1. Micro-evolutionary change hypothesis Multiple colonizing events Founder effects Genetic bottlenecks Genetic drift Natural Selection New abiotic environment New biotic environment Hybridization interspecific intraspecific

2. Micro-evolutionary change hypothesis  Summary Likely that most (if not all) invasive species go through micro- evolutionary changes Good evidence for hybridization being beneficial But Have evidence of micro-evolutionary changes for only a limited number of species Limited evidence that changes are beneficial A species that undergoes micro or macro evolutionary changes does not automatically become invasive Adaptation by natives in response to invasion

3. Vacant Niche Hypothesis Niche describes how an organism or population responds to the distribution of resources and competitors Fundamental niche (Hutchinson 1957) = theoretical limits of existence for a species along n resource axes Realized niche = actual limits of existence for a species Implies saturation of communities! Basic concept : Communities with greater diversity have no ‘vacant niches’ and are therefore less invasible.

4. Resource axis #1 Resource axis #2 Vacant Niche Hypothesis New realized niche – Species A, Species B Realized niche: Invader – Species C Realized niche: Invader – Species D

5. Vacant Niche Hypothesis SUMMARY: May have some utility for tropical oceanic islands Natural enemies should shift on to more similar new species more easily (enemy escape hypothesis) New life forms can be very successful (annual grasses in NV) BUT Many potential invaders lack pollinators, symbionts, etc. Actual demonstration of “vacant” niche is nearly impossible

6. Biodiversity hypothesis Basic concepts: High biodiversity confers high community stability Stable communities are not easily invaded Shares features with vacant niche hypothesis BUT does not require a vacant niche Uses niche concepts that: (1) Different species have different niches (2) As ↑ number species, ↑ filling of niche space Highly diverse communities more difficult to invade!

7. Theoretical evidence: Tilman (1999) Ecology 80: 1455-1474 ↑ number species ↑ filling of niche space Biodiversity hypothesis ↑ number species ↓ average resources availability Each species has a minimum average resource need = R* Corresponds with a minimum species diversity = N* At or below N*, species can invade

8. Biodiversity hypothesis Theoretical evidence: Tilman (1999) Ecology 80: 1455-1474 If do for all species in community, as diversity decreases, invasibility increases.

9. Evidence: Kennedy et al. (2002) Nature 417: 636-638 Question: At a small scale (field), how does diversity [species richness & density] influence invasion? Method: 147 plots seeded with up to 24 natives 13 aliens invaded naturally through time Biodiversity hypothesis Constructed communities ‘Neighborhood’ size = 40 x 125 cm

10. Evidence: Kennedy et al. (2002) Nature 417: 636-638 Question: At a small scale (field), how does diversity [species richness & density] influence invasion? Biodiversity hypothesis As ↑ native diversity: ↓ invader cover ↓ invader number ↓ invader maximum size no effect of species richness on mean invader size

11. Evidence: Kennedy et al. (2002) Nature 417: 636-638 Question: At a small scale (field), how does diversity [species richness & density] influence invasion? Biodiversity hypothesis Invasion decreased with increasing native species richness But what about native species density?

12. Evidence: Kennedy et al. (2002) Nature 417: 636-638 Question: At a small scale (field), how does diversity [species richness & density] influence invasion? Biodiversity hypothesis As diversity increased, crowding also increased As crowding increased, maximum invader size decreased

13. Evidence: Kennedy et al. (2002) Nature 417: 636-638 Question: At a small scale (field), how does native diversity [species richness & density] influence invasion? Invasion decreased with increasing native species richness Invader performance decreased with increasing crowding Biodiversity hypothesis

14. Diversity decreased invasion Is this an artifact of the manipulated experiment? Does the same pattern hold for natural situations?

15. Contrary evidence: Stolghren et al. (1999) Ecological Monographs 69: 25-46 Questions: (1)What is the relationship between native species richness and foliar cover and invasion of exotic plant species? (2)Are invasions patterns a matter of scale, or environment? Methods: Collected field data from 2 biomes Multi-scale vegetation sampling Biodiversity hypothesis

16. Evidence: Stolghren et al. (1999) Ecological Monographs 69: 25-46 Biodiversity hypothesis At small spatial scales: Cover of non-native species declined with increasing native diversity BUT only in the Central Grasslands Increasing native diversity increased non-native richness in the Colorado Rockies

17. Evidence: Stolghren et al. (1999) Ecological Monographs 69: 25-46 Biodiversity hypothesis At large spatial scales, areas of high native species richness were consistently more invaded than areas of low species richness. Areas with high diversity ALSO had the highest soil fertility and precipitation.

18. Resolving the conflict: Shea and Chesson (2002) Trends in Ecology & Evolution 17: 170-176 Why is the comparison of biodiversity between very different ecosystems valid? Biodiversity hypothesis Account for that range and then look at the biodiversity hypothesis! Different ecosystems (deserts  rainforests) vary in their extrinsic factors that influence ranges of biodiversity.

19. Resolving the conflict: Shea and Chesson (2002) Trends in Ecology & Evolution 17: 170-176 Within ‘clusters’ extrinsic factors (e.g. climate) are similar Biodiversity hypothesis Within ecosystems, more species = less invasible Across ecosystems, more diverse systems (more resources) = more invasible

20. Increasing biodiversity increases ecosystem stability which increases resistance to invasion (due to filled niche space= decreased resource availability). Biodiversity hypothesis Summary: Logical arguments & data to support the hypothesis But Logical arguments & data contrary to hypothesis Thus, biodiversity alone does not account for invasibility Assumes competition is dominant driver structuring communities Type of diversity examined Diversity patterns at different scales may explain paradox in part

21. Variable resource availability hypothesis a.k.a. Fluctuating resource hypothesis Background: Davis et al. (2000) Journal of Ecology 88: 528-534 There is a finite amount of plant resources (nutrients, light, water, ‘space’) at a given site in a given time. In most plant communities, at most times, resources are taken up by resident plants. Plant communities become susceptible to invasion whenever there is an increase in the amount of limiting resources.

22. Evidence: Davis et al. (2000) Journal of Ecology 88: 528-534 Gross resource supply Resource uptake Resource supply-uptake isocline Variable resource availability hypothesis a.k.a. Fluctuating resource hypothesis Resistant to Invasion A Easily Invasible B C Plant communities become more susceptible whenever there is an increase in the amount of limiting resources Invasion increases as: ↑ availability (A→B) ↓ uptake (A→C) Both (A→D) D

23. Evidence: Davis et al. (2000) Journal of Ecology 88: 528-534 Gross resource supply Resource uptake Resource supply-uptake isocline Variable resource availability hypothesis a.k.a. Fluctuating resource hypothesis Resistant to Invasion A Easily Invasible B C Plant communities become more susceptible whenever there is an increase in the amount of limiting resources D This is not a static attribute of the community, but rather a condition that will fluctuate over time!

24. Evidence: Davis & Pelsor (2001) Ecology Letters 4: 421-428 Question: How do fluctuations in resource availability influence competition and invasion? Methods: Desmodium canadense, Dalea purpurea, and Rudbeckia hirta were seeded into bare plots or plots established with non-native grasses Some plots weeded to reduce competiton Resource manipulated: water Variable resource availability hypothesis a.k.a. Fluctuating resource hypothesis

25. Evidence: Davis & Pelsor (2001) Ecology Letters 4: 421-428 Increasing the limiting resource (water) increased invasion for some species, even with high amounts of competition. Variable resource availability hypothesis a.k.a. Fluctuating resource hypothesis

26. Evidence: Lepš et al. 2002. Applied Vegetation Science Piper aduncum Native range: Central America Invaded range: Papua New Guinea Variable resource availability hypothesis a.k.a. Fluctuating resource hypothesis Invasive Piper should only be found where there are fluctuating resources.

27. Evidence: Lepš et al 2002. Applied Vegetation Science Piper aduncum Native range: Central America Invaded range: Papua New Guinea Invasive Piper should only be found where there are fluctuating resources. Variable resource availability hypothesis a.k.a. Fluctuating resource hypothesis River banks, abandoned gardens, landslide Where should resources fluctuate?

28. Example: Gundale et al. (2008) Ecography 31:201-210 Questions: Under what combination of soil resource conditions is invasion by cheatgrass favored or constrained? How is this influenced by fire? Methods Field and greenhouse experiments to determine if observed patterns were influenced by belowground factors Variable resource availability hypothesis a.k.a. Fluctuating resource hypothesis

29. Example: Gundale et al. (2008) Ecography 31:201-210 Variable resource availability hypothesis a.k.a. Fluctuating resource hypothesis

30. Resources naturally fluctuate over time. When availability of the most limiting resource is greater than resource uptake, the system is vulnerable to invasion. Variable resource availability hypothesis a.k.a. Fluctuating resource hypothesis SUMMARY: Conceptual appealing Flexibility to accommodate space, time, & many different resources Experimental evidence But Low predictive power Different invaders respond differently to different resources Have to know where/when availability increases

31. Basic concepts: Many invasive species have a “ruderal” life history strategy. “Ruderal” = small, very-short lived plants that grow and mature rapidly and that have a large reproductive effort, especially in response to stress These species are often associated with disturbed habitats Every system has a natural disturbance regime (fire return, flooding interval, etc) Changes in land use can alter the natural disturbance regime (more or less frequent, bigger or smaller events…) Disturbance and land use hypothesis

32. Empirical Evidence: Hobbs in Mooney & Hobbs (2000) Land use changes affect disturbance Disturbance and land use hypothesis Transition can be natural or deliberate, with deliberately different end states Transition to original or new state Change can be permanent or transitory Change can be abrupt or gradual

33. How can disturbance and land use changes enhance invasions? Disturbance and land use hypothesis Changing resource availability Changes in vegetation states provide opportunities for other species to exist Increases the probability of success for ruderals

34. Evidence: D’Antonio & Vitousek (1992) Without invasive species, typically when woodlands are disturbed, they eventually return back to woodlands Disturbance Recovery Disturbance and land use hypothesis With alien grasses there is a novel disturbance: fire Fire initiates a series of feedbacks that virtually precludes re-establishment of woody plants

35. Evidence: Kalin Arroyo et al. in Mooney & Hobbs (2000) Determined number of alien plants in 12 political regions of Chile Developed a land use index using data about agriculture use, urban areas and road density. disturbance Disturbance and land use hypothesis Both weedy non-native species AND total number of non-native species increased with development.

36. Evidence: Gelbard and Belnap (2003) Conservation Biology 17: 420-432 Examined the effect of road improvement on cover of non-native plants Disturbance and land use hypothesis 4-wheel drive tracks Graded roads Improved-surface roads Paved roads

37. Evidence: Gelbard and Belnap (2003) Conservation Biology 17: 420-432 Disturbance and land use hypothesis

38. Changes in land use cause changes in the extent and frequency of disturbance to an ecosystem which are then opened up for ruderal plant establishment. Summary: Consistent with ecological theories Evidence from a variety of ecosystems Empirical correlations But Is disturbance / land use the factor, or is it something associated with these? Species traits Resource availability Changes in competitive balance Temporary “vacant” niche What about plants that can establish without disturbance? Disturbance and land use hypothesis