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4 Effects of Variations in Pollinator Populations on Pollination Services
Pages 104-130

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From page 104...
... Shortages can be exacerbated by pollinator declines, but they can arise even when pollinator populations are stable or increasing over time. Some shortages, for example, those caused by the high seasonal demands of the California almond industry for honey bee colonies (for example, Norton, 2005)
From page 105...
... , the proportion likely varies among countries and regions and depends on dietary preferences, seasonal availability, cultural practices, and economic status of consumers. Major Crops Plant species grown as agricultural commodities display a wide variety of breeding systems.
From page 106...
... Listed in Table 4-1 are the relative area and value of several major crops known to benefit from pollinators. Because managed honey bees are often used to pollinate them, the extent to which managed and unmanaged pollinators provide adequate pollination service for optimal yields remains for the most part undetermined.
From page 107...
... Even in crops that are routinely pollinated by managed honey bees, wild pollinators also can contribute pollination services. Watermelon growers in California use managed honey bees, but their crops also receive substantial pollination from a diverse community of native bees (Kremen et al., 2002a)
From page 108...
... . Introduced honey bees do not buzz-pollinate, so high-bush blueberries depend largely on native wild bees for pollination (Buchmann, 1983; Free, 1993; MacKenzie, 1997; MacKenzie et al., 1996)
From page 109...
... calculated that the direct pollination provided by wild bees nationally to the hybrid sunflower seed industry was worth $1.9 million, the interspecific interaction between wild and honey bees was worth $10.4 million, and the direct contribution of honey bees was worth $13.8 million. Both proximity to natural habitat and crop rotation practices affect the amount of sunflower pollination provided by wild bees (Greenleaf, 2005; Greenleaf and Kremen, 2006b)
From page 110...
... rapa is self-incompatible and therefore requires pollinators for seed set. Although managed honey bees are used, many native species also pollinate canola crops (Morandin and Winston, 2005)
From page 111...
... Honey bees do not appear to be effective pollenators (Free, 1993)
From page 112...
... Monocultures with little genetic variation are susceptible to pests and diseases. Other Crops Ecologists have investigated a variety of crops, both in North America and elsewhere, to determine the degree of pollination services provided by wild bees.
From page 113...
... Strategies for improving pollinator habitats in and around agricultural areas are discussed in Chapter 6. Economic and Financial Consequences of Pollinator Shifts Honey bees are responsible for the greatest percentage of agricultural pollinator activity in North America, and they constitute the greatest percentage of the commercial pollination market.
From page 114...
... Predicting the direct, short-run economic consequences of population declines in honey bees -- the principal managed pollinator species -- is not straightforward. Microeconomic theory predicts two effects; both increase pollination fees.
From page 115...
... q0* Pollination Services FIGURE 4-1 Honey bee population declines raise bee production and maintenance costs, reducing the commercial supply of pollination services offered at all price levels from S0 to S1, and raising the market price and reducing the marketed quantity of honey bee colony rentals from e0 to ev.
From page 116...
... Statistical regression analysis of annual pollination fee data from Washington and Oregon shows that honey bee colony rental fees for pear, cherry, and apple rose by $4.40–$5.30 (in 2002 dollars) after 1991, when the varroa mite was widely
From page 117...
... , if each honey bee colony is rented out two to three times per year, the increase in pollination fees roughly equals the estimated annual cost of varroa mite control at $10–$15 per colony. By extrapolation from their Pacific Northwest data to the 2 million commercial hives in the United States, varroa mite control has increased honey bee colony rental fees by nearly $30 million annually (ignoring any honey or crop yield losses that could result from honey bee shortages)
From page 118...
... , and new work could focus on mite control, bee tolerance of mites, or crop pollination needs. Another potential response is the introduction of substitute pollinators -- propagated bumble bees or the imported Australian packaged honey bees, like those used to pollinate California almonds in 2005 (Sumner and Boriss, 2006)
From page 119...
... POLLINATORS IN NATURAL AREAS Pollinator Limitation of Seed Production As with agricultural systems, pollinator limitation in natural plant populations occurs through a variety of mechanisms that decrease pollinator abundance below that required by plants for full reproduction, and pollinator shortages are expected to depress fruit and seed set in local plants. Low seed set can be caused by other factors as well, including severe weather, inadequate concentrations of soil nutrients, disease, damage caused by herbivores, partial sterility, or shortages of neighboring plants with compatible pollen.
From page 120...
... FIGURE 4-4 Possible effects of declining pollinator populations and other factors on the abundance and persistence of plant species. Effects represented by black arrows are cause for concern because they could lead to local or global extinctions.
From page 121...
... , it is useful to distinguish between two types of pollen-related constraints on seed production: pollinator limitation (insufficient pollinator service) and pollen limitation (insufficient delivery of compatible pollen to receptive stigmas; see Thompson, 2001)
From page 122...
... reported a strong positive correlation between plant species richness and pollen limitation on a global scale, but there was no clear explanation for this association. The investigators hypothesized that competition for pollinators is more prevalent in species-rich communities, but they were unable to determine whether competition for pollinators is a natural and long-standing phenomenon or whether lower fecundity is a consequence of recent declines in pollinator populations.
From page 123...
... However, although many researchers have demonstrated apparent pollen limitation, few have investigated its effects on the demographics of plant species (but see Johnson et al., 2004; Ward and Johnson, 2005)
From page 124...
... . Although few North American plant species rely on a small number of pollinator species for seed production, many plants depend heavily on specific floral visitors, such as bumble bees, for effective pollination.
From page 125...
... suggests that low seed set or fruit production could be exacerbated by declining pollinator populations, but that might not be the case if alternative pollinators are present or if the plant is autogamous (self-pollinating) when pollinators are absent.
From page 126...
... Such a decline in wild plant populations because of pollination deficits might be in progress, but there is a dearth of published evidence for its occurrence in North America during recent decades. If all the consequences of declining pollinator populations in fact occur (which seems unlikely)
From page 127...
... . If there are no compensatory mechanisms for pollinator loss, and if reduced seed production from pollinator loss is directly reflected in smaller plant populations, these populations could enter an extinction vortex (Bond, 1994)
From page 128...
... Also important are the aesthetic benefits or cultural satisfaction of watching pollinators at work. This broad category of value applies not only to the pollinator species, but also to plant species that depend on them for reproduction and to animal species that depend on associated fruits and seeds for food.
From page 129...
... In natural areas, the consequences of pollinator shifts on ecosystem services and possible solutions to these problems are varied and complex. Relatively few plant species rely on a single pollinator species or even on a single category of pollinators for reproduction.
From page 130...
... More research is needed to assess ecological consequences of current and future pollinator declines.


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