In a broad sense, ecology represents the sum total of interrelations between organisms and their environment. In the case of orthopterans, the basic requirements of food and moisture; shelter, including protection from weather and from enemies; favourable habitats, involving special niches such as caves or deserts; as well as preferred seasons and conditions conducive to successful reproduction, are involved.
Mantids are the only orthopteran group that feeds almost entirely on insects, but some members of primarily plant-feeding groups also capture and devour insects. For instance, tree crickets (Oecanthus) regularly eat flowers, leaves, and other plant parts, in addition to many aphids and other weak insects. Some katydids are active predators of insects. Most cockroaches are typical scavengers, but some are specialized feeders. Cryptocercus, for example, digests cellulose in decaying logs with the aid of symbiotic protozoans in its intestines. All walking sticks and a majority of grasshoppers are plant feeders. Although many grasshoppers feed on a wide variety of plant species, some are restricted to a single plant species or one group of plants. Some orthopterans consume only certain parts of plants; for example, coneheaded katydids of the genus Neoconocephalus feed mainly on seeds of grasses. Plant preferences among some leaf-eating orthopterans change with the seasons of the year; in other leaf-eating groups feeding habits are dependent on the stage of the life cycle—i.e., nymphs do not eat the same plants as adults of the same species. Moisture requirements of orthopterans vary, as evidenced by the habitats they occupy. Some can absorb water from a drop on the cuticle (skin); others obtain it from water vapour in the air if the relative humidity is sufficiently high.
Shelter utilized by orthopterans ranges from general hiding places amid living plant foliage or dead leaves on the ground to special structures such as subterranean galleries in soil or the recesses in caves occupied by various crickets. Some Gryllacrididae are leaf rollers and produce silk to maintain the rolled shape of their hiding places. The loose bark of trees and logs and the water-filled leaf bases of bromeliads often shelter certain genera of cockroaches, some of which are semi-aquatic in their habits. In Africa a few cockroaches (Cyrtotria), of elongated and cylindrical body shape, are adapted to enter round holes in hollow plant stems where they sometimes live. With the exception of cockroaches, most conspicuous orthopterans are active by day (diurnal), although there are nocturnal species in every group. Although grylloblattids are essentially nocturnal, they are sometimes active on cloudy days or in winter. The majority of crickets and katydids are nocturnal, as are many walking sticks and some mantids; however, many mantids prey on insects that visit flowers by day.
The degree of moisture, types of vegetation, and altitude above sea level influence the location of orthopteran communities. Grasshoppers breed in the Himalayan Mountains at altitudes as high as 6,000 metres (about 18,000 feet), and each mountain altitudinal zone has distinctive species; fully winged, actively flying species are usually not restricted to a single zone but are found in adjacent ones. On the other hand, at high altitudes there are proportionately greater numbers of grasshoppers with short, nonfunctional wings or none at all.
Caves are a special habitat occupied by orthopterans on all continents. The long-horned grasshoppers and the crickets are the principal orthopteran representatives; nearly 200 species of these two groups have been found in caves. In addition, more than 30 cockroach species inhabit caves; and a third group, the grylloblattids, has at least one cavernicolous species in Japan and three in the U.S. Some of these orthopterans inhabit lava tubes and fissures resulting from past volcanic action. Air currents and high humidity in these tubes and fissures produce an “ice cave” condition. In the U.S., the grylloblattids and a few dozen cave crickets (Gryllacrididae) are the principal cavernicoles. It is noteworthy that a bone from a bison skeleton, found in a French cave in the 1920s, bore a prehistoric carving that depicted Troglophilus, a European cave cricket.
Usually the orthopteran species found on a given continent are distinct from those of other continents, especially if the land masses are well separated. For example, there are about 2,000 species and 500 genera of grasshoppers in Africa; although several of the genera are found in North America, none of the species is. Some species in North Africa, however, also inhabit southern Europe and western Asia. Explanations for distinct continental species are found not only in the far, overwater distances involved but also in the long periods of isolation that have occurred and in the different conditions that have prevailed in past geologic periods.
Oceanic islands have been populated, in part, by species that were transported by hurricanes, floating debris, birds, and in recent centuries by human activities. The 85 species of Hawaiian orthopterans include four distinct genera of crickets and katydids comprising about 45 of the 85 species. Evidently some of these evolved following the establishment of a few parent species. The remainder are believed to have been established as a result of the activities of man. Since cockroaches are scavengers, they are often found where man is found; i.e., in his buildings and campsites. Early man probably spread cockroaches as he moved about seeking food. Modern commerce has been even more helpful to these unwelcome travellers. An analysis of the distribution of 11 domiciliary species found in the U.S. and related species found elsewhere suggested that five reached America in shipping from West Africa; another African species might not have reached America directly; two probably came from Europe; two might have come from the Orient; and one was native to the West Indies. Thus man has played an important role in spreading cockroaches throughout the world.
The familiar, large, black field crickets of the U.S. are good examples of ecological differences among similar species. There are six native species in the eastern U.S.; several others occur in the western states, and at least two introduced species have become established in the Gulf States. Using the general appearance of dead specimens, the native eastern species are very similar and difficult to distinguish. For many years, the taxonomy of these species was unsettled. Entomologists using behaviour rather than morphology as a major taxonomic criterion have found that five of the six species have distinct songs; that four of them overwinter as nymphs, the other two chiefly as eggs; and that, to a considerable extent, the habitat preferences are different—i.e., one species lives in abandoned fields, another in leaf litter of open woodland. Laboratory attempts to crossbreed males and females of different species have been unsuccessful; the pair either failed to copulate or, if they did, produced unhealthy hybrids. Ecological differences also are important for other groups of closely related orthopterans.