BIOL 4120

Principles of Ecology

Phil Ganter

320 Harned Hall


The field above has many species of plant, all with similar resource needs.  Could the field support more of each species or are numbers limited?  Are the plants competing?

Lecture 13 Interspecific Competition

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Overview - Link to Course Objectives

Species Interactions

This lecture begins a series of lectures on species interactions.  Below is a general scheme for those interactions.  Some are familiar, but others may new.  The scheme is based on whether the individual organisms involved in the interaction are helped, hurt, or unaffected as a result of the interaction.  You can see that there are several +,- interactions

Hurt(-), helped(+), or not affected(0)

Name of interaction
Species A 
Species B 

* symbiosis (living in intimate contact) can occur in several of these interactions - Mutualism, Commensalism, Herbivory and Parasitism can all involve symbiosis

  • Amensalism has been added for the interaction in which one species is hurt, but the other does not benefit
    • As wild pigs forage, they often disturb the upper layer of soil and many organisms may be taken from their burrows and exposed to predation by the action of the pigs, although the harm that the burrowers suffer does not improve the pig's situation at all.
  • Allelopathy refers to the release of toxins in to the environment.  The term was first applied to plant releases of toxins but toxin release has been found by animals (sponge interactions), bacteria (bacteriocins), and fungi (mycosins), so it is a widespread phenomenon.
    • can be a situation in which a species is hurt by the release of toxins but the species that released the toxins gains no benefit (in which case it is amensalism)
    • can be that the species that releases the toxins does gain by their release, in which case it becomes a mechanism for competition or predation avoidance.

Interspecific Competition

Interspecific versus intraspecific

  • Review what was discussed in Lecture 11 on intraspecific competition
  • Interspecific Competition arises out of the need for a scarce resource, just as intraspecific competition does and the mechanism can be scramble or interference competition.

Lotka-Volterra model of Interspecific competition

  • Based on the Logistic model
    • adds the presence of another species or genotype to the braking effect of the population being modeled
    • now, the reduction in the growth rate is the sum of the numbers of the two species present
    • need a way to correct for differences between species (see below)
      • First, define the Lotka-Volterra interaction coefficient or (the Greek letter alpha)

      Interpretation of a

      • It is the effect an individual of another species has on an individual of a competing species of expressed as the equivalent number of the competing species.
        • If species A eats three times what species B does, then 5 individuals of species A would eat the same as 15 individuals of species B and would be 3
  • Next, we modify the logistic by including the effect of the presence of one species on another, using the relationship defined above
    • The growth rate of a population of a species in reduced not by just the number of that species present (here, N1 in the numerator on the expression on the right in the first equation) but also by the presence of another species (the following term)


  • So, what are the possible outcomes predicted by these equations?
    • Depending on the values of K's and a's, they can predict either:
      • Trivial equilibria
        • Called trivial because they predict an outcome that is obvious
        • One species drives the other out and you get the K number of the winning species
        • either Species A or B remains and the other goes locally extinct
      • Stable equilibrium
        • Displacing the system (adding or removing individuals of one or both species) takes you back to the same equilibrium point
        • this predicts that both species will continue to coexist in this environment indefinitely
      • Unstable equilibrium
        • Displacing the system takes you to one of two possible outcomes:
          • Species A wins
          • Species B wins
        • Which one occurs depends on what sort of displacement takes place
  • How can you tell what is predicted?
    • When the system gets to any of the equilibrium outcomes above, it hits a condition of no change
      • Any species which has lost is at N = 0 and the winner is at the predicted logistic equilibrium, so its dN/dt is 0
      • The equilibria with both species present are still both states of no net change, which implies that dN/dt for both species is 0
  • In the cases where both species are predicted to coexist, if dN/dt = 0, then for species 1:

  • or (dividing by r1N1, and then multiply by K1)

    For Species 2, the exact same algebra (with the exception that the subscripts change) will bring you to:  

  • We can analyze the situation graphically
    • each of the last two equations is the equation of a straight line, with one species as x, the other as y, K as the intercept, and a as the slope
    • Graph is in "species 1 and species 2 space" (see diagram axes) below

    • the idea of a zero isocline is a line that represents all of the combinations of species 1 and 2 (which are points in 2 dimensional space) that result in one species having a growth rate (dN/dt) of zero
      • make sure you are comfortable with this idea and also the fact that the lined region is where dN/dt is greater than 0 (where the population of species 1 will increase in number) and the checked region is where the growth rate is negative
        • to do this look at where K1 is and imagine that there are no species 2 present (so you are along the x axis) and then go beyond K1 to the right
        • this is obviously where the population must get smaller and it is also where the checked region is!
    • Now, do the same for species 2

  • Using these two expressions as a linear prediction of where each species will have a growth rate of 0, we can see under which conditions exclusion and coexistence are predicted.
    • We must superimpose the two graphs above, so that both isoclines appear on the same axes, then we can judge graphically what outcome is predicted
      • When one species has a zero growth isocline greater than another at all points, that species will be able to grow under some conditions where the other decreases.

      • Here, region B is where the population of Species 1 can get larger and Species 2 will get smaller
      • This growth and shrinkage will continue until Species 2 hits the x axis (where its population size is 0 and it has gone extinct)
      • Species 1 will then grow to its K1, which is its carrying capacity, and it is the winner
        • The situation is reversed if the isocline of Species 2 is farther from the origin than Species 1 (Species 2 is the winner when it hits K2)
      • and are equilibrium points, as the population sizes will not change unless something disturbs the system
  • Now, lets look at the situation where both are present and the zero growth isoclines do intersect.
    • Depending on the relative sizes of K1, K2, a1 and a21, either stable or unstable equilibria are predicted

    • the important areas here are 1 and 2
      • to understand why the top is unstable, go to these areas and predict how the populations will change
        • to do this, look at the zero isoclines to decide what will happen to each species
        • you will get a similar situation as when the lines don't intersect (one species will go extinct and the other will go to its carrying capacity
        • thus, when the equilibrium is unstable, one species wins and the other loses when the system is displaced from the equilibrium point represented by the intersection of the zero isoclines
      • in the lower diagram, the regions 1 and 2 are reversed (you can see why if you look at where they are with respect to the zero isoclines)
        • now if you predict where each species will go, you see that you will head back towards the intersection of the zero isoclines
        • thus, any disturbance will just result in return to the equilibrium point with both species present (unless the disturbance is so severe that one species is eliminated, which is a point from which it can not recover.

Competitive Exclusion Principle

  • When two species that require the same set of resources and one is better able to gain access to the scarce resource or resources, then one species will exclude the other from that locality
  • Related to the idea of a niche, a "space" describing all of the needs and abilities of a species
    • Restatement of the competitive exclusion principle is that no two species can occupy the same niche
    • some see a niche as a geometric space with each important abiotic factor or resource as an axis (so this space has many more dimensions than the three normal space has)
  • Called a principle, but it must be proven in each case, and so it is not a principle but a hypothesis
  • Competitive Exclusion Principle restated in terms of the niche is:  no two species can occupy the same niche.  Defining the principle this way brings up an observation and a question:
    • Competition is the mechanism of exclusion, so the only niche factors that influence exclusion are those that have something to do with the limiting resource or resources
    • Just how similar can two niches be before one of the species is excluded?  Look below under the "Coexistence and Competition" section!
  • Niches can be altered by presence of competitors or predators that reduce the total "space" occupied by a species
    • fundamental niche - maximum niche when no competitors, predators, parasites, etc. present
    • realized niche - actual space occupied by the species when other species are present

Factors that Influence Competition

Temporal Heterogeneity

  • Changes in a habitat over time may shift the competitive advantage from one competitor to another.
    • Seasonal changes in temperature or rainfall can favor different species
    • Changes in habitat due to age of habitat can alter competitive relationships
      • Tribolium
        • Two species of beetle living in stored grain and flour (important pest species)
        • Almost always, one species ousts the other, but not always the same species wins
          • mechanism of competition is predation of eggs and pupae by larvae and adult beetles
          • Some see this not as competition, rather as mutual predation, but the outcome is the same
        • Sporozoan infection first altered the outcome (T. confusum more resistant and won)
        • Abiotic conditions affected outcome
            • T. confusum won when flour dry, T. castaneum when wet
            • T. castaneum wins when grain is fresh, T. confusum when grain has dried .
      • Among my yeast, some yeast species are excluded from the rot by competitors under the conditions found in recently dead tissue but are competitive dominants when the tissue ages.
    • Disturbance from unusual events (storms, droughts, early freezes, HUMAN ACTIVITY) can alter competitive outcomes.

Spatial Heterogeneity

  • Gradients (Clines) can change competitive outcomes.
    • One species dominates one end of the gradient, the other species dominates the other end, and coexistence occurs in between.
  • Barnacles in the Rocky Intertidal - Connell
    • The tidal cycle sets up a gradient along the rocky shoreline, with the lower rocks covered by the tide for longer than the upper rocks
      • the rocks offer a hard substrate to which animals and algae can attach and feed on (or absorb nutrients from) the water that flows over them (sandy shorelines lack the opportunity for attachment)
      • Barnacles and mussels are both filter-feeding animals that benefit from the water flow
    • Chthalamus upper limit set by desiccation, lower limit by Balanus
      • Remove Balanus and Chthalamus grows
      • Remove Chthalamus and Balanus does not invade
    • Balanus upper limit set by desiccation, lower by starfish predation
      • Remove predators and Balanus invades
    • To refresh your memory about barnacles, remember that they are crustaceans that are sedentary as adults.  They lie on their backs on a rock and build a calcareous shell around themselves (some have stalks that attach them to the rocks).  When they feed, they extend their legs and use them as a net to filter food particles from the water.  I thank Arthur's Clipart Site for the following images.

Coexistence and Competition - Niche relationships

Resource Partitioning

first discussed in "Homage to Santa Rosalia or why are there so many kinds of animals?" Hutchinson, 1959

  • Asked an important question - if competition has the power to exclude all but the best competitors, why then are so many environments full of similar species
    • looked at a kind of bug found in ponds and found that more than one species of these bugs, which all look alike and feed in the same manner, occurred in the same ponds
  • question was why did not the best competitor force the other species out?
    • Concluded that species were Resource Partitioning - Species were monopolizing a portion of the resource but not the entire resource
    • In the case of the ponds, the species of bugs would not coexist if their feeding apparatus was too similar (they could not exceed a maximum similarity - called the Limiting Similarity)
      • Similarity can be expressed as a ratio between the size of two species (or the size of some body part important in dealing with the limiting resource)
        • Hutchinson measured this ratio as somewhere around 1:1.28 for his bugs in the ponds near Santa Rosalia and he called this ratio the Limiting Similarity
        • many went out and found the ratio and concluded that competition was the cause
      • Limiting similarity has bee criticized for non-experimental nature of findings and for the fact that the same ratio often occurs where no competition exists
  • These ideas about competition all assume that there is a limiting resource (usually 1, but two or more have been considered)
    • many have modeled resources as resource niche axes:
      • Resource Axis -- a line representing change in a resource, such as size, or sugar concentration, or concentration of toxins
      • Species Utilization Curve -- the degree to which a species can utilize a resource at some point on the resource axis
    • Different species can be represented as humps along the axis
      • humps (usually bell shaped, but not necessarily so) give each species range of resources sizes utilized and its optimum resource utilization point
        • Niche Overlap - the portion of the resource axis (or axes) shared by competing species
      • Ideally, if limiting similarity is correct, the overlap between species resources will not be larger than the limiting similarity ratio
    • Species Packing -- when all of the species on a resource axis are spaced so that each shows maximal overlap allowing coexistence (therefore, no additional species can be added without causing more overlap than limiting similarity would allow) the resource axis is called "packed"

Measuring Niche Overlap

  • What happens when the niche dimension is not a continuous variable, so that an axis can't be used to describe it?
  • this situation might describe a resource that is uniform (like suitable space) but occurs as patches separated by unusable space
  • here we can use a different measure of niche and a different measure of overlap
    • Niche breadth
      • Niche breadth is the degree to which a species utilizes all available patches or resources
      • there are many ways to measure this, but all are related to the simplest given below (Levin's niche breadth)

      • here the symbols are
        • S = number of resources or patches of resource
        • pi = proportion of a species utilizing the ith resource or patch
  • will range from 1.0 when the species is equally distributed across each resource or patch or 1/S when all members of a species are found on one patch or resource
  • the pi's are often measured as the proportion of different food types in the guts of the species of interest
  • Overlap between the breadth of two species (here designated species j and species k) may be as Proportional Similarity (between the two species)

here, pij and Pik are the proportions of the least-abundant species found on the ith patch or resource

As a rule of thumb, a PS of over 70% is considered to indicate active competition between the species, and lower values allow coexistence of the competitors

Competition and Evolution

Competition is an important influence on the success of organisms and so, competition will affect the fitness of organisms and characteristics of organisms that are important in competition will be subject to natural selection.  Below we consider two aspects of this relationship.

  • Character displacement
    • Best evidence for limiting similarity
    • When species are allopatric (look at lecture on modes of speciation). their utilization patterns (as reflected by their size or the size of their mouth parts) overlap (ratio is below 1:1.1 or so)
    • When they are sympatric, the overlap is reduced (indicated by larger ratios of the magnitude from 1.3 and 2) due to Character Displacement
      • so character displacement is the evidence of past competition forcing species that were too similar to become more dissimilar in order to coexist
    • Has been shown for Darwin's finches in the Galapagos islands
  • r and K selection
    • Remember that K selected species are thought to experience competition but r selected species simply move on to new, unexploited resource rather than compete for resource in a crowded patch
    • r-selected species are species that experience local patch extinction and live by colonizing new patches
      • they rarely experience carrying capacity, and often grow near to the maximal rate (~r when N is low for the logistic)
    • K-selected species occupy more long-lived patches, which can become saturated and so they experience carrying capacity (K)
    • When one characterizes species by this dichotomy, then "suites" of characters are associated with each type of selection
Character r-selected species K-selected species
Mortality density-independent density-dependent
Survivorship type III types I, II
Population Size variable constant
Competition minor keen
Life expectancy short long
Fecundity high low
First reproduction early late
Type of reproduction semelparous iteroparous
Body size small large

Cited Literature

Connell, J. H.  1961a.   Effects of competition, predation by Thais lapillus and other factors on natural populations of the barnacle Balanus balanoidesEcological Monographs 31:61-104

Connell, J. H.  1961b.  The influence of interspecific competition and other factors on the distribution of the barnacle Chthalamus stellatus.   Ecology 42:710-723.

Hutchinson, G. E. (1959). "Homage to Santa Rosalia or why are there so many kinds of animals?" American Naturalist 93(870): 145-159.


Mutualism, Commensalism, Competition, Allelopathy, Herbivory, Predation, Parasitism, Amensalism, symbiosis, Interspecific Competition, Lotka-Volterra Model, Trivial equilibrium, Stable Equilibrium, Unstable Equilibrium, Zero-Growth Isocline, niche, Competitive Exclusion Principle, Fundamental Niche, Realized Niche, Gradient (Cline), Resource Partitioning, Limiting Similarity, Resource Axis, Species Utilization Curve, Niche Overlap, Species Packing, Niche breadth, Proportional Similarity, Character displacement

Last updated February 17, 2007