The Effects Of Farmland Heterogeneity On The Biodiversity And Functional Diversity Of Above-ground Insects

The rapid expansion and intensification of agriculture has altered composition of communities and disrupted ecosystem services,yet it is also inevitable,given the high population of humans and their food demands.Thus,it is crucial to implement sustainable agricultural practices that can provide both sufficient food and also protection of ecosystem services and biodiversity.A major finding of agricultural ecology is that retaining some kinds of natural vegetation in proximity to agriculture conserves biodiversity;however,to keep such vegetation farmers must forego planting all their land,at some economic cost.Another idea is that the heterogeneity of crops could positively affect biodiversity,as heterogeneity generally allows for mor~e ecological niches.Having a high number of crops in relatively small crop fields,with a large amount of field margins(including some area for non-crop vegetation)is thus one potential strategy for preserving biodiversity.Yet,although research has supported the link between field margins and smaller fields and biodiversity,few studies have shown a direct association between crop compositional heterogeneity(the number and evenness of crop types)and organismal diversity.To address these issues and research gap,this study examined the effects of crop diversity and the length of field margins on the diversity(taxonomy-based species diversity)and functional diversity of insect communities.The study area was Guangxi Autonomous Province,People's Republic of China;this province is a major producer of sugar and subtropical fruits and vegetables for China.The study sampled 52 pre-existing biodiversity monitoring sites in agricultural areas surrounding the capital city of Nanning.I selected to survey four groups of beneficial insects,Hymenoptera(bees/wasps),Lepidoptera(butterflies),Odonata(dragonflies/damselflies)and Scarabaeinae(dung beetles),because they provide a range of ecosystem functions and the members of these groups are known to be sensitive to habitat changes and are therefore among the best indicators of habitat disturbance.In addition,they are easy to sample using methods such as sweep nets,pan traps and baited pitfall traps.For each taxon,species diversity(i.e.,Shannon-Wiener index)and total abundance were calculated as response variables,and to understand possible costs,I also looked at the abundance of the most common butterfly pest.As explanatory variables,I investigated Shannon-Wiener crop diversity,cumulative field margin length,and the percent of land planted in crops at a local,100-m.scale.Also,I included in General Least Square modeling the type of field margin(including six different structural types),and the percent of crops at a larger 500-m.landscape scale.The study found that dung beetle and dragonfly/damselfly diversity was strongly positively correlated with crop diversity,but was not related to field margin length.In contrast,bees/wasp and butterfly diversity was positively correlated with field margin length,but not related to crop diversity.The abundance of the pest butterfly Pieris canidia was negatively affected by crop diversity.Field margin type was consistently among the most important variables,with weedy margins having higher diversity.The presence of a vegetable crop on one side of the field margin,compared to non-vegetable monoculture on both sides,increased diversity in 3/4 insect group/season combinations for transects with rice,and 6/9 combinations with sugarcane or corn.Previous work on the influence of crop diversity on farmland biodiversity has shown mixed results for most taxa.Our results suggest that crop diversity can be a strong influence on insects that respond to differences among crop types in their soil attributes(e.g.,dung beetles),or in their pest load(e.g.,dragonflies).Our results confirm previous findings that field margins with floral resources such as weeds are useful for retaining a diversity of flower-visiting insects including pollinators,predators and parasitoids.Further,incorporating vegetable crops into monocultural rice and sugarcane or corn can have a positive influence on biodiversity in these subtropical agroecosystems.Thus,this study findings will be useful to make scientifically informed recommendations for the conservation and management of tropical and subtropical agroecosystems.