Biology Reference
In-Depth Information
Table 11.1
Seeds,
Urophora
, and
Larinus
found in flower heads, and estimates of seed production
for
C. stoebe
in Colorado (updated from Seastedt et al. 2007)
Sample
Number per flower head
a
Seed production
(number m
−2
)
Size
Seeds
Urophora
Larinus
Ye a r
2002
294
1.46 (0.180)
0.30 (0.041)
0.89 (0.025)
No data
2003
311
4.26 (0.335)
0.43 (0.053)
0.63 (0.036)
4,600
2004
429
3.06 (0.250)
1.15 (0.073)
0.10 (0.014)
1,030
2005
540
2.15 (0.151)
1.81
b
(0.065)
0.74 (0.018)
260
2006
288
1.80 (0.215)
0.57
b
(0.064)
0.57 (0.032)
0
2007
227
6.01 (0.441)
0.14
b
(0.031)
0.80 (0.042)
2,970
a
Values are means and SE from flower heads from a single population
b
Urophora
values after 2004 include estimates of
Urophora
present but consumed by
Larinus
.
Results obtained before 2005 are based on live
Urophora
or pupae fragments
plant competition. The study documented lower abundance of the weed after several
years in plots where competing vegetation remained intact, and using their estimates
we predict that the seed production and competition at our site will eventually result
in knapweed densities reduced to low levels of persistence.
While we are not certain if foliage herbivory reduces survivorship of imma-
ture plants, the combined impact of root feeders in addition to seed predators
best explains results of our long-term monitoring efforts. Corn et al. (2007) sug-
gest that insect effects are independent of climate. At our site, we have empha-
sized measurements on seed production. Our results appear to parallel those of
Schirman (1981) that suggest that plants produce more seeds in wetter years, or
in years that follow drought years. Plant size, seeds produced per flower head,
and seeds produced per unit area appear to be influenced by the amounts and
seasonality of precipitation. Both 2003 and 2007 were years of high spring mois-
ture that followed dry years, and both years resulted in pulses of plant growth
and seed production. The ability of this species to exploit spring rainfall and
increase seed production following drought supports the notion that spotted
knapweed is an effective opportunist (Hill et al. 2006), similar to diffuse knap-
weed (LeJeune et al. 2006; Seastedt and Suding 2007) and yellow star thistle
(Gerlach and Rice 2003).
11.5
Resolving Research Contradictions and Developing
a Management Framework
While factors identified to be responsible for the dominance and persistence of
Centaurea
spp. often seem contradictory, we believe that the collective findings
reported here can be reconciled to a conceptual model for
Centaurea
. This model
