Invertebrates 



Overview 



Invertebrates are impres- 

 sive in abundance and 

 diversity, living on land and in water and air. 

 Many species are borne to distant places on air 

 and water currents, and via modem transporta- 

 tion. 



Of the millions of species of animals world- 

 wide, about 90% are invertebrates, that is, ani- 

 mals without backbones (Opler, Powell, this 

 section). The arthropods, or jointed-leg inverte- 

 brates such as beetles, account for 75% of this 

 total. More than 90,000 described insect species 

 inhabit North America (Hodges, Powell, this 

 section); the Lepidoptera (butterflies and 

 moths) alone account for about 1 1.500 of these 

 (Powell, this section). 



Within an acre of land and water, hundreds 

 of different invertebrates form an ecological 

 web of builders, gatherers, collectors, predators, 

 and grazers, all interacting with each other and 

 each a necessary component of a healthy 

 ecosystem. The large macroscopic inverte- 

 brates — like bees, beetles, butterflies, grasshop- 

 pers, snails, and earthworms — are well known, 

 but other invertebrates are almost invisible 

 because they are extremely tiny or camouflaged 

 for protection. We have just begun to under- 

 stand the ecology of some commercially impor- 

 tant species, but we understand very little about 

 the behavior, communication, and function of 



many other invertebrates within various ecosys- 

 tems. 



Each individual invertebrate is a highly com- 

 plex, specialized animal. Some molt (change or 

 metamorphose) into several distinct life stages. 

 For example, some insects transform from egg 

 to larva, then to pupa, and finally emerge as a 

 terrestrial winged adult. Some aquatic inverte- 

 brates do not have pupal stages, and the larvae 

 (nymphs or naiads) grow progressively larger 

 by molts. Earthworms bear cocoons that each 

 contain about six miniature juveniles; they also 

 reproduce by fragmentation (architomy). 



Changes to the environment can disrupt 

 basic interactions of invertebrate species, there- 

 by affecting other organisms in the food chain. 

 Disruptions of natural food cycles may cause 

 drastic changes in the community structure and 

 ecological web of life. This is especially true of 

 the fauna that dwell in fragile ecosystems like 

 caves and springs (Webb, see box). Eventually 

 even humans are affected by changes to food 

 webs and destruction of beneficial habitat for 

 wildlife. 



Most invertebrates can survive extreme nat- 

 ural events like severe storms, blizzards, and 

 flooding. When confronted by unnatural distur- 

 bances, however, such as excessive siltation 

 from urban and highway developments, 

 eutrophication (excessive nutrients) by runoff 



