Download Character displacement

There have been many other laboratory studies of
competition. When the organisms studied are frequently
found naturally in small environments, the results are less
likely to have been affected by the laboratory situation.
One such experiment is the study of competition between two
species of yeast, Saccharomyces, the usual yeast used in
fermentation, and Schizosaccharomyces,
a different yeast genus.
The experiments were performed
by Gause. He already knew the
mechanism underlying the depression
of growth in competing populationsthe accumulation of ethyl alchohol
in the growth medium.
The evidence of that comes from other studies that show a
logistic growth curve for Saccharomyces seems to flatten
when the concentration of ethyl alchohol reaches
~6.5mg/cm3.
after Richards,
1928
As with any experiment of this type, the first thing we want
to know is how the two species grow when alone in
culture…
From the growth curves we can get K and r for each…
Saccharomyces
Schizosaccharomyces
K
13.00
5.80
r
0.22
0.06
Schizosaccharomyces has a lower growth rate and a smaller
carrying capacity than Saccharomyces. That suggests it
should do worse in competition. Gause grew the species
together. Here the two figures show growth alone and in
mixture…
Saccharomyces
Schizosaccharomyces
It is apparent that Schizosaccharomyces is more affected by
interaction than is Saccharomyces. Gause determined
competition coefficients for these species, and found that they
varied over time. Though this means that the system doesn’t
fully fit the Lotka-Volterra model, the coefficients and their
variation are of interest…
Species 1-Saccharomyces
Species 2
Age of culture
α2,1
α1,2
20 hours
4.79
0.501
30 hours
2.81
0.349
40 hours
1.85
0.467
As alcohol accumulates (as well as other waste products),
though Schizosaccharomyces is the weaker competitor, its
ability as a competitor is less affected than is Saccharomyces.
Competition has also been intensively studied in the field.
First, MacArthur’s studies of warblers coexisting in the
coniferous forest areas of New Jersey…
Five species of warblers coexist. All feed on insects in the
same or neighboring trees. Their diets almost completely
overlap.
Given similarity of diet and habitat, how can they achieve
coexistence? The answer is: by partitioning space on the
trees.
Concentrated feeding
is indicated by the
yellow shading.
Left half – time
Right - # of
observations
Based on numbers of observations in differing parts of the
trees, partitioning permitting coexistence appears to fit the
patterns for the blackburnian, black-throated green, and
possibly the bay-breasted warblers.
The myrtle warbler is much rarer and more generalized in its
feeding habits, and thus can co-exist.
The Cape May warbler seems to overlap extensively with the
black-throated green and blackburnian warblers, but is an
outbreak specialist. Between outbreaks of spruce budworm
(and possibly other insects) it declines in abundance, as
competition would predict. During outbreaks its population
size grows much more quickly than the competitors, and it
persists in the system.
Hutchinson, in an article about “the paradox of the plankton”
suggested that when the environment is unstable (varies a lot
over time, as for the warblers using insect outbreaks),
competitive exclusion might not occur. This may explain
many occurrences of apparent coexistence even when one
competitor seems clearly superior.
In many cases where we might expect competition could
occur, we are left to observe only the “ghost of competition
past”. Another classic set of observations you have previously
seen is the evolution of different bill sizes in the Galapagos
finches. There is indication of earlier competition when we
compare the bill sizes of species living alone on particular
islands, and what happens when they are found on the same
island.
Obviously, competition can only occur when the species occur
together (are sympatric), and cannot occur when they occur
separately (are allopatric).
First, the abstract view…
Ricklefs defines character displacement:
“Character displacement is divergence in the
characteristics of 2 otherwise similar species where their
ranges overlap, caused by the selective effects of
competition between species in the area of overlap.”
Character displacement:
1. Is an evolutionary process that results from genetic
changes in the species. When we observe the results, it
has already happened. We can only infer competition as
the cause.
2. It does not occur frequently.
3. Since it is evolutionary, and has already occurred, there is
no way to test it directly in experiments.
Character traits of two closely related species
differ more in sympatric regions than in
allopatric regions of their geographic ranges.
When Geospiza fortis and
G. fulginosa occur
without the other species
on islands, their beaks are
very similar in size, as are
the seed sizes they eat.
However, when they are
sympatric, their bill sizes
are shifted in opposite
directions. We generally
assume this has occurred
as a result of competition.
If we look at bill shape, as well as size, among Galapagos
finches, it is remarkable how distinct they are. The logical
explanation is that natural selection occurred as a result of
competition, making them distinct.
each polygon shows
a species.
each letter within a
polygon represents an
occurrence on an island
This logic has been questioned in recent years by
constructing ‘null’ communities, and determining whether
observed niche separations are larger than those you might
observe by chance. In this case Strong (1979) argues that
these differences are no larger than would be expected in a
randomly chosen group of birds from the ‘nearby’ mainland
of Ecuador.
This argument continues actively in ecology.
Another indirect evidence of competition comes from
looking at the community of granivorous (seed-eating)
rodents of the American southwest. If rodents are similar in
size, they probably eat about the same size seeds, and they
would compete if they lived in the same areas. If they were
different in size, they would not compete, or at least not very
intensively.
Jim Brown compared co-occurrence and lack thereof at 95
sites. His hypothesized that competition should lead to
similar sized rodents occurring together less frequently than
different sized rodents. He compared pairs of species. The
results in the table that follows are numbers of associations
that occurred more (+) or less (-) frequently than chance.
Here’s what he found:
Body mass ratio
Local coexistence
< 1.5
> 1.5
On a larger,
geographical scale
< 1.5
> 1.5
Association
+
27
0
65
28
44
72
13
60
This is pretty much what Brown predicted would occur if
competition among granivores was important in determining
whether they could co-occur.
The sort of pattern that Brown observed has been important
in inferring competition. The basic observation is of a
checkerboard pattern. In some patches you find one species,
in others you find the species with which it competes.
Jared Diamond, who developed many of the observations
important in the development of island biogeography, found
exactly this sort of checkerboard pattern in the distribution of
two flycatcher species (genus Pachycephala) on the islands
off New Guinea. Letters for islands identify which of the two
species is found there.
Each island on which flycatchers are found (not all islands
have them) has only one of the two species. None have both.
This is not a pattern likely to occur by chance. Rather, it is a
pattern logically explained by strong competition between
similar species.
Thus far, all examples of competition have involved closely
related species. Yet the definition of competition is based on
the use of shared resources, with no mention of competing
species necessarily being related. Can very distinct and
unrelated species compete strongly?
Jim Brown proved that the answer to this question is YES!
Brown studied competition between granivorous (seedeating) desert rodents and ants that also eat seeds. In at least
some drought years, the seeds they both like are in short
supply.
The results he got, scaled to set species abundance in control
areas (neither ‘species’ removed) at 1, show evidence of
competition…
ants removed
ants present
rodents removed
--1.71
rodents present
1.28
1
This is the best study of competition between taxonomically
divergent species, but not the only one.
Another important result from studies of competition is the
occurrence of competitive release. When, in some places,
one of two otherwise strongly competing species is absent,
the niche of the one present is frequently much broader. This
is exactly what you would expect – without the competitor,
the species present will occupy something much more like its
fundamental niche.
Anolis lizards on the Lesser Antilles show the effect of
decreasing intensity of competition (here measured by the
number of competing lizard species) on niche breadth,
measured by the diversity of heights at which they perch.
Variation in perch height is a direct indicator of niche breadth.
One last consideration before moving on to multi-species
communities…
We need to consider the evolutionary responses of species
involved in interactions. You’ve heard about the ‘ghost of
competition past’. How do species reduce the intensity of
competition? Here are a couple of ways…
1. Character displacement
2. Niche shifts
Consider how niche shift could reduce competition…
Niches of 2 competing species
%
use
5
7
Food Size (mm)
In evolutionary time,
the populations
(species)diverge to
avoid competition.
5
3
7
9
Food Size (mm)
How is a change in food use achieved?
Typically, by a change in morphology (e.g. mouth size,
position, jaw length, gape etc.) that influences what foods
are used or preferred.
Sometimes the circumstances of competition are affected by
other interactions involving participant species. An obvious
example is the affect a predator can have on competition.
Two examples:
1. Again considering the intertidal zone, Robert Paine studied
the effect of the predatory starfish, Pisaster ochraceous, on
the community of sessile invertebrates at Neah Bay,
Washington.
Paine removed starfish from some areas and left other areas
as controls.
There were approximately 15 different species of sessile
invertebrates present in control areas.
In areas where the starfish was removed, the diversity of
invertebrates declined from 15 to about half that, 7-8 species.
In these areas a single invertebrate came to dominate the
community by occupying space. That species was the
mussel, Mytilus californianus.
The starfish is a keystone species because so much of
community structure is determined by its presence or
absence. It prefers eating the mussel, but cannot eliminate it
because some mussels grow so large that it can’t eat them.
Size becomes a prey refuge.
2. Tadpoles compete with each other for food in ponds. When
tadpoles of different species live together in the same pond,
there are clear effects of competition:
Tadpoles are stunted by as much as 25% in body weight.
Their growth to metamorphosis is slower, takes longer. That
exposes them to predation for longer.
In these experimental studies, Morin (1981, Fig.19-20)
showed that newts eating tadpoles can reverse the effects of
competition. The newts reduce the number of tadpoles, and
thus the intensity of competition.
In the absence of newts, a species of toad tadpole dominated
the system, and spring peeper tadpoles were almost
eliminated.
Newts, however, like eating the toad tadpoles. With newts
present, all three tadpole species survived, and, with lower
tadpole densities, each species grew more rapidly…