Tree-like prickly pears and Galápagos tortoises. From fruit to young plant

The population on Española

Tree-like Opuntia galapageia var. galapageia grow on Española (in the 2008 study, O. megasperma var. megasperma). Flowers and fruits form in a crown of flattened segments, raised above the ground by the trunk. Fruits and falling segments provide tortoises with food and water. The crown creates a resting place sheltered from the sun. [Gibbs et al. 2008][2-WWW]

A plant map and signs of tortoise presence

In 2004, Gibbs, Marquez and Sterling surveyed a 60 ha part of Las Tunas. They mapped 319 prickly pears and determined developmental stage, dimensions, fruiting and neighbouring woody vegetation. Droppings around plants were used as an indicator of tortoise site use. [Gibbs et al. 2008]

Larger adult prickly pears were associated with more droppings. Their crowns provided attractive shelter, and large plants produced more fruit. However, young prickly pears appeared more often farther from adults in more open places. The strongest animal concentration and greatest recruitment probability thus had different spatial distributions. [Gibbs et al. 2008]

Dispersal and germination conditions

A tortoise eating a fruit can carry seeds beyond the cactus crown. Their subsequent fate depends on deposition site, light, moisture and competition. Trampling of small plants may also become important where animals frequently spend time. The authors considered these processes as possible explanations for the spatial separation of young and adult prickly pears. [Gibbs et al. 2008]

Seed excretion, germination and seedling survival are separate stages. The Española study primarily measured population distribution and structure. The authors identified the direct effect of gut passage on germination as a topic for further experiments. The tortoise–prickly pear relationship can therefore be divided into specific processes that can be studied separately. [Gibbs et al. 2008]

Restoration in a changing habitat

Comparison with earlier observations indicated an increase in the proportion and number of young plants during tortoise population restoration. Goat removal and changes in woody vegetation also affected the same landscape. Dense scrub can restrict tortoise movement and change the availability of open sites for seedlings. [Gibbs et al. 2008]

The field result describes recovery of the whole system after several interventions. The relationship between tortoise numbers and young cactus numbers gains meaning when combined with adult plant distribution, passage availability and scrub development. Reintroducing animals and changing vegetation cover affect successive parts of the same journey, from fruit to rooted seedling. [Gibbs et al. 2008]

Diversity that is visible and measured by DNA

The archipelago's prickly pears vary greatly in habit, segments and armature. Helsen and colleagues compared this variation with four DNA regions. Some markers showed little variation, and the resulting genealogies did not unambiguously reproduce all morphological divisions. The authors considered the young history of the lineages and limited gene flow between islands. [Helsen et al. 2009]

The resolution of a few markers defines the scope of this conclusion. A local population can have pronounced structural traits despite small differences in the sequences studied. For prickly pear conservation, the authors recommend considering both types of data. On Española, the ecological distinctness of a population helping create tortoise habitat adds another dimension. [Helsen et al. 2009][Gibbs et al. 2008]