Ecology – Biology 3500
Dr. Adams
POPULATION ECOLOGY UNIT
Chapter 9: Population Distribution and Abundance
Population Ecology -- Important characteristics
of populations include:
1. distribution (on Earth) -- why
organisms occur where they do (and why NOT elsewhere)
2. density -- within the population,
how many individuals occur within a unit area
3. abundance -- overall numbers
within the population as a whole
Distribution Limits: the environment limits the distribution of
species
Each species has a niche (what's a niche?)
Fundamental vs. Realized
niche
Animal example: Kangaroos in Australia (see pages 198-199)
Plant example: the genus Encelia and pubescence of
leaves
Encelia californica -- along
coast of CA; cool environment and relatively moist.
This species
has the least pubescent leaves (Why?)
Encelia actoni -- inland;
temps somewhat warmer but conditions also much drier.
Leaves more
pubescent than E. californica; gives leaves higher albedo
(reflectance)
In the hot Sonoran desert farther
east, E. frutescens, with sparsely pubescent leaves,
and E.
farinosa, with strongly pubescent leaves, grow side by side. Yet, both
species
leaves are
similar in temperature during the hot day. HOW? E. frutescens
has much
higher
transpiration rates. So, how does E. frutescens avoid too much
water loss? It
grows in a
different microhabitat, in the much deeper soils of seasonally wet washes
with much
deeper roots. E. farinosa grows mostly along banks with shallow
soils.
Intertidal barnacles (Balanus and Chthamalus)
and competitive exclusion (more in Chap. 13)
Distributions (Dispersion) Patterns on a Small Scale:
Tied with resource availability, distribution, abundance; age
structure can change distribution
Distributions can be:
1. random -- likely when? Resources incredibly
abundant and competitive interactions
reduced
(individuals "ignore" one another)
2. regular (uniform) -- sparse resources, individuals
"claim" exclusive use of a patch of
landscape (in
essence, individuals compete with each other to
claim "territory")
3. clumped -- when resources are clumped; food, water,
mates, etc. Creates a situation
where
individuals are "attracted" to one another. Obviously happens with social
groups.
Examples: Creosote bush
(adults) in the desert -- regular (uniform) distribution. WHY?
However, if you look at young individuals, they tend to be clumped. WHY?
See also the discussion of root systems of creosote on page 205.
Distribution Patterns on a Large Scale: always clumped
(WHY?)
Resources are NEVER uniformly distributed over large areas.
Many organisms clumped
around water sources, for instance.
Different species with different tolerances of various
conditions may have abundance gradients
that follow resource gradients:
moisture, temperature, salinity, etc. Can result in "hot spots".
This may also mean that in a given area, different species
with different needs/tolerances are
found at different places along the gradient (see bird and
plant examples in text on pages
206-208 -- crows; trees along elevational moisture gradients, Figs. 9.15, 9.17, 9.18).
Organism Size and Population Density
It should surprise no one that pop density necessarily
decreases with increasing organismal size,
all other factors being equal (though
rarely are other factors equal). These trends can be
seen across comparisons of major
taxonomic groupings (see page 209-210), and even within
species. Simply put, larger
size requires more resources, and therefore fewer individuals
can be supported by a given amount of
resources.
Abundance/rarity -- for reasons I will discuss in class, I do not
like to use the term "rare" except
under very specific
circumstances (circumstance "8" on page 211)
Based on three factors: 1. geographic range (extensive vs.
restrictive), 2. habitat tolerance
(broad vs. narrow), and 3. local
population size (large vs. small)
Chapter 10: Population Dynamics
Chapter 9 discusses populations from a static (snapshot)
point of view; however, populations are,
of course, dynamic.
Population size at some point: Nt = Nt-1 + B + I - D
- E
Dispersal: WHY do organisms disperse? Understand that
migration is directed dispersal.
Expand ranges (why is this important?), move away from
competition/dwindling resources, follow
food resources as they
move/experience pop. fluctuations, with changing weather/climate, etc.
Mechanisms for dispersal: many, many
organisms have very specialized dispersal mechanisms for
some part of the life history
(animals -- immatures, adults; plants -- seeds, runners, fruits; fungi
-- spores). You should be able
to provide EXAMPLES.
Dispersal of Expanding Populations -- Birds
provide many good examples (after all, have wings)
Example: Eurasian Collared Doves. Odd
in the sense that, unlike introduction of starlings or
English House Sparrows into new
countries, the Eurasian Collared Doves, once restricted to
Turkey in Europe, began around 1900
spreading into the rest of Europe, so that by 1980, every
country in Europe had been "invaded"
(see Fig. 10.3), all without human introduction or
apparently much other influence.
The
dispersal, although apparently reasonably steady, took place in small "jumps".
Only
young doves are active dispersers;
adults are highly sedentary. Each of these bursts of juvenile
dispersers averaged about 45 km. of
expansion yearly. Compared with the 300 to 500 km.
yearly expansion by Africanized
Honeybees in the Americas (see Fig. 10.1), 45 km is pretty
modest, but still impressive compared
to most other animals. Dinumma deponens in the U.S.
The RATE of expansion clearly differs for different species
(see Fig. 10.5), and is based upon
MANY different factors.
Also, UNDERSTAND that dispersal is likely taking place in all directions, but
the only direction in
which it
will be apparent will be away from the already established populations.
Plant Example: North American Trees -- Maple and
Hemlock
Both spread northward after the last
ice age, reaching present northern limits at different times,
based on different rates of dispersal.
Maple disperses faster than Hemlock (anyone know why?).
QUESTION: are we likely to see
these ranges begin to expand northward farther yet in the near
future, and, if so, WHY?
Dispersal in Response to Changing Food supply
Example: Small mammal populations (eg., lemmings,
voles) experience large population fluctua-
tions from year to year; these
fluctuations are thought to be rapid reproductive responses to
changes intheir food. Yet we
see rapid response in predator populations as well, and these are
almost exclusively due to dispersal
and NOT production of more offspring (preds. reproduce
slower)
Owl and
Kestrel densities match vole pop. fluctuations in Finland almost perfectly -- if
owls/
kestrels were exhibiting reproductive
responses to prey densities, there would be lag time. This
suggests owls/kestrels nomadic.
Not all preds can follow prey as well as these, because, after
all, wings can make a huge
difference!!
Dispersal in Water
Current is a natural disperser, but
true aquatic organisms are, of course, quite RESTRICTED as
to where
they can disperse. As far as dispersal is concerned, however, perhaps the
most
important
question here is "How do organisms PREVENT dispersal in water?"
As there are a huge number of
dispersal mechanisms, there are a large number of characteristics
that allow
organisms to maintain position within streams (be sure you know some).
Even so,
certain
flooding/weather events still move these organisms downstream -- drift.
Therefore, organisms that permanently
reside in a stream must offset drift with upstream
colonization cycles. Sometimes these upstream dispersal movements are
coordinated in
spectacularly
large groups (see migrating snails in the Rio Claro, Costa Rica, page 221).
Metapopulations -- subpopulations connected by moving
individuals. Example? Remember
the Glanville Fritillary populations
in Finland (Chapter 4).
North American Example: Parnassius
smintheus (an alpine butterfly)
The butterflies tend to leave small
subpopulations more than large, and move into larger pops.,
so that the
larger meadows have an even proportionally larger density of individuals.
With
the long
standing policy of fire suppression, and in the face of global climate change,
alpine
meadows are shrinking
in many places, leaving alpine species with nowhere to go.
Patterns of Survival
Survivorship Curves/Life Tables/Age Distribution pyramids
Follows cohorts through life
Types of
Survivorship Curves:
"I" represents high survivorship of young; typically occurs with parental care
of few young
"II" represents relatively equal chances of surviving/dying throughout life
"III" represents high young mortality; typically occurs with tremendous
numerical output of
young, but no parental care.
You should, of course, be
able to recognize examples of all of these, of which copious examples
are presented in the book (and I imagine you could think of more on your own).
Age
Distributions
These graphs give an indication not only of age structure, but in turn whether
or not a pop.
is likely to continue growth, remain level, or decline.
White Oaks (Illinois pop.) and Rio Grande Cottonwood (Belen, New Mexico pop.)
example
A Truly dynamic example:
Geospiza conirostris on Genovesa (Galapagos Islands)
Snapshots in
1983 and 1987 indicate droughts in 1977, 1984 and 1985 caused a lack of
young production during those years, and also killed some of the older birds.
Shows that
survivorship is typically MUCH MORE COMPLEX than a simple survivorship curve.
Life Tables: Rates of Population Change -- requires knowledge of a fecundity
schedule
Net Reproductive rate (per individual), geometric rate of
increase, generation time, per capita rate
of increase
You should be able to determine various questions asked about
life tables and to fill in life tables
when given nx and mx.