Short-term instabilities and long-term community dynamics

Competition in a temporally variable environment leads to sequences of short-term instabilities that in some cases are the mechanism of long-term coexistence; in other cases they promote long-term instability. Recent work associates long-term stability with a positive relationship between environmental and competitive effects and with population growth rates that are buffered against jointly unfavorable environmental and competitive events. Buffered growth rates arise from population subdivision over life-history stages, microenvironments or phenotypes. A distinct but related mechanism of long-term stability relies on population growth rates that are nonlinear functions of competition. New ways of understanding and investigating species diversity follow from these results.     

The biogeochemistry of a north-temperate grassland with native ungulates: Nitrogen dynamics in Yellowstone National Park

Nutrient dynamics of large grassland ecosystems possessing abundant migratory grazers are poorly understood. We examined N cycling on the northern winter range of Yellowstone National Park, home for large herds of free-roaming elk (Cervus elaphus) and bison (Bison bison). Plant and soil N, net N mineralization, and the deposition of ungulate fecal-N were measured at five sites, a ridgetop, mid-slope bench, steep slope, valley-bottom bench, and riparian area, within a watershed from May, 1991 to April, 1992.Results indicated similarities between biogeochemical properties of Yellowstone grassland and other grassland ecosystems: (1) landscape position and soil water affected nutrient dynamics, (2) annual mineralization was positively related to soil N content, and (3) the proportion of soil N mineralized during the year was negatively related to soil C/N.Grazers were a particularly important component of the N budget of this grassland. Estimated rates of N flow from ungulates to the soil ranged from 8.1 to 45.6 kg/ha/yr at the sites (average = 27.0 kg/ha/yr), approximately 4.5 times the amount of N in senescent plants. Rates of nitrogen mineralization for Yellowstone northern range grassland were higher than those measured in other temperate grassland ecosystems, possibly due to grazers promoting N cycling in Yellowstone.     

Resource pulses,species interactions,and diversity maintenance in arid and semi-arid environments

Arid environments are characterized by limited and variable rainfall that supplies resources in pulses. Resource pulsing is a special form of environmental variation, and the general theory of coexistence in variable environments suggests specific mechanisms by which rainfall variability might contribute to the maintenance of high species diversity in arid ecosystems. In this review, we discuss physiological, morphological, and life-history traits that facilitate plant survival and growth in strongly water-limited variable environments, outlining how species differences in these traits may promote diversity. Our analysis emphasizes that the variability of pulsed environments does not reduce the importance of species interactions in structuring communities, but instead provides axes of ecological differentiation between species that facilitate their coexistence. Pulses of rainfall also influence higher trophic levels and entire food webs. Better understanding of how rainfall affects the diversity, species composition, and dynamics of arid environments can contribute to solving environmental problems stemming from land use and global climate change.     

Habitat-specific demography: evidence for source-sink population structure in a mammal,the pika

Theory suggests that populations may persist in sink habitats that cannot support replacement-level birth rates. Although it is commonly believed that organisms that can actively select habitat should rarely occur in sinks, the frequency of use of sinks in free-ranging species is not well-documented. We found that a population of American pikas ( Ochotona princeps, Lagomorpha) inhabiting distinct alpine habitats (meadow and snowbed) in Wyoming, USA, had habitat-specific demographic rates that produced a source-sink population structure. Population size increased in both habitats in summer and declined in both habitats in winter, with populations in snowbeds increasing more during summer and decreasing more over winter. Birth rates were consistently higher in meadows and populations in meadows had a consistently higher finite rate of increase (lambda, from life tables) than did those in snowbeds, for which lambda was far below that needed for replacement. Patterns of immigration, population structure, and temporal variation in population size were as expected if meadows were functional sources and snowbeds functional sinks. Patterns of snowmelt differed between habitats, predicted the critical difference in birth rates between habitats, and are a likely primary cause of the differences in habitat-specific birth rates that we observed. This study provides a clear example of source-sink population structure for a mammal.     

The roles of harsh and fluctuating conditions in the dynamics of ecological communities

Harsh conditions (e.g., mortality and stress) reduce population growth rates directly; secondarily, they may reduce the intensity of interactions between organisms. Near-exclusive focus on the secondary effect of these forms of harshness has led ecologists to believe that they reduce the importance of ecological interactions, such as competition, and favor coexistence of even ecologically very similar species. By examining both the costs and the benefits, we show that harshness alone does not lessen the importance of species interactions or limit their role in community structure. Species coexistence requires niche differences, and harshness does not in itself make coexistence more likely. Fluctuations in environmental conditions (e.g., disturbance, seasonal change, and weather variation) also have been regarded as decreasing species interactions and favoring coexistence, but we argue that coexistence can only be favored when fluctuations create spatial or temporal niche opportunities. We argue that important diversity-promoting roles for harsh and fluctuating conditions depend on deviations from the assumptions of additive effects and linear dependencies most commonly found in ecological models. Such considerations imply strong roles for species interactions in the diversity of a community.     

The distribution of native and exotic plants in a naturally fragmented sagebrush-steppe landscape

We tested two general hypotheses for the diversity of native and exotic plants in an undisturbed, naturally fragmented sagebrush-steppe landscape in SE Idaho, USA, evaluating whether the MacArthur–Wilson hypothesis of island biogeography or a suite of environmental variables explained the distributions of native and exotic plants. We also tested a third hypothesis, which incorporated assumptions about the origin of exotic plants and their interaction with native plants. Of the three hypotheses we tested, the hypothesis that included exotic species best explained the diversity of the native plant community. The MacArthur–Wilson model of island biogeography did not explain the diversity of native (R ² = 0.13) or exotic plants well (R ² = 0.11), and the model fit the data poorly. A model of environmental variables better explained the diversity of native (R ² = 0.48) and exotic plants (R ² = 0.57), but it also fit the data poorly. Instead, proximity to a railroad explained the cover (R ² = 0.59) and richness of exotic plants (R ² = 0.63), which then explained the species richness of native plants (R ² = 0.34), and the model fit was adequate and had the lowest AIC value. This suggests that the transportation corridor had a significant, though indirect, effect on the native plant community, even in this undisturbed area. Moreover, explained variance, model fit, and the AIC model selection criteria favored the model with the railroad and exotic species over the M–W and environmental models. Since the habitat patches we studied were largely undisturbed by people and their activities, our results further suggest that the transportation corridor influenced the distribution of exotic plants by serving as a vector for colonization, rather than as a source of disturbance. Additionally, the results suggest that exotic plant species have had a negative effect on the diversity of the native plant community and have changed its composition. The results also support the inference that the nascent exotic plant community is influenced by source-sink (Pulliam in Am Nat 132:652–661, 1988) and assembly dynamics. In contrast, the native plant community appears to be more strongly influenced by environmental conditions associated with an elevational gradient, but there is evidence that the native community also has undergone directional change in species composition, associated with the invasion by non-native species.     

Herbivorous insects reduce growth and reproduction of big sagebrush (<Emphasis Type="Italic">Artemisia tridentata</Emphasis>)

Insect herbivores can reduce growth, seed production, and population dynamics of host plants, but do not always do so. Big sagebrush (Artemisia tridentata) has one of the largest ranges of any shrub in North America, and is the dominant and characteristic shrub of the extensive sagebrush steppe ecosystem of the western United States. Nevertheless, the impact of insect herbivores on big sagebrush, its dominant and characteristic shrub, is largely unknown. Occasional large effects of insect herbivore outbreaks are documented, but there is little knowledge of the impact of the more typical, nominal herbivory that is produced by the diverse community of insects associated with big sagebrush in natural communities. In 2008, we removed insects from big sagebrush plants with insecticide to evaluate whether insect herbivores reduced growth and seed production of big sagebrush. Removal of herbivores led to significant and substantial increases in inflorescence growth (22%), flower production (325%), and seed production (1053%) of big sagebrush. Our results showed the impact of insect herbivory in the current growing season on the growth and reproduction of big sagebrush and revealed an unrecognized, significant role of non-outbreak herbivores on big sagebrush.     

Pikas (Ochotona princeps : Lagomorpha) as allogenic engineers in an alpine ecosystem

Pikas (Ochotona princeps: Lagomorpha) build caches of vegetation (“haypiles”), which serve as a food source during winter in alpine and subalpine habitats. Haypiles appear to degrade over time and form patches of nutrient-rich soils in barren talus and scree areas. We sampled soils underneath and next to haypiles, and plants growing on and near haypiles in an alpine cirque in northwestern Wyoming, USA, to determine the effects of pika food caches on N, C, and C/N ratios in soils and plants. We found that (1) haypile soils had significantly higher carbon and nitrogen levels and lower C/N ratios than both adjacent soils and soils in the general study area, (2) two of three plant species tested (Polemonium viscosum and Oxyria digyna) had significantly higher levels of tissue percent N when growing on haypile soils, and (3) total standing plant biomass at the study site increased with soil percent N suggesting that vegetation was nitrogen limited. Pikas may therefore function as allogenic ecosystem engineers by modulating nutrient availability to plants. Received: 5 July 1997 / Accepted: 30 November 1997     

Conductance catheter-based assessment of arterial input impedance, arterial function, and ventricular-vascular interaction in mice

Global assessment of both cardiac and arterial function is important for a meaningful interpretation of pathophysiological changes in animal models of cardiovascular disease. We simultaneously acquired left ventricular (LV) and aortic pressure and LV volume (V(LV)) in 17 open-chest anesthetized mice (26.7 +/- 3.2g) during steady-state (BL) and caval vein occlusion (VCO) using a 1.4-Fr dual-pressure conductance catheter and in a subgroup of eight animals during aortic occlusion (AOO). Aortic flow was obtained from numerical differentiation of V(LV). AOO increased input impedance (Z(in)) for the first two harmonics, increased characteristic impedance (0.025 +/- 0.007 to 0.040 +/- 0.011 mmHg x microl(-1) x s, P < 0.05), and shifted the minimum in Z(in) from the third to the sixth harmonic. For all conditions, the Z(in) could be well represented by a four-element windkessel model. The augmentation index increased from 116.7 +/- 7.8% to 145.9 +/- 19.5% (P < 0.01) as well as estimated pulse-wave velocity (3.50 +/- 0.94 to 5.95 +/- 1.62 m/s, P < 0.05) and arterial elastance (E(a), 4.46 +/- 1.62 to 6.02 +/- 1.43 mmHg/microl, P < 0.01). AOO altered the maximal slope (E(max), 3.23 +/- 1.02 to 5.53 +/- 1.53 mmHg/microl, P < 0.05) and intercept (-19.9 +/- 8.6 to 1.62 +/- 13.51 microl, P < 0.01) of the end-systolic pressure-volume relation but not E(a)/E(max) (1.44 +/- 0.43 to 1.21 +/- 0.37, not significant). We conclude that simultaneous acquisition of Z(in) and arterial function parameters in the mouse, based solely on conductance catheter measurements, is feasible. We obtained an anticipated response of Z(in) and arterial function parameters following VCO and AOO, demonstrating the sensitivity of the measuring technique to induced physiological alterations in murine hemodynamics.