Causes Of Endangerment

Insects become endangered because of the same destructive forces faced by many other animals. According to the IUCN, the leading causes of animal endangerment are habitat destruction, displacement by introduced species, alteration of habitat by chemical pollutants (such as pesticides), hybridization with other species, and overharvesting. Many at-risk insects are threatened by more than one of these causes. For example, according to the Natural Heritage Program there are six tiger beetles and 33 butterflies that are imperiled or federally listed under the U.S. Endangered Species Act. The major threat to all six tiger beetles is habitat degradation and loss. Two of these beetles also are threatened by overcollecting. For the 33 butterflies, 97% are threatened by habitat loss, 36% by alien species, 24% by pollution, and 30% by overcollecting.

Insects as a group are not at risk because many species are generalists or widely distributed. A significant proportion of the total diversity of insects, however, is composed of species that are highly specialized or are restricted to one or a few small patches of habitat. The giant flightless darkling beetle, Polposipus herculeanus, for instance, lives only on dead trees on tiny Frigate Island in the Seychelles. The stonefly Capnia lacustra exists only in Lake Tahoe and is also the only stonefly in the world known to be fully aquatic in the adult stage. Another unusual stonefly, Cosumnoperla hypocrema, is known from only one intermittent spring in the Cosumnes River Basin in California.

Habitat Destruction

Agriculture, commercial development, outdoor recreation (including off-road vehicles), pollution, and water development rank as the most frequent causes of habitat degradation affecting federally listed endangered and threatened insect species in the United States (Fig. 1). Commercial and residential developments often are situated on sites that have naturally high diversity, such as along rivers or near bays and estuaries. Urban development in the southeastern United States and California has had particularly strong impacts on native insects because of the high rates of insect endemism

Commercial development Agriculture Outdoor recreation Pollutants Infrastructure development Water development Disruption of fire regimes Livestock grazing Mining, oil and gas, geothermal

Logging m

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FIGURE 1 Percentages of the 43 U.S. federally listed threatened and endangered insect species affected by different causes of habitat destruction or degradation, as of December 2001. It is important to note that the habitats of most listed species are being degraded by more than one cause. (Bar graph format modified from B. A. Stein et al., 2000. Data modified from D. S. Wilcove et al, 1998, Quantifying threats to imperiled species in the United States. Bioscience 48, 607-615.)

where these cities were built. The best known case is that of San Francisco, California, which now almost entirely covers what was once one of the major coastal dune ecosystems in western North America. Three dune butterflies, which were endemic to this region, are now extinct: Cercyonis sthenele sthenele, Glaucopsyche xerces (Fig. 2), and Plebeius icarioides pheres. Three other butterflies, Speyeria callippe callippe, Callophrys mossi bayensis, and Plebeius icarioides missionensis, are now limited to the San Bruno Mountains just south of San Francisco, the last remnant of the San Francisco hills ecosystem.

Conversion of natural habitats for agriculture, particularly for planted food and fiber crops (e.g., cotton), is one of the most extensive land uses and, according to Robert Pyle (a noted lepidopterist and author), has resulted in the greatest loss of native insect populations. The most serious losses of endemic insects to agricultural conversion have taken place in the tropics, but because of the lack of knowledge of insects in these regions, it is impossible to know the extent of this destruction.

Dams and other water development are implicated in the decline of 21% of federally listed insect species. Impoundments destroy habitat for native aquatic organisms, such as stoneflies, as well as some terrestrial insects. For example, the damming of the Columbia River in Oregon and Washington resulted in the destruction of much of the sand bar habitat of the tiger beetle, Cicindela columbica.

Although we have no numbers, insects most likely are lost to large-scale timber management. Studies have shown that there is higher invertebrate diversity, as well as endemism, in late successional forests than in younger stands, and less than 10% of U.S. native forests remain intact. Widespread use of off-road vehicles also threatens some species. For example, vehicles have crushed the larval burrows of the tiger beetle, Cicindela dorsalis, along beaches to such an extent that this

FIGURE 2 The Xerces blue butterfly (G. xerces), was one of the first butterflies in North America known to become extinct as a result of human interference. It was driven to extinction as San Francisco expanded over the butterfly's habitat. (Photograph courtesy of C. B. Barr and the Essig Museum of Entomology, University of California, Berkeley.)

FIGURE 2 The Xerces blue butterfly (G. xerces), was one of the first butterflies in North America known to become extinct as a result of human interference. It was driven to extinction as San Francisco expanded over the butterfly's habitat. (Photograph courtesy of C. B. Barr and the Essig Museum of Entomology, University of California, Berkeley.)

once widespread, abundant species has been eliminated throughout most of its range. Wetland draining also has taken its toll. The draining of fens in England caused the extirpation of the butterfly Lycaena dispar in 1851 and possibly other insects as well. Capping of springs led to the loss of the fritillary butterfly, Speyeria nokomis coerulescens, in the U.S. portion of its range.

The biggest unknown is, of course, the loss of tropical rainforest. Tropical rain forests may hold the majority of terrestrial insect diversity and are being converted to agriculture and other uses at an alarming rate. As rainforests around the world are clear-cut, insects are bound to go with them.

Alien Species

The introduction of various exotic organisms (whether intentional or not) has affected native insects, both directly and indirectly. For example, introduced plants may out compete native plants and, thus, lead to the loss of insect host plants or habitat. Introduced plant diseases also can wreak havoc on insect populations. A classic example involves the American chestnut. Mature examples of the tree disappeared throughout its range following the accidental introduction of chestnut blight. At least five microlepidopterans, including the chestnut borer, Synanthedon castaneae, are believed to have gone extinct because of the loss of their host plant. Some aquatic insect species are restricted to small mountain lakes in the United States and have been impacted by introductions of nonnative fish. On the Island of Oahu, a species of Megalagrion damselfly is uniformly absent in stream reaches where nonnative mos-quitofish in the family Poeciliidae have been introduced.

Intentional introductions of insects also many harm native insects. Over the past 50 years, nonnative insects often have been released to control nonnative pest insects. Although the damage to nontarget, native insects from these biological controls is rarely documented, some evidence is surfacing that it may be significant. For example, a parasitoid fly, Compsilura concinnata, that was released repeatedly in North America from 1906 to 1986 as a biological control against several pests, including the introduced gypsy moth, is implicated in the declines of four species of giant silk moths (Lepidoptera: Saturniidae) in New England. Another study in Hawaii found that 83% of parasitoids reared from native moths were former biological control agents.

Overcollecting

Although overcollecting has not been shown to harm healthy populations of insects, it may be an important threat to insect species with very small populations and is included in the list of threats to many of the federally protected insect species in the United States. The Endangered Species Act expressly forbids the collection of endangered or threatened species, and most insect conservationists feel that collecting from small populations should be done only for well-

designed, hypothesis-driven, scientific studies. It is not too much to ask that scientists rise to this standard when studying populations that are at risk.

Other Potential Threats

Pesticides and other pollutants are implicated in the decline of many native bees and some aquatic insects, although the degree of impact is not conclusive. Lights along streets and highways also have been implicated in losses of nocturnal insects, particularly large moths. Finally, even though we cannot specify the exact effects of climate change at this time, it could lead to endangerment of endemic insects with specific, narrow habitat requirements. A changing climate may be especially detrimental to species that cannot disperse, like the Uncompahgre fritillary butterfly (Boloria improba acrocnema), which is restricted to high mountain slopes in southern Colorado.

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