Using Pollination Deficits to Infer Pollinator Declines: Can Theory Guide Us?

Abstract

Authors examining pollinator declines frequently discuss pollination deficits, either as contemporary evidence that declines have occurred or as a possible negative consequence of future declines. Because pollination deficits can be measured in short-term studies, it would be useful if such studies could somehow replace painstaking documentation of insect population trends. I examine the legitimacy of this type of substitution with reference to evolutionary theory and natural plant populations. Operationally, pollination deficits are detected through pollen supplementation experiments. Although simple, these experiments are subject to subtleties of interpretation because of biases and nonlinear responses, which I discuss. Although it has been found that, in 62% of the natural populations studied, fruit or seed sets are at least sometimes limited by insufficient pollen, other research suggests that intact natural systems ought to arrive at an evolutionary equilibrium in which reproduction is limited equally by pollination and by maternal resources. Therefore, chronic severe pollination deficits may indicate that the pollinator service of a plant population has declined from some higher level in the past. However, there is no evidence of widespread declines, and, because of stochastic factors in nature, occasional shortfalls of pollination should be expected even at equilibrium. Although the effects of pollination deficits on plant population dynamics have been little studied, moderate declines in seed production may have relatively little effect on population growth rates because resources not expended on fruits and seeds may be reallocated to vegetative persistence or spread. It is therefore premature to conclude that pollinator declines are having strong effects on natural plant populations, but this mostly reflects a lack of data and is no cause for complacency. Theory must be supplemented by case studies; I give one example and recommendations.

Causes and Extent of Declines among Native North American Invertebrate Pollinators: Detection, Evidence, and Consequences

“…If pollinators are in decline, the thinking goes, then we should be able to measure the results of their absence as reductions in fruit or seed production in natural or agricultural ecosystems. Thomson (2001) critically reviews this idea, the thinking that educes it, and some of the work that has been used to support it. He concludes that expectations are really more complicated than we currently allow and that, although valuable if conducted properly, studies using pollination deficits to make the case for pollinator declines need to be interpreted with caution.…”

“…That service has been recently jeopardized by Old World parasitic mites and larval diseases. But how frequent, prolonged, and severe are pollination deficits in nature, and when do pollination deficits signal actual pollinator declines (Thomson 2001)?…”

Population Genetic Aspects of Pollinator Decline

“…Many environmental variables influence the long-and short-term survival of pollinator populations (Cane 2001, Marlin and LaBerge 2001, Roubik 2001, and thereby also the plants that they pollinate (e.g., Neff andSimpson 1993, Kevan andPhillips 2001), although this is not necessarily always the case (Thomson 2001). However, even if ecological factors are conducive to the long-term stability of a moderate-sized or small population, intrinsic genetic processes may lead to its eventual demise (Lande 1999).…”

Impact d’un apport supplémentaire de pollen sur la mise à fruit de Sambucus nigra subsp. canadensis (L.) R. Bolli

“…Le stigmate du sureau étant de type sec (Bolli 1994), donc naturellement moins adhérant que les stigmates de type humide, expliquerait l'augmentation observée à Lacolle en 2005 suite au traitement T4. Ce traitement rappelle le mouvement des insectes pollinisateurs en dispersant le pollen mature au sein d'une même inflorescence (Thompson 2001).…”

“…Chez les espèces anémophiles, une mise à fruit déficiente est habituellement causée par l'insuffisance de pollen associée à une faible densité végétale (Robertson et Ulappa 2004). Chez les espèces entomophiles, c'est un manque de pollinisateurs, attribuable à la destruction de leurs habitats ou aux conditions climatiques défavorables à leur activité, qui explique ce phénomène (Thompson 2001). Le nombre de grains de pollen atteignant le stigmate a également un impact sur le succès de la pollinisation (Lord et Russell 2002).…”