A survey of floral traits, breeding systems, floral visitors, and pollination systems of the angiosperms of the Juan Fernández Islands (Chile)

Abstract

A survey of the reproductive features of the angiosperm flora of the Juan Fernández Archipelago (Chile) is presented to provide a species-based review of reproduction and pollination, to identify generalizations associated with these systems, to understand the evolution of these features, and to utilize these data to promote conservation. The collection of original data was extensive, based on our own fieldwork, and was combined with data from existing literature. Data recorded include habit, sexual system, flower size, shape, and color, and the hypothesized pollination system of the first colonizers. In addition, the data on compatibility, presence and type of dichogamy, observed floral visitors, presence of floral rewards, and currently known pollination systems are summarized. Pairwise comparisons of different features are tested for statistical association. The flora is typically composed of perennials. The majority of the species have very small or small flowers. Inconspicuous flowers (i.e., a shape character describing flowers with no optical attraction) are widespread, as are dish-shaped flowers. Green is the most frequent flower color, followed by white and yellow. Most species are hermaphroditic, 9% are dioecious, and 9% are monoecious. Some 30% of the species are protandrous and 7% protogynous. Detailed studies of compatibility of about 14% of the flora indicate that 85% of these species are self-compatible (SC). Although most species studied are SC, their level of autogamy is low. Nevertheless, selfing mediated via geitonogamy is the most frequent mechanism of pollen transfer. Outcrossing is mainly achieved through dioecy and self-incompatibility (SI), promoted by dichogamy in the hermaphroditic flowers, and facilitated by wind pollination. About 55% of the species offer nectar rewards, and only 2% offer pollen rewards. Floral visitors are rare to uncommon. Two hummingbird species, one of them endemic, are considered as pollinators for 14 plant species. Flies, moths, and beetles are the native insect visitors to flowers, but they have been documented on only 11 plant species (7%). Even insect visits to these few species were rarely observed. Given the infrequent, irregular, and imprecise nature of native insect association with flowers, there is no certainty that any of the species are truly insect pollinated. Two species of introduced ants and a new endemic bee were recorded as well; however, neither is likely currently important to the pollination of the native flora species. About 9% of the extant flora is currently bird pollinated, and we hypothesize that 47% is wind pollinated. However, we propose that most of the colonizers were ancestrally either insect or wind pollinated. There is association between a number of current floral features and the hypothesized pollination of colonizers. Therefore, to a large extent the flower color, shape, and size of the extant flora may express the pollination syndromes of colonizers rather than representing extant pollination. In addition, the presence of nectar in many species of extant flora does not necessarily indicate biotic pollination. Thus, studies of the reproductive biology on oceanic island plants need to be conducted species by species before broad generalizations can be made, because the observed features can be misleading. Possible changes in the pollination system were assessed by comparison of species for which there are reliable data with the hypothesized pollination of their colonist progenitors. The wind-or bird-pollinated species have retained the pollination system of the colonizers. In other instances, species seem to bear a different pollination system: from ancestral insect systems to current hummingbird- or wind-pollination systems. The lack of alternative means of biotic pollination seems to have led in a number of instances to anemophily - in essence a default pollination system. The lack of strong selection pressure for wind pollination and the relative youth of the archipelago may help explain why the features associated with wind pollination in these species are not so obvious. Because there are many recorded extinctions of vascular plants from islands versus those from continental areas, it is imperative to invest additional effort in protecting the remaining island species. Conservation or restoration programs cannot be effective without a deep and broad understanding of the reproductive biology of the plants. In order to conserve these plants, programs must involve a combination of reproductive and environmental measures. The ultimate fate of some species may depend on preserving the plant-hummingbird relationship, including the web of organisms that affect both plant and pollinator. The populations of introduced animals and weeds must be controlled. Experimentally produced allogamous seeds would enhance diversity in restoration programs. In addition, the preservation of habitat seems to be the central challenge to indirectly protect the unique island species.