PLANTS EATEN BY JUVENILE DESERT TORTOISES IN THE CENTRAL MOJAVE DESERT
by E. Karen Spangenberg
Department of Biology, California State University, Dominguez Hills,
Carson, California, 90707
Paper presented at the National Biological Service
Desert Tortoise Symposium, 1995
Juvenile desert tortoises, four to five years of age, were observed in their natural habitat during May, June, and July, 1994 and 1995. During the hot, dry year of 1994, nineteen tortoises were observed to forage on 47 plants or dried stems of three species. During the same months in 1995, twenty-five juvenile tortoises were observed to forage on 833 plants comprised of 15 species. The latter year received above average rainfall during winter months crucial for spring plant productivity in the Mojave Desert. Forage plant selection changed as the season progressed and juvenile tortoises ate more dried grasses and dried annuals as wet green forage senesced. Juveniles were also observed to eat non-plant food items including bone, scat of various species, rocks, and lizards.
Neonate (first year of life where actual age is known) and juvenile (< 140 mm carapace length) desert tortoises (Gopherus agassizii) are the most vulnerable age classes, yet they are the least documented in terms of life history (Morafka 1994). Hatchling and juvenile tortoises are difficult to find in the wild because of their small and possibly secretive behaviors (Berry and Turner 1986). For those reasons, there is little information on these age classes in their natural habitat. In situ enclosure-raised tortoises provide the opportunity to study the behavior and physiology of young tortoises without compromising that behavior and biology through captivity in an artificial environment. A desert tortoise hatchery-nursery, established in natural habitat at the U.S. Army National Training Center (NTC), San Bernardino County, California, has provided researchers access to neonatal and juvenile tortoises since 1989.
A two-year study was undertaken at the NTC’s Ft. Irwin Study Site (FISS) to better understand the behaviors, activities budgets, survival, burrow and microhabitat use of young desert tortoises. Forage observations were made as a part of this study during late spring and early summer of 1994 and 1995 (Spangenberg 1996). Plant species and non-plant food items eaten by juvenile desert tortoises are reported here.
FISS is located in the southeastern corner of the NTC, approximately 40 km northeast of Barstow in San Bernardino County, California. The study site is situated on a 2° east facing slope of alluvial origin in the Central Mojave Desert at 35° 06′ 49″ N. Lat., 116° 29′ 27″ W. Long., elevation 650 m (Joyner-Griffith 1991; Morafka et al. 1996). Vegetation is a Mojave Desert creosote bush scrub (Vasek and Barbour 1988) with patches of perennial bunch grass (Pleuraphis ridgia). The dominant indicator species are creosote bush (Larrea tridentata), white bursage (Ambrosia dumosa), and rabbit thornbush (Lycium pallidum), Mormon tea (Ephedra spp.), desert senna (Cassia armata), and white rattany (Krameria erecta).
In April, 1990, a 60 m L x 60 m W x 2.4 m H, field enclosure was constructed at FISS. The enclosure was manually constructed to minimize disturbance of the habitat within and around the enclosure. A second field enclosure, FISS II, of similar dimensions and construction, was built 200 m southeast of the original FISS enclosure in Fall 1994 (Spangenberg 1996). Each year in May, from 1990 to 1993, and again in 1995, adult female tortoises were gathered from the surrounding 5 km2, brought into the original enclosure to deposit eggs, and returned to the collection points. Cohorts of neonates and juveniles ranging from 1 to 5 years-of-age and 40 to 85 mm straight line carapace length (CL) in size were maintained within the enclosure in the natural habitat without supplemental forage or water. All FISS tortoises were weighed and shells measured in March, June, and September of each year.
Tortoises were observed for 3 months (May, June, and July) in 1994 and 1995 as part of a study on time-activity budgets and microhabitat utilization by juvenile desert tortoises (Spangenberg 1996). Miscellaneous observations of juvenile tortoises at FISS were collected Spring 1994 through Fall 1995.
In April 1994, nineteen 3 and 4 year-old enclosure-raised desert tortoises; average weight 72.7 +/- 16.7 g (range: 53.5 to 105.0 g); and average CL 70.80 +/- 5.60 mm (range: 62.79 to 79.86 mm); were fitted with modified SM1 radio transmitters (AVM Instrument Company, 7Livermore, California) for radio telemetric observation in the field (Spangenberg 1996). Nine juvenile tortoises were released from the enclosure on the mornings of 9, 14, 15, May 1994, at random distances 200 to 300 m from the center of the FISS enclosure to the south, west, and north. The remaining 10 transmittered tortoises remained inside the enclosure with the rest of the FISS juvenile tortoises. Tortoises were observed 5 days each month between 15 May and 21 July, for a total of 30 days. Each tortoise was observed for equal number of hours and days.
In April, 1995, twenty-five 4 and 5 year-old juvenile tortoises; average weight 84.7 +/- 18.9 g (range: 60.4 to 118.9 g); and average CL 71.45 +/-5.92 mm (range: 63.82 to 83.29 mm); were observed 7 days each month between 15 May and 22 July, 1995, for a total of 63 days. Tortoises were fitted with AVM Company SM-1 transmitters and radiotelemetered with a Telonics TR-2 receiver (Mesa, Arizona) and a Telonics H-type antenna. Tortoises were in one of three study groups: actively released at random distances from FISS I enclosure, passively released through small holes in the fence at FISS II, the second enclosure, and retained inside the FISS I enclosure. Each tortoise was observed for equal number of hours and days.
Activities and microhabitat locations were observed once in each hour class between 0500 – 1000 PST and 1500 – 1900 PST. Midday hour class observations were not undertaken as temperatures were above thermal maximum for desert tortoises (Zimmerman et al. 1994). Activities were recorded as walking, foraging, walking-foraging (intermittently walking and foraging), resting (sitting with no movement or out of sight inside a burrow), and other (basking, digging on burrow mound, intraspecies or interspecies interaction, emerging or retreating into a burrow). Observation was done with binoculars. The data reported here were taken from foraging and walking-foraging observations as part of this larger study. When a tortoise was observed to forage, the plant species and the number of plants eaten were recorded. The part of the plant eaten was not consistently noted.
Temperature and Precipitation:
Temperature was recorded 3 times per day during each observation day, at 1 m above ground, 1 cm above ground, and just under the ground surface with a shielded thermometer bulb, at 0800 PST, 1200 PST and 1600 PST. Temperatures recorded at each daily time period were averaged by month. Monthly precipitation was determined from recordings at the NTC weather station at Bicycle Lake (35° 16′ N. Lat. and 116° 38′ W. Long.), Ft. Irwin, San Bernardino County, California, 21.25 km NW of the FISS. Historical average monthly precipitation was calculated from National Climate Data Center (NCDC) weather data recorded at Daggett Edison Plant, Station #2255 (34° 52′ 00″ N. Lat. and 116° 52′ 00″ W. Long.), San Bernardino County, California, 44.6 km SW of FISS (Earth Info 1994).
Plant Species List:
Systematic plant transects and quantification of vegetation was not untertaken for FISS. Plant species were collected and identified throughout 1994 and 1995. In July, 1995, annual dry biomass and frequency were recorded at FISS as part of overall vegetation monitoring at the NTC (Ferrus-Gracia unpublished data). A plant species list was compiled from these observations.
Seasonal temperatures and precipitation:
Mean air temperature (one m above ground) increased monthly from May to July in both 1994 and 1995. The ranges of average monthly temperatures for each daily recording period (0800, 1200, and 1600 PST) in 1994 were 28.2 to 40.1° C at 0800 PST, 34.25 to 52.9° C at 1200 PST, and 19.2 to 35.6° C at 1600 PST. The ranges of average monthly temperatures for each daily recording period in 1995 were 23.2 to 35.3° C at 0800 PST, 29.8 to 45.7° C at 1200 PST, and 29.5 to 43.8° C at 1600 PST. Mean monthly temperatures in 1994 were an average 5.6° C higher then mean monthly temperatures in 1995. Average temperatures during June, July, and August, 1994 were the highest recorded monthly maximum (40.5° C, 42.2° C, and 41.1° C, respectively) at Daggett Edison Plant weather station for 41 years of records (Earth Info 1994).
The 41-year average rainfall at Daggett Edison Plant Station (1953 to 1994), was 79.8 mm per year (Figure 1). Annual precipitation, October 1993 to September 1994, recorded at Bicycle Lake weather station was far below average (27.7 mm). Precipitation, October 1994 to September 1995, recorded at Bicycle Lake weather station was above normal (99.6 mm).
Over the two-year period with one year of negligible plant productivity and the other with good plant productivity, 97 plant species were collected and identified at FISS (Appendix A).
All released tortoises were observed to forage in 1994. Only seven out of 10 tortoises retained inside the enclosure were observed to forage. Out of 2141 separate 5 minute observation periods, only 49 (2.3%) of these periods included foraging behavior. The 19 tortoises were observed to take only 47 forage bites. Of these, 3 (6.4%) forage bites were of wet green forage ( Table 1). The remainder of all forage bites were of dried grasses, dried annuals and dried stems remaining from the 1993 growing season.
In 1995, twenty-five tortoises were observed to eat 833 plants from 15 different plant species (Table 2) and several non-plant items including scats, bones, rocks, and dead lizards (Callisaurus andCnemidophorus). For all forage observations combined, the 5 plants most eaten were Schismus barbatus (51.0%), Erodium cicutarium (13.3%), Cryptantha angustifolia (7.0%), Plantago ovata (6.9%), and Chaenactis fremontii (6.5%). Tortoises were also observed to eat, though in much smaller quantity, Camissonia boothii (1.7%), C. claviformis (1.2%), Malacothrix glabrata (0.8%), Pleuraphis rigida (0.7%), Chaenactis carphoclinia (0.6%), Loeseliastrum schottii (0.2%), Filago depressa (0.2%), Mentzelia albicaulis (0.1%), and Ambrosia dumosa (0.1%). Dried stems from 1994 were eaten 0.6% of the time. Plants unable to be identified accounted for 9.1% of plant foods eaten. Outside of hour class observations, one tortoise frequently foraged on Camissonia campestris.
Plants eaten varied by month during 1995. S. barbatus (dried stems and leaves) was the plant eaten the most each month (Figure 2). S. barbatus consumption increased each month, May to July, from 34% to 55% to 77% of total plants eaten. E. cicutarium was the second most eaten plant, but percentage eaten decreased May to June (21% to 13%). By July, none of the tortoises were observed to forage on E. cicutarium which had senesced and dried by this time. Consumption of C. fremontii and P. ovata deceased each month from May to July (10%, 5.1%, and 3.3% for P. ovata and 12%, 3.4% and 1.2% forC. fremontii). Consumption of Cryptantha angustifolia peaked in June (4.4%, 11%, and 6.7%).
In 1995, only 1.2% of all observation time was spent foraging. Foraging deceased from 1.3% and 1.4% of observation time in May and June to 0.8% during July. Each study group; actively released, passively released, and penned, showed similar decreases in the percentage time spent foraging May-June to July. Each group, had similar total combined time spent foraging during the three month study: actively released 1.0%, passively released 1.4%, and penned 1.3%.
The amount of precipitation, its seasonality, and resulting plant productivity and biomass influence juvenile tortoise foraging time budgets to a larger extent than whether juveniles are free-ranging or maintained inside a field enclosure. In the Mojave Desert heavy rains between late September and early January trigger vegetative growth of shrubs, herbaceous perennials and winter annuals during the following spring (Beatley 1974). Winter annuals and certain herbaceous perennials are restricted to seasonal growth dependent on fall and winter rains. Minimal precipitation and maximum seasonal summer temperatures during 1994 resulted in less foraging behavior compared to during 1995. In good years, tortoises increase activity but not necessarily the home range they traverse to search for forage and coversites. Home ranges were actually found to become larger when food availability became scarce (USF&WS 1994).
In 1994, the central Mojave Desert received below average rainfall, as a result, spring forage was negligible. In 1994, 19 tortoises at FISS were observed to forage only 47 plants of three plant species. Only three of these plants were green. Heavy precipitation events during January 1995, triggered the growth of winter annuals in abundance. Twenty-five juvenile tortoises in 1995 were observed to forage on 833 plants of 15 plant species. Foraging activity increased threefold from 1994 in the second year of the study. Walking-foraging activity doubled. For these reasons, tortoises increased in size and weight in 1995 from the previous season, whereas, in 1994, tortoises actually decreased in weight from the previous year (Spangenberg 1996). In both years, percentage increase of size and weight was greater for released juveniles than penned. Released tortoises had a wider variety of forage plants available to them (Karen Spangenberg personal observation). Long-term human and adult tortoise impacts to the enclosure including trampling of the substrate, uprooting of perennial shrubs, usurping juvenile burrows and consumption of available forage, has likely reduced forage availability for juveniles inside the enclosure.
Seasonal or monthly foraging activity also varied depending on rainfall regimes, annual variation in rainfall, and plant productivity. Plant phenology can vary from year to year depending on temperature and rainfall. A foraging study of adult desert tortoises at the Desert Tortoise Research Natural Area (DTRNA), western Kern County, California, found activity sharply declined in June in correlation with declining availability of succulent (wet-green) forage (Jennings,1993). Movements (activity levels) of gopher tortoises (G. polyphemus) were found to decrease with decreased herbaceous biomass (Diemer 1992). Aquirre et al. (1984) found bolson tortoises (G. flavomarginatus) also decreased movement during times of less abundant forage. Foraging activities of FISS juveniles during May and June were significantly more than foraging activity during July (Spangenberg 1996). Walking-foraging activity was greatest in May and levels declined to July. Diet selection shifted during this time from succulent annuals to dried grasses during 1995 (Figure 2).
Studies demonstrate desert tortoises are selective feeders and do not forage on plants in relationship to their abundance in the environment (Jennings 1993). Adult tortoises at the DTRNA ate 46 plant species and the top 6 species accounted for 70% of all bites. In a study I undertook as a volunteer with the Bureau of Land Management at Kramer Junction, San Bernardino County, California, four tortoises ate 33 species and the top 6 species accounted for 82% of all bites. The top 5 species of plants eaten by juvenile tortoises at FISS during 1995 accounted for 85% of all plants eaten.
Desert tortoises forage on a wide variety of plants which differ in different parts of its range and by individual preference (Esque 1995). Diet mixing in herbivores and omnivores can greatly increase digestibility of dry matter, organic matter, and energy sources (Bjorndal 1991) and make it advantageous to eat a variety of foods. Patchy habitat and individual preference might play a role in forage behavior as well as the need for variety in the diet.
Juvenile tortoises appear to be opportunistic feeders. Juveniles were observed to eat scat of several animal species, bones, and rocks (usually white in appearance). On several occasions lizards trapped in the 1.25 cm mesh surrounding the perimeter of FISS I enclosure were eaten by juvenile tortoises. Jennings (1993) reports of an adult feeding on a Gambelia carcass. Tortoises in Ivanpah Valley in the Eastern Mojave Desert, eat sphinx moth larvae (Hyles lineata) (Avery and Neibergs 1996). Captive juvenile tortoises immediately approached and either bit or bumped crickets (Acheta domestica) and king mealworms (Zophobas morio) during food choice trials (Okamoto 1995). I observed juvenile tortoises inside FISS I bite bird feathers of a downed bird inside the enclosure. Perhaps tortoises are actually scavengers but just not fast enough to get there first or opportunistic predators that rarely overtake their prey.
Nutrient content of desert plants vary with season and with plant part (Nagy et. al 1976; Adest et al. 1989; and McArthur et al. 1994). Tortoises have flexibility in digestive processing (intake and gut transit time) which helps maximize utilization of the limited resources of the desert environment (Bjorndal and Bolten 1993; Meienberger et al. 1993). E. cicutarium when succulent, contains three times more nitrogen per dry matter than S. barbatus. Low intake of S. barbatus (dried S. barbatus was the primary forage plant of all three tortoise groups in July, 1995) and slow transit time is required to maximize nitrogen assimilation. Variety in the diet and shifting of diet as the season progresses allows tortoises to maximize limited resources. It would be interesting to examine the nutrient and electrolyte content of the diet of juvenile tortoises and compare it to the diet of adult tortoises in the same area, given the distinct metabolic demands (Naegle and Bradley 1975) and vulnerabilities of juveniles.
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|Table 1. Number of forage plants eaten by juvenile desert tortoises during May, June, and July, 1994, at FISS, San Bernardino County, California. (Most dried stems and annuals were Malacothrix and Chaenactis which grew in 1993.)|
|Plant species||Number of plants eaten||Percentage total|
|Schismus barbatus (dry)||21||44.7|
|Schismus barbatus (wet)||3||6.4|
|Dried Malacothrix glabrata||8||17.0|
|Dried Amsinckia tesselatta||1||2.1|
|Dried annuals and stems||12||25.5|
|Table 2. Number of forage plants eaten by juvenile desert tortoises during May, June, and July, 1995, at FISS, San Bernardino County, California.|
|Plant species||Number of plants eaten||Percentage total|
Appendix A. Annuals, herbaceous perennials, and shrubs observed at Ft. Irwin Study Site (FISS), San Bernardino County, California. (Nomenclature based on Hickman, 1993.)
Gilia sp. 1
Gilia sp. 2