Galápagos - Paradise Still Not Lost
Chapter 2
Galapagos prickly pears and their importance in the diet of animals and in the island landscape


Today we continue our trip to the Galápagos Islands. In the previous episode we learnt a lot about general topics related to this extremely interesting archipelago. We analysed the importance of the garúa mists and their effect on flora and fauna. We also learnt about sea currents that constantly affect the climate of individual islands in the archipelago. We also got to know the most important ecological zones on the islands. And finally, we came to Bartolomé Island to admire the scenery and take the most famous photo, featured on most postcards from the Galápagos, with the iconic Pinnacle Rock. However, we were unable to stay there too long because the time spent on the Bartolomé island cannot be excessively long and usually is not more than two hours to protect the ecosystem and reduce human pressure as much as possible. However, it was enough to get to know a few local endemics (i.e., taxa occurring in a limited area, not found naturally outside of it).

So we now have a solid basis to go further and look at Galapagos prickly pears (Opuntia sp.). They are the ones that catch our eye practically at every step, starting from the moment we get off the plane on the island of Baltra (
Cover Figure). Prickly pears belong to succulents, i.e., plants that have adapted to life in conditions of limited water availability by developing water tissue (aqueous parenchyma) used to accumulate water and several other adaptations in structure and physiology. They are most common in deserts or other areas where there is a long-term lack of water.

A genus of prickly pear belongs to the cactus family (Cactaceae). It includes about 150 species, found mainly in North and South America: from Canada to southern Argentina, but many of them have also been introduced to other continents.
Prickly pears develop in very diverse forms: from very small to shrubs and trees, and it is mainly the latter two groups that can be seen in the landscape of the Galápagos Islands. Regardless of whether the shoots are erect or creeping, especially in the Galápagos, they are partially lignified (Figure 1) and, as usual, divided into flat segments (Figure 2). Prickly pears have thorns in the areoles (Figure 3-5), and there are neraby very small and brittle so-called glochidia. Glochidium is a very short, weak, and small thorn that is additionally equipped with microscopic backward-pointing hooks. When touched, these hooks easily break off and stick into the skin, and if not removed, they can even cause inflammation of the skin. Glochidia often grow in clusters, for example, at the base of thorns. On young opuntia shoots, impermanent, roller-shaped true leaves also grow.

Typical opuntia flowers are large, yellow, orange, or red fruits (
Figure 6) in some species are fleshy, green, or yellow to red, and in others they are dry. They are cylindrical or oval to spherical in shape and reach from 1 to 12 cm in length. In some species, they are thorny, and in others they are defenceless.

It is also very interesting that prickly pears produce two types of roots. The first is the surface network of fine roots, whose task is to collect up to the last drop of water after a rainstorm. However, there is also the "probe" root, whose task is to anchor in the rocks and search for water located deeper.
You should also remember that opuntias are generally long-lived. In the Galápagos Islands, it takes 50 years for them to bloom for the first time, and then they may breed for several more centuries, and it is possible that Charles Darwin saw some of them in person during his last visit.

There are about six species of opuntia in the Galápagos (depending on the views), but also numerous infrataxa (i.e., taxa in the ranks below the species, that is, subspecies, varieties, or forms). Examples that certainly cannot be omitted include Opuntia galapageia (
Figure 7-9) - endemic to the Galápagos, found by Charles Darwin in the lowlands of Santiago Island.
An interesting phenomenon observed on the islands of the Galápagos archipelago is the following relationship: on islands where turtles have never occurred (e.g., on Darwin, Wolf, and Genovesa islands), in other words, islands free of herbivores, local prickly pears are (generally) low, and with limp thorns. Therefore, the two facts appear to be related. However, another version of the interpretation of this phenomenon is as follows: soft thorns can make it easier for birds (e.g., one of Darwin’s finches, Geospiza scandens (common cactus finch or small cactus finch), Figure 10) to come for pollen and nectar, e.g., on the island of Genovesa, where it is not easy to find pollinating insects and in such conditions the possibility of carrying pollen by birds becomes particularly important. The common cactus finch also feeds on prickly pear seeds, but its slightly longer beak allows it to dig into fleshy prickly pear shoots for water and drink nectar from its yellow flowers.

Tall prickly pears also influenced the evolution of turtle shells, because 200 years ago, David Porter, the captain of the American frigate Essex, described the shells of individuals from the island of Espanola as “elongated, curled upward in the shape of a Spanish saddle”. This shape of the turtle shell helped it survive on flatter islands, where the scarcity of low plants is conspicuous (e.g., the island of Santiago is up to 10 m above sea level and is covered with dense and lush vegetation). Due to this shape of the carapax, the turtle can reach higher: it has a slit in the front of the carapax and a long neck and legs, due to which it can lift its head (when it supports itself) over 2 m high! This is how the turtle reaches for the coveted fruits of opuntia. Charles Darwin emphasised this in his “Diary of Observations” writing: “Turtles living on islands without water or in low lying and dry areas, feed mainly on succulent cacti”, and, of course, the most attractive are their fruits.

It should also be noted that by biting and trampling around, turtles reduce the density of small prickly pears but advantageously spread their seeds over a larger area, namely in their droppings: one serving of turtle faeces contains hundreds of seeds, usually of several taxa of prickly pear. It should also be remembered that the transition of a portion of food from the mouth to the anus of a turtle takes various lengths, but most often from 12 to 30 days, which in turn translates into a distance of 0.5 to 3 km from the position of the parent plant. In addition, such soaking of opuntia seeds in the turtle’s digestive tract, including the action of digestive juices in individual sections of its digestive tract, probably also facilitates their subsequent germination. Furthermore, “from the point of view of prickly pears”, each turtle is an “ecosystem engineer”, because movement and feeding at the same time reveal the soil in which prickly pears, as well as other plants, can take root after germination (
Figure 11).
Both native (Figure 12) and introduced prickly pears are found in the Galápagos Islands. All taxa of this genus endemic to the Galápagos Islands have a high category of threat, at least VU (Vulnerable taxon). Two taxa of prickly pear are critically endangered (CR category): O. saxicola (found only on Isabela Island) and O. megasperma var. mesophytica, which is only found on the island of San Cristóbal. Prickly pears with a very high risk, highly threatened with extinction (endangered, EN threat category) include the largest number of taxa, as many as six, with three varieties of the O. echiosi species: var. echios and var. gigantea (both found only on Santa Cruz Island) and var. inermis (found only on Isabela Island). Also endangered with extinction (EN category) are two varieties of O. megasperma var. megasperma (only found on Floreana Island) and var. orientalis (found only on the islands of Espanola and San Cristóbal). Opuntia galapageia var. galapageia also has the same risk category and is found only in the Pinta and Santiago Islands.
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The remaining six taxa of opuntia are also plants at risk of extinction (with the VU threat category). Two varieties of O. echios belong to this group: var. barringtonensis (found only on the small island of Santa Fé) and var. zacana (found only on Santa Cruz Island) and two varieties of O. galapageia var. macrocarpa  (found only on the small island of Pinzón) and var. profusa (found only on the islands of Isabela and Santiago). Apart from them, the following species are at risk of extinction (category VU): O. helleri (only found on the Genovesa, Marchena, and Wolf Islands) and O. insularis (only found on the Fernandina and Isabela Islands).

In addition, the flora of the Galápagos Islands includes introduced species of opuntia, alien, originating from other areas, artificially introduced by man to a non-native area to popularise them, often deliberately cultivated. Prickly pears introduced into the Galápagos archipelago include the escapee from cultivation, O. ficus-indica (in the wild, it occurs on the islands of Isabela, San Cristóbal, and Santa Cruz). Furthermore, four other prickly pear taxa have been recorded in the wild but are also cultivated in the archipelago: O. dillenii (in the wild, it occurs on the islands of Floreana, Isabela, San Cristóbal, and Santa Cruz), while the other three, i.e., O. leucotricha, O. subulata and O. microdasys var. albispina in the wild are found on the island of San Cristóbal.
Figure captions

Cover Figure. The coastal landscape with prickly pears in the Galápagos Islands differs mainly in two features: 1) the prickly pears grow in even greater density and 2) the shrub layer is less dense, but this does not change the fact that the traveler still needs to be vigilant not to encounter prickly pear thorns.

Figure 1. Old woody prickly pear shoots in Puerto Ayora (in the capital of the Galápagos Islands, located on Santa Cruz Island) are already making a huge impression on travellers, especially those who have just arrived here.

Figure 2. Very old prickly pear trunks sometimes take on a red colour, which is in particular contrast to the green colour of their flattened shoots.

Figure 3. Prickly pears have thorns in the areoles, and next to them there are very small and brittle so-called glochidia, i.e., very short, weak, and small thorns, additionally equipped with microscopic, backward-pointing hooks. Glochidia often grow in clusters, for example, at the base of thorns. The whole thing looks really dangerous and you can definitely get very hurt. All it takes is a moment of inattention!

Figure 4. The trunk of the prickly pear can be so thickly covered with thorns that it may not even be visible at all!

Figure 5. The photo shows the way thorns grow on opuntia shoots.

Figure 6. In August, you can already encounter immature fruits of prickly pears on the Galápagos Islands.

Figure 7. Prickly pears are truly ubiquitous in the landscape of the Galápagos Islands and there is no problem finding them.

Figure 8. Prickly pears in the Galápagos Islands grow easily even in places where the shrub layer is quite dense. For other plant species, such a state is a relatively serious limitation, whereas for opuntia, such a limitation is in principle minor.

Figure 9. It happens that opuntias grow very close to each other, so a wanderer without a machete basically has no chance.

Figure 10. One of the Darwin finches, Geospiza scandens on prickly pear.

Figure 11. Young prickly pears are also very easy to find in the Galápagos Islands; they are the next generations that spread from the seeds of their large parents.

Figure 12. Distribution of native species of prickly pears  in the Galápagos Islands.


Author:
Dr hab. prof. Jaros³aw Proæków
Department of Plant Biology, Institute of Environmental Biology, Wroc³aw University of Environmental and Life Sciences, Wroc³aw, Poland

Photograph credits: (cover picture, 2-9, 11) Jaros³aw Proæków; (10) Wmarissen, iStock
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