Altitudinal and Seasonal Distribution of Orthoptera in the Rocky Mountains of Northern Colorado

Abstract

Ninety—four species of Orthoptera (s. lat.) occur along an altitudinal transect from 5,000 ft (1,530 m) to above 14,000 ft (4,265 m) in the Front Range of the Rocky Mountains of Colorado, near 40° N. Lat. These were sampled, with observations on life histories in relation to altitude, at a series of altitudinally spaced stations during the seasons of 1949, 1958, 1959, and 1960, the stations being selected for maximum variety of Orthoptera at each altitude. Other observations of the past 35 years were added. Sampling was by a sweeping technique designed to be qualitatively exhaustive of species and age groups; it was thus quasiquantitative. Times of hatching, times of different juvenile stages, and times of maturing were determined for species at the different altitudes at which they occurred. These data are summarized by species, the 94 species being in the following families: Mantidae, 2; Phasmidae, 1; Tetrigidae, 4; Acrididae, 73; Tettigoniidae, 8; Gryllacrididae, 3; Gryllidae, 3. Numbers of resident species found in 1,000 ft altitudinal bands were as follows: 5—6,000 ft, 71; 6—7,000 ft, 49; 7—8,000 ft, 34; 8—9,000 ft, 25; 9—10,000 ft, 19; 10—11,000 ft, 15; 11—12,000 ft, 9; over 12,000 ft, 3. Larger average numbers of individuals per species occur at higher elevations. Many more species of low frequency occur at low altitudes than high, the total number of species known from a given high altitude station often being present in a single collection, while, at low altitude stations, even the most varied collection lacks species occasionally collected there. The reduction in species numbers with altitude is correlated with a similar reduction with latitude, and the geographic distribution of particular species of Orthoptera is correlated with altitude. Of the 18 species of Acrididae recorded north of 60° N. Lat. in North America nine are common to abundant in the area of study up to 10,000 ft, six of these to 11,000 ft or more. However, three strictly montane species from above 10,000 ft are not known from the far north. The altitudinal distribution of the 36 most common species provided a basis for testing various suggested schemes of altitudinal zonation. A few high plains species that feed only on forbs are apparently restricted in part by the distribution of host plants, and a few grass—feeders do not invade the grassy clearings in montane forest, but the majority of species occurring on the high plains do range into clearings in the lower montane forest up to about 7,000 ft. Pest species with wide altitudinal ranges are essentially indiscriminate plant feeders. The reduction in number of species of Orthoptera with altitude is much greater than the reduction in number of plant species, so variety of plants is not a limiting factor in their altitudinal distribution. A more important factor is apparently the length of growing season–in particular its abbreviation at the end. Evidence for this is the occurrence of early juvenile instars of some species above the altitude at which the species normally completes its life cycle. Evidence was found for the relations between phenology and altitude postulated by A. D. Hopkins in the times of maturing of species with wide altitudinal ranges, with events somewhat earlier than expected at the highest altitudes. Events that occur near the middle of the summer (hatching of eggs of species that overwinter as juveniles) are independent of altitude, but this too is the harmony with the Hopkins law.