1. 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

2. 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

3. 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

4. “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.

5. 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)

6. 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.

7. 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.

8. 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

9. Warblers Coexist in Simple Habitat
Bay-breasted warbler
Blackburnian 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.
Black-throated green warbler
Cape May warbler
Myrtle (Yellow-rumped) warbler

11. Warblers: Competition or Coexistence?
The five warbler species differed in many aspects of their foraging behavior
in what part of the spruce tree they frequented, whether they captured insects in flight,
Whether they foraged 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.

13. 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.

14. Resource Partitioning
Resource Partitioning - similar species share the same resources in different ways.
Arises in 2 ways:
(1) Ecological differences b/w established & competing populations may increase through natural selection
(2) only species that are dissimilar from established ones can succeed in joining an existing community
Bristly
foxtail
Indian
mallow
Resource partitioning - the subdividing of some category of similar resources that lets competing species coexist.
Each species is adapted to exploiting a different portion of the habitat
Drought-tolerant foxtail grasses have shallow roots fibrous roots that quickly abosrb rainwater. They grow where moisture in the soil varies from day to day.
Mallow plants, with a taproot system, grow in deeper soil that is moist early in the growing season but drier later.
Smartweed's taproot system branches in topsoil and soil below the roots of the other species. It grows where soil is continuously moist
Smartweed

15. 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?

16. 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.

17. Barnacles and Competition
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.
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?)

18. 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.

19. 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

20. 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.

21. Darwin’s Finches
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

22. 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.

23. On-going Evolution in Darwin’s Finches
1973 drought year
1977 wet year
Medium ground finch Geospiza fortis
Peter and Rosemary Grant have been studying finches on Daphne major since 1973, following every finch from birth to death. A drought in 1977 wiped out the plant that produced small seeds, and most medium ground finches died. Some of the largest individuals survived, and within a few generations, the average ground finch beak evolved to be 4% bigger. But in 1983 the island was clobbered by La Niña's soggy twin, El Niño, whose rains triggered a frenzy of small-seed plant growth. Ground finches with small beaks were more efficient at eating the new seeds and had more offspring, shrinking the average beak by 2.5% within a few years.
Daphne major

24. 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.