Principles of Ecology
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
Overview - Link to Course Objectives
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||
* symbiosis (living in intimate contact) can occur in several of these interactions - Mutualism, Commensalism, Herbivory and Parasitism can all involve symbiosis
Interspecific versus intraspecific
Lotka-Volterra model of Interspecific competition
Interpretation of a
- 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:
Competitive Exclusion Principle
Factors that Influence Competition
- 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
- 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.
- 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
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 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
Connell, J. H. 1961a. Effects of competition, predation by Thais lapillus and other factors on natural populations of the barnacle Balanus balanoides. Ecological 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
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