Journal of Ecology 2010, 98, 237–245
doi: 10.1111/j.1365-2745.2009.01606.x
Plant genetic diversity yields increased plant
productivity and herbivore performance
Agnieszka M. Kotowska, James F.Cahill Jr* and B. Andrew Keddie
Department of Biological Sciences, CW 405, Biological Sciences Centre, University of Alberta, Edmonton, AB T6G
2E9, Canada
Summary
1. Plant genotypic diversity has important consequences for a variety of ecosystem processes, yet
empirical evidence for its effects on productivity, one of the most fundamental of these processes, is
lacking. In addition, the performance of insect herbivores in response to high genotypic diversity is
unknown, despite previous work demonstrating differential herbivore performance among plant
genotypes.
2. We manipulated genotypic diversity of the annual plant Arabidopsis thaliana in both the presence
and absence of the generalist herbivore Trichoplusia ni under semi-natural growth conditions. We
used nine genotypes (eight ecotypes and one mutant) of A. thaliana known to differ widely in functional traits. Productivity and insect biomass were measured in monocultures and mixtures of all
nine genotypes grown at multiple fertilization levels and planting densities.
3. In both the absence and presence of herbivores, genotypic diversity increased plant productivity
and survival. This effect was, for the most part, independent of fertility or density. Sampling or
selection effects did not appear to be wholly responsible for these results as all genotypes were maintained in equal proportion and no single genotype became dominant for the duration of the experiment.
4. High diversity increased T. ni biomass and survival in all treatments. Insect biomass was positively, but not tightly, correlated to plant biomass, indicating that the higher herbivore performance
observed in genotypic mixtures was only partially due to higher productivity.
5. Synthesis. Our data support the idea that even within a single plant species, genotypic diversity
can exert strong influences on both the producer and herbivore communities. The exact mechanisms
responsible for these effects and the relative importance of genotypic diversity in natural communities warrant further investigation.
Key-words: Arabidopsis thaliana, biodiversity, genetic diversity, herbivory, populations, productivity, Trichoplusia ni
Introduction
In recent decades, there has been considerable interest in the
relationship between biodiversity and ecosystem function.
A large body of work (reviewed by Hooper et al. 2005) has
commonly demonstrated the positive effects of plant diversity
on ecosystem processes such as primary productivity (Jolliffe
1997, Hector et al. 1999; Tilman et al. 2001; Fargione et al.
2007), ecosystem stability (Loreau 2000), nutrient cycling
(Naeem et al. 1994; Tilman, Wedin & Knops 1996) and resistance to exotic plant invasions (Knops et al. 1999). Plant genotypic diversity, even without changes in species diversity, may
have important consequences for ecosystem function as
well (reviewed by Hughes et al. 2008), but empirical studies
*Correspondence author. E-mail: [email protected]
documenting such effects have only recently been conducted.
Here, we examine primary productivity at high and low genotypic diversity and determine whether concomitant changes in
insect performance impact upon levels of herbivory and plant
damage.
There are reasons to believe that plant genetic diversity may
alter the functioning of several fundamental ecological processes. For example, different plant genotypes have been
shown to vary in important functional traits, such as competitive ability (Cahill, Kembel & Gustafson 2005), forage quality
for insect herbivores (Karley et al. 2008) and resistance to
herbivory (Maddox & Root 1987; Wise 2007). Although the
variation among genotypes may be smaller than that among
species, it may nevertheless be large enough to impact
ecosystem function, as has been shown for species diversity.
Empirical evidence supporting this contention is growing and
2009 The Authors. Journal compilation 2009 British Ecological Society
238 A. M. Kotowska, J. F. Cahill & B. A. Keddie
studies have shown plant genetic diversity to improve ecosystem resistance to exotic plant invasions (Crutsinger, Souza &
Sanders 2008), enhance plant community resistance to extreme
climatic events (Reusch et al. 2005) and grazing (Hughes &
Stachowicz 2004), accelerate litter decomposition rates
(Schweitzer et al. 2005), maintain long-term species diversity
(Booth & Grime 2003; Vellend 2006), reduce plant disease
severity (Zhu et al. 2000) and impact arthropod diversity and
community composition (Wimp et al. 2005; Johnson, Lajeunesse & Agrawal 2006). However, few studies have been undertaken to demonstrate the existence of diversity–productivity
relationships at the genotypic level (but see Hughes &
Stachowicz 2004; Crutsinger et al. 2006), or the effects of such
relationships on insect herbivores. Genotypic diversity may, as
has been shown for interspecific diversity (Hooper & Vitousek
1997; Loreau & Hector 2001; Tilman et al. 2001; Hector et al.
2002; Cardinale et al. 2007; Tylianakis et al. 2008), potentially
enhance primary productivity through additive (i.e. sampling
effects) or non-additive mechanisms (i.e. complementarity,
selection effects).
Identifying the effects of biodiversity on ecosystem
processes requires measurement of responses at multiple
trophic levels, rather than primary productivity only, and
manipulation of more than one trophic group (Duffy et al.
2007). There is increasing evidence that plant genotypes,
even within a single species, can exert differential effects
on insect herbivore performance and community structure
(Service 1984; Cronin & Abrahamson 2001; Wimp et al.
2005). The effects of multiple plant genotypes grown in
mixture on herbivore performance, however, are unknown.
In particular, it is unclear whether herbivore performance
can be wholly predicted by genotype performance, or
whether genetic mixtures may influence herbivores in a
non-additive manner. For instance, insect herbivores may
achieve higher growth rates when grown on genetically
mixed, as opposed to single-food, diets (Bernays et al.
1994; DeMott 1998; Mody, Unsicker & Linsenmair 2007).
Changes in insect abundance or performance, whether positive or negative, can potentially ‘cascade’ back to affect the
plant community through higher or lower biomass consumption. For instance, Mulder et al. (1999) found a positive relationship between plant species diversity and invertebrate
herbivore damage, and notable changes in the diversity–productivity relationship upon exclusion of invertebrates from
experimental plots. Although the functional trait differences
among plant genotypes may be minor compared with those
among species, they may nevertheless be substantial enough to
cascade beyond one trophic level (Johnson 2008).
In this study, we examined how genetic diversity of the
model plant Arabidopsis thaliana (L.) Heynh. influenced primary productivity and insect herbivory by the generalist herbivore Trichoplusia ni (Hübner). The experiment was conducted
on rooftop facilities in the Department of Biological Sciences
at the University of Alberta, Edmonton, Canada. Nine genotypes (ecotypes) of A. thaliana were grown in monocultures or
mixtures of all nine genotypes. These non-naturally co-occurring genotypes were chosen to maximize differences in impor-
tant functional traits (J. F. Cahill, unpubl. data) and allow us
to detect genotypic effects. Both diversity treatments were replicated at two levels of each of fertilization, density and herbivory in a fully factorial design, for a total of eight treatment
combinations at each level of diversity. In total, 520 pots and
11 700 individual plants were used. By performing our study
under controlled conditions and restricting biota to only one
plant and one herbivore species, we were able to test if, (i) genotypic richness impacts primary productivity, (ii) genotypic richness impacts herbivore performance, and (iii) richness and
herbivory interact to differentially affect the plant community.
Materials and methods
STUDY SYSTEM
The model plant A. thaliana is a fast-growing, weedy annual with a
worldwide distribution. Because of its ability to self-fertilize and prolific seed production, identical individuals can be grown to serve as
replicates. We selected eight ecotypes and one mutant differing substantially in low-nutrient stress tolerance, palatability and competitive ability (J. F. Cahill, unpubl. data). Our goal in using these specific
genotypes was to maximize the potential functional variation within
a population rather than replicate the genetic diversity found in any
particular natural population. In doing so, we are providing a test of
whether diversity–function relationships could occur, with future
studies needed to determine their relative strengths within natural
populations or in other species.
The effects of diversity on insect herbivores and on herbivore
damage to producers were tested using the cabbage looper T. ni. This
species was chosen because it is an important agricultural pest
throughout North America, feeding on many species of crucifers,
including A. thaliana. In addition, the species is routinely bred in the
laboratory, allowing for relative uniformity among individuals in
terms of diet and growth rate.
EXPERIMENTAL DESIGN
The experiment was conducted on rooftop facilities at the
Department of Biological Sciences, University of Alberta, Edmonton, Canada. Plants were grown in genetic monocultures or mixtures
under all possible treatment combinations (eight in total) of fertilization, density and herbivory in a fully factorial experiment. Seedlings
were allowed to grow for 1 week before they were transplanted into
the appropriate treatment pots, and insects were added to herbivory
treatments 4 weeks after transplantation. Harvesting of plant
biomass took place c. 7 weeks after seed germination.
Diversity and density treatments
Individuals of A. thaliana were grown in monoculture (1 genotype per
pot) or in mixture (9 genotypes per pot) at a density of either 9 (low
density) or 36 (high density) individuals per 10-cm2 pot. In monocultures, all 9 or 36 individuals were of the same genetic line, while mixtures contained either 1 or 4 individuals of each line.
Fertilization treatment
Half of the pots in each diversity · density treatment combination
received 16 mg m)2 slow-release NPK 4 days after transplantation
into flats.
2009 The Authors. Journal compilation 2009 British Ecological Society, Journal of Ecology, 98, 237–245
Genotypic diversity and insect performance 239
Herbivory treatment
STATISTICAL ANALYSES
Cohorts of third instar T. ni larvae (mean weight 5.54 ± 0.08 mg
insect)1) were added to half of all diversity · density · fertilization
treatment combinations. Five larvae were added to each pot subjected to the herbivory treatment c. 5 weeks after seed germination,
before most plants had bolted. Insects were allowed to move freely
among the plants within a pot.
All analyses were performed using Linear Mixed Model procedures
in spss v. 14.0 (SPSS Inc. 2005). Plant and herbivore biomass were
square-root transformed prior to analysis to meet assumptions of
normality and homogeneity of variance. Diversity, density, fertilization and herbivory were treated as fixed factors and blocks were
included as random effects in all statistical models. These analyses
yielded similar results for individual plant biomass as for combined
pot biomass, thus analyses for individual biomass are not presented.
Individual herbivore biomass was estimated by dividing pot herbivore biomass by the number of insects weighed, as raw measurements
of individuals were not available. Direct comparisons between treatments (reported as per cent difference) were made using least squares
means, thus they were based on square-root transformed rather than
raw data.
The effects of the experimental treatments on both plant and
herbivore survival were determined using a generalized linear mixed
model (Proc Glimmix) in SAS 9.2 (SAS Institute Inc. 2008). In the
analysis of herbivore survival, the number of living herbivores
served as the response variable (Poisson error distribution; missing
insects were treated as dead), block served as a random effect, and
fertilization, density and genetic diversity served as the fixed effects.
In the analysis of plant survival, the number of living plants found
at the end of the experiment served as the response variable
(Poisson error distribution), block served as a random effect, and
fertilization, density, herbivore presence and genetic diversity served
as the fixed effects.
Replication
In total, 80 unique treatment combinations were obtained: eight mixture treatments (2 density · 2 fertilization · 2 herbivory = 8) and
72 monoculture treatments (9 genotypes · 2 density · 2 fertilization · 2 herbivory = 72). Monocultures were replicated five times
for each treatment combination, for a total of 72 treatments · 5 replicates = 360 pots (8100 individuals). All mixture treatment combinations were replicated 20 times, for a total of 8 treatments · 20
replicates = 160 pots (3600 individuals). In total, 11 700 individual
plants were used.
Growth conditions
The experiment was conducted in outdoor facilities under semi-natural growth conditions, where treatment pots were exposed to wind,
rain, hail and natural light. Five replicate 80 · 220 cm blocks, 90 cm
apart, were established to control for environmental conditions. Each
block contained 104 pots, corresponding to one of each of the nine
genetic monoculture · fertilization · density · herbivory treatments
(72 pots) and four replicates of each genetic mixture · fertilization · density · herbivory treatment (32 pots).
Seeds were obtained from the Arabidopsis Biological Resource
Center in Columbus, Ohio, USA (http://www.biosci.ohio-state.
edu/pcmb/Facilities/abrc/abrchome.htm). A seed increase was
conducted on lines under uniform growing conditions. The resulting offspring were then used in the study. Seeds were placed on
moist filter paper and placed in a refrigerator for 48 h prior to
being transplanted into the experimental pots at the beginning of
the growing season (May). We used a 3 : 1 sand : soil mix in all
pots, ensuring the low-fertilization treatment was, in fact, low in
nutrients. Soon after planting the seeds, we added fertilizer
[16 mg m)2 Osmocote slow-release 14 : 14 : 14 fertilizer (The
Scotts Company, Marysville, OH, USA)] to the fertilized treatments. Pots were weeded of volunteer plants after c. 1 week of
growth, such that only one seedling of a given genotype
remained in each planting location.
Harvest
Insects were confined to pots for 7 days before being removed
for enumeration (number alive, dead, and missing) and weighing
(fresh weight). Only insects found alive at time of harvest were
weighed and included in biomass analyses. Upon removal of larvae from herbivory treatments, all plants in the experiment were
allowed to grow for 1 week before each plant was clipped at
ground level and weighed (dry weight). As different ecotypes of
A. thaliana flower at different times (Pigliucci 2003), it was not
possible to measure seed set without introducing error related to
growing time. However, because seed production is highly correlated with above-ground biomass in this species (Cahill, Kembel
& Gustafson 2005), we assumed that biomass would serve as a
surrogate measure of overall plant fitness.
Results
PLANT PERFORMANCE IN THE ABSENCE OF T. NI
In the absence of any herbivores, plant genetic diversity
significantly increased whole-pot plant biomass by 17%
relative to genetic monocultures (Fig. 1a,b; Table 1).
High plant density and fertilization also increased
plant biomass (Fig. 1a,b; Table 1), such that maximum
biomass occurred in high density, fertilized mixtures
(Fig. 1a,b). No significant two- or three-way interactions
were observed (Table 1).
PLANT PERFORMANCE IN THE PRESENCE OF T. NI
To assess how insect damage to plants differed among diversity, density and fertilization treatments, herbivory was added
as a fixed factor to the analysis.
Plant survival
The proportion of plants surviving was significantly higher
in genetic mixtures than in monocultures, and this diversity effect was consistent across fertilization, density and
herbivory treatments as evidenced by the lack of any
significant two- or three-way interactions (Table 1). Plant
survival was not significantly different among fertilization
treatments, but fewer plants survived in high compared
with low density treatments and when plants were
exposed to insect herbivores (Table 1).
2009 The Authors. Journal compilation 2009 British Ecological Society, Journal of Ecology, 98, 237–245
240 A. M. Kotowska, J. F. Cahill & B. A. Keddie
way interaction between herbivore presence, fertilization and
density. Plant biomass reduction by herbivores was higher in
fertilized treatments (33%) compared with unfertilized (12%)
treatments at high planting density, whereas at low density,
this effect was reversed: herbivore suppression of plants was
greater at low (25%) than at high fertility (15%).
Plant biomass
Herbivory significantly reduced plant biomass in all treatment
combinations (Table 1): experiment-wide reduction in plant
biomass in herbivory treatments compared with no herbivory
treatments was 22%. While the addition of herbivores did not
negate the positive influence of high diversity on plant biomass (Table 1), it did decrease the size of the effect: in the
absence of herbivores, plant biomass was 17% higher in mixtures than in monocultures, but only 5% higher in their presence (Fig. 1). Total pot biomass was significantly higher in
high than in low density treatments (16%), and in fertilized
than in unfertilized treatments (50%; Table 1; Fig. 1c,d).
However, plant biomass was also dependent upon a three-
HERBIVORE PERFORMANCE
Herbivore survival
Insect survival was higher in the plant genetic mixtures compared with genetic monocultures and also higher when plants
were fertilized (Table 2). A greater number of insects survived
Density
Low
High
0.5000 (a)
(b)
0.4000
Diversity
Monoculture
Mixture
Absent
0.3000
0.1000
0.0000
0.5000 (c)
Herbivores
(d)
0.4000
Present
Plant biomass (g pot–1)
0.2000
0.3000
0.2000
Fig. 1. Above-ground plant biomass (mean±
1 SE) per pot (9 plants in low density or 36
plants in high density) as a function of diversity, density and fertility in the absence and
presence of herbivores.
0.1000
0.0000
Unfertilized
Fertilized
Unfertilized
Fertilization
Fertilized
Table 1. Plant population biomass and survival in all experimental pots in response to diversity, fertilization and density (fixed effects) when
herbivory treatments were excluded or included. Block was included as a random factor in all analyses. Subscripts under F ratios indicate
numerator and denominator degrees of freedom for each term in the mixed-model analysis
Herbivory excluded
Herbivory included
Biomass
Survival
Biomass
Source
F1,248
P-value
F1,500
P-value
F1,500
P-value
Diversity
Fertilization
Density
Herbivory
Diversity · Fertilization
Diversity · Density
Diversity · Herbivory
Fertilization · Density
Fertilization · Herbivory
Density · Herbivory
Diversity · Fertilization · Density
Diversity · Fertilization · Herbivory
Diversity · Density · Herbivory
Fertilization · Density · Herbivory
Diversity · Fertilization · Density · Herbivory
7.166
71.897
15.788
–
1.190
0.432
–
1.432
–
–
0.111
–
–
–
–
0.008
< 0.001
< 0.001
–
0.276
0.512
–
0.233
–
–
0.740
–
–
–
–
7.09
3.29
39.78
14.61
0.81
2.10
0.11
1.26
0.36
0.17
0.12
0.16
0.38
0.90
0.58
0.008
0.070
< 0.001
< 0.001
0.370
0.148
0.739
0.261
0.549
0.684
0.725
0.685
0.537
0.344
0.447
4.818
83.623
15.464
25.973
0.019
0.390
1.746
0.253
3.975
1.657
0.146
1.676
2.214
4.305
0.670
0.029
< 0.001
< 0.001
< 0.001
0.890
0.532
0.187
0.615
0.047
0.199
0.703
0.196
0.137
0.039
0.413
2009 The Authors. Journal compilation 2009 British Ecological Society, Journal of Ecology, 98, 237–245
Genotypic diversity and insect performance 241
Table 2. Herbivore survival, total biomass (all individuals in a pot) and estimated individual biomass in response to diversity, fertilization and
density (fixed effects). Block was included as a random factor in all analyses. Subscripts under F ratios indicate numerator and denominator
degrees of freedom for each term in the mixed-model analysis
Survival
Total insect biomass
Estimated individual
insect biomass
Source
F1,211
P-value
F1,211
P-value
F1,211
P-value
Diversity
Fertilization
Density
Diversity · Fertilization
Diversity · Density
Fertilization · Density
Diversity · Fertilization · Density
10.912
8.663
3.515
0.575
0.383
0.209
0.613
0.001
0.004
0.062
0.449
0.537
0.648
0.434
15.073
35.840
4.881
2.410
1.670
0.029
0.001
< 0.001
< 0.001
0.028
0.122
0.198
0.866
0.973
5.753
33.694
2.091
2.284
2.552
0.032
0.317
0.017
< 0.001
0.150
0.132
0.112
0.859
0.574
at high rather than low density, although this effect was only
marginally significant. There were no significant two- or threeway interactions (Table 2).
Table 3. Total insect biomass in monoculture in response to
genotype, fertilization and density treatments (fixed effects). Block
was included as a random factor in all analyses. Subscripts under F
ratios indicate numerator and denominator degrees of freedom for
each term in the mixed-model analysis
Herbivore biomass
Across all density and fertilization treatments, total herbivore
biomass was 19% higher when insects were reared on genetically diverse host populations than when reared on genetic
monocultures (Table 2). Overall, total insect biomass
increased by 31% with fertilization and by 11% when grown
at high density (Table 2).
Estimated individual insect biomass was 7% higher in mixtures than in monocultures and 18% higher in fertilized than
unfertilized treatments (Table 2). There was no significant
effect of density, nor any significant interaction terms
(Table 2).
Effect of genotype on herbivore performance
To examine whether herbivore performance varied with
plant genotype, we included genotype (monocultures only)
as a fixed factor in the statistical analysis. Total herbivore
biomass varied significantly among the nine A. thaliana
lines. The response to each genotype depended on fertilization (Fig. 3), although this genotype · fertilization interaction was only marginally significant (Table 3). In some
genetic monocultures (i.e. line 4, Fig. 3), fertilization actually decreased insect biomass, despite the overall positive
effect of fertilization on biomass in all treatment combinations (Table 3). The effects of density and all interactions
remained non-significant.
Discussion
PRIMARY PRODUCTIVITY
Genotypic richness of A. thaliana strongly influenced plant
survival and biomass both in the presence and absence of
insect herbivores. Plant biomass was 17% higher in
genetic mixtures than in monocultures in insect-exclusion
Total insect biomass
Source
Fd.f.
P-value
Genotype
Fertilization
Density
Genotype · Fertilization
Genotype · Density
Fertilization · Density
Genotype · Fertilization · Density
3.3548,113
7.4131,113
4.7311,113
2.2558,113
0.7418,113
0.0271,113
1.0248,113
0.007
< 0.001
0.462
0.068
0.996
0.942
0.271
treatments, 5% higher in mixtures in insect-inclusion
treatments and 11% higher in mixtures when all treatments were analysed together.
Our results are comparable with those of Crutsinger et al.
(2006), who found a 36% increase in productivity in 12-genotype mixtures of Solidago altissima in the field. This is higher
than the effect size seen here and raises the possibility that productivity may increase with richness in a linear fashion. Similarly, Johnson, Lajeunesse & Agrawal (2006) found a 27%
increase in fitness (fruit production) of Oenothera biennis in
mixtures containing up to eight genotypes compared with
monocultures. Because seed production is highly correlated
with above-ground biomass of A. thaliana (Cahill, Kembel &
Gustafson 2005), it is possible that growth at high genotypic
diversity may ultimately correspond to increased fitness of this
species. It must be noted that our results are based on a seminatural, mesocosm experiment and thus are not entirely comparable to results obtained in the field by Crutsinger et al.
(2006) and Johnson, Lajeunesse & Agrawal (2006). However,
the demonstration of positive genotypic diversity–productivity
relationships in both natural and artificial settings suggests that
these findings are biologically real, and may have important
consequences for community structure and function in natural
ecosystems.
2009 The Authors. Journal compilation 2009 British Ecological Society, Journal of Ecology, 98, 237–245
242 A. M. Kotowska, J. F. Cahill & B. A. Keddie
experiment (one generation) did not allow any genotype to
become numerous or dominant via enhanced reproductive success, thus the above definition may not apply. If, however,
mortality rates within a genotype are a function of diversity,
selection effects may occur through greater survival of highly
productive genotypes in mixture than in monoculture.
Recent work also sheds light on the importance of genetic
identity of neighbours to plant performance in diverse species
(Fridley, Grime & Bilton 2007) and genotypic assemblages
(Cahill, Kembel & Gustafson 2005). Such ‘neighbour effects’
may occur, for example, if a genotype with low fitness in monoculture expresses higher growth or lower mortality in the presence of particular neighbour genotypes in mixture. Similarly,
competitive asymmetry (Weiner 1990) between genotypes may
lead to performance in mixture (i.e. high productivity) which
cannot be predicted solely from performance in monoculture.
The identification of phenotypic plasticity and trait differences
associated with diversity effects has been previously omitted
from many biodiversity studies and constitutes a promising
direction for future research (Hughes et al. 2008).
Non-additive and additive mechanisms, outlined above, are
not mutually exclusive and may work in combination to drive
biodiversity–productivity relationships. However, a detailed
exploration of the relative contribution of each of these mechanisms to our results, although warranted, is beyond the scope
of this article.
HERBIVORE PERFORMANCE
Herbivore survival and biomass were both significantly higher
in mixtures than in monocultures of A. thaliana (Fig. 2). As
with primary productivity, the potential mechanisms responsible for this pattern fall into two broad categories. First, herbivore responses to varying plant diversity may be additive,
where observed biomass and survival of insects in genotypic
Density
Low
0.0500
High
(a)
(b)
Diversity
Monoculture
Mixture
0.0400
Insect biomass (g pot–1)
Unlike recent experiments manipulating plant genotypic
diversity only (Hughes & Stachowicz 2004; Crutsinger et al.
2006; Johnson, Lajeunesse & Agrawal 2006), the study
described here is novel in that it manipulates insect herbivore
presence or absence at two levels of diversity. By controlling
for the influence of herbivores, we were able to demonstrate
that the positive effect of genotypic diversity on plant performance is through plant-level interactions, and not necessarily
mediated by insects (i.e. through selective herbivory). This is in
contrast to the results of Hughes & Stachowicz (2004), who
detected differences between low- and high-diversity eelgrass
communities only after a grazing disturbance. A trade-off
between fast growth rates, as in A. thaliana, an annual, weedy
species, and resistance to disturbance, as in eelgrass (Hughes &
Stachowicz 2004), may explain this discrepancy. We emphasize
the need for concurrent manipulations of genotypic diversity
and arthropods in natural settings to elucidate the role of multiple trophic groups in structuring diversity–productivity relationships.
Although the design of this experiment did not allow us to
explicitly test the mechanisms responsible for higher plant biomass and survival at high genotypic diversity, several possibilities must be considered. First, mixtures may contain, by
chance, one or more highly productive genotypes which drive
total productivity. This ‘sampling’ effect (Huston 1997),
however, is not entirely applicable to this experiment: all nine
A. thaliana genotypes were present in equal proportions (1 ⁄ 9),
thus all genotypes present in monocultures were equally and
consistently represented in mixtures.
Rather, our results may be driven by non-additive mechanisms, which occur when productivity in mixture cannot be
predicted from each genotype’s performance in monoculture,
as in this study. For instance, complementarity (i.e. niche partitioning or facilitation) has been frequently demonstrated at the
species level (Tilman et al. 2001; Hector et al. 2002; Cardinale
et al. 2007), and may also play a role in diversity–productivity
relationships within populations. While it may be argued that
the functional differences among genotypes are smaller than
those among species, intraspecific variation is nevertheless
large enough to alter ecological processes (Hughes et al. 2008).
Given the global distribution of the genotypes employed in this
experiment, and the large among-genotype variability in
important functional traits such as competition (J. F. Cahill, unpubl. data), it is not unreasonable to suppose that niche partitioning or facilitation led to the high performance of
genetic mixtures. Variation in resource uptake strategies
may lead to reduced intraspecific competition and greater
overall resource capture in genetic mixtures. Decreased
ammonium concentrations in sediments in diverse eelgrass
communities (Hughes & Stachowicz 2004) suggest that niche
partitioning may occur at the genotypic scale in a similar
manner as at the species scale.
One alternative, although not mutually exclusive,
explanation is that of a ‘selection’ effect (Loreau & Hector
2001; Hector et al. 2002), which can occur if species or
genotypes with particular functional traits (i.e. high productivity) come to dominate a mixture over time. The duration of this
0.0300
0.0200
0.0100
0.0000
Unfertilized
Fertilized
Unfertilized
Fertilized
Fertilizer
Fig. 2. Mean fresh biomass (± 1 SE) of all insects in an experimental
pot in each diversity, density and fertility treatment combination.
2009 The Authors. Journal compilation 2009 British Ecological Society, Journal of Ecology, 98, 237–245
Genotypic diversity and insect performance 243
Fig. 3. Plant and insect biomass in monoculture replicates of each A. thaliana genotype at low and high density, and at each level of fertilization.
(a, b) Above-ground plant biomass (mean ± 1 SE) in monocultures in the absence of herbivores. (c, d) Summed insect fresh biomass (mean ± 1
SE) in monocultures. For reference, horizontal lines represent mean insect biomass in monoculture when fertilized (solid) or unfertilized
(dashed). Bracketed numbers are mean number of surviving insects per pot (pooled fertilization).
mixtures are wholly predictable from performance in monoculture. Under this scenario, high insect biomass in genotypic
mixtures is simply a result of greater plant quantity (see Primary productivity section, above), and we would expect to see
a tight correlation between herbivore and plant biomass.
Several lines of evidence indicate, however, that insect performance was not entirely dependent on primary productivity.
Comparison of plant biomass in herbivore-exclusion and
herbivore-addition treatments (roughly approximating plant
biomass ‘before’ and ‘after’ herbivory) reveals that insects were
unlikely to be limited by available biomass: the reduction of
plant biomass due to insect presence was only 18% in monocultures and 26% in mixtures. We also compared herbivore
biomass with plant biomass in each treatment. Herbivore
biomass reflected general patterns in primary productivity
(Fig. 1a,b; Fig. 2), but it did not match these patterns
perfectly, nor did it closely follow the productivity of each
genotype separately (Fig. 3). Linear regression analysis (not
shown) indicated that plant biomass measured in insect-exclu-
sion treatments explained only 22% of the variation in total
herbivore
biomass
(slope = 0.13919,
SE = 0.043,
F1,37 = 10.663, P = 0.002, r2 = 0.224). Collectively, these
results suggest that plant growth was an important, but not
absolute, predictor of herbivore biomass.
Alternatively, non-additive mechanisms, such as greater
food quality, may account for the disparities in insect performance observed between genotypic monocultures and mixtures. Trichoplusia ni is considered a generalist species, known
to feed on a multitude of agricultural crops and weeds (Cameron, Isman & Upadhyaya 2007), and as such may benefit from
a mixture of food items compared with only one or a few plant
species. A mixture of food items differing in nutritional quality
and toxin content may act in a complementary fashion to
improve herbivore growth rate and fitness, as has been demonstrated in laboratory feeding experiments performed on daphnids (DeMott 1998) and polyphagous insects (Bernays et al.
1994; Mody, Unsicker & Linsenmair 2007). Pfisterer, Diemer
& Schmid (2003) found a strong, positive relationship between
2009 The Authors. Journal compilation 2009 British Ecological Society, Journal of Ecology, 98, 237–245
244 A. M. Kotowska, J. F. Cahill & B. A. Keddie
plant species richness and grasshopper biomass in a natural
grassland ecosystem, indicating that the benefits of dietary
mixing are not limited to artificial settings.
Although evidence for the benefits of dietary mixing has largely been restricted to studies examining plant species richness,
similar principles may apply to genotypic richness. Host plant
genotypes have been shown to differentially affect performance and fecundity of aphids on Solidago clones (Moran
1981; Maddox & Cappuccino 1986), aphids on Rudbeckia laciniata (Service 1984) and stem gallers on Solidago altissima
(Cronin & Abrahamson 2001). Direct support for intraspecific
dietary mixing was recently demonstrated by Mody, Unsicker
& Linsenmair (2007), who found that the differences in food
quality among host plant conspecifics were large enough to
render mixed diets beneficial to lasiocampid caterpillars. In this
study, genotype had a significant effect on herbivore biomass,
suggesting that A. thaliana genotypes varied considerably in
nutritional quality or palatability (Fig. 3). A reasonable explanation for the results of our study is that in mixture, genotypes
acted in a complementary fashion to increase herbivore survival and biomass.
Interestingly, the relative palatability of each line changed as
a result of fertilization, as evidenced by a marginally significant
genotype · fertilization interaction in monocultures (Table 3).
Some genotypes which were highly palatable relative to other
genotypes at low fertilization were only somewhat palatable at
high fertilization, and vice versa. In some cases (i.e. lines 2 and
4, Fig. 3d), fertilization actually had a negative effect on herbivore biomass, despite the strong, positive effect of fertilization
in the study overall. The mechanism behind this interaction is
unclear, and, unfortunately, impossible to determine from the
data available.
Herbivores in natural systems have shown a variety of
responses to manipulations of plant species diversity (Knops
et al. 1999; Koricheva et al. 2000; Scherber et al. 2006a,
2006b, Unsicker et al. 2006). Studies examining plant intraspecific richness, however, have only recently been undertaken
and field experiments have thus far shown either an increase
(Crutsinger et al. 2008) or no change (Johnson, Lajeunesse &
Agrawal 2006) of insect herbivore abundance. Our study is
novel in that it demonstrates direct, positive effects of genotypic diversity on herbivore performance in semi-natural conditions, at multiple levels of resource availability and plant
density. Further work is needed to determine whether this
relationship is found for other insect species, and whether
non-additive mechanisms, such as complementarity (i.e. dietary mixing), can account for these results.
LEVELS OF HERBIVORY
Although insect biomass was consistently higher in mixtures
than in monocultures, levels of herbivory (as indicated by
plant performance in the presence of T. ni) did not significantly differ between the two treatments. Similarly, despite
greater insect biomass and survival at high than at low plant
densities, levels of herbivory did not differ between these two
treatments. Instead, herbivory was dependent upon an inter-
action between density and fertilization (Table 1): at high
density, herbivory was higher in fertilized treatments,
whereas at low density, herbivory was greater at low fertility
(Fig. 1). Higher survival of insects in fertilized and high density plots (Table 2) may account for these results. The interaction lends some support to the idea that changes at the
producer level can ‘cascade’ back and affect herbivory,
although it is apparent that changes in plant and herbivore
performance brought about by high diversity are not strong
enough to produce similar patterns.
Conclusions
By conducting this experiment in semi-natural, controlled conditions, we were able to directly demonstrate higher plant productivity and survival at high genotypic diversity. The
consistency of this result in both the presence and absence of
insect herbivores, and at multiple density and fertility levels,
suggests that this relationship is not necessarily mediated by,
or dependent upon, environmental factors such as fertility or
grazing disturbance. However, we did not attempt to elucidate
how richness might affect the population dynamics of either
producers or herbivores in natural populations and in field
conditions. Nevertheless, the strength of our findings suggests
that the effects seen here are biologically real and may have
important consequences for biodiversity–ecosystem function
relationships in both natural and agricultural systems. The
mechanisms responsible for the effects seen here are not readily
determined from our data, and warrant further investigation.
Future studies may benefit from measurements of phenotypic
traits and their plasticity in response to varying genotypic diversity to fully account for patterns seen in diversity–
productivity relationships.
Acknowledgements
We thank all members of the Cahill laboratory for providing comments
throughout the development of this project, and two anonymous referees for
their comments and suggestions. Grayson Alabiso-Cahill assisted with the collection of data, and funding was provided by an NSERC Discovery Grant to
J.F.C. and an undergraduate scholarship to A.M.K.
References
Bernays, E.A., Bright, K.L., Gonzalez, N. & Angel, J. (1994) Dietary mixing in
a generalist herbivore: tests of two hypotheses. Ecology, 75, 1997–2006.
Booth, R.E. & Grime, J.P. (2003) Effects of genetic impoverishment on plant
community diversity. Journal of Ecology, 91, 721–730.
Cahill, J.F., Kembel, S.W. & Gustafson, D.J. (2005) Differential genetic influences on competitive effect and response in Arabidopsis thaliana. Journal of
Ecology, 93, 958–967.
Cameron, J.H., Isman, M.B. & Upadhyaya, M.K. (2007) Trichoplusia ni
growth and preference on broccoli and eight common agricultural weeds.
Canadian Journal of Plant Science, 87, 413–421.
Cardinale, B.J., Wrigh, J.P., Cadotte, M.W., Carroll, I.T., Hector, A., Srivastava, D.S., Loreau, M. & Weis, J.J. (2007) Impacts of plant diversity on biomass production increase through time because of species complementarity.
Proceedings of the National Academy of Sciences of the United States of
America, 104, 18123–18128.
Cronin, J.T. & Abrahamson, W.G. (2001) Goldenrod stem galler preference
and performance: effects of multiple herbivores and plant genotypes. Oecologia, 127, 87–96.
2009 The Authors. Journal compilation 2009 British Ecological Society, Journal of Ecology, 98, 237–245
Genotypic diversity and insect performance 245
Crutsinger, G.M., Collins, M.D., Fordyce, J.A., Gompert, Z., Nice, C.C. &
Sanders, N.J. (2006) Plant genotypic diversity predicts community structure
and governs an ecosystem process. Science, 313, 966–968.
Crutsinger, G.M., Collins, M.D., Fordyce, J.A. & Sanders, N.J. (2008) Temporal dynamics in non-additive responses of arthropods to host-plant genotypic diversity. Oikos, 117, 255–264.
Crutsinger, G.M., Souza, L. & Sanders, N.J. (2008) Intraspecific diversity and
dominant genotypes resist plant invasions. Ecology Letters, 11, 16–23.
DeMott, W.R. (1998) Utilization of a cyanobacterium and a phosphorus-deficient green alga as complementary resources by daphnids. Ecology, 79,
2463–2481.
Duffy, J.E., Carinale, B.J., France, K.E., McIntyre, P.B., Thebault, E. &
Loreau, M. (2007) The functional role of biodiversity in ecosystems: incorporating trophic complexity. Ecology Letters, 10, 522–538.
Fargione, J., Tilman, D., Dybzinski, R., HilleRisLambers, J., Clark, C., Harpole, W.S., Knops, J.M.H., Reich, P.B. & Loreau, M. (2007) From selection
to complementarity: shifts in the causes of biodiversity-productivity relationships in a long-term biodiversity experiment. Proceedings of the Royal
Society B-Biological Sciences, 274, 871–876.
Fridley, J.D., Grime, J.P. & Bilton, M. (2007) Genetic identity of interspecific
neighbours mediates plant responses to competition and environmental
variation in a species-rich grassland. Journal of Ecology, 95, 908–915.
Hector, A., Schmid, B., Beierkuhnlein, C., Caldeira, M.C., Diemer, M., Dimitrakopoulos, P.G. et al. (1999) Plant diversity and productivity experiments
in European grasslands. Science, 286, 1123–1127.
Hector, A., Bazeley-White, E., Loreau, M., Otway, S. & Schmid, B.
(2002) Overyielding in grassland communities: testing the sampling effect
hypothesis with replicated biodiversity experiments. Ecology Letters, 5,
502–511.
Hooper, D.U. & Vitousek, P.M. (1997) The effects of plant composition and
diversity on ecosystem processes. Science, 277, 1302–1305.
Hooper, D.U., Chapin, F.S., Ewel, J.J., Hector, A., Inchausti, P., Lavorel, S.
et al. (2005) Effects of biodiversity on ecosystem functioning: a consensus of
current knowledge. Ecological Monographs, 75, 3–35.
Hughes, A.R. & Stachowicz, J.J. (2004) Genetic diversity enhances the resistance of a seagrass ecosystem to disturbance. Proceedings of the National
Academy of Sciences of the United States of America, 101, 8998–9002.
Hughes, A.R., Inouye, B.D., Johnson, M.T.J., Underwood, N. & Vellend, M.
(2008) Ecological consequences of genetic diversity. Ecology Letters, 11,
609–623.
Huston, M.A. (1997) Hidden treatments in ecological experiments: re-evaluating the ecosystem function of diversity. Oecologia, 110, 449–460.
Johnson, M.T.J. (2008) Bottom-up effects of plant genotype on aphids, ants,
and predators. Ecology, 89, 145–154.
Johnson, M.T.J., Lajeunesse, M.J. & Agrawal, A.A. (2006) Additive and interactive effects of plant genotypic diversity on arthropod communities and
plant fitness. Ecology Letters, 9, 24–34.
Jolliffe, P.A. (1997) Are mixed populations of plant species more productive
than pure stands? Oikos, 80, 595–602.
Karley, A.J., Hawes, C., Iannetta, P.P.M. & Squire, G.R. (2008) Intraspecific
variation in Capsella bursa-pastoris in plant quality traits for insect herbivores. Weed Research, 48, 147–156.
Knops, J.M.H., Tilman, D., Haddad, N.M., Naeem, S., Mitchell, C.E., Haarstad, J., Ritchie, M.E., Howe, K.M., Reich, P.B., Siemann, E. & Groth, J.
(1999) Effects of plant species richness on invasion dynamics, disease outbreaks, insect abundances and diversity. Ecology Letters, 2, 286–293.
Koricheva, J., Mulder, C.P.H., Schmid, B., Joshi, J. & Huss-Danell, K. (2000)
Numerical responses of different trophic groups of invertebrates to manipulations of plant diversity in grasslands. Oecologia, 125, 271–282.
Loreau, M. (2000) Biodiversity and ecosystem functioning: recent theoretical
advances. Oikos, 91, 3–17.
Loreau, M. & Hector, A. (2001) Partitioning selection and complementarity in
biodiversity experiments. Nature, 412, 72–76.
Maddox, G.D. & Cappuccino, N. (1986) Genetic determination of plantsusceptibility to an herbivorous insect depends on environmental context.
Evolution, 40, 863–866.
Maddox, G.D. & Root, R.B. (1987) Resistance to 16 diverse species of herbivorous insects within a population of goldenrod, Solidago altissima: genetic
variation and heritability. Oecologia, 72, 8–14.
Mody, K., Unsicker, S.B. & Linsenmair, K.E. (2007) Fitness related dietmixing by intraspecific host-plant-switching of specialist insect herbivores.
Ecology, 88, 1012–1020.
Moran, N. (1981) Intraspecific variability in herbivore performance and
host quality: a field study of Uroleucon caligatum (Homoptera: Aphidedae) and its Solidago hosts (Asteraceae). Ecological Entomology, 6,
301–306.
Mulder, C.P.H., Koricheva, J., Huss-Danell, K., Hogberg, P. & Joshi, J. (1999)
Insects affect relationships between plant species richness and ecosystem processes. Ecology Letters, 2, 237–246.
Naeem, S., Thompson, L.J., Lawler, S.P., Lawton, J.H. & Woodfin, R.M.
(1994) Declining biodiversity can alter the performance of ecosystems.
Nature, 368, 734–737.
Pfisterer, A.B., Diemer, M. & Schmid, B. (2003) Dietary shift and lowered
biomass gain of a generalist herbivore in species-poor experimental plant
communities. Oecologia, 135, 234–241.
Pigliucci, M. (2003) Selection in a model system: ecological genetics of flowering time in Arabidopsis thaliana. Ecology, 84, 1700–1712.
Reusch, T.B.H., Ehlers, A., Hammerli, A. & Worm, B. (2005) Ecosystem
recovery after climatic extremes enhanced by genotypic diversity. Proceedings of the National Academy of Sciences of the United States of America,
102, 2826–2831.
SAS Institute Inc. (2008) SAS Systems for Windows Version 9.2. SAS Institute
Inc., Cary, NC, USA.
Scherber, C., Mwangi, P.N., Temperton, V.M., Roscher, C., Schumacher, J.,
Schmid, B. & Weisser, W.W. (2006a) Effects of plant diversity on invertebrate herbivory in experimental grassland. Oecologia, 147, 489–500.
Schweitzer, J.A., Bailey, J.K., Hart, S.C. & Whitham, T.G. (2005) Nonadditive
effects of mixing cottonwood genotypes on litter decomposition and nutrient
dynamics. Ecology, 86, 2834–2840.
Service, P. (1984) Genotypic interactions in an aphid–host plant relationship:
Uroleucon rudbeckiae and Rudbeckia laciniata. Oecologia, 61, 271–276.
SPSS Inc. (2005) SPSS for Windows, Version 14.0. SPSS Inc., Chicago, IL.
Tilman, D., Wedin, D. & Knops, J. (1996) Productivity and sustainability influenced by biodiversity in grassland ecosystems. Nature, 379, 718–720.
Tilman, D., Reich, P.B., Knops, J., Wedin, D., Mielke, T. & Lehman, C. (2001)
Diversity and productivity in a long-term grassland experiment. Science,
294, 843–845.
Tylianakis, J.M., Rand, T.A., Kahmen, A., Klein, A.M., Buchmann, N.,
Perner, J. & Tscharntke, T. (2008) Resource heterogeneity moderates the
biodiversity–function relationship in real world ecosystems. PLoS Biology,
6, 947–956.
Unsicker, S.B., Baer, N., Kahmen, A., Wagner, M., Buchmann, N. & Weisser,
W.W. (2006) Invertebrate herbivory along a gradient of plant species diversity in extensively managed grasslands. Oecologia, 150, 233–246.
Vellend, M. (2006) The consequences of genetic diversity in competitive communities. Ecology, 87, 304–311.
Weiner, J. (1990) Asymmetric competition in plant-populations. Trends in
Ecology & Evolution, 5, 360–364.
Wimp, G.M., Martinsen, G.D., Floate, K.D., Bangert, R.K. & Whitham, T.G.
(2005) Plant genetic determinants of arthropod community structure and
diversity. Evolution, 59, 61–69.
Wise, M.J. (2007) Evolutionary ecology of resistance to herbivory: an investigation of potential genetic constraints in the multiple-herbivore community of
Solanum carolinense. New Phytologist, 175, 773–784.
Zhu, Y.Y., Chen, H.R., Fan, J.H., Wang, Y.Y., Li, Y., Chen, J.B. et al. (2000)
Genetic diversity and disease control in rice. Nature, 406, 718–722.
Received 24 March 2009; accepted 19 October 2009
Handling Editor: Rob Brooker
2009 The Authors. Journal compilation 2009 British Ecological Society, Journal of Ecology, 98, 237–245