1. Competition Since the early days of community ecology, interspecific competition has been virtually synonymous with community ecology However, it currently remains an important structuring force, but it is no longer the only force

2. Competition Studies trying to support or refute interspecific competition generally either fall into observational studies (which are looking for patterns) and experimental approaches Frequently the scale at which one is interested in examining is the limiting factor (or aspects of natural history)

3. Competition For example, the observed distribution of birds on the Bismark Islands has been interpreted as either competition or strictly chance

4. Competition Unfortunately, one cannot manipulate the distribution to test for the actual mechanisms Consequently, many studies take advantage of a simplified system to experimentally test mechanisms Problems?

5. Competition One of the common observational approaches to studying interspecific competition involves searching for negative correlations between abundances of ecologically similar species

6. Competition Such ‘complementary’ distributions are then attributed to present or past effects of competition (provided other mechanisms can be ruled out) The extreme result of such an interaction results in a ‘checkerboard’ distributional pattern

7. Competition

10. Another approach uses interspecific differences in morphology or resource use to infer possible competitive interactions These are then interpreted as evidence that species must differ by some fixed amount in order to avoid competitive exclusion

11. Competition This approach is central to concept of character displacement; where differences in the morphology of ecologically similar species are greater in sympatry than in allopatry

12. Competition

13. Anolis lizards where a single species occurs (green) vs. multiple (brown)

14. Competition You could also avoid competition by narrowing varying with changing the average (hence no displacement)

15. Competition Again the problem with observational studies such as this, it does not necessarily describe a single mechanism

16. Competition Let’s consider experiments Provide strong inferences about whether competition is occurring Of course, long-lived or rare species are particularly problematic (most field experiments <3 yrs) Response time may be relative long or particularly subtle

17. Competition When experiments are impossible, observational studies can be made more compelling by determining if observed patterns differ from those expected by chance Null Models

18. Competition Null models have been used to test for complimentary patterns (e.g. checkerboard), size ratios, species richness, or differences in resource use Unfortunately, frequently ecologists cannot agree from which ‘population’ to draw the random sample

19. Competition Mechanisms of interespecific competition (Schoener 1983) –1) comsumption –2) preemption –3) overgrowth –4) chemical interactions (allelopathy) –5) territorality –6) encounter

20. Competition Consumption: one species consumes a shared resource Preemptive: primarily among sessile organisms; competing for space Overgrowth: one grows over another (e.g. trees or corals)

21. Competition Chemical: release of chemicals to inhibit growth of competitor Territorial: aggressive encounters prohibiting use of space Encounter: non-territorial encounters can result in negative encounters for one or both (e.g. may abandon search or patch)

22. Evidence of Competition Much of the observational evidence comes from 1) regularly repeated patterns of co-occurrence of species with a low overlap (or conversely, the absence of combinations of species with high overlap) and 2) niche shifts in the presence of competition

23. Evidence of Competition Of course, in both cases we are inferring the mechanism If you are looking for strong competition, can you envision any problems?

24. Evidence of Competition Even with controlled experiments, interpretation and extrapolation of results must be cautioned Bender et al. (1984) discuss protocols and cautions with PRESS and PULSE experiments

25. Evidence of Competition Reynoldson and Bellamy (1970) documented the successful invasion of the flatworm Polycelis tenuis to a lake only inhabitated by the congeneric P. nigra P. nigra became only 10% of its original population size, without the total population remaining constant

26. Evidence of Competition Hairston (1980) examined two sympatric species of salamanders in the GSMNP and did removal experiments Removal of Plethodon jordani resulted in statistical increase in P. glutinosus Reverse did not show equal response, but did show an increase of juveniles

27. Evidence of Competition Menge (1972, 1976) conducted an experimental removal of competing starfish Pisaster ochracus and Lepasterisa hexactis and moving them to another rock (2x density) Numbers did not change when Pisaster removed, but size increased (and decreased where 2x)

28. Evidence of Competition Pisaster ochracus and is noteworthy for another classic study Dominant predator of 16 invert sp (many of which have high niche overlap) When Pisaster removed, 11 species disappeared from community with only 4 prey sp and 4 non-prey sp

29. Evidence of Competition When Pisaster removed, prey species abundance increased dramatically The dominant mussel outcompeted other prey species

30. Evidence of Competition Two large meta-analysis (Connell and Schoener) in 1983 Schoener (n=164) found 76% of all interactions showed some competition with 57% showing definite competition under all circumstances

31. Evidence of Competition Connell (n=72; 215sp.) demonstrated competition in >1/2 the species and 40% of the experiments Potential biases; were pairs randomly selected for the experiments? What about publishing biases? Bias in groups selected… phytophagous insects grossly underreprented in study

32. Evidence of Competition Gurevitch et al. (1992) examined 10 yrs of studies (n=137) Ranged from 1 to 36mo (x=4), 75%<1yr 80% showed impact on biomass Interesting impact of competition at various trophic levels…

33. Evidence of Competition There are also indirect effects of competition As previously stated, strong competitors may coexist in a sufficiently complex landscape or by a generalist predator Competitive release

34. Evidence of Competition E.g. sheep and rabbits in medieval pastures have been identified as the agents of maintaining a tremendous floristic diversity (by not allowing massive shading out of areas)

35. Evidence of Competition Additional competitors may surpress the effects of competition on a third party (competitive mutualism) Still other competitors (albeit weak) can slightly add more (diffuse comp) Finally, negative interactions of perceived competitors might simply be the result of a shared predator (apparent competition)

36. Evidence of Competition Consequently, binary interactions may be too simplistic and we may need to consider multiple interactions simultaneously (e.g. food webs) or consider the relative strength of varying effects (path analysis)

37. Evidence of Competition The end…

38. Competition Tyrannid flycatchers

39. Competition However, different size bills and bodies can also simply reflect diet specialization (i.e. just getting better) There are several clusters of similar species that have developed differences in foraging areas and that is an indirect indication that competition may have been at work (previously)

40. Competition Foraging relationship among several antbirds (Formicariidae)

41. Competition Other examples include three flatbilled flycatchers in Costa Rica who forage at different heights Across habitats, one species may replace another in this unoccupied ‘niche’ Other examples include niche expansion when others are absent

42. Competition In the absence of the other competitor, each species will expand its range up or down the mountain

43. Competition