61BL3313 Population and Community Ecology Lecture 09 Interspecific competition Spring 2013 Dr Ed Harris

61BL3313Population and Community Ecology

Lecture 09 Interspecific competition Spring 2013

Dr Ed Harris

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Announcements

announcements

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This time

Part II: Interspecific interactions

-introduction

-early experiments

-Lotka-Volterra

-resource competition

-spatial competition and colonization

-evidence of competition in nature

-natural experiments

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Part II: Interspecific interactions -introduction

The niche

Elton (1927) - subdivision within trophic grouping (carnivore, herbivore, etc.)

Grinnell (1917) - distribution of species across habitat types

Krebs (1994) - the role or ‘profession’ of an organism in the environment; its activities and relationships in the community

Begon (1986) - the limits, for all important environmental features, within which individuals of a species can survive, grow and reproduce

Hutchinson (1957) - N-dimensional hypervolume

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Part II: Interspecific interactions -introduction

fundamental niche - The fundamental niche is the largest ecological niche that an organism

or species can occupy

-It is based mostly on interactions with the physical environment and is always in the absence of competition

realized niche - that portion of the fundamental niche that is occupied after interactions

with other species: that is, the niche after competition

-the realized niche must be part of, but smaller than, the fundamental niche

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Part II: Interspecific interactions -early experiments

Tansley: competition shapes communities

-closely related plants living in the same area often were found in different habitats, e.g., different soils

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Tansley

For his experiment he selected two species of an herbaceous perennial, bedstraw, in the genus Galium (Rubiaceae).

One species, G. saxatile, is normally found on peaty, acidic soils, while the second species, G. sylvestre, is an inhabitant of limestone soils.

Tansley obtained soils from both areas, planted each species singly in each soil type and then placed the two species together in each soil.

He found that each species, when planted alone, was able to survive in both soils.

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Tansley

The fundamental niche for both species includes both acidic, peat-rich soil and limestone soil.

Growth and germination were best on the soil where the Galium species was normally found.

When grown together on limestone soil, G. sylvestre overgrew and outcompetedG. saxatile.

The opposite was true in the acidic peat soil.

At this early date, Tansley had established that competitive exclusion could be demonstrated, and that the results differedby environment.

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Gause - "the struggle for existence"

In a series of experiments with yeast (Gause 1932) and protozoans, Gause found that competitive exclusion is observed most often between two closely related species (two species in the same genus, for example), when grown in a simple, constant environment

When either Paramecium caudatum or P. aurelia was introduced alone, each flourished and grew logistically, leveling off at a carrying capacity

When placed together, however, P. caudatum diminished and eventually wentextinct, while P. aurelia grew to a steady level

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Paramecium grown seperately

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Paramecium grown together

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Lessons:

1. two closely related species were unable to coexist in the simple test-tubeenvironment

2. even though we declare P. aurelia the “winner,” notice that its steadystate of approximately 300 per 0.5ml sample is less than the carrying capacityof 500 when this species was grown alone

3. recall the definition of competition as a reciprocally negative interaction, meaning that competition has a negative effect, even on the winners

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Gause's theorem

A Two species cannot coexist unless they are doing things differently

B No two species can occupy the same ecological niche

Competitive exclusion principle

Species which are complete competitors, that is, whose niches overlap completely, cannot coexist indefinitely

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Part II: Interspecific interactions -Lotka-Volterra

Modeling interspecific competition

Lotka 1925 and Volterra 1926

-Modeling population growth based on the logistic growth equation

-To model competition between two species, Lotka and Volterra wrote two simultaneous equations, one for each species

-Each equation is based on the logistic equation, but includes a new term, the competition coefficient (αij), which describes the effect of one species on another

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N1 = the number of individuals of species oneN2 = the number of individuals of species two

r1 = the intrinsic rate of increase of species oner2 = the intrinsic rate of increase of species two

K1 = the carrying capacity of species oneK2 = the carrying capacity of species two

α12 = the competition coefficient: effect of species two on species oneα21 = the effect of species one on species two

t = time

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The value of the competition coefficient is usually between 0 and 1,for the following reasons:

- A competition coefficient of zero would mean that there is no competition between the two species

If that were the case, there is no reason to try to model this interaction

-If the competition coefficient were negative, the implication would be that species two actually benefits the growth rate of species one

The interaction between species one and two would then be mutualistic

-Notice that the number of individuals of both species one and two decreases the carrying capacity

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Part II: Interspecific interactions -resource competition

Dave Tillman and "mechanistic competition"

-resouce-based competition theory

-the idea that population growth is constrained by the depletion of critical resources, i.e., a population increases until the supply of a single critical resource becomes limiting

-for example, plant growth may continue until the amount of phosphorus, nitrogen, light, or soil moisture becomes limiting

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Dave Tillman and "mechanistic competition"

-E.g., if plant growth is constrained by phosphorus and a farmer addsphosphorus fertilizer, plant growth will continue until another resource, such as nitrogen, becomes limiting

-If the farmer adds nitrogen, then soil moisture may become the limiting factor

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According to what is now known as the R*-rule, for any given resource (R), if we determine the R*-value for each species when grown alone, the species with the lowest R* should competitively exclude all other species, given enough time and a constant environment.

In deriving their version of the R*-rule, Hansen and Hubbell (1980) assumed that two competitors are grown in a continuous culture with a continuous input of a nutrient (R) as well as an effluent rate, which is equivalent to a death rate, m.

The growth rates for two competing species were defined as...

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bi = maximum cell division rate (= rmax)

R = the concentration of the one limiting resource in the culture

Ki = half saturation constant for the limiting resource

m = death rate, here due to outflow

Ni = concentration of cells in the culture (population size)

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If we do an equilibrium analysis, and set dNi/dt = 0, the result is:

If we set Ki = R, then bi /2 = m

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Thus one solution is that growth stops when the concentration R equals the half-saturation constant

Conclusions:

(i) all competitors die out, or

(ii) one species survives while the second species dies out – that is, when competitive exclusion occurs

Which species survives depends on the critical parameter, R*, which we already saw in the equation above as R* = mKi /(b − m)

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Example of R* calculations based on Hansen and Hubbell (1980)

K,halfsaturation constant; m, mortality rate; b, maximal growth rate; ra, actual growth rate = b − m.

R* = mKi /(b − m) = mKi /ra

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Part II: Interspecific interactions -spatial competition and colonization

The idea that multiple species can coexist in a community without yielding to the superior competitors can traced to the competition–colonization trade-off idea first proposed by Levins

Recall that in a metapopulation, two species can coexist if one is a superior competitor and the other is a better colonize

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Remember also that in a metapopulation the increase in the proportion, P, of sites occupied by a species was based on the colonization rate, cP, times the proportion of sites occupied and available (1 − P), minus the local extinction or mortality rate, εP

When the equation below is set equal to zero and we solve for P:

we have the proportion of habitat sites occupied at equilibrium:

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The colonization rate necessary for equilibrium is then:

This basic idea has been generalized to multi-species situations by Tilman (1994) and others

Termed the “spatial-competition hypothesis,” this theory proposes stable coexistence for inferior competitors in a diverse community

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Part II: Interspecific interactions -evidence of competition in nature

The classic experimental demonstration of competition in the field was done by Joseph Connell (1961) on the barnacle species Chthamalus stellatus and Balanus balanoides

Balanus is consistently found on lower rock surfaces, usually near mean tide level or slightly above

Chthamalus, however, is found on the upper rocks, between mean high neap tide and mean high spring tide

While the adults of these two barnacle species have non-overlapping distributions, the larvae of both species settle over a wide variety of rock surfaces, showing a great deal of overlap

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The question Connell posed was, is the distribution of adults the result of competition, or is there a difference in the fundamental niches of the two species?

Connell performed a variety of experiments in which he moved the barnacles to different levels of the intertidal zone. He also experimentally removed one species or the other where the two were growing together, and observed theresults of putting the two species together.

He found that whenever he removed Balanus, Chthamalus was able to survive in the lower regions of the intertidal zone.

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However, in the presence of Balanus, Chthamalus was overgrown and eventually displaced.

In the upper regions of the intertidal zone, however, Balanus was unable to survive the long exposures to air during low tides.

Since Chthamalus was able to survive this exposure, it survives inthe upper intertidal zone.

Thus the two species occupy mutually exclusive microhabitats due to a combination of competition and differences in their fundamental niches.

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Competition in ants

Because both worker and soldier ants are numerous, easy to observe, and usually diurnal, aggressive interactions among ant species, demonstrating interference competition, can be documented throughout the world (Holldobler and Wilson 1990).

Placing a food bait of tuna or sugar water will provoke competitive interactions in a matter of minutes to hours.

Once bait is put out in the West Indies, where there are few ant species, there is a kind of predictable sequence, reminiscent of ecological succession (a kind of “ant succession”).

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Competition in ants

As described by Holldobler and Wilson (1990), first to arrive are workers of Paratrechina longicornis, known locally as “hormigas locas”(crazy ants).

These workers are very adept at locating food and often are the first to arrive at newly placed baits.

They fill their crops rapidly and hurry to recruit nestmates with odor trails laid from the rectal sac of the hindgut.

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Competition in ants

But they are also very timid in the presence of competitors. As soon as more aggressive species begin to arrive in force, the Paratrechina withdraw and search for new, unoccupied baits.

Paratrechina is an example of an “opportunist” species. They are poor com-petitors, but excellent dispersers.

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Competition in ants

Holldobler and Wilson also emphasize that territorial fighting and “ant wars” are common, especially among species with large colonies.

Numerous cases have been documented in which introduced ant species have eliminated other species over a few years’ time.

For example, on Bermuda Iridomyrmex humilis has been replacing Pheidole megacephala since the former was introduced in 1953, although the two species may be reaching equilibrium short of extinction of Pheidole (Lieberburg et al. 1975).

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Competition in ants

As a final example, the red imported fire ant (Solenopsis invicta) has virtually eliminated the native fire ant (S. xyloni) from most of its range in the United States (Holldobler and Wilson 1990).

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Part II: Interspecific interactions -natural experiments

maniplative field experiments have some drawbacks

(i) The outcome of the experiment often varies from year to year and season to season since weather and predators are uncontrolled.

(ii) Most field experiments are not run for enough time. This deficiency is, however, being remedied. For example, the National Science Foundation (NSF) is addressing this problem in its Long Term Ecological Studies (LTER) program.

(iii) The importance of large temporal and spatial scales cannot be addressed in contemporary time and space.

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maniplative field experiments have some drawbacks

(iv) A manipulation of two species may incorrectly ignore the importance of a third species.

(v) The kinds of experiments that might reveal important information, such as theremoval or introduction of a species in an ecosystem, are often “technically impossible, morally reprehensible and politically forbidden” (Diamond 1983).

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In order to solve these problems, Diamond (1983) extolled the virtues of “natural experiments” and other kinds of data gathered from field observations as opposed to experiments.

According to Diamond, natural experiments have three advantages:

First, they permit an ecologist to rapidly gather data. As an example, he described the work of Schoener and Toft (1983). They surveyed spider population on 92 small Bahamian islands, 48 of which lacked lizards and 26 of which were occupied by at least one species of lizard.

They found that spiders were ten times more abundant on the islands without lizards. The explanation was that lizards are both competitors with and predators on spiders.

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Diamond’s point, however, was that this natural experiment (lizards present on some islands, absent on others), would have been very difficult and time consuming to set up, and we would have waited a very long time (up to several years) before the spider populations reached new equilibrium values.

Using the natural experiments, Schoener and Toft completed their fieldwork in 20 days!

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Second, natural experiments allow ecologists to examine situations they would not be allowed to set up experimentally.

It is likely, for example, that the Bahamian government would have objected to having lizards removed from 48 islands.

In another example, Brown (1971) has shown that two species of chipmunk (genus Eutamias) divide the forest by altitude when they are sympatric on mountains in the Sierra Nevada range.

But on several mountains, probably due to chance colonization or extinction events, only one species is present.

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When only one species occupies the mountain, without its competitor, it is found at all elevations.

A field experiment, in which one species or the other was eliminated from an entire mountain, would never have been approved by the US Fish and Wildlife Service or by any granting agency.

Yet this natural experiment is an elegant demonstration of the phenomenon known as ecological release.

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Ecological release

In ecological release, a species occupies a broader niche or geographical area in the absence of a closely related competitor.

An example is the distribution of two species of Planaria in streams.

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Ecological release

When found alone in a stream (allopatric distribution) each species occupies a wide range of stream temperatures.

When both species are found in the same stream (sympatric distribution), however, the distribution of both species is restricted.

P. montenegrina is found from 5 to about 13.5°C, whereas P. gonocephala occupies the warmer portions of the stream from 13.5 to approximately 23°C (Beauchamp and Ullyott 1932).

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Niche partitioning

In niche partitioning, two or more species coexist while sharing one or more resources in such a way that the niche overlap apparently violates the competitive-exclusion principle.

Upon closer investigation, the resources, though shared, are used with different frequencies or are used in different ways so as to allow coexistence.

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Niche partitioning

For example, the root systems of coexisting annual plants can be shown to partition the soil by depth, thereby avoiding direct resource competition (Wieland and Bazzaz 1975).

In his classic study, MacArthur (1958) showed that five species of Dendroica warblers coexisted by foraging in different portions of trees in a coniferous forest.

Although there was overlap, each species spent the majority of its foraging time in a unique portion of the trees.

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Character dispalcement

Character displacement is defined as a situation in which two species, when living in separate geographical ranges (allopatric distributions), have nearly identical physical characteristics (i.e., beak sizes in birds, overall body sizes in lizards and snails, canine sizes in the cat family).

When sympatric, however, these physical or morphological characteristics diverge in one or both species.

This divergence minimizes competition for food and allows the two species to coexist.

Brown and Wilson (1956) appear to have introduced this idea.

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Character dispalcement

When examining the overall size and beak lengths of specimens of the eastern(Sitta tephronota) and western rock nuthatches (S. neumayer), they found that the allopatric populations were almost identical in both average size and in the range of sizes.

However, these two species become sympatric in Iran.

In sympatry, the eastern rock nuthatch is larger, while the western species has become smaller.

In this sympatric zone their beak and body sizes are completely non-overlapping.

This allows them to feed on different-sized prey items and therefore coexist.

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61BL3313 Population and Community Ecology Lecture 09 Interspecific competition Spring 2013 Dr Ed Harris