Frequency-dependent foraging is defined as the tendency of an individual to selectively forage on a certain species or
morph based on its relative frequency within a population.
[Levin 1972. Low frequency disadvantage in the exploitation of pollinators by corolla variants in Phlox. Am. Nat., 106: 453-460.] Specifically for
pollinators
A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains.
Insects are the maj ...
, this refers to the tendency to visit a particular floral morph or plant species based on its frequency within the local plant community, even if
nectar
Nectar is a sugar-rich liquid produced by plants in glands called nectaries or nectarines, either within the flowers with which it attracts pollinating animals, or by extrafloral nectaries, which provide a nutrient source to animal mutualist ...
rewards are equivalent amongst different morphs. Pollinators that forage in a frequency-dependent manner will exhibit
flower constancy
Flower constancy or pollinator constancy is the tendency of individual pollinators to exclusively visit certain flower species or morphs within a species, bypassing other available flower species that could potentially contain more nectar. This typ ...
for a certain morph, but the preferred floral type will be dependent on its frequency. Additionally, frequency-dependent foraging differs from density-dependent foraging as the latter considers the absolute number of certain morphs per unit area as a factor influencing pollinator choice. Although density of a morph will be related to its frequency, common morphs are still preferred when overall plant densities are high.
[Smithson, A and MR Macnair. 1997a. Density-dependent and frequency-dependent selection by bumblebees Bombus terrestris (L.) (Hymenoptera: Apidae). Biol J Linn Soc., 60: 401-417.]
Background
Floral traits, such as
corolla
Corolla may refer to:
*Corolla (botany), the petals of a flower, considered as a unit
*Toyota Corolla, an automobile model name
*Corolla (headgear)
A ''corolla'' is an ancient headdress in the form of a small circlet or crown.pollen transfer (and therefore, reproductive success) of many angiosperms
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants ...
, visitation behavior will impose frequency-dependent selection Frequency-dependent selection is an evolutionary process by which the fitness of a phenotype or genotype depends on the phenotype or genotype composition of a given population.
* In positive frequency-dependent selection, the fitness of a phenotyp ...
on the flower morphs that they visit.phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (biology), morphology or physical form and structure, its Developmental biology, developmental proc ...
, known as directional selection
In population genetics, directional selection, is a mode of negative natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. Under dir ...
. Alternatively, if rare morphs are preferred, this should promote phenotypic diversity, known as balancing or stabilizing selection
Stabilizing selection (not to be confused with negative or purifying selection) is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait value. This is thought to be the most common mechanism of a ...
.
Interest in frequency-dependent selection dates back to the time of Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
, who predicted that insects should demonstrate flower constancy and puzzled over the occurrence of deceptive orchid species. This phenomenon received little attention until the 1970s when Donald Levin suggested that one of the most important factors determining pollinator visitation behavior is the floral trait's frequency in the population relative to other floral elements.[Smithson, A and MR Macnair. 1997b. Negative frequency-dependent selection by pollinators on artificial flowers without rewards. Evolution, 51: 715-723.] Much less research has been conducted on frequency-dependent foraging on rewarding species, but experiments using bumblebees have illustrated that frequency likely plays a role in reproductive success of flowering plants.[Smithson, A and MR Macnair. 1996. Frequency-dependent selection of pollinators: mechanisms and consequences with regard to behaviour in bumblebees Bombus terrestris (L.) (Hymenoptera: Apidae). J. Evol. Biol., 9: 571-588.]
Experimental evidence
Researchers studying frequency-dependent visitation behavior seek to understand if pollinator preference is strong enough to induce fixation of traits or to maintain floral polymorphisms observed in natural populations.[Smithson, A. 2001. Pollinator preference, frequency dependence, and floral evolution. 237-258. In LD Chittka and JD Thomson (Eds.). Cognitive ecology of pollination: animal behaviour and floral evolution. Cambridge, UK: Cambridge University Press] Laboratory experiments use artificial flowers to test how pollinator preference varies with frequency. Typical experiments use two or more colored discs or artificial flowers (to represent flower morphs) that are arranged in various patterns and frequencies.