Micro-evolutionary change hypothesis

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 Basic concept : Communities with greater diversity have no ‘vacant niches’ and are therefore less invasible. Implies saturation of communities!

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

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. Biodiversity hypothesis • Theoretical evidence: • Tilman (1999) Ecology 80: 1455-1474 • ↑ number species ↑ filling of niche space • ↑ 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. Biodiversity hypothesis • 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 Constructed communities ‘Neighborhood’ size = 40 x 125 cm

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

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

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

13. Biodiversity hypothesis • 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

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

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

16. Biodiversity hypothesis • Evidence: • Stolghren et al. (1999) Ecological Monographs 69: 25-46 • 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. Biodiversity hypothesis • Evidence: • Stolghren et al. (1999) Ecological Monographs 69: 25-46 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. Biodiversity hypothesis 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? • Different ecosystems (deserts  rainforests) vary in their extrinsic factors that influence ranges of biodiversity. • Account for that range and then look at the biodiversity hypothesis!

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

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

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 limitingresources.

22. Variable resource availability hypothesis • a.k.a. Fluctuating resource hypothesis • Evidence: • Davis et al. (2000) Journal of Ecology 88: 528-534 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) Resource uptake Resistant to Invasion Resource supply-uptake isocline B A • Both (A→D) D Easily Invasible C Gross resource supply

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

24. Variable resource availability hypothesis • a.k.a. Fluctuating resource hypothesis • 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

25. Variable resource availability hypothesis • a.k.a. Fluctuating resource hypothesis • 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.

26. Variable resource availability hypothesis • a.k.a. Fluctuating resource hypothesis • 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.

27. Variable resource availability hypothesis • a.k.a. Fluctuating resource hypothesis 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. Where should resources fluctuate? River banks, abandoned gardens, landslide

28. Variable resource availability hypothesis • a.k.a. Fluctuating resource hypothesis 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

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

30. Variable resource availability hypothesis • a.k.a. Fluctuating resource hypothesis • Resources naturally fluctuate over time. When availability of the most limiting resourceis greater than resource uptake, the system is vulnerable to invasion. • 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. Disturbance and land use hypothesis • 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…)

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

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

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

35. Disturbance and land use hypothesis • 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. Both weedy non-native species AND total number of non-native species increased with development. disturbance

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

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

38. Disturbance and land use hypothesis • 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?