Changes in avian and plant communities of aspen woodlands over 12 years after livestock removal in the Northwestern Great Basin.

Riparian and quaking aspen (Populus tremuloides) woodlands are centers of avian abundance and diversity in the western United States, but they have been affected adversely by land use practices, particularly livestock grazing. In 1990, cattle were removed from a 112,500-ha national wildlife refuge in southeastern Oregon. Thereafter, we monitored changes in vegetation and bird abundance in years 1-3 (phase 1) and 10-12 (phase 2) in 17 riparian and 9 snow-pocket aspen plots. On each 1.5-ha plot, we sampled vegetation in 6 transects. Three times during each breeding season, observers recorded all birds 50 m to each side of the plot's 150-m centerline for 25 minutes. We analyzed data with multivariate analysis of variance and paired t tests with p values adjusted for multiple comparisons. In both periods, riparian and snow-pocket aspen produced extensive regeneration of new shoots (stems/ha and 7079 stems/ha, respectively). By phase 2, a 64% increase in medium-diameter trees in riparian stands indicated successful recruitment into the overstory, but this pattern was not seen in snow-pocket stands, where the density of trees was over 2 times greater. By phase 2 in riparian and snow-pocket stands, native forb cover had increased by 68% and 57%, respectively, mesic shrub cover had increased by 29% and 58%, and sagebrush cover had decreased by 24% and 31%. Total avian abundance increased by 33% and 39% in riparian and snow-pocket aspen, respectively, ground or understory nesters increased by 133% and 67% and overstory nesters increased by 34% and 33%. Similarly, ground or understory foragers increased by 25% and 32%, aerial foragers by 55% and 57%, and overstory foragers by 66% and 43%. We interpreted the substantial regeneration of aspen shoots, increased densities of riparian forbs and shrubs, and increased avian abundances as a multitrophic-level response to the total removal of livestock and as substantial movement toward recovery of biological integrity.

Flowering patterns of long-lived Heliconia inflorescences: implications for visiting and resident nectarivores.

Flowering patterns of four Heliconia (Heliconiaceae) species in Trinidad, West Indies were examined for their predictability and availability to the nectarivores that rely on Heliconia floral nectar. Principal flower visitors are trapling hermit hummingbirds; inflorescences are inhabited by nectarivorous hummingbird flower mites that move between inflorescences by riding in the hummingbirds' nares. Heliconia inflorescences flower for 40-200 days, providing long-term sources of copious nectar (30-60 µl per flower), but each Heliconia flower lasts only a single day. As an inflorescence ages the interval increases between open flowers within a bract; wet-season inflorescences produce open flowers more slowly than dry-season conspecifics.Estimated daily energy expenditures for hermit hummingbirds demonstrate that slow production of short-lived open flowers plus low inflorescence density preclude territorial defense of Heliconia by the hermits. Heliconia flowering patterns are viewed as a means of (i) regulating reproductive investment by the plants through staggered flower production over long periods of time, and (ii) maintaining outcrossing by necessitating a traplining visitation pattern by its hummingbird pollinators. I suggest that Heliconia exhibit a two-tiered pollination system by using hermit hummingbirds primarily for outcrossing and using hummingbird flower mites primarily for self-pollination.

Shifts of thermogenesis in the prairie vole (Microtus ochrogaster) : Strategies for survival in a seasonal environment.

The weight-specific oxygen consumption ([Formula: see text]) of prairie voles caught in winter is 24% higher at 27.5° C and 29% higher at 7.5° C than that of summer animals, thus affording a higher weight-specific thermogenesis in winter than in summer which may allow tolerance to lower thermal exposures. Coincident with the increase in weight-specific rates of oxygen consumption is a decrease in body weight. When total energetic cost to maintain an animal per unit time is calculated, the cost at 27.5° C is the same for both summer and winter animals. Further, the cost to maintain an animal at 7.5° C is less in winter than in summer. Arguments are presented suggesting that prairie voles compensate for increased weight-specific thermogenesis in winter by lowering body weight. The responses to thermal acclimation are quite different in summer and winter animals, thus implying different sorts of metabolic organization. Acclimation to 5° C effects a 26% increase in [Formula: see text] at 27.5° C of winter voles, and acclimation to 30° C does not change [Formula: see text]. In contrast, [Formula: see text] at 27.5° C of summer animals is unaffected by 5° C acclimation, and depressed 20% by 30° C acclimation. Thus, the animals are capable of considerable physiological adjustment to varying thermal conditions in different seasons.