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Biodiversity and exotic species invasion in riparian plant communities Rebecca Brown Eastern Washington University Riparian Areas Areas along rivers that are influenced by flooding Uniquely connected habitat from upstream to downstream reaches and aquatic to terrestrial environments

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Biodiversity and exotic species invasion in riparian plant communities l.jpg

Biodiversity and exotic species invasion in riparian plant communities

Rebecca Brown

Eastern Washington University


Slide2 l.jpg

Riparian Areas communities

  • Areas along rivers that are influenced by flooding

  • Uniquely connected habitat from upstream to downstream reaches and aquatic to terrestrial environments

  • Provide ecosystem services (wildlife habitat, erosion control, and water quality protection)


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REALLY diverse! communities

>180 plant species per 1000 m2

40 exotic plant species

(introduced by people to habitats outside of their natural range)



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Exotic Species areas?

  • Threat to global biodiversity

  • Expensive to remove

  • Understanding what makes habitats susceptible to species invasion is one of the fundamental questions in ecology


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Is invasibility related to diversity? areas?

“…the balance of relatively simple communities of plants and animals is more easily upset than that of richer ones; that is… more vulnerable to invasions”

Charles S. Elton 1958


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Is invasibility related to diversity? areas?

“…the balance of relatively simple communities of plants and animals is more easily upset than that of richer ones; that is… more vulnerable to invasions”

Charles S. Elton 1958

But what about riparian plant communities?


Slide8 l.jpg

Invasion areas?

Invasion

Diversity

Diversity

The diversity-invasibility debate

+

-

Mixed or no relationship

Stohlgren and Chong 2002 Wiser et al. 1998

Bruno et al. 2002

Burger et al. 2001

Sax 2002

Lonsdale 1999

(mostly observational, large scale)

Levine 2001

Lavorel et al. 1999.

Brown and Fridley 2003 Duncan 1996

Stohlgren et al. 1998

Brown and Peet 2003

Prieur-Richard et al. 2000 Stachowitz et al. 2002 Dukes 2002

Tilman 1997

Hector et al. 2001

Knops et al. 1999

(mostly experimental, small scale)


Why care l.jpg
Why care? areas?

Implications of diverse communities being resistant to exotic species invasion:

  • Loss of species diversity could cause more exotic species invasions

  • Argument for protecting seemingly unimportant species from extinction

  • Should diverse habitats be given priority for conservation?


Presentation overview l.jpg
Presentation areas?Overview

Why are there so many native and exotic species in riparian areas? How are diversity and invasibility related?

  • Conceptual framework - immigration hypothesis

    • What is the relationship between flooding, diversity, and invasion?

    • How does scale influence the relationship between diversity and invasibility in riparian areas?

    • What is the relationship between flooding and seed supply?

    • Do riparian zones have greater compositional similarity with distance due to high dispersal?

  • What implications does this have for the influence of dams on riparian plant communities?


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The diversity of a plant community represents a balance between two distinct sets of processes:

  • Extinction processes

  • competition

  • environmental stress

  • extreme disturbance

  • predation

  • Limits species richness

  • (the number of species)

  • Immigration processes

  • seed inputs

  • moderate disturbance

  • net resource availability

  • Increase immigration of

  • native and exotic

  • species


Immigration processes in riparian areas l.jpg
Immigration processes in riparian areas: between two distinct sets of processes:

  • Flood disturbance

  • Increased seed supply from flood waters

  • High net resource availability


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Seed Supply between two distinct sets of processes:

Conceptual relationship between disturbance, seed supply, and species richness


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Seed Supply between two distinct sets of processes:

Conceptual relationship between disturbance, seed supply, and species richness


Presentation overview15 l.jpg
Presentation between two distinct sets of processes:Overview

Why are there so many native and exotic species in riparian areas? How are diversity and invasibility related?

  • Conceptual framework - immigration hypothesis

    • What is the relationship between flooding, diversity, and invasion?

    • How does scale influence the relationship between diversity and invasibility in riparian areas?

    • What is the relationship between flooding and seed supply?

    • Do riparian zones have greater compositional similarity with distance due to high dispersal?

  • What implications does this have for the influence of dams on riparian plant communities?


Slide18 l.jpg

0.1 m between two distinct sets of processes:

1 m

0.31 m

3.1 m

10 m

Methods

  • 107 100-m2 vegetation plots

    • Percent cover by species recorded

    • Environmental data collected for each plot

    • Multiscale data

  • 1075 upland plots from Carolina Vegetation Survey


Methods 107 riparian plots stratified by geomorphic position l.jpg

Hupp and Osterkamp 1985 between two distinct sets of processes:

Methods107 riparian plots stratified by geomorphic position.

Landforms sampled:

  • Bar

  • Floodplain

  • Terrace(s)

  • Upland


Slide20 l.jpg

Bar between two distinct sets of processes:

Floodplain

Terrace


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Seed Supply between two distinct sets of processes:

Predicted position of geomorphic positions relative to immigration vs. extinction processes


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Native Species Richness Over Four Landforms between two distinct sets of processes:

Native Species Richness per 100 m2

Bar Floodplain Terrace Upland

n=35 n=56 n=16 n=1075

Floods often No floods


Slide23 l.jpg

Native Species Richness per 100 m between two distinct sets of processes:2

Bar Floodplain Terrace Upland

n=35 n=56 n=16 n=1075

Native Species Richness Over Four Landforms

A

B

B,C

D

Floods often No floods


Slide24 l.jpg

A between two distinct sets of processes:

Exotic Species Richness per 100 m2

B

C

D

Bar Floodplain Terrace Upland

n=35 n=56 n=16 n=1075

Exotic Species Richness Over Four Landforms

Floods often No floods


Presentation overview25 l.jpg
Presentation between two distinct sets of processes:Overview

Why are there so many native and exotic species in riparian areas? How are diversity and invasibility related?

  • Conceptual framework - immigration hypothesis

    • What is the relationship between flooding, diversity, and invasion?

    • How does scale influence the relationship between diversity and invasibility in riparian areas?

    • What is the relationship between flooding and seed supply?

    • Do riparian zones have greater compositional similarity with distance due to high dispersal?

  • What implications does this have for the influence of dams on riparian plant communities?


Presentation overview26 l.jpg
Presentation between two distinct sets of processes:Overview

Why are there so many native and exotic species in riparian areas? How are diversity and invasibility related?

  • Conceptual framework - immigration hypothesis

    • What is the relationship between flooding, diversity, and invasion?

    • How does scale influence the relationship between diversity and invasibility in riparian areas?

    • What is the relationship between flooding and seed supply?

    • Do riparian zones have greater compositional similarity with distance due to high dispersal?

  • What implications does this have for the influence of dams on riparian plant communities?


Slide27 l.jpg

Spatial scale effects between two distinct sets of processes:

Small scales:

Saturation, plant to plant competition

= negative relationship

Larger scales:

Variation in other factors (disturbance, seed supply, fertility)

= positive or no relationship


Relationship between diversity and invasibility at different scales l.jpg

p < .001 between two distinct sets of processes:

Relationship between diversity and invasibility at different scales

2

100 m

40

30

Exotic species richness

20

10

0

0

20

40

60

80

100

Native species richness


Relationship between diversity and invasibility at different scales29 l.jpg

p < .001 between two distinct sets of processes:

p = 0.001

Relationship between diversity and invasibility at different scales

2

2

1 m

100 m

4

40

3

30

2

Exotic species richness

20

1

10

0

0

0

20

40

60

80

100

0

5

10

15

20

Native species richness


Relationship between diversity and invasibility at different scales30 l.jpg

p < .001 between two distinct sets of processes:

p = 0.001

NS

Relationship between diversity and invasibility at different scales

2

2

2

1 m

0.1 m

100 m

3

4

40

3

30

2

2

Exotic species richness

20

1

1

10

0

0

0

0

2

4

6

8

10

0

20

40

60

80

100

0

5

10

15

20

Native species richness


Relationship between diversity and invasibility at different scales31 l.jpg

p < .001 between two distinct sets of processes:

p = 0.001

p = 0.02

NS

Relationship between diversity and invasibility at different scales

2

2

2

2

1 m

0.1 m

100 m

0.01 m

3

3

4

40

3

30

2

2

2

Exotic species richness

20

1

1

1

10

0

0

0

0

0

2

4

6

8

10

0

1

2

3

4

5

0

20

40

60

80

100

0

5

10

15

20

Native species richness


Presentation overview32 l.jpg
Presentation between two distinct sets of processes:Overview

Why are there so many native and exotic species in riparian areas? How are diversity and invasibility related?

  • Conceptual framework - immigration hypothesis

    • What is the relationship between flooding, diversity, and invasion?

    • How does scale influence the relationship between diversity and invasibility in riparian areas?

    • What is the relationship between flooding and seed supply?

    • Do riparian zones have greater compositional similarity with distance due to high dispersal?

  • What implications does this have for the influence of dams on riparian plant communities?


Presentation overview33 l.jpg
Presentation between two distinct sets of processes:Overview

Why are there so many native and exotic species in riparian areas? How are diversity and invasibility related?

  • Conceptual framework - immigration hypothesis

    • What is the relationship between flooding, diversity, and invasion?

    • How does scale influence the relationship between diversity and invasibility in riparian areas?

    • What is the relationship between flooding and seed supply?

    • Do riparian zones have greater compositional similarity with distance due to high dispersal?

  • What implications does this have for the influence of dams on riparian plant communities?


Slide34 l.jpg

Seed supply: between two distinct sets of processes:

The richness and abundance of seeds arriving at a site


Slide35 l.jpg

131 seed traps placed near vegetation plots between two distinct sets of processes:

Seeds collected 6 times over 3 years

Clay pads measure natural supply rates (don’t trap seeds)

Seed source not limited to flooding

Measuring seed supply


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Seed germination between two distinct sets of processes:


Slide37 l.jpg

No flood Flood between two distinct sets of processes:

No flood Flood

Seed Diversity and Abundance in Flooded and Unflooded Seed Traps

Chi2= 7.00, df = 1, P = 0.008 Chi2= 4.45, df = 1, P = 0.035


Presentation overview38 l.jpg
Presentation between two distinct sets of processes:Overview

Why are there so many native and exotic species in riparian areas? How are diversity and invasibility related?

  • Conceptual framework - immigration hypothesis

    • What is the relationship between flooding, diversity, and invasion?

    • How does scale influence the relationship between diversity and invasibility in riparian areas?

    • What is the relationship between flooding and seed supply?

    • Do riparian zones have greater compositional similarity with distance due to high dispersal?

  • What implications does this have for the influence of dams on riparian plant communities?


Presentation overview39 l.jpg
Presentation between two distinct sets of processes:Overview

Why are there so many native and exotic species in riparian areas? How are diversity and invasibility related?

  • Conceptual framework - immigration hypothesis

    • What is the relationship between flooding, diversity, and invasion?

    • How does scale influence the relationship between diversity and invasibility in riparian areas?

    • What is the relationship between flooding and seed supply?

    • Do riparian zones have greater compositional similarity with distance due to high dispersal?

  • What implications does this have for the influence of dams on riparian plant communities?


Distance decay of similarity l.jpg

1 between two distinct sets of processes:

High dispersal

Similarity

Low dispersal

0

Distance

Distance Decay of Similarity


Slide41 l.jpg

Measuring Similarity between two distinct sets of processes:

  • Sorensen similarity = 2a / (2a + b + c)

    • a = shared species

    • b = species in group 1 but not group 2

    • c = species in group 2 but not group 1


Measuring similarity l.jpg
Measuring Similarity between two distinct sets of processes:

  • Calculate Sorensen similarity between every possible pair of plots in each geomorphic position

  • Compare distance decay of similarity for each geomorphic position


Distance decay of similarity for 3 rivers l.jpg
Distance decay of similarity for 3 rivers between two distinct sets of processes:


Does this indicate higher dispersal in flooded sites l.jpg
Does this indicate higher between two distinct sets of processes:dispersal in flooded sites?

  • Possibly...

    Alternative explanation:

  • Flooded sites might have more homogeneous environmental conditions at the scales represented here than upland sites


Summary of results l.jpg
Summary of Results between two distinct sets of processes:

  • Diversity and invasion increase with increased flooding

  • The relationship between native and exotic species richness was positive at large scales and negative at small scales

  • Seed supply was higher in flooded areas

  • Riparian areas have higher compositional similarity with distance than uplands


Why are there so many native and exotic species in riparian areas l.jpg
Why are there so many native and exotic species in riparian areas?

  • Riparian areas are immigration-driven because of flooding which creates disturbance and increases seed supply and resource availability

  • This promotes high numbers of both native and exotic species


How are diversity and invasibility related l.jpg
How are diversity and invasibility related? areas?

Depends on the degree to which the community is immigration- or extinction- driven - and spatial scale

  • Positive relationship in immigration-driven communities sampled at large spatial scales

  • Negative relationship in extinction-driven communities or when sampling occurs at very small scales relative to the size of the plants


Broader implications l.jpg
Broader Implications areas?

  • In extinction-driven communities, the loss of species diversity could potentially permit more invasions

  • This is not the case, however, for all communities (such as riparian zones)

  • Invasibility depends on more than the number of species


Presentation overview49 l.jpg
Presentation areas?Overview

Why are there so many native and exotic species in riparian areas? How are diversity and invasibility related?

  • Conceptual framework - immigration hypothesis

    • What is the relationship between flooding, diversity, and invasion?

    • How does scale influence the relationship between diversity and invasibility in riparian areas?

    • What is the relationship between flooding and seed supply?

    • Do riparian zones have greater compositional similarity with distance due to high dispersal?

  • What implications does this have for the influence of dams on riparian plant communities?


Slide50 l.jpg

Dam impacts that may affect riparian plant communities areas?

Upstream Impoundment Downstream

  • Inundate riparian vegetation

  • Fragment riparian zone

  • Trap seeds

  • Store water and sediment

  • Reduced seed supply

  • Altered flood regime

  • Reduced sediment supply

  • Disturbed riparian habitat

  • Reduce or eliminate marine derived nutrients from migratory fish


Slide51 l.jpg
Dams areas?

Represent a large scale ‘experiment’

Baker Dam, Concrete, WA


Slide52 l.jpg

Seed Supply areas?

Predicted shift in species richness caused by dams


Slide53 l.jpg

Seed Supply areas?

Predicted shift in species richness caused by dams


Slide54 l.jpg

Flooding areas?

  • Dams:

    • Alter flood regime

    • Limit dispersal

  • Hypothesis: Reduced native and exotic species numbers below dams.

Disturbance

Seed

supply

Resource

addition

(sediment,

nutrients, water)

+ Native

and exotic

Plant species

Hypothesized impact of dams on riparian vegetation



Slide56 l.jpg

" areas?On average, we have constructed one dam every day since the signing of the Declaration of Independence."

Bruce Babbitt


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How does the effect of dams vary with dam size? areas?

< 1 m

1-5 m

5-50 m

>50 m



Study design l.jpg

Upstream reference sample areas?

Downstream sample (impacted

area)

Study Design

Upstream/downstream Comparisons of:

  • Riparian vegetation

    • Exotic richness

    • Native richness

    • Compositional similarity

    • Woody stem density

    • Species specific comparisons

  • Water chemistry

  • Geomorphology (sediment)

  • Diatoms

  • Macroinvertebrates

  • Fish

Impoundment


Study design61 l.jpg

Upstream reference sample areas?

Downstream sample (impacted

area)

Study Design

  • Multiple regression analysis to assess relationship between stressors (dam size) and response variables

  • Response = difference between downstream and upstream samples (dam impact)

Impoundment


15 study sites l.jpg
15 Study Sites areas?

  • Site Selection Criteria:

  • Comparable reaches

    • Geomorphology

    • Gradient

    • Land use

  • 1rst to 3rd stream order

  • Mostly forested



Slide64 l.jpg

Stressor-Response areas?Relationships

Response: downstream - upstream difference

Stressor (dam size variables): hydraulic residence time,

dam height, volume


Slide65 l.jpg

Riparian Vegetation areas?

More native species below big dams

Floodplains:

Fewer exotics below big dams and more below small dams

Log(volume)

Log(dam height)


Slide66 l.jpg

Riparian Vegetation areas?

  • More native species below big dams

  • Floodplains:

  • Fewer exotic species below big dams

  • more below small dams


Why fewer exotics below large dams l.jpg
Why fewer exotics below large dams? areas?

  • Consistent with the hypothesis that dams reduce immigration and thus the downstream dispersal of plants


Why more exotics below small dams l.jpg
Why more exotics below small dams? areas?

  • Dispersal from disturbed area around the dam site?


Slide69 l.jpg

Riparian Vegetation areas?

  • More native species below big dams

  • Floodplains:

  • Fewer exotic species below big dams

  • more below small dams


Slide70 l.jpg

Outlier: Stimson Run Reservoir areas?

  • Large dam for a water supply reservoir

  • Small, isolated watershed with no exotics upstream


Summary l.jpg
Summary areas?

Dam size influences dam effects on downstream exotic species richness (if exotic species are present within the watershed upstream)

Implications

All dams are not the same

Dams influence dispersal processes

Exotic species dispersal is a consideration for dam removals


Conclusions l.jpg
Conclusions areas?

  • Riparian plant communities are strongly driven by the flood regime which influences disturbance, seed dispersal, and ultimately plant community diversity and invasibility

  • Dams disrupt these processes and thus impact downstream plant communities

    • Changes in exotic species invasion

    • Changes in diversity are expected


Where do we go from here l.jpg
Where do we go from here? areas?

Effects of dams on riparian vegetation:

  • Need to expand the study:

    • More dams and more regions

    • Dam removals

  • Need to develop a more mechanistic understanding of the effects of dams on riparian plant communities


Research directions effect of dams on riparian vegetation in the pacific northwest l.jpg
Research directions: effect of dams on riparian vegetation in the Pacific Northwest

Elwha River, Olympic Peninsula, WA

  • Dam removal in 2008

  • Opportunity to assess dam impacts on seed dispersal and community diversity pre- and post-dam removal

  • Research consortium

Glines Canyon Dam, Photo courtesy NPS


Hydrologic alteration in the skagit river newhalem wa before and after lake ross l.jpg
Hydrologic alteration in the Skagit River, in the Pacific NorthwestNewhalem, WA before and after Lake Ross


Slide76 l.jpg

Polygonum cuspidatum in the Pacific Northwest(Japanese knotweed)


Hydrologic alteration may promote japanese knotweed dominance l.jpg

Seed in the Pacific Northwest

Seed Bank

A

Seedling

D

B

1 stem plants

Fragment

C

Large, clonal plants

E

Hydrologic alteration may promote Japanese Knotweed dominance

  • Low flow periods could promote seed germination

  • Loss of high magnitude floods may allow the exotic plant to become dominant


Hydrologic alteration may promote japanese knotweed dominance78 l.jpg

Seed in the Pacific Northwest

Seed Bank

A

Seedling

D

B

1 stem plants

Fragment

C

Large, clonal plants

E

Hydrologic alteration may promote Japanese Knotweed dominance

  • Low flow periods may promote seed germination

  • Loss of high magnitude floods may allow JK to become dominant

Implication: dam operations could be used as a tool for management of exotic plant species invasions and restoration of native riparian plant communities


Slide79 l.jpg

Acknowledgements in the Pacific Northwest

  • My Colleagues at the Patrick Center for Environmental Research and at the UNC Plant Ecology Lab

  • Advisors: Bob Peet, David Hart

  • Data: Carolina Vegetation Survey

  • Funding: Pennsylvania Growing Greener Program, Sigma Xi Grants in Aid of Research, The Southern Appalachian Botanical Society, The USDA Forest Service, The Nature Conservancy, The Graduate School at UNC, and The Ecology Curriculum at UNC


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