Paul Stapp

Extreme climatic events shape arid and semiarid ecosystems

6 Climatic changes associated with the El Nino Southern Oscillation (ENSO) can have a dramatic impact on ter- restrial ecosystems worldwide, but especially on arid and semiarid systems, where productivity is strongly lim- ited by precipitation. Nearly two decades of research, including both short-term experiments and long-term studies conducted on three continents, reveal that the initial, extraordinary increases in primary productivity percolate up through entire food webs, attenuating the relative importance of top-down control by predators, providing key resources that are stored to fuel future production, and altering disturbance regimes for months or years after ENSO conditions have passed. Moreover, the ecological changes associated with ENSO events have important implications for agroecosystems, ecosystem restoration, wildlife conservation, and the spread of disease. Here we present the main ideas and results of a recent symposium on the effects of ENSO in dry ecosystems, which was convened as part of the First Alexander von Humboldt International Conference on the El Nino Phenomenon and its Global Impact (Guayaquil, Ecuador, 16-20 May 2005).

Livestock as Ecosystem Engineers for Grassland Bird Habitat in the Western Great Plains of North America

Abstract Domestic livestock have the potential to function as ecosystem engineers in semiarid rangelands, but prevailing management practices largely emphasize livestock production and uniform use of vegetation. As a result, variation in vegetation structure might not occur at appropriate spatial and temporal scales to achieve some contemporary conservation objectives. Here, we introduce the utility of livestock as ecosystem engineers and address potential benefits and consequences associated with heterogeneity-based management practices for conservation grazing in the semiarid rangelands of the western North American Great Plains. To illustrate the potential value of this approach, we provide specific examples where engineering effects of livestock could alter vegetation heterogeneity at within-pasture (< 100 ha) and among-pasture (∼100 ha to thousands of hectares) scales to improve habitat for declining native grassland birds. Experimental evaluations of the efficacy of livestock to achieve desired modifications to vegetation structure are needed, along with the economic aspects associated with implementing heterogeneity-based management practices. Using livestock as ecosystem engineers to alter vegetation structure for grassland bird habitat is feasible in terms of application by land managers within the context of current livestock operations, and provides land managers important tools to achieve desired contemporary objectives and outcomes in semiarid rangelands of the western North American Great Plains.

Stable isotopes reveal strong marine and El Nino effects on island food webs

Stable isotope analysis is a powerful tool for unravelling the complex structure of food webs. This technique is particularly well suited for studies at ecosystem boundaries, where physical processes and mobile consumers link the dynamics of seemingly disparate systems. In coastal and insular environments, seabirds play a crucial role in transporting marine-based energy and nutrients to islands. Here we show using stable isotopes that nutrients from the ocean drive the dynamics of terrestrial food webs on small islands. The indirect effects of seabird-derived nutrients on plant productivity are particularly prominent during wet El Niño Southern Oscillation years on our Gulf of California study sites. During dry years that characterize the region, many terrestrial consumers are subsidized by carrion and prey from the ocean. Shifts in trophic structure related to El Niño Southern Oscillation could only be elucidated because of the distinct nitrogen isotope ratios associated with seabird islands. The contributions of seabirds and other marine sources are reflected in the isotope signatures of terrestrial consumers in ways that challenge conventional interpretations of stable isotope results in studies of food webs.

Patterns of extinction in prairie dog metapopulations: plague outbreaks follow El Ninì events

Outbreaks of many vector-borne human diseases are broadly correlated with climatic variation, but evidence of similar fluctuations in disease in natural host populations is rare. Here, we use 21 years of monitoring of black-tailed prairie dog (Cynomys ludovicianus) colonies to demonstrate a link between extinctions of colonies attributed to plague (Yersinia pestis) and climatic fluctuations associated with El Nini Southern Oscillation events that promote the growth of flea vector and rodent host populations. During epizootics, rates of extinction of the largest colonies (>16 ha) were nearly as high (>60%) as for the smallest ones (<3 ha), but only a third of intermediate-sized colonies were extirpated. The probability of extinction was influenced by the size and fate of adjacent colonies, but there was no predictable relationship between extinction probabilities and intercolony distance, indicating that spatial isolation does not reduce the vulnerability of colonies to plague. By causing sporadic extinctions of colonies, plague creates a metapopulation structure that has altered the dynamics of prairie dog colonies as they respond to a century of human persecution and habitat loss.

GENETIC STRUCTURE OF A METAPOPULATION OF BLACK-TAILED PRAIRIE DOGS

Abstract Habitat alteration, agricultural control, recreational shooting, and most recently, sylvatic plague (caused by Yersinia pestis) contributed to local extinctions and a steady decline of black-tailed prairie dog (Cynomys ludovicianus) throughout its range. As a consequence, prairie dogs currently live in metapopulations, where their overall persistence will depend on a balance between extinction of colonies and recolonization from extant colonies. Patterns of genetic similarity among colonies, as measured by neutral molecular markers, provide an estimate of the dispersal and gene flow among colonies within prairie dog metapopulations. We sampled 13 colonies of black-tailed prairie dogs in short-grass prairie of northern Colorado, 100-km east of Fort Collins, Colorado. We used historical records and genetic analysis to show that colonies undergo regular extinctions, which subsequently are recolonized by individuals from multiple source colonies. We examined 155 individuals for variation at 7 microsatellite loci and found moderate levels of genetic differentiation among colonies (Θ [=FST] = 0.118). We also used assignment and exclusion tests based on multilocus genotypes of individuals to determine the probability that individuals originated from the same colony in which they were captured. About 39% of individuals could not be assigned to colonies where they were captured, indicating they were either immigrants (adults) or the offspring of immigrants (adults and juveniles). We tested for genetic isolation by distance among colonies by comparing genetic distances to geographic distances between colonies. Akaikes Information Criterion for model selection revealed that dispersal most likely occurred along low-lying dry creek drainages connecting isolated colonies. Genetic distances between colonies were also related to ages of colonies; older colonies were more similar genetically than younger colonies. This underscores the importance of dispersal among prairie dog colonies and has important implications for persistence of prairie dog metapopulations, in which all colonies, regardless of size, are vulnerable to extinction from plague.

RESOURCES FROM ANOTHER PLACE AND TIME: RESPONSES TO PULSES IN A SPATIALLY SUBSIDIZED SYSTEM

As the theoretical bases for the dynamics of spatially subsidized communities emerge, ecologists question whether spatially subsidized communities exhibit similar structure or dynamics to communities that receive strongly pulsed resources. In both cases, communities may be structured by responses to resources that are potentially absent at any given point in time (pulsed communities) or space (subsidized communities), even if pulsed resources are part of the in situ productivity of the system or the subsidies arrive as a relatively constant input from a nearby system. The potential for significant spatial or temporal resource limitation, therefore, may be a key factor influencing in similar ways the persistence of populations, the structure and dynamics of communities, and the evolution of specific life history traits. In most complex systems, however, multiple resources may arrive for various trophic entities at various points in time and from various points in space, and thus it may be difficult to separate or compare the dynamics of spatially subsidized and pulsed systems. In this paper, we explore the effects of interactions between pulses and subsidies in plant and animal populations and communities on highly pulsed and variably subsidized islands in the Gulf of California. While many of the plant and animal communities on the unsubsidized islands in this system respond to pulses of rain in classic ways, responses to these rain pulses on islands subsidized by seabird guano or other marine resources are quite different and variable, and depend on a combination of life history characteristics, physiology, competitive interactions, and trophic relationships. These variable responses to rain pulses then translate into large differences in dynamics and community structure of subsidized vs. unsubsidized islands. Indeed, most systems experience both temporal pulses and spatial subsidies. When considered in tandem, complementary or synergistic effects of the multiple, temporally and spatially variable resources may emerge that help explain complex food web structure and dynamics.

Plague outbreaks in prairie dog populations explained by percolation thresholds of alternate host abundance

Highly lethal pathogens (e.g., hantaviruses, hendra virus, anthrax, or plague) pose unique public-health problems, because they seem to periodically flare into outbreaks before disappearing into long quiescent phases. A key element to their possible control and eradication is being able to understand where they persist in the latent phase and how to identify the conditions that result in sporadic epidemics or epizootics. In American grasslands, plague, caused by Yersinia pestis, exemplifies this quiescent–outbreak pattern, because it sporadically erupts in epizootics that decimate prairie dog (Cynomys ludovicianus) colonies, yet the causes of outbreaks and mechanisms for interepizootic persistence of this disease are poorly understood. Using field data on prairie community ecology, flea behavior, and plague-transmission biology, we find that plague can persist in prairie-dog colonies for prolonged periods, because host movement is highly spatially constrained. The abundance of an alternate host for disease vectors, the grasshopper mouse (Onychomys leucogaster), drives plague outbreaks by increasing the connectivity of the prairie dog hosts and therefore, permitting percolation of the disease throughout the primary host population. These results offer an alternative perspective on plagues ecology (i.e., disease transmission exacerbated by alternative hosts) and may have ramifications for plague dynamics in Asia and Africa, where a single main host has traditionally been considered to drive Yersinia ecology. Furthermore, abundance thresholds of alternate hosts may be a key phenomenon determining outbreaks of disease in many multihost-disease systems.

Habitat Selection by an Insectivorous Rodent: Patterns and Mechanisms across Multiple Scales

Patterns of habitat selection by small mammals reflect variation in availability of resources at a range of spatial and temporal scales. I investigated use of habitat by northern grasshopper mice ( Onychomys leucogaster ) in shortgrass prairie and related the patterns to distribution of vegetation and substrate and availability of arthropod prey. I used powder tracking to document use of microhabitats and live-trapping to estimate abundance on sites with different soil types and amounts of shrub cover (macrohabitats). At all spatial scales examined, mice used disturbances of soil (primarily pocket gopher mounds) and burrows more than expected based on abundance of these microhabitats, but showed no affinity for large shrubs. Furthermore, movement patterns suggested that mice concentrated activities in areas with high densities of mounds and burrows. Insect prey were more numerous on mounds than in other microhabitats and were generally more abundant in trapping areas where grasshopper mice were captured, especially in spring and early summer. Mounds and burrows provide arthropods with access to subterranean refuges, and the concentration and accessibility of prey, thus, may explain the intensive use of these microhabitats by mice. Microhabitat variables associated with mounds and burrows were better predictors of population density than was macrohabitat, which may reflect the spatial distribution and temporal predictability of insect prey. Although local abundance of grasshopper mice was best explained by availability of suitable foraging microhabitat, the relationship between these microhabitats and edaphic characteristics suggests that it may be difficult to separate the roles of microhabitat and macrohabitat for this wide-ranging species.

MICROHABITAT USE AND COMMUNITY STRUCTURE OF DARKLING BEETLES (COLEOPTERA :TENEBRIONIDAE) IN SHORTGRASS PRAIRIE : EFFECTS OF SEASON, SHRUB COVER AND S OIL TYPE

-I conducted pitfall-trapping studies from 1992 through 1995 to evaluate the effects of vegetation structure and soil texture on seasonal variation in the distribution, relative abundance and species composition of darkling beetles (Coleoptera: Tenebrionidae) in shrub-dominated areas of shortgrass prairie in N-central Colorado. Numbers of beetles captured in traps placed in shrub, cactus, grass and bare-ground microhabitats were used to assess microhabitat use and diel timing of activity, differences in the relative abundance of beetles between spring and summer and to compare beetle communities on sites with different shrub vegetation and soil types. Beetles were most numerous in summer on all sites, although some species showed peaks of activity in spring. All but two species (Eleodes extricata, E. hispilabris) were captured significantly more often in traps beneath shrubs, underscoring the importance of this microhabitat for thermal cover, food or protection from predators. Further, in shrub-dominated areas, beetles were more nocturnal than had been reported previously for adjacent grasslands, suggesting that shrubs kept temperatures on low-lying areas warmer at night than on open prairie. Fourteen tenebrionid species were captured, and on average, significantly more species were captured beneath shrubs than in other microhabitats. Each site had a suite of five resident species, of which only Eleodes obscura and Embaphion contusum were captured on all sites during all trapping periods. Species richness and beetle abundance, especially of the most common species (Eleodes obscura), were inversely related to shrub cover but were greater in areas with coarsely textured soils. Overall, tenebrionid communities in shrub-dominated areas were similar to those reported for open prairie, but small species tend to predominate in grasslands, perhaps because most refuges are relatively small. The prevalence of larger beetles in areas with sandy soils and large shrubs may be related to the number of suitable refuges, both belowground and in vegetation, as well as the availability of preferred feeding and oviposition sites.

Rodent communities in active and inactive colonies of black-tailed prairie dogs in shortgrass steppe

Abstract Black-tailed prairie dogs (Cynomys ludovicianus) alter shortgrass-steppe landscapes in ways that are expected to affect other mammals. I sampled rodent populations at 31 sites on the Pawnee National Grasslands, Colorado, including 18 active colonies, 6 colonies that had been unoccupied for >6 years (inactive), and 7 grassland sites without prairie dogs (controls). Rodents were livetrapped for 4 consecutive nights at each site between May and August 2004 to estimate relative abundance. I also measured vegetation and habitat characteristics. Northern grasshopper mice (Onychomys leucogaster) and 13-lined ground squirrels (Spermophilus tridecemlineatus) were captured on most (≥87%) of the sites and comprised 40% and 34% of individuals captured, respectively. Species richness ranged from 1 to 6 species, but most sites had only these 2 species. Grasshopper mice tended to be more abundant on colony sites than on controls, although differences were not statistically significant. Ground squirrels were least abundant on active colonies, and most abundant on inactive colonies, followed by controls. Habitat types did not differ in their abundance of any other species or in total rodent abundance; however, active colony area influenced total rodent abundance, with small colonies supporting fewer individuals. Controls supported the most rodent species, in part because these sites consistently had taller grass, which presumably provided habitat and food for less common species (Chaetodipus hispidus, Perognathus flavus, and Reithrodontomys megalotis). In shortgrass steppe, active colonies provided habitat for grasshopper mice, which may be involved in maintenance and spread of plague, but did not support consistently higher rodent species richness than the surrounding grasslands.