A Real Cliffhanger


Cliff faces are some of the most interesting habitat types on the planet. Few places in the world are as inhospitable. They are low in nutrient levels, they have limited space for root growth, and offer very little for recruitment. Cliffs do offer some benefits though. They are often sheltered from extremes in climate and can be inaccessible to large herbivores. With that in mind, it is understandable how they can be a haven for some very unique and equally extreme life forms.

One such life form that comes to mind is Borderea chouardii. This strange plant grows only on a couple cliff faces in the Pyrenees mountain range in Spain. It is critically endangered as it represents a relict population of a once tropical Tertiary environment. What makes it more interesting is the double mutualism it has formed with ants. As we have touched on a few times in the past, ants are often recruited as seed dispersers. Borderea chouardii does just that. In many of the observed cases of seed dispersal, researchers found that ants were the culprit. Interestingly enough, a majority of the remaining cases were due to the plant literally planting its own seeds. Known as "skototropism," the stems of the seed cases grow into dark crevices, which are perfect spots for seed to germinate and grow. Surprisingly, gravity plays a very small role in the reproduction of this species.

Let me back up for a bit here. I did mention this plant has a double mutualism with ant species after all. Based on years of observation, researchers found that ants actually served as the most efficient pollinator for Borderea chouardii. This is not a common thing. Generally speaking, ants do not make for effective pollinators. Most species have glands that secrete substances that destroy pollen. However, in a mountainous cliff setting, winged insects are relatively rare, so Borderea chouardii and ants have evolved together into this oddball double mutualism. To add an extra layer of complexity to the system, dare I mention that it isn't just one ant species that Borderea chouardii relies on, but rather 3. Two ant species serve as the pollinators while a a third ant species serves as a seed disperser. This is one risky plant species. The plant gets around the rarity of successful recruitment by living a long time. Individual plants can live upwards of 300 years, which is quite possibly the record for a non-clonal forb species.

Photo Credit: María B. García, Xavier Espadaler, Jens M. Olesen

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