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Where’s the Biology?

Grass?. 1 dN N dt. Lions?. (K-N). =. r. K. ??Energy/stress??. Where’s the Biology? . Wildebeest populations growth. competition for grass occurs. Individuals are energy stressed. Lions kill off weak individuals . From a single species to two species .

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Where’s the Biology?

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  1. Grass? 1 dN N dt Lions? (K-N) = r K ??Energy/stress?? Where’s the Biology? Wildebeest populations growth competition for grass occurs Individuals are energy stressed Lions kill off weak individuals

  2. From a single species to two species Effect * of species 1 on species 2 - + COMPETITION - PREDATION Effect* of species 2 on species 1 + PREDATION MUTUALISM * On per capita growth rate

  3. What is interspecific competition? Phenomenological: it is two species (interspecificcompetition) that have a negative effect on each other’s per capita growth rate Mechanistically this may involve: Mutual depletion of a limiting resource (exploitation), be it food, shelter, space Defense of a resource (Interference) Allelopathy – the production of toxins (or otherwise modify conditions) to the detriment of another species

  4. What are its effects and what evidence do we have that it occurs??

  5. Gause’s famous experiments with Paramecium showed for the first time experimentally that one species could eliminate (exclude) another species from its habitat

  6. Balanus outcompetes Chthamalus for space and restricts it to the upper tidal zone where Balanus is subject to desiccation

  7. Similarly, many species of ungulates occur on the African savannah, but appear to occupy different habitats Thompson’s gazelles in the young, succulent grass Zebras in the old, tall grass

  8. Removal of dominant competitor (K-rats) leads to ecological release for weak competitors Removal of K-rats Leads to rapid increase in small seed-eaters

  9. Gerbillus pyramidum Egyptian Gerbil ~40 g Gerbillus allenbyii Allenby’s Gerbil ~20 g ARE THEY GONE YET?? Let’s kick some Allenbyii butt Activity Pattern Control Gp removed Time (hrs after sunset)

  10. We know from an immense set of observations • and experiments that: • Species can reduce each other’s abundance • Species can force one another to extinction, • thereby influencing its geographic range, and • limiting its realized niche • But is competition simply haphazard?? • Or is is Repeatable and Predictable • That is, is there anything similar between: • Parameciumaurelia vs. P. caudatum • G. pyramidum vs G. allenbyii • Balanus vs. Chthamalus

  11. We have two options for attempting to answer this question: • Study as many examples as possible and • try to formulate generalizations • (2) Construct a model to help conceptualize • the process of Competition

  12. denotes a negative effect of another spp -X denotes a negative effect Of a species on itself INTRASPECIFIC COMPETITION Logistic growth embodies intraspecific (within a species) competition 1 dN N dt (K-N) N K r = = r (1 - ) K

  13. Lotka-Volterra Competition Equations also embody interspecific competition 1 dN1 N1 dt N1 K1 N2 K1 = r1 (1 - - 12 ) 1 dN2 N2 dt N2 K2 N1 K2 = r2 (1 - - 21 ) Other species reduce the growth rate of competitors exponential growth Limits to growth: Spp reduces their own growth rate

  14. 1 dN1 N1 dt N1 K1 N2 K1 Note: = r1 (1 - - 12 ) 1 dN1 N1 dt is the same as: = r1 (K1 – N1 – 12N2) K1

  15. 1 dN1 N1 dt N1 K1 N2 K1 = r1 (1 - - 12 ) There is only one new term: 12 and 21are called the Competition Coefficients 12 = the relative effect of an individual of species 2 on species 1’s per capita growth rate • For instance, grasshoppers and zebras compete for grass. One zebra • eats as much as 1,000 grasshoppers. Therefore ZG = 0.001. Does GZ = 1,000 ?? It may, but not necessarily. Also, the ’s represent the per individual competitive effect, not the population effect. If there are 3,000 more grasshoppers than zebras grasshoppers have a small per individual effect, but a larger population effect.

  16. N2 K1 12 K2 Species 2 drives Species 1 to extinction Species 1 drives Species 2 to extinction K1 12 K2 N1 K1 K2 21 K2 21 K1 One of 2 states arise based on starting conditions: Species 1 alone at K1 Or Species 2 alone at K2 K1 12 Stable coexistence at reduced density K2 K2 K1 12 K1 K2 21 K1 K2 21

  17. To download a version of Populus: http//www.cbs.umn.edu/populus/download/download.html

  18. LV Competition • Only 1 (in 4) solutions yields coexistence – Is coexistence the • unexpected outcome? • Is competitive exclusion random or does the theory indicate • some property (not identity) of the competing species that can • guide us? • Yes, as ’s get closer to 1 the conditions for coexistence get more restrictive – the biological interpretation is the more similar two species are the less likely they are to coexist • Two species that occupy the same niche (they are identical) cannot coexist = Competitive Exclusion Principle

  19. Competition as a structuring force in biological communities Small seed specialist Large seed specialist Frequency In the diet small large Seed Size Overlap = competition

  20. Medium seed eater Large seed specialist Small seed specialist Frequency In the diet small large Seed Size

  21. (2) Niche partitioning – there is a limit on the amount of similarity between adjacent species that results in a regular spacing of species’ morphology X Medium seed eater Large seed specialist Small seed specialist Frequency In the diet small large Seed Size Competition works as a biological filter to remove or prevent the invasion of species into biological communities

  22. Niche partitioning in two desert rodent communities

  23. Wildcat These coexisting Felids in Israel Caracal

  24. Diameters of canines for 3 species of coexisting cats in Israel (after Dayan et al. 1990) Wildcat, fem. Wildcat, male Jungle cat, fem. Caracal, fem. Jungle cat, male Caracal, male 5mm 6 7 8 9 Niche partitioning in the feeding appendages of Felids

  25. Small seed specialist Medium seed eater Frequency In the diet small large Seed Size

  26. Small seed specialist Medium seed eater Frequency In the diet small large Evolution Frequency In the diet small large

  27. If Competition is an evolutionary force in species’ ecology: • Predictions: • Species will differ greatly where the share an evolutionary history • Species will not differ much when they have not shared a evo. history Differences in species’ morphology will be greater in sympatry than in allopatry sympatry = occur together allopatry = occur separately

  28. (4) Character Displacement in Darwin’s Finches

  29. Conclusions: • In Ecological Time competition is a Species Taker • (2) Competition is a structuring force – dissimilar species can coexist • more easily • Influences geographic distributions • Niche partitioning • Niche expansion when there is release from competition - Ecology • Character Displacement - Evolution

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