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Ontogeny of plant defense and seedling defense

Ontogeny has emerged as a central factor underlying variation in plant defense and herbivory. Plant resistance traits, including secondary chemicals, spines, thorns and trichomes, can increase and decrease by orders of magnitude as plants develop from the seedling to juvenile to mature ontogenetic stages, and these developmental trajectories in resistance usually coincide with dramatic shifts in levels of herbivory, together driving dynamic temporal interactions between plants and their herbivore communities. Tolerance can also vary across ontogeny.

Since 2000, ontogeny has been a central focus of our research, including greenhouse, field, and meta-analytical studies. Some of our key findings include a general model for how resistance and tolerance vary across ontogeny, that ontogenetic trajectories are genetically variable within species, and that suites of defense traits vary independently across ontogeny.

As part of this research, it has become clear that very little is known about seedling defense, precluding us from understanding how defense varies across all of plant ontogeny. Thus, we have explicitly focused on the seedling stage in order to understand how seedlings deal with herbivory, and how seedling defense differs from that of older plants. The ultimate goals are to determine whether the evolution of plant defense is driven by selection imposed on seedlings, and whether defense is constrained by other priorities during this important life history stage.

Furthermore, because seedlings are simultaneously stressed by abiotic factors, such as water availability, we are looking at how drought interacts with herbivory to affect seedling performance. We are currently testing seedling tolerance of herbivory and drought in a suite of native Hawaiian plants, with particular focus on dry forest trees because dry forests are projected to experience the most severe droughts under climate change.

Collaborators: Julia Koricheva, Karina Boege, Deane Bowers, Carolina Quintero, Aaron Shiels, Mick Hanley, Curt Daehler, Matt Lurie

Funding: UK NERC, NSF, USDA APHIS

Publications:

PDF Hoan, R. P., R. A. Ormond, and K. E. Barton. 2014. Prickly poppies can get pricklier: ontogenetic patterns in the induction of physical defense traits. PLoS ONE 9(5): e96796.
PDF Barton, K. E.. 2014. Prickles, latex and tolerance in the endemic Hawaiian prickly poppy (Argemone glauca): variation between populations, across ontogeny and in response to abiotic factors. Oecologia 174: 1273-1281.
PDF Barton, K. E. and W. P. Haines. 2013. Koa looper caterpillars (Scotorythra paludicola, Geometridae) have lower fitness on koa (Acacia koa, Fabaceae) true leaves than on phyllodes. Proceedings of the Hawaiian Entomological Society 45: 141-147.
PDF Quintero, C., K. E. Barton, and K. Boege. 2013. The ontogeny of plant indirect defenses. Perspectives in Plant Ecology, Evolution and Systematics 15: 245-254.
PDF Barton, K. E.. 2013. Ontogenetic patterns in the mechanisms of tolerance to herbivory in Plantago. Annals of Botany 112: 711-720.
PDF Barton, K. E. and M. E. Hanley. 2013. Seedling-herbivore interactions: insights into plant defence and regeneration patterns. Annals of Botany 112: 643-650.
PDF Barton, K. E. and M. E. Hanley, editors. 2013. Special Issue: Seedling Herbivory. Annals of Botany 112: 643-765.
Koricheva, J. and K. E. Barton. 2012. Temporal changes in plant secondary metabolite production: patterns, causes and consequences. In The Ecology of Plant Secondary Metabolites: From Genes to Global Processes. Eds. G. R. Iason, M. Dicke and S. E. Hartley. Cambridge University Press.
PDF Boege, K., K. E. Barton, and R. Dirzo. 2011. Influence of tree ontogeny on plant-herbivore interactions. In Size and Age-Related Changes in Tree Structure and Function, Tree Physiology 4. Eds. F. C. Meinzer, B. Lachenbruch and T. E. Dawson. Springer, New York.
PDF Barton, K. E. and J. Koricheva. 2010. The ontogeny of plant defense and herbivory: characterizing general patterns using meta-analysis. American Naturalist 175(4): 481-493.
PDF Barton, K. E. 2008. Phenotypic plasticity in seedling defense strategies: compensatory growth and chemical induction. Oikos 117: 917-925.
PDF Barton, K. E. 2007. Early ontogenetic patterns in chemical defense in Plantago (Plantaginaceae): Genetic variation and trade-offs. American Journal of Botany 94: 56-66.

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Island plant defense

One of the paradigms of island biology is that island plants are defenseless against herbivores. This idea is mainly supported by anecdotal evidence that island plants are often decimated by introduced mammalian browsers and grazers. Yet, the few experimental or trait-based tests of this idea provide scant evidence to support it. Furthermore, many island plants express putative defense traits and have long-lasting interactions with native insect, bird, and reptilian herbivores. Since 2010, I have started new research to investigate defense in endemic island plants in order to gain new insights into this paradigm and more generally, into how defense evolves isolated islands.

This research involves four main projects:

  1. Examination of the functional ecology of the endemic Hawaiian prickly poppy pua kala (Argemone glauca), which expresses several putative defense traits, including prickles, latex, and epicuticular waxes (see The Poppy Project for more on this).
  2. Examinations of the koa (Acacia koa) - koa moth (Scotorythra paludicola) interaction.
  3. Investigations into leaf defense traits associated with deterrence of native psyllid galls in ʻōhiʻa lehua (Metrosideros polymorpha).
  4. Tests of seedling tolerance across a range of common/rare, herbaceous/woody native Hawaiian plant species. We are currently testing how drought affects seedling tolerance of herbivory.

Collaborators: Karina Boege, Ryan Hoan, William Haines, Aaron Shiels

Funding: NSF, USDA APHIS

Publications:

PDF Hoan, R. P., R. A. Ormond, and K. E. Barton. 2014. Prickly poppies can get pricklier: ontogenetic patterns in the induction of physical defense traits. PLoS ONE 9(5): e96796.
PDF Barton, K. E.. 2014. Prickles, latex and tolerance in the endemic Hawaiian prickly poppy (Argemone glauca): variation between populations, across ontogeny and in response to abiotic factors. Oecologia 174: 1273-1281.
PDF Barton, K. E. and W. P. Haines. 2013. Koa looper caterpillars (Scotorythra paludicola, Geometridae) have lower fitness on koa (Acacia koa, Fabaceae) true leaves than on phyllodes. Proceedings of the Hawaiian Entomological Society 45: 141-147.
PDF Haines, W. P., K. E. Barton, and P. Conant. 2013. Defoliation of the invasive tree Falcataria moluccana on Hawaii Island by the native koa looper moth (Geometridae: Scotorythra paludicola), and evaluation of five fabaceous trees as larval hostplants. Proceedings of the Hawaiian Entomological Society 45: 129-139.

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The poppy project

The Poppy Project examines defense in an apparently well-defended (but understudied) group of plants, the prickly poppies (genus Argemone). There are approximately 32 Argemone species, all of which are native to the Americas (center of diversity in Mexico) with a single endemic species in Hawaiʻi. Since 2010, we have been characterizing the defenses of the Hawaiian prickle poppy, pua kala (Argemone glauca). We have found that prickles, latex, and tolerance are highly variable across populations on different islands, in response to herbivory, and as plasticity with respect to light and water availability. All of these traits also vary across plant ontogeny. Our current work compares pua kala defense with that of its sister species, the Mexican poppy (Argemone mexicana), in an effort to test the island plant defense hypothesis, which predicts that island plants have weak defenses due to low herbivore selection pressure on islands.

In collaboration with Karina Boege (UNAM), we are using prickly poppies as a model system for testing whether defense traits show synchronous ontogenetic trajectories and whether the multi-functionality of defense traits can inform us about their ontogenetic patterns.

Collaborators: Karina Boege, Ryan Hoan

Funding: NSF

Publications:

PDF Hoan, R. P., R. A. Ormond, and K. E. Barton. 2014. Prickly poppies can get pricklier: ontogenetic patterns in the induction of physical defense traits. PLoS ONE 9(5): e96796.
PDF Barton, K. E.. 2014. Prickles, latex and tolerance in the endemic Hawaiian prickly poppy (Argemone glauca): variation between populations, across ontogeny and in response to abiotic factors. Oecologia 174: 1273-1281.

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Recruitment and population dynamics of a tropical foundation species

As anthropogenic alterations threaten natural ecosystems, much of the focus of conservation biology is on minimizing effects for the rarest species. Yet, the majority of individual plants in natural ecosystems come from a few common species. While common species might be at lower risk of extinction, they engineer environments, are involved in many biotic interactions, and contribute disproportionately to ecosystem function. Thus, declines in their abundance and distribution have broad ramifications for other species and for ecosystem functioning.

On each of the six largest Hawaiian Islands, the dominant tree species is ʻōhiʻa lehua (Metrosideros polymorpha), which has experienced a dramatic shift in its abundance over the past several decades. At present, the few studies that have been conducted on the population ecology, growth, and decline of ʻōhiʻa have been limited to a few locations and life stages. We are investigating the demography of ʻōhiʻa across the six largest islands to assess population dynamics and to identify factors associated with the widespread decline. Our preliminary results indicate that seedling recruitment is very low across most of the islands, and this points to a decline in populations over time. Current experimental work is testing whether drought and invasive species competition lead to lower seedling recruitment in ʻōhiʻa lehua.

Collaborators: Tiffany Knight

Funding: National Geographic Society (NGS), Pacific Islands Climate Science Center (PICSC)

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© , Kasey E. Barton <>, Department of Botany, University of Hawaiʻi at Mānoa

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