Synthesis of Kremen et al. 2002

Intensified land use in agriculture, for furnishing long-term demands for food, livestock and biofuels, has lead to reduction of natural and semi-natural habitats (Tscharnkte et al 2005, Bianchi et al. 2006, Tschanrtke et al. 2007). The conversion of these habitats severely impacts crops that entirely rely on pollinators (Klein et al. 2007) and natural enemies (Landis et al. 2000) for high yields. Natural habitat is also harmed by having reduced seed sets because these habitats also rely on these beneficial insects (Steffan-Dewenter and Tscharnkte 1999). As this intensification increases, landscapes changes from diverse crops and pastures in small fields, to large fields of one or two crops type with little to no non-crop habitat (e.g., corn and soybeans; Burel and Baudry 2003).

Natural and semi-natural habitats that can be retained in turn may provide for greater number of beneficial insects and the services that they provide (e.g., Holzschuh et al. 2007, Kremen et al. 2007, Winfree et al. 2008). These habitats are stable resources of perennial plants that are necessary for pollinators, predators and parasitoids, by supplying a continuous nectar resources, alternative prey sources and natural nesting habitats (Kremen et al. 2004, Winfree et al. 2008, Tschanrtke et al. 2007). High landscape heterogeneity with more non-crop habitat has a higher diversity of flowering and perennial plants and therefore a higher diversity of beneficial insects (Holzchuh et al. 2007, Klein et al. 2007, Tschanrtke et al. 2007).

Landscapes with high heterogeneity in land use/land cover type, should have higher levels of species diversity (Burel and Baudry 2003, Tscharkte et al. 2007). Billiter et al. (2008) found that there is a positive correlation between species richness of plants and arthropods. Positive influences on beneficial insects have been seen with the spatial arrangement and larger amounts of perennial habitat (Tscharnkte et al. 2007, Marini et al. 2009).
In this synthesis, we will be examining pollination services, the impacts of pest suppression on pollinators, and how natural habitat supplies necessary resources for these beneficial insects, and how the Kremen et al. (2002) paper considers near-by natural habitat essential for pollination of watermelon crops by native bees, which are seen as having the ability to provide equivalent service to that of managed honey bee populations.

Natural Habitat
Agricultural landscapes with diverse land uses may support high levels of biodiversity of insects and provide a range of ecosystem services (Swinton et al. 2007, Zhang et al. 2007). Insects comprise the most diverse and successful group of multicellular organisms on the planet, and they contribute significantly to vital ecological functions such as pollination, pest control, decomposition, and maintenance of wildlife species (Nee, 2004). A study by Losey and Vaughn (2006) shows that the value of insect services is almost $60 billion a year in the United States, which is only a fraction of the value for all the services insects provide. Indeed, these service-providing insects are under ever increasing threat from a combination of forces, including habitat fragmentation, land cover changes, invasion of non-native plants and animals, and overuse of toxic chemicals. The evidence indicates that beneficial insects are under a steady decline, associated with an overall decline in biodiversity (Kremen et al. 2002). New evidence indicates that in some situations, the most important species for providing ecosystem services are lost first. The overall, gradual decline in species, coupled with nonlinear changes in service levels, makes it difficult to pinpoint an optimal level of annual investment to conserve beneficial insects and maintain the services they provide (Bianchi et al. 2006).

Pollination
Animal pollination is essential for the successful reproduction of most flowering plants. Pollination provides for the transportation of pollen grains, eventually fulfilling both the ecosystem service of fruit and seed production (Kremen et al. 2002). Of the estimated 240,000 species of plants, nearly 220,000, including 70% of the crops that feed human begins depend on animals, mainly insects for pollination. Research shows that over 100,000 animal species including bats, bees, beetles, birds, butterflies, and flies are responsible for providing most pollination services that perpetuate our managed ad natural fields (Daily et al. 1997).
Pollination is perhaps the best ecosystem service performed by insects (Losey and Vaughan 2006). Estimates show that about one third of our food crops are pollinated by wild pollinators. Therefore, without wild pollinators farm yield would be adversely affected, and the survival of wild plants would be uncertain. Among insects, bees constitute the most dominant taxa in providing crop pollination (Zhang et al. 2007), pollinating more than two-thirds of the world’s 1,500 crop species (Kremen et al. 2002).

Pollination by bees, whether managed or native plays one of the most important roles in ensuring survival of food and non-food crops. In the US, pollination by native bees is very important. A study conducted by Cornell University in 2000 estimated the value of bee pollination in United States to be $14.6 billion (Morse and Calderone, 2000). In addition, pollination by honey bees which was imported from Europe also plays a great role in crop pollination. However, there have been great concerns about the invasion and success of the more aggressive African honey bee and the decline of native bees. Questions abound as to whether put more conservation efforts in protecting managed honey bees, or divert most of the resources in managing natural ecosystems upon which native bees depend. Another recent study by Kremen and Vaughan (2009) still on watermelon, a crop with high pollination demand, observed that wild bee community can provide sufficient pollination services for watermelon. Thus conserving native habitats can perpetuate the life of native pollinators and meet pollination needs of our crops. However, it is apparent that higher bee diversity, both managed and native correlates with higher diversity of flowering plants (Kremen et al. 2002).
Perhaps the importance of bee pollination in the US can be explained by watermelon production. Between 1997 and 2000, watermelon production in the US rose by 20% with the number of bee colonies rented in for watermelon production reaching 2000 (Morse and Calderone 2000).

Pest Control
The simplification of landscape composition and the decline of biodiversity may affect the functioning of natural pest control because non-crop habitats provide requisites for a broad spectrum of natural enemies, and the exchange of natural enemies between crop and non-crop habitats is likely to be diminished in landscapes dominated by arable cropland. Pest control has often been highlighted as an important ecosystem service provided by biodiversity (Mooney et al. 1995) and one that is threatened by modern agricultural practices (Naylor and Ehrlich 1997). Intensification of agriculture tends to simplify landscapes systems (Swift et al. 1996), reducing natural enemy diversity (Basedow 1990; Andersen and Eltun 2000). Although this is often known to destabilize arthropod populations (Swift et al. 1996), it often results in pest outbreak. A Study by Wilby et al. (2002) shows that with the increasing strength of species composition effects, pest control became less resistant, on average, to reduction of natural enemy species richness. Natural pest control is enhanced in complex patchy landscape with a high proportion of non-crop habitats as compared to simple large-scale landscapes with little associated non-crop habitat. Natural enemy populations were higher and pest pressure lower in complex landscapes versus simple landscapes (Wilby et al. 2002). Landscape-driven pest suppression may result in lower crop injury, although this has rarely been documented. Enhanced natural enemy activity was associated with herbaceous habitats in 80% of the cases (e.g. fallows, field margins), and somewhat less often with wooded habitats (71%) and landscape patchiness (70%). The similar contributions of these landscape factors suggest that all are equally important in enhancing natural enemy populations. Diversified landscapes hold most potential for the conservation of biodiversity and sustaining the pest control function (Bianchi 2006).

Conclusion and Synthesis
The finding of Kremen et al. (2002) shows that by having natural and semi-natural habitat near crop patches, that native bee community can provide pollination services that are equivalent to managed honey bees. Availability of critical resources influences the abundance and diversity of pollinators in the wild (Kremen et al. 2007). By having natural and semi-natural habitat, which has the ability of providing these necessary resources, near crop habitat, the amount and stability of pollination services generally increased (Kremen et al. 2002, Kremen et al 2004); but when agriculture is increased, there is a reduction in the diversity and abundance of pollinators by reducing and altering, in both space and time, the availability of floral resources (Kremen et al. 2007).

Bee foraging and pollination are affected by spatial arrangement of additional flowering patches, the diversity of flowers in these flowering patches, the spatial scale at which the flower visitor perceives variation flower abundances and the distribution of nesting sites (Kremen et al. 2007, Steffan-Dewenter et al. 2002, Tscharnkte et al. 2007). In agricultural fields, that are isolated from natural and semi-natural habitats, the diversity and abundance of native bees and the pollination services that they provides were below that necessary threshold to produce marketable products (Kremen et al. 2002). Overall complexity of the landscape is reduced during agricultural intensification because natural non-crop habitat is fragmented and is located in less productive areas at greater distances to farms. Therefore, managing for diversity of bees, though both farming practices and including natural and semi-natural habitat in close proximity, pollination requirements can be met (Kremens et al. 2002, Tscharnkte et al. 2007).


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