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Plant genotypic diversity reduces the rate
of consumer resource utilization
Scott H. McArt1,† and Jennifer S. Thaler1,2
1
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Research
Cite this article: McArt SH, Thaler JS. 2013
Plant genotypic diversity reduces the rate of
consumer resource utilization. Proc R Soc B
280: 20130639.
http://dx.doi.org/10.1098/rspb.2013.0639
Received: 12 March 2013
Accepted: 18 April 2013
Department of Entomology, and 2Department of Ecology and Evolutionary Biology, Cornell University, Ithaca,
NY 14853, USA
While plant species diversity can reduce herbivore densities and herbivory,
little is known regarding how plant genotypic diversity alters resource utilization by herbivores. Here, we show that an invasive folivore—the Japanese
beetle (Popillia japonica)—increases 28 per cent in abundance, but consumes
24 per cent less foliage in genotypic polycultures compared with monocultures of the common evening primrose (Oenothera biennis). We found
strong complementarity for reduced herbivore damage among plant genotypes growing in polycultures and a weak dominance effect of particularly
resistant genotypes. Sequential feeding by P. japonica on different genotypes
from polycultures resulted in reduced consumption compared with feeding
on different plants of the same genotype from monocultures. Thus, diet
mixing among plant genotypes reduced herbivore consumption efficiency.
Despite positive complementarity driving an increase in fruit production
in polycultures, we observed a trade-off between complementarity for
increased plant productivity and resistance to herbivory, suggesting costs
in the complementary use of resources by plant genotypes may manifest
across trophic levels. These results elucidate mechanisms for how plant genotypic diversity simultaneously alters resource utilization by both producers
and consumers, and show that population genotypic diversity can increase
the resistance of a native plant to an invasive herbivore.
Subject Areas:
ecology
Keywords:
biodiversity – ecosystem function, community
genetics, herbivory, plant – insect interactions,
diet mixing, Popillia japonica
Author for correspondence:
Scott H. McArt
e-mail: [email protected]
†
Present address: Department of Biology,
University of Massachusetts – Amherst,
Amherst, MA 01003, USA.
Electronic supplementary material is available
at http://dx.doi.org/10.1098/rspb.2013.0639 or
via http://rspb.royalsocietypublishing.org.
1. Introduction
Understanding how biodiversity affects the efficiency of resource use in food
webs is a central goal of community and ecosystem ecology [1,2]. Following
longstanding interest in how plant diversity affects herbivore populations
[3,4], recent analyses have begun to reveal how plant species diversity affects
resource utilization by herbivores [5–7]. At the same time, a growing body of
literature suggests that genotypic diversity within species can have pronounced
consequences on populations, communities and ecosystems [8]. However,
while several studies have now shown that plant genotypic diversity typically
increases the abundance of herbivores [9–12], how changes in herbivore abundance are linked to the consumption of plant resources remains unknown
[13,14]—an important gap in our understanding of how plant diversity alters
energy transfer across trophic levels.
While little theory has addressed how plant genotypic diversity can impact
herbivore populations or herbivory, at least four formal hypotheses have been
suggested for how species diversity impacts consumers. With regard to herbivore populations, the ‘resource concentration hypothesis’ [4] posits that fewer
specialist herbivores will accumulate in diverse plant assemblages owing to
reduced plant apparency, herbivore residence time and/or herbivore reproductive output. Alternatively, the ‘enemies hypothesis’ [4] posits that both
generalist and specialist natural enemies (i.e. predators of herbivores) will be
more abundant in diverse plant assemblages, and therefore suppress herbivore
populations in polycultures more than monocultures. Regarding the consumption of plants by herbivores, the ‘variance in edibility hypothesis’ [15,16] posits
that a resource base with more species is more likely to contain at least one
species that is resistant to consumption, which can then dominate in the presence of consumers and lead to less herbivory. This is analogous to the
& 2013 The Author(s) Published by the Royal Society. All rights reserved.
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changes in resource utilization at the consumer level related
to resource utilization at the producer level?
2. Material and methods
(a) Study system, plant propagation and
field establishment
(b) Herbivory surveys and plant productivity
We conducted two censuses of Japanese beetles (P. japonica) at
the peak of their abundance (once in late July and again in
Proc R Soc B 280: 20130639
We manipulated genotypic richness of O. biennis L (common evening primrose, Onagraceae), a native herbaceous plant that is
common to old fields and disturbed areas in eastern North
America. Oenothera biennis reproduces via a permanent translocation heterozygosity genetic system, which results in seeds that
are genetically identical to each other and the parent [29,30].
We collected O. biennis seeds from individual plants in 20 distinct
populations around Ithaca (New York). Each genotype used in
this experiment was determined to be unique using nine polymorphic microsatellite loci developed for O. biennis [31]. To
reduce maternal effects, we first grew the seeds in a common
garden in 2007, which was sprayed with insecticide at regular
intervals throughout the growing season, and we used seeds
collected from these plants (20 genotypes) for our experiment.
We cold stratified (48C, 4 days) all seeds for the field experiment in April 2010, sowed them into 96-well trays filled with
soil (Pro-mix ‘BX’ with biofungicide, Premier) and thinned germinated seedlings to a single individual per well. Plants were
watered ad libitum and fertilized weekly (21-5-20 NPK,
150 ppm) while in the greenhouse (14 L : 10 D cycle, five weeks)
and then field-hardened in an outdoor mesh cage (one week)
prior to planting in the field.
In May 2010, we established the field experiment in an
abandoned agricultural field near Ithaca, where the soil was
ploughed, but otherwise untreated. As we were interested in
invertebrate responses to plant genotypic diversity, we excluded
vertebrate herbivores such as deer and rabbits from our plots via
a 2.5 m fence (1.5 cm mesh) surrounding our entire experiment.
Using our pool of 20 O. biennis genotypes, we constructed two
treatments within this enclosure: genotypic monocultures (one
O. biennis genotype) and genotypic polycultures (seven O. biennis
genotypes). All plots contained seven equally spaced individual
plants arrayed in a ring 0.5 m in diameter, and plots were separated by 1.5 m. We clipped encroaching weeds by hand, every
two to three weeks, to ensure treatments remained consistent
throughout the summer. The original design included 120
plots, but owing to the loss of individuals within plots (which
was always only one plant per plot and showed no pattern
among genotypes or treatments), we restricted our analyses
to the 109 plots that experienced no mortality (monocultures:
n ¼ 55; polycultures: n ¼ 54). Every genotype appeared in
approximately 19 polycultures and there were three monocultures of each genotype (except for five O. biennis genotypes
that had two monocultures each owing to mortality). Oenothera
biennis is monocarpic, and every plant in the experiment bolted
and produced fruit in the first season, which is a typical response
of this plant in disturbed habitats such as our ploughed field [32].
Owing to its large size, we divided our experiment into four
spatial blocks to account for potential within-site environmental
variation, where each block contained the same proportion of
monocultures and polycultures. Although this common garden
is located within a homogeneous field, we have typically found
significant effects of spatial location within the garden for plant
productivity and arthropod responses [9,23] (also this study).
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selection effect [17] at the resource rather than consumer level.
Alternatively, the ‘balanced diet hypothesis’ [18,19] posits
that a resource base with higher diversity will result in greater
consumption from generalist herbivores via complementary
acquisition of deficient nutrients.
While some experimental support exists for each of these
hypotheses [19 –21], by far the most research has focused on
the resource concentration and enemies hypotheses. In a
review of 209 studies of 287 herbivorous and 130 predatory
arthropod species, Andow [3] found that 51.9 per cent of
the herbivorous species examined had lower population densities on plants in polycultures than monocultures (compared
with 15.3% of species having lower densities on monocultures). Furthermore, 52.7 per cent of predator species had
higher population densities in polycultures (compared with
9.3% having higher densities in monocultures). While the
resource concentration and enemies hypotheses are not
mutually exclusive, Andow determined that resource concentration was somewhat better at explaining these results.
Regardless, it is important to note that these two hypotheses
solely consider the population responses of animals to plant
diversity and do not explicitly link animal abundance to
plant damage [2,3]. This is important because the magnitude
of consumption, not simply the abundance of consumers,
controls energy transfer across trophic levels [22].
Whether mechanisms altering herbivore populations and
herbivory in response to plant species diversity carry over to
plant genotypic diversity has received limited attention. The
few studies that have focused on herbivore populations in
response to plant genotypic diversity have found no support
for either of the two major hypotheses related to plant species
diversity—the resource concentration or enemies hypotheses
[9–12]. Instead, most of these studies find that herbivore populations tend to increase in response to genotypic diversity,
which differs from the decreases that are typical in response
to plant species diversity [3,4]. Perhaps this is not surprising, because, due to the inherent differences in trait variation
within versus among plant species, fundamentally different
mechanisms are responsible for structuring broad-scale patterns
of arthropod diversity in response to plant genotypic versus
species diversity [9,10,23]. Because trait variation in plants is critically important to herbivores during both the selection of host
plants [4,24] and utilization of those hosts [25,26], mechanisms
altering herbivore populations and herbivory in response to
each type of plant diversity probably differ as well. Here, we
address this gap in the literature by linking patterns of herbivore
abundance, movement and consumption. In doing so, we provide a mechanism for how plant genotypic diversity increases
the abundance of herbivores yet reduces herbivory.
Using a manipulative field experiment, feeding bioassays
and behavioural observations, we test how genotypic diversity of the common evening primrose (Oenothera biennis)
influences the population size and consumption dynamics
of its dominant folivore, the Japanese beetle (Popillia japonica).
We focus on Japanese beetles—the most widespread and
destructive pest of turf grass, landscapes and nursery crops
in the eastern United States [27]—because they are responsible for more than 95 per cent of the leaf area consumed
on O. biennis in Tompkins County (New York) [28]. We
address three main questions in this study. (i) Is resource utilization by P. japonica altered in response to plant genotypic
diversity? (ii) What mechanisms are responsible for altering
resource utilization by P. japonica? (iii) Are diversity-mediated
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where NDRYMi measures the complementarity effect, Ncov
ðDRY; MÞ measures the selection effect, N is the number of genotypes in polyculture, Mi is the performance of genotype i in
monoculture and DRY ¼ RYOi 2 RYEi is the deviation from
expected relative performance of genotype i in the polyculture
[17]. Positive complementarity for fruit production implies that
genotypes had generally higher fruit production in polyculture
than monoculture, which is typically attributed to resource
partitioning or facilitation among genotypes. Negative complementarity implies that fruit production is generally lower across
genotypes in polyculture than monoculture. Positive selection
implies that highly productive genotypes produce proportionally
more fruits in polyculture than monoculture, whereas negative selection implies that less productive genotypes produce proportionally
more fruits.
This method for partitioning net biodiversity effects can also
be extended to herbivore damage. In this case, positive complementarity implies that genotypes received more damage on
average in polyculture than monoculture, whereas negative complementarity means that they received less damage. We interpret
negative complementarity for herbivore damage as evidence for
associational resistance [24,33] because proximity to genotypically
diverse neighbours buffered genotypes from receiving damage.
Positive selection indicates that highly attacked genotypes received
disproportionally more damage in polyculture than monoculture,
whereas negative selection indicates that less attacked genotypes
received disproportionately less damage. For both fruit production
and herbivore damage, we tested whether complementarity and
selection analyses were positive or negative by observing whether
95% CIs overlapped zero.
We tested how changes in herbivore damage were related to
changes in fruit production across treatments at both the plant
and patch levels. At the plant level, we tested for a genotypic
(c) Herbivore movement
Because movement among plants has been hypothesized to be a
key mechanism altering herbivore populations in response to
plant genotypic diversity [12], we conducted an experiment to
explicitly evaluate P. japonica movement. We quantified movement of P. japonica within and among O. biennis patches by
replicating the 2010 field establishment protocols (described
above) for a follow-up experiment in 2011 containing 17 of the
original 20 genotypes. Equal numbers of monocultures and polycultures were planted such that each genotype occurred in seven
polycultures and one monoculture (n ¼ 34 patches total). Similar
to the previous field experiment, monocultures were spatially
alternated with polycultures such that two monocultures and
two polycultures were present in nine blocks (the final block contained only one monoculture and polyculture each). In late July,
we observed beetle movement in each block for periods of 15 min
on three successive days. Observations were conducted at the
height of beetle activity and feeding (between 09.00 and 12.00
or between 15.00 and 18.00). We randomized which blocks
were visited at which time each day. Over the course of the
experiment, each patch contained six P. japonica adults on average, and we observed an average of nine movement events
(one beetle moving from one plant to another plant) in each
observation period. Thus, we were able to easily record whether
each beetle moved to a neighbouring plant within a patch or outside the patch, keeping track of the specific plants the beetle
moved between if movement occurred within each four-patch
block. We tested whether overall beetle movement within
versus between patches differed from a 50 – 50 expectation, and
whether within versus between patch movement differed
between diversity treatments, via Pearson x2 analyses.
(d) Fixed and sequential feeding bioassays
We conducted a two-part bioassay to test the resistance of
O. biennis genotypes grown in monocultures versus polycultures
to P. japonica. Because plant phenotypic traits such as biomass
[34,35], C : N ratio [36,37] and chemical defences against herbivores [38] can change when plants are grown in diverse compared
with homogeneous communities, we first hypothesized that individual plant resistance (and therefore consumption) could change
when genotypes grow in monocultures versus polycultures. Alternatively, owing to the high mobility of P. japonica within plant
patches (see §3), we hypothesized that sequential feeding on different genotypes in polyculture compared with sequential feeding on
different plants of the same genotype in monoculture could alter
rates of consumption.
During the peak of P. japonica abundance in late July 2010,
we collected individual leaves from six replicate plants of
each genotype in each treatment (6 leaves 20 genotypes 2
treatments ¼ 240 bioassays). The first fully expanded leaf from
each plant was cut at its petiole, placed in a Petri dish (9 cm
diameter) containing a moist sheet of filter paper and immediately transported back to the laboratory. One P. japonica adult
(collected from the wild) was placed in each dish and allowed
to feed for 24 hours at 208C. At the end of 24 hours, each
P. japonica adult was removed, and leaf area consumed (mm2)
was assessed on leaves. In this way, we assessed constitutive
resistance of individual plants grown in monocultures versus
3
Proc R Soc B 280: 20130639
DY ¼ NDRYMi þ NcovðDRY; MÞ;
correlation between reduced herbivore damage (mean damage
in monoculture minus mean damage in polyculture) and
increased production of fruits (mean number of fruits produced
in polyculture minus mean number of fruits produced in monoculture) via linear regression. At the patch level, we tested the
relationship between the net biodiversity effect (including complementarity and selection) for herbivore damage and fruit
production via linear regression.
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early August) by visually surveying every plant in the experiment (n ¼ 840 plants). In early September, when all beetles
were gone but leaves had not yet dropped from the plants, we
surveyed the quantity of beetle leaf damage. We did not observe
any leaves dropped due to herbivory over the course of the
growing season. We placed an acetate sheet printed with a
1 cm2 grid over each leaf of every plant in the experiment
(mean ¼ 109 leaves per plant), quantifying leaf area consumed
on each plant to the nearest square centimetre. In early October,
when plants stopped producing fruits, we counted the number of
fruits on each plant in the experiment (mean ¼ 148 fruits per
plant). Because O. biennis is monocarpic, the number of fruits
produced is an estimate of lifetime fitness. We note that there
is a strong positive relationship between above-ground plant biomass and number of fruits produced by O. biennis [23], so the
number of fruits produced is a good estimate of productivity.
We tested for differences in plot-level beetle abundance, leaf
area consumed and number of fruits produced between diversity
treatments by using restricted maximum likelihood (REML) to
estimate variance parameters in linear mixed models (LMMs)
with spatial block as a random effect (JMP PRO v. 9.0.2). We
log-transformed beetle abundance and damage data to improve
normality of the residuals. To compare the amount of leaf area
consumed between O. biennis genotypes in each treatment, we
tested for the effect of genotype, treatment and the genotype treatment interaction via LMM (REML) with spatial block and
plot nested within block as random effects. The electronic supplementary material (table S1) details all mixed models used to
analyse the data.
To understand the mechanisms by which O. biennis genotypic diversity affected the production of fruits, we used the
methods of Loreau & Hector [17] to calculate the net biodiversity
effect. The net biodiversity effect is calculated using the equation
We found 28 per cent more Japanese beetles (P. japonica) in
genotypic polycultures versus monocultures of O. biennis
(F1,103 ¼ 4.9, p ¼ 0.029; figure 1a; electronic supplementary
material, table S1). In contrast to these results, we found a 24
per cent decrease in the amount of leaf area consumed in polycultures compared with monocultures (F1,103 ¼ 4.0, p ¼ 0.044;
figure 1b). Because plants tended to be larger in polycultures
(see productivity results below), the percentage of leaf area consumed was probably even further reduced in polycultures
compared with this absolute measure of herbivory.
Genotypes differed in leaf area consumed by P. japonica
(F19,146 ¼ 21.1, p , 0.001), and there was also a significant
genotype treatment interaction for herbivory (F19,146 ¼ 2.2,
p ¼ 0.006). However, patterns of herbivory were largely
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3. Results
(a)
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0
(b) 120
leaf area removed (cm2)
polycultures by testing for differences in leaf area consumed
between treatments via LMM (REML) with genotype as a
random effect. Forty-one beetles did not initiate feeding or died
during this assay (n ¼ 22 monoculture, n ¼ 19 polyculture); however whether we included these zero values in our analysis did
not alter the direction or significance of results. Therefore, we present the data excluding these beetles for consistency with our
sequential feeding assay (see below).
In order to test whether sequential feeding on the same
versus different genotypes altered beetle consumption, we continued this initial bioassay. All beetles from the first assay were
immediately transferred to a new leaf in a new Petri dish and
allowed to feed for an additional 24 hours. Leaves for the
second feeding assay (day 2) were obtained from the field experiment in an identical manner to the first collection. In order to
mimic the way sequential feeding might occur within plant
patches in the field, beetles assigned to a monoculture sequential
feeding treatment were transferred to a leaf from a different plant
of the same genotype from the patch, whereas beetles assigned to
a polyculture sequential feeding treatment were transferred to a
leaf from a different genotype from the patch. Replication of genotypes across bioassay treatments was identical for each assay
(six replicate leaves per genotype per treatment) in order for
treatments to be balanced at each 24-hour time interval. Assignment of beetles to genotypes within the polyculture sequential
feeding assays was random with the exception that no two
sequences were identical, while source plants used for the monoculture sequential feeding assays were randomly chosen from
each patch. Leaf area consumed during the second assay was
assessed at the conclusion of the second 24-hour period (day 2),
and this design was repeated for one additional 24-hour period
(day 3) such that three Petri dish assays were conducted in
sequence for each beetle over 72 hours. Thus, beetles in the monoculture sequential feeding treatment consumed three leaves of the
same genotype over 72 hours, whereas beetles in the polyculture
sequential feeding treatment consumed three leaves from three
different genotypes over 72 hours.
We tested for differences in leaf area consumed over all three
assays by analysing the effect of treatment, day and treatment day interaction via LMM (REML), with genotype and beetle
as random effects. Twelve beetles died over the course of this
experiment and 83 beetles stopped feeding during the second or
third assays (n ¼ 45 monoculture sequence, n ¼ 50 polyculture
sequence). Whether or not we include these zero values did
not alter the direction or significance of any results (treatment
and day always p . 0.05, treatment day interaction always
p , 0.05). Therefore, to most completely assess the effects of
sequential feeding on rates of herbivory, here we present only
the data where full sequential feeding for all three assays occurred.
Popillia japonica abundance
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90
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30
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monocultures
polycultures
Figure 1. (a) The abundance of Japanese beetles (Popillia japonica) was 28%
greater in Oenothera biennis genotypic polycultures versus monocultures,
while (b) the amount of leaf damage was 24% less in polycultures versus
monocultures (mean + s.e.m.). Data were log-transformed to improve
normality and back-transformed to produce figure.
consistent by treatment: the magnitude of damage was reduced
on 16 out of 20 genotypes when grown in polycultures
(figure 2) and only one genotype received significantly
more damage in polycultures versus monocultures (see the
electronic supplementary material, table S2). When we partitioned the mechanisms for reduced herbivory in polycultures,
we found strong complementarity among genotypes for
reduced damage (95% CI ¼ 221.4 + 8.3; figure 2 inset) while
there was also weak but significant positive selection (95%
CI ¼ 4.1 + 3.2). Negative complementarity implies that
decreases in herbivore damage in polycultures are due to overall associational resistance [24,33] to herbivore damage among
the majority of plant genotypes, whereas positive selection
implies that genotypes that received the greatest amount of
damage in monoculture receive proportionally more damage
in polyculture compared with low-damage genotypes.
In our movement experiment, we observed that beetles
were more than twice as likely to move between plants within
a patch (i.e. within a monoculture or polyculture) compared
with leaving a patch to feed elsewhere. This pattern differed
significantly from a 50–50 expectation (within-patch n ¼ 144,
out-of-patch n ¼ 67; Pearson x 2 ¼ 28.1, p , 0.001), and there
was no difference in this within-patch bias for movement in
monocultures (within-patch n ¼ 62, out-of-patch n ¼ 29)
versus polycultures (within-patch n ¼ 82, out-of-patch n ¼ 38;
Pearson x 2 ¼ 0.001, p ¼ 0.98).
Given the strong complementarity for reduced damage
in O. biennis polycultures, we tested two mechanisms for
how complementarity could occur. We first tested whether
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comp
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S18 S1 S6 S36 S21 S7 S15 S11S20 S43 S9 A23S17A29 S38 A4 S32 S33 S5 S44
genotype
Figure 2. Least-squares mean values of damage on 20 O. biennis genotypes grown in genotypic monocultures (dark grey bars) versus polycultures (light grey bars;
error bars not shown for clarity). Reduced herbivory in polycultures resulted from negative complementarity (comp) and positive selection (sel) among O. biennis
genotypes for the amount of damage received (inset). Negative complementarity implies that decreases in herbivore damage in polycultures were due to beneficial
associational effects among plant genotypes [24,33], whereas positive selection implies that the most resistant genotypes in monocultures were even more resistant
in polycultures. Complementarity and selection analyses performed following the methods outlined by Loreau & Hector [17]; mean + 95% CIs shown.
3.2
*
leaf area consumed hr–1 (mm2)
individual plant resistance was different for genotypes when
grown in monocultures versus polycultures. When we performed a bioassay allowing beetles to consume leaf tissue
from the same genotypes grown in monocultures versus polycultures, we found no difference in consumption between
treatments (F1,178 ¼ 1.9, p ¼ 0.17; figure 3, day 1), implying
that constitutive plant resistance to herbivore damage was
not affected by diversity treatment. Next, owing to the high
degree of movement by P. japonica among plants within
patches, we tested whether sequential feeding on different genotypes in polycultures compared with sequential feeding on
different plants of the same genotype in monoculture resulted
in a reduced rate of herbivory. Consistent with this hypothesis,
we found that beetles increased their rate of consumption
when feeding on leaves from the same genotype in sequence
(monoculture sequence), whereas beetles tended to have a
reduced rate of consumption when feeding on leaves from
three different genotypes in sequence ( polyculture sequence)
over a period of 72 hours (treatment day interaction:
F2,279 ¼ 6.9, p ¼ 0.001; post hoc contrast day 3: p ¼ 0.010;
figure 3).
Consistent with previous studies in this system and others,
we found an 11 per cent increase in the productivity (number
of fruits produced) in O. biennis genotypic polycultures
(F1,103 ¼ 5.9, p ¼ 0.017), and there was a positive relationship
between plot-level fruit production and P. japonica abundance
(F1,103 ¼ 9.1, p ¼ 0.003, R 2 ¼ 0.13). Increased fruit production was a result of strong positive complementarity (95%
CI ¼ 158.0 + 63.0), while there was also weak but significant
negative selection (95% CI ¼ 29.2 + 7.5).
Because fruit production increased in polycultures
compared with monocultures, whereas herbivore damage
decreased in polycultures compared with monocultures, we
tested whether increases in plant productivity depended on
decreases in herbivore damage. At the plant level, we
found a negative relationship between the decrease in herbivory by genotypes in polycultures versus the increase in fruit
3.1
3.0
2.9
2.8
2.7
2.6
day 1
day 2
day 3
Figure 3. Leaf area consumed by P. japonica when fed leaves from different
plants of the same genotype (monoculture: black squares, solid line) or leaves
from different genotypes ( polyculture: open squares, dashed line) in
sequence (least-squares means + s.e.). Day 1 tested the effect of diversity
treatment on individual plant resistance to P. japonica, while days 1 – 3
tested the effect of genotype mixing in their diet. Data were log-transformed
to improve normality and back-transformed for the figure. *p , 0.05 post
hoc contrast between treatments for individual assays.
production in polycultures (F1,19 ¼ 22.3, p , 0.001, R 2 ¼ 0.55;
electronic supplementary material, figure S1). At the patch
level, we also found a negative relationship between the net
biodiversity effects for decreased herbivory versus increased
fruit production (F1,53 ¼ 11.3, p ¼ 0.002, R 2 ¼ 0.18; figure 4).
When we partitioned the net biodiversity effect in complementarity and selection, we found this latter result was
driven by the negative relationship between complementarity for reduced damage versus increased fruit production
(F1,53 ¼ 3.1, p ¼ 0.003, R 2 ¼ 0.16); there was no relationship
between selection for herbivore damage versus fruit production
(F1,53 ¼ 1.4, p ¼ 0.23).
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leaf area removed (cm2)
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300
100
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net biodiversity effect
(herbivore damage)
Figure 4. Relationship between net biodiversity effects [17] for leaf area
removed by herbivores (herbivore damage) and plant productivity (number of
fruits produced). Negative values along the x-axis correspond to less damage
in genotypic polycultures than predicted from monocultures, whereas positive
values along the y-axis correspond to greater productivity than predicted.
4. Discussion
In this study, we show that despite an increase in herbivore
abundance, the consumption of resources by herbivores is
reduced in plant genotypic polycultures. These results are consistent with recent analyses showing that herbivory can be
reduced in response to plant species [5–7] and genotypic
[13,14] diversity. Our results also support a growing recognition that plant genotypic diversity increases the abundance
of herbivores [9–12], which contrasts with the suppression of
herbivores that is typical in response to plant species diversity
[3,21]. By linking patterns of herbivore abundance, movement
and consumption in a single study, we provide a mechanism
to reconcile how plant genotypic diversity can increase the
abundance of herbivores, yet reduce consumption: by reducing
the rate of resource utilization.
Our results do not provide strong support for any of the
four main published hypotheses posed for altering herbivore
populations or herbivory in response to plant species diversity. Because we found greater numbers of P. japonica in
polycultures and they consumed less plant tissue, our data
do not support the resource concentration hypothesis [4]
or balanced diet hypothesis [18,19]. In this system and
others, the increased abundance of herbivores in response
to genotypic diversity has been found to track plant biomass
[9,10], which suggests herbivores are primarily responding to
the increase in available plant resources. We also find no support for the enemies hypothesis [4]; P. japonica adults are
relatively resistant to predation [27], and we did not find
an increase in predator abundance or richness in response
to O. biennis genotypic diversity in a previous study at the
same site [9]. Finally, we found there was positive selection
for plant damage in polycultures (figure 2), meaning that the
most resistant genotypes in monoculture were even more
resistant in polyculture. While this result is consistent with
the variance in edibility hypothesis [15,16], the increased resistance of particular O. biennis genotypes in polycultures (i.e.
positive selection for plant damage) was weak compared
with the strong overall associational resistance (i.e. negative
complementarity for plant damage) in genotypic polycultures
(figure 2 inset). This suggests that mechanisms other than
6
Proc R Soc B 280: 20130639
–500
variance in edibility were the primary drivers of reduced
herbivore damage in response to O. biennis genotypic diversity.
We tested two alternative hypotheses that could explain
how strong complementarity for reduced herbivory could
occur in O. biennis genotypic polycultures. First, in our bioassay with P. japonica (figure 3), we found that individual
plant resistance did not differ between genotypes grown in
monocultures versus polycultures (figure 3, day 1). This
suggests that individual plant quality traits that can change
when plants are grown in diverse species mixtures [36 –38]
were either not affected by genotypic diversity or unimportant for resistance to P. japonica. In support of this result,
we previously found no effect of O. biennis genotypic diversity
on the density of trichomes, leaf toughness or specific leaf area
of leaves from plants grown in monocultures versus polycultures ( p . 0.2 in each case; S.H.M. 2009, unpublished data).
Instead, in this study, we found that sequential feeding on
different O. biennis genotypes resulted in a reduced rate of herbivory compared with sequential feeding on the same genotype
(figure 3). Because our results show that P. japonica is a mobile
herbivore that preferentially moves and feeds on plants within
patches compared with moving out of patches, the reduced rate
of herbivory that beetles experience on a mixed-genotype diet
provides a probably mechanistic link between the opposing
patterns of increased P. japonica abundance and reduced
damage observed in O. biennis polycultures.
While sequential feeding on different plants is not typically
considered a mechanism of associational resistance to herbivores [24,33], this may be a common response of mobile
herbivores that feed on multiple neighbouring plants. For
example, in the most detailed study to date regarding the
population response of herbivores to plant genotypic diversity,
movement among plants was hypothesized to be a major
driver of aphid population growth [12]. Compensatory feeding
can occur when animals are restricted to diets suboptimal for
their target intake of different nutrients, such as suboptimal
protein : carbohydrate ratios [39,40]. Because the genotypes
used in this experiment are known to differ significantly in
nutritional characteristics such as C : N ratio [41], individual
P. japonica beetles may have compensated for suboptimal nutrition in single-genotype monocultures by consuming more
leaf tissue compared with mixed-genotype polycultures
(where diet mixing among genotypes occurred). Alternatively,
the genotypes we used in this experiment are also known to
differ significantly in the abundance of secondary compounds,
including ellagitannins and flavonoids [41]. Although diet
mixing among plant species that contain different toxins is
predicted to allow increased consumption by generalist herbivores [42], there is remarkably little experimental evidence
supporting this detoxification limitation hypothesis [43]. Furthermore, we are aware of no studies that have tested the
hypothesis for different plant genotypes, which may differ
more in the abundance of different toxins as opposed to their
presence or absence. Understanding how primary and secondary metabolites interact to affect consumption is an important
but understudied area of chemical and nutritional ecology
[44,45], and probably underlies the specific physiological
mechanism responsible for our results.
A few recent studies suggest that plant diversity can alter
the strength of top-down control over plant productivity
[5,6,14]. This is not surprising since plant diversity can
modify patterns of herbivory [7,13,14], and herbivory often
affects the outcome of plant competitive interactions such
rspb.royalsocietypublishing.org
net biodiversity effect
(fruit production)
700
Downloaded from http://rspb.royalsocietypublishing.org/ on June 18, 2017
We thank A. Agrawal, S. Cook-Patton, A. Kessler and S. Meldau for
helpful comments on the manuscript, and F. Vermeylan for advice on
statistical models. The NSF (IGERT small grant in Biogeochemistry
and Environmental Biocomplexity to S.H.M.), Grace Griswold Endowment (to S.H.M.) and USDA-NRI (2006-35302-1731 to J.S.T.) supported
this work. S.H.M. and J.S.T. designed the experiments, S.H.M. conducted
the experiments and analysed the data, and S.H.M. and J.S.T. wrote the
manuscript. The authors declare no competing interests.
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In summary, by linking behavioural observations of
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versus among species. Ultimately, a mechanistic understanding of consumption dynamics such as outlined in this study
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of resource use in food webs.
rspb.royalsocietypublishing.org
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above. This plot-level analysis is particularly useful for our
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plants in this experiment.
Instead of positive relationships between reduced herbivore
damage and increased plant productivity in polycultures, we
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electronic supplementary material, figure S1). In other words,
polyculture assemblages that were particularly good at increasing fruit production were also particularly bad at resisting
herbivore damage. We suggest these negative relationships
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