Opuntia cacti are a key species in the ecology of the Galapagos Islands, the pads forming a major food source for tortoises and land iguanas, and the fruits being eaten by iguanas, doves and mockingbirds. The flowers, fruits and seeds are eaten by cactus finches, which may also obtain water by pecking at the fleshy pads (3) (9). In return, these species help to disperse the seeds of the cactus, and also act as important pollinators of the flowers, particularly on islands lacking insect pollinators such as bees (3) (9) (11). Flowering of Opuntia echios has been reported to occur between November and February, with each plant producing a few flowers every day throughout the flowering season. The seeds take several months to mature, and the long flowering season means that the fruits ripen over a period of time rather than simultaneously (11). The species may also be able to reproduce vegetatively (3), an entirely new individual growing from a detached portion of the plant. It is thought that the Opuntia species of the Galapagos Islands are very long-lived, potentially reaching ages of 150 years or more (8).
The five varieties of Opuntia echios are quite variable in form, a phenomenon thought to result from adaptation to differing conditions on different islands (2) (3) (5) (6). The smallest, such as O. e. zacana, are shrubby and low-growing, never reaching more than a metre or two in height (2) (5) (7), while others, such as O. e. gigantea, grow into tall trees and are among the largest Opuntia species in the Galapagos (2) (3) (8). Opuntia echios usually has a well-developed trunk, which is spiny when young and later develops reddish, flaky bark (2) (3) (5). The more shrubby forms generally have softer spines, and tend to grow on islands where there are fewer browsing herbivores and lower competition with other plant species (3) (6) (9). Opuntia echios produces flat, fleshy pads, up to 45 centimetres long and 32 centimetres wide, covered in evenly spaced groups of 2 to 20 or more yellowish to brown spines. The spines, up to 12 centimetres in length, tend to be erect, stiff and sharp on young plants, and more bristly, almost hairlike, on mature individuals (2) (5). The pads of Opuntia echios produce large yellow flowers, up to ten centimetres across, which develop into the greenish “prickly pear” fruits which give the Opuntia cacti their common name. These roundish to oblong fruits may be over seven centimetres in length, and are covered in spines and short, barbed hairs, known as glochids (2) (3) (5) (9).
Endemic to the Galapagos Islands, the five varieties of Opuntia echios occur on different islands. O. e. echios is found on Daphne Major, Santa Cruz, Baltra and Las Plazas Islands, O. e. barringtonensis is found on Santa Fé Island, and O. e. gigantea is found on Santa Cruz Island, in the vicinity of Academy Bay. O. e. inermis is known only from Volcán Sierra Negra, on Isabela Island, and O. e. zacana is found on Seymour Island, where it is the only Opuntia species on the island (2) (5) (7).