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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

Biodiversity and exotic species invasion in riparian plant communities

Rebecca Brown

Eastern Washington University

slide2

Riparian Areas

  • 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)
slide3

REALLY diverse!

>180 plant species per 1000 m2

40 exotic plant species

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

slide5

Exotic Species

  • Threat to global biodiversity
  • Expensive to remove
  • Understanding what makes habitats susceptible to species invasion is one of the fundamental questions in ecology
slide6

Is invasibility related to diversity?

“…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

slide7

Is invasibility related to diversity?

“…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

Invasion

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
Why care?

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
PresentationOverview

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?
slide11

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
Immigration processes in riparian areas:
  • Flood disturbance
  • Increased seed supply from flood waters
  • High net resource availability
slide13

Seed Supply

Conceptual relationship between disturbance, seed supply, and species richness

slide14

Seed Supply

Conceptual relationship between disturbance, seed supply, and species richness

presentation overview15
PresentationOverview

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

0.1 m

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
slide20

Bar

Floodplain

Terrace

slide21

Seed Supply

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

slide22

Native Species Richness Over Four Landforms

Native Species Richness per 100 m2

Bar Floodplain Terrace Upland

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

Floods often No floods

slide23

Native Species Richness per 100 m2

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

A

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
PresentationOverview

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
PresentationOverview

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

Spatial scale effects

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

p < .001

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

p < .001

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

p < .001

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

p < .001

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
PresentationOverview

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
PresentationOverview

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

Seed supply:

The richness and abundance of seeds arriving at a site

slide35
131 seed traps placed near vegetation plots

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

slide37

No flood Flood

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
PresentationOverview

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
PresentationOverview

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

1

High dispersal

Similarity

Low dispersal

0

Distance

Distance Decay of Similarity
slide41

Measuring Similarity

  • 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
Measuring Similarity
  • Calculate Sorensen similarity between every possible pair of plots in each geomorphic position
  • Compare distance decay of similarity for each geomorphic position
does this indicate higher dispersal in flooded sites
Does this indicate higher 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
Summary of Results
  • 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
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
How are diversity and invasibility related?

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
Broader Implications
  • 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
PresentationOverview

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

Dam impacts that may affect riparian plant communities

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
Dams

Represent a large scale ‘experiment’

Baker Dam, Concrete, WA

slide52

Seed Supply

Predicted shift in species richness caused by dams

slide53

Seed Supply

Predicted shift in species richness caused by dams

slide54

Flooding

  • 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

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

Bruce Babbitt

study design

Upstream reference sample

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

Upstream reference sample

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
15 Study Sites
  • Site Selection Criteria:
  • Comparable reaches
    • Geomorphology
    • Gradient
    • Land use
  • 1rst to 3rd stream order
  • Mostly forested
slide64

Stressor-ResponseRelationships

Response: downstream - upstream difference

Stressor (dam size variables): hydraulic residence time,

dam height, volume

slide65

Riparian Vegetation

More native species below big dams

Floodplains:

Fewer exotics below big dams and more below small dams

Log(volume)

Log(dam height)

slide66

Riparian Vegetation

  • More native species below big dams
  • Floodplains:
  • Fewer exotic species below big dams
  • more below small dams
why fewer exotics below large dams
Why fewer exotics below large dams?
  • Consistent with the hypothesis that dams reduce immigration and thus the downstream dispersal of plants
why more exotics below small dams
Why more exotics below small dams?
  • Dispersal from disturbed area around the dam site?
slide69

Riparian Vegetation

  • More native species below big dams
  • Floodplains:
  • Fewer exotic species below big dams
  • more below small dams
slide70

Outlier: Stimson Run Reservoir

  • Large dam for a water supply reservoir
  • Small, isolated watershed with no exotics upstream
summary
Summary

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
Conclusions
  • 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
Where do we go from here?

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
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 may promote japanese knotweed dominance

Seed

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

Seed

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

Acknowledgements

  • 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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