Low abundance effects of alien plants on native plant richness

Maud Bernard-Verdier & Philip E. Hulme

EMAPi 2015 – September 24th

Invasion impact assessed at high alien abundances

Invasion impact assessed at high alien abundances

Very abundant or dominant

Absent or rare

?

Local abundance of the alien plant

Native richness

Native richness and abundance of the alien

• At what abundance do alien plants start having an impact?

• At the landscape scale, what implications for risk assessment and management?

Species A

Species B

Species C

a1 a2 a3

Criticalabundance values

Local abundance of the alien plant

Native richness

Study of a highly invaded landscape

Banks Peninsula New Zealand

40 % plant species are non-native

5 mi

Study of a highly invaded landscape

5 mi

Study of a highly invaded landscape

Extensive vegetation survey

Vegetation survey (1982-1985) by Hugh D. Wilson

Systematic 1000 x 1000 yards grid6 x 6 m plots

Extensive vegetation survey

Vegetation survey (1982-1985) by Hugh D. Wilson

Systematic 1000 x 1000 yards grid6 x 6 m plots

Abundance scores:

1. Rare2. Occasional3. Frequent4. Common5. Abundant6. Dominant

Identifying gradients of alien abundance

751 grassland plots466 vascular plant species

Species selection criteria:

≥ 3 abundance classes≥ 5 obs. per class

= 55 focal alien species

At what abundance are alien speciesassociated to a decrease in native richness?

Native richness along an alien abundance gradientN

ativ

e α

-ric

hn

ess

Contrasts with rare abundances (GLM)

= Effect size

Local abundance of the alien plant

e.g. Lolium perenne

Assigning critical abundancesEf

fect

siz

e co

mp

ared

to

rar

e(b

oo

tstr

ap 9

5%

CI) Critical abundance

= lowest abundance at which we observe a significantly negative effect followed by only negative effects

e.g. Lolium perenne

Occa

sio

na

l

Fre

qu

en

t

Co

mm

on

Ab

un

da

nt

Do

min

an

t

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Local abundance of the alien plant

0

10

20

30

40

50

Occ

asio

nal

Frequ

ent

Com

mon

Abu

ndan

t

Dom

inan

t

A range of critical abundances

55 Alien species : 11 critical abundances

Nu

mb

er o

f sp

ecie

s

All species

Negative effects

Critical

abundances

• 20% of aliens associated with a decrease in native richness

• >80% of critical abundances were below the highest abundance class

• Proportionally more frequent toward higher abundances

0.0

0.2

0.4

0.6

0.8

1.0

Occ

asio

nal

Frequ

ent

Com

mon

Abu

ndan

t

Dom

inan

t

Pro

po

rtio

n p

er c

lass

Occa

sio

nal

Fre

qu

en

t

Com

mo

n

Ab

und

an

t

Dom

inan

t

-3

-2

-1

0

1

2

3

Occa

sio

nal

Fre

qu

en

t

Com

mo

n

Ab

und

an

t

Dom

inan

t

-2

-1

0

1

2

Dactylis glomerata

Occa

sio

nal

Fre

qu

en

t

Com

mo

n

Ab

und

an

t

Dom

inan

t

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Lolium perenne

Low to high critical abundances

Achillea millefolium

Different shape of relationships between alien

abundance and native richness = different mechanisms?

High critical abundances are not the general rule

Effe

ct s

ize

What consequences at the landscape scale ?

Spread of potential impact across the landscape

Achillea millefolium Dactylis glomerata

Presence 233 611

Spread of potential impact across the landscape

Achillea millefolium Dactylis glomerata

Dominance

Presence 233 611

14 15

Spread of potential impact across the landscape

Above critical abundance

Dominance

Presence 233 611

14 15

171 15

Achillea millefolium Dactylis glomerata

Changes ranking of species

Scaling-up to γ-richness across plots

αi

αi

αi

αi

αi

αi

αiαi

αi

αiαi

αiγ

γ-richness : total number of unique species across plots

How do declines in local richness translate to the regional species pool?

Rar

e

Occ

asiona

l

Frequ

ent

Com

mon

Abu

ndan

t

Dom

inan

t

0

50

100

150

200

γ-richness

Trends in native γ-richness

Local abundance of the focal species

e.g. Lolium perenneCritical abundance

for α-richness

γ-richness

Rar

e

Occ

asiona

l

Frequ

ent

Com

mon

Abu

ndan

t

Dom

inan

t

0

50

100

150

200

Trends in native γ-richness

Expected given the number of plots in each abundance class

Local abundance of the focal species

Critical abundance for α-richness

e.g. Lolium perenne

239 4411664106100

γ-richness

Rar

e

Occ

asiona

l

Frequ

ent

Com

mon

Abu

ndan

t

Dom

inan

t

0

50

100

150

200

Trends in native γ-richness

Expected given the number of plots in each abundance class

Local abundance of the focal species

Critical abundance for α-richness

e.g. Lolium perenne

Effect Size of decrease in γ-richness

239 4411664106100

Loss in mean α-richness above critical abundances

Loss

in γ-r

ich

nes

s co

mp

ared

to

nu

llLoss in α vs. γ richness at critical abundance

• No correlation

• Loss in γ-richness always larger than loss in α-richness

Conclusions

• Negative relationship between native richness and alien abundance for 20% of alien species

• Low abundance effects are widespread

• Spatial extent is not directly an indicator of impact spread

• Species loss at the landscape scale is not proportional to loss at the local scale

Implications for management

• Low abundance alien species may be currently overlooked

• We might be underestimating impacts at the landscape scale

Prioritizing species based on impact at the landscape scale

• low vs. high critical abundances suggest different underlying ecological processes of invasion

- Different management targets

- Different management approaches

Bio-Protection Research CentrePO Box 85084Lincoln UniversityLincoln 7647, New ZealandP + 64 3 423 0932F + 64 3 325 3864 www.bioprotection.org.nz

Thank you

Hugh WilsonFederico TomasettoJennifer BuffordTyler BrummerWill Godsoe