Showing papers on "Species richness published in 1995"

Biodiversity: Population Versus Ecosystem Stability

Abstract: The relationships between biodiversity and stability were determined for both population and ecosystem traits in a long-term study of 207 grassland plots. Results demonstrate that biodiversity stabilizes community and ecosystem processes, but not pop- ulation processes. Specifically, year-to-year variability in total aboveground plant com- munity biomass was significantly lower in plots with greater plant species richness both for the entire 11-yr period and for the nine non-drought years. The change in total plant community biomass from before the drought to the peak of the drought was also highly dependent on species richness. For all three measures of total community biomass stability, multiple regressions that controlled for covariates showed similar significant relationships between plant diversity and stability. In contrast, year-to-year variability in species abundances was not stabilized by plant species richness for either all years or non-drought years. This difference between species vs. community biomass likely results from interspecific competition. When climatic vari- ations harm some species, unharmed competitors increase. Such compensatory increases stabilize total community biomass, but cause species abundances to be more variable. These results support both the predictions of Robert May concerning the effects of diversity on population stability and the diversity-stability hypothesis as applied to community and ecosystem processes, thus helping to reconcile a long-standing dispute.

Responses of Arctic Tundra to Experimental and Observed Changes in Climate

Abstract: We manipulated light, temperature, and nutrients in moist tussock tundra near Toolik Lake, Alaska to determine how global changes in these parameters might affect community and ecosystem processes. Some of these manipulations altered nutrient availability, growth—form composition, net primary production, and species richness in less than a decade, indicating that arctic vegetation at this site is sensitive to climatic change. In general, short—term (3—yr) responses were poor predictors of longer term (9—yr) changes in community composition. The longer term responses showed closer correspondence to patterns of vegetation distribution along environmental gradients. Nitrogen and phosphorus availability tended to increase in response to elevated temperature, reflecting increased mineralization, and in response to light attenuation, reflecting reduced nutrient uptake by vegetation. Nutrient addition increased biomass and production of deciduous shrubs but reduced growth of evergreen shrubs and nonvascular plants. Light attenuation reduced biomass of all growth forms. Elevated temperature enhanced shrub production but reduced production of nonvascular plants. These contrasting responses to temperature increase and to nutrient addition by different growth forms "canceled out" at the ecosystem level, buffering changes in ecosystem characteristics such as biomass, production, and nutrient uptake. The major effect of elevated temperature was to speed plant response to changes in soil resources and, in the long term (9 yr), to increase nutrient availability through changes in N mineralization. Species within a growth form were similar to one another in their responses to changes in resources (light or nutrients) but showed no consistent response to evelated temperature. Species richness was reduced 30—50% by temperature and nutrient treatments, due to loss of less abundant species. Declines in diversity occurred disproportionately in forbs, which are important for animal nutrition, and in mosses, which maintain soil thermal regime. There was no increased abundance of initially rare species in response to any treatment. During our 9—yr study (the warmest decade on record in the region), biomass of one dominant tundra species unexpectedly changed in control plots in the direction predicted by our experiments and by Holocene pollen records. This suggests that regional climatic warming may already be altering the species composition of Alaskan arctic tundra.

The elevational gradient of species richness: a uniform pattern?

Abstract: The decline in species richness with increasing elevation is widely accepted as a general pattern (Table 1). In as much as the elevational gradient is often claimed to mirror the latitudinal gradient, species richness is assumed to decrease monotonically (i.e. because of reduced temperature and consequent decrease in productivity). Perhaps because such a relation is intuitive, biologists have readily generalized the results of a few early studies of tropical birds as supporting a general biogeographic

Indirect Effects in Marine Rocky Intertidal Interaction Webs: Patterns and Importance

Abstract: To determine the patterns of occurrence and importance of indirect effects relative to direct effects in natural communities, I analyzed experimentally based studies from 23 rocky intertidal habitats. The vehicle of analysis was the construction of interaction webs, or the subset of species in food webs involved in strong interactions. The analysis focused on indirect effects involving changes in abundance, or interaction chains, since little information was available on other types of indirect effects (behavioral, chemical response, environmental). As expected, number of direct (= strong) interactions, indirect effects, interaction sequences producing indirect effects, and types of indirect effects (e.g., keystone predation, apparent competition, etc.) all increased with web species richness. Less expected, when these measures were adjusted to a per species basis, positive relationships with species richness were still observed for all measures but the number of types. In other words, with increasing web diversity, each species interacted strongly with more species, was involved in more indirect effects, and was part of more interaction pathways. The analysis identified 83 subtypes of indirect effect, including the seven previously identified types. Many of the 76 additional types could be reclassified into the seven types if the original definitions of these "classic" types were expanded to include interactions having similar effects but differing in the specific mechanism (e.g., both interference competition and inhibition of recruitment [preemption] have negative effects involving a spatial resource). Two new types of indirect effect, termed "apparent predation" and "indirect defense" were also identified, producing a total of 9 general types of indirect effect divided among 565 specific indirect effects. Of these, keystone predation (35%) and apparent competition (25%) were most common and exploitation competition (2.8%) was least common in these webs. Two methods of analysis suggested that indirect effects accounted for °40% of the change in community structure resulting from manipulations, with a range of 24—61%. The proportion of change due to indirect effects was constant with web species richness, indicating that strong direct interactions and indirect effects produce roughly the same level of alteration of community structure regardless of the level of web complexity. Several potential artifacts and biases were evaluated. Most importantly, neither variation in level of taxonomic resolution nor intensity of experimentation varied significantly with web size (species richness). Despite a bias toward manipulation of consumers over manipulation of basal species, some predator—initiated indirect effect types were scarce while some basal species—initiated types were common. While the frequency of exploitation competition may have been underestimated, it is unlikely that the frequency of this indirect effect would change dramatically: changes due to this effect should have been detected in many of the studies and reported; and the most intensively studied individual webs did not report frequencies differing much from the average. This analysis suggests investigators effectively identified and first manipulated those species responsible for most indirect effects and that more experiments added decreasing numbers of indirect effects. Moreover, the frequencies and importance of indirect effects may be more predictable than expected on the basis of theory.

Use of Amazonian Forest Fragments by Understory Insectivorous Birds

Abstract: We sampled understory insectivorous birds in Amazonian forest fragments from before isolation through 9 yr after isolation. We accumulated 3658 mist net captures of 84 insectivorous species in five 1-ha fragments and four 10-ha fragments. Abundance and species richness declined dramatically after isolation, even though fragments were separated from continuous forest by only 70-650 m. Three species of obligate army ant followers disappeared within the first 2 yr after isolation. Mixed-species flocks containing 13 commonly netted species disintegrated within 2-3 yr after isolation, although three species that dropped out of flocks persisted in fragments. Among insectivores not associated with flocks or army ants, only two species of edge specialists were unaffected by frag- mentation. Overall, loss of forest insectivores was not compensated for by an increase in nonforest or previously uncommon species. Secondary vegetation surrounding fragments strongly affected use of fragments after isolation. Fragments surrounded by Vismia, the dominant regrowth where felled forest was burned and temporarily used as cattle pasture, remained depauperate. In contrast, many species returned to fragments by moving through regenerating forest dominated by Cec- ropia, which occurred in areas where the felled forest was not burned. Both 1- and 10-ha fragments surrounded by Cecropia were used by ant followers by 5 yr after isolation. Mixed-species flocks reassembled in 10-ha fragments surrounded by Cecropia by 7-9 yr after isolation, and augmented their group territories by foraging in secondary forest outside fragments. Solitary species were more variable in their responses, although several species returned to 10-ha fragments surrounded by Cecropia. Terrestrial insectivores, such as Scle- rurus leafscrapers and various antbirds, did not return to any fragments, and appear to be the group most vulnerable to fragmentation. Ordination of the insectivore community showed that 1-ha fragments diverged from their pre-isolation communities more than did 10-ha fragments. Communities in 10-ha fragments surrounded by Cecropia were more closely associated with pre-isolation com- munities than those in fragments surrounded by Vismia. Over time, communities in 10-ha fragments surrounded by Cecropia became more like pre-isolation communities, although communities in other fragments generally continued to diverge.

Plant life-history attributes: their relationship to disturbance response in herbaceous vegetation.

Abstract: I Species composition and disturbance state (level of soil disturbance, grazing and water addition) were recorded for 120 (30-m2) plots in temperate grassland of the New England Tablelands (Australia). Three different classificatory schemes were used, based on (a) life-form (sensu Raunkiaer); (b) dispersal unit morphology and (c) vegetative reproduction. We analysed the effects of the different disturbance types and intensities on number and proportion of species belonging to these groups. The results were used to describe the spectrum of life-histories likely to be present at sites in different states of exogenous disturbance. 2 Soil disturbed sites had proportionately more therophytes, versatile/flat rosettes and wind dispersed species. Two features are likely to lead to success in soil disturbed sites: ability to colonize open space (e.g. therophytes, wind dispersal) and capacity to capture resources effectively (e.g. flat architecture of rosettes). Non-disturbed sites had more geophytes, chamaephytes, phanerophytes, proto-hemicryptophytes and erect rosettes, with greater numbers of vegetatively reproducing species. 3 Heavily grazed sites had higher proportions of therophytes and versatile/flat rosettes and species with mobile seeds than sites with light grazing. Moderately grazed sites had increased proportions of versatile and erect rosettes and more species with adhesive seeds (mainly grasses). Lightly grazed sites had a greater diversity in terms of evenness of all life-forms, dispersal morphologies and reproductive modes. 4 Water enriched sites had fewer geophytes and phanerophytes and some chamaephytes, erect rosettes and proto-hemicryptophytes. This loss was only partially compensated by a gain in versatile and partial rosettes but richness decreased overall. Water enrichment was unrelated to dispersal unit morphology or the potential for vegetative reproduction. 5 Of the three attributes examined, life-form was most useful in characterizing community response to different disturbance types. Traits related to regeneration (seed morphology and capacity of vegetative reproduction) were relevant to soil disturbance only. Our results support the use of such classifications for monitoring the effects of disturbance.

The assembly of experimental wetland plant communities

Abstract: The ability to predict the composition of communities from environmental factors is a central goal of community ecology. We carefully selected a pool of species and subjected it to a range of environmental factors to determine which factors were able to filter out subsets of species. We began with a pool of 20 species and sowed them into 120 wetland microcosms representing 24 different habitat treatments and monitored them for 5 yr. The treatments were fertility, water depth, fluctuations in water depth, soil texture, leaf litter, length of the initial growing season, and invasion by Typha. After 5 yr 14 species persisted; no rare species survived. The experimental communities differed from random expectation and were assembled by rules that constrained their organization. There were strong and consistent effects of fertility, water level, and leaf litter on community composition. Community assembly was modeled as a series of environmental filters. Some aspects of assembly were deterministic: trajectories were constrained within two pathway basins and species rank abundances were significantly concordant within treatments. Other factors indicated that assembly has a strong stochastic component: 50% of species were present only occasionally and we cannot accurately predict species ranks. Community stochasticity did not show any clear patterns among treatments.

A Modified-Whittaker nested vegetation sampling method

Abstract: A standardized sampling technique for measuring plant diversity is needed to assist in resource inventories and for monitoring long-term trends in vascular plant species richness. The widely used ‘Whittaker plot’ (Shmida 1984) collects species richness data at multiple spatial scales, using 1 m2, 10 m2, and 100 m2 subplots within a 20 m × 50 m (1000 m2) plot, but it has three distinct design flaws involving the shape and placement of subplots. We modified and tested a comparable sampling design (Modified-Whittaker plot) that minimizes the problems encountered in the original Whittaker design, while maintaining many of its attractive attributes. We overlaid the two sampling methods in forest and prairie vegetation types in Larimer County, Colorado, USA (n=13 sites) and Wind Cave National Park, South Dakota, USA (n=19 sites) and showed that the modified design often returned significantly higher (p<0.05) species richness values in the 1 m2, 10 m2, and 100 m2 subplots. For all plots, except seven ecotone plots, there was a significant difference (p<0.001) between the Whittaker plot and the Modified-Whittaker plot when estimating the total number of species in the 1000 m2 plots based on linear regressions of the subplot data: the Whittaker plot method, on average, underestimated plant species richness by 34%. Species-area relationships, using the Modified-Whittaker design, conformed better to published semilog relationships, explaining, on average, 92% of the variation. Using the original Whittaker design, the semilog species-area relationships were not as strong, explaining only 83% of the variation, on average. The Modified-Whittaker plot design may allow for better estimates of mean species cover, analysis of plant diversity patterns at multiple spatial scales, and trend analysis from monitoring a series of strategically-placed, long-term plots.

Diversity Patterns in Stream Benthic Invertebrate Communities: The Influence of Habitat Stability

Abstract: Invertebrate diversity patterns were examined in 11 freshwater habitats (10 streams and a windswept lake shore) of similar physicochemical nature but different thermal and hydrologic stability in the Cass-Craigieburn region, New Zealand. Species richness and density were markedly higher at the more stable sites, but species evenness peaked at sites of intermediate stability. Of the 20 environmental variables examined, a multivariate instability index incorporating temporal variation in depth, temporal variation in current speed, substrate stability, the Pfankuch channel stability index, temperature range, and stream reach tractive force was the single best predictor of the number of species, whereas epilithic pigment concentration was the single best predictor of invertebrate density. The pattern in species richness did not support any of three diversity hypotheses considered. In contrast, the pattern in species evenness suggested competitive exclusion may be occurring patchily and that Huston's dynamic equilibrium model may have some validity, at least at the level of the patch. However, the strong link between productivity and stability apparent in these habitats, and a lack of information on the effects of increased productivity on competition in stream benthic communities makes any firm assessment of the latter model difficult. The observed diversity patterns are, however, consistent with the idea that high diversity is maintained in these habitats by an interaction between low levels of disturbance and habitat patchiness.

Experimental Analysis of Intermediate Disturbance and Initial Floristic Composition: Decoupling Cause and Effect

Abstract: The intermediate disturbance hypothesis predicts that richness will be highest in communities with moderate levels of disturbance and at intermediate time spans following disturbance. This model was proposed as a nonequilibrium explanation of species richness in tropical forests and coral reefs. A second model of succession, initial floristic composition, states that nearly all species, including late seral species, are present at the start of suc- cession. This leads to the prediction that richness should be highest immediately following disturbance. We tested these predictions using plant species composition data from two long-term field experiments in North American tallgrass prairie vegetation. In contrast to one prediction of the intermediate disturbance hypothesis, there was a significant monotonic decline in species richness with increasing disturbance frequency, with no evidence of an optimum, in both field experiments. Species composition on an annually burned site was a subset of that of infrequently burned sites. The average number of species per quadrat and the number of grass, forb, and annual species were lowest on annually burned sites compared to unburned sites and sites burned once every 4 yr. The second prediction of the intermediate disturbance hypothesis, however, was supported. Richness reached a maximum at an intermediate time interval since the last disturbance. This contradicts the prediction from the initial floristic composition model of succession. These results also suggest that the two predictions of the intermediate disturbance hypothesis are independent and unre- lated. We propose that this may be explained by uncoupling the effects of disturbance as a single, relatively discrete event from system response to disturbance. From this perspec- tive, disturbance becomes an extinction-causing event in these grasslands, whereas recovery following disturbance is a balance between immigration and extinction.

Phylogeny, ecology, and the richness of parasite communities in vertebrates'

Abstract: The makeup of parasite communities is the result, among other factors, of interactions between the evolutionary history and ecological characteristics of hosts. This study evaluates the relative importance of some ecological factors (host body size, diet, habitat, latitude, and the mean number of parasite individuals per host) as determinants or correlates of parasite community richness in vertebrates, before and after controlling for potential effects of host phylogenetic relationships. Data were obtained from the literature on 596 parasite communities belonging to one of four distinct types: gastrointestinal parasite communities of fish, birds, or mammals, and ectoparasite communities of fish. There were positive correlations between the number of hosts sampled and mean species richness of the parasite community of each genus. In analyses treating host genera as independent statistical observations and using estimates of parasite species richness corrected for host sample size, positive correlations were observed between richness and host body size in gastrointestinal communities of all three groups of vertebrates. The mean number of parasite individuals per host also was correlated positively with species richness. In fish, richness increased with increases in the proportion of animal food in the host diet. Aquatic birds had richer parasite communities than their terrestrial counterparts, whereas marine fish had richer gastrointestinal parasite communities than freshwater fish. The richness of ectopar- asite communities on fish showed no association with any of the ecological variables investigated. Using host genera as independent points in the analyses may lead to biased results since some host lineages are descended from recent common ancestors, and are therefore not truly independent. The comparative analysis was repeated using phylogenetically indepen- dent contrasts derived from the phylogeny of hosts. Once the effects of host phylogeny were removed, somewhat different results were obtained: host body size showed no rela- tionship with parasite species richness in birds, and there was no evidence that habitat transitions resulted in significant changes in parasite species richness in any of the types of communities studied. Of the ecological factors studied, the comparative analyses suggest that only host body size can be an important determinant of parasite community richness in certain host groups. This study illustrates clearly the need to control for phylogeny in investigations of host-parasite interactions.

A hierarchical approach to evaluating the significance of soil biodiversity to biogeochemical cycling

Abstract: The significance of biodiversity to biogeochemical cycling is viewed most directly through the specific biogeochemical transformations that organisms perform. Although functional diversity in soils can be great, it is exceeded to a high degree by the richness of soil species. It is generally inferred from this richness that soil systems have a high level of functional redundancy. As such, indices of species richness probably contribute little to understanding the functioning of soil ecosystems. Another approach stresses the value of identifying “keystone” organisms, that is those that play an exceptionally important role in determining the structure and function of ecosystems. Both views tend to ignore the importance of biodiversity in maintaining the numerous and complex interactions among organisms in soils and their contributions to biogeochemical cycling. We describe some of those interactions and their importance to ecosystem function.

Influence of overstory composition on understory colonization by native species in plantations on a degraded tropical site

Abstract: Patterns of understory colonization by native and naturalized trees and shrubs were evaluated in 4.5-year-old plantations of three exotic tree species, Casuarina equisetifolia, Eucalyptus robusta, and Leucaena leucocephala, on a de- graded coastal grassland site with reference to overstory com- position and understory environmental conditions. 19 second- ary forest species were established in the plantation understories (with a total area of 0.52 ha ), while no natural regeneration occurred in unplanted, though protected, control areas. The majority of these species (90 %) and the total seedling popula- tion (97 %) were zoochorous, indicating the importance of frugivorous bats and particularly birds as facilitators of sec- ondary forest species colonization. Understory species rich- ness and seedling densities were affected significantly by overstory composition, the most abundant regeneration occur- ring beneath Leucaena and least under Casuarina. Understory colonization rates within mixed-species stands were interme- diate between those of single-species stands of the trees com- prising their overstories. Significant negative correlations were found between understory species richness and seedling den- sity, and forest floor depth and dry mass, especially for small- seeded ornithochorous species. Higher colonization rates near the peripheries of plantation plots relative to plot interiors were due in part to roosting site preferences by frugivores, particularly bats. The study results indicate that overstory species selection can exert a significant influence on subsequent patterns of colonization by secondary forest species and is an important consideration in the design of plantations for 'catalyzing' succession on deforested, degraded sites.

Global scale patterns of fish species richness in rivers

Abstract: Explanations of spatial and temporal variation in species richness is a central theme in community ecology. Until recently, most research has focused on small-scale phenomena, often emphasizing on local environmental factors and thus, poorly reflecting large-scale processes that organize species richness. In this paper, we analyze variations in species richness of indigeneous freshwater fish on a worldwide scale. We show that factors related to species-area and species-energy theories statistically explain most of the variation in freshwater fish species richness across continents. Historical events supposed to influence present distributions of fish are of little assistence in explaining variations in fish species richness at the global scale. Our model, which uses easily measured factors, should also be of practical value to aquatic conservation biology and natural resource management.

Composition, abundance, biomass, and production of macrofauna in a New England estuary: Comparisons among eelgrass meadows and other nursery habitats

Abstract: Quantitative suction sampling was used to characterize and compare the species composition, abundance, biomass, and secondary production of macrofauna inhabiting intertidal mud-flat and sand-flat, eelgrass meadow, and salt-marsh-pool habitats in the Nauset Marsh complex, Cape Cod, Massachusetts (USA). Species richness and abundance were often greatest in eelgrass habitat, as was macroinvertebrate biomass and production. Most striking was the five to fifteen times greater rate of annual macrofaunal production in eelgrass habitat than elsewhere, with values ranging from approximately 23–139 g AFDW m2 yr−1. The marsh pool containing widgeon grass (Ruppia maritima) supported surprisingly low numbers of macroinvertebrates, probably due to stressfully low dissolved oxygen levels at night during the summer. Two species of macroinvertebrates, blue mussels (Mytilus edulis) and to a lesser extent bay scallops (Argopecten irradians), used eelgrass as “nursery habitat.” Calculations showed that macroinvertebrate production is proportionally much greater than the amount of primary production attributable to eelgrass in the Nauset Marsh system, and that dramatic changes at all trophic levels could be expected if large changes in seagrass abundance should occur. This work further underscores the extraordinarily large impact that seagrass can have on both the structure and function of estuarine ecosystems. *** DIRECT SUPPORT *** A01BY070 00006

Disturbance and Diversity in Tropical Rain Forests: The Density Effect

Abstract: A null model for the effects of treefall gaps on tree species diversity of tropical rain forests is based on the well-known relationship between abundance and diversity: habitats supporting larger numbers of individuals can support more populations and more species than habitats supporting small numbers of individuals. Because seedling establishment and sapling density increase following canopy opening, gaps are often also sites of high species diversity. Both tree density and diversity are likely to be greater in areas of high stand-turnover rates. Thinning and non-catastrophic mortality reduce both density and diversity in areas of low gap frequency. Nevertheless, natural and anthropogenic disturbances, such as logging, have been hypothesized to both enhance and limit species diversity through changes in habitat heterogeneity, shifts in competitive balances among species, and creation of otherwise-rare habitats. Species-accumulation curves can be used to evaluate these alternatives by comparing observed species-abundance patterns with expected changes in diversity due to density effects alone. Topographic variation in density and diversity of shrubs and small saplings at the La Selva (Costa Rica) Biological Station preserve was analyzed as an example. The total number of species for each of 18 sites was estimated by fitting a two-parameter hyperbola (similar to the Michaelis-Menten equation) to sample data using a maximum-likelihood method. Although both density and diversity were higher on slopes than on ridges, total species richness was nevertheless higher on slopes than is predictable from density effects alone. High stand-turnover rates on slopes may enhance tree species diversity through increases in tree density and in habitat heterogeneity.

Prediction of neotropical tree and liana species richness from soil and climatic data

Abstract: We present an analysis of local species richness in neotropical forests, based on a number of 0.1 ha samples of woody plants collected by the late Alwyn Gentry. For each of 69 forests, soils were analysed and climatic data were collated. Using transformed independent variables and interaction terms, multiple regression equations were developed that explained the greatest possible amount of variation in species richness, and the best equations were selected on the basis of regression diagnostics. The best models are presented for (a) all neotropical forests, (b) forests west of the Andes (transandean) and (c) east of the Andes (cisandean), and for various subsets based on elevation and annual rainfall. For the whole dataset, and for most subsets, annual rainfall and rainfall seasonality were the most important variables for explaining species richness. Soil variables were correlated with precipitation — drier forests have more nutrient-rich soils. After the inclusion of rainfall variables, available soil nutrient concentrations contributed little to explaining or accounting for additional variation in species numbers, indicating that tropical forest species richness is surprisingly independent of soil quality. The results are consistent with the hypothesis that plants in mature tropical forests may obtain nutrients through the process of direct cycling, in which mineral nutrients are extracted from litterfall before they enter the soil. The strong relationship between community species richness and rainfall patterns has implications for biodiversity conservation. Wet forests with an ample year-round moisture supply harbour the greatest number of woody plant species and should be a focus of conservation efforts.

Environmental factors as forces structuring the fish community of the Elbe Estuary

Abstract: Smelt, flounder, eel, three-spined stickleback, twaite shad, ruffe and herring were the major species among 62 forming the fish community of the Elbe estuary. Species richness, species diversity, evenness and total fish biomass decreased in the upstream direction. Total abundance was greatest during summer. Salinity was the most significant physico-chemical factor affecting species richness and total fish biomass, whereas water temperature was the best predictor of total abundance. Marine fish species decreased with decreasing salinity, while the freshwater species roach and ide were absent at salinities > 15‰. Bream, blue bream, white bream, perch and pikeperch occurred more frequently in shallow marginal regions with lower current velocities, while herring and other marine species occurred closer to the deep central regions with high current velocities. Annual and seasonal variations of the community structure mainly reflected in population dynamics of smelt, flounder, twaite shad, three-spined stickleback and eel, especially those of 0-age smelt. Populations of smelt and ruffe and their importance in the fish community increased between 1989 and 1992, while those of twaite shad and eel decreased. At oxygen concentrations <3 mg I1, smelt and flounder were rare, but eels were at their maximum at 3.5 mg I−1. Changes of species composition and fish abundance were closely related to daytime and tidal cycle.

Effects of selective logging on tropical forest butterflies on Buru, Indonesia

Abstract: 1. The butterfly fauna of lowland monsoon forest on Buru, Indonesia was compared in unlogged forest and forest that had been selectively logged 5 years previously. 2. Seven variables relating to vegetation structure were measured in each habitat. Tree density and percentage cover of vegetation in the canopy and understorey were significantly higher, and vegetation cover 2 m above the ground was significantly lower, in unlogged forest. There were no differences between sites in the mean heights or girths of trees, but the ranges of both heights and girths were lower in logged forest. Percentage cover of vegetation at ground level was similar at the two sites. 3. Species richness, abundance and evenness of butterflies and an index of taxonomic distinctiveness were all significantly higher in unlogged forest. Two endemic species and a further four species with distributions restricted to Maluku Province were recorded only in unlogged forest. 4. Species abundance data for butterflies at both sites fitted a log-series distribution. Data for unlogged forest also fitted a log-normal distribution, whereas those for logged forest did not. This indicated the presence of a more complex butterfly community in unlogged forest. 5. These results indicate that the distributional pattern of species abundance of tropical butterflies may be used as an indicator of forest disturbance, and that selective logging of tropical forests in SE Asia may be associated with a significant decrease in biodiversity of butterflies, at least during the first 5 years of forest regeneration.

Interspecific aggression and habitat selection by Amazonian birds

Abstract: We report that interspecific territoriality is a common spacing mechanism among closely related bird species distributed along a primary successional gradient in the meander belt of an Amazonian whitewater river. In the first phase of the research, we mapped the territories of more than 330 bird species in large census plots encompassing the complete successional gradient. We found species pairs in over 20 genera that showed contiguous but non-overlapping territories, such that early stages of the successional gradient were occupied by one member of the pair, and later stages by the other. Other species pairs showed additional types of spatial relationships, including partially overlapping and completely overlapping territories. Using reciprocal heterospecific playback experiments, we tested for the existence of aggressive interactions between the members of species pairs (usually congeners) showing all three types of spatial relationship. Among 12 species pairs showing contiguous, non-overlapping territories, 10 showed evidence of interspecific aggression (approach to the playback speaker). In eight of these 10 cases, the responses were markedly asymmetric. Target individuals of one species approached the speaker, whereas individuals of the other species remained in place or moved away. The heavier species was consistently the aggressor. In six species pairs showing partial territorial overlap along the successional gradient, only one species displayed statistically significant interspecific aggressiveness, although some individuals in all six pairs approached or avoided the speaker. In species pairs distributed with completely overlapping territories, we found interspecific aggression in only one of five genera tested. We suggest that spatial segregation of congeneric bird species on habitat gradients (presumably including elevational gradients), is commonly underpinned by interspecific territoriality mediated through directed, asymmetrical interspecific aggression. Aggressive asymmetry suggests a despotic model of habitat occupancy, in which the larger species occupies the more productive end of habitat gradients, and the smaller species occupies less productive habitats. The pronounced successional gradients characteristic of Amazonia may explain much of the increased species richness, especially within genera, of Amazonian bird communities.

The Role of Traditional Agroforests in the Conservation of Rain Forest Bird Diversity in Sumatra

Abstract: Bird community composition and structure were compared among three different types of traditionally managed agroforests and the nearby primary forests in southwestern Sumatra during the breeding season. The agroforests were dominated respectively by rubber tree (Hevea brasiliensis), damar (Shorea javanica), and durian (Durio zibethinus) and were managed for cash crops (latex, resin, fruits, spices, coffee and timber). Although planted on formerly clearfelled and cultivated areas, these agroforests were highly diversified and structurally complex. Species relative abundance was obtained in 112 random samples of 50 individuals within limited census areas. The species were classified according to size, main diet, and observed habitat use. Eighty-three percent of all the regional pool of resident species were recorded at least once (excluding aerial, nocturnal, and aquatic species). Species richness, diversity, and equitability were all significantly lower (12% to 62% less) in every agroforest than in the primary forest, and the coefficients of similarity between the natural and managed forest communities were low (0.43–0.55). Less than half the lowland primary forest species known from western Sumatra were recorded in at least one agroforest. Among the 216 species occurring in samples, 56% significantly decreased in or disappeared from agroforests, whereas only 22% appeared or increased. The species most affected by the conversion of primary forests were large frugivores and large insectivores of the canopy and low understory, or terrestrial interior forest specialists. Species thriving in agroforests were small frugivores, foliage insectivores, and nectarivores often originally associated with gaps. The factors affecting biodiversity in agroforests may involve reduced tree height, canopy foliage volume, vertical diversity of the vegetation structure, floristic richness, and associated variety of food resources. High hunting pressure may also be a factor. These multipurpose, traditionally managed agroforests still are a valuable compromise between conservation of tropical forest biodiversity and sustainable use of natural resources. They also function as buffer zones between densely populated and protected areas and are one of the most successful examples of silviculture of dipterocarps. Yet they are not equivalent to primary forests.

Invasibility of experimental habitat Islands in a California winter annual grassland

Abstract: In an experimental test of plant community invasibility, we introduced seeds of a native ruderal, California poppy (Eschscholzia californica). at fixed density into experimental plots in a California winter annual grassland. Each of the 42 plots, which ranged in size from 2 m 2 to 32 m 2 , had been studied for 4 yr previous to the introduction, with the common observation that a subset of plots of each size consistently held more species than others. It was primarily in these more species-rich plots that establishment and reproduction by the experimental invader occurred. Success of the invader per plot, measured as the total number of plants germinating, producing seeds, or perennating, varied with plot size, but the statistical contribution of plot size was secondary to that of local species number. Contributing variables were the extent of small mammal disturbance (positive) and the degree to which a single resident plant species (in particular, Bromus diandrus) dominated a plot (negative). In contrast to theories of competitive exclusion via niche partitioning, species-rich plots were more invasible.

Mutualism and biodiversity in soils

Abstract: Most soil invertebrates and roots have developed strong interactions with micro-organisms to exploit the organic and mineral resources of soil. Micro-fauna are mainly predators of microorganisms whereas larger organisms interact with micro-organisms through the “external rumen” or facultative endosymbiotic digestive systems. Mobilisation of nutrient and organic resources through mutualism with soil microflora seems to be all the more efficient as the organisms are large (like e.g., roots, termites or earthworms) and temperature is high. In the humid tropics, part of the existing species richness may have originated from an increased base of resources resulting from the development of mutualistic relationships. Evidence for this process is given for earthworm communities. Consequences for soil function and the species richness of plants and consumers are discussed.

Forest Reorganization: A Case Study in an Old-Growth Forest Catastrophic Blowdown

Abstract: We studied the patterns and mechanisms of regeneration of a 400-ha wind- throw in an old-growth beech-hemlock forest caused by a tornado on 31 May 1985. Starting in 1986, and over a period of six growing seasons, we recorded percent cover and density of woody stems, and monitored seedling demography of nearly 5000 seedlings in the windthrow and adjacent forest. Plant community response to the disturbance was dramatic: by August of 1986, species richness, tree seedling density and total percent cover were significantly greater in the windthrow than in the adjacent forest. Shade-intolerant herbs (e.g., Erechtites hieracifolia) and shrubs (e.g., Rubus allegheniensis) established and rapidly increased in abundance during the first 3 yr, but began declining by the 5th yr of the study. Tree seedlings established in decreasing amounts through the 6 yr of the study, and the young tree canopy was dominated in 1991 by seedlings and sprouts that established prior to 1987. Fagus grandifolia, a shade-tolerant species that established via advanced regeneration, was dominant the first 3 yr, but was surpassed in the 5th yr by Betula alleghaniensis, a species of intermediate tolerance that established from seed germination just before or shortly after the disturbance. Tsuga canadensis seedling densities were initially high, but deer browsing prevented substantial growth and a drought in 1988 caused heavy mortality of browsed seedlings. Regeneration thus differed from the predictions of the gap and Hubbard Brook models of forest regeneration (which predict dominance by shade-intolerant species), and the severity model (which predicts dominance by shade-tolerant species). The differences point out important influences of availability of propagules and the impact of herbivory; and the need for more attention to models that incorporate multiple contingencies.

Herbaceous-Layer and Overstory Species in Clear-cut and Mature Central Appalachian Hardwood Forests

Abstract: The current interest among resource managers in ecosystem management necessitates a better understanding of the response of plant species diversity to forest management practices. This study attempted to assess the effects of one forest management practice-clear-cutting-on plant biodiversity in a mid-Appalachian hardwood forest by comparing species composition and diversity between two young (a20 yr following clear- cutting) and two "mature" (>70 yr following selective cutting) watersheds. Sampling was confined to the herbaceous layer (vascular plants l 1 -m in height) and woody overstory (stems ?2.5 cm diameter at 1.5-m height). The important tree species occurred on all watersheds, but the relative importance of these species varied greatly with stand age. Less shade-tolerant species, such as black cherry (Prunus serotina) and tulip poplar (Lirioden- dron tulipifera), are replaced by more-tolerant species, such as northern red oak (Quercus rubra) and sugar maple (Acer saccharum), as the stand matures. Analysis of tree species richness per plot suggests that the competitive thinning process decreases species evenness as the stand develops. Important herb-layer species included stinging nettle (Laportea canadensis), violets (Viola spp.), seedlings of striped maple (A. pensylvanicum), and several fern species. In sharp contrast with the trees, these species appeared to vary little with stand age. Species diversity (H') did not vary with stand age for either the overstory or the herbaceous layer. Detrended correspondence analysis showed a significant correlation between species composition of the two strata for the mature watersheds, but not the young, clear-cut watersheds. Thus, we suggest a temporal shift in processes influencing species composition following disturbance from allogenic factors (e.g., soil characteristics) to au- togenic factors (e.g., stand characteristics), which lead to a linkage between forest strata later in succession. The degree to which forest management alters species composition in these central Appalachian forest ecosystems may be tied to the degree of alteration of the link between strata.