(Note: Now the U.S. Geological Survey is using terminology from The Wildlands Project. This is NOT good! Do the following two quotes from the article raise any Red Flags? "Managing land use on private inholdings, on properties adjacent to public land, and on properties bordering intermountain corridors will be increasingly important." "Because of the geological, topographic, and biological uniqueness of each sky island, the policies for each mountain range will need to be custom-designed on a watershed by watershed basis.")

By Peter Warshall

University of Arizona

Office of Arid Lands Studies

4500 W. Speedway

Tucson, AZ 85745

The "sky islands" of Arizona and New Mexico in the southwestern United States form a unique complex of about 27 mountain ranges whose boundaries, at their lowest elevation, are desert scrub, grasslands, or oak woodlands (Figs. 1 and 2; Table 1). Since the last glaciation, these forested mountain ranges have become relatively isolated from each other.

Expanding desert grasslands and desert scrub in the valleys ("the sea" between the sky islands) have limited genetic interchange between populations and created environments with high evolutionary potential.

The resulting sky island ecosystems support many perennial streams in an arid climate, have a high number of endemic species, and harbor most game species as well as most threatened and endangered species in the Southwest.

The southwestern sky island "archipelago" is unique on the planet. It is the only sky-island complex extending from subtropical to temperate latitudes (compared to the Great Basin, the Venezuelan, and the African sky islands) with an exceptionally complex pattern of species of northern and southern origins.

The "continents" that have been the main sources of species for the archipelago are the Sierra Madre of Mexico and the Rocky Mountains of the United States, although the flora has been influenced by the Californian, Sonoran, Intermountain, Cordilleran, and Sierra Madrean Floristic Provinces (S. McLaughlin, University of Arizona, unpublished data).

The ecosystems of each mountain range are of major interest to resource managers concerned with preserving each sky island's unique biogeography and biological diversity as well as to the public for recreation.

Land uses sometimes conflict on the sky islands: camping, rock climbing, car-based tourism, military maneuvers, hunting, fishing, exotic grass and fish stocking, grazing, water-supply withdrawals, timber and fuelwood extraction, bird watching, critical habitat for threatened and endangered species, skiing, summer homes, mining, scientific research, sacred Native American ceremonies, and archaeological sites.

Most American sky islands are within the Gila River basin. About 15 additional sky islands are in Mexico and will not be discussed here.

Nevertheless, the cross-border management of sky islands is important for such tasks as reintroduction of the Mexican wolf (Canis lupus baileyi), maintenance of disjunct populations of rare plant species, and migration of the Mexican pronghorn (Antilocarpa americana mexicana), if it still occurs.

Status of Information

The floras of the largest sky islands of Arizona have been inventoried (S. McLaughlin, University of Arizona, unpublished data), including most insular, endemic, and rare species.

Certain inventory gaps (e.g., the Baboquivari, Galiuro, Santa Rita, Whetstone, and Patagonia mountains) exist. In addition, the number of sites for rare or insular plants, their abundance at each site, and other species diversity indices are lacking for many species of concern.

The areal extent, age class, structural characters, and regeneration rates of the five or six biotic communities on the sky islands are poorly known, especially for oak woodlands (McPherson 1992).

Fungi have been intensely inventoried on the Chiricahuas, though only partial inventories exist for ranges of mycorrhizal hypogeous fungi, truffles, and false truffles (States 1990; Nishida et al. 1992). The lichen flora, one of the most diverse and complex in western North America, is poorly inventoried for almost all the sky islands.

The highest sky islands, except the Peloncillos and the Animas, have been intensely inventoried for all groups of insects (C. Olson, University of Arizona, personal communication). Spider and pseudoscorpion distribution is poorly understood. The larger millipedes and scorpions have been extensively collected, but the micro-millipedes, the insular flightless beetles, and the flightless grasshoppers in the upper elevations are poorly known.

For instance, a 6-week survey on top of the Pinalenos yielded three new species of flightless beetles (Warshall 1986).

The land mollusks have been inventoried (Bequaert and Miller 1973), though their range extensions and taxa need review. The cienaga (wetland) mollusks are being studied by the Smithsonian.

Fish, birds, amphibians, reptiles, and mammals are well-inventoried and yield continuing surprises such as the recent discovery of the Ramsey Canyon leopard frog (Rana subaquavocalis).

Specific inventory and monitoring gaps in frequency and abundance of sensitive species remain.

Flora and Fauna

A major dividing line between the flora and fauna of southern and northern origins occurs in the sky island ecoregion.

The sky island complex harbors more than 2,000 plant species.

Of the more than 190 snail species in the Southwest, the sky islands support 3 endemic genera, and over 60 endemic species, including the genus Sonorella and the monotypic genus Chaenaxis.

More than 75 reptile species inhabit the sky islands, one of the most diverse herpetological regions in North America with several endemic races.

About 265 bird species occur within the sky island complex, including valley and riparian species. About 30 are of subtropical origin and have their northern limits within the sky island complex.

The sky islands are the most diverse U.S. area for mammals; some 90 native mammals inhabit the area from the chaparral community to higher elevations; at least 6 are endemic subspecies.

Trends and Management

Researchers have begun measuring biological trends in six major categories concerning inventory, monitoring, preservation, and restoration that are most pertinent to sky island forested ecosystems.

A discussion of these categories follows.

Endemics and Insular Species

With new investigative techniques, there has been growing respect for the genetic diversity of this area, especially late-Cenozoic and Pleistocene relict faunal populations (see examples, Tables 2, 3).

For instance, recent genetic analyses on the Mt. Graham red squirrel (Tamasciuris hudsonicus grahamensis) and the lemon lily (Lillium parryi) showed that both populations are more highly divergent from closely related populations than previously thought.

Increasingly, however, local and insular species have hybridized with introduced races and species; hybridization is particularly evident among fish (e.g., hybrids of the Apache trout, Oncorhynchus apache, with the rainbow trout, O. mykiss), but can also be found among white-tail deer (Odocoileus virginianus versus hemionus), pronghorn (Antilocapra americana versus mexicana), turkeys (Meleagris merriami versus mexicana), and bighorn sheep (Ovis mexicana versus nelsoni). Selected rare, unique, threatened, and endangered species and subspecies whose critical habitat includes the sky islands are listed in Tables 2 and 3. Figure 3 compares the Coronado National Forest to other forests.

The number of sensitive plant and animal species from the sky island ecoregion has increased over the last 20 years. (Sensitive means that the population's viability is of concern and requires monitoring or active protection.)

The increase is, in part, the product of more detailed knowledge.

For instance, a recent review of Erigeron pringlei split this fleabane into four species, creating a new endemic, E. heliographis, on the Pinalenos. Nevertheless, the Coronado Forest reports 56 sensitive plants, among the largest number reported from any national forest, including 1 on the federal endangered list and 3 candidate species. McLaughlin (University of Arizona, unpublished data) suggested that the local extirpation of six plant species in the last century was related to either global warming, habitat alteration, or both.

Seven insects are listed by the Coronado National Forest as species of concern (U.S. Forest Service, unpublished memo).

About 12 fish species are considered vulnerable, including 9 federally listed and 7 living in sky island drainages (Table 3).

Within this national forest, there are 11 amphibians whose population viability is of concern, though none is on the federal list; 8 dwell in the valleys or lower drainages of the sky islands.

They are sensitive to upstream watershed alterations.

Fourteen sky island reptiles are considered sensitive but not federally listed.

There are about 55 bird species of concern, 5 federally listed, and about 20 whose population viability is of significant concern (Table 3).

About 30 mammals are of concern, 4 federally listed. The grizzly bear, jaguar, ocelot, Tarahumara frog, and gray wolf have been extirpated from the sky island archipelago.

Not counting the extirpated species and the 11 bats of concern, there are 13 mammal species and subspecies dwelling on the sky islands that have low populations of concern (Table 3).

*This list is incomplete because of revisions since publication in 1988. Only animals inhabiting sky islands within or above the oak-pine woodlands are included. Some wetland species have not been included. The species must have been on federal lists. Various -- more than two sky-island ecosystems; T -- threatened; E -- endangered; C -- candidate in Arizona or New Mexico; ( ) -- listing by Arizona Fish and Game only; federal status not yet determined.

Distribution

Some of the most interesting aspects of sky island ecosystems and history are why some mountains lack a particular species ("holes"), why some species skip mountain ranges or appear as an exception in an otherwise species-poor flora or fauna ("outliers"), and why some species, even mobile animals such as birds, end their distribution on a particular sky island (Warshall 1986).

For example, why are there no chipmunks on the Huachucas?

Why does the Mexican chickadee (Parus sclateri) stop on the Chiricahuas, but only 35 miles away, the mountain chickadee (P. gambeli) inhabits the Pinalenos?

Why are there no voles on the Catalinas?

Colonization of the sky islands by exotic species is increasing with over 60 non-native plants having established regenerative populations in the Arizona sky islands.

Major issues include limiting introductions of buffel grass (Pennisetum cilaris) and exotic lovegrasses (Eragrostis spp.) by the U.S. Department of Agriculture, as well as controlling and restoring habitats swamped by exotic forbs such as Euryops multifida on the Pinalenos and Catalinas.

Fifteen non-native fish species have been added to the five or six native freshwater fish families, with consequent hybridization, predation, and competition throughout springs and drainages.

The Central Arizona Project has become a new corridor for exotic fish, some of which are invading the last strongholds of natives.

Three feral exotic mammals may have colonized the sky island complex. The opossum (Didelphis spp.) colonization is believed to be a mix of released Virginia opposum (D. virginiana and range-expanding Mexican opossum (D. marsupialis). It has been reported but not confirmed that the European ferret (Mustela putorius) has become feral in the Huachucas.

Over the last 50 years, about 12 birds and mammals of southern origin have been recorded colonizing more northern sky islands. No animal species is known to have retreated south except the extirpated jaguar, ocelot, and thick-billed parrot. A few species such as the Abert's squirrel (Sciuris aberti) have been introduced for hunting and have then expanded their range.

The monitoring of these changes will be an important barometer to how habitat changes, species introductions, and climate interact with ecosystem management practices.

Vertical Migration

Each sky island has a unique ecosystem with a stack of life zones ranging from arid to boreal (Fig. 2).

Many species migrate vertically to feed and breed at various elevations.

The Pinalenos contain the most stacked life zones in the shortest vertical distance of any mountain in North America.

By traversing five biotic communities in a few hours, bears can feed on Opuntia (prickly pear cactus) fruit in the morning and grass roots growing in semi-alpine meadows in the afternoon.

Assuring minimal viable habitat size and the appropriate forest age-class structure to support animal populations with vertical migration is an unstudied aspect of forest ecosystem management.

In addition, various biotic communities are remnants of colder climates with small relict acreage. For instance, only about 243 ha (600 acres) of spruce-fir (Picea engelmannii-Abies lasiocarpa) forest are left within the sky island complex.

This forest type, found only on the Pinalenos, is critical habitat for the endemic and federally listed endangered Mt. Graham red squirrel, which also inhabits the transition to mixed conifer forests (Douglas fir-white fir; Pseudotsuga menziesii-Abies concolor) at lower elevations.

These two plant associations, heavily logged and cleared, will not become mature enough to supply the minimum viable habitat to ensure the squirrel population's survival for 2 centuries.

Annual growth rates, seeding rates, and regeneration cycles have become less predictable with the unknown effect of global warming and fire risk, requiring rethinking of the minimum size required for viable habitats.

Special Habitats

Special habitats and plant associations (e.g., high-elevation cienagas, limestone outcrops, perennial streams, talus slopes) create islands of habitat within the sky island ecosystem, increasing biological richness.

For instance, talus slopes support a series of endemic land snails; the rock cliffs and outcrops support plant species such as the fleabanes Erigeron lemmonii, E. heliographis, and E. pringlei, found nowhere else in the world. The perennial streams support seven rare native fish species.

Of the special habitats, the cienagas (swampy, marshy cover) and perennial streams require the most monitoring, protection, and restoration.

Special Interest Game

The densest populations of most game species are found on the sky islands.

For instance, the densest populations of black bear (Ursus americanus) and mountain lion (Felis concolor) south of the Mogollon Rim are on the Pinalenos.

In general, over a 20-year period, both species increased with population troughs occurring from rancher depredation and drought. White-tailed deer (Odocoileus virginianus) populations have increased, while mule deer are less stable. Javelina (Tayassu tajacu) are stable or declining, having suffered from canine distemper after the drought of 1989.

Band-tailed pigeon (Columba fasciata), a species dependent on sky island forests, has had a long-term decline as have two subspecies of turkey.

Corridors

For many land animals, corridors of animal movement between sky islands have been through riparian zones.

Increasing habitat fragmentation from increased subdivisions around the base of the sky islands is further isolating some populations, especially in the Tucson area, which separates the Santa Catalina and Rincon mountains from the Tucsons and the Santa Ritas.

This structural change in migration patterns has not been studied but is believed to be the most significant threat to "safe passage" corridors between sky islands.

In summary, the single best indicator of ecosystem management has been the increasing number of threatened and endangered populations (USFS 1993).

This trend requires increasing acreage of critical or otherwise protected habitat; increased monitoring and control over the introduction and spread of exotic grasses, fish, and gamebirds, and the reintroduction of locally extirpated mammals and tree species in restoration projects.

Other Issues

Fire management is planned to reduce catastrophic fires from fuel build ups, to allow natural burns required by certain species, and to increase fire suppression to maintain remnant old-growth forest biodiversity. Experimentation and debate about fire management are widespread, however.

Another trend is toward the restriction of cattle to prevent overgrazing and trampling, to protect sensitive plant species, and to protect and promote recovery of wetland and riparian habitats.

A third trend is toward upstream rehabilitation in specific watersheds where flooding endangers sensitive plants.

In addition, there is increasing urban pressure on the Forest Service to clear more habitat for recreation such as camping and skiing (on the Catalinas) and to expand roads into the sky islands for greater access and uses that can conflict with habitat protection (USFS 1993).

Managing land use on private inholdings, on properties adjacent to public land, and on properties bordering intermountain corridors will be increasingly important.

The final trend is the unknown impact of global warming on the biseasonal (winter and summer) rainfall pattern of the southwestern sky islands. This trend is of special importance because of the large number of relict and insular species and subspecies in the region.

Because of the geological, topographic, and biological uniqueness of each sky island, the policies for each mountain range will need to be custom-designed on a watershed by watershed basis.

References

Bequaert, J.C., and W.B. Miller. 1973. The mollusks of the arid Southwest. University of Arizona Press, Tucson. 271 pp.

Marshall, J.T. 1957. Birds of the pine-oak woodland in southern Arizona and adjacent Mexico. Pacific Coast Avifauna 32. 125 pp.

McPherson, G.R. 1992. Ecology of oak woodlands in Arizona. In Ecology and management of oak and associated woodlands. Rocky Mountain Forest and Range Station, Gen. Tech. Rep. RM-218. Fort Collins, CO.

Nishida, F.H., W.J. Sundberg, J.A. Menge, J.S. States, R.E. Tulloss, and J. Cifuentes Blanco. 1992. Studies in the mycoflora of the Chiricahua Mountains. I. Preliminary report on species distribution, ecology, and biogeographical affinities. Pages 35-39 in A.M. Barton and S. Sloane, eds. 1992 Chiricahua Mountains Research Symposium, Proceedings. Southwest Parks and Monument Association, Tucson, AZ.

States, J.S. 1990. Mushrooms and truffles of the Southwest. University of Arizona Press, Tucson. 232 pp.

USFS. 1993. Executive summary, Coronado National Forest plan five year review report, October 1987 through September 1991. U.S. Forest Service, Tucson, AZ. 33 pp.

Warshall, P. 1986. Biogeography of the high peaks of the Pinalenos. Reprinted from the Environmental Data Book, U.S. Forest Service, Coronado National Forest, by Maricopa Audubon Society, Phoenix, AZ. 18 pp.

http://biology.usgs.gov/s+t/noframe/r119.htm