handouts on table near entrance. readings: melbourne et al (2007) ecology letters 10 (1): 77-94

• Handouts on table near entrance

• Readings:

Melbourne et al (2007) ECOLOGY LETTERS 10 (1): 77-94

3) What makes a species invasive?d) Variable resource availability hypothesis

Davis et al. (2000) JOURNAL OF ECOLOGY 88 (3): 528-534Basic concepts:• In most plant communities at most times, most of the

resources that are available are taken up by the plants

Gross resource supply

Davis et al. (2000)Basic concepts:• In most plant communities at most times, most of the resources that

are available are taken up by the plants• Plant community becomes more susceptible whenever there is

an increase in the amount of unused resources

Resist

Easily

are available are taken up by the plants• Plant community becomes more susceptible whenever there is an

increase in the amount of unused resources↑ availability (A→B)

Resist

Easily

increase in the amount of unused resources↑ availability (A→B)↓ uptake (A→C)

Resist

Easily

increase in the amount of unused resources↑ availability (A→B)↓ uptake (A→C)Both (A→D)

Resist

Easily

• Changes in availability& uptake naturallyoccur through time

Resist

Easily

• Changes in availability& uptake naturallyoccur through time

• Assumes invaders haveaccess to resources

Resist

Easily

Davis et al. (2000)Evidence:• Conducted experiment in relatively unproductive grassland• Imposed 2 resource gradients

(1) Disturbance gradient to ↓ uptake

High disturbance(Low uptake)

Low disturbance(High uptake)

(1) Disturbance gradient to ↓ uptake(2) Fertility gradient to ↑ supply

High supply

Low supply

Low uptakeHigh uptake

• Seeded 54 alien species• Measured cover of aliens

High supply

Low supply

Low coverof aliens

• Seeded 54 alien species• Measured cover of aliens

High supply

Low supply

Low coverof aliens

High coverof aliens

SUMMARY: Variable resource availability hypothesis• Conceptual appealing• Flexibility to accommodate space & time; many different resources• Experimental evidence

But• Low predictive power

Different invaders respond differently to different resourcesHave to know where and when availability increases in complex

world to predict susceptibility

But• Low predictive power

Different invaders respond differently to different resourcesHave to know where and when availability increases in complex

world to predict susceptibility• Many invaders don’t fit model

Doesn’t appear to be any changes in resource availabilityOther factors (e.g. allelopathy) seem to be more important

3) What makes a species invasive?e) Competition hypothesis

Basic concept:• Invasives are inherently better at getting resources

i.e. better competitors

Basic concept:• Invasives are inherently better at getting resources

i.e. better competitors

Realized niche – Species A, Species BFundamental niche: Invader – Species C

Resource axis #1

Evidence: Pattison et al. (1998)• Tropical rainforest in Hawaii• Studied 4 native species & 5 invasive species

Included trees, shrubs, & herbsIncluded a pair congeners (Bidens sandwicensis native & B. pilosa

alien)• Grew in different light environments representative of rainforest

Light is a limiting factor in rainforests• Expectations: Invasives better at utilizing light

Evidence: Pattison et al. (1998)• What happens with ↑ shade?

“Relative growth rate” (RGR) = how fast plants grow“Leaf area ratio” = how leafy plants are

↑ shade

Evidence: Pattison et al. (1998)• Shade ↓ RGR

↑ shade

Evidence: Pattison et al. (1998)• Shade ↓ RGR of all species

↑ shade

Evidence: Pattison et al. (1998)• Shade ↓ RGR of all species, BUT invasives had >RGR

↑ shade

Evidence: Pattison et al. (1998)• Shade ↓ RGR of all species, BUT invasives had >RGR

↑ shade

SunPartial shade

Evidence: Pattison et al. (1998)• Shade ↓ RGR of all species, BUT invasives had >RGR (esp. @

higher light)

↑ shade

ShadePartial shade

Evidence: Pattison et al. (1998)• Shade ↓ RGR of all species, BUT invasives had >RGR• Shade ↑leafiness of all species

↑ shade

ShadePartial shade

Evidence: Pattison et al. (1998)• Shade ↓ RGR of all species, BUT invasives had >RGR• Shade ↑leafiness of all species, BUT invasives had > leafiness

(esp. @ low light)

↑ shade

ShadePartial shade

ShadePartial shadeSun

Evidence: Pattison et al. (1998)• Shade ↓ RGR of all species, BUT invasives had >RGR• Shade ↑leafiness of all species, BUT invasives had > leafiness• Leaf photosynthesis:

“Light compensation point” = lowest light with + photosynthesis“Apparent quantum yield” = how efficiently plants turn light into

chemical energy for photosynthesis

Higher light compensation point for natives

Higher light compensation point for natives, OR invasives tolerate greater shade

Invasives tolerate greater shadeInvasive greater quantum yield

Invasives tolerate greater shadeInvasive greater quantum yield, OR invasives more light

efficient

Invasives tolerate greater shadeInvasives more light efficient“CO2 assimilation” = photosynthesis

Invasives tolerate greater shadeInvasives more light efficientInvasives have greater photosynthetic rates

ShadePartial shade

Evidence: Baruch & Goldstein (1999)• Broad survey in Hawaii along elevation gradient of tropical

rainforests• 34 native species• 30 invasives species• Included trees, shrubs, & herbs• Included 83 populations (i.e. >1 population for some species)

Evidence: Baruch & Goldstein (1999)Invasives had• Bigger leaves

Evidence: Baruch & Goldstein (1999)Invasives had• Bigger leaves• More nutrients

• Cost less to build

• Cost less to build• Higher photosynthesis

• More efficient N use

Evidence: Baruch & Goldstein (1999) Invasives had• Bigger leaves• More nutrients• Cost less to build• Higher photosynthesis• More efficient N use

Overall, invasives are better suited than natives in capturing and utilizing light in the light limited tropical rainforest, especially in high light environments that characterized disturbed habitats

Evidence: Melgoza et al. (1990) Oecologia 83:7-13• Field study of invasive annual grass Bromus tectorum competition

with 2 native species Chrysothamnus viscidiflorus and Stipa comata• Studied plants in

(1) recently-burned area without Bromus(2) recently-burned area with Bromus(3) area that had burned >12 years prior (also had Bromus)

• Examined competition for soil water

Evidence: Melgoza et al. (1990) Oecologia 83:7-13• Greater water stress for natives when Bromus is present• Degree of water stress imposed by Bromus in the first year after

burn is similar to that 12 years after burn

Evidence: Melgoza et al. (1990) Oecologia 83:7-13Natives have• Greater water stress with invasives• Less biomass production with invasives

Evidence: D’Antonio et al. (2001)• Worked with 2 invasive species in Hawaii in seasonally dry

woodlands:Schizachyrium condensatum – native to mainland USMelinis minutiflora – native to Africa

• Both C4 bunchgrasses

• Both C4 bunchgrasses• Observed that Melinis appears to replace Schizachyrium

sdec inc

Evidence: D’Antonio et al. (2001)• Seed banks NOT involved

Melinis seed is present in unburned areas without adult plants at densities similar to that of Schizachyrium

Evidence: D’Antonio et al. (2001)• Seed banks NOT involved• Competition involved

Field experiment where removed Melinis plants from around Schizachyrium

Field experiment: removed Melinis plantsMeasured change in number of tillers for Schizachyrium through

Field experiment : removed Melinis plantsMeasured change in tillers for Schizachyrium

Melinis competition

Field experiment : removed Melinis plantsMeasured change in tillers for SchizachyriumAfter 5 months: when ↓ Melinis competition, ↑ Schizachyrium

Melinis competition

Field experiment : removed Melinis plantsMeasured change in tillers for SchizachyriumAfter 5 months: when ↓ Melinis competition, ↑ SchizachyriumTrend continues through at least 14 months

Melinis competition

Additional field & glasshouse studies indicate Melinis is the better competitor for both light and N

Dominance of Schizachyrium in unburned areas is simply because it appeared to get there first!

SUMMARY: Competition hypothesis• Conceptually appealing• Strong evidence for a number of species in many different habitats

(although sometimes invoked without concrete evidence)

SUMMARY: Competition hypothesis• Conceptually appealing• Strong evidence for a number of species in many different habitats

(although sometimes invoked without concrete evidence)

But• Hard to generalize (and hence predict)

Critical resource(s) varies with different environmentsSpecies characteristics that make for better competitor varies with

type of resourceEven for any 1 resource, various ways to be a better competitor

• Why hasn’t evolution already come up with the strategy in situ?• Not all invaders fit model; other factors seem to be important for

some species

3) What makes a species invasive?f) Micro-evolutionary change hypothesisReading: Leger and Rice (2003) Ecology Letters 6:257-264

Micro-evolutionary change hypothesis• Frequent colonizing events are a central feature of invasive

plants

3) What makes a species invasive?f) Micro-evolutionary change hypothesis

Micro-evolutionary change hypothesis• Frequent colonizing events

Founder effects = founders of a new population carry only a fraction of the total genetic variation of the source populations

Founder effectsGenetic bottlenecks = loss of genetic variation when population

size drastically decreases; often associated with catastrophic events that result in mass mortality

Founder effectsGenetic bottlenecksGenetic drift = loss of genetic variation by chance when

populations are small and do not have complete, random interbreeding

Founder effectsGenetic bottlenecksGenetic drift

• Natural selection: strong selective forces often apply to successful colonizers

• Natural selectionNew abiotic environment – rapid adaptive responses over short

times and within short distances to new environment

• Natural selectionNew abiotic environmentNew biotic environment – shifts in relative proportions of

competition vs. defense pressures

• Natural selectionNew abiotic environmentNew biotic environment

• Hybridization – a natural process that occurs in plants

• Hybridization – a natural process that occurs in plants↑ genetic diversity

• Hybridization – a natural process that occurs in plants↑ genetic diversity↓reproductive barriers

• Hybridization – a natural process that occurs in plants↑ genetic diversity↓reproductive barriersTransfers or originates adaptations

• Hybridization – a natural process that occurs in plants

2834 plant species in the British IslesOf these, 715 (25%) are hybrids

74 are native X alien21 are alien X alien95 (13% of hybrids) involve aliens

• HybridizationInterspecific – often with other species in new environment

• HybridizationInterspecificIntraspecific – often with populations from native range that would

not normally occur

Hybridization Evidence:• Stabilized introgressants

Introgression = back cross with 1 or more parentsP1 X P2 → F1

F1 X {P1, P2} → F2 introgressant

Introgression = back cross with 1 or more parentsStabilized = viable, fertile hybrids

Hybrids form new Intraspecific taxa

Hybrids form new intraspecific taxaHybrids form new Species – Note: all examples are alien X alien

Hybridization Evidence:• Stabilized introgressants• Allopolyploids = hybrid between different species in which

chromosomes of both parents are retained

Hybridization Evidence:• Stabilized introgressants• Allopolyploids

Hybrids form new Species – Note: both Tragopogon’s are alien X alien

Specific Example: Rhododendron ponticum in British Isles*Milne & Abbott (2000) Molecular Ecology 9:541-556• Natural distribution: south of Black Sea with disjunct

populations in Lebanon, Spain, & Portugal• Extensively naturalized throughout British Isles

Specific Example: Rhododendron ponticum in British Isles*Milne & Abbott (2000) Molecular Ecology 9:541-556• Natural distribution: south of Black Sea with disjunct populations in

Lebanon, Spain, & Portugal• Extensively naturalized throughout British Isles• Origin unclear:

Earliest known introduction (1763) from SpainBut subsequent introductions likely, especially from Black Sea

areaAlso can’t tell from morphological information where it came from,

but know from morphology that had to hybridize at some timeR. ponticum cultivated along with other introduced species

Specific Example: Rhododendron ponticum in British Isles*Milne & Abbott (2000) Molecular Ecology 9:541-556• Most individuals from naturalized populations had genotypes from

Spain (88%), followed by Portugal (10%)• No genotypes from Black Sea region

Specific Example: Rhododendron ponticum in British Isles*Milne & Abbott (2000) Molecular Ecology 9:541-556• Most individuals from naturalized populations had genotypes from

Spain (88%), followed by Portugal (10%)• No genotypes from Black Sea region• Small number of individuals had hybridized with at least 3 other

speciesOccurrence of R. catawbiense genotypes most common in

Scotland (coldest area of British Isle)Introgression with catawbiense appears to have conferred cold

tolerance into ponticum

SUMMARY: Micro-evolutionary change hypothesis• Likely that most (if not all) invasive species go through founder

events, experience genetic bottlenecks & drift, and undergo selectionIn other words micro-evolutionary changes

• Good evidence for hybridization being beneficial

SUMMARY: Micro-evolutionary change hypothesis• 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• For only a subset of these, have evidence that micro-evolutionary

changes have been beneficial• A species that undergoes micro- (or even macro-) evolutionary

changes does not automatically become invasive

handouts on table near entrance. readings: melbourne et al (2007) ecology letters 10 (1): 77-94

plant communities

plants plant community

species invasive

basic concepts

b slide

b c d slide

entrance slide

b c slide

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