Pesticide Impact on Wildlife Ecology
The term wildlife, as used here, shall include insects, spiders, mammals, birds, fish, amphibians, reptiles, and plants. Each species fills a certain niche, which includes its specific food, cover, water, space, and breeding site preferences. The location where a species can meet all of its living requirements becomes that species' habitat. Wildlife habitats are not just the Grand Canyon, ancient forests of the Pacific Northwest, or rich coastal marshes off of the eastern seaboard; they exist across the American landscape. Wildlife habitats, large and small, native and man-made, exist in urban settings, in agricultural fields, and in the wilderness.
Wildlife ecologists and natural resource managers study the needs and habits of wildlife. An important goal of wildlife research is to discover and understand the critical factors that affect survival and sustainability of viable populations. Most wildlife will adapt and flourish, given sufficient quantity of quality habitat, even in the presence of people. While ecological studies may pinpoint very specific requirements for individual species, the lives of plants and animals and their habitats can be integrated collectively into a matrix (ecosystem).
Knowledge of the biological and ecological relationships of any given plant or animal, and the role that species plays in the ecosystem, is required to evaluate the potential impact of a specific pesticide on a specific species. The impact of a specific pesticide may be negative, neutral, or positive to a species or its habitat as the chemical's residues move through the soil, water, food, or air. The interaction of wildlife, its habitat, and pesticides is evaluated by scientists trained in wildlife ecology, population dynamics, physiology, and environmental chemistry.
Pesticide Poisoning of Wildlife
Pesticides are applied in many forms via various delivery methods to forests, rangeland, aquatic habitats, farmland, rights-of-way, urban turf, and gardens. Their widespread use makes contact with pesticide residues inevitable for some wildlife. Pesticide poisonings to wildlife may result from acute or chronic exposure. Additionally, pesticides may impact wildlife via secondary exposure or through indirect effects to the animal or its habitat.
Short exposures to some pesticides may kill or sicken wildlife. Examples of acute wildlife poisoning include fish kills that are caused by pesticide residues carried to ponds, streams, or rivers by surface runoff or spray drift, and bird die-offs caused by foraging on pesticide-treated vegetation or insects, or by consumption of pesticide-treated granules, baits, or seeds. These types of poisonings generally can be substantiated by analyzing tissues of affected animals for the suspected pesticide or by investigating impacts on biochemical processes (e.g., cholinesterase levels in blood and brain tissue). In general, acute poisoning to wildlife takes place over a relatively short time, impacts a very localized geographical area, and is linked to a single pesticide.
Exposure of wildlife over an extended period of time to pesticide levels not immediately lethal may result in chronic poisoning. The most well-known example of a chronic effect in wildlife is that of the organochlorine insecticide DDT (via the metabolite DDE) on reproduction in certain birds of prey. DDT and other organochlorine pesticides such as dieldrin, endrin, and chlordane have been implicated in bird mortality resulting from chronic exposure. The reduction of these compounds in the 1970s, and early 1980s, has resulted in decreased organochlorine residues in most areas, and reproduction in birds, such as the bald eagle, has greatly improved. Organochlorine pesticides used in some foreign countries may pose risk to migratory birds which overwinter there.
Pesticides may impact wildlife through secondary poisoning when an animal consumes prey species that contain pesticide residues. Examples of secondary poisoning are (1) birds of prey becoming sick after feeding on an animal that is dead or dying from acute exposure to a pesticide, and (2) the accumulation and movement of persistent chemicals in wildlife food chains.
A pesticide may affect wildlife in ways other than direct or secondary poisoning. Pesticides may impact wildlife indirectly when a part of its habitat or food supply is modified. For instance, herbicides may reduce food, cover, and nesting sites needed by insect, bird, and mammal populations; insecticides may diminish insect populations fed on by bird or fish species; insect pollinators may be reduced, thereby affecting plant pollination. The study of indirect effects is an emerging area and one that may be difficult to investigate.
Investigating Pesticide Effects on Wildlife
Not all pesticides have detrimental effects on all wildlife, nor do pesticide residues necessarily lead to serious consequences for wildlife. The potential impact must be evaluated by simultaneously considering the availability of the pesticide, or its degradation product(s), the toxicological properties of the pesticide, and the ecological characteristics of the exposure. Due to the complexity of these issues, many scientific disciplines must play a role in both the studies and the interpretation of results. The results from scientific studies aid numerous federal and state natural resource agencies to assess and manage the effects of pesticides on wildlife, including endangered species.
The degree of direct impact a pesticide has on wildlife is determined by the sensitivity of a species to the chemical and the degree to which the species is exposed. Just how safe are pesticides to wildlife? The following questions help to summarize the complexity that biologists and toxicologists face when attempting to evaluate pesticidal effects on wildlife.
What level of a pesticide residue or its breakdown product (metabolite) is introduced into a wildlife habitat through direct application or via the transportation of residues in air, water, food, or soil?
How long does the pesticide remain in the environment?
Is the animal or plant exposed to the pesticide by mechanisms including dermal contact, inhalation, or consumption of contaminated food or water?
Is the pesticide capable of producing biochemical effects, illness, or death through either single or multiple exposures?
Authored by Fred Whitford, et al.
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