An observation: “Tens Rule” From Williamson & Fitter (1996) Ecology 77:1661-1666

1. Invasive plants: THE 10 hypotheses • 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

2. Why are only some species invasive?

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

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

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

6. Why are only some species invasive? • Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002) • 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

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

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

9. Why are only some species invasive? • Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002) • Reproductive system (tend to be: self-compatible & monoecious w/ asexual reproduction) • Flowering & fruiting periods (tend to be: long) • Juvenile period (tend to be: short) • Seed production (tend to be: high) • Germination cues (tend to be: present) • 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

10. Why are only some species invasive? • Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002) • Reproductive system (tend to be: self-compatible & monoecious w/ asexual reproduction) • Flowering & fruiting periods (tend to be: long) • Juvenile period (tend to be: short) • Seed production (tend to be: high) • Germination cues (tend to be: present) • 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

11. Why are only some species invasive? • 10 hypotheses compiled from literature: • Vacant niche hypothesis • Belowground chemical warfare hypothesis • Environmental change hypothesis • Variable resource availability hypothesis • Competition hypothesis • Microevolutionary change hypothesis • Escape from biotic constraints hypothesis • Biodiversity hypothesis • Disturbance and land use hypothesis • Anthropogenic hypothesis

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

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

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

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

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

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

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

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

20. Why are only some species invasive? • 1) Vacant Niche Hypothesis • 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

21. Why are only some species invasive? • 2) Belowground Chemical Warfare Hypothesis (aka allelopathy) • 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

22. Why are only some species invasive? • 2) Belowground Chemical Warfare Hypothesis • 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

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

24. Why are only some speciesinvasive? • 3) Environmental <Global> Change Hypothesis • Global Changes • Increasing atmospheric CO2 has 2 direct effects on plants: • Increases photosynthesis → more C for growth • Closes leaf stomates → uses less water Keeling & Whorf (2004) CDIAC

25. Why are only some speciesinvasive? • 3) Environmental <Global> Change Hypothesis Global Changes – Increasing atmospheric CO2 From Smith et al. (2000): Red brome (Bromus madritensis ssp. rubens)

26. Why are only some speciesinvasive? • 3) Environmental <Global> Change Hypothesis Global Changes – Increasing atmospheric CO2 From Dukes in Mooney & Hobbs (2000) – Fig. 5.1: Stimulation of growth by elevated CO2 for invasives vs. noninvasives

27. Why are only some speciesinvasive? • 3) Environmental <Global> Change Hypothesis • Global Changes • Increasing atmospheric CO2 • Evidence in some specific cases • But not all invasives benefit • Similar results for other global change factors (N deposition, warming)

28. Why are only some species invasive? • 10 hypotheses compiled from literature: • Vacant niche hypothesis No support • BCWhypothesisSpecific cases, but how many? • Environmental change hypothesis Limited support • Variable resource availability hypothesis • Competition hypothesis • Microevolutionary change hypothesis • Escape from biotic constraints • Biodiversity hypothesis • Disturbance and land use hypothesis • Anthropogenic hypothesis

29. Why are only some species invasive? • 10 hypotheses compiled from literature: • Vacant niche hypothesis No support • BCWhypothesis Specific cases, but how many? • Environmental change hypothesis Limited support • 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

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

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

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

33. Why are only some species invasive? • 10 hypotheses compiled from literature: • Vacant niche hypothesis No support • BCWhypothesis Specific cases, but how many? • Environmental change hypothesis Limited support • Variable resource availability hypothesis Limited support • Competition hypothesis General support, but exceptions • Microevolutionary change hypothesis Specific cases ; how many? • Escape from biotic constraints General support; but exceptions • Biodiversity hypothesis Conflicting support

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

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

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