
(a) Patterns of
dispersion.
(b) Temporal
changes in spatial
patchiness of the
grass Festuca
ovina in a transect
through English
heathland
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 5.5
(3rd ed.):
Dispersal of
Eucalyptus seeds
from edge of
forest (a) & from
single tree (b).
(c) Seed dispersal
contours of
tropical tree.
(d) Seed densities
against dispersal
distances for 4
species
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 5.12 (3rd ed.): Dispersal distances to site of
first breeding for outbreeding great tits.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 6.5: Changing densities of adult
cicadas and percentage predation
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 5.3 (3rd ed.): Incidence (fraction J of islands occupied) of
supertramp and high-S bird species against bird species richness (S)
on islands in the Bismarck archipelago (see also Fig. 21.19, 4th ed.).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 6.13:
Spread of the
Colorado
potato beetle,
Leptinotarsa
decemlineata,
in Europe
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 5.10
(3rd ed.):
Dispersal
dimorphisms
in animals
(left) and
plants (right).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 5.9
(3rd ed.):
Dispersal
stages in
marine and
freshwater
habitats.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 2.19 (Brewer 1994): Limits to dispersal and
range expansion.
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Figure 2.20 (Brewer 1994): Opossum (Didelphis)
range expansion in eastern North America
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 2.21 (Brewer 1994): Distribution of zebra
mussels in eastern North America.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 5.14 (3rd ed.):
Migration patterns in
relation to life cycles
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Defining populations
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Figure 5.16 (3rd ed.): Development and movement of an
outbreak of the desert locust, Schistocerca gregaria, in
Saharan Africa 1985-1989.
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