Pesticides and herbicides
Chemical toxins are the most common way to control agricultural pests like insects, rodents, and fungi. However, these toxins also have serious detrimental effects on wildlife, including migratory species.
For sedentary populations, it is relatively simple to discern the causes and effects of toxic exposure. But for migratory populations, symptoms might not be observed until the animals have traveled far away from where the initial exposure occurred.
- A complete understanding of migratory connectivity is needed to understand the carry-over effects of chemical exposure and the ultimate causes of population declines.
- Chemicals like DDT are still used outside the United States, and can reduce the lifespan and reproductive success of species that are familiar summer breeders in the USA.
- In the 1990s, fewer and fewer Swainson’s hawks returned from winter migration. Wintering ground activities were the likely culprit, but nobody knew exactly where this was. In 1995, the hawks were tracked to Argentina, where 20,000 hawks were found dead or dying from pesticide-laden grasshoppers. Conservation efforts could not begin to succeed without migratory connectivity knowledge.
- Declines in dickcissel populations could not be addressed until their wintering locations were discovered in Venezuela, where it became known that local farmers killed thousands annually hoping to reduce crop damage.
The hazards of mechanization
Modern technology has made it possible to mechanize almost every aspect of agriculture from planting to harvesting. Unfortunately, mechanical farming is extremely dangerous for birds that are prone to nesting in agricultural fields.
- Every year, tens of thousands of tricolored blackbird chicks are killed by silage harvesting just before they fledge.
- Bobolinks and many other grassland species are also killed during harvest.
- Mechanized applications expand the use of chemical pest control, reducing food supplies for breeding birds.
- It is essential to know where breeding populations winter to accurately estimate carry-over effects.
The conversion of native habitat to monocultured fields homogenizes vegetation, protective cover, and food resources. Domesticated grazing animals also wreck havoc on many types of habitat, including ecologically important riparian zones. Land conversion and fragmentation has negative consequences at all stages of a migratory animal’s annual cycle.
- Loss of important stop-over habitat during migration may force populations to graze in cultivated fields, increasing exposure to pesticides
- Loss of wintering grounds can mean death for populations of the monarch butterfly, or negative carry-over effects for birds.
- Changes in migratory behavior can weaken the condition of migrating individuals and contribute to population decline.
Migratory connectivity knowledge helps managers protect the most important landscapes and mitigate long-term effects of habitat fragmentation on connected populations.
- Fish farm effluents contain excrement and chemicals and can cause severe declines in water quality of pristine rivers and lakes
- When wild salmon fry migrate past fish farms, they are at risk of contracting deadly parasites
- Some fish farms completely block important migratory headwaters
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