Ecology Competition

7/31/2019 Ecology Competition

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11Competition


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11 Competition

Case Study:Competition in Plants thatEat Animals

Competition for Resources

General Features of Competition Competitive Exclusion

Altering the Outcome of Competition

Case Study Revisited

Connections in Nature:The Paradox ofDiversity


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Case Study: Competition in Plants that Eat Animals

Charles Darwin was the first to provideclear evidence of carnivory in plants.

Plants use a variety of mechanisms to

eat animals.

The Venus flytrap has modified leaves

that attract insects with nectar. Theinner surface has touch-sensitive hairs;if an insect trips those hairs, the leafsnaps shut in half a second.


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Figure 11.1 A Plant that Eats Animals


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Pitcher plants lure insects into a pitcher-shaped trap.

The inside of the pitcher has downward-

facing hairs, which make it easy for theinsect to crawl in, but hard to crawl out.

About halfway down, many pitchers have alayer of wax that sticks to the insectsfeet, causing it to tumble into a vat thatcontains water or digestive juices.


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Why do some plants eat animals?

Competition among plants can beintense where soil nutrients are scarce.

In nutrient-poor environments, carnivoryin plants has evolved multiple times.

Carnivory may be an adaptation to low-nutrient environments, to avoidcompeting with other plants.


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In experiments with pitcher plantsSarracenia alata, Brewer (2003) removednoncarnivorous competitor plants. Somepitcher plants were also deprived of prey

(starved).

Growth rates increased when competitorswere removed.

But with neighbors intact, and pitcherscovered, the growth rate was not reducedas expected.


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Figure 11.2 Competition Decreases Growth in a Carnivorous Plant


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Introduction

A. G. Tansley did one of the firstexperiments on competition in 1917.

He wanted to explain the distribution of

two species of bedstraw: Galiumhercynicum, which was restricted toacidic soils, and G. pumilum, restricted to

calcareous soils.


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Introduction

Tansley found that if grown alone, eachspecies could survive on both acidic andcalcareous soils.

But when grown together, soil typedetermined which would survive.

Tansley inferred that competitionrestricted the two species to particularsoil types in nature.


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Introduction

Interspecific competition is aninteraction between two species in whicheach is harmed when they both use the

same limiting resource.Intraspecific competition can occurbetween individuals of a single species.


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Organisms compete for resourcesfeatures of the environment that arerequired for growth, survival, orreproduction, and which can beconsumed to the point of depletion.

Concept 11.1: Competition occurs betweenspecies that share the use of a resource thatlimits the growth, survival, or reproduction ofeach species.


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Examples of resources that can beconsumed to depletion:

Food.

Water in terrestrial habitats. Light for plants.

Space, especially for sessile

organisms. For mobile animals, space for refuge,nesting, etc.

S C


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Figure 11.3 Space Can Be a Limiting Resource


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Species are also influenced by factorsthat are not consumed, such astemperature, pH, salinity.

These factors are not considered to beresources.

Physical factors affect populationgrowth rates but are not consumed ordepleted.


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Experiments using two species ofdiatoms (single-celled algae that makecell walls of silica, SiO2) were done byTilman et al. (1981).

When each species was grown alone, astable population size was reached andsilica concentrations were reduced.

When grown together, the two speciescompeted for silica, and one speciesdrove the other to extinction.

Fi 11 4 C ti O i C D l t R (P t 1)


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Figure 11.4 Competing Organisms Can Deplete Resources (Part 1)

Fi 11 4 C ti O i C D l t R (P t 2)


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Figure 11.4 Competing Organisms Can Deplete Resources (Part 2)


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Competition should increase in intensitywhen resources are scarce.

Competition in plants might be expectedto increase in importance when theyare growing in nutrient-poor soils.

Using a perennial grass species, Wilsonand Tilman (1993) were able todemonstrate this.


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The grass species was transplanted intoplots that had been growing with andwithout nitrogen fertilizer added.

Each plot type had 3 treatments:

1. Neighbors left intact.

2. Neighbor roots left intact but neighborshoots tied back.

3. Neighbor roots and shoots both removed.


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Treatment 1 would include bothaboveground and belowgroundcompetition, which did not differ

between the two plot types.

Belowground competition (treatment 2)was most intense in the nitrogen-limited

plots.

Figure 11 5 A Resource Availability Affects the Intensity of Competition


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Figure 11.5 A Resource Availability Affects the Intensity of Competition


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Aboveground competition was estimatedby subtracting competition in treatment2 from competition in treatment 1.

Aboveground competition for lightincreased when light levels were low.

Figure 11 5 B Resource Availability Affects the Intensity of Competition


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Figure 11.5 B Resource Availability Affects the Intensity of Competition


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How important is competition in ecologicalcommunities?

Results from many studies have been

compiled and analyzed to answer thisquestion.

Schoener (1983) found that of 390 speciesstudied, 76% showed effects ofcompetition under some conditions; 57%showed effects under all conditions tested.


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Connell (1983) found that competitionwas important for 50% of 215 species in72 studies.

Gurevitch et al. (1992) analyzed themagnitude of competitive effects foundfor 93 species in 46 studies. They

showed that competition had significanteffects on a wide range of organisms.


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Potential biases in these analyses includefailure of researchers to publish studiesthat show no significant effects, and a

tendency for investigators to studyspecies they suspect will showcompetition.

Still, they document that competition iscommon, though not ubiquitous.


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General Features of Competition

As far back as Darwin, competitionbetween species has been seen as aninfluence on evolution and species

distributions.

Concept 11.2: Competition, whether direct orindirect, can limit the distributions andabundances of competing species.


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Exploitation competition:Speciescompete indirectly through their mutualeffects on the availability of a shared

resource.Competition occurs simply becauseindividuals reduce the availability of a

resource as they use it.

Examples: The pitcher plants and thediatoms


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Interference competition: Speciescompete directly for access to aresource.

Individuals may perform antagonisticactions (e.g., when two predators fightover a prey item, or voles aggressively

exclude other voles from preferredhabitat).


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Interference competition can also occur insessile species.

Example: The acorn barnacle oftencrushes or smothers nearby individualsof another barnacle species as it grows.As a result, it directly prevents the other

species from living in most portions of arocky intertidal zone.


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Allelopathy: A form of interferencecompetition in which individuals of onespecies release toxins that harm other

species.Spotted knapweed, an invasive plant inNorth America, has been very

successful and caused great economicdamage to rangeland.


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Cattle do not eat spotted knapweed,giving it an edge over native plants thatcattle do eat.

It also releases a toxin called catechininto surrounding soils, which has beenshown to reduce germination and

growth of native grasses.

Figure 11.6 Chemical Warfare in Plants (Part 1)


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g ( )

Figure 11.6 Chemical Warfare in Plants (Part 2)


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g ( )


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For a resource in short supply,competition will reduce the amountavailable to each species.

In many cases the effects of competitionare unequal, or asymmetrical, and onespecies is harmed more than the other.

Example: When one species drivesanother to extinction.


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Competition can also occur betweendistantly related species.

In experiments with rodents and ants that

eat the same seeds, Brown andDavidson (1977) set up plots with fourtreatments:


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1. Wire mesh fence excluded seed-eatingrodents.

2. Seed-eating ants were excluded by

applying insecticides.

3. Both rodents and ants were excluded.

4. Undisturbed control plots.


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Where rodents were excluded, antcolonies increased by 71%.

Where ants were excluded, rodents

increased in both number and biomass.

Where both were excluded, the number

of seeds increased by 450%.

Figure 11.7 Ants and Rodents Compete for Seeds


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When either rodents or ants wereremoved, the group that remained ateroughly as many seeds as rodents and

ants combined ate in the control plots.In natural conditions, each group wouldbe expected to eat fewer seeds in the

presence of the other group than it couldeat when alone.


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Competition can also limit distribution andabundance of species.

Connell (1961) examined factors that

influenced the distribution, survival, andreproduction of two barnacle species,Chthamalus stellatusand Semibalanus

balanoides, on the coast of Scotland.


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Distribution of larvae of the two speciesoverlapped throughout the upper andmiddle intertidal zones.

Adult distributions did not overlap:Chthamaluswere found only near thetop of the intertidal zone; adultSemibalanuswere found throughout the

rest of the intertidal zone.

Figure 11.8 Squeezed Out by Competition


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Using removal experiments, Connellfound that competition with Semibalanusexcluded Chthamalusfrom all but thetop of the intertidal zone.

Semibalanussmothered, removed, orcrushed the other species.

However, Semibalanusdried out andsurvived poorly at the top of the intertidalzone.


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Competition can also affect geographicdistribution.

A natural experimentrefers to a situation

in nature that is similar in effect to acontrolled removal experiment.


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Chipmunk species in the southwesternU.S. live in mountain forests.

Patterson (1980, 1981) found that when a

chipmunk species lived alone on amountain range, it occupied a broaderrange of habitats and elevations than

when it lived with a competitor species.

Figure 11.9 A Natural Experiment on Competition between Chipmunks


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Competitive Exclusion

If the overall ecological requirements of aspeciesits ecological nicheare verysimilar to those of a superior competitor,

that competitor may drive it to extinction.

Concept 11.3: Competing species are morelikely to coexist when they use resources indifferent ways.


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In the 1930s, G. F. Gause performedlaboratory experiments on competitionusing three species of Paramecium.

Populations of all three Parameciumspecies reached a stable carryingcapacity when grown alone.

When paired, some species drove othersto extinction.

Figure 11.10 Competition in Paramecium(Part 1)


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Figure 11.10 Competition in Paramecium(Part 2)


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C i i E l i


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P. aureliadrove P. caudatumtoextinction. They may have been unableto coexist because both fed on bacteria

floating in the medium.P. caudatumand P. bursariawere able tocoexist, although they were clearly in

competitionthe carrying capacity ofboth species was lowered.

C titi E l i


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P. caudatumusually ate bacteria floatingin the medium, while P. bursariausuallyfed on yeast cells that settled to the

bottom.Unless two species use availableresources in different ways, one can go

extinct.

C titi E l i


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The competitive exclusion principle:Two species that use a limiting resourcein the same way can not coexist.

Field observations are consistent with thisexplanation of why competitiveexclusion occurs in some cases, but not

others.

C titi E l i


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Resource partitioning:Species use alimited resource in different ways.

Example: Four species of Anolislizards

on Jamaica live together in trees andshrubs and eat similar food.

Schoener (1974) found that the lizardsused the space in different ways,resulting in a reduction in competition.

Figure 11.11 Resource Partitioning in Lizards


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C titi E l i


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Competition was first modeled by A. J.Lotka (1932) and Vito Volterra (1926).

Their equation is now known as the

Lotka

Volterra competition model.

2

12

22

2

1

21

11

1

1

1

K

NNNr

dt

dN

K

NNNr

dt

dN

Competiti e E cl sion


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N1 = population density of species 1

r1 = intrinsic rate of increase of species 1

K1 = carrying capacity of species 1

and = competition coefficientsconstants that describe effect of one

species on the other:

B 11 1 Wh t D th C titi C ffi i t d R t?


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Box 11.1 What Do the Competition Coefficients and Represent?

is the effect of species 2 on species 1; is the effect of species 1 on species 2.

measures the extent to which the use of

resources by an individual of species 2decreases the per capita growth rate ofspecies 1.

When = 1, individuals of the two

species are identical in their effects.

B 11 1 Wh t D th C titi C ffi i t d R t?


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Box 11.1 What Do the Competition Coefficients and Represent?

When < 1, an individual of species 2decreases growth of species 1 by asmalleramount than does an individual

of species 1.When > 1, an individual of species 2

decreases growth of species 1 by a

largeramount than does an individual ofspecies 1.



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The LotkaVolterra model supports theidea that competitive exclusion is likelywhen competing species require very

similar resources.The model can be used to predictchanges in the densities of species 1

and 2 over time. Then those changescan be related to the way in which eachspecies uses resources.

Bo 11 2 When Do Completing Pop lations Stop Changing in Si e?


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Box 11.2 When Do Completing Populations Stop Changing in Size?

Population density of species 1 does notchange over time when dN1/dt= 0.

This can occur when

rearranging:

01

1

21

K

NN

1

1

2

1N

KN

Box 11 2 When Do Completing Populations Stop Changing in Size?


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Box 11.2 When Do Completing Populations Stop Changing in Size?

Using a similar approach for species 2,we find that dN2/dt= 0 when

These two equations describe straight

lines written with N2 as a function of N1.

122 NKN

Figure 11.12 Graphical Analyses of Competition


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The straight lines are zero populationgrowth isoclines: The population doesnot increase or decrease in size for any

combination of N1 and N2 that lies onthese lines.

Zero growth isoclines can determine the

conditions under which each species willincrease or decrease.



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This graphical approach can be used topredict the end result of competitionbetween species.

The N1 and N2 isoclines are plottedtogether. There are four possible waysthat the N1 and N2 isoclines can be

arranged relative to each other.

Figure 11.13 A, B Outcome of Competition in the LotkaVolterra Competition Model


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When the isoclines do not cross,competitive exclusion results.

Depending on which isocline is above the

other, either species 1 or species 2always drives the other to extinction.

Figure 11.13 C, D Outcome of Competition in the LotkaVolterra Competition Model


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In only one case, the two species coexist.

Although in this case, competition still hasan effect: The final or equilibrium density

of each species is lower than its carryingcapacity.



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Coexistence occurs when the values of ,, K1, and K2 are such that the followinginequality holds:

If and are equal, and close to 1, thespecies are equally strong competitors,and have similar effects on each other.

1

2

1

K

K



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Example: If = = 0.95

Coexistence is predicted only within anarrow range of values for the carryingcapacities, K1 and K2.

053.195.0

2

1 K

K



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Example: If = = 0.1

Coexistence is predicted within a muchbroader range of carrying capacities.

101.02

1

KK



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Environmental conditions can results in acompetitive reversalthe species thatwas the inferior competitor in one habitat

becomes the superior competitor inanother.

Concept 11.4: The outcome of competitioncan be altered by environmental conditions,species interactions, disturbance, andevolution.



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Example: Presence of herbivores canlead to competitive reversals.

When ragwort flea beetles were

introduced to western Oregon, thebiomass of ragwort, an invasive species,decreased, and its competitor speciesincreased.

In the absence of the flea beetles,ragwort is a superior competitor.

Figure 11.14 Herbivores Can Alter the Outcome of Competition


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te g t e Outco e o Co pet t o

Disturbances such as fires or storms cankill or damage individuals, while creatingopportunities for others.

Example: Some forest plant speciesrequire abundant sunlight and are foundonly where disturbance has opened thetree canopy.

As trees recolonize and create shade,these plants can not persist in the patch.



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g p

Such species are called fugitive speciesbecause they must disperse from oneplace to another as conditions change.

The brown alga called sea palm coexistswith mussels, a competitively dominantspecies, in the rocky intertidal zone

because large waves sometimesremove the mussels, creating temporaryopenings.



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g p

On shorelines with low disturbance rates,competition runs its course, andmussels drive sea palms to extinction.

Figure 11.15 Population Decline in an Inferior Competitor


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g p

Competition has the potential to causeevolutionary change, and evolution hasthe potential to alter the outcome ofcompetition.

This interplay has been observed in manystudies.


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Figure 11.16 A Competitive Reversal (Part 1)


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Figure 11.16 A Competitive Reversal (Part 2)


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Individuals were also tested for signs ofevolutionary change.

Blowflies raised in competition with

houseflies had evolved to becomesuperior competitors and alwaysoutcompeted the houseflies.

The underlying mechanisms of this andthe associated genetic changes are notknown.



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Natural selection can influence themorphology of competing species andresult in character displacement.

Natural selection results in the forms ofcompeting species becoming moredifferent over time.

Figure 11.17 Character Displacement


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In two species of finches on theGalpagos archipelago, the beak sizes,and hence sizes of the seeds the birdseat, are different on islands with bothspecies.

On islands with only one of the species,

beak sizes are similar.

Figure 11.18 Competition Shapes Beak Size (Part 1)


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Figure 11.18 Competition Shapes Beak Size (Part 2)


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Experimental studies have alsodemonstrated character displacement.

The morphology of sticklebacks (fish)

varies the most when different specieslive in the same lake.

Individuals whose morphology differed

considerably from their competitors grewmore rapidly than did those withmorphology similar to that of theircompetitors.

Figure 11.19 An Experimental Test of Character Displacement


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Case Study Revisited: Competition in Plants that Eat Animals


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In the experimental studies on pitcherplants (S. alata), the results suggestedlittle competition between the pitcherplant and its noncarnivorous neighborsfor soil nutrients.

But competition for light was more

important. When shaded by neighbors,pitcher height increased at the expenseof pitcher volume.



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When neighbors were removed, S. alatagrowth rate increased, but only whenthey were able to capture animal prey.

When neighbors were left intact, lightavailability had no effect on S. alatagrowth rates when prey were excluded.

When prey was available, growth rateincreased as light increased.

Figure 11.20 Interaction between Light and Prey Availability


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S. alatacompetes with its neighbors forlight but avoids competition for soilnutrients by eating animal prey.

When light levels are low, S. alatagrowslittle and requires few nutrients, thusprey deprivation has little effect.

In high light levels, S. alatagrows moreand requires nutrients, thus preydeprivation matters.

Connections in Nature: The Paradox of Diversity


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In spite of competition, naturalcommunities contain many speciessharing scarce resources.

Resource partitioning is one explanationfor this.

Other mechanisms include environmentalvariation and disturbance. Species maycoexist if different species are superiorcompetitors under differentenvironmental conditions.



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In the pitcher plant studies, Brewerwanted to know whether resourcepartitioning in the form of differentmethods of nutrient acquisition couldexplain the coexistence of carnivorousand noncarnivorous plants.



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When pitcher plants were deprived ofprey, they should have experiencedmore severe competitive effects, orcompensated for reduced nutrients byincreasing production of roots orpitchers.

Neither of these outcomes occurred.



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S. alatais tolerant of fire and useschanges in light levels as a cue forgrowth.

Itgrows primarily when its competitorsare absent or reduced (e.g., after a fire).

This growth strategy may allow S. alatato

persist with noncarnivorous plants thatcan outcompete it for both light andscarce soil nutrients.

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