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Plant Pollinator Relationships

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on January 13, 2007 at 6:45:33 pm


National Academies of Science


Status of Pollinators in North America (2006)

!!! Below, see excerpt on climate's plausible impact on pollinator populations and interactions with plants.

Full text online at <http://orsted.nap.edu/openbook.php?record_id=11761&page=69>




Climate Change

Global, regional, and local climate changes can alter or disrupt plant-pollinator relationships. Included in the global climate change forecast are shifts in temperature and precipitation, concentrations of carbon dioxide (CO2) and ozone, and ultraviolet light levels. All are important to plant growth and flowering, and those changes could alter plant and pollinator phenology and distribution along altitudinal and latitudinal gradients, generate changes in plant and pollinator mutualisms and community compositions, and cause local extinctions.


There is evidence that the latitudinal and altitudinal ranges of some plants and pollinators have changed in the past 30 years, presumably in response to global warming (Walther, 2004). For example, some butterflies in Britain and North America have expanded ranges north (Hill et al., 1999; Parmesan et al., 1999; Crozier, 2003), and others in Montana (Lesica and McCune, 2004), Spain (Wilson et al., 2005), and Norway (Klanderud and Birks, 2003) have contracted ranges at

lower altitudes and latitudes.


An increase in atmospheric CO2 could alter production of nectar (reviewed by Davis, 2003). Typically, elevated CO2 concentrations alter nectar volume and secretion rate, sometimes negatively and sometimes positively, but not sugar concentration orcomposition (for example, Lake and Hughes, 1999). Increases in CO2 could benefit at least one species of melon (Cucumis melo). Average nectar volumes per flower were significantly higher, sometimes by as much as 100 percent (Dag and Eisikowitch, in greenhouses enriched with CO2. No comparable

greenhouse or field studies seem to have addressed the potential for CO2 enrichment to affect pollen production.


Elevated intensities of ultraviolet-B radiation (UV-B; wavelengths between 280 and 320 nanometers) result from diminished concentrations of atmospheric ozone and can delay flowering and diminish lifetime flower production in some plants. Sampson and Cane (1999) reported idiosyncratic responses in flowering phenology and flower production in two annual plants, traits that could affect plant competition for pollinator services, and plant and pollinator reproductive success.

Stephanou and colleagues (2000) reported that UV-B increased nectary size in another species, which apparently resulted in an observed increase in pollination, but no differences were reported in honey bee foraging behavior on brassicaceous nectar plants exposed to and protected from UV-B (Collins et al., 1997).


In the Washington, D.C. area, Abu-Asab and colleagues (2001) reported that 89 plant species had advanced flowering time by an average of 4.5 days (although 11 species showed later flowering times). Primack and colleagues (2004) used herbarium specimens of the same individual plants in the Arnold Arboretum in Boston, Massachusetts, to compare flowering times from 1885 to 2002. Plants flowered 8 days earlier from 1980 to 2002 than they did from 1900 to 1920. Flowering by agricultural species also is influenced by global warming: a 40-year study of white clover (Trifolium repens) revealed that flowering has

advanced by 7.5 days per decade since 1978 (Williams and Abberton, 2004).


Several studies demonstrate that pollinator phenology can be influenced by changing global temperatures. The first appearance of most British butterflies has advanced in the past two decades; peak appearance also occurs earlier, and multibrooded species exhibit longer flight periods (Roy and Sparks, 2000). Forister and Shapiro (2003) documented a similar change in California butterflies. The mean date of first flight trended toward earlier dates for 16 species (70 percent of the fauna studied), and the trend was statistically

significant for four of them (average shift of 24 days); seven species showed trends toward later appearance that were not statistically significant. Some Spanish butterflies (8 of 19 species studied from 1988 to 2002) also showed significant advances in mean flight dates (Stefanescu et al., 2003). If the phenology of flowering and pollinator activities does not change synchronously, there is the potential for disruption of coordinated interactions. Plants might

flower before or after the period of seasonal activity of their pollinators and different groups of pollinators might respond differently to a change in temperature. A record-early spring in Japan resulted in drastic decreases in seed set of two species normally pollinated by bees, but not in two others pollinated by flies (Kudo et al., 2004).


A long-term study of life cycles of Mediterranean plants and animals showed that the phenology of plant leafing out, flowering, and fruiting changed at different rates, and all were different from changes recorded for butterfly emergence and the arrival of migratory birds (PeƱuelas et al., 2002). The authors suggested that these changes could alter ecosystem structure and function. Migrating

pollinators (for example, hummingbirds that overwinter in Mexico and reproduce in the United States) depend on corridors with flowers that bloom at the appropriate times during spring and fall migrations.


If the timing of the migration does not coincide with flowering, the plants could suffer a loss of pollinators and the pollinators could face energetically expensive migratory flights with no opportunity to forage and replenish metabolic fuel along the way. Thus, the evidence indicates that plants and their pollinators could respond differentially to climate change. Depending on the degree of variations in their responses, the consequences of climate change

could range from subtle to dramatic. Alterations in nectar abundance or concentration could change the foraging behavior of pollinators, increasing or decreasing pollination of one flower by another of the same plant (geitonogamy); changing the quantities of pollen collected or deposited or the distances that pollen is transported--all can have significant effects on plant mating systems and genetic parameters.


Changes in floral abundance could in turn influence the abundance and distribution of pollinators. The loss of synchrony that could result from differential responses in phenology of plants and pollinators could be important and possibly result in the loss of some historical mutualisms or the creation of new ones. It appears that this area of research warrants more attention, in view of the potential for climate change to disrupt plant-pollinator interactions significantly in the future.


The combined effects of climate change and other environmental changes (such as habitat fragmentation) have not been assessed for most pollination systems, but Warren and colleagues (2001) reported that 34 of 46 British butterfly species that might be expected to respond positively to climate warming at their northern climatic range margins in fact declined, as negative consequences of habitat loss outweighed the positive responses to climate warming over the

past 30 years. Although half of the habitat generalists that also were mobile species increased their distributions, the other generalists and 89 percent of the habitat specialists declined in distribution, suggesting that the diversity of pollinators could decline substantially in the face of the combined pressures of climate change and habitat loss. The potent combination of

environmental changes could cause substantial harm to many plant-pollinator interactions.




Just as different species of pollinators differ in the degree to which their diversity and populations have declined, the causes that underlie decline vary widely. Some mortality is particularly important in a narrow range of pollinators; in managed pollination systems, there is clear evidence of reductions in pollinator numbers caused by introduced parasites and pathogens. The evidence indicates that these agents of mortality also could operate in wild pollinator declines. Other causes of mortality affect a cross- section of

pollinators (albeit to different extents); habitat degradation and habitat loss, in their many manifestations, have contributed to declines in many vertebrate and invertebrate pollinators.



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