Proceedings-Ecology and Management of Annual Rangelands; 1992 May 18-21; Boise, ID
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This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. SEED USE BY DESERT GRANIVORES William S. Longland ABSTRACT and high-elevation deserts, because most bird species are resident only from spring through fall, and they eat in Three western Great Basin study areas that have under sects during much of this time, while ants are relatively gone extensive disturbance were monitored for species com inactive above ground when temperatures are cold. By position ofgranivorous rodents. Disturbed habitats at two contrast, nonhibernating rodent species are active forag of these sites (Red Rock and Flanigan) recovered with na ers for seeds all year. Furthermore, birds and ants glean tive or introduced perennial plants and were dominated seeds largely from the soil surface; rodents dig for buried by heteromyid rodent species, which are important seed seeds as well (Johnson and Jorgensen 1981). Thus, rodents dispersal agents for many desert plants. At one of these are often the main consumers of desert plant seeds. In sites, Indian ricegrass-a native perennial grass-rapidly addition to consuming seeds, though, large numbers of dominated the disturbed area, perhaps because of strong seeds are cached by desert rodent species in three families interactions between this grass and local heteromyid rodents. (Heteromyidae-the kangaroo rats, kangaroo mice, and The third site (Noble), which lacks heteromyids, has become pocket mice; Muridae-the New World mice, voles, and infested with an introduced annual weed (medusahead). The woodrats; and Sciuridae-the ground squirrels and chip seed dispersal activities ofheteromyids may be important munks). The heteromyids are a diverse group that is well in obtaining desirable responses to disturbance on desert represented in most desert localities. rangelands. Heteromyids and other arid-land rodents can harvest substantial fractions of a given plant species' seed produc INTRODUCTION tion. The degree of granivory probably depends both on seed density and on desirability of particular seed types. Granivorous, or seed-eating, animals are the most abun Thus, seeds of some plant species are greatly reduced dant and diverse herbivores in North American deserts. by rodents, while others may be barely touched (table 1). This high abundance and diversity is probably due to the From a plant's perspective, it is what the rodent does with generally greater availability and suitability of seeds as the harvested seed that is important. food compared to other types of plant materials in deserts. After seeds are harvested, rodents may consume them Desert plants produce large seasonal flushes of seeds, or cache them in one of two ways (Price and Jenkins 1986). which remain dormant and retain their nutritional quality "Larderhoarding" -practiced by most desert rodents for substantially longer periods than other aboveground refers to placing large caches in a centrally located burrow plant parts (Janzen 1971). Even with the seasonal nature or nest. "Scatterhoarding"-practiced mainly by hetero of desert seed production, seeds are available to desert myids and to a lesser degree by sciurids-refers instead granivores in the soil seed pool year-round; this is well to placing numerous smaller caches in shallow depres illustrated by occasional spring flushes of annual plants sions on the ground surface and covering them with soil. from seeds lying dormant in the soil over one or more win ters. These properties of desert plant seeds (seasonal pro duction in massive quantities and nutritional retention over time) also make them an ideal food for storing for fu Table 1-Previous studies documenting percentages of plant seed ture use. As a consequence, granivorous diets have been production harvested by rodents adopted by various groups of desert animals, and desert plants, having coevolved with these granivores, often ex Plant type Seeds hibit adaptations that either reduce levels of seed preda (site) harvested Source tion or allow them to capitalize on granivore activities for Percent dispersing their seeds. Here, I concentrate on the latter Annual grasses 93 Pearson 1964 coevolutionary relationship-seed dispersal. (California Central Valley) Various groups of rodents, birds, and seed-harvester Annual grasses 30-65 Borchert and Jain 1978 ants comprise the granivore guild in North American (Southern California) deserts (Brown and others 1979). Birds and ants are to Erodium cicutarium 95 Soholt 1973 a large extent seasonal granivores, especially in northern (Mojave Desert) Larrea tridentata 87 Chew and Chew 1970 (Chihuahuan Desert) Oryzopsis hymenoides 46 McAdoo and others 1983 (Great Basin Desert) Desert plants 30-80 Nelson and Chew 19n Paper presented at the Symposium on Ecology, Management, and Res toration of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992. (Mojave Desert, NV) William S. Longland is Research Ecologist, U.S. Department of Agricul Forbs <1 Pulliam and Brand 1975 ture, Agricultural Research Service, Conservation Biology of Rangelands, (Arizona grassland) Reno, NV 89512. 233 It is through scatterhoarding activities that desert rodents heteromyid species, while at the third site, which is domi are most likely to have a positive effect on seedling recruit nated by an introduced annual weed species, heteromyids ment; larderhoards are generally placed too deep under and other scatterhoarders are rare. I also discuss data ground for seedlings to successfully emerge if they germi from two experiments: (1) a field study illustrating strong nate. Thus, the net effect of rodents on seedling recruitment interactions between heteromyids and a native animal of a particular plant species is largely determined by num dispersed grass species, and (2) laboratory seed preference bers of seeds that are removed from the germinable seed tests with captive heteromyids showing that these rodents pool by consumption and larderhoarding versus numbers prefer seeds of this native grass to those of two introduced that persist in scatterhoards for later germination (Price annual grasses. My aim is to illustrate the potential im and Jenkins 1986). Scatterhoards that are not recovered portance of scatterhoarding granivores for desert range for future consumption may benefit seeds in three different lands by showing examples of desirable successional re ways: (1) buried seeds may have a higher probability of sponses to disturbance where these animals are common germination and establishment than unburied seeds and undesirable responses where they are lacking. Cer (Vander Wall1990), (2) buried seeds are not vulnerable to tainly, numerous ecological and historical factors combine consumption by nonscatterhoarding granivores (birds and to determine such responses, but plant/animal interactions ants) that harvest seeds only from the soil surface (Price involving native granivores are one such factor that has and JenkinS 1986), and (3) seeds of certain desert plant spe received little attention. cies may have enhanced germinability when they have been handled by scatterhoarding rodents (La Tourette and others METHODS 1971; McAdoo and others 1983; Reynolds and Glendening 1949). In this regard, heteromyid rodents, being very com Field studies were conducted at two northwestern mon and avid scatterhoarders in North American deserts, Nevada sites (Red Rock, Washoe Co., Reno NW Quad.: are very important components of arid rangeland commu T21N.R18E.S14; and Flanigan, Washoe Co., Flanigan nities. Although heteromyids have been known to have Quad. T27N.R18E.S2) and a northeastern California site important effects on range vegetation for some time (for (Noble, Lassen Co., Shaffer Mtn. Quad. T30N.R15E.S27). example, Reynolds 1950), results oflong-term experiments The Red Rock and Flanigan sites were burned in 1985 in the Chihuahuan Desert have recently highlighted the and 1988, respectively, while the Noble site was disturbed ecological significance of these "keystone" granivores; ro by extensive grazing of sheep over several decades preced dent exclusion experiments have shown that heteromyids ing this study. Prefire vegetation at Red Rock consisted of directly affect the species composition and physiognomy big sagebrush (Artemisia tridentata), Mormon tea (Ephe of the local plant community (Brown and Heske 1990). dra viridis ), desert peach (Prunus andersonii), and various Table 2 lists a sample of plant species that have been herbaceous species seeded after a previous fire, especially found germinating from rodent scatterhoards; this list in crested wheatgrass (Agropyron desertorum), which domi cludes grasses, shrubs, and trees and both native and intro nates the postfire vegetation. Prefire vegetation at Flanigan duced species. The native species listed appear to be depen consisted largely of big sagebrush, scattered shrubs of other dent on harvesting by animals for dispersal since their seeds native species, and infrequent bunches of Indian ricegrass lack external appendages that could facilitate dispersal by (Oryzopsis hymenoides); Indian ricegrass dominates the other means (for example, wind or adhesion to fur), while postfire community at Flanigan. The Noble site was domi the introduced species (cheatgrass) has such appendages nated by low sagebrush <Artemisia arbuscula) before dis to facilitate dispersal. This is to be expected, since native turbance and remains so in adjacent undisturbed areas; plant species