2 Competition

Chapter 13 Competition 

 • Forms of Competition  

• Evidence for Competition in Natural Systems  

• Mathematical and laboratory Models  

• Competition and Niches   

Chapter Concepts  

• 13.1 Individuals can compete with other individuals, of their own and of different species, in a number of different ways.  

• 13.2 Field and mesocosm studies show that resource limitation and competition are widespread.  

• 13.3 Mathematical and laboratory models provide a theoretical foundation for studying competitive interactions in nature.  

• 13.4 Competition can have significant effects on species coexistence and the direction of evolution 

Ecological Interactions  

• 9 categories of interactions among organisms.  

• Based on whether interaction is positive, neutral, or negative to each participant 

REALLY EASY QUESTION TO GIVE ON FINAL 

"What effect does the interaction with species A have on species B" 

	Positive (+)	Neutral (O)	Negative(-)
Positive (+)	mutualism	commensalism	Exploitation (predation, parasitism, herbivory for example)
Neutral (O)	-------	Neutralism	Amensalism
Negative (-)	--------	--------	competition

Down the column is "What effect does the interaction with species B have on species A?" 

Forms of Competition  

• Intraspecific: competition between individuals of the same species.  

• Interspecific: competition between individuals of two species.  

• Resource limitation: limited supply of food, space, etc. leads to:  

-Interference competition: direct aggressive interaction between individuals. 

 -Exploitative competition: competition to secure resources first. 

Sexual Selection – from lecture 8  

• Two forms of sexual selection  

• Intrasexual Selection: Individuals of one sex compete among themselves for mates. • Intersexual Selection: Individuals of one sex consistently choose mates among members of opposite sex based on a particular trait.  

Forms of Competition cont’d  

• Interference competition in damselfish.  

• Territorial for shelter areas in coral. Fish fight aggressively for access to vacant territory.  

Figure caption of image:Territorial reef fish can compete intensely for space 

 

• Exploitative competition in white pines.  

• Young trees grow much faster in plots where root area of larger trees experimentally limited. 

Figure caption of image: Competition in a forest can be as intense as competition on a coral reef. However, much of the competition in a forest takes place underground, where the roots of plants compete for water and nutrients.  

interference competitions vs exploitative competition

Forms of Competition: Interference Competition 

Interference competition is a direct interaction between individuals 

Individual A has a negative effect on individual B and vice versa 

Forms of Competition: Exploitative Competition 

Exploitative competition is an indirect interaction between individuals 

Individuals harm each other by taking up resources before others are able to. 

(Sharing limited resource eg nitrogen) Both indiviuals draw resources from a shared and limited resource pool. 

The more resources an individual has the higher its fitness 

Three Sisters – Indigenous teaching – from lecture 2

● Corn provides tall stalks for the beans to climb so that they are not out-competed by sprawling squash vines. ● Beans provide nitrogen to fertilize the soil while also stabilizing the tall corn during heavy winds. ● Beans are nitrogen-fixers meaning they host rhizobia on their roots that can take nitrogen, a much needed plant nutrient, from the air and convert it into forms that can be absorbed by plant roots. ● The large leaves of squash plants shade the ground which helps retain soil moisture and prevent weeds.  

 

Evidence of Competition in Natural Settings 

 • Intraspecific Competition in Plant Populations  

• Self-thinning: as stand of trees develop, more and more biomass composed of fewer and fewer individuals 

COPY DIAGRAM INFO slide 11 

Logistic Population Growth – from lecture 12 

DIAGRAM INFO slide 12 

Evidence of Competition in Natural Settings cont’d • Intraspecific Competition in Plant Populations • Self-thinning rule: plot of average mass of individual plant with density, gives slope ~(-3/2). 

COPY DIAGRAM INFO SLIDE 13 

Intraspecific Competition Among Planthoppers  

• Competition in many herbivorous insects has been difficult to show.  

• However, repeatedly demonstrated in Homoptera  

• Prevalence of competition in Homoptera may be due to their habit of aggregating, rapid growth, and the mobile nature of the plant fluids they eat.  

-Demonstrated intraspecific competition within populations of planthopper Prokeleosia marginata.  

           -Probably result of limited resources (the food that will be limiting, the fluid within the plant) 

• Enclosed planthoppers (P. marginata) in cages with seedlings 

As the population density of the planthopper was increased, the following was observed: lower survivorship, increased development time, reduced body size 

Interference Competition Among Song Sparrows  

• Study of male song sparrows (Melospiza melodia) on Mandarte Island (BC).  

-Males territorial; sing and display to attract females.  

-There are more males than territories, so ~20% of males do not have a territory. Called floaters and hide in territories of other males.  

-If territorial male sees floater on his territory, produces song and chases floater.  

• Are some territorial males more likely to be intruded than others? 

Intrusion pressure by floaters varies as a function of the age of the territorial male 

Higher values mean more intrusions by floaters 

The very young and the very old males experiences the most intrusions 

Males of middle ages were least likely to be challenged 

Waiting to increase the chances of survival 

Is Competition a Common Ecological Interaction?  

• In early 1980s, importance of competition in natural populations was hotly debated topic.  

• Evidence reviewed by:  

-Schoener: concluded competition found 90% of studies and 76% of species.  

-Connell: found evidence of competition in 40% of studies and 50% of species.  

-More recent analysis by Gurevitch (1992) suggests apparent discrepancy due to variation among taxa and trophic levels.  

• Overall best evidence suggests competition important factor in natural systems generally 

This shows that how you interpret competition can change how you see competition happening 

Is Competition a Common Ecological Interaction? Cont’d  

• Plant ecologists were certain about importance of competition, but debated whether competition strongest when resources rare or common.  

-Grime (1973) argued competition among plants will be unimportant in areas of low resource.  

-Newman (1973) and Tilman (1987) argued competition will be important at both high and low resource levels, but will be mostly below-ground competition when resources low and above-ground competition at high resource levels. 

b) Plant Life Histories – from lecture 9  

• Grime (1977, 1979)  

• Three important variables exerting selective pressures in plants are: 

 • Intensity of disturbance  

• Intensity of abiotic stress  

• Intensity of competition (when stress and disturbance are low)  

Disturbance: (i.e., processes that destroy biomass)  

Stress: (i.e., external constraints that limit the rate of production 

From diagram : competitive species such as birch predominate under conditions of low disturbancec and low stress 

Stress tolerant species predominate under conditions of low disturbance and high stress (Beech for example) 

Ruderals are dominant under conditions of high disturbance and low stress (annual grass for example) 

is Competition a Common Ecological Interaction? Cont’d  

• Role of resource level on strength of competition in plants.  

From diagram: Grime hypothesized that competition is highest in the most productive areas 

Newman and others hypothesized that competition in general is constant across a productivity gradient....but competition for soil resources decreases....and competion for light increases 

Is competition a common ecological interaction continued

• Global, collaborative focal plant studies from around the world found no evidence that competition varies with resource level. 

Diagram info: Compeition was variable at low productivities. But there was no overall relationship between competition on productivity. The numbers represent different study sites. 

Reader and colleagues (1994) found no evidence that the strength of competition varied as a function of habitat productivity. Note that the different numbers in the graph indicate data drawn from the twelve study sites, and that there are often multiple samples per site.  

Mathematical and Laboratory Models  

• Provide theoretical foundation for study of competitive interactions in nature. Modelling Interspecific Competition 

 • Try to predict how interspecific competitors influence population growth rates of other species.  

• Approach: take model we have and add term to account for new factor. e.g. we added term for intraspecific competition to exponential model to get logistic model. 

-Now, add term for interspecific competition. 

Modelling Interspecific Competition Lotka-Volterra Model of Interspecific Competition:  

• Logistic equations for 2 populations (denoted by subscripts 1 and 2) 

• Here, level of intraspecific competition expressed as ratio of population size to carrying capacity (N/K). Based on idea that resources decline as population gets larger, leading to increased competition for resources. 

• Similarly, we expect resources will decrease as result of interspecific competition. 

Lotka-Volterra Model of Interspecific Competition  

• Introduce competition coefficients:  

• α12: the effect that one individual of species 2 has on population growth rate of species 1  

• α21: the effect that one individual of species 1 has on population growth rate of species 2

• In general, model predicts coexistence can occur if interspecific competition is weaker than intraspecific competition for both species.  

• To see this, consider both populations when they stop growing by setting dN/dt =0  

If rmax or N=0 , population already extinct; trivial solution we can ignore. We just need to determine when that occurs. 

• Doing this for both populations we predict population growth stops when: 

• These equations are simple lines that indicate lines along which population growth is zero: isoclines of zero population growth 

Graphical Representation of Lotka-Volterra Competition Model 

Diagram info The zero growth isocline for species 1 

When N1 falls to the right of the isocline, the environment cannot surrpot the entire poplatoin of species 1 and N1 decreases 

When N1 falls to the left of isocline the environment can support more of species 1 and N1 grows 

When N1 falls on the isocline, the encironment is at capacity with respect to species 1. N1 neither increases nor decreases. 

the zero growth isocline for species 2 

When N2 falls below isocline N2 increases 

When N2 falls above the isocline N2 decreases 

When N2 falls on the isocline it neither increases nor decrease

Lotka volterra competition model 

Depending on whether or not the resources can sustain your populations and they can remain stable or not 

LOOK at slide 32 and 33 when studying' 

Laboratory Models of Competition  

• Experiments with Paramecia  

• Gause demonstrated resource limitation with Paramecium caudatum and Paramecium aurelia in presence of two different concentrations of Bacillus pyocyaneus.  

-When grown alone, carrying capacity determined by intraspecific competition.  

-When grown together, P. caudatum quickly declined.  

     -Reduced resource supplies increased competition. 

Laboratory Models of Competition cont’d   paramecia examples

• Laboratory Models of Competition  

• Experiments with Paramecia  

• Species grown alone. 

Diagram info: 

P.aurelia attained greater population size than P.caudatum both..... when growth in half strength growth medium....and when grown in full-strength growth medium 

 

• Experiments with Paramecia  

• Species grown in competition.  

• Competitive exclusion of P. caudatum  

Diagram info: When grown together, P. aurelia survives both at low and high resource supply...while P.caudatum quickly declines 

Competitive Exclusion and Mechanisms of Coexistence

• Competitive exclusion principle: complete competitors cannot coexist.  

• Some potential mechanisms of coexistence:  

-Spatial heterogeneity in strength of competition.  

-Non-equilibrium conditions  

-Incomplete exploitation of resources  

-Competitive equivalence.  

-Variation in competitive ability within species 

Like running a race and you're going at the same speed, you wont win but you wont lose 

But if you run against Usain bolt you will lose, he will outcompete you? 

Mechanisms of Coexistence: Variation in competitive ability within species. 

• Tribolium (flour beetles) live in flour. In some environments, T. castaneum competitively excludes T. confusum 

Diagram info 

When grown separately at 34 degrees C and 70% relative humidity, populations of T.confusum and T. castaneum both did well. 

When grown together at 34 degrees selcius and 70% relative humidity T.confusum died off after 430 days, while T. castaneum persisted. 

Competition depends on what you're looking at and what resource you're competing for 

• In other environments, T. confusum competitively excludes T. castaneum. 

Diagram info: 

Grown separetly at 24 degrees and 30% relative humidity T confusum populations did well, while T. castaneum populations died off in about 500 days. 

Grown together at 24 degrees celcius and 30% relative humidity T. castaneum populations died off in less than 400 days, while T. confusum persisted

Competition and Niches  

• Fundamental niche: range of conditions under which species can survive.  

• Realized niche: range of conditions under which species typically found.  

• Competition can have short-term effects on species’ distributions by restricting realized niche.  

• Strong and pervasive competition may lead to evolutionary response of change in dimensions of fundamental niche.  

Graph stuff: The species can potentially persist only in this combination of conditions 

In the presence of a competitor, a species can often only persist near optimum growth conditions. As a result, the edges of the conditions defined by the fundamental niche, though suitable for growth without competition, are often unsuitable with compeition. This reduced area is the realized niche. 

Figure captionTwo dimensions of a (a) fundamental niche, and (b) realized niche of a hypothetical species. The niches of most species will be defined by a larger number of dimensions.  

Niches and Competition Among Plants  

• Created mesocosms  

• Competition restricts realized niche for each of two species of bedstraw (Galium spp.) to a narrower range of soil types than used when no competition 

 

Though G.saxtile is largely confined to acidic soils and G. sylvestre to pasic soils, the seeds of both species germinate on both soils 

Barnacle competition from lecture 10

Barnacle competition – From lecture 10 The battle for space between barnacles 

Competition for space between barncales Balanus balenoids and Chthamalus stellatus is of the interference type, and results eventually in the smaller species being displaced (Chthamalus) 

Niche Overlap and Competition Between Barnacles  

• Lower limit of Chthamalus within intertidal zone affected by presence of Balanus 

Diagram info: 

The upper intertidal zone removing Balanus had little effect on survival of Chtamalus 

The middle intertidal zone a much higher percentage of Chthamalus survived where Balanus was removed 

Niche Overlap in Barnacles • Environmental factors that restrict distribution of Chthamalus to upper intertidal zone. 

Diagram:  

Desiccation prevents Chthamalus from inhabiting higher levels.  

Zone inhabited by adult Chthamalus 

Competition with Balanus excludes Cthamalus from middle intertidal zone. 

Chthamalus is very vulnerable to predation in lower intertidal zone. 

Competition and Niches of Small Rodents  

Competition and Niches of Small Rodents  

• Competition among rodents in Chihuahuan Desert.  

-Large scale: 20 ha study site with 24 50 x 50 m study plots.  

-Well-replicated in both space and time.  

-Long term: ongoing since 1977.  

• 3 species of large granivorous rodents; 4 species of small granivorous rodents; 2 species of small insectivorous rodents 

 

• Competition among rodents in Chihuahuan Desert.  

-Predicted if competition among rodents is mainly for food, then small granivorous (eats seeds) rodent populations would increase in response to removal of larger granivorous rodents.  

         -Insectivorous rodents would show little or no response.  

         -Results supported hypothesis 

Competition and Niches of Small Rodents slide 50 and 51

ompetition and Niches of Small Rodents cont’d 

DIagram info on slide 50: 

Dipodomys numbers remained high on control plots throughout the study. 

Removal kept dipodomys numbers at or near zero on the dipodomys removal plots 

In response, numbers of small granivores rodents increased on the removal plots relative to the control plots 

Meanwhile numbers of insectovores rodents did not differ on control and removal plots 

Competition and Niches of Small Rodents cont’d 

 DIagram info slide 51: Dipodomys numbers were immediately reduced by the removal of procedures on plots.  

Small granivore numbers increased very quickly on the dipodomys removal plots. 

Meanwhile, numbers of insectivorous rodents again did not change in response to dopodomys removal 

Character Displacement 

 • Character displacement: competition between species living together (in sympatry) may lead to evolution of some meaningful difference between them, even though they are nearly identical when living apart (in allopatry). 

Character Displacement cont’d 

 • Criteria necessary to show character displacement:  

-Chance should be ruled out.  

-Phenotypic differences must have genetic basis.  

-Phenotypic differences in sympatry should result from evolutionary shifts. 

 -Phenotypic differences should relate to resource use.  

-Sites of sympatry and allopatry should be similar.  

-Should be evidence that similar phenotypes compete for resource. 

Character Displacement in Sticklebacks 

Intermediate species 

Allopatric species have a broad range of gill racker lengths...while sympatric species have diverged into benthic and limnetic forms 

Size-adjusted gill raker length (this is a measure of phenotype related to feeding.)  

IDK LOOK AT DIAGRAM ON SLIDE 54 

Character Displacement in Sticklebacks cont’d 

When grown alone (hollow circles) growth rates are independent of phenotype 

When grown in competition, fish with phenotypes more similar to competitors grow slowest. 

The fish most closely resemble benthic species