Giant Galapogean Prickly Pear (Opuntia echios var. gigantea) on Santa Cruz, Galapagos Islands
Prickly pear cacti are usually small bushy like plants lacking any sort of central trunk, however, in the Galapagos prickly pears have become towering trees with tall central, woody-looking lignized trunks, some of which that I saw personally where at least over 40 feet tall (+12 meters), although the average was more like 12-20 ft (~4-6 meters). This peculiar morphology has occurred due to an evolutionary arms race between the cacti and their predators, the giant Galapagos tortoises (Chelinoidis nigra sp.). On islands without any tortoises living in the arid regions, the cacti do not grow towering trunks, but more closely resemble prickly pear species that live in the rest of the world; however, on tortoise occupied islands, the cacti grow large and have 'bark', with their pads held high and out of reach. But the tortoises haven't been totally evaded, as they too have evolved their own modifications, with the tortoise populations that one finds in the arid regions being characterized by the 'saddle-back' morphology, which means their shells look like saddles, with the anterior region flared dorsally, which allows their incredibly long necks to extend upward and forage for the elevated prickly pear pads. In fact, many researchers have appropriately compared the Galapagos tortoises to giraffes and sauropods, who have also increased their neck lengths dramatically in order to reach food (Many hypothesize other roles for long necks in these species, most importantly, for sexual purposes. In sauropods and giraffes this view has begun to fall out of favor due to recent work, although at least in giraffes and tortoises, it is known that neck demonstrations play a role in sexual displays and combat to some degree).
This particular forest of cacti was adjacent to the rocky shore line, including mangroves. To see a field of giant woody cacti next to tropical mangroves and black basaltic lava rocks is a truly alien and mesmerized sight. I suggest you see it one day.
For a discussion on why I find Opuntia so neat morphologically, developmentally, and functionally: see this image: www.flickr.com/photos/31867959@N04/7457728654/in/set-7215...
***This photo previously labeled these individuals as Opuntia echios var. echios, however, after searching the scientific literature I've discovered this designation was innaccuate. Opuntia echios is the only Opuntia species on Santa Cruz, however, there are two recognized varieties, var. echios in the north and var. gigantea in the south. These varieties were named for their morphological differentiation, however, recent genetic work with both microsatellites and ribosomal genes have determined that these varieties do not differ genetically, with over 95% of existing genetic variability existing within localities rather than between localities of different varieties. This is interesting, because despite the lack of genetic divergence between the varieties, var. echios are small, shrub like plants with dense covering of spines, while the var. gigantea are arboreal (tree-like) plants with large robust trunks and and multiple branchings containing more sparsely spined pads. Is there a genetic basis to this differention that has evolved within very recent times, such that both populations retain overall similar genomes, or is this morphological variability simply due to regional environmental differences, which has been observed in other cacti? Who knows. There are many intermediate individuals at hybrid zone localities, but this is compatible with both models. TIme will tell...
Helsen, P., P. Verdyck, A. Tye, S. Van Dongen. 2009. Low levels of genetic differentiation between Opuntia echios varieties on Santa Cruz (Galapagos). Plant Systematics and Evolution 279:1–10.
Helsen, P., R. A. Browne, D. A. Anderson, P. Verdyck, S. Van Dongen. 2009. Galápagos' Opuntia (prickly pear) cacti: extensive morphological diversity, low genetic variability. Biological Journal of the Linnean Society 96(2): 451–461.