Showing papers in "Ecology in 1992"

The Problem of Pattern and Scale in Ecology: The Robert H. MacArthur Award Lecture

Abstract: This book is the second of two volumes in a series on terrestrial and marine comparisons, focusing on the temporal complement of the earlier spatial analysis of patchiness and pattern (Levin et al. 1993). The issue of the relationships among pattern, scale, and patchiness has been framed forcefully in John Steele’s writings of two decades (e.g., Steele 1978). There is no pattern without an observational frame. In the words of Nietzsche, “There are no facts… only interpretations.”

Partialling out the spatial component of ecological variation

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.

Are trophic cascades all wet? differentiation and donor-control in speciose ecosystems'

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

The Orgins and Evolution of Predator‐Prey Theory

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.

Corvid Density and Nest Predation in Relation to Forest Fragmentation: A Landscape Perspective

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.

Complex Interactions in Metacommunities, with Implications for Biodiversity and Higher Levels of Selection

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

Salt Marsh Plant Zonation: The Relative Importance of Competition and Physical Factors

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.

Complex Dynamics in Ecological Time Series

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.

Fine Root Production Estimates and Belowground Carbon Allocation in Forest Ecosystems

Abstract: We compared published estimates of the net fine root production (FRP) in forest sites to litterfall and aboveground net primary production (ANPP) to test whether annual rates of fine root and aboveground production vary together at global scales. We also compared FRP estimates to theoretical upper limits as defined by our previously published relationship between total root allocation (TRA, carbon allocated to FRP plus live—root respiration) and litterfall. Estimates of the carbon content of FRP in the total data set ranged from 25 to 820 g°m—2°yr—1 and were not correlated with annual litterfall of ANPP. Different methods used for estimating fine root production, however, showed contrasting results. Estimates derived using either sums of seasonal changes in fine root biomass ("Sequential Core" method) or differences between annual maximum and minimum fine root biomass ("Maximum — Minimum" method) were not correlated with either litterfall of ANPP. Sequential Core estimates were often high relative to predicted TRA values, whereas Maximum — Minimum estimates were generally <50% of TRA. The small number of FRP estimates derived from root—free cores ("Ingrowth Core" method) were not correlated with measures of aboveground production but were all well below predicted TRA values. In contrast to results of other methods, FRP estimates derived using ecosystem N budgets ("N Budget" method) were positively correlated with both litterfall and ANPP. Comparing FRP estimates based on N budgets with previous results of forest soil C budgets suggested that annual fine root production increases with aboveground production and that °1/3 of TRA is used for production of fine roots.

Fish predation and benthic community structure - the role of omnivory and habitat complexity

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 ...

Stable Isotopes and Planktonic Trophic Structure in Arctic Lakes

Abstract: Actual food—web structure or function is difficult to determine based on visual observation, gut analyses, or the feeding interactions expected from a given list of species. We used C and N stable—isotope distributions to define food—web structure in arctic lakes, and we compared that structure with results based on more traditional analyses. Although zooplankton species composition was similar across the eight lakes studied, the food—web structure varied greatly. In some lakes the copepod predator Heterocope fed on the herbivorous copepod Diaptomus as expected in a conventional food web. In most lakes, however, °15N data were consistent with Heterocope functioning as an herbivore rather than a predator. These inferences were supported by evidence from carbon isotopes and energy—flow data. Our study indicates that only two or three trophic levels exist in the macrozoopolankton of these lakes, in comparison to five or six trophic levels reported in temperate lakes. Isotope analyses showed that actual food—web structure is poorly predicted from simple consideration of species lists and potential trophic interactions.

Internal Cycling of Nitrate in Soils of a Mature Coniferous Forest

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

Life History Traits of Open- vs. Cavity-Nesting Birds

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.

The Effect of Leaf Litter on Early Seedling Establishment in a Tropical Forest

Abstract: The presence of leaf litter of different depths within a tropical forest creates many different microsites for plant establishment. The amount and distribution of leaf litter within a forest can influence patterns of plant establishment. In this study, we de- termined the spatial variability in leaf litter in the forest understory, and investigated how different litter depths (bare, 1, 6, and 12 cm) affected the establishment of several tropical tree species in both growth house (sun and shade) and field (gap and understory) experiments in the semideciduous tropical forest of Barro Colorado Island, Panama. The tree species used in this study (Aspidospermum cruenta, Ceiba pentandra, Cordia alliodora, Gustavia superba, Luehea seemannii, Ochroma pyrimidale) were chosen to represent a range of seed masses and a gradient in the light requirement for establishment of the species. The spatial distribution of leaf litter was not correlated between adjacent sampling points within the forest understory, suggesting that the establishment environment for seedlings, with respect to litter, is highly variable at scales of 1-20 m. The presence of litter affected five of the six species, but the nature and the magnitude of the effect were species specific. The smaller seeded shade-intolerant species had fewer seedlings establishing under leaf litter than on bare ground. The species ranged from strongly negatively affected (Luehea) to moderately negatively affected (Cordia, Ochroma) to affected only by extreme amounts of litter (Ceiba). The presence of litter influenced Gustavia, one of the larger seeded shade-tolerant species, but did not affect Aspidospermum, the other larger seeded species. The effect of litter on Gustavia depended on the light environment. Gustavia had more seedlings establishing under litter in the sun, but the presence of litter had no effect in the shade. Differences among the smaller seeded shade-intolerant species in the amount they were negatively influenced by litter were not correlated with seed mass. Data from our field study were consistent with our growth house results for the shade- intolerant species. Additional data from the field study indicated that these species with similar habitat requirements differed in the developmental stage at which they were affected by the presence of litter. Luehea had fewer seeds germinating under litter while the other two species, Ochroma and Cordia, were affected only after germination. Interspecific comparisons done for each light level and litter depth indicated that the presence of litter caused reversals in the relative ranking of species success. For example, Gustavia preferentially established under relatively deep litter depths in the sun where Luehea could not establish. In conclusion, the presence of litter can potentially increase seedling diversity within the forest by creating heterogeneity in the establishment envi- ronment and by causing reversals in species' rankings.

The demography of fine roots in a northern hardwood forest

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.

Community Regulation: Under What Conditions Are Bottom-Up Factors Important on Rocky Shores?

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)

How Distribution and Abundance Influence Fertilization Success in the Sea Urchin Strongylocentotus Franciscanus

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.

Effect of Shrubs on Recruitment of Quercus Douglasii and Quercus Lobata in California

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.

Effects of Moose Browsing on Vegetation and Litter of the Boreal Forest, Isle Royale, Michigan, USA

Abstract: Large mammalian herbivores can influence the dynamics and structure of ecosystems by selectively removing tissues of specific plant species. The plant community composition can be altered as animals feed on some species but not others, changing the biomass, production, and nutrient cycling of an entire ecosystem. We used four paired moose (Alces alces) exclosures and browsed plots (built between 1948 and 1950) on Isle Royale, Michigan, to examine the influence of moose on aboveground biomass, production, and annual litterfall of boreal vegetation in 1987. Tree biomass was significantly greater (X = 230 vs. 150 Mg/ha, df = 3, P < .05), shrub biomass was significantly less (X = 1.9 vs. 3.1 Mg/ha, P < .05), and herb biomass was significantly less (X = 0.2 vs. 0.8 Mg/ha, P < .05) in exclosures than in browsed plots. Tree production was greater in exclosures than in browsed plots (X = 7.9 vs. 5.0 Mg.ha—1.yr, P = .05), but there was no difference in the production per unit biomass between exclosures and browsed plots. Shrub production in exclosures was similar to that of browsed plots (X = 3.5 vs. 2.3 Mg.ha—1.yr—1, P < .05), despite total vegetation biomass differences between paired plots. There was significantly greater herb litter produced in the browsed plots than in the exclosures (X = 0.7 vs. 0.1 Mg.ha—1.yr—1, P < .05). Moose browsing prevented saplings of preferred species from growing into the tree canopy, resulting in a forest with fewer canopy trees and a well—developed understory of shrubs and herbs. In addition, browsing may have altered the eventual balance of white spruce (Picea glauca) was balsam fir (Abies balsamea), causing an increase in the former and a decrease in the latter. Thus, browsing by moose influences in long—term structure and dynamics of the boreal forest ecosystem, which has important implications for forest ecosystem management, especially where the population dynamics of moose are regulated.

Plant and Soil Controls on Mycorrhizal Fungal Communities

Abstract: A field experiment was conducted to examine the relative importance of soil factors and plant species on communities of vesicular-arbuscular mycorrhizal (VAM) fungi. Populations of VAM fungal spores were studies in 4-yr-old monocultures of five successional grass species grown in gradient of soil mixtures ranging from pure subsurface sand to pure sandy loam topsoil. A total of 19 species of VAM fungi were found across all treatments. Of the 12 most abundant VAM fungal species, 6 species had a significant dependence on both soil mixture and host species, while 2 were dependent only on soil and 2 only on host. To our knowledge, these are the first results indicating that even closely related hosts (five grasses) may cause divergence in VAM fungal communities on initially identical soils. Cluster analysis of the similarity of fungal communities by host plant species showed that fungal communities in the two late successional grasses to be most similar to one another and least similar to the fungal communities in the early successional grass species. Cluster analysis of the similarity of fungal communities by soil mixture showed the fungal communities in the sandy end of the soil gradient diverged predictably from the fungal communities in the black soil end of the gradient. These results support the hypothesis that soil factors and plant species may be of equal importance in regulating the species composition of VAM fungal communities.

Root Communication Mechanisms and Intracommunity Distributions of Two Mojave Desert Shrubs

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.

Experimental Studies of Seedling Recruitment from Contrasting Seed Distributions

Abstract: To examine how selection may act on seed dispersal, we experimentally manipulated seed distributions and compared levels and patterns of seedling recruitment in three altered distributions, relative to the nonmanipulated, leptokurtic distribution. We picked up, then redistributed the wind-dispersed seeds of two isolated parents of Tachigalia versicolor, a tropical tree species growing on Barro Colorado Island (BCI), Panama. Three experimental distributions were created in long columns adjacent to or well beyond the undisturbed column of the natural distribution: (1) a nonleptokurtic "even" distribution of uniform density equal to the mean density throughout the natural column, (2) a "mixed" distribution with the same total number and kurtosis as the natural column, but composed of an equal proportion of seeds from the two parent trees, and (3) an "extended-tail" distribution spread in low uniform density up to 1.8 km beyond the natural distribution. Thus, the study addressed the relative importance of density, distance, and genetic makeup on offspring recruitment. The results of the 2 yr observations of seed and seedling survival indicate that, relative to the natural distribution, seedling recruitment at 2 yr was greater in the even distribution, was minimally affected by a mixture of genotypes from overlapping distributions of two parent trees, and was very infrequent from seeds dispersed to a wide variety of distant sites. Although the mixture of genotypes from overlapping distributions did enhance the probability of survival at one site, that effect occurred only during the seedling and not the seed stage. For the extended-tail and mixed distributions, in which offspring of both parents occurred at both sites, the impact of site differences (survival in Site A tail > Site B tail; survival in Site A mixed > Site B mixed) exceeded that of parent differences (Parent A > Parent B). The major differences among distributions arose in the seed and early seedling stages (<2 mo), and were due to heavy mortality primarily from seed predators (postdispersal by terrestrial mammals) and mammalian herbivores eating young seedlings. Mammalian predation and herbivory were particularly heavy in the extended-tail distributions. It ap- peared that either predator satiation and/or incomplete searching occurred in the three, much more dense distributions adjacent to the parent trees. In contrast, density-dependent mortality at the spatial scale of -iM2 experimental quadrats was limited to very high densities and occurred only in early seedling stages (0-2 mo). Distance effects on the probability of survival were inconsistent both between sites and among distributions. However, the absolute number of seedlings surviving at 2 yr declined with distance from the parent in all three distributions, except the even distribution at Tree A. Additionally, seedling recruitment at any distance < 100 m from the parent for all three distributions greatly exceeded recruitment at extended-tail (100-1800 m) sites. In summary, offspring recruitment was enhanced by the high cohort density at a large spatial scale (i.e., within a circle of 100 m radius) because of predator satiation. At the same time, it was enhanced slightly by the evenness of the distribution within this scale because of density-dependent mortality at a smaller spatial scale (i.e., 1 M2). Therefore, we conclude that selection on dispersal by Tachigalia versicolor on BCI should act to increase the uniformity of the seed distribution, but not to increase dispersal distance to the point that the density of the seed shadow is diluted.

Effects of the Argentine Ant on Arthropod Fauna of Hawaiian High‐Elevation Shrubland

Abstract: Human-caused biological invasions by an alien species are a worldwide phenomenon. They are particularly significant on isolated oceanic islands and represent a serious threat to endemic biota. The Argentine ant (Iridomyrmex humilis) has become established in portions of the high-elevation shrubland of Haleakala National Park, Maui, Hawaii, over the past 25 yr. This ecosystem lacks native ants but possesses many locally endemic and rare arthropod species. Pitfall trapping and under-rock surveys were conducted to determine the effects of I. humilis on the local arthropod fauna. More than 180 taxa were sampled, mostly Arthropoda. Presence of the Argentine ant is associated with reduced populations of many native and non-native arthropod species, including important predator species and major pollinators of native plants. Effects of ant invasion were particularly severe at higher elevations of Haleakala volcano where endemic species normally exist at low densities. Some taxa, primarily alien species, were more abundant in the presence of ants. Invasion of the Argentine ant has locally reduced the abundance of many endemic species in the shrubland ecosystem. Although the spread of this ant species is slow, I. humilis appears to have the potential to invade a much larger area of Haleakala National Park than it now occupies. Active management of Argentine ant populations will be nec- essary if the endemic fauna is to be preserved.

Long‐Term Soil Water Dynamics in the Shortgrass Steppe

Abstract: To assess the temporal and spatial dynamics of soil water at a shortgrass steppe site in northcentral Colorado, we evaluated the precipitation regime for a 33-yr period and ran a simulation model for this period. Small precipitation events accounted for a large fraction of the total number of events and represented a source of water with small interannual variability. The difference between wet and dry years was related to the occurrence of a few large events. Average daily precipitation was concentrated during the warmest months of the year with a maximum in late spring. Water in the surface soil layers had a short residence time and no seasonal pattern. Intermediate layers reflected the seasonal pattern of precipitation. Maximum soil water availability occurred in the late spring, but this was also the period with the highest interannual variability. The wettest layer was at 4-15 cm of depth. The frequency of wet conditions decreased above this layer because of the strong influence of evaporation and below because recharge was infrequent. No deep percolation events were recorded. During dry years distribution of soil water was very shallow and during wet years wet conditions reached to depths of 120-135 cm. This shallow distribution of soil water matches the distribution of processes and struc- tural elements in the steppe suggesting that there is a cause-effect relationship between them. We speculate that the pattern of water availability interacts with biotic constraints and determines the rate of ecosystem processes. The depth distribution of water in dry and wet years is compared to the root distribution of grasses, shrubs, herbs, and succulents to suggest the response of each group to modal and extreme conditions. Comparison of long- term soil water patterns and traits of the major species allows us to suggest why Bouteloua graci/is is the dominant species in the shortgrass steppe.

Riparian vegetation recovery patterns following stream channelization: a geomorphic perspective

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.

Biogeochemistry of adjacent conifer and alder-conifer stands

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.

Resistance and resilience of lotic algal communities: importance of disturbance timing and current'

Abstract: We examined effects of disturbance timing on resistance and resilience of epilithic algal communities growing in fast- (29 cm/s) and slow- (12 cm/s) current outdoor experimental stream channels in Kentucky, USA that were either left undisturbed (control) over 33 d following a simulated spate, or were subjected to an additional spate after either 9, 18, 27, or 33 d. On day 33, all channels were subjected to a final spate to assess effects of short-term disturbance history on resistance, independently of seasonal influences. Suc- cession proceeded from a sparsely populated community dominated by a small, monora- phid diatom (Achnanthes minutissima) immediately after the initial spate, to dominance by dense floating mats of filamentous green algae (Zygnematales: Mougeotia and Spirogyra) and Synedra spp. by day 21-24. Resistance was generally lower in slow-current commu- nities, both in terms of cell-density reduction and displacement of taxonomic structure, than in fast-current communities. Resistance in slow-current communities varied tem- porally, with communities least resistant on day 18, when community composition and physiognomy was changing rapidly, and on day 33, when green algal mats began to senesce. On day 33, slow-current communities that had not been recently disturbed (control, D9) exhibited greatest spate-induced loss of algal biomass. Additionally, slow-current com- munities with high pre-disturbance phaeophytin content (an indicator of algal senescence) also changed most in diatom assemblage structure across the final spate. No such rela- tionship was noted in fast current, suggesting that autogenic factors influenced communities in slow current more than those in fast. Resilience was higher in slow-current communities than in fast current, with disturbed communities reaching biomass and taxonomic structure similar to controls after 3-9 d. High resilience in slow current resulted from enhanced reproduction in some populations following spate-induced biomass reduction and pre- sumed release from nutrient and light limitation, and low shear stress relative to fast- current channels where biomass accrual was limited by current. Interactions between dis- turbance timing, successional state, and habitat affect the susceptibility of epilithic algal communities to disturbance and likely influence temporal and spatial heterogeneity in stream ecosystems.

Habitat Heterogeneity and The Functional Signifiance of Fish in River Food Webs

Abstract: had little effect on algae or invertebrates associated with gravel. Gravel-dwelling heptageniid mayflies were behaviorally inhibited from using tops of stones in fish enclosures, and stone surfaces had more chironomid tubes in fish enclosures than in fish exclosures. However, no effects on epilithic algae or densities of invertebrates comparable to those of biota on boulder-bedrock substrates were detected. These spatially varying predator effects in a river parallel results from marine benthic systems, where strong effects of large predators documented for rocky intertidal habitats and unvegetated soft bottoms are not conspicuous in seagrass beds.