Transcript
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
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