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Research into phylogenetic patterns of plant invasions produces a fascinating side serving of science

Immersed in their work on the phylogenetic patterns of plant invasions, a couple of post-docs (Drs Serban Proches and John Wilson) at the C·I·B continued their discussion over lunch. As they ate, they thought that the same phylogenetic techniques applied to invasive species would also apply to what they were currently doing: eating. The breadth of the human diet has been emphasized before, but a broad diet (in terms of number of species) does not mean that most groups of plant are eaten. Implicit in previous work is the idea that although humans consume many plant species, food plants are clustered in a few particular families.

In a paper published in the February 2008 issue of BioScience they showed this was not the case, and used fast food to elegantly illustrate the point. A McDonald’s Big Mac with French fries and a cup of coffee is a perfect example of a "globalised" meal. From potatoes that were first domesticated in South America, to mustard that was developed in India, onions and wheat that originated in the Middle East and coffee from Ethiopia, the meal contains approximately 20 different species from 12 plant families from all eight global centres of cultivated plant diversity. This observation provoked some interest in the media around the world, including an excellent graphic rather aptly copyright to Die Burger newspaper (a Cape Town Afrikaans-language daily).

Fig 1: A "global" Big Mac, fries and coffee

As dated phylogenies become increasingly available, there has been a flurry of studies linking phylogenies to spatial distribution of species, provoking somewhat of a resurgence in community ecology. In particular, it has been suggested that alien species with close indigenous relatives in the introduced range may have reduced chances of successful establishment and invasion (Darwin’s Naturalization Hypothesis). In a recent paper in Global Ecology and Biogeography, researchers at the C·I·B argued that the phylogenetic pattern will change depending on the spatial and phylogenetic scales considered. Expectations and observations from invasion biology and the study of natural communities are that at the spatial scale relevant to competitive interactions; closely related species will be spatially separated, whereas at the regional scale, species in the same genera or families will tend to co-occur more often than by chance. These patterns in the relatedness of indigenous and naturalized plants should also depend on the continental / island setting, spatial occupancy levels, and on the group of organisms under scrutiny.

The researchers developed various techniques to look at this problem in natural systems, and are applying the methods to look at plant invasions. But as a side-project, they decided to work on lists of food plants. They found that worldwide, humans have access to a greater range of food plants than any other species. Although humans tend to eat more species in certain families and fewer in others—for example, in Rosaceae we eat over 250 out of a total of ~3,000 species, including apples, pears, plums, cherries, raspberries, while we only regularly eat ~25 of the ~30,000 orchid species, including vanilla—in general, this is not the case. The proportion of edible species in most families is similar to random expectations, and there is only a weak phylogenetic pattern in edibility. They argue that the remarkable breadth of the human diet is the result of humans’ huge geographic range, diverse food-collection methods, and the ability to process normally inedible items. Humans are thus generalist feeders in the broadest sense. Cross-cultural analyses of diversity in the plant diet of humans could represent a fascinating new field of research linking ecology, anthropology, history, and sociology.

The researchers examined more than 7,000 plant species eaten by people to determine the origins and evolutionary relationships of the various plants that comprise humankind’s menu. The study also argues that steps to protect the diversity of human food plants may have to be taken as loss of indigenous knowledge gradually leads to more uniform diets for the world’s population overall. Individually we are probably eating a greater range of plant species than our ancestors. However, the loss of regional cuisine and indigenous knowledge of, for instance, the Nama on what local Northern Cape plants are potentially sources of food, may mean that as a species our diet is becoming increasing focussed on a few plant species, and indeed a few varieties of those species.

Proches, S., Wilson, J.R.U. & Cowling, R.M. (2006) How much evolutionary history in a 10 x 10 m plot? Proceedings of the Royal Society of London Series B—Biological Sciences, 273, 1143–48.

Proches, S., Wilson, J.R.U., Richardson, D.M. & Rejmánek, M. (2008) Searching for phylogenetic pattern in biological invasions. Global Ecology and Biogeography, 17, 5–10.

Proches, S., Wilson, J.R.U., Vamosi, J.C. & Richardson, D.M. (2008) Plant diversity in the human diet: weak phylogenetic signal indicates breadth. Bioscience, 58, 151–59.