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).