1. An observation: “Tens Rule” From Williamson & Fitter (1996) Ecology 77:1661-1666 Only ~10% of imported species escape to wild Only ~10% of species that make it to the wild become naturalized Only ~10% of naturalized species become pests Invasive plants: THE 10 hypotheses

2. Why are only some species invasive?

3. If so few species become invasive, does a plant species become invasive because it has certain characteristics? Why are only some species invasive?

4. Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002) 1.Reproductive system Dioecious (male & female flowers on separate plants) vs. Monoecious (on same plant) Why are only some species invasive?

5. Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002) 1.Reproductive system Dioecious vs. Monoecious Self-incompatible pollen vs. Self-compatible pollen Why are only some species invasive?

6. Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002) 1.Reproductive system Dioecious vs. Monoecious Self-incompatible pollen vs. Self-compatible pollen Some type of asexual reproduction Apomixis – produce viable seed without fertilization Vegetative reproduction – regenerate from stem or root fragments Clonal propogation – new individuals produced through rhizomes Why are only some species invasive?

7. Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002) 1.Reproductive system (tend to be: self-compatible monoecious w/ asexual reproduction) Why are only some species invasive?

8. Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002) 1.Reproductive system (tend to be: self-compatible & monoecious w/ asexual reproduction) 2.Flowering & fruiting periods (tend to be: long) 3.Juvenile period (tend to be: short) 4.Seed production (tend to be: high) 5.Germination cues (tend to be: present) 6.Light requirements (tend to be: highly efficient, plastic, & competitive) Why are only some species invasive?

9. Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002) 1.Reproductive system (tend to be: self-compatible & monoecious w/ asexual reproduction) 2.Flowering & fruiting periods (tend to be: long) 3.Juvenile period (tend to be: short) 4.Seed production (tend to be: high) 5.Germination cues (tend to be: present) 6.Light requirements (tend to be: highly efficient, plastic, & competitive) BUT Very few invasive plants have all these traits Some invasive plants have few or none of these traits Why are only some species invasive?

10. Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002) 1.Reproductive system (tend to be: self-compatible & monoecious w/ asexual reproduction) 2.Flowering & fruiting periods (tend to be: long) 3.Juvenile period (tend to be: short) 4.Seed production (tend to be: high) 5.Germination cues (tend to be: present) 6.Light requirements (tend to be: highly efficient, plastic, & competitive) BUT Very few invasive plants have all these traits Some invasive plants have few or none of these traits THUS must be more than just the species characteristics Why are only some species invasive?

11. 10 hypotheses compiled from literature: 1.Vacant niche hypothesis 2.Belowground chemical warfare hypothesis 3.Environmental change hypothesis 4.Variable resource availability hypothesis 5.Competition hypothesis 6.Microevolutionary change hypothesis 7.Escape from biotic constraints hypothesis 8.Biodiversity hypothesis 9.Disturbance and land use hypothesis 10.Anthropogenic hypothesis Why are only some species invasive?

12. Native, fundamental niche – Species A Resource axis #1 Success Why are only some species invasive? 1) Vacant Niche Hypothesis

13. Native, fundamental niche – Species A Resource axis #1 Success Resource axis #2 Success Why are only some species invasive? 1) Vacant Niche Hypothesis

14. Native, fundamental niche – Species A Resource axis #1 Resource axis #2 Why are only some species invasive? 1) Vacant Niche Hypothesis

15. Native, fundamental niche – Species A Resource axis #1 Resource axis #2 Why are only some species invasive? 1) Vacant Niche Hypothesis

16. Native, fundamental niche – Species A Native, fundamental niche – Species B Resource axis #1 Resource axis #2 Why are only some species invasive? 1) Vacant Niche Hypothesis

17. Native, fundamental=realized niche – Species A Native, fundamental=realized niche – Species B Resource axis #1 Resource axis #2 Why are only some species invasive? 1) Vacant Niche Hypothesis

18. Native, fundamental=realized niche – Species A Native, fundamental=realized niche – Species B Invader, fundamental niche – Species C Invader, fundamental niche – Species D Resource axis #1 Resource axis #2 Why are only some species invasive? 1) Vacant Niche Hypothesis

19. Native, new realized niche – Species A, Native, new realized niche – Species B Invader, realized niche – Species C Invader, realized niche – Species D Resource axis #1 Resource axis #2 Why are only some species invasive? 1) Vacant Niche Hypothesis

20. Appealing ecological theory, BUT Actual demonstration of “vacant” niche is nearly impossible Many potential invaders lack pollinators, symbionts, etc. May have some utility for tropical oceanic islands Why are only some species invasive? 1) Vacant Niche Hypothesis

21. Allelopathy = one plant releases chemicals that are toxic to another In natural environment, invader releases allelochemicals: But the other members of the plant community have evolved with the invader Thus other plants are relatively immune to the allelochemicals In new invaded environment, invader releases allelochemicals: Now the allelochemicals are novel to the other members of the plant community Thus other plants are susceptible to damage by the allelochemicals Why are only some species invasive? 2) Belowground Chemical Warfare Hypothesis (aka allelopathy)

22. Callaway & Aschehoug (2000): C. diffusa releases chemicals that are NOT toxic to species in native Caucasus habitat, but chemicals are toxic to species in new Montana habitat Bais et al. (2003) Science 301:1377-1380 Identified the specific allelochemical: (–)-catechin that has higher concentration in C. diffusa soils and inhibits germination and growth of Montana grasses Why are only some species invasive? 2) Belowground Chemical Warfare Hypothesis

23. Excellent support for some species (C. diffusa) BUT How many other species? Why are only some species invasive? 2) Belowground Chemical Warfare Hypothesis

24. Global Changes Increasing atmospheric CO 2 has 2 direct effects on plants: Increases photosynthesis → more C for growth Closes leaf stomates → uses less water Keeling & Whorf (2004) CDIAC Why are only some species invasive? 3) Environmental Change Hypothesis

25. Global Changes – Increasing atmospheric CO 2 From Smith et al. (2000): Red brome (Bromus madritensis ssp. rubens) Why are only some species invasive? 3) Environmental Change Hypothesis

26. Global Changes – Increasing atmospheric CO 2 From Dukes in Mooney & Hobbs (2000) – Fig. 5.1: Stimulation of growth by elevated CO 2 for invasives vs. noninvasives Why are only some species invasive? 3) Environmental Change Hypothesis

27. Global Changes Increasing atmospheric CO 2 Evidence in some specific cases But not all invasives benefit Similar results for other global change factors (N deposition, warming) Why are only some species invasive? 3) Environmental Change Hypothesis

28. 10 hypotheses compiled from literature: 1.Vacant niche hypothesis No support 2.BCW hypothesis Specific cases, but how many? 3.Environmental change hypothesis Limited support 4.Variable resource availability hypothesis 5.Competition hypothesis 6.Microevolutionary change hypothesis 7.Escape from biotic constraints 8.Biodiversity hypothesis 9.Disturbance and land use hypothesis 10.Anthropogenic hypothesis Why are only some species invasive?

29. 10 hypotheses compiled from literature: 1.Vacant niche hypothesis No support 2.BCW hypothesis Specific cases, but how many? 3.Environmental change hypothesis Limited support 4.Variable resource availability hypothesis Limited support In most plant communities at most times, most of the resources that are available are taken up by the plants Plant community becomes susceptible whenever there is an increase in the amount of unused resources (↑ availability, ↓ uptake, or both) Changes in availability & uptake naturally occur through time Assumes invaders have access to variable resources Why are only some species invasive?

30. 10 hypotheses compiled from literature: 1.Vacant niche hypothesis No support 2.BCW hypothesis Specific cases, but how many? 3.Environmental change hypothesis Limited support 4.Variable resource availability hypothesis Limited support 5.Competition hypothesis General support, but exceptions Why are only some species invasive?

31. 10 hypotheses compiled from literature: 1.Vacant niche hypothesis No support 2.BCW hypothesis Specific cases, but how many? 3.Environmental change hypothesis Limited support 4.Variable resource availability hypothesis Limited support 5.Competition hypothesis General support, but exceptions 6.Microevolutionary change hypothesis Specific cases ; how many? Why are only some species invasive?

32. 10 hypotheses compiled from literature: 1.Vacant niche hypothesis No support 2.BCW hypothesis Specific cases, but how many? 3.Environmental change hypothesis Limited support 4.Variable resource availability hypothesis Limited support 5.Competition hypothesis General support, but exceptions 6.Microevolutionary change hypothesis Specific cases ; how many? 7.Escape from biotic constraints General support; but exceptions Why are only some species invasive?

33. 10 hypotheses compiled from literature: 1.Vacant niche hypothesis No support 2.BCW hypothesis Specific cases, but how many? 3.Environmental change hypothesis Limited support 4.Variable resource availability hypothesis Limited support 5.Competition hypothesis General support, but exceptions 6.Microevolutionary change hypothesis Specific cases ; how many? 7.Escape from biotic constraints General support; but exceptions 8.Biodiversity hypothesis Conflicting support Why are only some species invasive?

34. 10 hypotheses compiled from literature: 1.Vacant niche hypothesis No support 2.BCW hypothesis Specific cases, but how many? 3.Environmental change hypothesis Limited support 4.Variable resource availability hypothesis Limited support 5.Competition hypothesis General support, but exceptions 6.Microevolutionary change hypothesis Specific cases ; how many? 7.Escape from biotic constraints General support; but exceptions 8.Biodiversity hypothesis Conflicting support 9.Disturbance and land use hypothesis Special case of #4 & #10 Why are only some species invasive?

35. 10 hypotheses compiled from literature: 1.Vacant niche hypothesis No support 2.BCW hypothesis Specific cases, but how many? 3.Environmental change hypothesis Limited support 4.Variable resource availability hypothesis Limited support 5.Competition hypothesis General support, but exceptions 6.Microevolutionary change hypothesis Specific cases ; how many? 7.Escape from biotic constraints General support; but exceptions 8.Biodiversity hypothesis Conflicting support 9.Disturbance and land use hypothesis Special case of #4 & #10 10.Anthropogenic hypothesis Broad support, few exceptions Why are only some species invasive?

36. 10 hypotheses compiled from literature: 1.Vacant niche hypothesis No support 2.BCW hypothesis Specific cases, but how many? 3.Environmental change hypothesis Limited support 4.Variable resource availability hypothesis Limited support 5.Competition hypothesis General support, but exceptions 6.Microevolutionary change hypothesis Specific cases ; how many? 7.Escape from biotic constraints General support; but exceptions 8.Biodiversity hypothesis Conflicting support 9.Disturbance and land use hypothesis Special case of #4 & #10 10.Anthropogenic hypothesis Broad support, few exceptions Any missing hypotheses? Is summary accurate for each hypothesis? Why are only some species invasive?