Genetic diversity and connectivity shape herbivore load
within an oak population at its range limit
TA¨HTI POHJANMIES,1,6, AYCO J. M. TACK,2,3 PERTTI PULKKINEN,4 SAKINA ELSHIBLI,5
PEKKA VAKKARI,5 AND TOMAS ROSLIN1
1Spatial Foodweb Ecology Group, Department of Agricultural Sciences, P.O. Box 27 (Latokartanonkaari 5),
FI-00014 University of Helsinki, Helsinki, Finland
2Metapopulation Research Group, Department of Biological and Environmental Sciences, P.O. Box 65 (Viikinkaari 1),
FI-00014 University of Helsinki, Helsinki, Finland
3Department of Ecology, Environment and Plant Sciences, Stockholm University, Lilla Frescati, SE-106 91 Stockholm, Sweden
4Haapastensyrja¨ Breeding Station, Finnish Forest Research Institute, Haapastensyrja¨ntie 34, FI-12600 La¨ylia¨inen, Finland
5Vantaa Research Centre, Finnish Forest Research Institute, PL 18, FI-01301 Vantaa, Finland
Citation: Pohjanmies, T., A. J. M. Tack, P. Pulkkinen, S. Elshibli, P. Vakkari, and T. Roslin. 2015. Genetic diversity and
connectivity shape herbivore load within an oak population at its range limit. Ecosphere 6(6):101. http://dx.doi.org/10.
1890/ES14-00549.1
Abstract. Host genetic diversity and genotypic identity have been reported to affect the abundance,
species richness and species diversity of associated herbivore communities. Recent work, however,
suggests that these effects are highly context-dependent and that the magnitude and direction of the effects
may vary with e.g., spatial factors and the amount of genetic variation present in the host population. Here,
we use observational data on a Finnish oak (Quercus robur) population to examine whether low genetic
diversity within peripheral populations reduces the impact of host genotype on associated herbivore
communities. We first compared measures of genetic variation within Finnish oak populations with those
recorded in more central parts of the species’ range, confirming that genetic variation within the Finnish
populations is comparatively low. Despite this result, we found consistent imprints of host genetic diversity
on herbivore communities: herbivore load, but not the species richness, increased with host genetic
diversity in both years and both spatial scales examined. Spatial connectivity of hosts increased herbivore
diversity as well as abundance. While the similarity of herbivore communities increased with the genetic
similarity among hosts, the effect of geographic distance was stronger. Overall, our findings identify a
major role for spatial context in structuring oak-associated herbivore communities—but we still trace
detectable imprints of host genotype at multiple spatial scales even in this peripheral, genetically
impoverished oak population.
Key words: community genetics; extended phenotype; genetic variation; Quercus robur.
Received 29 December 2014; revised 23 February 2015; accepted 4 March 2015; final version received 10 April 2015;
published 25 June 2015. Corresponding Editor: D. P. C. Peters.
Copyright:  2015 Pohjanmies et al. This is an open-access article distributed under the terms of the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the
original author and source are credited. http://creativecommons.org/licenses/by/3.0/
6 Present address: Department of Biological and Environmental Science, P.O. Box 35 (Survontie 9 C), FI-40014 University
of Jyva¨skyla¨, Jyva¨skyla¨, Finland.
 E-mail: tahti.t.pohjanmies@jyu.fi
INTRODUCTION
The extent to which the herbivore communities
of plants represent an extended phenotype of the
host genotype has been the subject of recent
debate (Hughes et al. 2008, Bailey et al. 2009,
Tack et al. 2012). According to the framework of
community genetics, genetic variation within a
v www.esajournals.org 1 June 2015 v Volume 6(6) v Article 101
population of one species may influence the
dynamics and diversity of associated species—a
notion for which support has been found in
several empirical studies (Hersch-Green et al.
2011). In particular, genetic diversity at the level
of plant individuals (i.e., heterozygosity; e.g.,
Tovar-Sa´nchez et al. 2013) or plant populations
(e.g., Crutsinger et al. 2006, Johnson et al. 2006)
has been proposed to affect the abundance and
diversity of associated arthropod communities,
and the similarity among plant individuals to
affect the similarity of the associated communi-
ties (e.g., Bangert et al. 2006b). The effects of
intraspecific genetic variation of host plants may
occur at various levels and thus arise from
various mechanisms. For example, host genetic
diversity may increase the abundance of associ-
ated arthropods at the level of individual plants
e.g., by herbivore preference (cf. Gripenberg et al.
2010, Kalske et al. 2014), or at the level of plant
populations e.g., by increased resource availabil-
ity (Crutsinger et al. 2006).
Recent work (Tack et al. 2012) stress that the
realized effects of host plant genotype should be
compared with other forces that may shape
communities in their natural environment. In
particular, it has been proposed that spatial
effects—the relative position of a host plant with
respect to congeneric individuals within the
landscape—may dwarf genetic effects (Tack et
al. 2010). In addition to spatial effects, the relative
effects of host genotype may vary with the
amount of genetic variation present in the host
population (Bangert et al. 2006b). However,
studies in community genetics have consistently
focused on systems with ample genetic variation
(e.g., hybrid systems). This status quo under-
mines the generality of current theory.
Oaks (Quercus spp.) are as a genus character-
ized by high levels of genetic variation within
and among species and within conspecific
populations (Kremer and Petit 1993). Throughout
their range, oaks harbor a diverse community of
insect species. They may be considered founda-
tion species in the temperate forests of the
northern hemisphere, where their abundance is
currently in decline (Lindbladh and Foster 2010).
The pedunculate oak (Quercus robur) grows in
northern Europe at the northern limit of its
natural range. As a likely consequence, the
diversity of chloroplast DNA (cpDNA) in the
Q. robur populations of northern Europe has been
found to be lower than in central Europe (Petit et
al. 2002). In addition to the marginal location,
low genetic variation in north European Q. robur
populations may derive from small population
sizes, the absence of other, interfertile oak
species, and the historic fragmentation of habi-
tats (Vakkari et al. 2006).
Here, we use observational data from a natural
system to examine the effects of genetic variation
in Finnish populations of Quercus robur at their
northern range edge on the associated commu-
nity of galling and leaf-mining herbivores. In
particular, we ask: (1) Will the genetic diversity at
the individual level (i.e., heterozygosity) affect
the structure of the herbivore assemblage occu-
pying the trees? (2) Will host genetic diversity
affect the herbivore assemblage at a neighbor-
hood scale, i.e., clusters of trees within the
landscape? (3) Will the genetic similarity among
hosts increase the similarity of their herbivore
communities? (4) Are the results observed here
conditional on their setting at the range margin
of the host, i.e., are the oaks of Finland
genetically more uniform than are oaks at the
core of their European range?
METHODS
Study design
Quercus robur occurs naturally in the south-
western archipelago and coastal zone of Finland
(Ferris et al. 1998). To quantify the level of genetic
diversity characterizing Finnish Q. robur popula-
tions, we measured the genetic variation within
three tree stands in south-western Finland: one in
Inkoo, one in Salo, and one on the island of
Wattkast (Fig. 1A). All three stands are scattered
in structure and located in a mosaic of agricul-
tural areas, water, and bare rock. Samples of trees
were genotyped using 15 nuclear microsatellite
loci (for details, see Appendix A). Genetic
variation within each population was measured
as allelic richness (A), calculated in Arlequin
v3.5.1.3 (Excoffier and Lischer 2010).
We examined the community-level effects of
host genotype within the Q. robur population on
Wattkast, focusing on two spatial scales: individ-
ual trees and tree neighborhoods. One hundred
trees were randomly selected (Fig. 1B) and
surveyed for c. 20 galling and leaf-mining
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POHJANMIES ET AL.
herbivores in both 2006 and 2007. Tree neighbor-
hoods were defined as including a minimum of
three trees located within a maximum distance of
120 m of each other, corresponding to the
average dispersal distance of one of the miner
species, Tischeria ekebladella (Appendix A in Tack
and Roslin 2011). Eleven such neighborhoods
were identified in the data, with the number of
trees in a neighborhood ranging from 3 to 12
(Fig. 1C). The neighborhoods were mutually
exclusive, and together included a total of 60
out of the 100 trees surveyed. For further details
on the study design and the herbivore surveys,
see Appendix B.
Fig. 1. Locations of the study populations, surveyed trees, and tree neighborhoods. (A) Locations of the three
oak populations in south-western Finland within which levels of genetic variation were quantified. The locations
are marked on the map with black stars. (B) A map of the island of Wattkast. All individual oak trees on the
island are marked on the map with black dots, and the 100 trees surveyed for herbivores with white dots. The
dashed square marks the area enlarged in (C). (C) A close-up of a part of Wattkast. Tree neighborhoods, as
identified for the analyses, are marked on the map with different shades of grey. Background maps copyrighted
by the National Land Survey of Finland.
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POHJANMIES ET AL.
At the level of individual trees, herbivore
communities were described by total species
richness, total herbivore abundance, and the
Shannon-Wiener index of diversity. Adopting
the approach of Tovar-Sa´nchez et al. (2013), we
described the genetic diversity of individual trees
as level of heterozygosity, which we measured as
internal relatedness (IR; Aparicio et al. 2006).
Values of IR can vary between 1 and 1, with
negative IR values indicating higher heterozy-
gosity and positive values higher homozygosity.
For a more intuitive interpretation of the results,
we used (IR) in the analysis, so that higher
values indicate higher heterozygosity.
For the neighborhoods, total species richness
and total herbivore abundance were calculated
by adding up the data over the trees in the
neighborhoods (total number of unique species
encountered and sum of herbivore abundances).
These values were rarefied to correct for unequal
neighborhood sizes: all unique combinations of
three trees were sampled for neighborhoods of
size .3, species richness and herbivore abun-
dance were calculated across each such sample,
and the resultant values were averaged. These
averages were then used as estimates of neigh-
borhood-level metrics. The genetic diversity
within neighborhoods was described as allelic
richness (A), corrected for the variation in the
numbers of trees in the neighborhoods by the
rarefaction method of Hurlbert (1971). The
method was used to calculate estimates of the
expected number of alleles at each locus in a
random sample of 3 individuals from the
neighborhoods of size .3. Thus, the measures
of genetic variation, species richness, and herbi-
vore load were all treated with effectively similar
corrections for unequal neighborhood sizes.
As previous work (Tack et al. 2010) has
reported spatial effects to be of major importance
in our study system, we accounted for such
imprints in all of our analyses. The connectivity
of individual trees was described with a metric
adopted from Tack et al. (2010). In principle,
connectivity reflects the expected immigration of
herbivores to a patch at maximum patch occu-
pancy (Tack et al. 2010). At the neighborhood
level, connectivity was described as the connec-
tivity value averaged across the trees in the
patch.
Statistical analyses
To test for an effect of host genotype on
herbivore community structure, we applied three
sets of statistical analyses: First, we constructed
generalized linear models of the herbivore
community descriptors (abundance, species rich-
ness, and Shannon-Wiener diversity at the
individual and neighborhood level) as functions
of host connectivity and genetic diversity. A log-
link and a Poisson distribution were assumed for
each response variable, except for the Shannon-
Wiener index, which was modeled by standard
linear regression. Separate models were con-
structed for the individual (n ¼ 100) and the
neighborhood (n ¼ 11) levels, with all models
fitted in R v.2.15.3 (R Core Team 2013). To check
for spatial autocorrelation in the observed IR
values, we used a Mantel test implemented in R
using the vegan package (Oksanen et al. 2013).
No significant spatial autocorrelation was detect-
ed (P ¼ 0.23).
Second, to examine whether species responded
similarly to individual host genetic diversity, we
constructed a generalized linear mixed model
(GLMM) of species-specific abundances in indi-
vidual trees (19 species3 100 trees3 2 years) as a
function of species identity, tree connectivity, IR,
and their interactions as fixed effects, and tree
identity as a random effect. A Poisson distribu-
tion was assumed for the species abundances.
The model was fitted using proc Glimmix in SAS
for Windows (SAS Institute, Cary, NC, USA),
version 9.2.
Survey year was included as an explanatory
variable in all of these models, whereas data on
three species (Cynips longiventris, Neuroterus
numismalis, and Tischeria dodonea) not encoun-
tered in both years were excluded. The number
of leaves examined per tree during the surveys
was also included as a covariate in models fitted
at the individual tree level, and a species3 year
interaction was added as a fixed effect in the
GLMM.
Third, to examine whether genetically more
similar hosts shared more similar herbivore
communities, we used Mantel tests between
community dissimilarity and genetic distance,
between community dissimilarity and geograph-
ic distance, and a partial Mantel test between all
three matrices. The tests were implemented using
the vegan package for R. These tests were
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POHJANMIES ET AL.
performed for data from year 2006 and 2007,
respectively. For individual trees, genetic dis-
tance was measured with Rousset’s aˆ, computed
with SPAGeDi v.1.4 (Hardy and Vekemans 2002).
At the neighborhood level, genetic distance was
measured by pairwise FST, calculated in Arlequin
with default settings. Geographic distance be-
tween two neighborhoods was defined as the
distance between their centroids. For construct-
ing the community dissimilarity matrices, data
on species abundances were transformed into
relative abundances by dividing each species-
specific abundance with the total herbivore
abundance observed on the tree/in the neighbor-
hood. The community dissimilarity matrices
were computed using the vegan package for R
with the Bray-Curtis index as the community
dissimilarity measure. Sites in which no species
had been observed were excluded from the data,
leaving at the individual level n¼ 92 trees in 2006
and n ¼ 93 trees in 2007. There were no empty
sites at the neighborhood level.
RESULTS
As expected, genetic diversity among Finnish
oak populations proved comparatively low. The
average allelic richness varied between 9.37 in
Wattkast and 13.44 in Inkoo. The numbers of
alleles per locus in Finnish populations were, on
average, lower than those recorded in more
central populations in Europe (Appendix C:
Table C1).
Despite low overall variation, genetic diversity
left a statistically significant imprint on herbivore
abundance, and these patterns were similar at
both individual and neighborhood scales: higher
individual heterozygosity and higher neighbor-
hood-level allelic richness were associated with
higher herbivore abundance (Table 1). A consis-
tent imprint of genetic diversity was detected at
the level of individual herbivore species: higher
heterozygosity was found to increase the species-
specific abundances of herbivores. In the GLMM,
the species 3 heterozygosity interaction and the
three-way species 3 connectivity 3 heterozygos-
ity interaction were non-significant (F18,3607 ¼
0.61, P ¼ 0.90, and F19,3607 ¼ 0.44, P ¼ 0.98,
respectively), suggesting that most species re-
spond to heterozygosity in the same way. When
these non-significant interactions were dropped,
the main effect of heterozygosity proved statis-
tically significant (F1,97 ¼ 5.17, P ¼ 0.03). In
contrast to the impact of genetic diversity on
herbivore abundance, there was no detectable
impact of genetic diversity on either species
richness or Shannon-Wiener diversity (Table 1).
Unlike genetic diversity, spatial context had a
Table 1. Generalized linear models of herbivore richness, abundance and diversity as functions of host
connectivity, host genetic diversity, survey year, and number of leaves examined. Each response was separately
modeled at the level of (A) individual trees and (B) neighborhoods of multiple trees.
Response Variable Estimate SE Z/t P
A) Individual level (n ¼ 100)
Richness Genetic diversity 0.16 0.18 0.86 0.39
Connectivity 0.01 0.002 7.65 ,0.0001
Year 0.18 0.06 2.97 0.003
No. leaves examined 0.002 0.0002 9.33 ,0.0001
Abundance Genetic diversity 0.45 0.06 7.03 ,0.0001
Connectivity 0.02 0.001 26.74 ,0.0001
Year 0.37 0.02 18.24 ,0.0001
No. leaves examined 0.004 0.0001 62.61 ,0.0001
Shannon-Wiener Genetic diversity 0.07 0.19 0.37 0.71
Connectivity 0.010 0.002 5.50 ,0.0001
Year 0.16 0.07 2.46 0.02
No. leaves examined 0.001 0.0003 4.18 ,0.0001
B) Neighborhood level (n ¼ 11)
Richness Genetic diversity 0.31 0.44 0.69 0.49
Connectivity 0.02 0.01 2.80 0.01
Year 0.02 0.13 0.16 0.88
Abundance Genetic diversity 1.37 0.13 10.60 ,0.0001
Connectivity 0.04 0.002 26.19 ,0.0001
Year 0.30 0.03 9.40 ,0.0001
 Z-values for richness and abundance modeled with Poisson regression; t-value for Shannon diversity modeled with
standard linear regression.
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POHJANMIES ET AL.
clear effect on all of the descriptors of herbivore
community structure at both the individual and
the neighborhood level, with higher connectivity
increasing the richness, diversity, and abundance
of herbivores (Table 1). For individual herbivore
species, tree connectivity and the species 3
connectivity interaction also had significant
effects (F1,97 ¼ 7.67, P ¼ 0.01, and F18,3644 ¼ 4.46,
P , 0.0001, respectively), suggesting that host
connectivity increases herbivore abundance in
general, but that different species respond to it in
different ways.
On average, genetically more similar trees and
tree neighborhoods shared more similar herbi-
vore communities (Table 2). However, a similar
effect was observed for geographic distance: on
average, the closer to each other two trees/
neighborhoods were located, the more similar
were their herbivore communities. This correla-
tion was stronger than that between genetic and
community similarity in all but one case, the
neighborhood level in 2006 (Table 2). Reflecting
this, the association of genetic and community
similarity weakened to non-significant when all
three distance matrices were compared, i.e.,
when the geographic distances were accounted
for (Table 2).
DISCUSSION
We detected a clear imprint of host genotype
on the associated herbivore communities both at
the scale of individual trees and of tree neigh-
borhoods: herbivore abundance was strongly
affected by both host genetic diversity and spatial
connectivity. Importantly, the effects of host
genotype emerged despite our finding that
Finnish oak populations sustain less genetic
variation than do oak populations at the core of
their range, and despite the suggestion that the
relative importance of genetic versus environ-
mental effects may be smaller within genetically
impoverished populations (Bangert et al. 2006b).
However, in contrast to the majority of previous
studies (Crutsinger et al. 2006, Johnson et al.
2006, Hersch-Green et al. 2011), we detected no
impact of host genetic diversity on the diversity
of the herbivore community.
While comparisons of allelic richness between
samples of different size are admittedly prob-
lematic (Leberg 2002), our data offer convincing
evidence that the genetic diversity of Finnish
populations is lower than that characterizing
more central stands. In itself, the loss of genetic
diversity from populations at the range margin is
consistent with theory (Eckert et al. 2008) and
matches with patterns observed in terms of
cpDNA variation within Q. robur populations
(Petit et al. 2002). While maternally inherited
cpDNA may show somewhat different levels of
variation than nuclear microsatellites (Petit et al.
2005), the levels of nuclear microsatellite diver-
sity detected in the present study suggest that the
historical colonization processes have affected
both (Ferris et al. 1998). In terms of average allelic
richness, genetic variation within the Q. robur
population on Wattkast was also the lowest
measured among the three Finnish populations.
Despite all these considerations, the genetic
variation present within the population is appar-
ently sufficient to cause detectable effects on
herbivore load.
In terms of the imprints of host genetic
diversity detected, our results were strikingly
Table 2. Correlations between herbivore community dissimilarity, geographic distance, and genetic distance
among (A) individual trees and (B) tree neighborhoods measured using Mantel and partial Mantel tests.
Community dissimilarity vs
2006 2007
r P r P
A) Individual level
Genetic distance 0.10 0.02 0.14 0.002
Geographic distance 0.36 0.0001 0.39 0.0001
Genetic distancejGeographic distance 0.03 0.28 0.05 0.15
B) Neighborhood level
Genetic distance 0.68 0.003 0.45 0.01
Geographic distance 0.68 0.01 0.60 0.002
Genetic distancejGeographic distance 0.34 0.03 0.001 0.50
Note: Shown are the Mantel statistics computed (r) and their significance (P) based on 10,000 permutations of the community
dissimilarity matrix.
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POHJANMIES ET AL.
consistent over species and spatial scales: the
heterozygosity of individual hosts as well as the
allelic richness within tree neighborhoods had a
statistically significant, positive effect on both the
overall abundance and the species-specific abun-
dances of herbivores. These results agree with
previous findings that host genotypic diversity at
the patch or population level increases arthropod
abundance (Crutsinger et al. 2006, Johnson et al.
2006), and that the heterozygosity and genotype
of individual hosts may affect herbivore abun-
dance to significant degrees (Evans et al. 2012,
Kalske et al. 2014). Effects of heterozygosity have
previously been reported e.g., by Tovar-Sa´nchez
et al. (2013), who found total arthropod biomass
to decrease with increasing heterozygosity of the
host and interpreted this as a potential sign of
weaker defense in more homozygous hosts. Our
results, conversely, suggest that higher heterozy-
gosity may lead to higher susceptibility. We
hypothesize that the galler and leaf-miner species
in our study may respond positively to hetero-
zygosity, which, assuming that heterozygosity is
associated with vigor, matches with numerous
observations testing the plant vigor hypothesis
(Price 1991, Cornelissen et al. 2008).
While the imprints of host genetic diversity on
herbivore abundance were clear, herbivore species
richness and diversity were not detectably affect-
ed by host genetic diversity at either spatial scale.
This finding contrasts with several previous
reports of herbivore diversity increasing with
population-level genotypic diversity. Such pat-
terns have been explained by additive and non-
additive effects (Crutsinger et al. 2006, Johnson et
al. 2006, Tack and Roslin 2011), i.e., by sampling
among host genotypes with specific communities
(additive effects) and synergistic effects realized in
mixtures of multiple host genotypes (non-additive
effects). It seems likely that genetic diversity
within an individual host does not produce the
structures, functions, or interactions that create
either additive or non-additive effects. Nonethe-
less, we also failed to find any effects of host
genetic diversity on species richness at the
neighborhood level—but then again, we mea-
sured neighborhood-level genetic diversity by
average allelic richness, which is not strictly
analogous to genotypic diversity (Hughes et al.
2008). Thus, the current results do not suffice as
evidence for a lack of host genetic diversity
reflecting into a lack of effect on herbivore
diversity. Instead, they suffice to demonstrate that
what limited genetic diversity there is, reflects into
detectable variation in herbivore abundance.
The general association of host genetic related-
ness and community similarity, coined the ‘genetic
similarity rule’, has been observed, for instance, in
cottonwood-based systems (Bangert et al. 2006a,
b) and in tropical epiphytic bromeliads (Zytynska
et al. 2012). In the present study, we found genetic
similarity among hosts to increase the similarity of
their herbivore communities, but also geographic
proximity among hosts increased community
similarity. With the effects of geographic distances
accounted for, the association of community
similarity and genetic relatedness generally dwin-
dled into statistical non-significance. This result
suggests that in our study system, imprints of
spatial context may override those of host
genotype. Such an interpretation is further sup-
ported by previous findings from oak-based food
webs in Finland, where effects of host genotype
have been proposed to be secondary to those of
landscape configuration (Tack et al. 2010). Nota-
bly, the absence of a genetic similarity pattern
cannot be directly ascribed to the lower genetic
diversity of Quercus robur at the range margin:
Gossner et al. (2015) recently found that within
the core area of the oak distribution (Bavaria,
southern Germany) the degree of genetic similar-
ity among mature oaks was not reflected in the
associated insect communities.
Importantly, however, the effects of the spatial
context failed to smudge all imprints of host
genotype. While host connectivity proved a
significant factor influencing herbivore abun-
dance, richness, and diversity, the effects of host
genotype on herbivore abundance remained
statistically significant even when connectivity
was included in the models. Moreover, while
our results suggest that different herbivore species
respond to host connectivity in different ways (as
likely due to differences in dispersal ability),
responses to host heterozygosity proved consis-
tent across all species. Thus, both landscape
context and host genotype seem to come with
independent effects, and both may be important
in determining herbivore community structure.
These results agree closely with previous obser-
vations that individual variation in host plant
quality is strong enough to cause local adaptation
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POHJANMIES ET AL.
by herbivore populations to their hosts, but that
the strength of such adaptation will vary with the
landscape context (Tack and Roslin 2010).
While our paper primarily focuses on the
impact of genetic diversity and spatial connec-
tivity on herbivore community patterns, we note
that the results can also be interpreted from the
perspective of tree fitness. As low-level herbivory
may strongly affect the fitness of oak trees
(Crawley 1985), and as our results reveal a
strong link between genetic diversity, habitat
configuration, and herbivore load, they suggest
that tree fitness will vary predictably across the
landscape. Exploring the impact of genetic
diversity and the position of a tree within the
landscape on tree fitness and the evolution of tree
resistance will thus offer an interesting avenue
for future research.
In conclusion, our study reveals how imprints of
host genotype and spatial context may be found
even in genetically impoverished host populations
at the margins of their range. By doing so, it points
to intriguing interactions between host diversity
and herbivore communities across landscapes in
any part of the host’s range.
ACKNOWLEDGMENTS
The study was supported by the Academy of
Finland (grant number 138346 to T. Roslin). We thank
all the fieldworkers and all the landowners for their
kind permission to sample oaks and insects.
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SUPPLEMENTAL MATERIAL
APPENDIX A
Details on the genotyping procedures
The Inkoo oak population consists of approx-
imately 100 trees, the Salo population of approx-
imately 300 trees, and the Wattkast population of
approximately 1900 trees. In Inkoo, all trees with
diameter at breast height .5 cm (n¼ 96), and in
Salo, 35 randomly selected trees were sampled
for genotyping in the autumn of 2004. In
Wattkast, 194 randomly selected trees were
sampled in September 2011.
Sampled individuals were genotyped using 15
nuclear microsatellite loci (Dow et al. 1995,
Steinkellner et al. 1997, Kampfer et al. 1998).
DNA was extracted from leaf samples using the
E.Z.N.A. extraction protocol (E.Z.N.A. SP Plant
Mini Kit D5511-02; Omega Bio-Tek, Norcross,
GA, USA). Twelve pairs of primers were used to
amplify the microsatellite loci in the extracted
DNA. The PCR amplification reactions were
performed in a volume of 13 lL. Four different
multiplex reaction mixtures were made using
QIAGEN Multiplex PCR Kit. Forward primers
were fluorescently labeled with Beckman’s
WeI1RED (Beckman Coulter, Brea, CA, USA).
Genotyping was carried out using a CEQ 8000
genetic analyzer (Beckman Coulter); 1 lL of
amplified DNA and 24 lL of DNA Size Standard
were loaded into 96-well plates. Allele visualiza-
tion and scoring were performed using the
fragment analysis module CEQ 8000 software
(Beckman Coulter).
APPENDIX B
Additional details of the study design
The presence and abundance of more than
twenty insect herbivore species were surveyed on
the oaks of Wattkast in 2006 and 2007. Data was
collected on a comprehensive sample of trees
selected by stratified random sampling, as aimed
at covering trees across the island and represent-
ing different spatial settings (see, e.g., Gripenberg
and Roslin 2005, Gripenberg and Roslin 2008,
Tack et al. 2010).
For each tree, galling and leaf-mining herbi-
vores were surveyed by randomly selecting
twenty primary shoots from each tree, then
recording the number of leaves in the shoot and
the presence and abundance of each herbivore
species. 20 of the species surveyed were observed
within the sample of 100 trees used in the present
study (Table B1). Given large variation in
abundance between generations of gallers, the
sexual and asexual generations of Andricus
pseudoinflator and Neuroterus quercusbaccarum
were recorded and treated as separate taxa.
Table B1. List of insect herbivore species encountered in a sample of 100 oak trees on Wattkast in 2006 and 2007,
and their feeding mode (from Tack et al. 2010).
Order Family Genus and species Feeding mode
Diptera Cecidomyiidae Macrodiplosis dryobia Galler
Homoptera Triozidae Trioza remota Galler
Hymenoptera Cynipidae Andricus callidoma Galler
Hymenoptera Cynipidae Andricus curvator Galler
Hymenoptera Cynipidae Andricus pseudoinflator Galler
Hymenoptera Cynipidae Cynips longiventris Galler
Hymenoptera Cynipidae Neuroterus numismalis Galler
Hymenoptera Cynipidae Neuroterus quercusbaccarum Galler
Hymenoptera Tenthredinidae Profenusa pygmaea Miner
Lepidoptera Coleophoridae Coleophora spp. Miner
Lepidoptera Eriocraniidae Dyseriocrania subpurpurella Miner
Lepidoptera Gracillariidae Caloptilia alchimiella Miner
Lepidoptera Gracillariidae Phyllonorycter spp. Miner
Lepidoptera Nepticulidae Ectoedemia albifasciella Miner
Lepidoptera Nepticulidae Stigmella spp. Miner
Lepidoptera Tischeriidae Tischeria dodonea Miner
Lepidoptera Tischeriidae Tischeria ekebladella Miner
Lepidoptera Bucculatricidae Bucculatrix ulmella Other
Lepidoptera Heliozelidae Heliozela sericiella Other
Lepidoptera Tortricidae Ancylis mitterbacheriana Other
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POHJANMIES ET AL.
APPENDIX C
Table C1. Level of genetic variation encountered in Finnish oak populations compared with European
populations. Numbers of alleles detected in the three oak populations studied here (top part of table) as
compared with results for the same loci detected in Quercus robur populations elsewhere in Europe (lower part
of table). Note that sample sizes (n) vary.
Location n
Locus
A36 A9 A1-5 Z104 Q13
Finland, Wattkast 194 15 13 13 22 11
Finland, Salo 35 11 12 12 24 12
Finland, Inkoo 96 9 11 11 14 5
Mean 11.67 12 12 20 9.33
France (Streiff et al. 1998) 183 19 14 17 29 12
Germany (Degen et al. 1999) 228 17 14 15 33 . . .
85 13 11 12 28 . . .
UK (Cottrell et al. 2003) 388 20 12 20 31 12
58 28 16 20 25 15
Romania (Curtu et al. 2007) 65 16 13 14 30 8
Turkey (Yu¨cedag˘ and Gailing 2013) 50 15 . . . 12 17 . . .
50 15 . . . 12 17 . . .
Mean 17.88 13.33 15.25 26.25 11.75
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