Oikos 123: 886–896, 2014
doi: 10.1111/j.1600-0706.2013.00946.x
© 2014 The Authors. Oikos © 2014 Nordic Society Oikos
Subject Editor: Susan P. Harrison. Accepted 24 November 2013
Plant damage and herbivore performance change with latitude
for two old-field plant species, but rarely as predicted
Tania N. Kim
T. N. Kim ([email protected]), Dept of Biological Science, Florida State Univ., Tallahassee, FL 32306-4295, USA. Present address:
Great Lakes Bioenergy Research Center, Univ. of Wisconsin, Madison, WI 53726-4084, USA.
A long standing hypothesis in biogeography is that latitudinal gradients in plant defenses (LGPD) should arise because
selection for plant defenses is greater in the tropics compared to temperate areas. Previous studies have focused on
plant traits thought to confer resistance, yet many traits may not actually confer resistance (putative resistance) or
interact to influence herbivore performance. In this study, I used a multi-trophic approach to examine relationships
between latitude, herbivore pressure, and plant resistance (measured as the growth rates of herbivores) of two oldfield plant species (Solanum carolinense and Solidago altissima) using a field survey across a 12 degree gradient in the
eastern US combined with laboratory bioassays measuring the performance of generalist and specialist herbivores.
I used structural equation modeling to examine the direct and indirect pathways by which latitude influences herbivore
pressure and plant resistance. A latitudinal gradient in plant damage was observed in the expected direction for
S. caroliense (damage decreased with latitude), but the opposite relationship was observed for S. altissima. Damage
to both plant species was mediated by herbivore abundances, which was in turn influenced by predator abundances.
Resistance to herbivores also varied with latitude but the form of the relationship was dependent on herbivore and
plant species. There were direct, non-linear relationships between latitude and resistance (for Spodoptera exigua and
Schistocerca americana feeding on S. altissima; S. exigua and Manduca sexta feeding on S. carolinense). Herbivore
growth rates were also mediated by the density of S. carolinense for Leptinotarsa juncta and S. americana feeding on
S. carolinense. There was no relationship between plant resistance and herbivore pressure and no indication of feedbacks.
Results from this study indicate that latitudinal variation in plant resistance is complex, possibly constrained by
resource availability and tradeoffs in plant defenses.
Latitudinal gradients in biological traits and processes have
fascinated biogeographers for decades (Darwin 1859,
Pianka 1966). Latitudinal variation in climatic, historical,
and other correlated features of the environment (e.g. area,
energy, spatial domain) can be strong drivers of biodiversity, evolution and ecosystem productivity (Hillebrand
2004). Consequently, the intensity of interspecific interactions may vary with latitude as well. For example, species
interactions, including herbivory, competition and predation, tend to be stronger at lower latitudes compared to
higher latitudes (reviewed by Schemske et al. 2009).
Because herbivore pressure on plants (i.e. plant damage and
herbivore abundance) is generally greater in the tropics, it
had been hypothesized that low latitude plants should
evolve more effective plant defenses compared to high latitude plants as a result of increased selection for these
defense traits (Coley and Aide 1991, Coley and Barone
1996, Fig. 1). A prediction of this hypothesis is that there
should be a latitudinal gradient in plant defenses (LGPD)
with low latitude plants being better defended against
herbivores than high latitude plants.
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Recent empirical studies have looked for evidence of latitudinal gradients in herbivory and plant defenses but results
are inconsistent (reviewed by Moles et al 2011a). In a global
survey of habitats from 14 countries (including rainforests,
deserts, tundra, and temperate forests), Moles et al. (2011b)
examined latitudinal gradients of 14 plant resistance traits
using standardized protocols and phylogenetic contrasts.
Only two of the measured plant traits followed the predicted
pattern of greater levels at low latitudes compared to high
latitudes; six traits had the opposite pattern with greater levels at high latitudes and six traits showed no relationship
with latitude. Mixed results have also been shown in continental-scale studies within single habitat types. The predicted
patterns of higher herbivory and plant defenses at low
latitudes were observed in salt marshes of North America
and Europe (Pennings et al. 2001, 2007) but the reverse
pattern was observed in hardwood trees in North American
forests (Adams and Zhang 2009) and there were no latitudinal gradients in damage or plant defenses for Acacia
shrubs in Eucalyptus forests in Australia (Andrew and
Hughes 2005). These inconsistent findings, at both global
Predator
abundance
g
j
f
n
Damage
b
Latitude
a
Herbivore
abundance
c
d
e
k
h
m
Plant resistance
l
Plant
abundance
i
Figure 1. Conceptual model of the relationships between latitude,
herbivore pressure and plant resistance. Black, solid arrows represent relationships described by the traditional latitudinal gradient
in plant defenses hypothesis, which predicts a negative relationship
between latitude and herbivore abundance, a positive relationship
between herbivore abundance and damage, and a positive relationship between damage and resistance. Gray arrows represent
other relationships that might influence damage and plant resistance such as top–down and bottom–up processes. Dashed arrows
represent feedbacks. Gray arrows represent relationships are not
examined in traditional latitudinal gradient studies.
and continental scales, suggest that the predicted pattern
of LGPD may only apply to some habitat types and some
plant traits.
One reason for inconsistent findings could be that the
underlying LGPD hypothesis is too simple. According to
the LGPD hypothesis (black, solid arrows in Fig. 1), herbivore abundance should decrease with latitude (path a),
herbivore abundance positively influences damage (path b),
and the resulting damage positively influences selection for
or the production of plant defenses (path c). However, the
LGPD hypothesis does not account for feedbacks between
plant defenses and herbivore abundances (dashed arrows in
Fig. 1). A large body of literature has demonstrated that
plant resistance can influence subsequent damage (path d),
and variation in resistance has the potential to influence
herbivore population dynamics (path e, Karban and
Baldwin 1997, Underwood 1999). Reduction in herbivore
densities could in turn reduce damage, potentially leading to
positive latitudinal relationships with herbivores, damage or
plant defenses. Latitudinal relationships may therefore be
dynamic, making it unlikely that there would be a simple
relationship at any one point in time. A simple and static
hypothesis may therefore not effectively describe latitudinal
variation in plant defenses given the often complex and
dynamic relationship among species.
Another reason for inconsistent findings could be different (and imperfect) measures of resistance. Although the
plant traits typically measured in latitudinal gradient studies
are thought to provide resistance against herbivores (e.g.
phenolics, toughness, C:N), not all putative defense traits
actually result in resistance to herbivores or do so equally
amongst different herbivore species (Carmona et al.
2011, Johnson and Rassman 2011). Also, interactions
between different chemical compounds in plants are common and these interactions may result in non-additive (or
even unpredictable) effects on herbivores (Hay et al. 1994,
Steppuhn and Baldwin 2007). Additionally, biochemical
changes in insect guts can help avoid toxicity of plant
defenses or alter the efficiency of insect digestion (Jongsma
and Bolter 1997, Chown and Nicolson 2004) rendering
plant defenses ineffective. In order to better understand the
relationships among latitude, herbivore pressure, and plant
resistance traits, we need to ensure that our measure of
resistance is ecologically meaningful. Rather than examining
relationships between latitude and individual plant traits,
the use of bioassays that measure changes in herbivore growth
rates after feeding on plants can be used to assess plant
resistance more holistically. Bioassays integrate the effects of
multiple plant traits on herbivores and the ability of herbivores to digest plant material, thus revealing the efficiency of
those traits in deterring herbivores from future plant
damage. Bioassays may also allow us to examine the ecological (and evolutionary) consequences of those traits for herbivore populations.
Finally, previous studies examining LGPD make little
attempt to incorporate other factors that might influence
plant damage and resistance (e.g. predator and plant abundances, gray arrows in Fig. 1). For example, predators
can indirectly influence damage by suppressing herbivore
abundance (path f, top–down effects, Hairston et al. 1960)
or directly influence damage by altering herbivore feeding
behavior and physiology (path g, trait-mediated indirect
effects, Abrams 1995). These consumptive and/or nonconsumptive effects can influence the production of plant
defenses (Griffin and Thaler 2006). Alternatively, the
abundance of plants might indirectly influence damage by
affecting herbivore abundances (path h, bottom–up effects)
or directly influence damage by modifying the feeding
behavior or physiology of herbivores (path l ). If competition
for resources constrains the production of defenses
(Herms and Mattson 1992), then plant density can directly
influence the growth rates of herbivores (path i). Finally,
predator and plant abundances can also vary with latitude
(path j and k, respectively). Because other trophic interactions can interact with herbivore pressure to influence damage and plant resistance, incorporating their effects using a
multi-trophic approach may be required to better understand why LGPD have been found in only a few systems
(Johnson and Rasmann 2011, Marczak et al. 2011).
In this study, I used a multi-trophic approach to examine
relationships between latitude and plant resistance to herbivory using the growth rates of herbivores to measure resistance for two common old-field plant species (Solanum
carolinense and Solidago altissima) spanning a broad
geographic range ( ⬎ 12 degrees). Specifically, I asked
1) are there latitudinal gradients in herbivore abundances
and damage (i.e. herbivore pressure) and 2) does plant
resistance (measured as herbivore growth rate) vary with
latitude? Because of the many ways in which latitude
can affect damage and plant resistance (e.g. through
variation in herbivore, predator, and/or plant abundances), I
used structural equation modeling (SEM) to ask 3) are latitudinal gradients in damage or resistance simply mediated
by herbivore abundances or are more complex indirect
pathways involving plant and/or predator abundances also
important?
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Methods
Study system
Solanum carolinense (Carolina horsenettle) and Solidago
altissima (tall goldenrod) are perennial herbaceous plants
native to the eastern United States but considered invasive in
other parts of the US, Europe and Asia (Werner et al. 1980,
Wise et al. 2008). Both species co-occur in early successional
old-field habitats and support a diversity of generalist and specialist insect herbivores. The primary herbivores of S. carolinense are specialist leaf chewing insects including the false potato
beetle Leptinotarsa juncta, tobacco hornworm Manduca sexta
and eggplant flea beetle Epitrix fuscula. Common herbivores
of S. altissima include generalist sap suckers (e.g. red goldenrod aphids Uroleucon sp., spittle bugs Clastoptera sp.) and specialist feeders (e.g. gall-making midges Eurosta solidaginis and
Rhopalomyia solidaginis, leaf miners Microrhopala vittata and
leaf chewers Trirhabda virgata). Both plant species also host
generalist insects including the beet army worm Spodoptera
exigua, cabbage looper Trichoplusia ni and grasshoppers (Melanoplus sp., Schistocerca americana, Aptenopedes sp.). Both
plants have chemical and physical traits suspected to deter herbivores (Bosio et al. 1990, Cipollini and Bergelson 2002), and
herbivores are known to affect plant performance (Root 1996,
Wise and Sacchi 1996), population growth (Carson and Root
1999, Underwood and Halpern 2012), and natural selection
(Meyer 1993, Wise and Cummins 2006) of both plant species. All species hereafter will be referred to by genus name.
Field survey
In 2008, I surveyed 18 old-fields at six locations spanning
more than 12 degrees of latitude from Florida to New York,
USA (Fig. 2). At each location, I surveyed three early successional fields (⬍ 5 years post disturbance). Field sizes were at
least 50 ⫻ 50 m in area, and separated by 2 km. At each field,
I estimated the local abundance of Solanum and Solidago
using two 50 ⫻ 1 m belt transects, separated by 20 m. I measured leaf damage (percent area removed on all leaves) and
recorded herbivores found on 20 randomly selected Solanum
and Solidago plants within each belt transect. I characterized
the insect herbivore and predator communities at the fieldlevel with sweep net sampling across the entire length of
both belt transects. I identified herbivores and predators to
the species or morphospecies level. I started field surveys in
Florida (late July) and ended in New York (early August),
completing all surveys within a two week period. This period
corresponded to the middle of the growing season (peak herbivore activity and plant growth) at each location.
Herbivore performance
To measure plant resistance, I performed no-choice bioassays in the laboratory using plant material collected
from the field and measured herbivore growth rates. Note
that I did not measure plant quality (the chemical and
nutritional components of the leaves) but rather the net
effect of these quality traits on herbivore growth rates.
Because the effectiveness of plant defenses against herbivores can vary with herbivore species (e.g. generalists may
be more affected by variation in defenses than specialists),
I examined the performance of several herbivore species
known to feed on Solanum and Solidago. I collected five
plants per species at each field and dried plant material in a
forced-draft oven at 60°C until constant weight. Plant
material was then ground to a fine powder using a mortar
Figure 2. Locations of surveyed sites. Six locations were surveyed across a latitudinal gradient in the eastern USA including areas near
Tallahassee, FL; Rome, GA; Knoxville, TN; Dublin, VA; Boyce, VA; and Ithaca, NY. Three fields per location were surveyed yielding
18 old-fields.
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and pestle. While low-temperature drying can result in
some loss of chemical compounds in the leaves (e.g. phenolics and volatiles, Abascal et al. 2005, Harbourne et al.
2009), low-temperature drying preserves the nutritional
content of the leaves and is commonly used in agronomy
and ecosystem sciences. Moreover, even with some loss of
chemical compounds in leaves, a relative comparison
of resistance to herbivores was possible because plant material was prepared identically across all sites and latitudes.
For the bioassays, I reconstituted the plant material
(0.5 g) in 10 ml of agar solution (1%) to make a standard
insect diet following Pennings et al. (2007). I used a control
diet consisting of only agar and water to determine how agar
influenced herbivore growth. While the use of standard
diets eliminates some physical defenses produced by
Solanum and Solidago (trichomes were destroyed but
Solanum spines remained intact), both these plant species
are known to produce a range of phenolics to deter herbivores (Bosio et al. 1990, Cipollini and Bergelson 2002). A
large amount of plant material was required for the diet,
therefore I combined plant material from all individuals of
each plant species at each field. The performance of herbivores on Solanum and Solidago diets was therefore assessed at
the field-level rather than the individual plant-level. Generalist herbivores (Schistocerca and Spodoptera) were tested on
Solanum and Solidago diets separately. Specialist herbivores
were tested only on their respective host plants; Solanum
diet was fed to Leptinotarsa and Manduca, and Solidago diet
was fed to Trirhabda. Grasshoppers were obtained from a
laboratory colony and raised on lettuce and barley. Both caterpillar species were obtained from commercial rearing facilities and fed on artificial diets. Leptinotarsa were obtained
from a laboratory colony and raised on Solanum foliage
from plants collected in north Florida. Trirhabda beetles
were obtained from various fields throughout New York and
Virginia. One month prior to the bioassays, both beetle species were fed on a mixture of live plants collected from the
different latitudes and grown in the greenhouse to avoid
bias based on prior feeding experience.
The performance of all herbivores except Trirhabda
was assessed by the relative growth rates (RGR) of insect
larvae. Seven to ten larvae per site per insect species were
used for the bioassays and were fed either Solanum
or Solidago diet from each site. Because RGRs can decrease
with increasing size (see Rose et al. 2009 for a plant
body size example), I used 2nd instar larvae. For Spodoptera
bioassays where a large number of individuals were needed
(280 individuals), I supplemented with 3rd instar larvae
(approximately 40 individuals). Prior to the start of the
bioassays, larvae were starved for three hours to remove
food content in their digestive tracts and each larva was
weighed separately. Standardized pieces of diet (4 cm3)
were offered to each larva in 75–150 ml soufflé cups
lined with moist filter paper. Herbivores fed on the diet
for three days. After three days of feeding, the diet
was removed, and the larvae starved for an additional
three hours and weighed. The RGRs were calculated as ln
(final larva mass) – ln (initial larva mass). To account for
agar effects which varied amongst herbivores, the final RGR
for each individual was calculated as the RGR (diet) –mean
RGR of the herbivore species (agar). For Trirhabda feeding
on Solidago diet, adult females were used because larvae
were not available. Because the growth rates of adult
insects vary little, performance was characterized using the
number of eggs laid after feeding on diet for three days.
Bioassays were carried out in four temporal blocks because
of the large number of bioassays needed to be performed and
not all herbivores were available at the same time.
Blocks were done between February–June 2009 using
Leptinotarsa, Manduca, Spodoptera and Schistocerca. A separate experiment was done using Trirhabda adults in
September 2011. Because the supply of insects was limited,
a subset of latitudes was used for the Schistocerca bioassays
(Florida 30°N, Georgia 34°N, northern Virginia 39°N,
New York 42°N) for both Solanum and Solidago. Similarly,
only three of the six latitude locations were used for bioassays
using Trirhabda (Florida 30°N, southern Virginia 37°N,
New York 42°N). The final number of bioassays was
870 (180 using Leptinotarsa, 180 Manduca, 252 Spodoptera,
168 Schistocerca and 90 Trirhabda).
Statistics
Field was the experimental unit so damage and the relative
growth rates of herbivores were averaged across plant individuals within each field for each plant species (n ⫽ 18 for
damage; n ⫽ 9 to 18 for herbivore growth rates). The abundance of plants and herbivores were summed across both belt
transects for each field (n ⫽ 18). The mean damage per field
was log-transformed to meet the assumptions of normality.
General linear models
To examine the effects of latitude on herbivore abundances
and plant damage, I used separate GLMs using Gaussian error
in R (ver. 2.12). I also used separate GLMs to examine relationships between latitude and the RGR of each herbivore species.
Preliminary results revealed non-significant effect of temporal
blocks therefore temporal block was not included as a variable
in the model (Supplementary material Appendix 1–2). The
RGRs for individuals from all temporal blocks were then averaged per field. After visual examination of the data, some of the
relationships with latitude appeared non-linear therefore
I included a second order polynomial latitude term in the
models. Linear and quadratic latitude terms were centered to
reduce multicollinearity between these two variables.
Structural equation modeling (SEM)
I used SEM to examine the direct and indirect relationships
between latitude, herbivore pressure and plant resistance
(Fig. 1). According to the LGPD hypothesis (black arrows in
Fig. 1), I predicted 1) a negative relationship between latitude and foliar-feeding herbivore abundance (path a),
2) a positive relationship between foliar-feeding herbivore
abundance and damage (path b), and 3) a positive relationship between damage and plant resistance (path c). This
model is hereafter referred to as the ‘LGPD model’. I tested
an ‘alternative model’ that included relationships between
latitude (both linear and quadratic terms), plant, and predator
abundances (all paths in Fig. 1). Herbivore abundances can
be driven by top–down (predator abundances, path f) and
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bottom–up factors (host-plant abundances, path h), which
may themselves vary with latitude (paths j and k, respectively). Damage can also be influenced by predators (path g)
through non-consumptive effects on herbivores. Plant density can influence resistance (path i) and damage (path l) if
plants are competing for limited resources.
While explicitly examining the importance of feedbacks
between plant resistance and herbivore abundance using
dynamic models is beyond the scope of this paper, SEM
allowed me to incorporate reciprocal interactions and a feedback loop in the alternative model to address the potential
for feedbacks in this system (paths d, m and n). An assumption of models with feedbacks (non-recursive models) is that
they represent a static feedback process at equilibrium (Grace
et al. 2007). Non-recursive models have the potential to
be under-identified (model degrees of freedom ⬍ 0), therefore two conditions (order and rank conditions) must be
met to avoid identification issues (Kline 2011). The order
condition states that the number of excluded variables for
each endogenous variable must equal or exceed the total
(A)
number of endogenous variables minus 1. The rank condition states that each variable in the feedback loop must have
a unique set of variables directly influencing them from outside of the loop. In order to satisfy the order and rank conditions, I dropped paths m and n from the initial alternative
model because I was not explicitly interested in bottom–up
effects on predators and top–down effects on host-plants.
This modified alternative model (Supplementary material
Appendix 3) satisfied both the order and rank conditions
therefore the model was not under-identified. Separate
SEM analyses were performed for each herbivore and
plant species combination, comparing the fits of the LGPD
model against the alternative model (14 SEMs in total).
Parameter estimation and model fitting was performed
using maximum likelihood procedures in AMOS (ver. 22).
I assessed model fit using χ2 and associated p-values,
comparative fit index (CFI), and root mean square error of
approximation (RMSEA). Good model fit is indicated by
p-values ⬎ 0.05, CFI values ⬎ 0.95 and RMSEA values
ⱕ 0.05. To improve the fit of the alternative model (Fig. 1 all
Solidago
8
45
Gallers
40
Hemiptera
35
Orthoptera
30
Lepidoptera
25
Coleoptera
7
Richness
(number of species)
Abundance
(number of individuals)
50
20
15
5
4
3
10
2
5
1
0
FL
GA
TN
S.VA N.VA
(B)
0
NY
FL
GA
TN
FL
GA
TN
S.VA N.VA
NY
Solanum
50
8
Hemiptera
40
Lepidoptera
7
Coleoptera
6
Richness
(number of species)
45
Abundance
(number of individuals)
6
35
30
25
20
15
10
5
4
3
2
1
5
0
0
FL
GA
TN
S.VA N.VA
NY
Field locations
S.VA N.VA
NY
(increasing latitude)
Figure 3. Latitudinal gradients in the total abundance and richness of herbivores on the plants Solidago altissima (A) and Solanum
carolinense (B) (n ⫽ 60 plants per field location). Locations include FL ⫽ Florida, GA ⫽ Georgia, TN ⫽ Tennessee, S.VA ⫽ Virginia (south),
N.VA ⫽ Virginia (north), NY ⫽ New York, USA.
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paths), I removed non-significant pathways (p ⬎ 0.05), starting with the greatest p-value. I also evaluated the residual
covariances between variables to determine whether paths
should be added. Because the ratio of sample size to number
of parameter estimates was less than the suggested ratio of
5 (Grace 2006), I confirmed path coefficients estimated
via maximum likelihood with parameters estimated using
Bayesian methods which are less sensitive to small sample
sizes (Lee and Song 2004). Path coefficients where the
95% confidence intervals included zero were considered
non-significant and therefore removed from the model. I
compared models using the Akaike information criterion
(AIC) and deviance information criterion (DIC) where
models with lower AIC and DIC (⬎ 2) are considered better
fit models (Burnham and Anderson 2002). This step-wise
elimination process continued until all paths remaining in
the model where significant or when AIC and DIC values
increased after a variable was removed. For models where
ΔAIC or DIC ⬍ 2 (models of similar fit), I selected the
most parsimonious model (Burnham and Anderson 2002).
Results
Figure 4. Latitudinal gradients in herbivory (% leaf damage)
for Solanum carolinense (gray) and Solidago altissima (black). Lines
represent significant relationships with latitude and damage
(Solanum: F1,15 ⫽ 6.760, plinear ⫽ 0.02; Solidago: F1,15 ⫽ 4.760,
pquadratic ⫽ 0.04).
Field surveys
Solanum received significantly greater leaf damage (15.0%)
than Solidago (7.1%, t ⫽ ⫺4.08, DF ⫽ 31, p ⬍ 0.001).
Solidago damage increased, non-linearly, with latitude
(quadratic F1,15 ⫽ 4.76, p ⫽ 0.04, Fig. 4). On the other hand,
damage to Solanum decreased with latitude (linear
F1,15 ⫽ 6.76, p ⫽ 0.02), mostly due to Solanum plants
in New York that experienced very little damage (Fig. 4).
The total abundance of herbivores per field did not vary
with latitude (linear F1,15 ⫽ 0.01, p ⫽ 0.91, quadratic
F1,15 ⫽ 1.04, p ⫽ 0.32). Sweep net samples revealed that herbivores within the sampled fields were mostly Hemipterans
(77% of the abundance of herbivores). Herbivore counts
on plants revealed that Hemipterans made up a large
proportion of herbivores observed on Solidago (53% of herbivores on Solidago, Fig. 3); however most of herbivores
observed on Solanum were Chrysomelid beetles (92.2% of
herbivores on Solanum, Fig. 3). When examining foliarfeeding herbivores only, there was a positive linear relationship with latitude (F1,15 ⫽ 9.99, p ⫽ 0.01).
Laboratory bioassays
Overall, latitude did affect mean herbivore growth rates,
but latitude effects depended on diet specialization, herbivore species, and host plant. Generalist herbivores generally
lost weight whereas specialists gained weight (Fig. 5, 6), suggesting that specialists were able to meet their nutritional
requirements feeding on Solanum or Solidago, whereas
generalists did not. For Solanum specialists, Manduca
performed best on mid-latitude plants (Fig. 5, quadratic
F1,15 ⫽ 7.29, p ⫽ 0.01) but there was no relationship between
latitude and the RGR of Leptinotarsa, (linear F1,15 ⫽ 0.04,
p ⫽ 0.84, quadratic F1,15 ⫽ 0.03, p ⫽ 0.85). For generalists
feeding on Solanum, Spodoptera performed poorly on midlatitude plants (quadratic F1,15 ⫽ 10.77, p ⫽ 0.01). There
was no relationship between latitude and the RGR of
Schistocerca (linear F1,9 ⫽ 0.45, p ⫽ 0.51, quadratic
F1,9 ⫽ 0.03, p ⫽ 0.85).
For generalist herbivores feeding on Solidago (Fig. 5),
Spodoptera also performed poorly on mid-latitude plants
(similar pattern to a Solanum diet, quadratic F1,15 ⫽ 11.33,
p ⬍ 0.01). However, unlike Solanum, a positive, slightly
non-linear relationship was observed between the RGR
of Schistocerca and latitude (linear F1,9 ⫽ 21.20, p ⬍ 0.01,
quadratic F1,9 ⫽ 9.11, p ⫽ 0.01), indicating that northern
Solidago plants were more nutritious and palatable to
Schistocerca. For the specialist leaf feeding beetle, Trirhabda,
a negative relationship was observed between the number
of eggs and latitude (linear F1,6 ⫽ 6.05, p ⫽ 0.04).
Structural equation models
The models describing the LGPD hypothesis were very
poor fits to both the Solanum and Solidago data (Supplementary material Appendix 4–5). Instead, the alternative
models that included relationships between predator and
plant abundances improved model fit (ΔAIC ranged from
12.2 to 43.3, Supplementary material Appendix 4–5).
Latitude indirectly influenced damage to both Solanum
and Solidago through variation in the abundance of
foliar-feeding insect herbivores, but not in the direction
predicted by the LGPD (Fig. 7, 8). Herbivores were also
negatively influenced by predator abundances, indicating
top–down control and predators were positively influenced by latitude. For Solidago, predators also had direct
effects on damage (without any changes in the abundance
of herbivores, Fig. 8b). Solidago damage was also directly
influenced by the density of Solidago (positive relationship) but not through variation in herbivore abundances
(Fig. 8b–c).
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Figure 5. Relationships between latitude and the relative growth rates of Solanum specialists (A) Manduca sexta, (B) Leptinotarsa juncta
and generalists (C) Spodoptera exigua and (D) Schistocerca americana on Solanum diet. Growth rates for each species are averaged at each
site. p-values of significant latitude effects (both linear and quadratic terms) on mean growth rates. NS ⫽ non-significant latitude effects.
Solanum and Solidago resistance to herbivory varied
with latitude, but this variation was not dependent on herbivore pressure. For the two of the three assayed herbivores on
Solidago (Spodoptera and Schistocerca), there were direct
relationships between latitude and plant resistance (Fig. 8).
For both caterpillar species feeding on Solanum (Spodoptera
and Manduca), there were direct, non-linear relationships
with latitude. For the other two Solanum herbivores
(Leptinotarsa and Schistocerca), herbivore growth rates were
mediated by the density of Solanum. For all assayed herbivores on Solanum and Solidago, there was no relationship
between damage and plant resistance and no indication
of feedback loops and reciprocal interactions between predictor variables (all relationships were unidirectional).
Discussion
The latitudinal gradient of plant defenses (LGPD) hypothesis predicts that because herbivore pressure (i.e. herbivore
abundance and damage) is generally greater at low latitudes,
low latitude plants should have evolved better defenses than
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plants at higher latitudes. In this study, foliar-feeding
herbivore abundance and plant damage did vary with latitude though not necessarily in the expected direction
(i.e. greater damage and herbivore abundance in lower latitudes). Furthermore, herbivore abundance and damage
were influenced by top–down and bottom–up processes,
respectively, and damage in the field did not influence resistance. There was no general pattern of herbivore responses
across latitude; the exact relationship between herbivore
growth rates and latitude depended on plant and herbivore
species. Together, these results suggest that the LGPD
hypothesis is too simple to explain the complex realities of
plant–insect interactions.
Relationships between herbivore abundance,
damage and resistance are complex
The idea that low latitude plants experience greater herbivore pressure than high latitude plants forms the foundation
of the LGPD, but this pattern was not observed in this
study. Low latitude sites had lower abundances of foliarfeeding insects and this relationship resulted in opposite
damage patterns to Solanum and Solidago (Fig. 4) even
though both plant species are similar in life-history. For
Solidago, the positive relationship between herbivore
abundance and damage may be driven by an increase in
the abundance of Trirhabda and Microrhopala (two specialists on Solidago) with latitude. The ranges of both beetle
species are limited to northern regions of the US
and can reach outbreak levels causing high levels of foliar
damage (Carson and Root 2000, Uriarte 2000). Trirhabda
and Microrhopala were not found in fields at lower
latitudes but within their ranges, their abundances generally
increased with latitude (sweep net: F1,7 ⫽ 8.83, p ⫽ 0.02,
plant surveys: F1,7 ⫽ 18.49, p ⫽ 0.003). For Solanum, on
the other hand, the unexpected negative relationship
between herbivore abundance and damage can be partially
explained by the fact that not all herbivores exert the same
amount of damage. Solanum herbivore body sizes varied by
an order of magnitude: from Epitrix (2–3 mm in length) to
Manduca (10–70 mm). The abundance of large-bodied
herbivores (Manduca and Leptinotarsa) decreased with latitude (sweep net: F1,16 ⫽ 5.021, p ⫽ 0.03, plant surveys:
F1,16 ⫽ 15.19, p ⫽ 0.001) while the abundance of smallbodied herbivores (Epitrix) increased with latitude (sweep
net: F1,13 ⫽ 4.27, p ⫽ 0.06, plant surveys: F1,13 ⫽ 7.39,
p ⫽ 0.02). This variation in species composition could
explain why there was an overall negative relationship
between damage and herbivore abundance. These results
suggest that latitudinal variation in herbivore species composition (or identity of herbivores present) may better characterize selection pressures on plants than the total
abundance of herbivores alone (Salazar and Marquis 2012).
In order to make the connection between herbivore
pressure and plant resistance, we need a measure of
resistance that is ecologically and evolutionarily meaningful.
Previous studies have measured latitudinal variation in
plant traits thought to confer resistance and inferred effects
on herbivores (reviewed by Moles et al. 2011a). However,
some of these traits may not actually confer resistance nor do
so equally for all herbivores (Carmona et al. 2011). This was
evident in the bioassays where herbivore species responded
differently to the same diet. For example, at mid-latitudes,
Manduca generally performed well on Solanum but
Spodoptera performed poorly. Variation in responses suggests
that measuring plant traits alone may not be an adequate
measure of overall resistance. Furthermore, some herbivore
species had consistent responses across latitude (Leptinotarsa
and Schistocerca) while other herbivore species had varying
responses across latitude (Manduca, Spodoptera). Because the
responses of different herbivore species vary both within
sites and across latitude, the ecological and evolutionary consequences of resistance for both plants and herbivores may
be difficult to generalize and predict.
In this study, factors other than herbivore abundance
influenced damage and resistance which could explain
why the expected direction of the LGPD was not observed
(Fig. 7, 8). For example, Solidago damage increased with
predator abundance and the density of Solidago. Variation
in predation risk (i.e. predator abundance) and the foraging
environment (i.e. density of plants) could influence the
feeding behavior and physiology of herbivores, potentially
influencing the amount and distribution of damage to
Figure 6. Relationships between latitude and (A) the number
of eggs per clutch for a Solidago specialist (Trirhabda virgata)
(B) the relative growth rates of generalists Spodoptera exigua and
(C) Schistocerca americana on Solidago diet. Growth rates for
each species and number of eggs are averaged at each site. p-values
of significant latitude effects (both linear and quadratic terms).
host plants without changing herbivore densities (Belovsky
et al. 2011). Predators also had consumptive effects on herbivores. The indirect benefit of predators to plants may not
require the production of plant defenses to deter herbivores.
Instead, plants may invest in other traits (e.g. floral resources,
shelter) to attract insect predators, rather than chemical and
physical defenses. Finally, there were direct effects of latitude
on damage and resistance. These direct linkages suggest that
there were other unmeasured factors, such as climate and
893
Figure 7. Final models examining relationships between latitude, damage, and plant resistance to Solanum specialists (A) Manduca sexta,
(B) Leptinotarsa juncta, and generalists (C) Spodoptera exigua, and (D) Schistocerca americana. Field-level characteristics were identical in all
four SEMs (predator abundances, foliar-feeding herbivore abundances, Solanum density, and mean Solanum damage (log-transformed)).
Only resistance to each herbivore species (measured as the relative growth rates (RGR) of each herbivore species) varied in each SEM.
Solid arrows are significant positive relationships and dashed arrows are significant negative relationships. Arrow thickness is scaled to the
standardized path coefficients (Supplementary material Appendix 6). χ2- and p-values denote model fit.
resource availability, that influenced damage and resistance
to both plants species because latitude cannot directly
influence damage and resistance. Because damage and resistance can be affected by a range of biotic and abiotic factors,
the LGPD hypothesis may be too simple to describe the
complexities of plant–herbivore interactions.
Constraints to selection
The LGPD hypothesis suggests that variation in herbivore
pressure should lead to changes in resistance to herbivores,
however I did not observe a relationship between herbivore
pressure and plant resistance. One reason for this lack of relationship could be that responses to selection pressures
may be constrained. For example, resource availability may
limit the ability of plants to produce defenses even if herbivore abundances and damage is high. In two of the feeding
assays, Solanum density (which varied with latitude) affected
herbivore growth rates (Leptinotarsa and Schistocerca) but
these relationships were not influenced by herbivore
abundance. If plants from high-density fields experienced
greater competition for resources, this could limit the production of plant defenses, resulting in lower resistance and
greater performance of some herbivore species (Kim 2012,
Halpern et al. in press). Furthermore, because multiple
herbivores are known to feed on Solanum and Solidago in
the field (⬎ 30 species described on Solanum (Wise 2007)
894
and ⬎ 100 species described on Solidago (Root 1996)), diffuse coevolution could constrain the evolution of resistance
to all herbivores (Wise and Rausher 2013).
Another reason for the lack of latitudinal concordance
between herbivore pressure and plant resistance may be
that tolerance varies with herbivore pressure instead of resistance. The degree to which plants in a particular location
express tolerance versus resistance could influence herbivore
abundances (Stinchcombe 2002, Fornoni 2011). Both
Solanum and Solidago are known to tolerate damage by
herbivores and tolerance has been shown to vary with environmental context such as successional stage (Hakes and
Cronin 2012) or intraspecific density (McNutt et al. 2012).
To my knowledge, no study has examined latitudinal variation in tolerance, which is likely to be an important missing
component in latitudinal gradient studies (Johnson and
Rasmann 2011).
Summary and future directions
In summary, latitudinal relationships between damage and
herbivore growth rates were observed, but not in the
predicted directions. The LGPD hypothesis suggests that
plant defenses should be influenced by latitudinal variation
in herbivore pressure. However, plant responses may be
influenced by other factors such as predator abundances
or constrained by resource availability. To further our
multiple points in time and through manipulative experiments may be required. Revisiting the original LGPD
hypothesis to incorporate some of the inherently dynamic
and complex nature of natural selection and population
dynamics through the use of mathematical models would
be one fruitful approach.
Acknowledgements – I gratefully acknowledge M. Cipollini (Berry
College, Georgia, USA), G. Crutsinger and L. Souza (Univ. of
Tennessee, Tennessee, USA), M. Wise and D. Carr (Blandy
Experimental Farm, Virginia, USA), and S. Campbell (Cornell
Univ., New York, USA) for logistical support during latitudinal
field surveys. I thank J. Capinera (Univ. of Florida, Florida, USA)
for providing grasshoppers for bioassays. I thank J. Stanford for lab
and greenhouse assistance. This manuscript was greatly improved
by comments from N. Underwood, B. Spiesman, J. Grinath and
A. Hakes. The Robert K. Godfrey Endowment Award for the Study
of Botany (Florida State Univ., Florida, USA) helped fund this
research.
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Figure 8. Final models examining relationships between
latitude, damage and plant resistance to Solidago specialist
(A) Trirhabda virgata, and generalists (B) Spodoptera exigua, and
(C) Schistocerca americana. Field-level characteristics were identical
in all four SEMs (predator abundances, foliar-feeding herbivore
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growth rates (RGR) of Spodoptera, and Schistocerca and number
of Trirhabda eggs) varied in each SEM. Solid arrows are significant
positive relationships and dashed arrows are significant negative
relationships. Arrow thickness is scaled to the standardized
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