Transcript Distribution of Species

Distributions of Species
1. How do we represent, methodologically, the
distribution of species on the earth?
2. What factors influence the distribution of:
a. Individuals?
b. Populations?
c. Species?
Range Maps – what information do they give us?
There are three basic types:
Outline maps
Dot maps
Contour maps
Outline maps show a range
as an area, typically
shaded, within a boundary.
The boundary line defines
the limits of the known
distribution of the species.
Often much guesswork
involved.
Outline map indicating range of the sooty
orange-tip butterfly in Asia.
Dot maps indicate
points on a map
where a species
has been
recorded.
A dot map showing the locations where the
presence of Sonoran Desert canyon
ragweed (Ambrosia ambrosioides) has
been documented by the collection of a
voucher specimen.
Dot and outline maps can
be combined. A contour is
drawn to enclose the dots,
each of which represents a
documented location.
This is a combination dot and outline
map of the distribution of the southern
festoon butterfly (Zerynthia polyxena).
Contour maps of the geographic
range of the blue jay (Cyanocitta
cristata).
Distribution of Individuals
The most basic unit of distribution are individual organisms. It’s generally very
difficult to examine that.
With some organisms, we can get an idea about the distribution of individuals by
looking at aerial photographs.
Juniper distribution
Distributions tend to be
complex. Individuals
often are found in
clumps separated by
gaps.
As the edge of the range
is approached,
individuals tend to be
more sparsely
distributed, with larger
gaps.
In Rapoport’s data on palms, the density is much
lower at the edge of the distribution and gets
progressively greater as one moves into the
range.
A depiction of range is scaledependent.
In this depiction of the
distribution of Fremont’s
leather flower, we can see the
range represented on
progressively smaller scales,
culminating with the
distribution of individual plants
within a small space.
Abundance and
distribution can vary over
space and time. This
variation can differ among
populations.
In this example, Population A is intermittently presently and locally
extinct. Populations B and C never go locally extinct, but vary
dramatically in abundance over time.
Distribution of Populations
Thomas Malthus, in his 1798 Essay on the
Principle of Population, showed that all
populations have the capacity to increase in
numbers exponentially.
The process by which populations change in
size can be expressed relatively simply as:
r=b+i–d–e
where r, the per capita rate of increase, is
equal to the sum of the per capita birth rate (b)
and the per capita immigration rate (i), minus
the per capita mortality rate (d) and the per
capita emigration rate (e).
Thomas Malthus
When populations
are growing
exponentially, the
rate of growth
increases with the
population size.
Human population grew at a near
exponential rate until recently.
No organisms
actually continue to
grow at exponential
rates. Eventually,
they reach a carrying
capacity at which the
environment is
supporting the most
individuals possible.
The concept of the ecological
niche goes way back, but it was
refined by G. Evelyn Hutchinson in
1957 to show how organisms are
affected by the physical
environment.
This diagram depicts a niche in
two dimensions. Each dimension
reflects an organisms ability to
utilize a given range of a physical
factor. The resulting twodimensional figure is a
visualization of the organism’s
niche with respect to salinity and
temperature.
Imagine a fish that utilizes prey size in the
frequencies indicated…..
… and has O2 requirements as indicated
on the Y-axis here. This figure now
represents a niche in two dimensions.
If we add a third
resource, salinity, we
can view the niche in
three dimensions.
The geographic range of an
organism can be viewed as a
spatial reflection of its niche.
The dashed line on this map of
Arizona indicates the northern
extent of the distribution of
saguaro cactus. The markers
represent meteorological
stations. The solid points are
stations that have never recorded
a period of 36 hours without a
thaw. The crosses are stations
where such periods have been
recorded.
What does this tell us about the
distribution of saguaro cactus?
The term “niche” first appeared in the
ecological literature in 1917. At that
time, it was used to describe an
organism’s physical location in the
environment. Charles Elton, in 1927,
was the first to use the term in its
modern context as the “ecological
role” of an organism. This concept is
sometimes referred to as an
organism’s functional niche.
Conditions such as interspecific
competition may limit organism’s to
only a portion of their fundamental
niche. The range of resources
actually utilized by an organism is
known as its realized niche.
One of the earliest studies
examining the ecological niche
was done by Joe Connell
working with barnacles on the
Scottish coast.
At Connell’s study site in
Scotland, two species of
barnacle (Chthalamus
stellatus and Balanus
balanoides) were commonly
found. Although both species
settle randomly throughout the
intertidal zone, adult
Chthalamus are always found
higher in the intertidal zone
than are Balanus.
Connell performed a series of experiments:
1.
He moved rocks bearing Chthalamus from the upper to the
lower intertidal to see whether the species could survive in the
low intertidal zone. He also moved rocks bearing Balanus
from the low intertidal to the high.
2.
He removed Balanus from rocks in the low intertidal and
Chthalamus from rocks in the high intertidal. These
experiments were designed to show whether each could grow
in the other tidal zone if its competitor were absent.
Niche variables alone are not sufficient to explain patterns of distribution and
abundance.
1. Too simplistic to assume that conditions are equally favorable for a
species at all locations where it occurs. Some locations are undoubtedly
more favorable than others. In these locations, birth rates will exceed
death rates. The surplus individuals can migrate from these “source
habitats” to other locales. Other locations may be so unfavorable than
death rates exceed birth rates. These sites will be “sink habitats”, and
can only continue to exist if they are supplied with immigrants.
2. There may be sites that are inhabited DESPITE unfavorable
environmental conditions. Typically, such situations are explained by the
history of the organism at the site.
3. Other sites may be inhabited intermittently. Local populations are
variable, and may occasionally go extinct. The study of such isolated
subpopulations has become an area of great interest. The greater
population (comprised of the subpopulations) is known as a
metapopulation.
While there might be some
migration from sink to source
habitats, sink habitats must be
supported by an influx of
individuals from source
habitats.
This is central to
metapopulation theory.
The distribution pattern of most
individuals tends to be heterogeneous,
with some locations having high densities
and others much lower.
Such a distribution pattern is called
clumped or aggregated.
It can be assumed that
most of the variation in
numbers from one
location to another
within a distribution
results from differences
in the degree of
suitability of the
environmental variables.
Common species are
usually several orders of
magnitude more
common at some sites
than at others. This can
be examined by looking
at results of
standardized census
routes of the North
American Breeding Bird
Survey.
For both species, only one individual was recorded
on the majority of routes. More than 100 were
recorded on a limited number of routes.
Rare species may
be uncommon
throughout their
range, but there is
still variation. Nancy
Rabinowitz identified
three “forms” of
rarity.
We may also see temporal
variation in distribution and
abundance. This typically results
from temporal variation in niche
parameters.
An example can be seen in African
locusts. Source populations exist in
the darkly shaded areas. These
populations are permanent. When
conditions are good, they undergo a
population “explosion” and spread
across South Africa.
Similar fluctuations can
be seen in other
organisms.
Voles and lemmings of
northern Canada show
fluctuations in density
ranging across several
orders of magnitude.
They lack the dispersal
ability of locusts. Instead,
they show changes in
their habitat utilization.
Birds that prey on these
rodents also show
changes in their
geographic ranges,
shifting far to the south in
poor years..
Winter range of the snowy owl in poor years
is indicated by the “dotted” line. Typical
range is the darkly shaded area to the north.
Range Boundaries
It appears that the boundary of a
species’ range is probably set by
multiple environmental factors,
some biotic and some abiotic.
However, there have been few
detailed studies.
Physical factors that play a role in
limiting distributions include:
1. Temperature regime.
2. Water availability.
3. Soil and water chemistry.
As the saguaro cactus approaches the
northern edge of its range in the Sonoran
desert, it appears to be strongly limited by
temperature regime.
However, ecologists have moved away from Leibig’s law of the
minimum, and recognized that distributions are influenced by multiple
factors.
Many birds seem to be limited in the northern extent of their distribution
by cold temperatures. This may not be simply the physiological effect
of temperature, but its impact on food availability.
Many birds seem to be
expanding their winter ranges
northward as wintertime food
becomes more available in the
form of bird feeders.
Timberline is the
upper elevational
limit of trees on
mountains.
Geographically,
timberline seems to
be related to the
mean or maximum
temperature during
warm months of the
growing season.
This varies with
latitude and
elevation.
Locally, factors
such as wind, snow
depth, and energy
balance seem to
play a role.
Trees at timberline
may live for a long
time, with very slow
growth and
infrequent
reproduction.
Bristlecone pines
may live for
thousands of years,
but grow very
slowly.
In some places they
provide a “fossil
timberline”,
indicating the
elevation that was
once suitable for
tree growth.
Bristlecone pine
The desert pupfish Cyprinodon nevadensis lives in desert
streams. Its distribution is strictly limited by temperature. In
the outflow stream from a hot spring, it can be found only in
areas below 42 C.
Some specialized organisms can be found in places that are not habitable for
most animals. Great Salt Lake of Utah supports only two macroscopic inverts, the
brine “shrimp” (Artemia salina) and the larvae of the brine fly (Ephydra cinerea).
Many other inverts are found in the freshwater streams that empty into the lake,
but cannot tolerate the high salinity of the lake itself.
Disturbances of various types also
influence local and broadscale
distribution patterns.
Natural disasters may wipe out entire
populations.
If they occur regularly, however, they
become a natural part of the
environment.
Situations where there is a regular
pattern of colonization and
replacement of species following a
disturbance illustrate secondary
succession (i.e., succession on a
location previously inhabited by living
things).
Fire is an example of a disturbance
that has come to be required in many
ecological communites.
The longleaf pine community is an
example of a fire-controlled system.
Periodic disturbance can
be severe enough to
prevent the expansion of
species into areas where
they could otherwise
survive.
Historically, grassland fires
have helped prevent the
spread of woody
vegetation into prairie
habitats.
In recent years, fire
suppression has led to a
decrease in the amount of
grassland habitat in
southern Texas..
Smaller scale disturbances,
particularly biological ones
like predation, often have the
effect of removing dominant
species and allowing less
dominant species to coexist.
This may create a “mosaic”
(patchwork) of subhabitats
that supports greater
diversity.
Robert Paine found the
predation by sea stars on
rocky coastlines removed
selectively removed the
dominant mussels and
allowed less effective
competitors to coexist with
them.
In many cases, distributions
are limited less by physical
factors than by biological
interactions.
The three most significant of
these interactions are:
1. Competition.
2. Predation
3. Mutualism
African lions and hyaenas may compete for
food.
A summary of biological interactions between two species.
Effect on Species 2
Benefits
Effect on
Species 1
Not
Affected
Harmed
Benefits
Not Affected
Harmed
Mutualism
Commensalism
Parasitism
(or predation)
Commensalism
No Interaction
Amensalism
Parasitism
(or predation)
Amensalism
Competition
Types of Competition
We can classify competitive interactions in a number of ways. The most
obvious dichotomy is intraspecific competition, between individuals
belonging to the same species, and interspecific competition between
individuals of different species.
Another extinction can be drawn between exploitation competition, in
which the actions of one species (or individual) reduces the availability of
a resource to another species and individual, and interference
competition, in which the actions of one species (or individual) actively
interferes with the abilities of another to use a resource.
Exploitative competition may be consumptive in
nature. In America’s southwestern deserts, seedeating ants compete with rodents like kangaroo
rats for a limited number of seeds. This is an
example of exploitation competition.
Seeds taken by one species are not
available for use by the other.
Another form of exploitation competition occurs when
organisms compete for space. This is common in
plants. If one plant occupies a position in the landscape,
it cannot be filled by another plant. This is referred to as
preemptive competition.
Other examples of preemptive
competition may be seen in
intertidal invertebrates……
Exploitation competition may
be exemplified by territoriality.
In the Cascade Mountains of
Washington, two sympatric
species of tree squirrels
maintain interspecific
territories. This ensures both
species an adequate food
supply. The territory is
actively defended.
Red squirrel – Tamiasciurus hudsonicus
Chickaree – Tamiasciurus douglasii
Interference competition may also
take different forms. This include
chemical interference, sometimes
called allelopathy. Allelopathic
organisms release a chemical that
has a deletorious effect on other,
nearby organisms. Black walnut
trees are known for the production
of juglone, an allelopathic
chemical that interferes with the
ability of other plants to establish
themselves nearby.
We may also see interference competition in hummingbirds, who may
chase other bird away from sources of nectar.
If two species are “complete
competitors”, one will win out. This is
known as competitive exclusion.
It is thought that competitive exclusion
may have played a role in the
extinction of many prehistoric
mammals. For example, the marsupial
family Borhyaenidae in South America
may have gone extinct after the
development of the Panama land
bridge enabled North America sabertooths to enter South America.
Rainbow trout
Smoky Mountain stream
Brook trout
Competitive exclusion can also
be seen in Appalachian streams
where the native brook trout
can no longer compete with
introduced rainbows. Brookies
are typically found only in the
upper reaches of streams that
are unavailable to rainbows.
The non-overlapping ranges of
five species of large kangaroo
rates in the southwestern U.S.
probably results from competitive
interactions.
Evidence for competitive
exclusion can be seen in the
range of chipmunks on
mountain ranges in the
American southwest. On
most ranges, two species
exist, and overlap only
slightly in distribution.
On the ranges where only a
single species is found, its
range has expanded to
include virtually all
elevations and all habitats.
Neotamias dorsalis
Predation is defined (in your
text) as any interaction
between two species in which
one benefits and the other is
harmed
(McCall’s Note: Many
authors would distinguish
between predation, where
one organism is killed and
devoured, and parasitism,
where the other organism is
typically not killed. Herbivory
is also typically treated
separately.)
There are a couple of ways in
which predator-prey
interactions can influence
distribution patterns:
1. Predators that rely on
specific prey are restricted by
the distribution of the prey.
2. Predators may limited prey
distributions by killing and
eating prey organisms.
In coastal California, the
checkerspot butterfly is
limited to locales
supporting its host plant,
Plantago hookeriana.
It’s more difficult to see examples where
the distribution of a prey population is
limited by predators.
Sometimes this may be seen when
predators are introduced into areas where
they do not normally occur.
This may be deliberate – a form of
biological control of a pest species.
The introduced pricky pear
cactus (Opuntia stricta) was a
serious pest on grazing lands
in Australia. The introduction
of the moth Cactoblastis
cactorum, whose larvae feed
on prickly pear, has been
successful in controlling it.
The introduction of large,
predatory fish has often
resulted in the elimination of
certain prey species.
The introduction of Cichla
ocellaris resulted in the
extinction of many species
native to Lake Gatun in
Panama.
A similar result was seen after
the introduction of the huge
Nile perch (Lates niloticus) into
Africa’s Lake Victoria.
The lake trout, Salvelinus
namaycush, was almost
eliminated from the Great
Lakes after the sea lamprey
(Petromyzon marinus) was
allowed to reach the lakes
following the construction of
the Welland Canal
Similarly, parasites probably
play a major role in limiting
distributions.
There are many examples
of organisms expanding
their range after the
elimination of a parasite or
pathogen.
Similarly, ranges have been
contracted after the
introduction of a new
parasite or disease.
The introduction of avian
malaria into the Hawaiian
Islands has played a role in
the elimination many of the
islands’ native species,
particularly at low
elevations.
Mutualisms are interspecific interactions
in which both species benefit.
There are many examples, including
plants and their pollinators, cleaner fish
and their hosts, and others.
If these interactions are obligate, they
play a major role in controlling
distributions.
These figures indicate the distributions of
North American hummingbirds (A) and
the nine species of red tubular flowers
that are regularly visited by hummingbirds
at one Arizona site (B).
Note that, despite their close, mutualistic
relationship, there is little relationship
between geographic ranges of the
specific plants and their pollinators.
In the U.S., the yellow warbler is found in
a variety of shrubby and forested
habitats, but it Panama it is restricted to
mangrove swamps and offshore islands.
MacArthur attributed this distribution
pattern to diffuse competition in Panama
– the combined negative effects of
competition with many other bird species.