Transcript competition 2004

COMPETITION
We wish to know:
• What happens when two individuals, populations or
species are sustained by the same resource
• Do populations compete for resources, or are their needs
sufficiently different that they can be said to occupy nonoverlapping niches
• What are the consequences of competition for the
distribution and abundance of species with similar needs
• If there is any relationship between competition for a
shared resource and evolution by natural selection
Competition
• Competition occurs when two species each require
a resource that is in short supply, so that the
availability of the resource to one species is
negatively influenced by the presence of the other
species.
• A “-/-” interaction
• Competition is linked to two important ideas
– struggle for existence (Darwinian evolution)
– the uniqueness of a species’ niche
Evidence of Competition
• “bottle” experiments: grow two similar
species in a simple environment
• observations of natural populations:
conduct detailed studies of similar species
apparently sharing the same niche (overlapping in resource use)
• field experiments with natural populations:
use cages, transplants, etc, to determine
competitive outcome in nature
“bottle” experiments
• place two similar species in a simple environment
in a test-tube, bottle or garden
• competitive exclusion is always the outcome
• winner may be consistent over a wide range of
environmental conditions, or vary with
environment
• basis for competitive exclusion principle: no two
species can indefinitely occupy the same niche.
The Niche Concept
And NUH is the letter I use to spell Nutches,
Who live in small caves, known as Niches, for hutches.
These Nutches have troubles, the biggest of which is
The fact there are many more Nutches than Niches.
Each Nutch in a Nich knows that some other Nutch
Would like to move into his Nich very much.
So each Nutch in a Nich has to watch that small Nich
Or Nutches who haven't got Niches will snitch.
Dr. Seuss - On Beyond Zebra (1955)
Test-tube Competition
Paramecium is a single-celled protozoan
ciliate. It feeds by engulfing bacteria and
reproduces by binary fission. It grows
readily in test-tubes of bacterial broth,
making it an extremely simple laboratory
study organism.
Classic studies of competition examined the
population growth patterns of two species
(P aurelia and P caudatum), alone and
together.
Results led to the competitive exclusion
principle: No two species of similar
requirements can long occupy the same
niche.
Test-tube
Competition
Grown separately, each
species exhibits “S-shaped”
population growth. An initial
rapid increase slows as the
population reaches the
“carrying capacity” of the
environment (test tube).
Grown together, P caudatum
invariably loses, and P
aurelia emerges as the
winner. Only one species
can survive in this simple
environment.
Observations of Natural
Populations
• Field observations of similar, often closely-related
species.
• Classic study of five warbler species in spruce
forest -- very similar species, very uniform habitat
• many specific differences in foraging ecology and
habitat use indicates that these five species occupy
different niches
• suggest that competition is rare in nature because
of niche differences among species and
complexity of habitat
Warblers Coexist in Simple Habitat
Bay-breasted warbler
These five species of
warblers, similar in
size, all insectivorous,
co-exist in a very
uniform habitat, the
spruce forests on
Maine, from spring
until autumn.
Blackburnian warbler
Black-throated green warbler
Cape May warbler
Myrtle (Yellow-rumped) warbler
Warblers: Competition or
Coexistence?
A careful study of foraging behavior revealed
many differences among the five species.
They differed in what part of the spruce tree they
frequented, and whether they captured insects in
flight, from needles, under bark, etc.
In the end, it was concluded that enough
differences could be found to explain
coexistence, essentially by arguing that each
species’ niche was sufficiently different.
The Niche Concept
The niche of a species includes all
aspects of its habitat, how it
makes a living, and where it is
found.
It appears that even superficially
similar species, on close
inspection, really differ in
resource use: how, when, where,
and what types of resources are
utilized.
Experiments with Natural
Populations
• Two barnacles on rock walls in inter-tidal:
Balanus occurs lower, and Chthamalus higher, in
the inter-tidal zone.
• Is competition an on-going force determining the
spatial distribution of these two species, or do they
represent the “ghost of competition past”, so that
today they occupy different niches and have no
influence upon one another?
Barnacles and Competition
Barnacles as larvae float in the
plankton, before settling on a rock
face and transforming into their adult
form, which filter feeds on small
plankton. Finding open space on
rock surfaces, and holding their own
against other species, is critical to
their survival.
A classic study in Scotland found
one species, Balanus, in inter-tidal
and sub-tidal zones. Another,
Chthamalus, occurred only in the
upper inter-tidal.
Barnacles and Competition
Observational evidence
suggested that the
distributions of Balanus and
Chthamalus only partly
overlapped. Were they each
adapted to live in different
regions of the inter-tidal zone
(occupy different niches?)
Further observation revealed that
larvae of both species settled
throughout the inter-tidal zone.
Larvae of Balanus died out in the
upper regions, larvae of Ch. Dies
out in the lower reaches.
When larvae that had settled on
flat rocks were transplanted (ie,
the rocks were moved), and other
species prevented from
colonizing, Balanus still died out
in the upper inter-tidal, but Ch.
thrived in the lower inter-tidal.
Competition in
Barnacles
Many studies find
competition to be
asymmetrical. One species
(in this case, Balanus)
appears capable of excluding
the other (Chthamalus) from
all regions that Balanus can
occupy. Chthamalus coexists
by virtue of its ability to live
in physically harsher
environments, where Balanus
cannot live.
Mechanisms of Competition
• Individuals of two populations may
compete by using more effectively and
depleting the resources that are critical to
both. Eg, nest sites, food, soil minerals.
Indirect competition
• Individuals of one species may interfere
with or directly harm individuals of a
second species. Eg, combat, release of
chemicals. Direct competition
Competition and Specialization
• If we begin with two very similar species,
competition is expected to be strong.
• Individuals whose resource use (character trait)
differs, in the direction of low overlap, should be
favored by natural selection.
• Over time, character traits in the two populations
will diverge.
• Although thought to be a common phenomenon, it
is difficult to distinguish character displacement
from ecological release.
Darwin’s
Finches
The fourteen species of Galapagos
finches provide a classic link
between theories of competition
and adaptive radiation.
http://www.rit.edu/~rhrsbi/Galapago
sPages/DarwinFinch.html#anchor72
5315
This group includes seed-eating
finches, insect-eating finches, and a
range of body sizes, and even a
woodpecker finch that uses a cactus
spine to probe for insects in
crevices.
Their differences have been cited as
evidence of character displacement
Character
Displacement in
Galapagos Finches
Geospiza fortis and G
fuliginosa differ more in beak
size when they occur together
on the same island, compared
to when they are sole
occupants of an island.
Summary: Consequences of
Competition
• Influences the distribution and abundance of
natural populations.
• Favors evolutionary specialization, which can lead
to reduced niche overlap
• affects biological diversity
– in the short run, competition is likely to reduce
the number of co-occurring species
– in the long run, competition is likely to increase
the number of species by encouraging
evolutionary specialization.