“Population variation, environmental gradients, and the evolutionary ecology of plant defense against herbivory”

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Philip G. Hahn, Anurag A. Agrawal, Kira I. Sussman, and John L. Maron (Jan 2019)

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Climate and herbivory drive positive growth-defense correlations in plants

Rethinking the growth-defense paradigm using milkweed populations

Tetraopes femoratus on showy milkweed (Asclepias speciosa) near Missoula, MT.
(Credit: P. G. Hahn)

Plants have evolved many defensive traits to protect themselves from the damaging effects of herbivory, such as chemical toxins, thorns, or sticky latex. Energy spent on producing defenses presumably comes at a cost to other functions like growth, resulting in a growth-defense tradeoff. These tradeoffs are best documented at the species level, where species either grow fast but have low defenses, or grow slow but produce high levels of defense. The growth-defense paradigm, however, has received less support when comparing different populations of a single species that grow in different resource environments.

Monarch caterpillar on showy milkweed plant.
(Credit: P. G. Hahn)

Researchers at the University of Montana and Cornell University teamed up to test the hypothesis that populations of showy milkweed (Asclepias speciosa) can have higher levels of both growth and defense when they grow in more favorable climates, compared to less favorable sites. To test this hypothesis, the researchers visited milkweed populations across a strong climate gradient in the western USA and measured their growth and defensive traits. Herbivorous beetles increased in abundance and were more damaging to the plants at more favorable locations. Plants were also larger and better defended in the more favorable climates. These results show that climate and herbivores can cause growth and defense to be positively correlated across populations, in opposition to the growth-defense tradeoffs that are commonly found when comparing different species. This study highlights the different patterns of growth and defense that can occur among species vs. among populations of a single species.


Milkweed population in central Washington, USA.
(Credit: P. G. Hahn)

Abstract

A central tenet of plant defense theory is that adaptation to the abiotic environment sets the template for defense strategies, imposing a tradeoff between plant growth and defense. Yet, this tradeoff, commonly found among species occupying divergent resource environments, may not occur across populations of single species. We hypothesized that more favorable climates and higher levels of herbivory would lead to increases in growth and defense across plant populations. We evaluated whether plant growth and defense traits co-varied across 18 populations of showy milkweed (Asclepias speciosa) inhabiting an east-west climate gradient, spanning 25° of longitude. A suite of traits impacting defense (e.g., latex, cardenolides), growth (e.g., size), or both (e.g., SLA, trichomes) were measured in natural populations and in a common garden, allowing us to evaluate plastic and genetically based variation in these traits. In natural populations, herbivore pressure increased towards warmer sites with longer growing seasons. Growth and defense traits showed strong clinal patterns and were positively correlated. In a common garden, clines with climatic origin were only recapitulated for defense traits. Correlations between growth and defense traits were also weaker and more negative in the common garden compared to the natural populations. Thus, our data suggest that climatically favorable sites likely facilitate the evolution of greater defense at minimal costs to growth, likely because of increased resource acquisition.