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Astragalus
sinuatus
Insect
herbivory is common and has been shown to drastically
limit the reproductive success of many plant species.
Most research examining the affects of herbivory has focused
on common plants, while very little attention has been
given
to understanding how herbivory impacts the reproductive
success of rare plant species.
Astragalus sinuatus
is a Washington endangered plant species.
Its entire distribution is restricted to an 8km square
area in the shrub-steppe habitat of Eastern Washington.
Ms. Julie Combs's Master's Degree research determined that in areas of
high Bromus tectorum cover, A. sinuatus
is most likely limited by B. tectorum, an invasive
annual grass. However, in areas of low B. tectorum
cover, A. sinuatus is more likely limited by insect
seed predators. |
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Julie Combs uses an aspirator to manually remove
a specialist insect herbivore.
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Astragalus sinuatus. Aim your mouse at this photo
to see Bombus sp. pollinating Astragalus sinuatus
flowers, with Bromus tectorum, a non-native annual
grass, growing in the background.
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To
examine whether competition with B. tectorum inhibits seedling germination and survivorship of A. sinuatus, seeds were added to plots with and without B. tectorum.
Overall seedling germination did not differ between control
and removal treatments. But overall seedling and juvenile
establishment after the two-year monitoring period was
significantly higher in plots where B. tectorum was removed. These results show that insect herbivores
may strongly affect demographic processes of A. sinuatus.
Primary
seed predators were tortricid moth larvae and two species
of specialist beetles: the seed weevil, Tychius semisquamosus LeConte, and the seed beetle, Acanthoscelides fraterculus Horn. An insect reduction experiment using insecticide and
manual removal techniques suppressed insect herbivores,
resulting in 164-345% greater viable seed production. Insect
herbivory indirectly led to a seven- to eleven-fold greater
incident of fungal attack on plants exposed to herbivores.
These results show that insect herbivores directly and indirectly
decrease viable seed output of A. sinuatus. |
In her dissertation work, Ms. Combs is investigating the hypothesis that rare plants are more susceptible to insect damage compared to more widespread, co-occurring, congeneric species. Some researchers have suggested that herbivory may be one causal factor of rarity; however, there is very little empirical evidence to either support or discard this hypothesis. The few studies that are published show conflicting results. To investigate this question, Ms. Combs is using rare-common, co-occurring pairs within the genus Astragalus (Fabaceae) to investigate the degree to which herbivory may limit rare plant populations. In addition, she is also investigating how pollination affects reproductive failure (seed loss) and success (seed-set) in a rare-common sympatric, congeneric pair. She is using several field sites located in the shrub-steppe plant communities of eastern Washington and Oregon where common and rare Astragalus species co-occur. Ms. Combs will finish her dissertation in 2011.
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Field assistant Kelley Craig records flower production data |
