If you — like me — live in Canada, you might think of the prickly pear cactus as a scruffy, low-growing cluster of flat, spiny pads. In the Galapagos, though, these so-called Opuntia cacti take on wholly different forms.
 
 This is a Plains prickly pear cactus, native to Canada, usually found in prairie and grassland regions of western Canada. (Photo credit: Cephas)


Prickly pear cacti are abundant and easy to see in the Galapagos, as they are the dominant vegetation in most of the archipelago’s dry habitats. There are 14 types spread across the islands. They seem to represent six species, some with multiple variations, though their exact taxonomic arrangement is not entirely clear. Complicating matters is the fact that the shape of a given type of prickly pear cactus appears to depend more on which island it inhabits than who it is related to.

On Genovesa, for example, the prickly pears resemble their Canadian cousins, often growing in low mats of jumbled pads. On North Seymour, they grow slightly taller, with the pads seemingly stacking end-to-end to take the form of a shrub. On Pinta, they grow taller still, with a cluster of pads forming a mushroom-shaped cap atop a thick, central trunk. On Santa Cruz, the cacti grow to a towering height of 12 metres, with the pads sprouting like tree branches from a massive trunk covered in reddish bark.
 
 
Prickly pear cactus (Opuntia helleri) on Genovesa Island (Photo credit: Thomas Schoknecht)

 

 Prickly pear cactus (Opuntia echios var zacana) on North Seymour Island (Photo credit: David Berkowitz)
 
 
 
Prickly pear cactus (Opuntia galapageia subvar. barringtonensis) on Santa Fe (Photo credit: Justin Peter)
 
 
The prickly pears of the Galapagos, like Darwin’s Finches, descend from an ancestor that arrived on the archipelago from the South American mainland. As the cacti came to inhabit the various islands, they adapted to meet the unique demands of their new homes. Since each island is different, the cacti became different, too. This process is called adaptive radiation, and it was the inspiration behind Charles Darwin’s Theory of Evolution.

Darwin’s attention was drawn to the birds. The differences within the finches and mockingbirds — especially how beak shapes vary to take advantage of different foods — sparked his curiosity about the process of diversification. He may not have realized that the cacti told a similar story. But if food was the driver of radiation in the birds, what pressure has given rise to the varied forms of the prickly pear?

The answer may, very simply, be predators.
 
 
 
 Common Cactus-Finch feeding on prickly pear flowers
 
 
 
 The peak of prickly pear bloom is during the rainy season (Photo credit: Justin Peter)
 

Cacti face the threat of predation wherever they occur, hence their typically prickly demeanour. The fleshy pads and fruits of the prickly pears are prized sources of food and moisture in the arid environments they inhabit. Two of the most ferocious prickly pear predators are Galapagos Giant Tortoises and the Galapagos Land Iguanas. These voracious herbivores will eagerly munch on a juicy cactus, so the cacti need a defensive strategy.
 
 
 
Galapagos Giant Tortoises munching on cactus pads
 
 
Perhaps unsurprisingly, on islands with tortoises and land iguanas, like Santa Cruz, the cacti tend to be tall. They are also particularly spiny, especially when they are young and growing close to the ground. This leaves the earth-bound reptiles to forage on fallen pads and fruit, allowing the cacti to grow intact and unthwarted.
 
 
 
 Prickly pear cactus (Opuntia galapageia var. gigantea) on Santa Cruz Island (Photo credit: Dallas Krentzel)
 
 
 
 A young prickly pear is densely armed with spines (Photo credit: Justin Peter)
 

On islands where there are no tortoises or iguanas, like Genovesa, the cacti can be short. They need not even invest in a heavy armament, and their spines are accordingly small and soft. In this way, the simple variation in the populations of cactus-eaters, possibly combined with competition for light, has likely produced the great variation in the height and form of the Galapagos prickly pears that can be seen today.

Remote island chains like the Galapagos provide a unique opportunity to see the effects of evolution unusually clearly. They act as living laboratories, isolated from the rest of the world. Each island represents a separate experiment, with its own set of experimental conditions. The way that species adapt and change to suit these varied conditions shows us evolution in action, just as it did for Darwin almost 200 years ago.

Whether you’re inspired by birds, cacti, or any other aspect of nature, a visit to the Galapagos is one you’ll never forget. It is a glimpse into one of the fundamental processes of our world, and a unique haven for species found nowhere else. It is surely a must-see for any naturalist.
 
See all upcoming departures to the Galapagos Islands here.