Many of the iNat sightings are from deciduous trees (cottonwoods and willows) where porcupines are more visible during winter leaf-off.
Previous research did not emphasize the importance of these deciduous species.
Taylor commented that "Occurrences in junipers, willows. black walnuts, aspens, and cottonwoods are apparently limited to a very few records out of several hundred available. No evidence is at hand that the porcupine, in the Southwest proper, feeds to any extent on these last-named trees…"
Brown and Babb only reported 5 porcupines in riparian deciduous trees out of their total 214+ observations, and McCarthy only reported 4 in these trees out of his total 56 observations.
It is possible that the preponderance of iNat porcupines in these trees is due to observer bias, with the Willow lake and Petrified Forest hosting large numbers of hikers and nature enthusiasts. However, it should be noted that many other areas of the state (including the Grand Canyon and areas around Flagstaff) also host large numbers of recreationalists without reporting large numbers of porcupines. However, as stated above, deciduous trees leaf-off state does make porcupines easier to spot.
Looking at iNat observations of live porcupines on the ground, it does look like they are most active in June, with elevated activity through October.
Porcupine Distribution
McCarthy reported a continuation of the observations by Brown and Babb, i.e. that porcupines are sparsely spread throughout the habitats where they have been reported. While this is true as far as it goes, it does appear that there are certain areas of either greater porcupine population density or greater observer bias in photographing them. About half of the iNat observations are from two discrete locations: Willow lake in Prescott, and Petrified Forest National Park near Holbrook.
McCarthy noted that porcupines commonly occur in habitats that are not dominated by conifer trees. That certainly continues to be the case in the iNat data. Taylor's original paper noted that national forests were the preferred habitat of porcupines, but in more recent years they appear to be more common in deciduous forests, grasslands, and other non-conifer forest habitats.
There are areas of apparently good habitat that do not support porcupine populations. The Prescott National Forest, despite extensive stands of ponderosa pine with mixed oak understory, has consistently been noted as not having many porcupines. Brown and Babb reported 7, but interestingly these were all from grasslands, not the forests areas. Based on personal communication with employees of the Forest, no porcupines have been observed recently on that forest.
Taylor noted: "The porcupine…appears to attain its greatest numbers in parts of the San Juan (Colorado), Carson and Cibola (New Mexico), Coconino and Tusayan (Arizona) national forests. On some forests where conditions seem as favorable as on those mentioned, as the Santa Fe, Manzano, Apache, Kaibab, and Sitgreaves, porcupines are for the most part scarce or of little economic importance. In general as one goes southward porcupines become less numerous. They are decidedly scarce on the Lincoln, Gila, Crook, Tonto, and Prescott forests."
Another area of apparently suitable habitat is the upper Verde river, which has an extensive stand of cottonwood and willow trees surrounded by wildlands. Surveyors, who look for Yellow Billed Cuckoos throughout this area each month of the growing season, report that they have never seen a porcupine. Yet porcupines are well known from the cottonwoods and willows around nearby Willow lake in Prescott.
Each of the previous authors have speculated that mountain lion predation may control porcupine abundance. It may be that mountain lions are less present around Willow lake in Prescott and in Petrified Forest National Park, and more abundant along the upper verde and in the conifer forests of Prescott National Forest. The present study cannot cast any light on that hypothesis.
Another hypothesis for the patchy distribution of porcupines is habitat fragmentation by roads and other human development. As discussed above, the present study did not find a high proportion of porcupine roadkill, but incidental observations and discussions suggests that porcupines are commonly killed on roads but those observations were not documented in iNaturalist.
If porcupine populations are small and patchy in distribution, and if migrations between populations is difficult and uncertain, then porcupine populations may be reproductively isolated.
Taylor: "A noteworthy feature of porcupine distribution is its lack of uniformity. In some regions the animals will be fairly abundant, while in others, perhaps not far away, they will be scarce, although conditions appear to be equally favor-able."
Uldis Roze, in "The North American Porcupine," suggested that porcupines are dependent on a species-specific microbiome to digest their high cellulose diet of rough plant matter. This is based on observations that when porcupines are introduced to a new area they consume the fecal pellets of resident porcupines in an apparent attempt to inoculate their microbiome. Porcupines eat a wide variety of plant species, but individual porcupines are documented preferring certain plants, possibly based on their ability to digest them.
If these ideas are correct, then porcupines may have difficulty colonizing areas that do not currently support porcupines. It may take awhile to develop a "taste" for plants in different areas. If so, porcupine populations may be at risk of long term decline in Arizona. Small and isolated populations may die out, and if nearby porcupines cannot safely travel and cannot easily digest the different plants in those areas, it may be difficult or impossible to replace extirpated populations.
Taylor: "The porcupine must occasionally, if not regularly, make long trips across country. It must possess considerable capacity to adapt itself to whatever dens, natural burrows, rocky shelters, or vegetative cover it can find in the non-timbered areas into which it roams. The obvious wanderlust of the animal must tend to insure the species the widest possible geographic and ecologic range. Foster reports occasional porcupines found in badger holes in the treeless Williamson valley, Yavapai county, Arizona."
The large continuous band of conifers across the national forests of Arizona should continue to provide habitat for sustainable porcupine populations. Hopefully the few scattered iNat observations across this area are few and scattered due to lack of observers and not lack of porcupines. If porcupines are not doing well in this bastion of habitat they indeed face an uncertain future in Arizona.
The American Southwest, including parts of Texas, NM, and Arizona marks the southern extent of porcupines except for a few endangered populations in the mountains of Mexico. As the climate warms, it is possible that porcupines find Arizona's environment increasingly challenging. However, Taylor states that porcupines are limited by food availability, not climatic extremes.
Brown, David E., and Randall D. Babb. "Status of the Porcupine (Erithizon dorsatuh) in Arizona, 2000–2007." Journal of the Arizona-Nevada Academy of Science 41.2 (2009): 36-41.
McCarthy, Michael. "Porcupines (Erethizon dorsatum) in Arizona, 2011–2015." Journal of the Arizona-Nevada Academy of Science 47.1 (2017): 19-22.
Roze, Uldis. The North American porcupine. Cornell University Press, 2009.
Taylor, Walter Penn. Ecology and life history of the porcupine (Erethizon epixanthum) as related to the forests of Arizona and the southwestern United States. No. 3. University of Arizona, 1935.
