Our example concerns an interaction between two species of seed-eating rodents that occur in the deserts of the southwest U.S. This particular study took place in the Chiracawa Mtns of Arizona. It is a notable study because of it is very well replicated, and has been ongoing for over 20 yrs now (12 when these data were collected). the experiment consists of 24 study plots, each a quarter hectare in area. all plots are fenced to keep livestock out. In half the plots, the most abundant of the rodents, kangaroo rats were trapped and removed. In the other half kangaroo rats were trapped (to count density) and released within the plot (control). The experiment was done to determine whether competition from kangaroo rats limits the size of deer mice populations. What is the null hypothesis? Results are shown for two time intervals: 2-years shortly after the experiment was set up. And two years, 10 years later. results are number of individuals trapped in a 2-3 day survey. Kangaroo rats competitively exclude deer mice? Bigger, more abundant, more aggressive.
Competitive exclusion. With total resource overlap complete competitors cannot coexist Do you believe it? What if we raised the resource supply? Lowered the initial densities? What are the criteria for 2 or more species to coexist? Lets try to beat the competitive exclusion principle, prove it wrong using our virtual rabbits.
Note, the simulation shows coexistence with complete resource partitioning. This is not necessary. In fact coexistance often occurs in nature with some degree of resource overlap. The crucial point is that members of each spp compete more among themselves than with the other species. Not completely, but more.
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Why are ecological interactions important? Interactions can affect distribution and abundance. Interactions can influence evolution.
Ecological effects of competition What are the following? Intra specific competition – Inter specific competition –
Ecological effects of competition Intra specific competition – between individuals of the SAME species Inter specific competition – between individuals of DIFFERENT species
Competition – two species share a requirement for a limited resource reduces fitness of one or both species
# deer mice captured 0 .2 .4 .6 .8 1 k rats excluded control 1978-80 1988-90 (Heske, E. J., J. H. Brown, and S. Mistry 1994) What is the effect of kangaroo rat competition on deer mice?
competitive exclusion principle If two species have the same niche, the stronger competitor will eliminate the other competitor. “ Complete competitors cannot coexist.”
Why do kangaroo rats exclude deer mice? What if you put more seeds in the environment? What if you added another resource to the environment?
Answers Complete competitors cannot coexist. Competitive exclusion is reached more slowly with higher resource abundances. Stable coexistence requires niche differentiation , such that members of each species compete more strongly among themselves than with members of the other species. (intraspecific > interspecific)
What is the niche? <ul><li>set of conditions </li></ul><ul><li>within which an organism </li></ul><ul><li>can maintain a viable </li></ul><ul><li>population </li></ul><ul><li>multi-dimensional </li></ul><ul><li>with as many </li></ul><ul><li>dimensions as their </li></ul><ul><li>are limiting conditions </li></ul>temperature light intensity okay salinity ecological niche
predict the outcome of interspecific competition A classic interspecific competition experiment two species of Paramecium P. caudata P. aurelia Gause (1934)
Barnacles on a Rocky shore <ul><li>Describe the distribution of the two species of barnacles </li></ul><ul><li>Where are the harsher conditions, what are they? </li></ul>
Fundamental niche depends on physical (abiotic) conditions. Realized niche depends on biotic as well as abiotic conditions. What is the realized niche of each barnacle? What is the fundamental niche of each?
growth rate Location in intertidal zone low high middle How can we determine the niche of each barnacle? Where do they grow when allowed to compete? Balanus niche Chthamalus niche Balanus and Chthamalus
growth rate Location in intertidal zone low high middle Chthamalus alone Balanus alone Why are these niches different? Balanus fundamental niche Chthamalus fundamental niche
growth rate Location in intertidal zone low high middle Chthamalus alone Balanus alone Why are these niches different? Removal experiments – remove each species and see where the other grows Balanus fundamental niche Chthamalus fundamental niche
growth rate Location in intertidal zone low high middle Chthamalus alone competitive release – niche of the competitively-inferior species expands in the absence of the competitively-superior species fundamental niche realized niche Chthamalus with Balanus competitive release
Streams with only Planaria species A Streams with only Planaria species B Streams with both Planaria species What are the fundamental and realized niches for each species?
The niche of a species may contract in the presence of a competitor species. This phenomenon leads to resource (niche) partitioning and coexistence among functionally similar species. The narrower niche resulting from competition is called the realized niche . When the dominant competitor is removed, the niche of the inferior competitor can expand by competitive release .
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