Ecology of the Saguaro: II
NPS Scientific Monograph No. 8
NPS Logo


Biotic Factors

Rodents and birds

Rodents are a major cause of first-year saguaro mortality. In the absence of other sources of food and/or moisture, most if not all of the species of rodents that occur commonly in the principal saguaro habitats at Saguaro National Monument and elsewhere in the Sonoran Desert will feed upon saguaro seedlings (Tables 13 and 14). In addition to direct destruction by consumption, a substantial number of young saguaros are uprooted or buried by intensive digging, especially by the Harris and round-tailed ground squirrels.

TABLE 13. Consumption of saguaro seedlings by species of rodents occurring commonly in saguaro habitats at Saguaro National Monument.

All rodents, live-trapped during mid-summer 1971, were held in doors without food or water for 24 hr prior to feeding. Each animal was then provided with a single pot containing 15-day-old seedlings growing in native soil. In every instance, all seedlings offered were eaten within 24 hr. These results are similar and not significantly different from those of previous experiments. Experiments of similar design conducted in the field laboratory at Navojoa with rodents from southern Sonora (see Table 14) produced results similar to those obtained at Tucson, Arizona.

Species Seedlings consumed
Digging activity
(Soil disturbance)

Spermophilus tereticaudus1756overturned
Citellus harrisi654overturned
Neotoma albigula565slight
Dipodomys merriami2494overturned
Perognathus baileyi933overturned
Perognathus penicillatus2424overturned
Peromyscus eremicus246.0none

TABLE 14. Native rodents and lagomorphs occurring in saguaro habitats in the Mohave Desert, Sonoran Desert, and in thornscrub in southwestern Sonora, Mexico. Grasshopper mice (Onychomys), cotton rats (Sigmodon), and gophers (Thomomys) are not listed.

Speciesa Subspecies in area of
Southwestern Sonora
Centwestern Arizona—
Centwestern Sonora
(Sonoran desertscrub)
Northwestern Arizona
(Mohave desertscrub)

Plant eaters
Desert Cottontail
Sylvilagus auduboni
a. goldmania. arizonaea. arizonae
Antelope Jackrabbit
Lepus alleni
a. palitansa. allenia. alleni
Blacktail Jackrabbit
Lepus californicus

c. eremicusc. eremicus
White-throat Woodrat
Neotoma albigulab
a. melanuraa. albigulaa. albigula
Sonoran Woodrat
Neotoma phenax

Desert Woodrat
Neotoma lepidab

l. devial. devia
Cactus Mouse
Peromyscus eremicus
e. anthonyie. eremicuse. eremicus
Riparian Mouse
Peromyscus merriami
m. goldmanim. merriamim. merriami
Deer Mouse
Peromyscus maniculatus

m. sonoriensism. sonoriensis
Canyon Mouse
Peromyscus crinitusb

c. pallidissimusc. pallidissimus
Fulvous Harvest Mouse
Reithrodontomys fulvescens
f. tenuisf. fulvescens
Sonora Harvest Mouse
Reithrodontomys burti
Western Harvest Mouse
Reithrodontomys megalotis

m. megalotism. megalotis
Rock Squirrel
Citellus variegatus
v. grammurusv. grammurusv. grammurus
Roundtail Ground Squirrel
Citellus tereticaudus
t. neglectust. neglectust. neglectus
Sonora Antelope Squirrel
Citellus harrisib

h. harrisih. harrisi
Desert Cliff Chipmunk
Eutamias dorsalis

dorsalis (Sonora)

Seed gatherers
Merriam Kangaroo Rat
Dipodomys merriamib
m. merriamim. merriamim. merriami
Desert Kangaroo Rat
Dipodomys desertib

d. deserti
Bannertail Kangaroo Rat
Dipodomys spectabilis

s. perblandus
Ord Kangaroo Rat
Dipodomys ordi

o. ordio. chapmani
Bajada Pocket Mouse
Perognathus baileyib
b. baileyib. baileyi
Thornscrub Pocket Mouse
Perognathus goldmani
g. goldmani

Sinaloa Pocket Mouse
Perognathus pernix
p. pernix

Desert Pocket Mouse
Perognathus penicillatusb

p. priceip. penicillatus
Rock Pocket Moose
Perognathus intermediusb

i. lithophilusi. lithophilus
Arizona Pocket Mouse
Perognathus amplusb

a. rotundusa. pergracilis
Little Pocket Mouse
Perognathus longimembrisb

I. bombycinus

aScientific names, vernacular, and ranges for mammals in saguaro habitats follow Hall and Kelson (1959) the most recent reference, and communication from Drs. E. Lendell Cockrum (UA) and James L. Patton (UCB).
bOther subspecies in area(s) in addition to tabulated. And other subspecies.

We have found, however, that only the white-throated woodrat can subsist entirely on a diet of live saguaro tissues (Fig. 27). This is because the species of Neotoma are able to metabolize and neutralize oxalates, compounds that are lethal to other rodents. The story is well told by Schmidt-Nielsen (1964).

Fig. 27A. Juvenile saguaro at Saguaro National Monument (east) severely damaged by white-throated woodrat (Neotoma albigula) and subsequent freezing. Woodrat tunnels often extend like a winding staircase completely through the stem. Such damage occurs on both juvenile and adult saguaros and increases vulnerability to freezing and to wind-caused stem breakage. Photographed 15 Sept. 1968.

Seedlings of saguaros (and those of other cacti) that are not well hidden on the desert floor are vulnerable "canteens" that are approximately 90% water. In the desert, such cactus "feeding" rises sharply during the hot-dry months of May and June that precede the summer rains. Experimentally, however, we have found that under the hot-dry stress when ample free water and other fresh green plant material (grasses and forbs, both with and without seeds, and leaves) are available at the same time of year (and any other time), the same ground squirrels (Citellus) and desert mice (Peromyscus, Onychomys) refuse cactus of any species, age, or succulence. Because of oxalate poisoning, only one of these mammals (Neotoma) is able to incorporate the saguaro (and cacti in general) into its diet as a substantial food item. Desert rodents in general, and certain larger mammals as well (Fig. 27B), irregularly ingest limited amounts of cactus tissue primarily or wholly for its water content rather than for its caloric value. Packrats can and do have it both ways, saguaros included.

Fig. 27B. Juvenile saguaro on the Cabeza Prieta Game Range (west of Organ Pipe Cactus National Monument). In the more arid portions of the saguaro's range, and elsewhere during periods of severe drought, jackrabbits utilize the succulent stem tissues as a source of moisture. Similar consumption by desert bighorn sheep has been observed (Simmons 1969). Although such damage does not in itself usually kill the saguaro, such "girdling" increases the plant's vulnerability to destruction by wind and freezing. Photographed 3 Feb. 1971.

Destruction of recently emerged seedlings by cactus wrens has been observed. However, uprooted plants are not eaten. There is no doubt that the similar intensive foraging and digging activities of the curve-billed thrasher, gilded flicker, and gila woodpecker also result in destruction of young seedlings.

In nonrocky habitats, seedlings are rarely found near the base of adult saguaros, and almost never become established there. Such stations are subject to intense disturbance by foraging animals. Generally, the probability of seedling survival increases with distance from reproductive adult saguaros.


Young saguaros are subject to predation by a diversity of plant-eating insect species, none of which are known to feed exclusively on saguaros. The list of predators shown in Table 15 is by no means complete. Our own field observations and reports by others (Turner et al. 1966; Mann 1969) indicate that a complete listing of insect predators on young saguaros would include additional families, and many times the number of species identified. Although some insect-caused mortality of young saguaros takes place during every month of the year, the heaviest predation occurs during the humid summer months immediately following germination, and again with the arrival of warm weather in April and May.

TABLE 15. Insect consumers of young saguaro cacti, collected in saguaro habitats at Saguaro National Monument (east). Insects were placed in closed containers with young saguaros. All species fed, consuming the entire succulent portions of the stems of one or more plants.

SpeciesMonth collected Stage

Cactobrosis fernaldialis (Hulst)June, Julylarva
Feltia subterranea (Fabricus)Junelarva
Heliothis zea (Boddie)April, Maylarva
Orthodes alfkeni (Grote)July, Dec., Feb.larva
Peridroma margaritosa (Hawthorn)June, Julylarva
Spodoptera exigua (Hubner)Aprillarva
Melanoplus sp.Julynymph
Heleastus sp.Julynymph
Gryllidae 2 spp.Julyadult
Aneflus protensu (Le Conte)Julyadult

During the first weeks following germination, the succulent seedlings are particularly vulnerable to destruction by the newly hatched larvae of lepidopterous insects. Immediately after hatching, the larva enters at the base or at the apex of the seedling to feed on the succulent inner tissues. Leaving the epidermis largely intact, the larva then moves on to an adjacent plant. The remaining withered epidermis presents the appearance of death resulting from lack of moisture. Some of the previously reported summer drought-kill based on observations of desiccated seedlings (Steenbergh and Lowe 1969) may be more correctly attributable to such destruction by insects. Subsequent observations indicate that insects are second only to rodents as a cause of seedling death during the first weeks following germination.

By the end of the first summer growth period, the relatively tough epidermal tissues protect the seedling from entry by small insect larvae and the well-developed spines protect the tender apex. However, the seedling remains vulnerable to destruction by larger insect predators: grasshoppers, crickets, beetles, and the larger larvae of certain moths.

Cutworms (noctuid moth larvae) appear to be the most common insect-consumers of saguaros that have survived beyond the first summer (Steenbergh and Lowe 1969; Table 15). Leaving only the roots and spines of demolished plants, cutworms frequently make a meal of several adjacent young plants (incidentally relieving competition for the survivors). Consumption of an entire 4-year-old plant (1.5 cm; 0.6 inch ht) within a 24-hr period was observed. The ability of the young saguaro to survive insect-caused damage increases with age and size—the saguaro outgrows the individual consumptive capacity of most predatory insects within the first 5 years of life.

Gerstaeckeria turbida (Lec.), a weevil reported by Turner et al. (1966) to be the principal insect responsible for the deaths of transplanted (cultured) young saguaros at Saguaro National Monument, was not observed during our investigations on causes of mortality in seedlings naturally germinated on-site in saguaro habitats.

A few species of large insects do occasionally invade larger juvenile saguaros. At Saguaro National Monument (east) the destruction of transplanted saguaros up to 30 cm (1 ft) tall by the larva of a large weevil [Cactophagus validus (LeConte)] has been observed. Our observations on naturally established saguaros, however, indicate that insect-caused deaths of such larger juvenile saguaros rarely occur in nature. Further, our observations strongly suggest that such destructive invasion of large juvenile saguaros by insects is limited to weak, moribund, or dead individuals damaged by freezing or other factors.

Experimental exclosures

Seed broadcasting and experimental exclosures were used on-site in both the east and west units of Saguaro National Monument (Tables 19-22; Figs. 19B, 29, 30). Details on the structure and placement of the wire exclosures are given in Steenbergh and Lowe (1976).

TABLE 16. First-year survivorship of 231 saguaro seedlings naturally germinated July 1967 from seeds broadcast within seven 1-m2 plots at Saguaro National Monument. Location-habitat symbols are: SE = east monument, SW = west monument, F = flats, H = rolling hills, RN = rocky north-facing slope, and RS = rocky south-facing slope. Data graphed in Fig. 28.


East Monument
West Monument
N = 73
N = 23
N = 17
N = 10
N = 49
N = 26
N = 33
Live% Live% Live% Live% Live% Live% Live%

Aug. 131

939.1 1058.8 770.0 816.3

Aug. 232

1973.1 2987.9
Aug. 333 1013.7

Aug. 838 1013.7 834.8 1058.8 770.0 816.3

Aug. 939

1765.4 2369.7
Aug. 1545 1013.7 834.8 1058.8 330.0 816.3

Aug. 1848

1034.5 2163.6
Aug. 2151

830.8 2060.6
Aug. 2252 912.2 730.4 847.1 330.0 816.3

Sept. 768

519.2 1339.4
Sept. 869 45.5 626.1 635.3 330.0 48.2

Sept. 1980

13.8 721.2
Sept. 2081 22.7 626.1 635.3 220.0 24.1

Oct. 394

13.8 13.0
Oct. 495 22.7 521.7 15.9 110.0 00.0

Oct. 18109 22.7 521.7 15.9 110.0

00.0 13.0
Nov. 1123 22.7 521.7 00.0 110.0

Nov. 15137 11.4 521.7


Nov. 28150



Nov. 29151 11.4


Dec. 13165 11.4 521.7


Dec. 19171 11.4 521.7


Jan. 4187 11.4 313.0


Jan. 10193 11.4 313.0


Jan. 24207 11.4 313.0

Feb. 7221


Feb. 8222 11.4

Feb. 21235 11.4 313.0

Mar. 6249 00.0 313.0

June 25360


TABLE 17. Regression equations for post-germination survival of saguaro seedlings germinated July 1967; probit conversion of percent survival of seedlings (Y) on time in days from 1 July (X) by least squares. Habitat symbols as in Table 16; data in Table 16 is graphed in Fig. 28.

HabitatN Equationr

SEF18 Probit Y = -0.006 X + 3.897 -0.882
SEHS15 Probit Y = -0.010 X + 5.225 -0.828
SEHN6 Probit Y = -0.016 X + 4.645 -0.922
SERS2.0 Probit Y = -0.003 X + 4.617 -0.89.0
SERN8 Probit Y = -0.025 X + 6.230 -0.943
SWRS7 Probit Y = -0.040 X + 6.762 -0.968
SWRN8 Probit Y = -0.040 X + 7.299 -0.982

TABLE 18. Post-germination survival and establishment rates of saguaro seedlings (N = 231) naturally germinated July 1967 in representative habitats at Saguaro National Monument; predicted survival and establishment rates estimated by regression equations in Table 17. Habitat symbols as in Table 16.

"N" is the original number of seedlings under observation. Survivorship is shown as time in days to .0.0.01 survival (1 survivor per 1000 seedlings). Establishment rate is expressed as number of seedlings per 1000 surviving to the end of the first year of life (15 July 1968). Data in Table 16, graphed in Fig. 28.

HabitatN Survivorship
To 0.001
1st year


Fig. 28. Post-germination first-year survivorship of saguaro seedlings (N = 231) naturally germinated July 1967 in representative habitats at Saguaro National Monument; percent survival on time in days from 1 July. Data in Table 16, regression equations in Table 17. (click on image for an enlargement in a new window)

Comparison of 1968 seedling survival in open (unprotected) and exclosure (protected) plots clearly shows that vertebrate predators are a primary cause of saguaro seedling mortality during the first 5 pre-winter months of the first year of life (Tables 19-22; Figs. 29 and 30).

TABLE 19. First-year survivorship in protected (exclosure) plots of 1018 saguaro seedlings naturally germinated July 1968 from seeds broadcast within five 0.25 m2 (2.7 ft2) plots. Seedlings were continuously protected from predation by vertebrate animals (mammals and birds) by 0.5-inch (1.3 cm) mesh exclosures. Habitat symbols as in Table 16. Data graphed in Figs. 29 and 30.

East Monument
West Monument
N = 283
N = 232
N = 237
N = 30
N = 151
N = 85
Live% Live% Live% Live% Live% Live%

July 3130 283100.0232100.0

Aug. 131

237100.030100.0 151100.0

Aug. 737 24084.8

Aug. 838

22195.3 22795.830100.0 14596.0

Aug. 1444 22579.5

30100.0 14294.085100.0
Aug. 1545

20688.8 18578.1

Aug. 2151 22579.5

30100.0 13891.485100.0
Aug. 2252

15568.8 16167.9

Sept. 465 22077.713056.0 208.4

Sept. 566

30100.0 12985.485100.0
Sept. 1879 16458.08436.2 73.0

Sept. 1980

2893.3 12381.58498.8
Dec. 9161 15053.08436.2 73.0

Dec. 10162

2686.7 9764.28498.8
Jan. 6189 15053.0

Jan. 8191

8034.5 73.02686.7 9764.28195.3
Feb. 3217 14250.28034.5 73.02273.3 9059.67992.9
Mar. 3245 13848.88034.5 73.02273.3 9059.67689.4
April 1274 13848.88034.5 73.02273.3 9059.67588.2
May 6309 13848.87532.3 62.5

May 7310

2273.3 8958.97588.2
June 2336 13748.47431.9 62.52170.0 8958.97284.7
July 1365 13748.4

July 2366

2066.7 8958.96272.9
July 3367

7431.9 62.5

July 31395 13748.47431.9 52.1

Aug. 1396

1860.0 7952.35767.1

TABLE 20. First-year survivorship in unprotected (open) plots of 250 saguaro seedlings naturally germinated July 1968 from seeds broadcast within five 0.25 m2 (2.7 ft2) plots; habitat symbols as in Table 16. Data graphed in Figs. 29 and 30.

East Monument
West Monument
N = 114
N = 27
N = 37
N = 13
N = 59
Live% Live% Live% Live% Live%

July 31
30 114100.0

Aug. 131

27100.0 37100.0 13100.0

Aug. 737 9482.5

Aug. 838

2281.5 3081.1 1184.6

Aug. 1444 8171.0

Aug. 1545

933.3 2362.2

Aug. 2151 2521.9

Aug. 2252

622.2 513.5

Sept. 465 43.5414.8 12.7

Sept. 566

Sept. 1879 00.013.7 00.0

Sept. 1980

Dec. 10162



TABLE 21. Comparison of post-germination survival and establishment rates of saguaro seedlings plots 1268) in paired protected (exclosure) and unprotected (open) 0.25 m2 (2.7 ft2) plots for seedlings naturally germinated July 1968. Exclosure plots were covered from the date of seedling emergence with .0.5-inch (1.3-cm) mesh hardware-cloth cages (30 cm; 11.8 in ht X 61 cm; 24.0 in dia.) to exclude birds and mammals (Fig. 19B).

Survival and establishment rates estimated using regression equations in Table 22. Habitat symbols as in Table 16. "N" is the original number of seedlings under observation. Survivorship is shown as time in days to 0.001 survival (1 survivor per 1000 seed lings). Establishment rate is expressed as number of seedlings surviving to the end of the first year of life (15 July 1969). Data in Tables 19 and 2.0, graphed in Figs. 29 and 30.

HabitatSurvivorship and Establishment
N To 0.001
1st year
N To 0.001
1st year

SEF 114 76.5 <0.1 283 1442.0 498.6
SERS 27 100.2 <0.1 232 968.8 223.4
SERN 37 74.2 <0.1 237 462.9 60.6
SE-(all) 178 85.9 <0.1 752 806.9 122.1
SWF 13 129.0 <0.1 30 1316.4 629.0
SWRN 59 91.8 <0.1 85 1001.7 737.8
SWRS 82.1a <0.1a 151 1128.4 463.1

aEstimated values based on analysis of 1967 data.

TABLE 22. Regression equations for post-germination survival of saguaro seedlings germinated July 1968; probit conversion of percent survival of seedlings (Y) on time in days from 1 July (X) by least squares.

Habitat symbols as in Table 16. Data in Tables 19 and 20, graphed in Figs. 29 and 30.

HabitatN Equationr

SEF4Probit Y = -0.103 X + 9.805-0.979
SERS5Probit Y = -0.056 X+ 7.471-0.920
SERN4Probit Y = -0.109 X + 9.971-0.973
SE (all)13Probit Y = -0.077 X + 8.558-0.837
SWF4Probit Y = -0.046 X + 7.836-0.959
SWRN5Probit Y = -0.100 X + 11.059-0.977
SEF14Probit Y = -0.003 X + 5.788-0.846
SERS14Probit Y = -0.004 X + 5.736-0.724
SERN14Probit Y = -0.007 X + 5.108-0.701
SE (all)42Probit Y = -0.004 X + 5.541-0.442
SWF10Probit Y = -0.004 X + 6.675-0.941
SWRS14Probit Y = -0.004 X + 6.423-0.884
SWRN10Probit Y = -0.006 X + 7.870-0.962
SW (all)34Probit Y = -0.005 X + 6.924-0.668

Fig. 29. First-year post-germination survivorship in open and protected field plots of saguaro seedlings (N = 338) germinated July 1968 at Saguaro National Monument (west).

As in the east monument, high rates of survival within exclosures indicate that mammals and birds are a primary cause of first-year seedling mortality in all habitats. Relative survivorship within exclosures on north-facing and south-facing slopes is reversed from that observed in the east monument (see Tables 21 and 22; Fig. 30).

In this arid environment the higher rate of survival on the north-facing slope is attributed to the more favorable moisture relationships that prevail in that habitat during critical drought periods.

Regression equations in Table 22, data in Tables 19 and 20. (click on image for an enlargement in a new window)

The life expectancy of seedlings protected by exclosures that effectively excluded all vertebrate animals (but not insects) was approximately 10 times that of seedlings subject to natural predation (Table 21).

Within exclosures, consumption by insects was the principal cause of seedling deaths. However, despite relatively mild winter minimum temperatures, there were four freeze-caused seedling deaths within the two north-slope plots.

The relatively high rate of survival within the east monument "flats" exclosure is especially noteworthy. In that habitat, the pre-winter July to November climatic environment is highly favorable for seedling survival. There, also, it appears that the foraging activities of birds and rodents are most detrimental to seedling survival.

Although seedling survival in rocky habitats was improved vastly by the use of exclosures, the relative suitability of north-facing and south-facing slopes for seedling survival was not altered by the exclusion of vertebrate animals. The differences in the relative suitability of these habitats for pre-winter (July to November) seedling survival must be attributed to other factors, namely, consumption by insects and, to a lesser extent at Saguaro National Monument (west), to differences in moisture availability.

<<< Previous <<< Contents >>> Next >>>

Last Updated: 21-Oct-2005