Sciurus aberti (Abert's squirrel)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Biology and Ecology
- Natural Food Sources
- Latitude/Altitude Ranges
- Air Temperature
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Impact Summary
- Economic Impact
- Environmental Impact
- Threatened Species
- Risk and Impact Factors
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Sciurus aberti Woodhouse, 1853
Preferred Common Name
- Abert's squirrel
International Common Names
- English: tassel-eared squirrel
- Spanish: ardilla de Abert
Summary of InvasivenessTop of page
Abert’s squirrels (S. aberti) are modest-sized tree squirrels native to Mexico and parts of the USA including Colorado, Utah and Wyoming. Although recorded as native to Arizona and New Mexico it was introduced to isolated areas throughout the 1900s. The species is listed as a species of Least Concern on the IUCN red list and protected as non-game or game mammals throughout the southwestern USA and northwestern Mexico. Abert’s squirrels are restricted principally to ponderosa pine (Pinus ponderosa) forest but they have been reported from other types of forest in places such as above the treeline in the San Francisco Peaks of Arizona, indicating that the species can thrive in other types of forest. Distribution of ponderosa pine forests in the semi-arid regions throughout its range are fragmented by deserts. It is considered aesthetically pleasing and is also consumed by humans, and more than 25 translocations have occurred since 1900, primarily by USA state governments. Introductions to isolated habitats have established animals in 90% of the efforts. Furthermore, the species has spread through hostile habitats >70 km from introduction sites. Natural range expansion continues to occur. In areas where other tree squirrels co-occur, Abert’s squirrel introduction can result in decline of native species. As a result, Abert’s squirrels represent an invasive species with potential to reduce native biodiversity.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Chordata
- Subphylum: Vertebrata
- Class: Mammalia
- Order: Rodentia
- Family: Sciuridae
- Subfamily: Sciurinae
- Genus: Sciurus
- Species: Sciurus aberti
Notes on Taxonomy and NomenclatureTop of page
Sciurus aberti Woodhouse 1853 consists of 6 subspecies: S. a. aberti Woodhouse 1853 found in much of Arizona, Utah and western New Mexico, S. a. barberi Allen 1904 found in western Chihuahua (Mexico), S. a. chuscensis Goldman 1931 found in northeast Arizona and extreme northwest New Mexico, S. a. durangi Thomas 1893 found in Durango and Chihuahua (Mexico), S. a. ferreus True 1894 found in Wyoming, Colorado, and northern New Mexico and S. a. kaibabensis Merriam 1904, isolated on the Kaibab Plateau on the north rim of the Grand Canyon in northern Arizona. This last subspecies, S. a. kaibabensis, is externally distinctive and possesses a white tail and black venter and had been considered a separate species (S. kaibabensis Merriam 1904); however, genetic evidence suggests little differentiation from nearby populations and thus this population is only considered a unique subspecies (Lamb et al., 1997).
DescriptionTop of page
Abert’s squirrels are large bodied arboreal squirrels with a long, bushy tail. Body mass typically ranges from 550 to 750 g (Fitzgerald et al., 1994) but individuals can approach 1 kg in autumn when food is plentiful. Total body length ranges from 450 to 580 mm, tail length from 200 to 300 mm, and hind foot 65 to 75 mm (Fitzgerald et al., 1994). The dorsum is grizzled gray to charcoal often with a medial reddish band and separated by a black horizontal line from the white venter (Thorington et al., 2012). A slate coloured tail, frosted with white and conspicuous ear tufts are particularly prominent, especially in winter (often >25 mm: Nash and Seaman, 1977). The tufts can be present throughout the year in some populations or individuals but not in others. Melanism is common, especially in the northerly populations in Colorado. S. a. kaibabensis on the north rim of the Grand Canyon in Arizona typically has a black venter and white tail (Thorington et al., 2012), although these traits have been occasionally reported in other populations (Nash and Seaman, 1977; Keith, 2003; Allred, 2010).
DistributionTop of page
Native to Mexico and parts of the USA including Colorado, Utah and Wyoming, Abert’s squirrels appear to be continuing to expand their range gradually in many areas (Davis and Brown, 1989). Although recorded as native to Arizona and New Mexico it was introduced to isolated areas throughout the 1900s. In New Mexico, individuals were translocated to the Manzano Mountains (1929 and 1940), Sandia (1940), and Sacramento Mountains (1955 and 1969). Individuals were introduced to numerous mountain ranges in Arizona in 1941.
Little is known about the species in Mexico (Yensen and Valdés-Alarcón, 1999).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Mexico||Present||Native||Not invasive||Allred, 2010||Western Chihuahua and Durango|
|USA||Present||Present based on regional distribution.|
|-Arizona||Widespread||Invasive||Davis and Brown, 1989; Allred, 2010||Native but introduced to numerous locations in 1941, including Bradshaw Mountains, Granite Dells, Hualapai Mtns, Mt. Logan, Santa Catalina Mtns, Pinal Mtns, Pinaleno Mtns and believed to have spread from these sites to the Superstition, Rincon and Mazatzal Mtns.|
|-Colorado||Widespread||Native||Not invasive||Allred, 2010|
|-New Mexico||Widespread||Davis and Brown, 1988; Allred, 2010||Native but also introduced to the Manzano (1929 and 1940), Sandia (1940), and Sacramento Mtns (1955 and 1969)|
|-Utah||Localised||Native||Not invasive||Allred, 2010||Introduced to Dixie National Forest in 1898 and 1908 but success is unknown|
|-Wyoming||Localised||Native||Not invasive||Allred, 2010|
History of Introduction and SpreadTop of page
Introductions are thoroughly documented in Davis and Brown (1988) and Allred (2010) and include almost exclusively S. a. aberti, except where noted below. In 1898 to 1908 individuals were moved likely from Arizona to the Dixie National Forest’s Markagunt Plateau in Utah but the status of the population has not been documented recently; individuals were seen for a few years post introduction but have not been reported in at least 50 years. Individuals from elsewhere in New Mexico were translocated to New Mexico’s Manzano Mountains (1929 and 1940) and Sandia Mountains (1940); the source of introductions to the Sacramento Mountains (1955 and 1969) is unknown. The first two locations retain healthy populations of S. aberti; however, the status of the third population is not known. Arizona Game and Fish Department translocated animals in 1941 from the Fort Valley Experimental Forest in northern Arizona to numerous isolated mountain ranges (Bradshaws, Granite Dells, Hualapais, Pinals, Pinalenos, and Santa Catalinas). A population was established on Mingus Mountain in Arizona; however, the source, agent and reason are not known. S.a. kaibabensis was introduced from the Kaibab Plateau to Mt. Logan in 1971 and 1977 to establish populations that remain today. Introduced populations have spread to the Superstition, Rincon and Mazatzal Mountains of Arizona most likely through natural dispersal acrossed relatively hostile habitat.
IntroductionsTop of page
|Introduced to||Introduced from||Year||Reason||Introduced by||Established in wild through||References||Notes|
|Natural reproduction||Continuous restocking|
|Arizona||Arizona||1941||Hunting, angling, sport or racing (pathway cause)||Yes||Davis and Brown (1988)||Deliberate|
|New Mexico||Arizona||1929-1940||Hunting, angling, sport or racing (pathway cause)||Yes||Davis and Brown (1988)||Deliberate|
|Utah||Arizona||1898-1908||Hunting, angling, sport or racing (pathway cause)||Yes||Davis and Brown (1988)||Deliberate|
Risk of IntroductionTop of page
Risk of introduction likely remains small as state and federal agencies have ceased such routine translocations and the introductions may have led to declines in native species of tree squirrels (Arizona gray squirrels (Sciurus arizonensis) and Mt. Graham red squirrels (Tamiasciurus hudsonicus grahamensis)) in Arizona. Natural spread is likely to be slow and limited as most habitats are isolated (Davis and Brown, 1989). Many consider this species to be aesthetically pleasing; therefore, there may be some potential for movement through the pet trade by individuals.
HabitatTop of page
As tree squirrels, Abert’s squirrels inhabit areas with forest although they have occasionally been reported from places such as above the treeline in the San Francisco Peaks of Arizona (Cooper, 1987). Abert’s squirrels are often suggested to be obligates of ponderosa pine (Pinus ponderosa) forests that contain abundant conifer seeds and moderately closed canopies for efficient foraging and protection from predators (Keith, 1965; Allred, 2010). Recent evidence, especially for introduced populations, suggests that S. aberti can use and persist in more diverse coniferous forests (Edelman and Koprowski, 2009). Mosaic, heterogenous and unevenly aged forests are particularly favoured (Dodd et al., 2006). Dodd et al. (2006) found that the maintenance of 24 to 42% of the area in high quality dense mesoreserves with a matrix composed of mixed forest management prescriptions was most beneficial to Abert’s squirrels and also provided fire-resistance. The dependence on ponderosa pine forest is likely overstated; however, Abert’s squirrels clearly are able to persist nearly exclusively on this single tree species and associated fungi (Keith, 1965; Dodd et al., 2003; Allred, 2010). Squirrels nest in trees either in cavities or bolus nests constructed of leaves, grass and twigs of trees (Allred, 2010; Edelman et al., 2009) and appear to favour the dense growth of twigs that result from parasitic mistletoe infections known as witch’s brooms (Farentinos, 1972a; Garnett et al., 2006). Prather et al. (2006) elegantly modelled abundance and recruitment in populations. Squirrel density was highest for models that included high local-scale basal area and canopy cover >60% at the 65-ha spatial scale. For squirrel recruitment, high local-scale basal area and variable canopy cover over extents of approximately 160–305 ha were most strongly related to positive recruitment.
Habitat ListTop of page
|Terrestrial – Managed||Managed forests, plantations and orchards||Principal habitat||Harmful (pest or invasive)|
|Managed forests, plantations and orchards||Principal habitat||Natural|
|Urban / peri-urban areas||Secondary/tolerated habitat||Natural|
|Urban / peri-urban areas||Secondary/tolerated habitat||Productive/non-natural|
|Terrestrial ‑ Natural / Semi-natural||Natural forests||Principal habitat||Harmful (pest or invasive)|
|Natural forests||Principal habitat||Natural|
Biology and EcologyTop of page
Chromosome number is 2n = 40, with 14 metacentric, 24 submetacentric, a metacentric X, and an acrocentric Y chromosome (Nadler and Sutton, 1967). Assessments of intrapopulation genetic variation are still needed; however, S. aberti does show high variation in T-cell receptor genes (tcra and trb: Wettstein et al., 1990) and MHC class II genes (Wettstein and States, 1986). Variation in mtDNA and the cytochrome b gene is enough to determine phylogenies for the subspecies (Wettstein et al., 1994, 1995). No hybrids with other species are known.
S. aberti demonstrates a strong seasonal pattern of reproduction (Thorington et al., 2012). Males have scrotal testes from late autumn through early summer, after which the testes become small and are withdrawn into the abdominal cavity (Keith, 2003; Allred, 2010). Breeding takes place primarily between mid-February and June (Keith, 2003) but can occur in just about any month (Allred, 2010). Females are in estrus for less than a day (Brown, 1984), when males are attracted to and compete for access to them through a linear dominance hierarchy (Farentinos, 1972c, 1974). Females mate with multiple males and occasionally solicit copulations from low ranking males (Farentinos, 1980). A single litter is produced averaging 2.9-3.4 young each year, typically in late spring or early summer at a 40-46 day gestation (Keith, 2003; Thorington et al., 2012). Paternal care is not evident and young remain with their mother until natal dispersal at about 10 weeks of age and can reproduce at about 10-12 months of age (Brown, 1984; Allred, 2010; Thorington et al., 2012).
Physiology and Phenology
Abert’s squirrels have long gastrointestinal tracts, especially the caecum, suggesting adaptation to the low quality, low digestibility cambium food habits, especially during periods of seed and fungal shortage (Murphy and Linhart, 1999). The plush winter pelage, grown in autumn, is molted in spring to help cope with thermal challenges faced by this species in high elevation conifer forests (Allred, 2010). Abert’s squirrels possess the ability to be heterothermic to exploit the thermally challenging, seasonal environments in their range (Golightly and Ohmart, 1978). Mean body temperatures are 39.0 to 40.7°C (Patton et al., 1976; Golightly and Ohmart, 1978); however, hyperthermia was experienced during intense activity in summer and winter, which induced resting in cool microclimates (Golightly and Ohmart, 1978). Hypothermia occurred when in nests during the day or night, which induced basking in the sun upon exit; tail position also appeared to influence body temperature (Golightly and Ohmart, 1978). Production of young is timed with the emergence of buds and fungi in spring and tree seeds and fungi in summer (Allred, 2010).
Abert’s squirrels generally live 3-4 years in the wild; maximum ecological longevity for males is 7 years and for females is 8 years (Allred, 2010). An individual raised in captivity survived for 9 years (Allred, 2010).
Abert’s squirrels are strictly diurnal and do not hibernate (Thorington et al., 2012). Although active all year round and throughout the day, seasonal and diel patterns have been recorded (Allred, 2010). Nests and shaded or sun-soaked areas are used to behaviourally thermoregulate over the course of a day or season (Golightly and Ohmart, 1978). Individuals exit nests soon after sunrise and bask in the sun for variable periods of time. Activity occurs throughout the day with a mid-day lull, especially in summer, that may include returning to the nest, basking in sun or cooling in the shade. Individuals return to the nest at variable times in the evening but almost always before darkness (Hall, 1981; Halloran, 1993). Rain and winds can hasten return to the nest and appear to be the most disruptive weather influences (Golightly and Ohmart, 1978).
Population Size and Structure
Abert’s squirrels are relatively solitary animals that do not defend territories and do tolerate overlap and occasional nesting in pairs or small groups (Farentinos, 1974; Edelman and Koprowski, 2007; Allred, 2010). Densities reported fluctuate considerably from 2.5 to 5 sq/km2 (Keith, 1965), 8 to 16 (Brown, 1984), 7 to 33 (Dodd et al., 1998), 1 to 30 (Ramey, 1973), 31 to 56 (Farentinos 1972a), 12 to 124 (Lawson, 1941 in Keith, 2003), 30 to 124 (Trowbridge and Lawson, 1942 in Keith, 2003), and 247 sq/km2 (Patton, 1975).
Sex ratios for Abert’s squirrel populations demonstrate considerable variation. Brown (1984) summarized data across studies for 853 animals and found 56.7% were males. In contrast, Mearns (1907), Stephenson (1975), and Dodd et al. (1998) examined a total of 614 squirrels and found that 54% were females. Combining these data gives a male to female ratio of 52:48 (Keith, 2003). Autumn populations consistently were composed of about 45% young each year, ranging from 21-56% (Brown, 1984).
Winter mortality of females can be especially high in years of heavy snowfall (Lema 2001). Snow cover was most influential to trends in Abert’s squirrel abundance. Annual winter mortality is negatively related to number of days with snow cover of 10 cm or more and explained more than 70% of variation in annual mortality in Arizona with a maximal 66% mortality occurring in a winter with 85 days of heavy snow cover (Stephenson and Brown, 1980). Squirrel numbers in spring were a function of population levels the previous autumn and snow-induced mortality during winter (Stephenson and Brown, 1980).
Precipitation is also important to recruitment. In the driest winter and spring ever recorded for Arizona (42% of normal precipitation), Dodd et al. (2003) found that only seven out of 160 adult females displayed signs of reproduction (mean recruitment was 0.04 juveniles per adult female on nine study sites). In contrast, in years of 75% and 90% of normal precipitation, recruitment was 0.83 and 1.79. Thus weather in the form of heavy winter snowfall and winter/spring drought can radically alter productivity in Abert’s squirrels (Keith, 2003).
Keith (2003) succinctly summarized that fluctuations in Abert’s squirrel abundance results from: 1) abundance and quality of their foods (Patton, 1974, Brown, 1984, Pederson et al., 1987, States et al., 1988), 2) elevated mortality due to deep, persistent snows in winter (Stephenson and Brown, 1980, Brown, 1984), 3) poor recruitment during droughts (Dodd et al., 2003), and 4) differential avian predation among habitats with different densities of trees (Lema, 2001; Sieg, 2002).
The availability of different foods varies among seasons, years, and habitats. Abert’s squirrels do not store or cache large amounts of food for future use (Keith, 1965; Halloran, 1993); however, scatterhoarding of conifer cones is known (Allred, 2010). Abert’s squirrels are primarily herbivores and mycophagists (Allred, 2010). In many areas the species feeds almost exclusively on tissues of the ponderosa pine to include phloem of the inner bark of terminal shoots, seeds, pollen and emerging apical buds (Keith, 1965). In areas with little ponderosa pine (Pinus ponderosa), Engelmann spruce (Picea engelmannii) cambium is consumed (Edelman and Koprowski, 2005). Feeding on phloem is conducted by clipping the terminal branch tips and excising a 5-8 cm segment that is completely stripped of bark and the cambium removed resulting in a neatly ‘cleaned’ white segment of wood that is discarded (Brown, 1982, 1984). Trees are selectively attacked and tend to have more palatable tissues with less secondary compounds and thus more available nutrients (Snyder 1992, 1993; Allred, 2010). Other foods that are available and consumed in its native range are epigeous and hypogeous fungi, mistletoe (Arceuthobium) tissues and acorns (Quercus). Mycophagy can compose 50-95% of the diet in some years but estimates of relative contribution of species are lacking (Keith, 2003; Allred, 2010). In an area outside its native range, Abert’s squirrels feed on much the same tissues but expand the diversity of species beyond the uncommon ponderosa pine (Edelman and Koprowski, 2005). Animal matter is occasionally taken to include bird eggs and nestlings, scavenging of dead vertebrate carcasses, insects and larvae, and gnawing bones and antlers (Allred, 2010).
The species has a strong association with ponderosa pine (P. ponderosa) as noted elsewhere; however, appears able to persist in areas with very small amounts.
Natural Food SourcesTop of page
|Food Source||Food Source Datasheet||Life Stage||Contribution to Total Food Intake (%)||Details|
|Apical buds of Abies lasiocarpa||All Stages||<2%|
|Apical buds of Pinus ponderosa||All Stages||1-63%|
|Apical buds of Pinus strobiformis||All Stages||<2%|
|Apical buds of Pseudotsuga menziesii||All Stages||0-15%|
|Fruits of Arceuthobium||All Stages||0-10%|
|Leaves of Cirsium||All Stages||<2%|
|Leaves of Robinia||All Stages||<2%|
|Phloem of Pinus ponderosa||All Stages||0.50%|
|Phloem of Picea engelmannii||All Stages||0-12%|
|Phloem of Pinus edulis||All Stages||<2%|
|Pollen of Pinus ponderosa||All Stages||<2-10%|
|Pollen of Pseudotsuga menziesii||All Stages||0-12%|
|Seeds of Juniperus osteosperma||All Stages||<2%|
|Seeds of Lupinus||All Stages||<2%|
|Seeds of Pinus edulis||All Stages||<2%|
|Seeds of Pinus flexilis||All Stages||<2%|
|Seeds of Pinus ponderosa||All Stages||5-50%|
|Seeds of Pinus strobiformis||All Stages||0-33%|
|Seeds of Pseudotsuga menziesii||All Stages||0-15%|
|Seeds of Quercus gambelli||All Stages||<15%|
ClimateTop of page
|B - Dry (arid and semi-arid)||Tolerated||< 860mm precipitation annually|
|BS - Steppe climate||Tolerated||> 430mm and < 860mm annual precipitation|
|C - Temperate/Mesothermal climate||Preferred||Average temp. of coldest month > 0°C and < 18°C, mean warmest month > 10°C|
|Cf - Warm temperate climate, wet all year||Tolerated||Warm average temp. > 10°C, Cold average temp. > 0°C, wet all year|
|Cs - Warm temperate climate with dry summer||Preferred||Warm average temp. > 10°C, Cold average temp. > 0°C, dry summers|
|D - Continental/Microthermal climate||Preferred||Continental/Microthermal climate (Average temp. of coldest month < 0°C, mean warmest month > 10°C)|
|Ds - Continental climate with dry summer||Preferred||Continental climate with dry summer (Warm average temp. > 10°C, coldest month < 0°C, dry summers)|
Latitude/Altitude RangesTop of page
|Latitude North (°N)||Latitude South (°S)||Altitude Lower (m)||Altitude Upper (m)|
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Absolute minimum temperature (ºC)||-30|
|Mean annual temperature (ºC)||4.4||15.6|
|Mean maximum temperature of hottest month (ºC)||20||32|
|Mean minimum temperature of coldest month (ºC)||-15||-4.5|
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
|Accipiter cooperii||Predator||All Stages||not specific|
|Accipiter gentilis||Predator||All Stages||not specific|
|Aquila chrysaetos||Predator||All Stages||not specific|
|Bubo virginianus||Predator||All Stages||not specific|
|Buteo jamaicensis||Predator||All Stages||not specific|
|Buteo swainsoni||Predator||All Stages||not specific|
|Canis latrans||Predator||All Stages||not specific|
|Ceratophyllus vison||Parasite||All Stages||not specific|
|Citellinema quadrivittati||Parasite||All Stages||not specific|
|Crotalus||Predator||All Stages||not specific|
|Dermacentor andersoni||Parasite||All Stages||not specific|
|Eimeria ontarioensis||Parasite||All Stages||not specific|
|Eimeria tamiasciuri||Parasite||All Stages||not specific|
|Eimeria toddy||Parasite||All Stages||not specific|
|Enterobius sciuri||Parasite||All Stages||not specific|
|Eumolpianus eumolpi||Parasite||All Stages||not specific|
|Falco peregrinus||Predator||All Stages||not specific|
|Hystrichopsylla dippiei||Parasite||All Stages||not specific|
|Lynx rufus||Predator||All Stages||not specific|
|Orchopeas caedens||Parasite||All Stages||not specific|
|Orchopeas neotomae||Parasite||All Stages||not specific|
|Oropsylla idahoensis||Parasite||All Stages||not specific|
|Pituophis||Predator||All Stages||not specific|
|Puma concolor||Predator||All Stages||not specific|
|Urocyon cinereoargenteus||Predator||All Stages||not specific|
|Vulpes vulpes||Predator||All Stages||not specific|
|West Nile virus||Pathogen||All Stages||not specific|
|Yersinia pestis||Pathogen||All Stages||not specific|
Notes on Natural EnemiesTop of page
Endo- and ectoparasitic loads are generally low in Abert’s squirrels (Patrick and Wilson, 1995) as is disease (Allred, 2010). Allred (2010) provides a summary of natural enemies of S. aberti. A diversity of medium and large carnivores appear to feed on Abert’s squirrels opportunistically, although documented killing and feeding is rare; the most thorough study estimated the species to be about 5% of the diet of mammalian carnivores (Canis latrans, Lynx rufus, Puma concolor) (Turkowski,1980). Although most raptors large enough will occasionally attack an Abert’s squirrel, only Accipter gentilis demonstrates a strong dependence on the species as prey (Drennan and Beier, 2003).
Means of Movement and DispersalTop of page
Abert’s squirrels possess reasonable vagility and despite the supposed link to ponderosa pine (Pinus ponderosa) forests, 20% of museum records are from lower elevation pinyon-juniper forests (Hoffmeister, 1986) and individuals can persist in non-ponderosa pine (P. ponderosa) forests (Pederson et al., 1976; Davis and Brown, 1989; Edelman et al., 2009) and have been observed crossing open areas more than 1 km from the nearest forest (Cooper, 1987). Home ranges are typically 2.5 to 34 ha with male ranges sometimes greater than females (Allred, 2010; Edelman and Koprowski, 2006). Individuals are known to routinely cover distances of 1.5 km during the breeding season (Farentinos, 1979; Keith, 2003). Juveniles are documented to disperse about that same distance from their natal area (Farentinos, 1972c). Home ranges or distances moved generally expand in the mating season, especially those of males (Farentinos, 1979; Edelman and Koprowski, 2006). Ranges in poor habitat (Allred, 2010) and recently thinned habitat (Pederson et al., 1976) tend to increase relative to those in high quality or unthinned forests (Allred, 2010). The species has naturally colonized areas that are isolated by hostile habitats over the last century (Davis and Bissell, 1989; Davis and Brown, 1989) and has shown its invasive abilities by continued dispersal and range expansion following translocations (Davis and Brown, 1989). Rate of range expansion is related to isolation distance with 8 to 12 km crossed in about 10 years, 23 km in about 20 years, 29 km in 30 years and 57 km in 40 years (Davis and Brown, 1989).
A series of officially sanctioned, governmental introductions is thoroughly documented in Davis and Brown (1988) and Allred (2010). In 1898 to 1908 individuals were moved from Arizona to the Dixie National Forest in Utah but the status of the population has not been documented recently. Individuals from New Mexico were translocated to that state’s Manzano Mountains (1929 and 1940), Sandia Mountains (1940) and possibly the Sacramento Mountains (1955 and 1969); the first two locations retain healthy populations of S. aberti; however, the status of the third population is unknown. Arizona Game and Fish Department translocated animals in 1941 from the Fort Valley Experimental Forest in northern Arizona to numerous isolated mountain ranges (Bradshaws, Granite Dells, Hualapais, Pinals, Pinalenos, and Santa Catalinas). A population translocated successfully to Mingus Mountain in Arizona is not well documented. S.a. kaibabensis Merriam 1904 was introduced from the Kaibab Plateau to Mt. Logan in 1971 and 1977 to establish populations that remain in 2015. Intentional introductions have mostly been to establish new populations or additional populations for hunting and conservation.
Impact SummaryTop of page
|Cultural/amenity||Positive and negative|
|Economic/livelihood||Positive and negative|
|Environment (generally)||Positive and negative|
Economic ImpactTop of page
The economic costs of Abert’s squirrels have not been quantified. Clipping and girdling of branch tips during feeding does appear to decrease growth rates of feed trees; however, a careful controlled experimental study has not been completed (Skinner and Klemmedson, 1978; Soderquist, 1987). Abert’s squirrels can remove all seeds from an individual tree and have been documented to remove up to 74% of seeds from a local site (Larson and Schubert, 1970; Snyder, 1990; Allred et al., 1994; Rushton et al., 2006). These potential costs are to some extent, at least, mitigated by its economic, social and environmental value detailed in the Uses section.
Environmental ImpactTop of page
Impact on Habitats
S. aberti feed heavily, and in some areas exclusively, on two primary food resources (tissue and seeds of ponderosa pine (Pinus ponderosa) and fungi) and so the impacts on habitats are primarily related to the impacts in accessing energy (Keith, 1965, 2003; Allred, 2010). Abert’s squirrels do feed on the tree-parasitic mistletoes (Littlefield, 1984) and will nest in mistletoe and mistletoe-caused witches brooms (Garnett et al., 2006), however, because of the limited movements relative to much more vagile frugivorous and granivorous birds, the impact on tree health is typically discounted (Hudler et al., 1979). A major food source, especially in winter is the phloem from twigs and branch tips of ponderosa pine (P. ponderosa) (Keith, 1965). Clipping and girdling of branch tips during feeding does appear to decrease growth rates of feed trees; however, a carefully controlled experimental study has not been completed (Skinner and Klemmedson, 1978; Soderquist, 1987). Abert’s squirrels can remove all seeds from an individual tree and have been documented to remove up to 74% of seeds from a local site (Larson and Schubert, 1970; Snyder, 1990; Allred et al., 1994; Rushton et al., 2006). The leaf litter and cone debris are considerable (Pearson, 1950; Skinner and Klemmedson, 1978) and likely are important in providing fuel for the frequent ground fires that characterize open ponderosa pine forests (Pearson, 1950; Keith, 1965). Furthermore, the branch trips are quite high in nitrogen relative to other litter components and likely speed nutrient transfer (Skinner and Klemmedon, 1978).
Impact on Biodiversity
Abert’s squirrels likely have both positive and negative effects on biodiversity in their native and introduced range. Perhaps the greatest positive role for Abert’s squirrels is the spread of tree seeds (Allred, 2010) and fungal spores (Kotter and Farentinos, 1984a,b). Both of these functions likely enhance diversity of tree and fungi in local areas and the species that depend on these organisms and outweigh the costs of seed loss (Larson and Schubert, 1970; Snyder, 1990; Allred et al., 1994) and perhaps tree damage (Skinner and Klemmedson, 1978; Soderquist, 1987). In areas of introduction, Abert’s squirrels have been linked to the decline of uncommon native species in two isolated mountain ranges. Introduced to the Santa Catalina Mountains of southern Arizona near Tucson in 1941, the species naturally colonized the nearby Rincon Mountains in subsequent years (Davis and Brown, 1988). Introductions ceased when concern over the decline of native Arizona gray squirrels (Sciurus arizonensis) was voiced (Layne, 1960). After introduction to the Pinaleno Mountains of southeastern Arizona, the species spread and is now implicated in the decline of the endangered Mt. Graham red squirrel (Tamiasciurus hudsonicus grahamensis) (Rushton et al., 2006). In the latter case, the mechanism appears to be related to competition for food and space (Rushton et al., 2006).
Threatened SpeciesTop of page
|Threatened Species||Conservation Status||Where Threatened||Mechanism||References||Notes|
|Tamiasciurus hudsonicus grahamensis (Mount Graham red squirrel)||USA ESA listing as endangered species USA ESA listing as endangered species||Arizona||Competition - monopolizing resources||Sanderson and Koprowski, 2009|
Risk and Impact FactorsTop of page Invasiveness
- Proved invasive outside its native range
- Abundant in its native range
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Pioneering in disturbed areas
- Capable of securing and ingesting a wide range of food
- Highly mobile locally
- Benefits from human association (i.e. it is a human commensal)
- Has high genetic variability
- Ecosystem change/ habitat alteration
- Negatively impacts forestry
- Threat to/ loss of endangered species
- Threat to/ loss of native species
- Damages animal/plant products
UsesTop of page
The economic value of Abert’s squirrels has not been quantified. Abert’s squirrels passively disperse seeds when accessing conifer cones and actively through their occasional scatterhoarding of conifer cones (Allred, 2010). Additionally, this species is an effective dispersal agent of fungal spores, especially those that form symbiotic relationships with trees and facilitate tree survival and growth (Kotter and Farentinos, 1984a,b). Abert’s squirrel hunting is popular in some states in the US with between 22,000 and 106,000 squirrels taken in Arizona each year (Keith, 2003; Arizona Game and Fish Department, 2014) and about 1300 individuals harvested in Colorado (Keith, 2003); however, economic gains have not been calculated.
Abert’s squirrels are enjoyed by the general public and are often considered to be attractive to humans due to tasseled ears, mild behavior and tolerance of humans (Allred, 2010). A children’s book focuses on the education value of the species (Allred and Iverson 2007). Popular amongst hunters in the USA, the species has attracted 5000 to 12,000 hunters in Arizona since 2006 (Arizona Game and Fish Department, 2014).
Services provided by Abert’s squirrels in native and introduced range include the passive and active dispersal and planting of seeds when accessing or scatterhoarding conifer cones (Allred, 2010) and fungal spores, especially those that form symbiotic relationships with trees and facilitate tree survival and growth (Kotter and Farentinos, 1984a,b). Fungal spore densities ranged from 200 million to 1 billion spores/g of faeces (Kotter and Farentinos, 1984b). Furthermore, squirrels facilitate transfer of nutrients, particularly nitrogen through their feeding and clipping activities; clippings have 2x the nitrogen content of other litter (Skinner and Klemmedson, 1978).
Detection and InspectionTop of page
The species is conspicuous in size and pelage, most often nests in large bolus nests of conifer needles, leaves and twigs, and provides characteristic feeding signs on clipped branches due to its phloem feeding behaviour (Allred, 2010). No other species leaves this unique feeding sign. Transect or plot based sampling for feeding signs and individuals is the best means of detection (Brown, 1982); however, remote wildlife cameras also show promise (Edelman et al., 2005; Swann and Perkins, 2013). A number of other techniques are reviewed by Allred (2010) and include hunter success, mark-recapture trapping, nest counts, track pans and tracks in snow. These methods can provide information on presence and in some case abundance.
Similarities to Other Species/ConditionsTop of page
Abert’s squirrels are unique among North American squirrels in possessing prominent tufts or tassels on their ears during all but the summer months. Only three species of tree squirrel are found within the native or introduced range of the Abert’s squirrel: Tamiasciurus hudsonicus, Sciurus arizonensis and Sciurus niger (Thorington et al., 2012). T. hudsonicus is less than 50% the size of an Abert’s squirrel (250 g compared to 650 g) and has a strong reddish colour to the entire dorsum. S. arizonensis is about the same body size as the Abert’s squirrel but lacks elongated ears or tufts as well as the black horizontal line that separates the gray dorsum from the cream to buff venter. S. niger has been introduced and may be naturally expanding its range in Colorado, Wyoming and New Mexico but lacks elongated ears and tufts, and lacks a horizontal black line that divides a yellow-orange to orange venter from a grizzled brown agouti dorsum.
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Early Warning Systems
The conspicuous nature of this species suggests that biologists and interested lay people will likely note its presence early in any natural or human-facilitated event.
No methods have been developed to remove Abert’s squirrels in any portion of their natural or naturalized range.
Public awareness of the spread and potential invasiveness of the species is quite low and attempts to increase awareness should be made.
No attempts have been made to eradicate the species anywhere where it is distributed; however, Arizona has recently liberalized hunting regulations to increase take in some sites of past introductions.
Habitat for expansion seems limited in the arid west of North America where the species is found, although the species continues to gradually increase its range (Davis and Brown, 1988).
Standard techniques for eradication such as trapping and shooting have been used on other squirrel species with limited success and none have been tried on this species.
US and Mexican regulations prohibit movement of animals without prior government approval.
Monitoring and Surveillance (Incl. Remote Sensing)
Remote cameras and surveys for feeding sign/clippings (Brown, 1982) are the best available methods for monitoring this species.
Mitigation methods have not been studied in any of the areas of introduction.
No studies have assessed restoration of forest ecosystems that have been invaded by S. aberti. Abert’s squirrels likely help with inoculation of degraded soils with fungal spores (Kotter and Farentinos, 1984a,b) and also disperse seeds during foraging and feeding activity (Keith, 1965; Edelman and Koprowski, 2005). Because this species can use relatively open forests following fire and forestry, the species may have a positive role in forest restoration in such areas.
ReferencesTop of page
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ContributorsTop of page
01/07/15 Original text by:
John Koprowski, University of Arizona, Tuscon, AZ 85721, USA
Distribution MapsTop of page
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