I think every biologist has a favorite kind of organism. For some people it is a charismatic animal, like an elephant, or a whale. For me it is silverswords. I first learned about these plants in my first year of graduate school. Someone was giving a lecture on adaptive radiations, and showed photographs of a strange-looking plant on a Hawaiian volcano. It had a silvery-gray rosette with long straight leaves, and was growing out of the bare lava (here, Argyroxiphium sandwicense ssp. macrocephalum).
Then he showed another photograph of a different species, this one more shrub-like and growing in a bog, but with something of the same spirit in it (Argyroxiphium kauense). I learned that these two species were part of a group of more than thirty, and were all descended from a single ancestor that colonized the Hawaiian islands a few million years ago. From an unremarkable California tarweed came the entire Hawaiian silversword alliance – the genera Argyroxiphium, Dubautia, and Wilkesia (Asteraceae). The man giving the lecture was Rob Robichaux, a silversword expert. That very day I decided I was going to see them myself, and study them.
That was 2009. I got funding from National Geographic’s Young Explorers program in 2010 with the help of my doctoral supervisor Brian Enquist, and went to Hawaii in 2011 for fieldwork under the guidance of Rob Robichaux (below) and Bruce Baldwin (two below), with extra help from Emma Wollman.
The silversword alliance is commonly thought of as one of the most spectacular and diverse adaptive radiations anywhere on the planet, but direct measurements of functional variation among species were somewhat limited. I was intrigued by the apparent diversity of their leaves’ venation networks, as passingly described by Sherwin Carlquist in 1959. I focused the project on understanding how functionally diverse this set of thirty-odd species really was, given that they occupy all the major habitats and islands of the Hawaiian archipelago.
Five years later, the work is all done and the paper is finally out. You can read it, “Variation and macroevolution in leaf functional traits in the Hawaiian silversword alliance (Asteraceae)”, in the Journal of Ecology (link, PDF).
The short message from the paper is that the clade really is a world-class example of adaptive radiation. The variation in traits among the clade is closely matched to global ranges of trait variation across all species – and this variation seems to evolve very quickly with few evolutionary constraints. These findings provide a quantitative perspective on a charismatic group of plants, and hopefully support arguments for conservation of this endemic and often threatened flora.
It has been a dream to work on this group of plants alongside some of the botanists who love them best. A few species in the clade are now extinct, and several others are either threatened or endangered – for example, this Argyroxiphium sandwicense ssp. sandwicense individual high on a cliff face, protected from non-native sheep browsing – one of fewer than 40 individuals known to exist when Rob became involved with the ongoing state conservation efforts.
Or this Dubautia latifolia individual that I was only able to sample from a herbarium collection – no more than a few dozen individuals are known to be alive in the wild.
Or this Wilkesia hobdyi, in cultivation at the National Tropical Botanical Garden on Kaua`i.
It was humbling getting to work with such rare species, touching individuals of something shaped by millions of years of evolution and now so close to being gone. I had never held an USFWS endangered species recovery permit before this project, and felt very much the obligation to take good care of the plants I was studying, but also the challenges faced in conserving and restoring populations of these native species on an archipelago now run over by non-native species.
Yet among these challenges, the beauty of the habitats and forms of these species remains incredible. Here are a few examples of other species in the clade:
Dubautia platyphylla, growing on the dry slopes of Haleakalā volcano.
Dubautia menziesii, on cinder cones at the summit of Haleakalā.
Dubautia reticulata, a tall tree-like species growing at The Nature Conservancy’s Waikamoi Preserve.
Wilkesia gymnoxiphium, a spindly rosette growing near Waimea Canyon.
Dubautia linearis, a tall shrub in the high plateau between Mauna Loa, Mauna Kea and Hualālai.
I’m not sure what resonates with me so deeply about these plants. Maybe it is the tenacity and hope their continued existence represents. Maybe it is how their underlying form becomes multiplied and reflected on different island habitats. Maybe it is the feeling of touching a soft and silvery leaf. But it is a deep sort of affinity that persists across thousands of miles of ocean, and I am sure our paths will cross again soon.