1. Interspecific Competition II Getting back to plants… Although competition clearly applies to plants, most of the theory is zoologically based

2. The Problem with Plants… All plants have essentially same requirements: Light Water Nutrients These are not typically discrete packages Shortage of one may affect ability to compete for other needed resources

3. The Problem with Plants… Cause and effect are very difficult to establish without experimentation Coexistence explanations: Frequently focus on demonstrating that spp are different in some characteristics Not conclusive in terms of mechanisms~

4. SIX MECHANISMS OF COEXISTENCE IN PLANTS 1. Resource Partitioning Niche separation Intraspecific competition > Interspecific competition 2. Spatial Segregation

5. Example: Solidago (Begon et al. 1996 p. 102) Soil moisture and interspecific comp. appear to determine each realized niche

6. Six Mechanisms of Coexistence in Plants 3. Recruitment Limitation The dispersal/competitive ability trade-off 4. Pest Pressure These four mechanisms all assume that coexistence is an equilibrium condition

7. Six Mechanisms of Coexistence in Plants 5. The Storage Effect 6. Density-Independent Mortality Overlapping generations Asynchronous recruitment These two mechanisms are based on nonequilibrium as the condition

8. Mechanisms of Coexistence Very difficult to demonstrate Too much emphasis on adult plants? Maybe preemptive competition for regeneration niche the key? Need to consider indirect abiotic effects? Tilman et al. (1982) experimental work with planktonic algae

9. Two algae and abiotic Influences on competition Asterionella formosa requires less SiO 2 at lower temperatures Synedra ulna requires less SiO 2 at higher temperatures Coexistence: temperature fluctuations, alternating competitive displacement (Tilman et al. 1982)

10. Theory and Real Result (Begon et al. 1996 p. 104)

11. Experimental Work Vital To Elucidate mechanisms of coexistence Particularly due to history of agricultural research, standard experimental designs developed These include additive and substitutive designs

12. Experimental designs for plants Simple Additive Substitutive or Replacement Series Full Additive

13. Some Experimental Results: Replacement series design 2 Species, 4 densities (Begon et al. 1996 p. 86) Competition uneven A. fatua less affected than A. barbata

14. Two Botanical Measures of Competition Relative Yield Total (RYT): Yield of X in mixture Yield of X in monoculture Relative Resource Total (RRT): Measure of performance in mixture Both try to measure the extent to which 2 spp utilize different resources

15. Theoretical explorations of competitive exclusion and coexistence The logistic model of two-species competition….

16. Outcomes of the model (Begon et al. 1996 p. 108) Competitive exclusion Unstable coexistence Stable coexistence