Skurski T, Maxwell, BD, and Rew...
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The Ecological Society of America Conference, Albuquerque, New Mexico. August 2-7, 2009. Skurski T, Maxwell, BD, and Rew LJ Quantifying changes in density and spatial extent of multiple Centaurea maculosa Lam (spotted knapweed) populations. Exotic species vary in their rate of establishment, spread and impact at both the local and landscape scale. Changes in exotic species’ density and spatial extent over time are often used as surrogate measures of impact; hence, it is important to quantify these rates across a range of environments. We studied the spatial and temporal growth of Centaurea maculosa populations across four different natural plant communities. At each site we measured changes in C. maculosa presence/absence and density in 33 0.25 m 2 plots (132 plots total). Temporal population growth rate was measured as the change in population density (N) over the change in time (t) or δN/δt, where δN = N t /N t-1 or λ. Spatial growth of populations was measured in two ways: (1) proportional change in spatial extent over time, and (2) local colonization and extinction rates. The probability of colonization was measured as the number of plots unoccupied in t that became occupied in t + 1 divided by the number of plots observed. The probability of extinction was measured as the number of occupied plots in t that became unoccupied in t + 1, divided by the number of plots occupied in time t. We also examined the relationships between temporal growth, spatial growth and habitat suitability for C. maculosa (generated from a logistic model of presence/absence data and environmental variables). We hypothesized that not all populations of C. maculosa would increase in density and spatial extent. We also hypothesized a positive relationship between the population growth rate (λ) and habitat suitability. Of the 132 plots, 7 unoccupied in t became occupied in t +1, and 7 occupied in t became unoccupied in t +1, meaning the probability of extinction equals the probability of colonization for these populations. Twenty-one percent of populations had a λ <1, 61 percent had a λ > 1, and 18 percent were stable. Populations with λ >1 were typically increasing in spatial extent as well. We found a significant positive relationship between λ and habitat suitability. Of the 13 patches studied, 8 decreased in spatial extent and 5 increased in spatial extent. These results highlight the fact that not all exotic species consistently increase in density and spatial extent in all habitats. The positive relationship between population growth rates and habitat suitability predictions suggest these predictions could be used for exotic plant management prioritization.
Transcript of Skurski T, Maxwell, BD, and Rew...
The Ecological Society of America Conference, Albuquerque, New Mexico. August 2-7, 2009.
Skurski T, Maxwell, BD, and Rew LJ
Quantifying changes in density and spatial extent of multiple Centaurea maculosa Lam (spotted
knapweed) populations.
Exotic species vary in their rate of establishment, spread and impact at both the local and
landscape scale. Changes in exotic species’ density and spatial extent over time are often used
as surrogate measures of impact; hence, it is important to quantify these rates across a range of
environments. We studied the spatial and temporal growth of Centaurea maculosa populations
across four different natural plant communities. At each site we measured changes in C.
maculosa presence/absence and density in 33 0.25 m2 plots (132 plots total). Temporal
population growth rate was measured as the change in population density (N) over the change
in time (t) or δN/δt, where δN = Nt /Nt-1 or λ. Spatial growth of populations was measured in
two ways: (1) proportional change in spatial extent over time, and (2) local colonization and
extinction rates. The probability of colonization was measured as the number of plots
unoccupied in t that became occupied in t + 1 divided by the number of plots observed. The
probability of extinction was measured as the number of occupied plots in t that became
unoccupied in t + 1, divided by the number of plots occupied in time t. We also examined the
relationships between temporal growth, spatial growth and habitat suitability for C. maculosa
(generated from a logistic model of presence/absence data and environmental variables). We
hypothesized that not all populations of C. maculosa would increase in density and spatial
extent. We also hypothesized a positive relationship between the population growth rate (λ)
and habitat suitability. Of the 132 plots, 7 unoccupied in t became occupied in t +1, and 7
occupied in t became unoccupied in t +1, meaning the probability of extinction equals the
probability of colonization for these populations. Twenty-one percent of populations had a λ
<1, 61 percent had a λ > 1, and 18 percent were stable. Populations with λ >1 were typically
increasing in spatial extent as well. We found a significant positive relationship between λ and
habitat suitability. Of the 13 patches studied, 8 decreased in spatial extent and 5 increased in
spatial extent. These results highlight the fact that not all exotic species consistently increase in
density and spatial extent in all habitats. The positive relationship between population growth
rates and habitat suitability predictions suggest these predictions could be used for exotic plant
management prioritization.
2008.1 2009.1 2008.2 2009.2
-20
24
68
10
Population growth rate (Nt/Nt-1)
~ patch & year, Site 3
Year/patch
Po
pu
latio
n g
row
th r
ate
(N
t/N
t-1
)
2007-2008
2008-2009
0.5 1.0 1.5 2.0 2.5 3.0
02
46
810
12
14
Population growth rate (Nt/Nt-1) 2008-2009
~ proportional change in spatial extent (m^2)
Proportional change in spatial extent (m^2)
Popula
tion g
row
th r
ate
(N
t/N
t-1)
Site Specific Results
Site 1
Site 2
Site 3
Site 4
Tanya C. Skurski, Bruce D. Maxwell, and Lisa J. Rew. Montana State University
Quantifying changes in density and spatial extent of multiple Centaurea maculosa Lam. (spotted knapweed) populations
Travel scholarship provided by:
C. m
acu
losa
hab
itat
su
itab
ility
Introduction•Not all non-native plant (NIS) populations increase in density and/or spatial extent across all environments.•Knowing which populations are increasing in number of individuals and spatial extent can aid in prioritization of management.•Establishing a relationship between NIS habitat suitability and invasiveness can make population prioritization and management more efficient.
Definitions• Invasiveness : the degree to which a plant population or meta-population can be characterized as consistently increasing in
density and/or spatial extent• Monitoring for Invasiveness: measuring populations or meta-populations across a range of environments to determine change
in density and spatial extent over time. • Centaurea maculosa: non-native biennial to short-lived perennial; often found in open, disturbed areas; considered a serious
problem in the rangelands of the northwestern United States.
Objectives1. Quantify change in density of C.
maculosa populations across a range of environments
2. Quantify change in spatial extent of C. maculosa populations across a range of environments
3. Determine relationship between invasiveness (increasing in density and/or spatial extent) and habitat suitability
Conclusions• Not all C. maculosa populations consistently
increase in density and spatial extent across different environments.
• There is a significant relationship between population growth rate and habitat suitability, suggesting habitat suitability predictions can serve as a useful management prioritization tool.
• There is a significant relationship between spatial and temporal growth rates of C. maculosa, indicating that the most invasive populations will consistently increase in both density and spatial extent.
Methods• Four sites across a range of habitat
suitability- a function of slope, aspect , elevation, vegetation type, distance from road, distance from trail, and disturbance type, as determined by Rew et al. 2005
• Three distinct C. maculosa populations studied at each site
• Six 0.25m2 plots per population• 30 plots/site• 120 plots total
• Spatial extent measured in three replications with Trimble Geo XT
• Density measured by counting the number of individuals per 0.0625 m2
• Density analyzed per 0.25 m2
• Temporal population growth rate calculated as the change in population density (N) over the change in time (t)
or δN/δt, where δN = Nt /Nt-1 or λ. • Spatial population growth was
calculated as a proportion: area m2
t / area m2t-1
Slope: 0%Aspect: 90 EElevation: 1515m
Slope: 35-40%Aspect: 90 EElevation: 1720-1780m
Slope: 0-10%Aspect: 0 NElevation: 1670m
Slope: 12-15%Aspect: 220 WElevation: 1540-1560m
Patch 3
Patch 2
Patch 1
Patch 2
Patch 1
Patch 3
Patch 2
Patch 3
Patch 1
*Patch 3 densities not available
p < 0.001
Literature CitedRew, LJ; Maxwell BD; Aspinall, R (2005) Predicting the occurrence of non-indigenous species using environmental and remotely sensed data. Weed Research 53,236-241
Combined Results
Artemesia tridentataPseudoroegneria spicatum
Festuca idahoensisBalsamorhiza sagittata
Pseudotsuga menziesiiSymphoricarpos albus
Phleum pratensePoa pratensis
Pseudoroegneria spicataFestuca idahoensis
Koeleria cristataArtemisia frigida
Pseudoroegneria spicataFestuca idahoensisBouteloua gracilis
Hesperostipa comata
1 2 3
0.6
0.7
0.8
0.9
1.0
1.1
1.2
Change in spatial extent
2007 to 2009, Site 4
Patch
Pro
po
rtio
na
l ch
an
ge
in
are
a (
m^2
)
1 2 3
0.6
0.8
1.0
1.2
1.4
Change spatial extent
2007 to 2009, Site 3
Patch
Pro
po
rtio
na
l ch
an
ge
in
are
a (
m^2
)
1 2 3
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Change in spatial extent
2007 to 2009, Site 1
Patch
Pro
po
rtio
na
l ch
an
ge
in
are
a (
m^2
)
1 2 3
0.0
0.5
1.0
1.5
Change in spatial extent
2007 to 2009, Site 2
Patch
Pro
po
rtio
na
l ch
an
ge
in
are
a (
m^2
)
p < 0.001
0.5 1.0 1.5 2.0 2.5 3.0
010
20
30
40
50
Population growth rate (Nt/Nt-1) 2007-2008
~ proportional change in spatial extent (m^2)
Proportional change in spatial extent (m^2)
Popula
tion g
row
th r
ate
(N
t/N
t-1)
0.1 0.2 0.3 0.4
010
20
30
40
50
Population growth rate (Nt/Nt-1) 2007-2008
~ habitat suitability
Habitat suitability
Popula
tion g
row
th r
ate
(N
t/N
t-1)
0.1 0.2 0.3 0.4
02
46
810
12
14
Population growth rate (Nt/Nt-1) 2008-2009
~ habitat suitability
Habitat suitability
Popula
tion g
row
th r
ate
(N
t/N
t-1)
p < 0.001
p < 0.001
2008.1 2009.1 2008.2 2009.2 2008.3 2009.3
-20
24
68
10
Population growth rate (Nt/Nt-1)
~ patch & year, Site 4
Year/patch
Po
pu
latio
n g
row
th r
ate
(N
t/N
t-1
)
2007-2008
2008-2009
2008.1 2009.1 2008.2 2009.2 2008.3 2009.3
-20
24
68
10
Population growth rate (Nt/Nt-1)
~ patch & year, Site 2
Year/patch
Po
pu
latio
n g
row
th r
ate
(N
t/N
t-1
)
2007-2008
2008-2009
Fundingsupport from:
2008.1 2009.1 2008.2 2009.2 2008.3 2009.3 2008.4 2009.4
010
20
30
40
50
60
Population growth rate (Nt/Nt-1)
~ patch & year, Site 1
Year/patch
Popula
tion g
row
th r
ate
(N
t/N
t-1)
2007-2008
2008-2009Patch 2
Patch 1
Patch 3
Patch 4
p < 0.001
p =0.192
