Herbivores can exert strong selective pressures on plant defensive traits, the consequences of which can directly or indirectly impact plant-herbivore abundances, population dynamics, and community composition. Plant responses to herbivore selection pressures involve the evolution of resistance and/or tolerance traits. Few studies have jointly examined resistance and tolerance in plant populations and none have done so in nature. As a result, the following questions remain:

Are resistance and tolerance mutually exclusive defensive strategies? Are plants best defended when they are both resistant and tolerant? How does the evolution of resistance affect the evolution of tolerance? Also unknown are the spatial distribution of resistance and tolerance traits, how the costs, benefits and selection gradients for these traits vary environmentally, and whether particular regions of the plant genome are associated with spatial variation in resistance or tolerance.
		Trirhabda larvae

Participants in this project include my graduate student Alyssa Hakes and Dr. Michael Hellberg. We have only recently begun our research on this subject, therefore, I can only describe our general research approach. Our work is being conducted in a coastal salt-marsh landscape and focuses on the dominant plant, Spartina alterniflora and its associated herbivores and pathogens.

Alyssa Hakes		Cordgrass Marsh at Cameron Parish, LA

In the first phase of the project, we are conducting an extensive survey and experiment to estimate S. alterniflora resistance to and tolerance of herbivory by a suite of herbivores. Cordgrass clonal diversity, traits putatively associated with resistance (leaf toughness and phenolic levels), costs of resistance and tolerance, and a number of site-specific environmental variables are being measured at multiple sampling stations within the marsh. Recent advances in spatial statistics and landscape genetics will be employed to discover patterns in the spatial distribution of plant defenses, and whether genetic similarity, defensive traits, and environmental variables are spatially correlated within them. The mechanistic and genetic bases for these patterns will be explored experimentally in the second phase of this study (due to begin in the spring of 2006).

Planthoppers	Planthopper Damage	Ischnodemus Bugs

Specifically, experiments will be conducted to 1) determine the broad-sense heritability of plant defensive traits, 2) evaluate the cause and effect relationships between environmental variables and defensive traits, and 3) test the hypotheses that environmental stress levels are negatively correlated with defense levels, and defenses have similar effects on different types of herbivores. The final phase of the project will involve large-scale field experiments using different resistant and tolerant S. alterniflora clones to test theories regarding the evolution of within-habitat variability in plant defensive traits. This will be accomplished through a partnership with the Department of Natural Resources and National Resources Conservation Service which are conducting salt marsh restoration projects in coastal Louisiana.

In addition to providing the first landscape-level study of the causes for phenotypic and genotypic variation in resistance and tolerance in plants, this project should make significant contributions to the fields of restoration and conservation biology, and the management of plant pests in forestry and agriculture. Owing to sediment depletion, global warming and other anthropogenic causes, Louisiana coastal marsh habitat is disappearing at an alarming rate. Numerous restoration projects using S. alterniflora are underway, but to date projects have not considered the defensive abilities or genetic diversity of plant stocks. Our studies will provide a large-scale test of whether plant restoration or conservation projects can benefit from managing for diversity in defensive traits as well as in clonal variability.

Addendum:

This research project suffered a serious setback on September 24, 2005 when hurricane Rita passed directly over our field site in Cameron Parish. The devastation to the City of Cameron was complete. The status of our study plots is unknown.

NOAA satellite image, Sept 24, 2005	The town of Cameron, 1 mile from field site