The production of vegetables and other crops, while distinct from animal agriculture, involves practices that result in the unavoidable mortality of wild animals living in or near the fields. This impact is generally focused on small, ground-dwelling species, such as field mice, voles, shrews, birds, and insects, that inhabit agricultural landscapes. Understanding the scale of this harm requires an objective look at the physical and chemical processes inherent to modern crop production.
Mechanisms of Animal Mortality in Crop Fields
The use of heavy machinery in agriculture is a primary cause of direct animal fatalities in crop fields. Tillage equipment, such as plows and cultivators, disrupts the soil structure, often crushing or dismembering small mammals, reptiles, and amphibians living in burrows. High-speed harvesting machinery, like combine harvesters, also causes direct death by striking or running over animals attempting to flee the rapidly moving equipment.
Chemical agents used to protect crops represent a second major mechanism of mortality. Pesticides, including insecticides and fungicides, are designed to eliminate organisms that threaten the crop yield, leading to the direct death of vast numbers of insects and other invertebrates. Herbicides, while targeting plant life, can eliminate the food sources and shelter for many small animals, leading to indirect fatalities from starvation or exposure.
Rodenticides are sometimes intentionally deployed to control specific pests, resulting in direct poisoning. Secondary poisoning occurs when predators, such as birds of prey or carnivores, consume poisoned prey, accumulating lethal doses of the toxin. The continuous cycle of planting and harvesting also constitutes habitat destruction by periodically removing protective cover, which exposes animals to predators or the elements, leading to displacement or death.
Methods for Estimating the Scale of Animal Deaths
Quantifying the total number of animals killed during crop production is a complex task, requiring researchers to rely on limited data and modeling to derive estimates. One common methodology involves conducting surveys of animal populations in test plots before and after specific farming activities, such as tillage or harvest. Researchers estimate mortality rates by counting animal carcasses found or tracking the population density decline of small mammals.
Early, often-cited studies focused on rodents, such as field mice and rats, due to their high population densities and susceptibility to machinery. These studies estimated mortality rates by tracking radio-collared animals during harvest or by extrapolating from observed population declines. Research has estimated animal deaths ranging from approximately six to forty small mammals per acre of grain harvested, though these figures are highly specific to the species, region, and machinery used.
The variability of these estimates underscores the difficulty in establishing a definitive number. Most scientific models focus primarily on small mammals, like voles and shrews, because their ground-level habitation makes them most vulnerable to equipment. The population dynamics of these species, which can experience rapid increases in density, also mean that the number of animals present in a field fluctuates significantly throughout the growing season.
Researchers must use ranges and approximations rather than single figures because mortality is highly dependent on local conditions and farming practices. One estimate, based on averaging mortality rates from rodent studies, suggested a rate of about 15 field animal deaths per hectare per year. The lack of comprehensive, up-to-date empirical studies across diverse crop types means that all current large-scale estimates rely heavily on extrapolating limited localized data.
Variables Influencing Mortality Rates
The wide range of mortality estimates is largely explained by the variability in agricultural practices and environmental factors. The type of crop being grown significantly influences the impact on wildlife populations. Crops that require extensive soil disturbance, such as root vegetables or those grown in flooded conditions like rice, typically result in higher mortality due to the greater disruption of subsurface habitats.
Conversely, perennial crops like tree fruits or nut orchards, which involve less frequent and less intensive soil work, tend to have lower associated mortality rates. The farming method is another major factor. Conventional tillage systems cause high mortality by physically turning the soil and destroying burrows. No-till or minimal-till practices, which leave the soil structure largely intact, are associated with lower animal mortality because they reduce physical trauma to ground-dwelling species.
The application of irrigation can also influence mortality, potentially causing localized drowning or forcing animals to evacuate their burrows, increasing their exposure to predators. Geographic location and seasonality introduce further variation. Migration patterns or seasonal rodent population booms will affect the number of animals present in a field when farming activities are performed.