Transcript Does competition occur in natural populations? Yes, there is strong

Does competition occur in natural populations?
Yes, there is strong evidence that competition
between species is a significant process.
Sometimes effects are subtle.
In this case study, competition occurs, yet both
species still coexist.
Menge conducted an experiment in which the large
starfish was removed from a group of tidepools.
Another group of pools was left as a control.
CONDITION
RESPONSE OF SMALL STARFISH
Large starfish
removed
30% gain in body weight
Control
No change in weight
INTERFERENCE COMPETITION
• Caused by direct contact among
individuals
• The resource in short supply in this
type of competition is usually space,
not food
• Over a period of 2 years, Connell (1961) observed a
strong form of interference in 2 species of barnacles
• Barnacles are sessile filter feeders that live on waveswept rocky shores.
• Space is limiting, not food.
• One species is large (15 mm); the other is small (6-7 mm).
• Where they live together, the large species grows
on top of and crushes the smaller one and crushes it.
Ricklefs (2001, Fig. 19.16)
Grant - interference in rodents
Grass
Forest
Deer mouse
alone
Vole alone
Vole
Deer mouse
Vole excludes deer mouse from grassland
(attacks)
In both the barnacles and rodents, competitive
exclusion was caused by interference competition.
But, the exclusion was local and both species
persisted in the larger landscape.
We can generalize this result…
Suppose we have 2 species & 2 habitats.
Habitat 1
A wins, B loses
Habitat 2
B wins, A loses
In a mixture of these two habitats:
Both species persist.
Our perception of the relationship between
the 2 species is scale dependent...
Spatial Scale
Result
within 1 habitat
competitive
exclusion
landscape
(both habitats)
persistence
How common is competition in natural populations?
• Schoener (1985) reported about 100 cases
competition occurred.
• But, there are some species where it is not
not important.
• e.g., insects appear not to be resource limited
for long periods of time.
• In tropical beetles, Strong (1984) found no
evidence for competition among species
• The beetles lived on & ate large leaves of
banana and other plants
• But, they only consume 1-2% of each leaf.
• In controlled lab experiments, Strong
showed that beetles do not affect one
another’s survival, numbers or growth.
• Instead, the beetles are limited in nature
by egg, larval & pupal mortality caused
by parasites & predators.
For example, in aquatic ecosystems, tadpoles
(larvae) of frogs compete for resources.
In addition, tadpoles are eaten by predatory
salamanders.
When tadpoles of different species live
together in the same pond…
1. Tadpoles are stunted by as much as 25%
in body weight.
2. The larval stage is longer.
This is the result of competition.
Morin - Predators influence the
outcome of competition
But, salamanders eat the tadpoles
Therefore, the salamanders reduce the #
of tadpoles & this reduces the competition
among the species.
Only when the predator is absent, is it possible
to see effects of competition among species.
Ricklefs (2001,
Fig. 19.20)
P.J. Morin 1981
Science 212:1284-1286
Because competition occurs in species
that share resources, we expect competition
to occur in species that are related.
Does competition ever occur in species
that are not taxonomically similar?
It can happen as long as the species
share the same resource.
Brown (1966) discovered competition
between 2 very different groups of species
… rodents and ants!
Both live in deserts & both rely on seeds
for food. They are “granivores”.
Both use similar types of seeds, which may
be scarce in years with little rain.
Brown’s Results
1. Remove rodents:
Ant colonies increase 71% vs. controls
2. Remove ants:
Rodents increase 28% vs. controls
There are probably many cases of
competition in unrelated species:
For nectar in flowers…
bees & hummingbirds
For insects…
birds & insectivorous rodents
Experiments in ecology are important.
We think that the results tell us about ecological
processes.
In competition studies, we also rely on nonexperimental evidence.
This type of evidence is called “indirect”
evidence.
But this indirect evidence can also be
persuasive.
An example of this kind of evidence is from
studies where the use of resources by a species
is compared under 2 conditions…
• Where the species lives with a potential
competitor.
• Where the same species lives alone.
The Question….
Does this species change its use of
resources when its potential competitor
is absent?
MacArthur, Diamond & Karr
wren
Panama:
ant bird
yellow
warbler
yellow
warbler
But, on an island:
yellow warbler
Therefore, competitive release
Another line of evidence comes from
patterns in the distribution of species
In a groups of islands…
a “checkerboard” distribution...
Diamond (1975) observed this type
of distribution in 2 species of dove
on 20 islands near New Guinea.
Species
M
N
M+N
Number of islands
14
6
0
This result is not likely due to chance.
Strong competition between species could
cause this pattern.
REVIEW…
There are 2 types or mechanisms of
competition between species:
- exploitative
- interference
Exploitative refers to resource (food)
consumption.
Interference is direct contest for space.
I distinguished 2 types of studies that
provide evidence for competition...
1. Experimental manipulations:
- Rodent & ant removals
- Tadpole - salamander interactions
2. “Indirect”…the evidence is inferred from
a pattern:
- Competitive release
- Checkerboard pattern
The pattern we see can be explained by
interspecific competition.
Both checkerboard & release appear to be
consistent with or caused by interspecific
competition.
But what if the islands differed in some
unknown way? Some had a predator &
other islands did not.
Some islands may have had a resource
that was missing on other islands…
Do you see the problem?
Co-evolution of Competitors
Ricklefs 2001: chapter 20
How do species avoid competition?
Evolve different niches.
This would be favoured by natural selection.
Natural Selection
Darwin’s Definition:
individuals are selected
according to their ability
to survive and reproduce.
Ricklef’s Definition:
change in the frequency
of genetic traits in a population
through differential survival
& reproduction of individuals
bearing those traits.
http://www.literature.org/authors/darwin-charles/
the-origin-of-species/