1) The study tested whether plant traits like height, trichome density, chemical composition, leaf area, specific leaf area, and dry matter content can predict levels of invertebrate leaf herbivory using 23 plant species. 2) Preliminary results from a pilot study with 8 species found a negative correlation between specific leaf area and herbivory, but this correlation was not significant when tested on the 23 species. 3) The final model found that only the presence of secondary defense compounds in leaves was a useful predictor of herbivory levels, which aligns with expectations about plant chemical defenses and herbivore interactions.

1. Background
Methods
Results
Discussion
Leaf herbivory is best predicted by plant chemistry
Specific Leaf Area (cm^2/g)
LeafHerbivory(%)
The interaction between herbivores and plants is influential in structuring
and regulating global ecosystems. Theoretical data predicts, however, that
primary and secondary producers will not react in the same way to
increasing temperature (1). Models which provide an understanding of the
herbivore-plant relationship will therefore be essential in dealing with the
impacts of global climate change.
A prominent hypothesis in plant ecology posits that key plant traits can be
used to model herbivory (2). There are debates, however, over whether
traits can substitute species identification without masking important
information from models. Our study was designed to contribute to the body
of literature assessing trait-based models of invertebrate leaf herbivory.
This study tested whether plant height, trichome density, chemical
composition, leaf area, specific leaf area, and leaf matter dry content, traits
that appears to structure plant-environment relationships within regions
and across bioregions, are able to predict the impact of herbivores on plant
performance. In this study, we asked:
1) Are plant height, trichome density, chemical composition, leaf
area, specific leaf area and leaf matter dry content useful
predictors of invertebrate leaf herbivory?
2) Does invertebrate leaf herbivory predict the overall impact of
herbivores on plant biomass?
1.A
1.B
2.A
2.B
Figure 1: a) Mean percent leaf herbivory is
shown for all species. Species abbreviations
are the first letters of the genus and species
1.b) Categories of trichome density follow the
procedure described in (3). S referrers to
species with secondary defense compounds.
2.a) Correlation between species mean
specific leaf area and mean percent herbivory
in the pilot study done on 8 species using the
same methodology in 2013
(r= -0.85, p=0.008). Each point is a species.
2.b) Mean specific leaf area, calculated across
treatments.
We tested these questions on 8 species found in tall grass prairie or old
fields in a pilot study in May-Sept. 2013 and expanded the study to 23
species in May-Sept. 2014.
Plants were grown in field plots protected from aerial invertebrate
herbivores or exposed to invertebrate herbivores, using mosquito nets that
were intact (Fig. 1A) or mosquito nets with holes cut into them. Plants were
grown for 12 weeks at the Koffler Scientific Reserve (10 replicates with all
species included in each replicate). In the second month of growth, plants
were surveyed weekly for height, number of leaves and leaf herbivory. 2
leaves per plant were sampled on the 7th week of experiment to obtain
measurements for leaf traits.
We then used multiple regression models to determine the relationship
between leaf herbivory and the measured traits.
Pilot study (Box 2)
Specific leaf area is negatively correlated with percent leaf herbivory
(r=-0.85, p=.008, Fig. 2 A).
Specific leaf area was not related to the overall effect of herbivore
exclusion on biomass
D. canadense was the only species to exhibit a plastic response of SLA
to herbivory; SLA was higher in the herbivory treatment (p=0.048).
Current study (Box 1)
Multiple regression analysis determined that the best herbivory model would
contain only data relating to chemical composition of leaves (p=.004).
-Project supervisor Dr. Benjamin Gilbert.
-Funding provided by NSERC USRA, CGCS Internship
-Koffler Scientific Reserve and U of T Earth Science Greenhouse Staff
Acknowledgments
Although specific leaf area and invertebrate leaf herbivory were correlated in
the pilot study, preliminary data suggests that the correlation was found to
be non-significant upon analysis with a sample of 23 species.
Similarly, the correlation between leaf herbivory and plant height, leaf dry
matter content, and leaf area is also non-significant.
The model with the strongest predictive power considered only the presence
of secondary defense compounds. This is an expected result that implies
the experimental community is behaving as expected.
We note that our study was done on young plants, and that ontological shifts
in leaf traits and plant-herbivore interactions may be common (4).
Future work will consider ontological changes, leaf carbon and nitrogen
concentration, as well as herbivores that feed on non-leaf tissues.
A
B
C
Image 1: A) Experimental enclosure (herbivores excluded). B) Ants on Asclepias tuberosa. C)
Broad-nose weevil on Desmodium canadense.
Species list
Current study: Amorpha canescens Andropogon gerardii Aster laevis Aster oolentangiensis Asclepias syriaca
Asclepias speciosa Ceanothus americanus Cirsium discolor Desmodium canadense Desmodium illinoense
Eupatorium altissimum Euphorbia corollata Lupinus perennis Monarda punctata Nicotiana rustica Parthenium
integrifolium Psoralea onobrychis Psoralea tenuiflora Pycnanthemum pilosum Silphium terebinthinaceum
Satureja vulgaris Veronica catenata
Pilot study: Desmodium illononse, Desmodium canadense, Monarda fistulosa, Monarda puncata, Asclepias
tuberosa, Asclepias syriaca, Liatris aspera, Coreopsis lanceolata,
Literature Cited:
1. O’Conner, M., Gilber, B., Brown, J. B. 2011. The American Naturalist 178: 627-37.
2. McGill, B., Enquist, B., Weiher, E & Westob, M. 2006. Trends in ecology & evolution 21: 178-85.
3. Carmon, D., Lajeunesse, M.J. & Johnson, M.T. 2011. Functional Ecology 25: 358-367.
4. Boege, K. & Marquis R. J. 2005. Trends in ecology & evolution 20(8): 441-448