Endangered Cacti in the Chihuahuan Desert: I. Distribution Patterns

We mapped the geographical distributions of 93 endangered species of cacti from the Chihuahuan Desert Region. We divided the region into grids of 30 minutes latitude by 30 minutes longitude and calculated species frequencies for each grid. The grids with the highest species richness values are aggregated in areas of moderate elevation, particularly towards the southeastern and, to a lesser extent, the eastern margins of the Chihuahuan Desert Region, in northern San Luis Potosí and the southern portions of Coahuila, Nuevo León, and Tamaulipas. This vast area constitutes the most important nucleus of cactus species concentration in the continent. Species richness decreases toward the western segment of the Chihuahuan Desert Region and from the Cuatro Ciénegas region to the north and northwest. Another important area is the Queretaroan-Hidalgoan Arid Zone, where several grids containing an important assemblage of endangered species occur. Climatic factors, such as minimum temperatures and mean annual precipitation, explain the current distribution patterns of these plants, and the recent Pleistocene climatic episodes appear to have played a determinant role in the existence of areas of high species concentration and in the proliferation of narrow endemics. Special actions are urgently needed to conserve the endangered Cactaceae of this region. We propose that a carefully selected network of small areas would be an appropriate approach for the conservation of these plants. But species richness cannot be taken as the sole criterion in the determination of protected areas. Additional criteria, such as degree of endemicity, degree of threat to species and areas, habitat diversity, and biogeographic congruence with other plant and animal groups, should be analyzed before these areas are selected. Propagation in botanical gardens using scientific criteria and commercial propagation would be additional methods of conservation.     

Biogeography of Recently Extinct Marine Species: Implications for Conservation

At least fifteen marine and coastal animal species have become extinct since the end of the Pleistocene. Analyses of the number of marine biogeographical provinces occupied by these species show that, contrary to the prevailing view that extinction is usually associated with a small range, at least five species (33%) had large ranges encompassing parts of two or more provinces. At least eight species occurred in areas that served as geographical refuges for taxa during the Pliocene and early Pleistocene. These refuges, in which primary planktonic productivity is generally very high, include the northwestern Atlantic and North Pacific. Extinctions in the northwestern Atlantic (four species) may be partly responsible for the subsequent success of human-introduced species in subtidal and open-coast intertidal habitats. Regions of high productivity deserve priority among marine areas to be protected from overexploitation and habitat destruction.     

Response of Avian Communities to Historic Habitat Change in the Northern Chihuahuan Desert

Abstract: Throughout much of the northern Chihuahuan Desert, the grasslands that were widespread at the time of European settlement have been replaced by desert shrublands. Little is known about the effects of this change on avian communities. We analyzed historic U.S. Government Land Office records to assess large-scale changes in vegetation cover from the 1880s to the present day. We studied vegetation and avian communities in one grassland habitat type and four desert shrubland habitat types to examine (1) how breeding-bird communities may have changed in response to habitat conversion from grassland to desert shrubland and (2) whether breeding-bird communities differ among the four desert shrubland habitat types that compose Chihuahuan Desert scrub in this region. To estimate the characteristics of 1880s black grama (    Bouteloua eriopoda ) grassland, we focused on plots located within extensive patches of present-day black grama and compared the avian communities found there with those in desert shrubland. Species richness was higher in desert shrubland than grassland. Among the desert shrubland habitat types, species richness was consistently highest in mesquite. Avian abundance patterns differed among the four desert shrubland habitat types. At least 30% of the avian community in each habitat pair was distinct. Conversion of grassland to shrubland in south-central New Mexico has likely been accompanied by a major turnover in the avian community. Remaining tracts of black grama provide habitat for species that may be uniquely adapted to the northern Chihuahuan Desert and should be protected.     

Indices of Grassland Biodiversity in the Chihuahuan Desert Ecoregion Derived from Remote Sensing

Abstract: We used a relatively simple and direct remote-sensing approach to determine biodiversity values in arid ecosystems and thus identify potential conservation sites. We developed indices based on regression models between grass, shrub, litter, exposed-soil groundcover components, and Landsat thematic mapper satellite imagery reflectance values over a reference site in the northern Chihuahuan Desert in New Mexico. This site supports low-disturbance desert grasslands that have been excluded from livestock grazing for 55 years and moderate-disturbance grasslands that have been under a continuous grazing regime for over 100 years. Greater richness and abundance of noninvasive and nonruderal plant species were associated with the low-disturbance grasslands that had lower shrub abundance, increased litter and grass cover, and lower exposed soil. Using the thematic mapper indices, we computed an additive grassland biodiversity index such that, as exposed soil and shrub values go down, litter and grass values go up, as does the biodiversity index. When the biodiversity index was applied to the reference-site landscape, grasslands previously identified for their high conservation value were detected. As a further test, we applied the indices to a site in Chihuahua, Mexico, that supports similar grasslands but for which there are few other data on condition and conservation values. The soil, grass, and shrub indices were moderately effective in describing the range of variation at the Mexico site, but the litter equation was not. Still, higher biodiversity value in terms of nonruderal plant diversity tended to correspond to higher grass cover and lower soil exposure and a higher overall biodiversity index. Some localized calibration with geologic substrate may be required along with an assessment of the temporal constraints, but generally the index shows promise for quickly and efficiently detecting desert grasslands of high biodiversity conservation value.     

Chihuahuan Desert kangaroo rats: nonlinear effects of population dynamics, competition, and rainfall

Using long-term data on two kangaroo rats in the Chihuahuan Desert of North America, we fitted logistic models including the exogenous effects of seasonal rainfall patterns. Our aim was to test the effects of intraspecific interactions and seasonal rainfall in explaining and predicting the numerical fluctuations of these two kangaroo rats. We found that logistic models fit both data sets quite well; Dipodomys merriami showed lower maximum per capita growth rates than Dipodomys ordii, and in both cases logistic models were nonlinear. Summer rainfall appears to be the most important exogenous effect for both rodent populations; models including this variable were able to predict independent data better than models including winter rainfall. D. merriami was also negatively affected by another kangaroo rat (Dipodomys spectabilis), consistent with previous experimental evidence. We hypothesized that summer rainfall influences the carrying capacity of the environment by affecting seed availability and the intensity of intraspecific competition.     

Hierarchical analysis of vegetation dynamics over 71 years: soil-rainfall interactions in a Chihuahuan Desert ecosystem

Proliferation of woody plants in grasslands and savannas is a persistent problem globally. This widely observed shift from grass to shrub dominance in rangelands worldwide has been heterogeneous in space and time largely due to cross-scale interactions among soils, climate, and land-use history. Our objective was to use a hierarchical framework to evaluate the relationship between spatial patterns in soil properties and long-term shrub dynamics in the northern Chihuahuan Desert of New Mexico, USA. To meet this objective, shrub patch dynamics from 1937 to 2008 were characterized at patch and landscape scales using historical imagery and a recent digital soils map. Effects of annual precipitation on patch dynamics on two soils revealed strong correlations between shrub growth on deep sandy soils and above-average rainfall years (r = 0.671, P = 0.034) and shrub colonization and below-average rainfall years on shallow sandy soils (r = 0.705, P = 0.023). Patch-level analysis of demographic patterns revealed significant differences between shrub patches on deep and shallow sandy soils during periods of above- and below-average rainfall. Both deep and shallow sandy soils exhibited low shrub cover in 1937 (1.0% +/- 2.3% and 0.3% +/- 1.3%, respectively [mean +/- SD]) and were characterized by colonization or appearance of new patches until 1960. However, different demographic responses to the cessation of severe drought on the two soils and increased frequency of wet years after 1960 have resulted in very different endpoints. In 2008 a shrubland occupied the deep sandy soils with cover at 19.8% +/- 9.1%, while a shrub-dominated grassland occurred on the shallow sandy soils with cover at 9.3% +/- 7.2%. Present-day shrub vegetation constitutes a shifting mosaic marked by the coexistence of patches at different stages of development. Management implications of this long-term multi-scale assessment of vegetation dynamics support the notion that soil properties may constrain grassland remediation. Such efforts on sandy soils should be focused on sites characterized by near-surface water-holding capacity, as those lacking available water-holding capacity in the shallow root zone pose challenges to grass recovery and survival.     

The Biogeography of Globally Threatened Seabirds and Island Conservation Opportunities

Seabirds are the most threatened group of marine animals; 29% of species are at some risk of extinction. Significant threats to seabirds occur on islands where they breed, but in many cases, effective island conservation can mitigate these threats. To guide island‐based seabird conservation actions, we identified all islands with extant or extirpated populations of the 98 globally threatened seabird species, as recognized on the International Union for Conservation of Nature Red List, and quantified the presence of threatening invasive species, protected areas, and human populations. We matched these results with island attributes to highlight feasible island conservation opportunities. We identified 1362 threatened breeding seabird populations on 968 islands. On 803 (83%) of these islands, we identified threatening invasive species (20%), incomplete protected area coverage (23%), or both (40%). Most islands with threatened seabirds are amenable to island‐wide conservation action because they are small (57% were <1 km²), uninhabited (74%), and occur in high‐ or middle‐income countries (96%). Collectively these attributes make islands with threatened seabirds a rare opportunity for effective conservation at scale. La Biogeografía de Aves Marinas Amenazadas Globalmente y las Oportunidades de Conservación en Islas     

Conservation Genetics in the Management of Desert Fishes

Abstract: The status and security of fishes in North American deserts have steadily declined in this century due to man's activities in this naturally fragile region. We address genetic aspects of the population structure of desert fishes as applicable to conservation and recovery programs by developing two zoogeographic models of isolation and gene flow. In the Death Valley model populations are isolated, with no chance of natural gene flow among them. Genetic diversity within populations tends to be low, but genetic divergence among populations within a species is high. In the Stream Hierarchy model, a complicated hierarchical genetic structure exists and is a function of geographic proximity and connectivity of habitats. Within-habitat genetic diversity tends to be higher, and among-habitat differentiation lower, than in the Death Valley model. These two systems must be recognized as distinct and managed differently. We also suggest three areas of experimentation needed to better understand and manage genetic stocks of desert fishes: relationships between heterozygosity and fitness, experimental mixing of similar stocks to examine effects of increased heterozygosity, and analysis of the relative roles of genetic adaptation and phenotypic plasticity in local differentiation.     

Wind Characteristics of Mesquite Streets in the Northern Chihuahuan Desert, New Mexico, USA

Past research has shown that the most important areas for active sand movement in the northern part of the Chihuahuan Desert are mesquite-dominated desert ecosystems possessing sandy soil texture. The most active sand movement in the mesquite-dominated ecosystems has been shown to take place on elongated bare soil patches referred to as “streets”. Aerodynamic properties of mesquite streets eroded by wind should be included in explaining how mesquite streets are more emissive sand sources than surrounding desert land. To understand the effects of wind properties, we measured them at two flat mesquite sites having highly similar soil textures but very different configurations of mesquite. The differences in wind properties at the two sites were caused by differences of size, orientation, and porosity of the mesquite, along with the presence of mesquite coppice dunes (sand dunes stabilized by mesquites growing in the dune and on its surface) found only at one of the two sites. Wind direction, u_* (friction velocity), z₀ (aerodynamic roughness height) and D (zero plane displacement height) were estimated for 15-m tower and 3-m mast data. These aerodynamic data allowed us to distinguish five categories with differing potentials for sediment transport. Sediment transport for the five categories varied from unrestricted, free transport to virtually no transport caused by vegetation protection from wind forces. In addition, “steering” of winds below the level of the tops of mesquite bushes and coppice dunes allowed longer parallel wind durations and increased wind erosion for streets that aligned roughly SW–NE. U.S. Government right to retain a non-exclusive royalty-free licence in and to any copyright is acknowledged.     

Avian Conservation under the Endangered Species Act: Expenditures versus Recovery Priorities

Abstract: Budget constraints require the U.S. Fish and Wildlife Service to prioritize species for recovery spending. Each listed species is ranked according to the degree of threat it faces, its recovery potential, and its taxonomic distinctness. We analyzed state and federal government expenditures for recovery of threatened and endangered birds ( n = 85 species) from 1992 to 1995 to determine if the priority system was being followed. Although recovery spending correlated with priority rank, priority rank explained <5% of the variation in spending. A small number of the same moderately ranked species dominated expenditures each year (41–79% of total annual budgets). Species with wide distributions, high recovery potential, and captive breeding programs received the most funding, and more funding than their priority ranks dictated. Island species received significantly less funding than expected based on priority rank. Twelve species, 10 of which resided on islands, received <$5000 at least once from 1992 to 1995. Recovery spending was unrelated to degree of threat, taxonomic distinctness, and migratory status. There also was no relationship between land-purchase expenditures and priority ranks. To improve the relationship between recovery spending on threatened and endangered birds and their priority rank, significant changes need to be made within the private sector ( less litigation and special-interest lobbying  ), U.S. Congress (increased budget and reduced earmarking  ), and the U.S. Fish and Wildlife Service (restructuring of regional offices and increased accountability).     

Protected Areas and Prospects for Endangered Species Conservation in Canada

Abstract:  Reserve networks figure prominently in conservation strategies that aim to reduce extinction rates. We tested the effectiveness of the current reserve network at protecting species at risk in Canada, where relatively extensive wilderness areas remain. We compared numbers of terrestrial species at risk included in existing reserves to randomly generated networks with the same total area and number of reserves. Existing reserve networks rarely performed better than randomly selected areas and several included fewer endangered species than expected by chance, particularly in the most biologically imperiled regions. The extent of protected area and density of species at risk were unrelated at either broad (countrywide) or finer spatial scales (50 × 50 km grids), although there was a tendency for the most threatened regions of the country to have few or no protected areas (1.5% of areas with >30 endangered species were in reserves). Although reserves will play a useful role in conserving endangered species that occur within them, reducing extinction rates in a region with much of the world's remaining wilderness will require integrating conservation strategies with agricultural and urban land-use plans outside formally protected areas.     

Conservation Genetics of the Endangered Isle Royale Gray Wolf

Abstract. The small group of wolves on Isle Royale has been studied for over three decades as a model of the relationship between large carnivores and their prey. During the last ten years the population declined from 50 individuals to as few as 12 individuals. The causes of this decline may be food shortages, disease, or reduced genetic variability. We address the issues of genetic variability and relationships of Isle Royale wolves using allozyme electrophoresis, mtDNA restriction-site analysis, and multilocus hypervariable minisatellite DNA analysis (genetic fingerprinting). Our results indicate that approximately 50% of the allozyme heterozygosity has been lost in the island population, a decline similar to that expected if no immigration had occurred from the mainland. The genetic fingerprinting data indicate that the seven sampled Isle Royale wolves are as similar as captive populations of siblings. Surprisingly, the Isle Royale wolves have an mDNA genotype that is very rare on the mainland, being found in only one of 144 mainland wolves ThFF suggests that the remaining Isle Royale wolves areprobably derived from a single female founder     

Evolving patterns of agricultural frontier expansion in Mexico’s Chihuahuan Desert: a political ecology approach

ABSTRACT

This study sought to understand why and how the area of irrigated agriculture in Janos County, in northwestern Chihuahua, Mexico, has tripled since 1987. Expansion of agricultural frontiers has often been attributed to state policies and programs or von Thünian land-rent dynamics. Here, a political ecology approach relying on qualitative interviews with landowners was used to understand the drivers and patterns of conversion from native rangeland to crops. Results show that socioeconomic patterns of agricultural frontier expansion evolved over time, with different drivers at different phases. State policies, especially the land reform, were initially important for settling the agriculture frontier in small farms. Subsequently, economic differentiation, intensification, and growth of existing farms drove expansion. Since 2010, expansion has been driven by investment-oriented land purchases by absentee owners and larger-than-family farms. The Janos case details what may be common but underappreciated temporal dynamics at agricultural frontiers.