Blossey & Notzold, 1995

Evolution of Increased Competitive Ability in Invasive Nonindigenous Plants: A
Hypothesis
Bernd Blossey; Rolf Notzold
The Journal of Ecology, Vol. 83, No. 5. (Oct., 1995), pp. 887-889.
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Journal of
Ecology 1995,
83,887-889
FORUM
Evolution of increased competitive ability in invasive
nonindigenous plants: a hypothesis
BERND BLOSSEY* and ROLF NOTZOLD
International Institute of Biological Control, European Station, 1, Chemin des Grillons, CH-2800 Delimont,
Switzerland; Zoologisches Institut, Christian-Albrechts-Universitat, Olshausenstr. 40, 0-24098 Kiel, Germany
Introduction
There are very few valid generalizations about invasive
species, so that it is only possible to make weak,
probabilistic predictions about which species will
invade (Gilpin 1990; Daehler & Strong 1993). A
phenomenon that has not received much attention,
however, is the observation that, in alien environments,
plants tend to be more vigorous and taller,
producing more seeds than in their native distribution
(Crawley 1987). The seed production of Chrysanthemoides
monilijera (native to South Africa) in
Australia and of Acacia longijolia (native to Australia)
in South Africa is an order of magnitude higher where
the plants are aliens (Noble 1989). The vigour and
success of aliens in areas where they have been introduced
has been attributed to both, more favourable
environments, and to the release from natural phytophagous
enemies (Crawley 1987).
These alternatives can be evaluated according
to two theories relating to resource allocation. The
optimal defence hypothesis predicts that plants with
limited resources will show trade-offs in biomass
allocation among maintenance, growth, storage,
reproduction, and defence (Coley et al. 1985; Bazzaz
et al. 1987; Fagerstrom 1989; Herms & Mattson 1994;
Lerdau et al. 1995). The environmental constraint
hypothesis (Bryant et al. 1988), however, predicts that
the evolution of defence mechanisms against herbivory
will result in only minor reductions in growth.
Resource availability is then the primary force determining
whether there is surplus photosynthate available
for allocation to secondary metabolism.
Optimal defence theory and invasions: development
of the evolution of increased competitive
ability (EICA) hypothesis
The theoretical framework of the optimal defence
hypothesis can be applied to invasive plants. Invas-
*Present address: New York Cooperative Fish and Wildlife
Research Unit, Department of Natural Resources, Cornell
University, Fernow Hall, Ithaca, NY 14853, USA. Fax:
607 2551895. E-mail: bb22@cornell.edu
iveness of nonindigenous plants is then seen as a result
of shifts in biomass allocation patterns. In the absence
of herbivores, selection will favour genotypes with
improved competitive abilities and reduced resource
allocation to herbivore defence. Competitive abilities
can be maximized by increasing vegetative growth
or reproductive efforts depending on which is more
important for success in a particular new environment.
This hypothesis predicts (a) that, under identical
growing conditions, individuals of a species taken
from an area where they have been introduced will
produce more biomass than individuals taken from
the species native range; and (b) specialized herbivores
(i.e. those with potential for introduction as biological
control agents) will show improved performance on
plant individuals originating from an area where
plants have been introduced.
In preliminary experiments we tested the predictions
of the EICA hypothesis using a comparison
of the growth of Lythrum salicaria L. (purple loosestrife)
plants from two locations (one with and one
without natural herbivory). In addition, we evaluated
the performance of herbivorous insects which had
been studied for their potential as biological control
agents against L. salicaria, on plants from both
locations. Purple loosestrife is an Eurasian wetland
perennial introduced into North America in the early
1800s. Its spread across the temperate part of the
continent has degraded many prime wetlands. Large,
monotypic stands eliminate native plant communities
and threaten endangered plant and animal species
(Thompson et al. 1987; Malecki et al. 1993). Once
established, L. salicaria populations persist over decades
in North America and tend to encroach upon
adjacent areas (Thompson et al. 1987). In contrast,
populations in Europe are rapidly invaded by other
plant species and L. salicaria is a regular but
infrequent component of mixed wetland communities
(Shamsi &Whitehead 1974; Blossey 1991). In Europe,
displacement from communities is promoted by specialized
insects which devastate tissues, both above
and below ground, but these insects were absent from
North America until 1992 when three species were

888
B. Blossey &
R. Notzold
Table 1 Mean ( k SE) dry biomass and height at the end of development time or pupal weights in experiments
the growing (N = 20 per group) purple with the leaf-feeder (Table 2). However, excised leaves
loosestrife plants from Ithaca (US) and Lucelle (Switzerland),
Plants were grown in a common garden under were maintained in Petri dishes for raising larvae of
identical conditions
the leaf-feeder and this might have altered their quality
as food for the herbivores. In addition, the results
Ithaca Lucelle P* of our experiments might reflect different defence
investments of purple loosestrife. Foliage is easily
Dry biomass (g)
1991 29.3 & 1.9 16.3 & 1.4 i0.001 replaceable, generally within a few days, whereas root
1992 96.0 k 12.3 28.7 & 4.3 < 0.001 tissue is essential for over-wintering survival, com-
Plant height (cm)
petitive success in the following growing season and
1991 99.7 k 4.4 82.6 k 2.0 0.001 long-term survival. If foliage were less well defended
1992 177.8 k 4.8 109.7 7.4 i0.001
only minor differences would be expected between
*Probability value (t-test).
European and North American plants. This would
explain why larval performance of the leaf-feeder is
not significantly different on plants from Ithaca or
introduced as biological control agents (Hight 1990; Lucelle.
Blossey 1993, 1995; Malecki et al. 1993).
In fall 1990 seeds were collected from at least 20
individual well-spaced plants from populations in
Ithaca, New York, USA and Lucelle, Switzerland.
Alternative explanations?
Seeds were shipped to Christian-Albrechts University, Though we can not exclude the possibility, we believe
Kiel, northern Germany. In spring 1991 seeds were that maternal effects offer no explanation for the large
germinated on a mixture of sand and potting soil. differences observed among plants from Ithaca and
After establishment seedlings were transferred indi- Lucelle. In general, environmentally induced varividually
into clay pots (10 cm in diameter) and kept ation has been demonstrated for species with larger
outdoors in a common garden 30 km south of Kiel. seeds where seeds with higher weight had higher
Plants were grown in commercial potting soil (real viability, germinated faster, and produced more vigsoil,
mixture of Sphagnum moss and compost, nutri- orous seedlings (Nelson et al. 1970). L. salicaria has
ents added by manufacturer) and placed at random. extremely small seeds (0.5mg) and such carry-over
In 1991 and 1992 final plant height and dry biomass effects seem highly unlikely especially if plants are
of 20 plants from each site were determined. We also grown for more than one growing season as in our
tested larval performance of a root-feeding weevil on experiments (Nelson et al. 1970).
potted plants and of a leaf-feeding chrysomelid beetle We also believe that phenotypic plasticity cannot
on foliage of plants from both locations.
be the explanation for the observed differences in
The results of our preliminary tests confirmed pre- growth between introduced and native populations of
dictions that plants from the area with low herbivore a plant species or between European and American
pressure show increased vegetative growth. Plants purple loosestrife. In the absence of herbivory phenofrom
Ithaca grew taller and produced more biomass typic plasticity should lead to increased vegetative
in both years (Table 1). At the same time survival growth not only where species are aliens but also in
of the root feeder was higher and larval weight was their native lands where low and high levels of herincreased
if larvae were raised on plants from Ithaca bivory fluctuate over time but are generally unprecompared
to larvae on plants from Lucelle (Table 2). dictable. Phenotypic plasticity in plant growth should
There were no significant differences in survival rates, reflect the currently experienced herbivore pressure.
Table 2 Performance of a leaf-feeder (Galerucella pusilla) and a root-feeder (Hylobius transversovittatus) on plants from Ithaca
(US) and Lucelle (Switzerland). Survival and development time data of G. pusilla are means (+ SE) of five replicates with 20
larvae each; weight data are means ( k SE) of 19 pupae in each group. Weight data for H. transversovittatus are means (+ SE)
of 32 larvae on plants from Ithaca and 15 larvae on plants from Lucelle
Ithaca Lucelle P*
Galerucella pusilla
Survival (%)L,-Imago 34 k 9.8 40 & 11.4 0.700
Development time (days) L,-Imago 25.9 k 0.5 25.7 & 0.4 0.760
Pupal weight (mg) 4.4 & 0.3 3.9 k 0.1 0.186
Hylobius transversovittatus
Survival (%) 62.7 18.1 iO.Ol**
Larval weight after one year (mg) 42.2 + 5.1 32.6 k 5.8 0.040
*Probability value (t-test).
**Chi-square test x2 = 25.785.

889 The observation that plants grow taller where they
Competitive ability are aliens supports the EICA hypothesis.
in invasive
nonindigenous
plants
Conclusions
The results of our preliminary tests with one population
from each continent supported the two predictions
of the 'evolution of increased competitive
ability' hypothesis. However, the origin of introduced
plant species is not generally well known and differences
might exist in biomass allocation patterns
among populations in the native and alien range of a
plant. Multiple introductions are likely for L. salicaria
(Thompson et al. 1987) but all populations in the
native range experience the impact of the same herbivores
over much of its distribution (Dieckmann
1963; Silfverberg 1974; Blossey 1993). Comparisons
of more populations from both continents and other
plant species are needed to investigate whether we are
just comparing populations with different biomass
allocation patterns or whether plants from areas of
introduction consistently produce more biomass and
are less well defended. These experiments are currently
under way for L. salicaria.
Acknowledgements
We thank A. Blossey, P. McEvoy, R. Malecki, M.
Williamson, and three anonymous reviewers for
improving earlier versions of this paper.
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