Paper on tarweeds published in journal Ecology

After about 4 years of work, my colleague Ian Pearse and I have published a paper on the evolution of stickiness (and lack thereof) in the tarweeds. The tarweeds comprise a wide range of amazing annuals in CA, including many that are critical resources for birds and pollinators. Beyond those benefits, we have shown previously that the sticky tarweed species support predatory insects by catching them food in their sticky hairs in the form of insect meat: insects stop to rest on a tarweed plant and get stuck, then their carcasses are fed upon by a suite of beneficial, predatory insects. A collaborator of mine showed elegantly that other plants - namely the serpentine columbine, go as far as to lure these insects with attractive smells, an interaction he calls their 'siren song'.

This new paper takes the sticky plant story to another level. One of the thing that makes tarweeds so interesting is how diverse the the different species are, and among the same species, how different populations can have radically different plant traits. For example, among many species of tarweed, there are populations of early-bloomers and late-bloomers. Individuals from both populations germinate at the same time, but the early-bloomers flower in the spring while the late-bloomers wait until fall. Early-bloomers are small, not sticky, and short-lived. Late-bloomers are much larger, sticky and fragrant (both as a result of resin production largely lacking in the early-bloomer populations). This occurs in virtually all the tarweed species, yet until now there had been no explanation as to why.

Our hypothesis was that the early-bloomers sacrifice size and longevity in order to avoid their herbivores in time; each species of tarweed has a specialized herbivore (a moth caterpillar) that can consume entire plants. They are active in the late summer and fall, but rare or absent in the spring. The early-bloomers lack the sticky-mediated defense that the late-bloomers have - catching insects in their sticky hairs to use as bait for predatory insects that kill the caterpillars. In this study we also showed that the early-bloomers lack the ability to regrow tissues eaten by caterpillars, while the late-bloomers are able to recover from damage effectively.

Thus, there are two alternate strategies employed by tarweed for dealing with herbivores: avoid them or confront them. Avoiding them means almost certain reproduction, but at a low rate - these smaller plants only produce a few fruits, but are rarely sterilized by caterpillars. Late-bloomers employ a riskier strategy: grow big and store energy reserves to regrow lost tissues, and employ the help of bodyguards to protect them from caterpillars by offering food to the bodyguards in the form of insect carrion. In a good year with low caterpillar abundance, these late-bloomers could thrive, but in a year of high abundances they could be wiped out.

One last piece of this story is that in all cases the early-bloomers have apparently evolved directly from the late-bloomers at a given location. Meaning: early-bloomers are more closely related to late-bloomers living nearby than to other early-bloomers nearby. Weird, right? For a long time the early-bloomers and late-bloomers were considered different subspecies (vernalis, meaning spring, for early, and densiflora, meaning densely leaved, for the late). In reality they are not subspecies (if they were the early-bloomers would breed with early-bloomers, and late with late), but symptoms of a repeating and rapid evolutionary response to caterpillars, embodied in two distinct strategies.