Abstract: A method is proposed to partition the variation of species abundance data into independent components: pure spatial, pure environmental, spatial component of environmental influence, and undetermined. The new method uses pre-existing techniques and computer programs of canonical ordination. The intrinsic spatial component of com- munity structure is partialled out of the species-environment relationship in order to see if the environmental control model still holds. The method is illustrated using oribatid mites in a peat blanket, forest vegetation data, and aquatic heterotrophic bacteria. In this latter example, the new method is shown to be complementary to another approach based on partial Mantel tests.

Abstract: Trophic cascades mean runaway consumption, downward dominance through the food chain. Especially vulnerable are the autotrophs. Standing crop and coverage of the plant community are reduced wholesale when one or a few species of potent herbivores are not suppressed. In archetypical trophic cascades, overwhelming effects propagate down through three trophic levels. Primary carnivores or diseases, by suppressing herbivores, switch the substrate from open and virtually bare to well occupied by plants. The discovery that these potent forces extend through four levels, in some instances (Carpenter and Kitchell 1988, Power 1990b), is surely one of the most important in all of ecology of the last decade; secondary carnivores, by suppressing primary carnivores, unleash herbivores that clear the substrate and greatly decrease standing crop of plants. In true trophic cascades, pervasive topdown influence combines with the always strong bottom-up influence through the food chain to produce acute intertwining between population, community, and ecosystem processes (Carpenter and Kitchell 1988, Oksanen 1990, Power 1992; M. J. Wiley, personal communication). True trophic cascades imply keystone species (Paine 1980), taxa with such top-down dominance that their removal causes precipitous change in the system. But all trophic interactions do not cascade, and simple top-down dominance is not the norm of communities or ecosystems. This point is central to the issues of this Special Feature, to Hunter and Price's (1992) proposal that the null hypothesis of food webs should be bottom-up forces, and Power's (1992) search for appropriate models of population dynamics of consumer and consumed. My premise is that true trophic cascades in the community sense are a relatively unusual sort of food web mechanics. I argue that, over the full range of ecological communities, evidence is that these cascades are restricted to fairly low-diversity places where great influence can issue from one or a few species; the majority of examples of true trophic cascades have algae at the

Abstract: Predator-prey theory is traced from its origins in the Malthus-Verhulst lo- gistic equation, through the Lotka-Volterra equations, logistic modifications to both prey and predator equations, incorporation of the Michaelis-Menten-Holling functional response into the predator and prey equations, and the recent development of ratio-dependent functional responses and per-capita rate of change functions. Some of the problems of classical predator-prey theory, including the paradoxes of enrichment and biological con- trol, seem to have been caused by the application of the principle of mass action to predator- prey interactions. Predator-prey models that evolved from logistic theory or that incor- porate ratio-dependent functional responses do not have these problems and also seem to be more biologically plausible.

Abstract: The abundance and distribution of corvids (Common Raven, Corvus corax; Hooded Crow, C. corone; Jackdaw, C. monedula; Black-billed Magpie, Pica pica; and European Jay, Garrulus glandarius) were studied across a gradient from a landscape dom- inated by agricultural land to a landscape dominated by forest in south-central Sweden. The total density and the predation rate on dummy nests increased as the proportion of agricultural land increased. This supports the suggestion that the density of corvids increases as forest becomes fragmented and intermixed with agricultural land, causing an increase in nest predation in small forest fragments. However, the different species responded differently to the proportion of forest in the landscape. The two forest-living corvids, the Jay and Raven, were absent from small forest frag- ments in a matrix of agricultural land and mainly preyed upon dummy nests inside the large forest fragments. Thus, both of these species were habitat specialists restricted to forests. Three corvids basically inhabit agricultural land, the Hooded Crow, Jackdaw, and Magpie. The Jackdaw and the Magpie showed a strong preference for agricultural land and they mainly preyed upon dummy nests in agricultural land. On the other hand, the density of Hooded Crows was higher in landscapes with a mixture of agricultural land and forest than in landscapes dominated by either agricultural land or forest, indicating a use of both agricultural land and forest. Further, Hooded Crows regularly preyed upon nests in both forest habitat and agricultural land; it is a habitat generalist. Therefore, the Hooded Crow was the most important species in the corvid family, causing increased predation pressure close to forest-farmland edges and in small forest fragments surrounded by agricultural land.

Abstract: Two common features of biological communities are (a) complex interactions among species, which make community dynamics sensitive to initial conditions, and (b) spatial heterogeneity, which fragments large-scale ecological systems into a mosaic of patches, hereafter termed a metacommunity. This computer simulation study examines the effect of complex interactions on the global and local dynamics od metacommunities. Patches are physically identical and differ only in the initial proportion of species that colonize the patches. The random variation is then magnified by deterministic interactions that cause patches to follow different trajectories based on initial conditions. After a period of interaction, individuals from all patches join a global pool of dispersers that colonize a new generation of patches. Complex interactions can have at least two important effects on metacommunity dynamics. First, the number of species coexisting in the metacommunity can greatly exceed the number of species coexisting in any single patch, despite the fact that the patches are physically identical, the species do not differ in colonization ability, and stochastic effects are absent after the colonization stage. Second, complex interactions provide a new source of variation upon which natural selection can operate at the patch level, providing a mechanism for the evolution of more » functionally organized communities. 59 refs., 11 figs., 2 tabs. « less

Abstract: In Carpinteria Salt Marsh, Salicornia virginica (pickleweed) grows at lower marsh elevations than does Arthrocnemum subterminalis (Parish's glasswort). Standing biomass of both species was greatest immediately adjacent to their abrupt border, suggesting that conditions for plant growth were best here. We utilized field experiments, in which growth rates of naturally occurring and transplanted individuals of both species were measured in four marsh zones, to investigate the role of edaphic factors and competition in maintaining this zonation pattern. The frequency of flooding, and hence soil waterlogging, was greatest at lower marsh elevations, whereas salinity was highest at higher marsh elevations. Consequently, it was not clear, a priori, which part of the marsh had the most severe physical conditions. In our field experiments, both Salicornia and Arthrocnemum grew better in the two middle marsh zones (high Salicornia zone and Arthrocnemum zone) than in either the low marsh (low Salicornia zone), where flooding was frequent and soils were waterlogged, or the high marsh (transition zone), where soil salinity was extremely high during much of the year and plant water potentials very low. However, Salicornia appeared better able to tolerate flooding, and so persisted in the low Salicornia zone, whereas Arthrocnemum appeared better able to tolerate high salinities, and so persisted in the transition zone. Interspecific competition was most important in the relatively benign middle marsh zones, where each species excluded the other from a portion of this prime habitat. In this marsh, flooding, soil salinity, and competition all interacted to determine plant zonation patterns, but the relative importance of these factors varied at different elevations.

Abstract: Although the possibility of complex dynamical behaviors-limit cycles, quasiperiodic oscillations, and aperiodic chaos-has been recognized theoretically, most ecologists are skeptical of their importance in nature. In this paper we develop a meth- odology for reconstructing endogenous (or deterministic) dynamics from ecological time series. Our method consists of fitting a response surface to the yearly population change as a function of lagged population densities. Using the version of the model that includes two lags, we fitted time-series data for 14 insect and 22 vertebrate populations. The 14 insect populations were classified as: unregulated (1 case), exponentially stable (three cases), damped oscillations (six cases), limit cycles (one case), quasiperiodic oscillations (two cases), and chaos (one case). The vertebrate examples exhibited a similar spectrum of dynamics, although there were no cases of chaos. We tested the results of the response-surface meth- odology by calculating autocorrelation functions for each time series. Autocorrelation pat- terns were in agreement with our findings of periodic behaviors (damped oscillations, limit cycles, and quasiperiodicity). On the basis of these results, we conclude that the complete spectrum of dynamical behaviors, ranging from exponential stability to chaos, is likely to be found among natural populations.

Abstract: In population models, omnivorous predation (i.e., predation on > 1 trophic level) generally has a destabilizing influence, whereas habitat heterogeneity tends to stabilize both predatory and com ...

Abstract: We re—examined the longstanding dogma that cavity—nesting birds have larger clutch sizes than open—nesting species because of lower nest predation rates, which allow longer developmental periods. We provide data on nesting success of open—nesting species plus excavator and nonexcavator species (cavity—nesters that do vs. do not create their own cavities) of birds that coexist in the same habitat and use natural nest sites; studies were conducted in central Arizona in high—elevation forest drainages from 1987 to 1989. In comparisons among species, nest success increased in the order open—nesting < nonexcavator ° excavator species. Length of the nestling period increased, whereas number of broods decreased, with increased nest success across these nest types. Contrary to conventional expectation, clutch size did not increase directly with increased nest success and length of the nestling period, and these patterns were not simply a result of phylogenetic effects. A literature review of data on adult survival in land birds indicated that nonexcavators have significantly lower adult survival than the other two nest types. The lower adult survival of nonexcavators was associated with greater annual productivity than for the other two nest types. Annual productivity appeared to be associated with nest site attributes, whereas number of broad attempts was related to nest failure rate. Clutch size was a byproduct of annual productivity and number of brood attempts. Thus, the long—standing paradigm relating clutch size directly to nesting failure and developmental period was not supported.

Abstract: Gross rates of N mineralization, immobilization, and nitrification were mea- sured by '5N isotope dilution in a 10-yr-old conifer plantation and in a mature conifer forest Gross rates revealed nutrient cycling characteristics that differ from expectations based on more common measures of net rates Although net mineralization rates were somewhat higher in the young forest than in the old forest, gross mineralization rates in the old forest were 2-3 times as high as gross mineralization rates in the young forest, indicating more rapid turnover of inorganic-N pools in the old forest Net mineralization rates were < 14% of gross mineralization rates Smaller N03 pool size and lower net nitrification rates in the old forest than the young forest might lead to the conclusion that the old forest is a non-nitrifying ecosystem and that nitrate is important only in the N cycle of the young forest However, gross nitrification rates were similar in both young and old forests Microbial assimilation of N03 was also significant in both forests, indicating a rapid turnover of a small but important N03- pool Microbial assimilation may be an important pathway for N03 retention in forest ecosystems

Abstract: The production, development, and mortality of fine roots in a northern hardwood forest was monitored for 1 yr using minirhizotrons. Roots were divided into two strata based upon their depth in the soil, 30 cm. Cohort analyses of roots produced in the spring of 1989 revealed that while almost 50% of fine roots at both depths survived after 346 d, the number of white roots in each cohort declined very rapidly. Virtually all roots had turned brown after 346 d. The probability of a surviving white root turning brown was much greater than the probability that it would die at all times of the year, and the bulk of root mortality was accounted for by brown roots. Analysis of root length production and mortality showed that total annual length mortality at the 30 cm depth. Fine root production and mortality occurred simultaneously throughout the year, and production was slightly greater than mortality at both depths. Total root length peaked in the summer at both depths, and overwinter production and mortality was rather low. Production of white and brown root length indicated that roots near the soil surface were undergoing much more rapid rates of browning than deep roots. Loss of root length between sampling dates was largely due to roots that died and rapidly decayed or otherwise disappeared.

Abstract: Do top-down (e.g., trophic interactions) or bottomup (e.g., nutrients) effects control communities? In recent years, an increasing number of ecologists have argued that, as with most ecological controversies (Schoener 1987), this dichotomy is artificial and counterproductive. Both top-down and bottom-up factors can have important effects on community structure (e.g., Carpenter 1988, Hunter and Price 1992, Power 1992). Thus, it seems more appropriate to ask "how do bottom-up and top-down effects interact and influence each other," and "what are the mechanisms underlying variation in each?" These rather simple questions still mask enormously complex issues. However, in contrast to earlier, "single-factor" approaches, recent work dealing directly with the multi-factorial nature of natural communities has offered important insights. Among these are that bottom-up processes can underlie variations in community structure that can have important consequences for the influence of top-down factors (e.g., Carpenter and Kitchell 1984, 1987, Oksanen 1988, Persson et al. 1988, Power 1990). For instance, the level of primary production may determine the number of trophic levels, which in turn could have major effects on community structure (Oksanen et al. 1981, Fretwell 1987, Persson et al. 1988). It is important to define what is meant by "control." As used here, control means having a major quantitative and/or qualitative effect on community structure. Quantitative effects are changes in abundance (numbers, cover, biomass), while qualitative effects are changes in community composition and/or the nature of interactions among community components. For example, increased production might increase the abundance of herbivores without changing species composition (i.e., a quantitative but not qualitative change). On the other hand, predation might reduce prey abundance, allowing both increases in abundance of competitors already present (a quantitative change)

Abstract: Many organisms reproduce by releasing gametes into the environment. How- ever, very little is known about what proportion of released eggs become fertilized. We examined the influence of spawning group size, degree of aggregation, position within an aggregation, and water flow, on in situ fertilization in the sea urchin Strongylocentrotus franciscanus. This study was conducted at a depth of 9 m on the west coast of Vancouver Island, British Columbia, Canada. Males were simulated by syringes filled with sperm; females were simulated by sperm-permeable containers filled with eggs. Individuals were placed 0.5 or 2.0 m apart within a 2 x 2 or 4 x 4 (group size of 4 or 16 individuals) experimental array. The results indicate that group size, degree of aggregation, position within a spawning group, and water flow all affect fertilization success. Fertilization success ranged from 0 to 82%. Increases in group size and aggregation, decreases in flow velocity, and central and downstream positions within an aggregation all lead to increases in fertil- ization success. Thus, individual reproductive performance is dependent on, and highly sensitive to, population parameters and environmental conditions.

Abstract: Biologically derived safe sites appear to strongly affect spatial and temporal patterns in terrestrial plant communities; however, few experimental field studies have been conducted to investigate the importance of such interactions or the mechanisms by which they operate. Quercus douglasii and Quercus lobata are winter-deciduous oaks en- demic to California. I examined associations between seedlings of these oaks and several species of shrubs and conducted field experiments that tested for facilitative effects of two shrubs, Salvia leucophylla and Artemisia californica, on seedling survival. Naturally oc- curring Q. douglasii seedlings were relatively common (280 seedlings/ha) and were strongly associated with shrub canopies. In two experimental plantings, 30% and 55% of Q. douglasii seedlings that emerged under shrubs survived for > 1 yr, whereas no seedlings in the open survived in either experiment. Survival of Q. douglasii where shrubs were removed was similar to survival in the open grassland, indicating that association between shrubs and oak seedlings was not due to shared microsite requirements. Q. douglasii seedling survival was not different between the two shrub species. Only three Q. lobata seedlings were found in 26 study plots, although reproductive adults were present. The seedlings were under shrub canopies. However, the survival of Q. lobata was not facilitated by shrub cover or shrub simulation and no other evidence was found for nurse-shrub interactions with this oak species. Artificial shade was critical to the survival of Q. douglasii seedlings in simulated shrub environments. When grown in artificial shade, photosynthetic capacities and root elon- gation rates of Q. douglasii seedlings were significantly higher than those of Q. lobata. These differences may partially explain why nurse-plant interactions exist between shrubs and Q. douglasii seedlings but not between the same shrubs and Q. lobata seedlings. Causes of acorn and seedling mortality differed between the shrub and open grassland habitats. Under shrubs, acorn predation was the primary cause of mortality, whereas mortality due to shoot herbivory was much more frequent in the open grassland. Thus, propagules under shrubs that avoided predation until shoot emergence benefitted from canopy shade and protection from herbivores. The shade tolerance and nurse-plant utilization of Q. douglasii seedlings have parallels with shade tolerance and late successional roles of other oak species in temperate deciduous forests, and, as in these forests, may affect the long-term spatial dynamics in California woodlands.

Abstract: Experimental studies using root observation chambers to observe the effects of encounters between individual roots on root elongation rates have revealed that the interactions among roots of Ambrosia dumosa and Larrea tridentata are more complex than simple competition for a limiting resource. Larrea roots inhibited elongation of either Larrea or Ambrosia roots in their vicinity, and Ambrosia roots inhibited elongation of contacted roots on other Ambrosia plants only. The purpose of the study reported here was to test the hypothesized involvement of inhibitory substances released by roots in these interroot encounters by attempting to remove such substances by adsorption to activated carbon. The presence of activated carbon caused a significant decrease in the inhibition of elongation of neighboring roots by Larrea roots, but activated carbon had no effect on the intraspecific responses of Ambrosia roots. These results support the hypotheses that the interaction mechanism of Larrea roots in- volves the release of a readily diffusible, generally inhibitory substance by Larrea roots into the soil, rather than a simple depletion of water or nutrients from around Larrea roots, and that the intraspecific, self-nonself-recognizing interaction mechanism of Ambrosia roots is mediated by contact and is fundamentally different from that of Larrea. These findings may enhance our understanding of Mojave desert community structure. The root-mediated allelopathy of Larrea may play a role in producing and maintaining the commonly occurring, regular distributions of Larrea. The complex communication mechanism of Ambrosia roots appears to constitute a detection and avoidance system that may allow this shrub to grow in clumped intraspecific distributions with little or no intra- specific competition for water. The interspecific interference between Larrea and Ambrosia in the field may be mechanistically asymmetrical due to their different root communication mechanisms.

Abstract: Hundreds of kilometres of West Tennessee streams have been channelized since the turn of the century. After a stream is straightened, dredged, or cleared, basin- wide ecologic, hydrologic, and geomorphic processes bring about an integrated, character- istic recovery sequence. The rapid pace of channel responses to channelization provides an opportunity to document and interpret vegetation recovery patterns relative to otherwise long-term, concomitant evolution of river geomorphology. Nearly 150 sites along 15 streams were studied in the Obion, Forked Deer, Hatchie, and Wolf River basins. Channels of these streams, except that of the Hatchie River main stem, have undergone major modi- fications along all or parts of their courses. This paper presents the eco-geomorphic analyses and interpretation of a large multidisciplinary study, with special reference to the inter- related hydrogeomorphic aspects of channel recovery. Quantitative plant ecological anal- yses were conducted to infer relative bank stability, to identify indicator recovery species, and to determine patterns of vegetation development through the course of accelerated channel evolution. Binary-discriminant and ordination analyses show that distinctive ri- parian-species patterns reflect a six-stage model of channel evolution and can be used to infer channel stability and hydrogeomorphic conditions. Woody vegetation initially estab- lishes on low- and mid-bank surfaces at the same location and time that bank accretion begins, and corresponds to the site of initial geomorphic restabilization. The linkage of channel bed aggradation, woody vegetation establishment, and bank accretion all lead to recovery of the channel. Pioneer species are hardy and fast growing, and can tolerate moderate amounts of slope instability and sediment deposition; these species include river birch (Betula nigra), black willow (Salix nigra), boxelder (Acer negundo), and silver maple (Acer saccharinum). High stem densities and root-mass development appear to enhance bank stability. Tree-ring analyses suggest that on average 6 5 yr may be required for recovery after channelization.

Abstract: We studied avian breeding and molting activity in relation to rainfall, tem- poral fluctuations in food resource abundance, and food exploitation by birds, in four arid and semiarid tropical habitats in Venezuela. Twice a month we used mist nets to monitor changes in breeding and molting conditions of captured birds and forced them to regurgitate to determine their diet and feeding guild membership. Food abundance was assessed by measuring the flowering and fruiting seasonality of marked plants and by evaluating ar- thropod abundance with four different trapping methods. Flowering activity was limited largely to the wet season. Fleshy fruits, although produced year-round, were also more abundant in the rainy period. Arthropod abundance followed the same general pattern with numbers highest in the wet season and lowest in the dry season. Birds of all feeding guilds predominantly bred and molted during the wet season, synchronously with the highest abundance of most food resources. However, the diet analysis revealed a higher occurrence of arthropods coupled with a sharp decrease in the intake of vegetable matter during the birds' breeding season. Consequently, we suggest that arthropod abundance is a crucial factor governing the timing of breeding activities, even in species that normally include a high proportion of nectar and fruits in their diet. We also postulate that, in tropical habitats receiving > 1500 mm of rain per year, breeding in nectarivores and frugivores in the dry season may be related to the lower reduction in arthropod numbers over the less severe drought period.

Abstract: To determine the long-term effect of alder on soil fertility, biogeochemical fluxes were measured and calculated for two pairs of adjacent, 55-yr-old stands dominated by conifers, primarily Douglas-fir (Pseudotsuga menziesii), and by conifers and nitrogen- fixing red alder (Alnus rubra). At a low-fertility site in the Wind River Experimental Forest in southwestern Washington, biomass of the alder-conifer stand (289 Mg/ha) exceeded that of the conifer stand (171 Mg/ha), and the aboveground net primary production (ANPP) of the alder-conifer stand (10.3 Mg-ha-I yr- 1) was more than twice that of the conifer stand (4.8 Mg ha-I 'yr-l). At a more fertile site in the Cascade Head Experimental Forest in western Oregon, both biomass and ANPP were higher than at Wind River, and biomass and ANPP were higher in the conifer stand (584 Mg/ha and 19.2 Mg-ha-l yr-l1) than in the alder-conifer stand (342 Mg/ha and 10.7 Mg-ha-l yr-1). Nitrogen accretion in the alder-conifer stand at Wind River averaged 54 kg-ha- -yr-' for the 52 yr since stand establishment, with a current rate of N fixation of 75 kg ha-I.yr-1. For the alder-conifer stand at Cascade Head, N accretion averaged 73 kg ha-l yr- for 55 yr, with a current N-fixation rate of 85 kg-ha- lyr-1. The cycling of all nutrients appeared very malleable under the influence of alder. At Wind River, return of nutrients in fine litterfall in the alder-conifer stand ranged from 1.5 (P) to 7.9 (N) times those in the conifer stand; whereas at Cascade Head, these ratios ranged from 1.7 (S) to 4.2 (N). Nutrient-use efficiencies (kilograms of ANPP per kilogram of nutrient uptake) were generally lower for the alder-conifer stands at both sites. Denitrifi- cation appeared negligible (<0.3 kg-ha-l yr-1) in all stands. Leaching of organic plus inorganic N ranged from -5 kg-ha-l yr-1 for the conifer stand at Wind River, to 50 Kg -ha-l-yr-1 for the alder-conifer stand at Cascade Head.

Abstract: Two of the central hypotheses of the triangular model of primary plant strategies were tested by a novel technique involving seven grasses of contrasted ecology grown in pure stands and an additive mixture on an experimental matrix of crossed gradients of mineral nutrient stress and vegetation disturbance. The experimental design allowed reductions in vegetative and reproductive vigor resulting from interspecific competition to be distinguished from those arising from direct effects of nutrient stress and vegetation disturbance. It was also possible to determine the extent to which competitive suppression of each species was affected by stress and disturbance. In isolation, all species showed maximum vegetative and reproductive vigor at high soil fertility and low disturbance. In the mixture, absolute reductions in biomass and flowering due to competition were greatest at high soil fertility and low disturbance, and the species of most extreme strategy became restricted to areas of the matrix broadly consistent with those predicted by strategy classification. When standardized for differences in biomass in pure stands, the effect of competition remained relatively constant across the stress-disturbance matrix for all species except Poa annua, which was less restricted by competition at high intensities of stress. There were marked and consistent differences between species in their susceptibility to competition. At both high and low soil fertility, two species of natural occurrence on infertile soils (Festuca ovina, Bromus erectus) were poor competitors relative to Arrhenatherum elatius, a widespread dominant of productive grasslands. The effect of competition was least severe on flowering of annuals in low-stress portions of the matrix. If competition is assessed simply as the percentage of reduction in biomass between pure and mixed stands it appears that competition intensity is constant across different intensities of stress and disturbance. However, observations that maximum reductions in biomass coincided with low stress and low disturbance, that competition decreased in importance as a factor reducing yield and flowering (relative to stress and disturbance) as stress and disturbance intensities increased, and that there was a consistently inferior competitive ability of plants from infertile soils at all positions on the matrix all support the hypothesis that competition declines in importance as a vegetation determinant in the vegetation of infertile soils.

Abstract: Fugitive species often live under harsh physical conditions, yet their ability to cope with environmental extremes has received little attention. We studied the association of fugitive plants and stressful physical conditions using salt marsh plants that live in hypersaline bare patches. In New England salt marshes, primary space is dominated by dense monospecific stands of perennial turfs, but disturbances frequently leave bare space that rapidly becomes hypersaline. Elevated salinities of bare patches result form direct exposure of soil to solar radiation and evaporative water loss. A number of fugitive plants that are rare in undisturbed salt marsh vegetation are commonly associated with bare patches before being displaced during secondary succession. Greenhouse studies show that plants that dominate undisturbed vegetation are stunted by high substrate salinities, whereas the photosynthesis and growth of patch—dependent fugitives are relatively independent of typical salinity variation. Moreover, in the field, watering more than doubled the survivorship and growth of seedlings in patches and more strongly affected dominants than patch—dependent fugitives. Our results show that physical conditions in salt marsh bare patches can limit plant colonization, and suggest that high salt tolerances may permit fugitive plants to utilize bare patches as refugia from competitors. Since fugitive plants are commonly restricted to physically stressful habitats, the ability to withstand harsh physical conditions is likely often an important aspect of their biology.

Abstract: We compared rates and directions of benthic aquatic macroinvertebrate succession following eight spates of varying magnitude that occurred in different seasons over 3 yr in Sycamore Creek, a Sonoran Desert stream. A consistent cycle of seasonal change in assemblage composition occurred each year, little altered by spates. Changes reflected variations in presence or absence rather than relative abundance of taxa. Seasonal patterns were confirmed by plotting temporal changes in densities of common taxa. In- vertebrate abundance (mostly oligochaetes and mayflies) peaked in spring. "Summer" dominants included the gastropod Physella virgata and the caddisfly larva Cheumatopsyche arizonensis. Assemblage composition remained relatively consistent during spring over 3 yr when high discharge was prolonged, whereas there was a major change in autumn community structure between 1984 and 1986, probably reflecting low discharge during a drought in 1986. Drying apparently influenced assemblage composition more than spates, possibly by altering habitat availability and the intensity of biotic interactions as surface stream volume shrank. Assemblage resistance to disturbance by spates was variable. Similarly, resistance of individual common taxa varied within and among taxa, and like assemblage resistance, was not simply a function of spate magnitude or timing (season). Resilience was generally high. Succession rate (degree of change in assemblage composition) declined during suc- cession in all but spring sequences, which displayed no consistent trend. The two summer sequences had highest initial succession rates (in first 30 d postspate), possibly reflecting higher water temperatures, and also exhibited late-successional increases in succession rate. Spatial variation in assemblage composition was uncorrelated with any physical variable measured. Factors known to influence ecosystem-level processes such as primary productivity (e.g., inorganic nitrogen flux, days since spate) also affected community-level aspects such as aquatic invertebrate assemblage composition in Sycamore Creek. Discharge and water temperature had lesser but detectable effects, and probably contributed to the marked seasonality in assemblage composition. Further comparisons of collective properties of ecosystems and communities within other biomes may identify "common denominators" that characterize responses to disturbance and environmental change. This will remove the different perceptions about stability we gain by using response variables that are assessed only at a community or ecosystem level.

Abstract: Drift activity of stream invertebrates typically is greatest during the nighttime hours in running waters throughout the world. Such diel periodicity may be an adaptive response that minimizes exposure to visually hunting, drift-feeding fishes. I tested this risk- of-predation hypothesis by examining drift behavior of mayflies in a series of Andean mountain and piedmont streams that vary in the abundance of drift-feeding fishes. Drift was primarily nocturnal in piedmont streams with natural populations of visually hunting predators. In contrast, mayfly drift activity did not differ between day and night in mountain streams that historically lack drift-feeding fishes. However, in naturally fishless Andean streams containing introduced rainbow trout, nocturnal peaks in drift were observed for the mayfly Baetis, suggesting a rapid evolutionary change in behavior in response to an exotic predator. When drift periodicity was examined along a gradient of predation regimes, activity was found to be increasingly restricted to the nighttime hours as predation risk became more intense. Diel periodicity was observed even when fish were experimentally excluded, suggesting that nocturnal activity has evolved as a fixed behavioral response to predation, and is not a direct ecological consequence of diurnal feeding by fishes. These observations support the hypothesis that predation risk is important in determining the timing of prey drift behavior.

Abstract: Foraging behavior and physiological adaptations for diving were studied in Thick-billed Murres, Uria lomvia, in the field and laboratory. Electronic, light-emitting diode, and capillary recording devices were used to measure foraging behavior. Individual dives were a flattened U shape in profile, and occurred in bouts lasting 15 min. Dive patterns were nocturnal; most dives occurred between 2000 and 0400. Murres probably concentrate their foraging effort at times when prey is most available as it migrates closer to the surface in the evening as part of the deep scattering layer. Although dives averaged 18 m in depth and 55 s in duration, most time-at-depth was spent between 21 and 40 m. Thus, murres made a large number of shallow, short-duration dives. Maximum dive depth was 210 m, while maximum dive duration was 224 s. Descent and ascent rates averaged 0.94 and 0.85 m/s, respectively. Hematocrit, hemoglobin, blood volume, and pectoralis myoglobin levels were measured in the laboratory as 52.8%, 18.0 g/100 mL, 12.3% body mass, and 1.9 g/100 g, respectively. Total useable oxygen store was calculated as 44.8 mL/kg, giving an estimated aerobic dive limit (ADL) of 47 s. Murres exceeded the calculated ADL in 48% of their dives. Long-duration diving is probably a more efficient foraging strategy for murres given their relatively small size and limited oxygen storage capabilities. The observed dive depths raised questions of potential problems with decompression sickness (bends) and lung collapse.

Abstract: The purpose of this study was to examine the effect of abundant native large herbivores on ecosystem function of a spatially and temporally heterogeneous temperate grassland. Net aboveground primary production (ANPP), large herbivore consumption (C), and dung deposition (D), an index of nutrient flow from herbivores to the soil, were measured in grassland and shrub-grassland habitat on winter, transitional, and summer range used by herds of elk (Cervus elaphus) and bison (Bison bison) in northern Yellowstone National Park. Temporary exclosures (5-7 per site) were moved every 4 wk during the snow-free season to determine ANPP and C. Data were collected during 1988, a year of drought and unusually high elk and bison population levels, and 1989, a climatically near- average year, with dramatically fewer elk and bison. All three processes, ANPP, C, and D, varied widely among sites: ANPP range: 16-589 g/m2, C range: 0-306 g/m2, and D range: 0-68 g/m2. An average of 45% of ANPP was consumed by herbivores. Production and consumption, and consumption and dung de- position were positively correlated across all sites. In addition, sites were grazed when plants were growing. There was a 19% reduction in ANPP from 1988 to 1989, likely caused by death or injury to plants during the 1988 drought. Drought also appeared to be partially responsible for reductions in elk and bison from 1988 to 1989, which were coincident with declines in C and D. Results indicate direct effects and suggest indirect effects of a single-season drought on grassland function that will persist for several years after the event.

Abstract: The genus Greya is closely related to the yucca moths, and Greya species interact with their host plants in many of the same ways as yucca moths. Females both pollinate and oviposit in the flowers of their host. Unlike yucca moths, however, pollinating Greya species share flowers with co—pollinators that do not oviposit on the host. We studied the interaction between Greya politella (Walsingham) (Lepidoptera: Prodoxidae) and Lithophragma parviflorum (Hook.) Torr. & Gray (Saxifragaceae) to evaluate the effect on seed output of a pollinating seed parasite against a background of co—pollinators. Flowers were visited and pollinated mostly by bombyliid flies, solitary bees, and G. politella. Bombyliid flies alone composed 68—88% of the 5522 visits recorded over 2 yr. Although both male and female G. politella visited the flowers and probed for nectar, pollination by this species occurred only as females oviposited through the corolla tube, thereby passively transferring to the stigma pollen adhering to the abdomen. Visitation to flowers by all pollinators averaged 0.3—1.9 visits/h during daylight hours. Consequently, most flowers were visited multiple times during the several days that stigmas were receptive, and 77% of the flowers tagged during the 4 yr had some developing seeds. Pollination did not depend upon visitation by G. politella. Flowers receiving G. politella eggs had the same probability of producing some seed and the same mean number of developing seeds as flowers visited only by other insects. Most flowers received the eggs of only one G. politella female, and the larvae ate 15—27% of the developing seeds. The final number of mature seeds remaining in attacked flowers did not differ from unattacked flowers except at one site in 1 yr, in which the values for attacked plants were marginally lower. Other sources of variation affecting seed output masked the effects of seed consumption by Greya. Overall, G. politella females have the potential to be mutualistic with L. parviflorum: they are effective pollinators, generally visit most plants and about half the flowers in the populations, and impose a fairly small cost on seed output. Nonetheless, the abundant and effective co—pollinators, which do not eat the developing seeds, swamp Greya's mutualistic effects. Under the current conditions at Granite Point, the relationship between G. politella and L. parviflorum may be mostly commensalistic. The evolution of specialization to G. politella as an exclusive pollinator would seem to be possible only in L. parviflorum populations in which effective co—pollinators were either rare or unpredictable. That is, the potential for the evolution of specificity in this mutualism appears to depend upon the community context in which the interaction takes place rather than upon the simple outcome of the pairwise interaction between Greya and Lithophragma.

Abstract: Ratio-dependent models of predators and prey are approximations of the biology of resource acquisition and allocation and their consequences for population birth and death rates. A demand-driven functional response model that has a physiological basis in mass (energy) dynamics is reviewed (i.e., the metabolic pool model), and its obvious links to the logistic model are outlined. To demonstrate the utility of this approach, a distributed maturation time age-structure model of the dynamics of A. J. Nicholson's classic laboratory population data on the sheep blow fly (Lucilia cuprina Weidman) is developed. The model provides sufficient information on the dynamics of the intermediate life stages to show that the blow fly oscillations were due to the effects of larval competition for food on size, fecundity, and pupation success. These results agree with Nicholson's conclusions. The advantage of this model, in contrast to prior models, is that the dynamics emerge by considering the processes of resource acquisition and allocation as they affect growth, reproduction, and survival. No explicit time delays, which automatically lead to oscillations, were included. Lastly, the notions of the metabolic pool model are found in Nicholson's original model for equilibrium popu- lation density. The metabolic pool paradigm in an age-structure setting is used to model the tri-trophic dynamics of Acyrthosiphon aphids in an alfalfa ecosystem. The model explains the role of the various natural enemies in the regulation of the aphids.

Abstract: This laboratory study of Daphnia magna had two major aims: (1) to examine the effect of food quantity on offspring size and number and how this effect varies with genotype and maternal size, age, and metabolic demand; and (2) to examine the relationship of investment per offspring to total reproductive investment. Differences in the response of offspring size to food quantity between the two clones in this study and between Daphnia studies in the literature are explained by a nonlinear model that explicitly considers the effect of food quantity relative to the metabolic demands of the mother. According to this model, offspring size covaries positively with food quantity/maternal demand at very low food levels because of reproductive constraints, but it covaries negatively with food quan- tity/maternal demand at middle food levels as an adaptive response, and it remains constant at very high food levels because of a lower limit on viable offspring size. Effects of genotype (clone) and ration level on trade-offs between egg size and number and on the maternal size-dependence of egg mass, brood mass, and brood size are described. Strong genotype-environment interactions are evident for egg size and number. Trade-offs between egg size and number are seen at high and low ration levels, but only in second (or third) to sixth broods, and not in first broods, which had relatively high variation in total mass. Results of this study that contradict predictions or assumptions of optimal offspring investment theory include: (1) at both ration levels both egg size and number were strongly positively correlated with brood mass, independently of maternal mass, (2) although, at low ration, variability of brood mass usually exceeded the variability of egg mass, as expected, the opposite was true at high ration, and (3) the interclonal difference in offspring size was not correlated with survival ability under starvation. Offspring size covaried positively with maternal size and age, partly independently of culture conditions. This relationship is explained as the result of selection in growing parthenogenetic populations for increased fecundity in young small females, which are constrained to produce relatively small eggs because of spatial limitations of the brood pouch.