Flower constancy

Abstract: Mechanical and ethological isolation between species is widespread in angiosperms with specialized animal-pollinated flowers, being recorded in 29 species groups belonging to 27 genera and 16 families. Mechanical isolation occurs in two forms. (i) The common type, designated the Salvia type, operates when two or more species of flowers are adapted for different groups of pollinators with different body sizes and shapes. (ii) In the Pedicularis type two flower species have the same species of pollinator but pick up pollen from different parts of the pollinator's body. Four forms of ethological isolation are recognized. (i) In the Aquilegia type, which is widespread, ethological isolation is a side effect of mechanical isolation. (ii) The flower-constancy type, as the name suggests, is based on flower-constant foraging behavior. (iii) In the Ophrys type, floral scents attract male bees or wasps and play a role in their mating behavior; different species of flowers, often orchids, have different scents and attract different sets of hymenopteran species. (iv) The monotropy type occurs in plants pollinated by hymenopterans with species-specific or group-specific flower preferences for nutritive purposes (monotropic and oligotropic bees and fig wasps). Three modes of origin of floral isolation are confirmed by evidence: (i) mechanical and ethological isolation arising as a by-product of allopatric speciation, (ii) ethological isolation developing by selection for reproductive isolation per se, and (iii) mechanical isolation arising as a by-product of character displacement. Mode of origin i accounts for the Salvia and Aquilegia types of isolation in nine known species groups and for the Ophrys type in one group. Mode of origin ii accounts for the flower-constancy type of ethological isolation in two species groups. Mode of origin iii explains mechanical isolation in two groups. Sympatric origin of floral isolation by hybrid speciation and by flower constancy has been proposed, but these modes are undocumented and improbable.

Abstract: On their first 2 foraging trips out of the hive, young Boinhus x'iwtcans workers visited, on the average, 4 or 5 different kinds of rewarding as well as unrewarding flowers, and few of each kind in succession. But, after 3-7 foraging trips, most of the bees specialized on jewelweed, which was the most numerous flower available with high nectar reward. When jewelweed specialists became numerous, and the food rewards in jewelweed declined, the bees resampled the reward spectrum. They again visited, and continued to vist, at least 3-4 different kinds of flowers on successive foraging trips in an enclosure where patch size was limited. Flowers in open inflorescences (aster, goldenrod) were handled appropriately from the start, but handling accuracy at zygomorphic flowers jewelweedd, turtlehead) was initially 40-50% at the first 10 flowers encountered, and increased to >90%Xe in 60-100 flower visits. It is concluded that the most important problem faced by the foraging bees attempting to enhance food intake is that of assessing the resources, which often change rapidly. Individual bees specialize on flowers yielding rewards that are "perceived" to exceed some minimum. However, the difference between perceived and actual rewards is, in part, determined by handling skills that are affected by foraging experience. Thus, optimal foraging in the bees must be explored from the per- spective of long- rather than short-term energy balance.

Abstract: The majority of species of flowering plants rely on pollination by insects, so that their reproductive success and in part their population structure are determined by insect behaviour. The foraging behaviour of insect pollinators is flexible and complex, because efficient collection of nectar or pollen is no simple matter. Each flower provides a variable but generally small reward that is often hidden, flowers are patchily distributed in time and space, and are erratically depleted of rewards by other foragers. Insects that specialise in visiting flowers have evolved an array of foraging strategies that act to improve their efficiency, which in turn determine the reproductive success of the plants that they visit. This review attempts a synthesis of the recent literature on selectivity in pollinator foraging behaviour, in terms of the species, patch and individual flowers that they choose to visit. The variable nature of floral resources necessitate foraging behaviour based upon flexible learning, so that foragers can respond to the pattern of rewards that they encounter. Fidelity to particular species allows foragers to learn appropriate handling skills and so reduce handling times, but may also be favoured by use of a search image to detect flowers. The rewards received are also used to determine the spatial patterns of searches; distance and direction of flights are adjusted so that foragers tend to remain within rewarding patches and depart swiftly from unrewarding ones. The distribution of foragers among patchy resources generally conforms to the expectations of two simple optimal foraging models, the ideal free distribution and the marginal value theorem. Insects are able to learn to discriminate among flowers of their preferred species on the basis of subtle differences in floral morphology. They may discriminate upon the basis of flower size, age, sex or symmetry and so choose the more rewarding flowers. Some insects are also able to distinguish and reject depleted flowers on the basis of ephemeral odours left by previous visitors. These odours have recently been implicated as a mechanism involved in interspecific interactions between foragers. From the point of view of a plant reliant upon insect pollination, the behaviour of its pollinators (and hence its reproductive success) is likely to vary according to the rewards offered, the size and complexity of floral displays used to advertise their location, the distribution of conspecific and of rewards offered by other plant species, and the abundance and behaviour of other flower visitors.