Threatened Plant Seeks an Efficient Pollinator

A Tenerife lizard (Gallotia galloti) visiting the flowers of the Pico de El Sauzal (Lotus maculatus).

The Plant and the Problem

The Pico de El Sauzal (Lotus maculatus) is a legume endemic to the Canary Islands and is classified as Critically Endangered on the IUCN Red List. At present, the species appears to survive in the wild only as a small natural population on the north coast of the island of Tenerife.

The Island Council of Tenerife, as the public authority responsible for its conservation, has made a major effort to protect the species, both in its natural population and through several reintroduction programmes carried out along the island's north coast. However, it remains very difficult to confirm natural regeneration within the various restored populations.

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Given that the natural regeneration of this species depends largely on seed production, it was essential to determine whether the plants were being pollinated. Owing to the colour of its flowers and other floral traits, the Pico de El Sauzal (Lotus maculatus) has traditionally been regarded as a bird-pollinated species. However, until recently, no birds had ever been observed visiting its flowers. We therefore set out to investigate whether this threatened plant relies on any animal species for the production of fruits and seeds.

Our experiments

The first step was to carry out an exclusion experiment, in which flowers were enclosed in small fabric bags to prevent any interaction with animals. This allowed us to determine that the flowers must be visited by animals in order to produce fruits containing seeds.

In other words, the plant depends on animal pollinators to reproduce successfully. Without pollination, the flowers wither and die without producing either fruits or seeds.

Flowers bagged to prevent interactions with animals

Secondly, we observed which animals were visiting the flowers. In doing so, we discovered that the flowers were visited primarily by the western honey bee (Apis mellifera), a solitary bee (Lasioglossum arctifrons), and the Tenerife lizard (Gallotia galloti). As expected, no visits by birds were recorded.

We also observed that honey bees acted as nectar thieves, accessing the nectar from the outside at the base of the flower without coming into contact with either the anthers or the stigma, which is essential for effective pollination. In contrast, the solitary bee and the Tenerife lizard were the only two species capable of touching the anthers or stigma during their visits and therefore had the potential to act as pollinators.

An "outlandish" question and a surprising answer

Following our observations, the following question arose: could lizards transport pollen attached to their scales? To answer this, we captured lizards and checked whether they carried pollen on their head and neck. In addition, we evaluated whether the plants that received more visits from any of the three floral visitors produced a higher quantity of fruit. Surprisingly, not only did we find pollen grains on the lizards' bodies, but the plants that received more floral visits from these animals produced more fruit than those that received fewer visits. On the other hand, frequent visits by honeybees decreased fruit production, whereas the number of visits made by the solitary bee had no effect on fruiting. We conclude, therefore, that in the absence of pollinating birds, the conservation of this 'Critically Endangered' plant depends on its mutualistic interaction with lizards. Learn more here.