Modelling the population dynamics of the Mt. Graham red squirrel: Can we predict its future in a changing environment with multiple threats?

The Mt. Graham red squirrel (Tamiasciurus hudsonicus grahamensis; MGRS) is among the most critically endangered mammals in the United States and is isolated on the periphery of the species’ range, potentially increasing its conservation priority. To investigate potential threats to the population and provide a tool for land managers, we developed a spatially explicit population dynamics model. We tested model predictions using available range-wide data from the literature and field work specific to the MGRS. A general model input data set using mean life history values overpredicted MGRS abundance. However, we found significant correlation with known squirrel abundance using a general data set with curtailed fecundity and survival. A model with MGRS-specific data provided the best fit to observed population size. We investigated potential impacts of two major threats to the MGRS: competition from introduced Abert’s squirrels (Sciurus aberti) and increased levels of predation. Predation and particularly competition could have significant effects on the future population of the MGRS. Careful attention must be used to model the viability of fringe populations as peripheral populations can have a different life history than populations found in the range core.     

Persistence of Gunnison’s prairie dog colonies in Arizona, USA

Although the five species of prairie dogs (Cynomys spp.) are recognized as important components of grassland ecosystems in western North America, they have experienced major population declines due to poisoning, outbreaks of sylvatic plague, recreational shooting, and habitat conversion. From May 2000 to October 2001, we investigated 270 colonies of Gunnison’s prairie dog (Cynomys gunnisoni) in Arizona. Because these colonies were classified as active in previous surveys (1987, 1990-1994, 1998), we were able to examine their persistence. Most (70%) of the colonies became inactive between the initial and recent surveys, with colony extinctions spanning our study area. Colony persistence was positively associated with the persistence of the nearest neighboring colony but was not associated with major vegetation type, distance to nearest neighboring colony, or initial size of the colony. The amount of area occupied by individual colonies varied between surveys, sometimes dramatically. We found little evidence that the reduction in active colonies was due to poisoning, recreational shooting, or habitat conversion. Rather, direct and indirect evidence suggest plague is the primary factor negatively impacting Gunnison’s prairie dog populations in Arizona. Currently, there is no way to control or prevent plague outbreaks in Gunnison’s prairie dog populations. To mitigate the effects of localized plague outbreaks on the overall population of this species, we suggest that Gunnison’s prairie dog be reintroduced to public lands throughout its historical range.     

Autochthonous Chagas disease in the southern United States: A case report of suspected residential and military exposures

Chagas disease is a parasitic infection that can result in a progressive dilated cardiomyopathy. Here, we present the epidemiologic details of a suspected locally acquired transmission case originating from the southern United States. This is the first published report of Chagas disease in a young, healthy United States veteran with repeat triatomine exposures in Arizona. Military personnel and Arizona residents should be aware of their Chagas disease transmission risks.     

Mexican wolves,elk, and aspen in Arizona: Is there a trophic cascade?

In 1998, Mexican gray wolves (Canis lupus baileyi) were introduced into the Blue Range Wolf Recovery Area (BRWRA) that spans adjacent portions of Arizona and New Mexico. In 2009 we selected three mixed-conifer sites on the Apache National Forest, within the BRWRA of east-central Arizona, to characterize long-term age structure of aspen (Populus tremuloides) and to check for the possible occurrence of a tri-trophic cascade involving Mexican wolves, Rocky Mountain elk (Cervus elaphus nelsoni), and aspen. These mixed-conifer sites included (a) a refugium site, (b) an old-growth site, and (c) a site thinned in 1991–1992. The refugium site was inaccessible to elk and cattle whereas the old-growth and thinned sites were accessible to elk, but not cattle. Age structure results indicated that aspen recruitment (i.e., the growth of sprouts/seedlings into tall saplings, poles, and eventually trees) at the refugium site had been ongoing over a period of many decades. In contrast, aspen recruitment at the old-growth and thinned sites decreased significantly (p < 0.05) during the two most recent decades when elk populations, as indexed by annual harvest levels, were relatively “high”. From 2000 to 2008, only 2.9 Mexican wolves per 1000 elk were present on the Apache National Forest compared to 9.3 western gray wolves (Canis lupus occidentalis) per 1000 elk in Yellowstone National Park where tri-trophic cascades involving wolves, elk, and aspen have been reported. The low number of Mexican wolves relative to their primary prey (elk) suggests that an ecologically effective density of wolves has not become established in east-central Arizona. Furthermore, the lack of recent aspen recruitment in stands accessible to elk indicates an absence, to date, of a tri-trophic cascade.     

Arizona pine (Pinus arizonica) stand dynamics: local and regional factors in a fire-prone madrean gallery forest of Southeast Arizona,USA

In southwestern North America, large-scale climate patterns appear to exert control on moisture availability, fire occurrence, and tree demography, raising the compelling possibility of regional synchronization of forest dynamics. Such regional signals may be obscured, however, by local, site-specific factors, such as disturbance history and land use. Contiguous sites with similar physical environments, lower and middle Rhyolite Canyon, Arizona, USA, shared nearly the same fire history from 1660-1801, but then diverged. For the next 50 years, fires continued to occur frequently in lower Rhyolite, but, probably as result of flood-induced debris deposition, largely ceased in middle Rhyolite. We related stand dynamics of Arizona pine (Pinus arizonica) to fire history and drought severity and compared the dynamics in the two sites before and after the divergence in fire frequency. Fires occurred during unusually dry years, and possibly following unusually moist years. Arizona pine exhibited three age structure peaks: two (1810–1830 and 1870–1900) shared by the two sites and one (1610–1640) only in middle Rhyolite. The latter two peaks occurred during periods of unusually low fire frequency, suggesting that fire-induced mortality shapes age structure. Evidence was mixed for the role of favorable moisture availability in age structure. As expected, moisture availability had a prominent positive effect on radial growth, but the effect of fire was largely neutral. The two sites differed only moderately in stand dynamics during the period of divergence, exhibiting subtle age structure contrasts and, in middle Rhyolite only, reduced growth during a 50-year fire hiatus followed by fire-induced release. These results suggest that, despite local differences in disturbance history, forest responses to regional fire and climate processes can persist.     

Reaction of lizard populations to a catastrophic wildfire in a central Arizona mountain range

In April 1996, the Lone Fire denuded over 90% of the vegetation in 130 km² around the Four Peaks area of the Mazatzal Mountains in central Arizona. To understand the reaction of a relatively immobile guild of species to a wildfire, we pit-trapped lizards from 1996 to 1999 in both burned and unburned interior chaparral and Madrean evergreen forest. In 26,214 trap nights, we found relative abundance was up to 10 times greater in burned than unburned vegetation. Species richness and diversity values were also greater in burned sites. Our data indicate a rapid settlement of burned areas primarily by individuals that survived the fire. Increased capture rate, diversity and richness values in 1998 and 1999 indicate that many species of lizards may even prefer early successional stages in chaparral and Madrean evergreen forests. Resident species of Teiidae and Sceloporus undulatus were more adapted to disturbed habitats than other resident Phrynosomatidae or Crotophytidae.     

Economic feasibility of introducing pulsed‐flow aquaculture into the irrigation system of cotton farms in Arizona

Abstract

The objective was to evaluate the economic feasibility of introducing fish culture into irrigated cotton production on farms in central Arizona. Water as a production variable was calculated only for the additional quantity required to keep water in the ditches during the growing season for fish. Raising tilapia in pulsed‐flow culture systems on Arizona cotton farms is economically feasible. Production function estimates indicate that profits can be increased through additional use of feed. At any ditch capacity, a density of six fish/m³ provided the optimal economic results, as the value of marginal product (VMP) equalled the price of fingerlings stocked/m³. This optimum fish production scenario would increase the net income for a typical irrigated cotton farm by 7 per cent. Increasing the initial size of the fingerlings improved the percentage of fish reaching marketable size and perhaps will increase economic returns, a question to be studied by future research.     

Analysis of hyperspectral reflectance data for monitoring growth and development of lesquerella

Seed oil from lesquerella (Lesquerella fendleri (Gray) Wats.) is currently being developed as a biorenewable petroleum substitute, but several issues related to crop management and breeding must be resolved before the crop will be commercially viable. Due particularly to the prominent yellow flowers exhibited by lesquerella canopies, remote sensing may be a useful tool for monitoring and managing the crop. In this study, we used a hand-held spectroradiometer to measure spectral reflectance over lesquerella canopies in 512 narrow wavebands from 268 to 1095 nm over two growing seasons at Maricopa, Arizona. Biomass samples were also regularly collected and processed to obtain aboveground dry weight, flower counts, and silique counts. Partial least squares regression was used to develop predictive models for estimating the three lesquerella biophysical variables from canopy spectral reflectance. For model fitting and model testing, the root mean squared prediction errors between measured and modeled aboveground dry weight, flower counts, and silique counts were 2.1 and 2.3 Mg ha⁻¹, 251 and 304 flowers, and 1018 and 1019 siliques, respectively. Analysis of partial least squares regression coefficients and loadings highlighted the most sensitive spectral wavebands for estimating each biophysical variable. For example, the flower count model heavily emphasized the reflectance of yellow light at 583 nm, and contrasted that with reflectance in the blue (483 nm) and at the red edge (721 nm). Because of the indeterminate nature of lesquerella flowering patterns, remote sensing methods that monitor flowering progression may aid management decisions related to the timing of irrigations, desiccant application, and crop harvest.