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 performedby Gause. He already knew themechanism underlying the depressionof growth in competing populations-the accumulation of ethyl alchoholin 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 5.80r 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 2Age 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 – timeRight - # 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 speciesdiffer 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 showsa species.

each letter within a polygon represents anoccurrence 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 Association - +

Local coexistence < 1.5 27 0> 1.5 65 28

On a larger, < 1.5 44 13geographical scale > 1.5 72 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 (seed-eating) 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…

rodents removed rodents present ants removed --- 1.28 ants present 1.71 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

%

5 7

Food Size (mm)

use

5 7

3 9

Food Size (mm)

In evolutionary time,the populations (species)diverge to avoid competition.

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…