Prickly pear  (Opuntia helleri)

Classified as Vulnerable (VU) on the IUCN Red List (1) and listed on Appendix II of CITES (2).

Forming large, sprawling thickets, Opuntia helleri is one of the characteristic cactus species of the northern Galapagos Islands (3) (4). The stem is divided into broad, fleshy pads up to 37 centimetres long and 22 centimetres wide, covered with evenly-spaced clusters of 7 to 28 yellowish-white or brown spines (4). In contrast with the strong, hard spines of many cactus species, the spines of Opuntia helleri are soft and flexible (4) (5). The pads produce striking yellow flowers, up to eight centimetres wide, from which large, green, fleshy fruits develop. Commonly known as “prickly pears”, these fruits are generally ovoid, covered in clusters of small spines and usually contain multiple seeds. Although generally a low-growing species, older specimens of Opuntia helleri will sometimes grow into a taller shrub or tree-like form up to two metres in height, developing a woody trunk with a coating of reddish-brown bark (4).

Endemic to the Galapagos, Opuntia helleri is restricted to the northern islands of Darwin, Genovesa, Marchena and Wolf (4).

Opuntia helleri is scattered throughout the arid lowlands, increasing in abundance towards the coast (4) (6).

Interestingly, Opuntia helleri relies on birds to aid pollination, as the northern islands occupied by this cactus are not inhabited by any suitable insect pollinators (7). A number of bird species visit Opuntia helleri, including doves, cactus finches and mockingbirds. The soft-spines allow easy access to this species’ flowers, and while feeding on the pollen, nectar and even petals, the birds receive a dusting of pollen which is transferred to, and fertilises, the other flowers visited (4) (7). This adaptation to bird pollination is only possible because the absence of large, herbivorous reptiles, such as giant tortoises, from the northern Galapagos Islands, allows Opuntia helleri to survive without the protection of hard, sharp spines (8) (9).

Despite producing flowers all year round, Opuntia helleri’s main flowering season occurs between November and February (9). Interestingly, even if the flowers are not pollinated, fruits and seeds may still develop. This is due to a remarkable process called apomixis, in which the female, unfertilised gametes develop into an embryo without needing to fuse with male gametes (10). Whether pollination occurs or not, fruits take several months to mature. Eventually dropping off the pads, many of the fruits are consumed by mockingbirds and cactus finches, which later pass the seeds in their faeces, thereby helping Opuntia helleri to disperse its offspring (7).

Flowering only occurs in larger specimens of Opuntia helleri (8), but individuals of any size can reproduce simply by dropping their pads. In a process called vegetative reproduction, an entirely new plant may grow from the detached portion, giving rise to large thickets of cacti, all of which are genetically identical (7).

The main threats affecting the Opuntia species found in the Galapagos are competition with invasive plants for space, light and nutrients; overgrazing by invasive herbivores such as goats and pigs; and expanding urban development and agriculture. Overgrazing is probably the most significant of these threats, since the consumption of dropped pads prevents vegetative reproduction, and means that only the larger plants capable of flowering are able to reproduce (8).

The Charles Darwin Foundation, a Galapagos conservation organisation, is working to restore damaged populations of Opuntia species. By conducting detailed surveys, they aim to determine the status and vulnerability of each species and develop specific conservation action plans to help preserve them (8).

For more information on Conservation in the Galapagos see:

• The Charles Darwin Foundation:
  www.darwinfoundation.org