Mad cow disease is good for grassland birds: Exploring novel socioeconomic links to wildlife ecology

Background/Question/Methods   Many prairie bird species in North America, which have adopted agricultural grasslands as surrogate habitat, have experienced dramatic population declines in recent decades. The dangers of using such habitat are well-known, ranging from pesticide use to direct mortality from hay harvest. Although many proximate causes of population reduction have been identified in these habitats, very few studies have examined the ultimate causes of such outcomes. Many agricultural grasslands in North America are hayfields that supply forage for cattle operations, so any factor which affects cattle supply would likely in turn affect grassland birds; however the stability and timing of this relationship is less intuitive. We examined the role of a major pressure on global beef markets (outbreaks of bovine spongiform encephalopathy (BSE)), and the timing of any subsequent effects on North American hayfields and grassland birds. To do this, we obtained data on all BSE cases over the past 20 years and then focussed on the five countries with the highest prevalence (UK, Ireland, France, Switzerland, and Portugal) and acquired annual records for each focal country on all cattle imports and exports to and from North America. We obtained US and Canadian statistics for the amount of hayfield harvested in each of those years, as well as an annual index of grassland bird abundance derived from the Breeding Bird Survey. We used d-sep tests to examine the plausibility of several directed acyclic graphs (DAGs), which varied only in the lag time between each step. Each DAG expressed BSE as influencing both imports and exports, which in turn affect the amount of forage needed to maintain livestock; the amount of hay harvested then affects the grassland bird population index.

Results/Conclusions   We found support (χ2 = 7.23, pr|data = 0.512) for the DAG that included a one-year lag between each step wherein: 1) imports increase and exports decrease in response to BSE outbreaks in the previous year, 2) the hay harvest in North America is then reduced two years after the outbreak as fewer cattle remain in North America, which ultimately yields 3) a positive response in grassland bird populations in the year after hay harvest reduction – three years after the BSE outbreak. Our results illustrate the importance of socioeconomics in conserving North American avifauna – such knowledge will allow us to improve management recommendations and better predict future periods of adversity to bird populations.