Frequency-dependent foraging is defined as the tendency of an individual to selectively forage on a certain species or
morph
Morph may refer to:
Biology
* Morph (zoology), a visual or behavioral difference between organisms of distinct populations in a species
* Muller's morphs, a classification scheme for genetic mutations
* "-morph", a suffix commonly used in tax ...
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 mutualists ...
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 color, flower shape, size and scent, appear to have evolved primarily for the purpose of attracting pollinators and many pollinators have learned to associate these floral signals with the reward that is present there. As pollinators are essential in the process of
pollen transfer (and therefore,
reproductive success
Reproductive success is an individual's production of offspring per breeding event or lifetime. This is not limited by the number of offspring produced by one individual, but also the reproductive success of these offspring themselves.
Reproduct ...
) 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 th ...
, visitation behavior will impose
frequency-dependent selection Frequency-dependent selection is an evolutionary process by which the fitness (biology), fitness of a phenotype or genotype depends on the phenotype or genotype composition of a given population.
* In positive frequency-dependent selection, the fit ...
on the flower morphs that they visit.
If pollinators selectively visit a particular morph, this will cause this morph to increase in frequency, and may ultimately lead to the fixation of this
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
, 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 fr ...
, 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.
Since this time, attention has focussed on understanding how obligately pollinated, unrewarding species can persist as they offer pollinators no incentive to visit.
[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.
It is predicted that if pollinators do not exhibit frequency-dependent foraging, morph preference will not correlate with the relative frequency of that morph. Instead, this preference may depend on some frequency-independent quality, such as an