The following are the major threats cited in decline and loss of insect species and assemblages.
Many insects depend on very intricate and specialized ecological conditions for their survival, so that critical habitat and resource parameters can be very subtle. Whereas destruction of a forest, for example, is an obvious form of habitat loss, relatively small changes in vegetation composition or microclimate may lead to decline of ecologically specialized insects. Many lycaenid butterflies, for example, depend on a tripartite association whereby their caterpillars have obligate mutualistic relationships with particular species of ants, as well as specific larval food plants, so that both of these are critical resources, in addition to the need for nectar sources for the adult butterflies. On a broader scale, many insects are limited to or associated with particular vegetation types, so that any process that diminishes forests, grasslands, heathlands, alpine meadows, mangroves, and many other habitats may harm them. The area needed for many insect populations to be self-sustaining is not large—colonies of many butterfly species can thrive on areas of less than a hectare—so that the widespread pattern of habitat fragmentation through agricultural and urban conversion so damaging to many other taxa may not necessarily be harmful for insects. But, by the same token, even limited habitat destruction or change might exterminate the entire population or species.
Despite wide supposition to the contrary, many insects do not disperse readily or far. Some butterflies are reluctant to traverse even narrow bands of open ground between sheltered or shaded habitats, so even apparently unobtrusive habitat fragmentation (such as by construction of an access road) may have severe demographic and genetic consequences through promoting isolation.
Habitat loss is the paramount threat concern in insect conservation and is potentially universal. Many insect conservation programs stress the need for habitat security and management as the most important single conservation measure. The latter aspect is critical; simply that an insect is represented in a high-quality reserve such as a National Park does not in itself guarantee its well-being, because conditions may continue to change through succession or management for other priorities. For example, in Britain, several butterflies declined following changed grassland management involving removal of grazing by domestic animals or rabbits. This led to the decline of attendant ant species because of changes in ground microclimates from denser overlying vegetation; particular grazing regimes are an integral part of habitat management for such taxa. Simply "locking up" a habitat in a reserve may be a vital first step in ensuring security, but is not an end point in conservation practice.
Replacement of native flora by exotic plants has characterized much human endeavor. In Australia, native grasslands in the south east are regarded as among the country's most endangered ecosystems, having been reduced to around 1%
of their original extent. There is still "plenty of grassland," but most of it is composed of exotic grass species introduced to improve pasture quality for domestic stock. Many insects (including wingless morabine grasshoppers and some Lepidoptera) that depend on native grass species are now of considerable conservation concern, as representing putative remnant populations confined to small patches of their original much wider range. Introduced plants, be they agricultural or forestry crops, weeds or ornamentals, provide opportunity for exotic herbivores to establish and thrive— often as insect pests demanding control in order to protect commercial interests. Classical biological control of introduced pest weeds and arthropods has led to numerous introductions of insect consumers, be they herbivores, predators, or parasitoids. The practice has aroused concern among insect conservationists, because of the propensity of some such taxa to invade natural environments and attack native species, rather than being restricted to the (predominantly) agroecosystem environments where their impacts are needed. Protocols for screening for safety of biological control agents continue to improve, but some recent pest management practices need careful appraisal. Neoclassical biological control (whereby exotic natural enemies are introduced to combat native pest species) is a highly controversial practice, for example, as witness the recent debate over the possible side effects of exotic wasp parasitoids against innocuous native grasshoppers coexisting with the few destructive rangeland species in North America. In such situations, lack of host specificity is a prerequisite, as the agent is to attack "new species"; the agents are thereby seen as predisposed to become invasive and attack a wider host spectrum. For classical biological control, much concern has arisen from isolated island environments such as Hawaii, where there is strong suggestion that extinctions of sensitive native insects have resulted from nonspecific agents invading natural environments. A tachinid fly, Compsilura concinnata, introduced to combat gypsy moth (an introduced major forest pest in North America), is known to attack a wide variety of native Lepidoptera, with recent concerns for its effects on some giant silkmoths (Saturniidae).
Invasive social Hymenoptera, particularly ants such as the Argentine ant, Linepithima humile, and bigheaded ant, Pheidole megacephala, and vespoid wasps (such as Vespula in New Zealand), are known to outcompete native species and to disrupt the structure of natural communities in many parts of the world. As with other invasive taxa, many of these insects are extremely difficult to eradicate once they become established, and continuing spread is a major conservation concern.
Exploitation of insects as a threat revolves largely around issues of "overcollecting," a highly controversial and emotive theme in insect conservation. Collector demands for rare butterflies and beetles, in particular, have led to the listing of a number of species on schedules of the Convention on
International Trade in Endangered Species (CITES) as either prohibited in trade (a few species such as Queen Alexandra's birdwing butterfly, Ornithoptera alexandrae) or for which numbers in trade must be monitored. Much protective legislation for insects prohibits or restricts take of specimens. Individual rare insects can command sums of many thousands of dollars on the black market or more openly in dealers' catalogs. Trade in insects is the predominant aspect of exploitation and has three main components (as nominated by Collins and Morris for swallowtail butterflies): the low-value high-volume trade (mainly in common species, for the souvenir trade and general collector supply), the high-value low-volume trade (of very rare species for collectors), and the live trade (mainly of long-lived showy species for display in butterfly houses). The second of these is the major conservation concern, with potential for illicit measures to circumvent protective measures for exceedingly scarce taxa and which has led to development of butterfly farming (or butterfly ranching) activities to help satisfy demand for high-quality reared specimens. This approach was pioneered in Papua New Guinea and has major conservation benefits in helping to reduce human pressures on primary forest habitats. Rearing butterflies for sale through a centralized government-supported agency has provided income sufficient to curtail needs for continued agricultural development in places and has allowed people to recognize forests as resources on which their sustainable incomes depend.
However, and despite widespread assertion to the contrary, there is very little evidence that overcollecting is a common threat to insect species or populations. It is almost always subsidiary to changes to habitats. Very small, isolated populations of highly desirable species may indeed be "tipped over the brink" by imposition of any additional pressures and mortality, but measures to prohibit take must be seen as a responsible action. They should be balanced against the possible loss of information to be gained from hobbyists, who have contributed most of the information available on collectable insects such as butterflies.
Pesticides are a special category of environmental pollutants, in that they are chemicals designed specifically to kill insects, rather than simply the by-product of industrial and other manufacturing processes. Pesticides can have nontarget effects, with the practice of greatest conservation concern being aerial spraying of insects in noncrop environments, either accidentally or purposely. Wiest's sphinx moth, Euproserpinus weisti, was almost exterminated when its last known site in Colorado was sprayed with malathion, for example, and aerial use of fenitrothion against massing plague locusts in Australia before they reach cropping areas remains controversial and a stimulus to develop alternative management strategies.
Other forms of chemical contamination, of both land and freshwater environments, have been documented as harmful to insect assemblages, and the more widespread acid rains in the northern hemisphere have undoubtedly threatened insects in forest environments. Many local pollutant effects could be cited as occasional threats. Concerns have been expressed widely over effects of insecticides on dispersive pollinators such as honey bees that can forage 5 km or more from their hives.
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