[singlepic id=33 w=320 h=240 float=right]When are multiple species better than one? This question has intrigued ecologists for over a century, and has relevance both for conservation and agriculture. Recently, Ben Werling, Claudio Gratton and their coauthors published work examining the benefits of diversifying potato agroecosystems, both at the top and bottom of the food chain. Looking down from the top of the food chain, the team asked the question: “If and when does having multiple predator species improve control of insect pests?” To answer this question, Ben worked with David Lowenstein – then fresh from the Bronx – and Dr. Cory Straub to examine the effects of predator diversity on predation of Colorado potato beetle, a notorious potato pest. Their results, recently published in the Journal of Insect Science, provide evidence that multiple predator species are better than one – but only under certain conditions. Specifically, they found that the benefits of predator diversity were greater at low prey density. They also suggest a simple mechanism for this effect. Specifically, ecologists hypothesize that competition amongst predators will be lessened in diverse communities, which contain species that all attack different types of prey, compared to lower diversity communities, where individuals are more likely to compete for the same type of prey. Given this, it is reasonable to expect that predator diversity will be more important at lower prey density, where predators are competing for scarcer resources. This suggests that conserving multiple predator species on farmland could be important for keeping pest populations that are currently at low levels from escaping control. In other words, diverse predator communities may help keep pests “down for the count.”
[singlepic id=32 w=320 h=240 float=left]Entomologists have also shown that diversity at the bottom of food chains – at the plant level – also affects the severity of pest problems. Specifically, pest numbers are often lower in more diverse cropping systems compared to those dominated by a single plant species. However, the mechanisms underlying these patterns are often less then clear. On the one hand, increasing plant diversity can have direct effects on pests by making it harder to find – and easier to lose – their host plants. On the other hand, diverse plants may provide more of the resources that predatory insects need to survive and increase their ability to control pests. Ben Werling and Claudio Gratton teamed up with Cory Straub and Jason Harmon – both postdocs at the time – to examine how diversifying potato fields by planting strips of prairie grasses affect both pests and their natural enemies. Their findings – recently published in Biological Control – suggest that prairie grasses increase the abundance of spiders and harvestmen, leading to increased predation of Colorado potato beetle. However, these benefits were limited to the area immediately adjacent to grassy strips. This suggests that planting prairie grasses on farmland could increase natural pest control – but, as in opening a business – location matters. In particular, interspersing patches of crop and natural habitat within a crop field could allow natural enemies to benefit from resources in non-crop areas, while minimizing the distance they have to travel to control pests in crops.
[singlepic id=31 w=400 h=300 float=none]Above, a native warm-season prairie habitat adjacent to potato field at Arlington Agricultural Research Station in Wisconsin.