Generalization vs. specialization in the pollination system of Hormathophylla spinosa - Cruciferae

Ecology, April, 1999 by Jose Maria Gomez, Regino Zamora

INTRODUCTION

Most theoretical and empirical studies on plant-pollinator interactions use specialization as a central idea. Nevertheless, recent studies point out that generalization is at least as frequent as specialization in pollination systems, with increasing evidence indicating that most flowering plants are pollinated by a moderate to high number of insect species, and that most floral visitors usually visit many different hosts (Schemske 1984, Herrera 1996, Waser et al. 1996).

Several factors favor the maintenance of these types of generalized pollination systems. A high spatial variability in the distribution and abundance of pollinator species might restrict the potential evolutionary response to particular pollinators in those plant species with gene flow among subpopulations (Brantjes 1981, Thomson 1981, Aker 1982, Howe 1984, Herrera 1988, Horvitz and Schemske 1990, Eckhart 1991). Nevertheless, a geographic structure of specialization can appear when plants without or with limited gene flow among populations are pollinated by different species in different populations, with the result of each population being locally specialized to one or a few pollinators (Thompson 1994). Generalization is also favored within each plant population by temporal unpredictability in pollinator assemblage (Herrera 1988, 1996, Horvitz and Schemske 1990, Eckhart 1992, Fishbein and Venable 1996, Waser et al. 1996, and references therein). Indeed, when local floral visitor assemblages change between years, pollinator-mediated selective pressures become inconsistent, and the appearance of morphological and physiological traits of plants shaped to the demand of particular visitors is unlikely (Herrera 1988, Petterson 1991).

In addition to the importance of the spatiotemporal heterogeneity in promoting generalization, we should also consider that there is an opportunity for specialization only when different pollinators, due to interspecific differences in individual effectiveness, produce different outcomes from the plant's viewpoint [TABULAR DATA FOR TABLE 1 OMITTED] (Schemske and Horvitz 1984, 1989, Herrera 1987, 1989, Eckhart 1991, 1992, Stanton et al. 1991, Fishbein and Venable 1996). A different scenario can develop when very different pollinator species have the same effect on plant fitness (Feinsinger 1983). Indeed, there is evidence suggesting that taxonomically unrelated pollinators can forage at flowers in similar ways, and thereby have similar individual effectiveness (Motten et al. 1981, Waser 1982, Feinsinger 1983, Vaughton 1992, Conner et al. 1995, Fishbein and Venable 1996). In this case, the evolutionary process might be expected to result in a more generalized response to whole groups of species interacting in a similar way.

In the present paper, we study the interaction between Hormathophylla spinosa (Cruciferae) and its pollinators in the high mountains of the Sierra Nevada (Granada province, SE Spain). We quantified the number of species visiting the flowers of this plant by surveying the entire assemblage without any prior selection of species (Waser et al. 1996). Moreover, to test whether H. spinosa is locally specialized to only a few taxa, despite being visited by many different species throughout its distribution area, we analyzed the spatial variability of the main visitors by studying the pollination assemblage in three different populations. Additionally, we assessed year-to-year fluctuations in the local visitor assemblage during 4 yr for all three plant populations. Finally, we quantified the per-visit pollination effectiveness for the main visitors to determine whether they interacted similarly with the plant. By studying simultaneously multiple sources of potential variation in pollination success, we seek to ascertain whether this plant species has a generalized pollination system and, if so, the possible factors by which this generalization is maintained.

MATERIALS AND METHODS

Plant natural history

Hormathophylla spinosa (L.) Kupfer (Cruciferae) is an abundant stunted shrub of the high mountains of southern Spain and the western Mediterranean (Kupfer 1993). In the Sierra Nevada, this plant grows at an altitude of between 1600 and 3340 m.

During flowering, from June to August (Gomez 1993), a typical individual of this mass-flowering species produces [approximately]8000 flowers grouped in inflorescences, each bearing 4 to 22 flowers (Gomez and Zamora 1992), with 6.1 [ or -] 1.2 flowers (mean [ or -] 1 SD) open at the same time in each inflorescence. Each flower lasts 3.6 [ or -] 1.9 d. The white to pink flowers are actinomorphic, hermaphroditic, and slightly protandrous. The flower has four nectaries that produce traces of nectar (0.039 [[micro]liter] per flower per day) and 769 [ or -] 460 pollen grains per anther (Gomez and Zamora 1992). Each flower invariably has four ovules. H. spinosa can produce seeds by geitonogamy (Gomez and Zamora 1996).

Study area

The study was carried out in the Sierra Nevada for 4 yr (1988-1991). We selected three populations of H. spinosa above the timberline, situated at 2160 m (Population A), at 2550 m (Population B), and at 3130 m (Population C) (see Gomez 1993 for a full description).