Using pollination deficits to infer pollinator declines: Can theory guide us?

Authors examining pollinator declines frequently discuss pollination defici ts, either as contemporary evidence that declines have occurred or as a pos sible 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 e volutionary theory and natural plant populations. Operationally, pollinatio n deficits are detected through pollen supplementation experiments. Althoug h simple, these experiments are subject to subtleties of interpretation bec ause of biases and nonlinear responses, which I discuss. Although it has be en found that, in 62% of the natural populations studied, fruit or seed set s are at least sometimes limited by insufficient pollen, other research sug gests that intact natural systems ought to arrive at an evolutionary equili brium in which reproduction is limited equally by pollination and by matern al resources. Therefore, chronic severe pollination deficits may indicate t hat the pollinator service of a plant population has declined from some hig her level in the past. However, there is no evidence of widespread declines , and, because of stochastic factors in nature, occasional shortfalls of po llination should be expected even at equilibrium. Although the effects of p ollination deficits on plant population dynamics have been little studied, moderate declines in seed production may have relatively little effect on p opulation growth rates because resources not expended on fruits and seeds m ay be reallocated to vegetative persistence or spread. It is therefore prem ature to conclude that pollinator declines are having strong effects on nat ural plant populations, but this mostly reflects a lack of data and is no c ause for complacency. Theory must be supplemented by case studies; I give o ne example and recommendations.