Ecology Letters, (2003) 6: 257–264
REPORT
Invasive California poppies (Eschscholzia californica
Cham.) grow larger than native individuals under
reduced competition
Elizabeth A. Leger*
and Kevin J. Rice
Department of Agronomy and
Range Science and the Center
for Population Biology,
University of California, Davis,
One Shields Avenue, Davis, CA
95616, USA
*Correspondence: E-mail:
[email protected]
Abstract
Invasive plants can be larger and more fecund in their invasive range than in their native
range, although it is unknown how often this is a result of a genetically controlled shift in
traits, a plastic response to a favourable environment, or a combination thereof. Here we
present data from common garden experiments that compare the size and fecundity of
native and invasive California poppies, Eschscholzia californica Cham. Individuals from 20
populations, half from California (native) and half from Chile (invasive), were grown
both with and without competition from other plants in a container experiment and at
two field locations. There were no differences in survival between native and invasive
plants at any location. We found significant increases in size and fecundity in invasive
populations at two of three locations when poppies were grown without competition
from other plants. Our results indicate that genetic shifts in traits have occurred in
invasive populations, and that the invasive plants are better at maximizing growth and
reproduction in open environments.
Keywords
California poppy, Chile, competition, Eschscholzia californica, evolution of increased
competitive ability, fecundity, invasive species, plant size, trade-offs.
Ecology Letters (2003) 6: 257–264
INTRODUCTION
When organisms move to new locations, they often evolve
in the face of new selection pressures. For example,
mainland organisms colonizing island habitat often change
after they become established: seeds can lose their dispersal
ability, birds and insects can evolve flightlessness, and
mammals often undergo size shifts (Foster 1964; Carlquist
1974). How often does evolutionary change accompany
colonization and range expansion in human introduced
invasive species? Despite engaging in the study of invasive
species for some decades now, the answer to this question is
still largely unknown. While it is clear that invasive species
often behave very differently in their introduced ranges, it is
not clear whether this behaviour is most commonly
attributable to a plastic response to a new environment, to
a genetically controlled shift in traits, or both.
Some researchers have proposed that invasive individuals
are often larger in their introduced range than in their native
range (Elton 1958; Crawley 1987; Blossey & Nötzold 1995).
Recently, however, others have challenged the idea that an
increase in the size of invasive individuals is a pervasive
phenomenon (Thebaud & Simberloff 2001). What is the
evidence suggesting that invasive individuals are indeed
larger than their native counterparts? Two types of studies
have been performed to address this question: observational
studies and common garden experiments. Observational
studies have been conducted to compare the maximum size
of native and invasive individuals of the same species both
at home and in the invasive range using size information
published in regional floras (Crawley 1987; Thebaud &
Simberloff 2001). Results of these studies provide similar
evidence for the increased size of invasive populations: 43
and 36% of plants in Crawley’s, and Thebaud and
Simberloff’s studies, respectively, were larger in the introduced range, while the rest were either smaller or not
different. These types of studies, of course, have their
limitations; among them being the unknown quality and
consistency of plant size measurements in published floras.
In addition, mean or median plant size may be a more
2003 Blackwell Publishing Ltd/CNRS
258 E. A. Leger and K. J. Rice
meaningful measure of the size differences between invasive
and native individuals than the maximum size recorded for a
species (Thebaud & Simberloff 2001).
These studies do not attempt to differentiate between a
plastic response to a new environment and a genetic change
in invasive populations. Common garden experiments can
make this distinction by providing a comparison of both
native and invasive individuals of the same species grown
together in the same environment. Common garden
comparisons of invasive and native plants have been carried
out only a handful of times (Pritchard 1960; Blossey &
Nötzold 1995; Blossey & Kamil 1996; Willis et al. 2000;
Siemann & Rogers 2001). These studies have compared the
size of invasive and native individuals of seven different
species. In three species comparisons, plants from invasive
populations were larger than plants from native populations
whereas in the remaining four comparisons there were no
differences.
There have been no attempts to measure differences in
phenotypic plasticity (the ability of a genotype to produce
different phenotypes in variable environments) between
native and invasive populations. We present a series of
common garden studies conducted under both field and
controlled conditions designed to compare the size and
fecundity of invasive and native plant populations, and to
test for differences in performance under variable growing
conditions. Many invasive species colonize disturbed
habitats (Hobbs & Huenneke 1992) which are characterized
by temporary increases in resource availability (Davis et al.
2000; Davis & Pelsor 2001) and reductions in resident
species diversity (Levine & D’Antonio 1999; Lyons &
Schwartz 2001). Our design tested whether invasive and
native plant populations respond differently to disturbance.
We predicted that invasive populations might grow larger
and exhibit greater fecundity under disturbed conditions
created by vegetation removal.
MATERIALS AND METHODS
Our study focuses on the comparison of native California
poppies, Eschscholzia californica Cham. (Papaveraceae), with
populations that are invasive in Chile. Endemic to western
North America, E. californica is an invasive species in other
areas of the world, chiefly those with Mediterranean
climates (Stebbins 1965). Eschscholzia californica grows across
a wide range of environmental conditions in its native range,
often occupying open, naturally disturbed environments
(Cook 1962). It also grows quite well in human disturbed
environments and is commonly planted along roadsides in
California. Variable in its home range, E. californica
populations differ significantly in morphology and lifehistory characteristics throughout its range. At one point,
the genus was split into 112 different species (Green 1905),
2003 Blackwell Publishing Ltd/CNRS
but only 10 species are currently recognized, and much of
the previously noted variation is now included within
E. californica (Hickman 1993). There are three common
variants of E. californica that are relevant to this study: first,
plants from coastal environments that are perennial and
usually short in stature with prostrate growth and yellow
flowers; second, perennial plants from non-desert, inland
areas that generally grow taller and have orange flowers,
sometimes called E. californica var. crocea (Benth.); and third,
an annual form that occurs in desert regions, sometimes
called E. californica var. peninsularis (Greene) (Munz 1963).
These varietal differences persist when plants are grown in
common environments (Boucher 1985).
The introduction of poppies into Chile most likely
occurred from multiple sources during the mid-1800s to
early 1900s. There are reports of intentional introductions
into botanic gardens in both coastal and inland cities, and
purposeful spread of seeds along railroad tracks and in
private gardens (Frias et al. 1975; Arroyo et al. 2000). It is
also likely that there were accidental introductions through
the import of alfalfa seed, as E. californica was a common
seed contaminant in alfalfa grown in California, and trade in
agricultural products between the two areas expanded
rapidly during the 1850s (Gillis 1885; Hillman & Henry
1928). All plants in Chile appear to be perennial varieties
and grow primarily in human disturbed environments (Frias
et al. 1975).
California and Chile have very similar climates, topographies, vegetation types and share many of the same
invasive species (Mooney 1977; Arroyo et al. 2000). Seeds
used in this experiment were collected from poppies
across a wide but similar range of environments in both
areas, sampling both coastal and inland areas over 4 of
latitude (Table 1). As all plants in Chile appear to be
perennial, collections from California used in this experiment were limited to perennial populations. Differentiating
native populations in California from horticultural varieties
used for revegetation, horticulture and highway beautification is not a straightforward task, particularly for a
species as widely planted as E. californica. Our criteria for a
population to be considered native were (1) plants not
growing along a roadside and (2) presence of other native
vegetation not likely to be used in seed mixes or for
revegetation (i.e. stands of native bunchgrasses or other
native species not of horticultural interest). Seeds were
collected from plants growing on coastal bluffs, sand
dunes, rocky slopes, river washes, etc. As all populations
in Chile are exotic, seed collection was much more
straightforward: seeds were collected along road-cuts,
railroad tracks, dry riverbeds and agricultural fields. At
each population, seeds were collected from 25 to 30
individual plants with a minimum distance of 2 m
between target plants. Seed from individual plants were
Size differences in invasive Eschscholzia 259
Table 1 Collection date and location of Eschscholzia californica seeds used in this study
Country of origin
Region
Collection date
Collection site
Latitude and longitude
California,
California,
California,
California,
California,
California,
California,
California,
California,
California,
California,
California,
Chile
Chile
Chile
Chile
Chile
Chile
Chile
Chile
Chile
Chile
Coast
Coast
Coast
Coast
Coast
Inland
Inland
Inland
Inland
Inland
Inland
Inland
Coast
Coast
Coast
Coast
Coast
Inland
Inland
Inland
Inland
Inland
July 1999
July 1999, 2000
July 1999
July 1999
July 1999
2000
June 2000, 2001
July 2001
May 2000
May 2000
July 2001
July 1999
December 2000
December 1999
December 2000
December 2000
December 2000
December 1999
December 2000
December 1999
December 2000
December 2000
Fort Bragg
Albion
Bodega Bay
Plaskett Creek
Morro Bay
Lodoga* Bear Valley
Snow Mountainà
Cedar Grove
Dry Creek Austin Creekà
Gilroy
Los Vilos
Valparaı́so
San Antonio
Pichilemu
Constitución
Ovalle
La Aguada
San Felipe
Talagante
San Fernando
3926¢N; 12348¢W
3912¢N; 12345¢W
3819¢N; 12303¢W
3555¢N; 12128¢W
3520¢N; 12051¢W
3918¢N; 12229¢W
3904¢N; 12224¢W
3920¢N; 12245¢W
3844¢N; 12039¢W
3841¢N; 12053¢W
3834¢N; 12303¢W
3703¢N; 12132¢W
3153¢S; 7129¢W
3258¢S; 7130¢W
3333¢S; 7132¢W
3423¢S; 7159¢W
3527¢S; 7229¢W
3035¢S; 7111¢W
3138¢S; 7111¢W
3246¢S; 7043¢W
3340¢S; 7055¢W
3435¢S; 7058¢W
USA
USA
USA
USA
USA
USA
USA
USA
USA
USA
USA
USA
*Bulk collection of seeds from plants grown from wild collected seeds.
All populations used in all experiments, except: seeds used in container experiment only, àseeds used in field experiment only.
kept separate and stored in paper envelopes at room
temperature (15–25 C) until planting.
Seeds were planted in three different common gardens
in California: a container experiment and two field
experiments planted at a coastal and an inland site. Within
each common garden, plants were grown in two different
environments: either alone or with competition from other
plants. Twenty populations were used in each experiment:
10 invasive populations from Chile and 10 native
populations from California. The 10 populations from
each country included five coastal and five inland
populations. Seed from 18 populations were used in all
three experiments: the seed supply from two populations
was exhausted in the container experiment, and two
separate populations were substituted for them in the field
experiments (see Table 1).
Container experiment
Nineteen individuals from each of the 20 populations were
randomly selected and their seeds were sown in three-gallon
tree pots filled with UC mix (composed of 25% each of
Canadian sphagnum, white pumice, redwood compost, and
washed sand, with 1.7 kg oyster shell calcium and dolomite
lime added per cubic metre of soil). Seeds from each
individual plant were represented twice: once growing alone
in a competition free environment and once growing with
competition. An individual from 13 of the 20 populations
(chosen at random) was sown in one of 26 extra pots, both
with and without competition, for a total of 786 individual
pots. Pots were placed in a complete randomized design on
outdoor benches in Davis, California (3832¢31¢¢ N,
12145¢48¢¢ W).
Seeds were weighed prior to planting. As seeds of
E. californica can be dormant, all seeds were soaked in a 5 mg
per 100 mL)1 solution of giberellic acid overnight before
planting. This treatment is known to increase time to
emergence slightly, but does not significantly affect growth
rate or leaf size of poppy plants (Fox et al. 1995).
Horticultural poppy seeds were planted in four positions
in half of the pots to create a competitive environment, and
pots were watered throughout the growing season. The
experiment was conducted from April 2001 to August 2001,
when plants were harvested. Both above and below-ground
biomass were dried at 65 C to a constant weight. Fecundity
was estimated by first counting the total number of seed
capsules per plant. We then estimated the total number of
seeds produced per plant by collecting five capsules (unless
the plant made fewer than five flowers; then all capsules
were collected), counting the seeds, and multiplying the
2003 Blackwell Publishing Ltd/CNRS
260 E. A. Leger and K. J. Rice
average number of seeds produced per capsule by the total
number of capsules produced.
Field experiments
Eight individuals were randomly selected from each of the
20 populations (half invasive and half native, half from the
coast and half from inland locations) and planted in
common gardens in two locations. Seeds from the same
individuals were planted in both field locations and some of
these individuals were also planted in the container
experiment, as they were randomly sampled from the same
pool of maternal families. One common garden was in an
agricultural field in the central valley in Yolo County,
California (3832¢23¢¢ N, 12147¢18¢¢ W), and the other in an
open lot on the coast in San Mateo County, California
(3732¢36¢¢ N, 12230¢42¢¢ W). Both areas were tilled and
levelled in November 2001. Weed matting was anchored to
the ground in both sites, and circles of varying diameter
were cut into the weed mats: 25 cm for each plant assigned
to a competition treatment and 12.5 cm for plants assigned
to reduced competition treatments. Seeds were sown
directly in the ground spaced 45 cm from each other in a
complete randomized design, with no treatment to break
dormancy. Seeds from each individual were planted both
with and without competition, totaling 320 plants at each
location. Competition was provided by the background
vegetation present at each site, which was primarily invasive
forbs and grasses in both locations. Potential competitors
were hand weeded from the reduced competition treatment.
Because of their large size and vigorous growth, competitors
in the coastal garden were trimmed every few weeks.
Repeated sowing in the coastal location of some seeds
during November and December was necessary because of
extensive seedling damage by slugs snails, and small
mammal burrows. This herbivory and disturbance were
curtailed using bait and deterrence methods. Plants in the
inland garden were harvested as they senesced during a 3week period in June 2002. Plants in the coastal garden
were harvested at the end of July 2002. Above-ground
biomass was collected and weighed as in the container
experiment, and total number of seed capsules produced
per plant was counted as a measure of reproductive
output.
Analyses
was used to measure differences in survival between
invasive and native plants; analyses were conducted on
survival per population nested within country of origin. Our
measure of survival combined germination success and
survival at the end of the experiment. We tested for a
relationship between seed size and final plant size in the
ANOVA
2003 Blackwell Publishing Ltd/CNRS
container experiment using linear regression, and used
logistic regression to determine if there was an effect of seed
size on survival. Because of intense seedling herbivory at the
coastal common garden, we performed two survival analyses
for that location. First, we compared survival between
invasive and native seedlings that died early because of
herbivory and small mammal disturbance. A second analysis
was conducted on seeds planted after disturbance and
herbivory were controlled. Standard transformations failed
to make size and fitness data conform to the assumptions of
normality and homogeneity of variances required for
parametric ANOVA; therefore, size and fitness data were
rank-transformed prior to performing ANOVA (Conover &
Iman 1981). Analyses of size and fitness were performed
with country of origin and growing environment (with or
without competition) as fixed effects, and with population
nested within country as a random effect. Planting date was
included as a covariate in the coastal common garden
experiment. F ratios and significance tests were conducted
using the expected mean squares (EMS) method in JMP IN
4.0.2 (SAS 2001), which calculated an appropriate mean
square denominator for each F-test in our nested mixed
models. All analyses were conducted using JMP IN 4.0.2
and significance was measured at the P ¼ 0.05 level.
RESULTS
Survivorship
There were no statistically significant differences in survivorship between invasive and native populations in any of the
experiments (Fig. 1). There was no statistically significant
relationship between seed size and plant survivorship with
competition (r2 ¼ 0.0092, P ¼ 0.1822; n ¼ 393) or without
competition (r2 ¼ 0.0026, P ¼ 0.8882; n ¼ 393) in the
container experiment.
Size and fecundity
Initial seed size (mean 1.719 mg, SE 0.0158 mg) did not
significantly covary with the final size of plants grown with
competition (P ¼ 0.1336, r2 ¼ 0.01; n ¼ 283) or without
competition (P ¼ 0.4174, r2 ¼ 0.0026; n ¼ 318) in the
container experiment. There was a significant interactive
effect of growth environment (with or without competition)
and country of origin on shoot size, and number of seed
capsules produced in both the container experiment and the
coastal field site (Tables 2 and 3). Plants from Chile had larger
shoots and made more seed capsules than plants from
California only when grown without competition (Fig. 2). In
the container experiment, there was a significant interaction
between total number of seeds produced and the country of
origin, such that invasive plants made more seeds only when
Size differences in invasive Eschscholzia 261
DISCUSSION
100
90
Chilean
Californian
Percentage of survival
80
70
60
50
40
30
20
10
0
Containersa
Coastalb
Coastalc
Inlandd
(Initial planting) (Later planting)
Figure 1 Percentage survival of invasive (Chilean) and native
(Californian) Eschscholzia californica in three different common
gardens. Two separate analyses are presented for the coastal
garden: one is of survival after intense herbivory and small
mammal disturbance early in the experiment (b), and the other is
survival of seeds replanted in these locations (c). No differences are
significant; bars are mean values and SE based on population
averages. (a) F ¼ 0.0703, P ¼ 0.7939, d.f. ¼ 1,18; (b) F ¼ 0.0204,
P ¼ 0.8881, d.f. ¼ 1,18; (c) F ¼ 0.6018, P ¼ 0.4480, d.f. ¼ 1,18,
(d) F ¼ 0.0589, P ¼ 0.8110, d.f. ¼ 1,18.
grown without competition (Table 2). Root size was only
significantly effected by growing environment and population
of origin, and no interaction terms were significant, although
the tendency (P ¼ 0.0857) is to show the same interaction as
the other responses in the container experiment.
In the inland field experiment, neither shoot size nor
number of seed capsules per plant were significantly affected
by any of the factors, including competition treatment
(Table 3). Interaction terms were also not significant,
however interaction plots of size and number of seed
capsules are shown in Fig. 2 for comparison with other
experiments.
We found size and fecundity differences between invasive
and native populations of E. californica grown in a common
environment, and these differences were only found when
plants were grown in an environment with reduced
competition. Under reduced competition, plants from
invasive populations exhibited a greater plastic increase in
size and fecundity than did native populations. These
differences were found in two of three common gardens: no
differences between native and invasive populations were
found in the inland location.
The most obvious difference between the inland experiment and the others is that sample size in this experiment is
smaller, variance is high, and consequently power to detect
differences between groups is low. As in the coastal common
garden, there was high mortality at this site during the seedling
stage, but no re-seeding was conducted for a variety of
reasons. Immediately after planting and during seeding
emergence, the area experienced repeated and prolonged
frosts, which were followed by heavy rain that caused
temporary flooding of the area, and finally, the site experienced exceptionally low temperatures accompanied by
significant snowfall. In contrast to the coastal garden, these
very unusual weather events eliminated any window of
opportunity in which to replant seeds within a reasonable
amount of time. Summers at this location are always hot and
dry. All these factors, combined with the fact that competition
had no significant effect on plant size at this location when it
had such a large effect in the other gardens, may indicate that
abiotic factors were a primary limitation for growth and
survival in this experiment. It is possible that under these
conditions, neither invasive nor native populations are
superior, nor do poppy plants benefit much from disturbed
conditions.
The capacity of invasive poppy populations to exhibit
greater plasticity in above-ground growth and reproduction
under reduced competition in some environments does not
appear to be related to allocation differences. One might
Table 2 Size, number of seed capsules, and seed production in container experiment. Analyses are
ANOVA
performed on rank-transformed
data
Shoot mass
Source of variation
d.f.
F ratio
P-value
Number seed capsules
Total number of seeds Root mass
d.f.
d.f.
F ratio P-value
Country
1
2.73
0.1157
1 1.45
Competition
1 124.79
<0.0001
1 70.07
Population (country)
18 12.18
<0.0001 18 13.37
Country · competition
1
9.65
0.0058
1 4.97
Competition · population (country) 18
0.6094
0.8936 18 1.27
Error
559
558
0.2441
<0.0001
<0.0001
0.0385
0.1997
F ratio P-value
1 7.76
1 33.11
18 4.90
1 7.06
18 1.86
453
0.0119
<0.0001
0.0007
0.0147
0.0170
d.f.
F ratio P-value
1 0.74
1 58.90
18 7.30
1 3.29
18 0.87
560
0.4008
<0.0001
<0.0001
0.0857
0.6195
2003 Blackwell Publishing Ltd/CNRS
262 E. A. Leger and K. J. Rice
Table 3 Shoot size and number of seed capsules in field experiments. Analyses are ANOVA performed on rank transformed data, with planting
day included as a covariate in the coastal field experiment
Coast
Source of variation
Inland
Shoot mass
Number seed capsules
Shoot mass
d.f.
d.f.
d.f. F ratio P-value d.f. F ratio P-value
F-ratio P-value
Country
1
5.79
Competition
1 130.34
Population (country)
18
2.78
Country · competition
1
4.95
Competition · population (country) 18
0.98
Planting day
1 12.26
Error
177
0.0258
<0.0001
0.0178
0.0352
0.4900
<0.0006
P-value
1
7.8879
0.0106 1
1 144.04
<0.0001 1
18
1.79
0.1126 18
1
5.85
0.0236 1
18
1.25
0.2279 18
1 10.98
<0.0011 –
177
85
expect that a greater allocation to growth and reproduction
in the invasive populations would be accompanied by a
concomitant reduction in allocation to roots. However, in
the container experiment, the trend (P ¼ 0.0857) is for
invasive plants to also have larger roots than native plants
when grown without competition (Table 2).
Although the phenotypic differences in size and fecundity
between country of origin in these plants are likely to be
heritable, they could also be affected by parental influences
that are not entirely genetic. Parental environment can effect
many traits of progeny including germination, plant size,
flowering time and expression of herbivore defences (Roach
& Wulff 1987; Rossiter 1996; Mousseau & Fox 1998; Agrawal
et al. 1999). Seed size is a commonly measured parental effect
that can influence plant size and survival, yet there was no
statistically significant effect of seed size on either of these
factors in the container experiment. The expression of a
parental effect can depend on the environment in which the
offspring are grown (Rossiter 1998), and seed size has been
shown to affect plant attributes in the field when no effects
were detected in greenhouse experiments (i.e. Stanton 1984).
However, previous experiments with poppies in the same
coastal common garden location also showed no effect of
seed size on plant growth or survival (E.A. Leger, unpublished
data). We think that it is unlikely that parental effects would
explain the increase in size and fecundity of invasive plants in
this experiment. In order for a parental environmental effect
to significantly influence these results, it would have to be
pervasive and sizeable enough in both California and Chile to
outweigh all the differences in habitat type, resource
availability and climate that are represented in the sites that
were sampled.
A potential lack of herbivores and pathogens in the
invasive range is often proposed as a cause for invasive
species success. An increase in growth caused by a release
from herbivore and pathogen pressure in the invasive range,
sometimes called the enemy release hypothesis (ERH) or the
natural enemies hypothesis (reviewed in Maron & Vila 2001;
2003 Blackwell Publishing Ltd/CNRS
F ratio
1.29
0.79
0.88
0.93
0.78
–
Number seed capsules
0.2702
0.3861
0.6078
0.3467
0.7228
–
1
1
18
1
18
–
85
0.87
0.74
1.03
1.84
0.38
–
0.3630
0.3998
0.4725
0.1918
0.9889
–
Keane & Crawley 2002), is often cited as a possible cause of
a plastic increase in the size of invasive individuals. If release
from enemies were the main factor controlling success of
the invasive poppy, one would not expect to find genetic
differences in size or fecundity in areas where enemy
pressure is equal. Thus, the ERH alone cannot explain the
dominance of invasive populations in disturbed environments.
Alternatively, genetically controlled increases in size
and ⁄ or fecundity are predicted by the evolution of
increased competitive ability (EICA) hypothesis, which
suggests that the large size of invasive plants will result from
selection for less-resistant individuals, which hypothetically
shift resource use from potentially costly resistance traits to
growth (Blossey & Nötzold 1995). This increase in size is
predicted to coincide with a decrease in resistance to
specialist herbivores. Our results show a genetic shift
towards larger size of invasive populations, as predicted by
the EICA hypothesis, but we have no data on specialist
herbivore preference or performance on our plants. It is
interesting, however, that invasive plants were not decimated upon return to their natural environment, which
indicates that either (a) invasive plants are not less resistant
to natural enemies than natives, (b) invasives are more
tolerant of damage than are natives, or (c) enemies were not
present in sufficient numbers to reveal differences in
resistance. Without further experimentation, it is not
possible to differentiate between these possibilities.
Conclusions from a single year study of survival and
reproduction in a perennial plant are necessarily somewhat
limited. However, a primary focus of this study is on the
relative ability of invasive poppy populations to take
advantage of ephemeral, human disturbed habitats, where
the ability to grow and reproduce successfully in one season
is likely to be very important. Further experimentation could
determine the nature of the selection pressures causing these
shifts in the invasive populations. For example, are changes
in the invasive populations the result of selective pressures
Size differences in invasive Eschscholzia 263
(a) 8
45
Chilean
Number of flowers
Weight (g)
40
Californian
7
6
5
4
No
competition
(b) 120
Chilean
No
competition
With
competition
No
competition
With
competition
No
competition
250
Number of flowers
Californian
Weight (g)
With
competition
300
100
80
60
40
20
200
150
100
50
0
0
With
competition
No
competition
(c) 18
80
Chilean
16
70
Number of flowers
Californian
14
Weight (g)
25
15
With
competition
ber of seed capsules produced by Eschscholzia
californica plants grown from seeds collected
from native (Californian) and invasive (Chilean) populations, when grown with and
without competition from other plants, in
three different common garden environments. Bars are mean values and SE.
(a) Container experiment, (b) coastal field
experiment, (c) inland field experiment.
Interactions in experiment (a) and (b) are
significant (P < 0.05), but not in experiment
(c), where interaction plots are shown for
comparison with other experiments.
30
20
3
Figure 2 Above-ground plant size and num-
35
12
10
8
60
50
40
30
6
4
20
With
competition
unique to Chile, or are these changes a result of the selective environment associated with generic man-made disturbances? In areas where fields are plowed and roads are
graded, selection for short-term size increases and seed
production may outweigh other factors that limit the growth
of native poppies in naturally disturbed sites in the wild. It
would be instructive to compare ruderal roadside populations from California with native populations from naturally
disturbed sites to see if they also grow larger and produce
more seeds in the first year.
This and other studies have demonstrated that invasive
individuals can indeed be on average larger and more fecund
than native individuals. In this study, the differences
between invasive and native poppy populations suggest an
increased capacity in the invasive plants to grow and
No
competition
reproduce quickly in disturbed environments. Whether this
phenomenon is common, and whether or not it is most
often attributable to a chance effect of founding populations, selection for pre-adapted individuals, or the evolution
of new weedy genotypes, is still unknown.
ACKNOWLEDGEMENTS
We would like to thank those who assisted with this project,
particularly Paul and Clare Leger and Matt Forister, and
Maraya Cornell and the members of the Rice Lab for field
assistance. Thanks to Dr Jose ÔKongoÕ Farina and Dr
Lafayette Eaton for assistance and advice in Chile. This
manuscript was improved by suggestions from Erin
Espeland, Matt Forister, Rick Karban, John Maron and
2003 Blackwell Publishing Ltd/CNRS
264 E. A. Leger and K. J. Rice
John Randall. This work was financially supported by the
Jastro-Shields Research Scholarship award at the University
of California, Davis, and by the UC Davis Botanical Society.
EAL was financially supported during this project by the
Department of Agronomy and Range Science at UC Davis
and the IGERT for biological invasion studies at UC Davis.
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Manuscript received 7 October 2002
First decision made 15 November 2002
Manuscript accepted 13 December 2002