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 non-overlapping 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 (over-lapping 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 CompetitionParamecium 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 Blackburnian warbler

Black-throated green warbler

Cape May warbler

Myrtle (Yellow-rumped) 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.

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 CompetitionBarnacles 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

http://www.rit.edu/~rhrsbi/GalapagosPages/DarwinFinch.html#anchor725315

The fourteen species of Galapagos finches provide a classic link between theories of competition and adaptive radiation.

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.