Thorny Diets, Part I: Neneo & Guanacos in Patagonia

By Bruce G. Marcot, Ecology Picture of the Week:

In the wind-swept Patagonian steppe country of southern Argentina thrive a shrub of thorns and a relic of the camel family, the guanaco.  

Although densities of ungulate grazers in the steppe country has been quite low during much of the Holocene epoch (the last 12,000 years), the guanaco occurred in high numbers up through the 1970s.  This likely resulted in local grass-like (graminoid) vegetation evolving poor forage quality specifically as a defense against grazing by guanaco (Adler et al. 2005).  

And beyond just poor forage quality, this shrub -- locally called neneo and scientifically called Mulinum spinosum -- also evolved a thicket of thorns as an adaptation to thwart overbrowsing.  The thorns are actually spines formed from its deciduous leaves!

One study (Bahamonde et al. 1986) showed that neneo is the most common shrub consumed by guanacos in Patagonia, comprising 23% of their annual diet items ... but apparently the plant can sustain such grazing intensity and still thrive.  Most of the grazing on neneo by guanacos is during non-summer months when lush grasses and forbs are far less available and when neneo is not reproducing.  

However, guanacos are not the only grazers of neneo in Patagonia.  Domestic sheep and introduced red deer (Cervus elaphus) also consume neneo, although at least the plant is a more minor part of the deer's annual diet (3%) than the guanaco's diet.  

Guanacos and red deer also graze on neneo in different places.  Guanacos prefer gallery forests and shrub communities at intermediate elevations along slopes, whereas red deer use low meadows in spring and intermediate height vegetation the rest of the year.  This means that they assert different grazing pressures on neneo, and the plant might respond differently.  For example, neneo is known to invade lands overgrazed particularly by domestic livestock, mostly sheep, once grass cover has been greatly reduced.  

Further, neneo has evolved seeds that are far less palatable than are seeds of some introduced plants.  This results in fewer neneo seeds being removed and destroyed by seed-eating animals of the steppe such as some insects, birds, and small mammals.  This may be an evolutionary adaptation of neneo to granivory (seed-eating), as neneo seeds have greater levels of phenols and toxic concentrations of iron and copper, and lesser levels of dry matter digestibility, and phosphorous and nitrogen nutrients, than do seeds of some introduced plants (Folgarait and Sala 2002).  How neneo might further respond in terms of its density, distribution, and further evolution of anti-grazing adaptations remains to be seen.

One final note: neneo also has value to indigenous Mapuche people of southern Argentina and Chile, who traditionally travel over a kilometer to gather the plant to treat urinary ailments and leg weakness (Estomba et al. 2006).  There is far more to this plant than thorns! 

  --Bruce G. Marcot

Image and text © Bruce G. Marcot, Ph.D. Research Wildlife Ecologist,
who produces the Ecology Picture of the Week website.