Community Trophic Structure Research Articles

A comparative sink web analysis of two bird species in two habitats: trophic structure, functionality, aggregation and system-level indication

Changes in the trophic structure of communities are good indicators of ecosystem stress or environmental change. Here, we compare the sink webs of two bird species (great tit, Parus major, and European blackbird, Turdus merula) in two habitats (a forest and an orchard), in order to detect the differences in their trophic status. The webs are functionally aggregated in three steps, based on energetics. Our main conclusion is that tits are less sensitive to habitat change from the natural to the agricultural.

Trophic structure patterns of Brazilian reef fishes: a latitudinal comparison

AbstractAim To investigate how reef fish trophic structure responds to latitudinal changes, using a simple model: the extensive Brazilian coast.Location Six Brazilian tropical and subtropical coral and rocky coastal reefs, and the oceanic island of Atol das Rocas, between latitudes 0° and 27° S.Methods Underwater visual census data collected by the authors (five locations) or obtained from the literature (two locations) were used to estimate the relative abundance of 123 fish species belonging to 33 reef‐associated families. Cryptic species were excluded from the analysis. Fishes were grouped in eight trophic categories: roving herbivores, territorial herbivores, mobile invertebrate feeders, sessile invertebrate feeders, omnivores, planktivores, piscivores and carnivores. After a series of detailed predictions based on phylogeny, physiological constraints and anthropogenic impacts was established, the community trophic structure was analysed along a latitudinal gradient and among coastal, mid‐shore and oceanic sites.Results The trophic structure of Brazilian reef fish assemblages clearly changed with latitude. Roving herbivores such as scarids and acanthurids were proportionally more abundant at low latitudes. The browsing herbivores kyphosids followed an opposite latitudinal pattern. The parrotfish genus Sparisoma, more plastic in its feeding habits than Scarus, presented wider distribution. The relative abundance of territorial herbivores did not decrease towards higher latitudes. Mobile invertebrate feeders were the most important (in low latitudes) or the second most important trophic guild (in high latitudes) at all coastal sites. Sessile invertebrate feeders did not show any clear latitudinal trend, despite an expected increase in abundance towards low latitudes. Omnivores dominated high latitude reefs (27° S) and planktivores the oceanic island Atol das Rocas. Piscivores and carnivores were proportionally better represented in high latitudes.Main conclusions Latitudinal patterns seem to be influenced by phylogeny, physiological constraints (mainly related to temperature), and also by anthropogenic impacts. Grazing scarids and acanthurids are largely restricted to tropical reefs and show an abrupt decline beyond 23° S. This does not reflect the amount of algae present, but probably temperature‐dependent physiological constraints. Other herbivores seem to overcome this through symbiotic microbial digestive processes (kyphosids), manipulating the structure of algal turfs or increasing animal protein from within the territory (pomacentrids). Omnivores dominate the southern sites Arraial do Cabo and Arvoredo, being more adapted to environment constraints related to seasonal and/or stochastic shifts. Large carnivores (including piscivores) extend farther into high‐latitude habitats, apparently not constrained by thermal thresholds that limit the herbivores. Overfishing and/or ornamental harvesting certainly has been modifying local fish communities, but could not be detected properly at the large‐scale patterns found in this study. The data presented put in evidence for the first time how reef fish trophic structure behave in the extensive south‐western Atlantic latitudinal gradient.

Habitat fragmentation resulting in overgrazing by herbivores

Habitat fragmentation sometimes results in outbreaks of herbivorous insect and causes an enormous loss of primary production. It is hypothesized that the driving force behind such herbivore outbreaks is disruption of natural enemy attack that releases herbivores from top-down control. To test this hypothesis I studied how trophic community structure changes along a gradient of habitat fragmentation level using spatially implicit and explicit models of a tri-trophic (plant, herbivore and natural enemy) food chain. While in spatially implicit model number of trophic levels gradually decreases with increasing fragmentation, in spatially explicit model a relatively low level of habitat fragmentation leads to overgrazing by herbivore to result in extinction of the plant population followed by a total system collapse. This provides a theoretical support to the hypothesis that habitat fragmentation can lead to overgrazing by herbivores and suggests a central role of spatial structure in the influence of habitat fragmentation on trophic communities. Further, the spatially explicit model shows (i) that the total system collapse by the overgrazing can occur only if herbivore colonization rate is high; (ii) that with increasing natural enemy colonization rate, the fragmentation level that leads to the system collapse becomes higher, and the frequency of the collapse is lowered.

The role of cryptobenthic reef fishes in coral reef trophodynamics

An examination of the trophic status of a cryptobenthic reef fish community from the central Great Barrier Reef was carried out to evaluate the potential role of cryptobenthic fishes in coral reef ecosystems. Using frequency of occurrence data, dietary analyses revealed a diverse range of trophic groups, although detritivory (in 10 out of 16 species and 39.3% of individuals) and carnivory (5 of 16 species and 40.5% of individuals) clearly dominate as trophic modes. Herbivory (1 species; 2.4% of individuals) is only a minor component in the community trophic structure. Of the 18 dietary categories identified, detritus and copepods were the only constituents represented in all 16 species examined. Although the degree of dietary specialization varied among taxa, the 2 most abundant species, Eviota queenslandica and Istigobius goldmanni, utilised the broadest range of dietary items. Morphology reflected the trophic partitioning among fishes: carnivores were invariably 38 mm TL with GLRs generally exceeding >1.0. The trophic composition and numerical strength of the cryptobenthic fish fauna suggests that cryptobenthic reef fishes have the potential to make a significant contribution to reef trophodynamics along a number of trophic pathways. This prompts a re-evaluation of the roles of reef fishes in the functioning of coral reefs, particularly those related to the recycling of primary production through detrital pathways.

Long-term trends in the trophic structure of the North Sea fish community: evidence from stable-isotope analysis, size-spectra and community metrics

Fishing has wide-ranging impacts on marine ecosystems. One of the most pervasive signs of intensive fishing is "fishing down the food web", with landings increasingly dominated by smaller species from lower trophic levels. Decreases in the trophic level of landings are assumed to reflect those in fish communities, because size-selective mortality causes decreases in the relative abundance of larger species and in mean body size within species. However, existing analyses of fishing impacts on the trophic level of fish communities have focused on the role of changes in species composition rather than size composition. This will provide a biased assessment of the magnitude of fishing impacts, because fishes feed at different trophic levels as they grow. Here, we combine body size versus trophic level relationships for North Sea fishes (trophic level assessed using nitrogen stable-isotope analysis) with species–size–abundance data from two time-series of trawl-survey data (whole North Sea 1982–2000, central and northern North Sea 1925–1996) to predict long-term trends in the trophic structure of the North Sea fish community. Analyses of the 1982–2000 time-series showed that there was a slow but progressive decline in the trophic level of the demersal community, while there was no trend in the trophic level of the combined pelagic and demersal community. Analyses of the longer time-series suggested that there was no trend in the trophic level of the demersal community. We related temporal changes in trophic level to temporal changes in the slopes of normalised biomass size-spectra (which theoretically represent the trophic structure of the community), mean log2 body mass and mean log2 maximum body mass. While the size-based metrics of community structure showed long-term trends that were consistent with the effects of increased fishery exploitation, these trends were only correlated with trophic level for the demersal community. Our analysis suggests that the effects of fishing on the trophic structure of fish communities can be much more complex than previously assumed. This is a consequence of sampled communities not reflecting all the pathways of energy transfer in a marine ecosystem and of the absence of historical data on temporal and spatial changes in the trophic level of individuals. For the North Sea fish community, changes in size structure due to the differential effects of fishing on species and populations with different life histories are a stronger and more universal indicator of fishing effects than changes in mean trophic level.

RESPONSE OF ANTARCTIC TERRESTRIAL MICROARTHROPODS TO LONG-TERM CLIMATE MANIPULATIONS

The terrestrial biota of the Antarctic Peninsula region are experiencing marked changes in climate, especially rising temperatures, precipitation, and UV-B radiation—a combination unique worldwide. These changes, combined with the inherent simplicity of terrestrial communities, have led to their use as “model systems” to predict the future climate change responses of biota at lower latitudes. However, studies integrating responses at different levels of the community trophic structure are lacking. We report here the consequences on the soil microarthropod community of a four-year, multivariate, climate-manipulation experiment carried out over vegetation near Palmer Station, Anvers Island, western Antarctic Peninsula. The experiment used a multifactorial randomized-block design, deploying filters to raise temperatures and reduce ultraviolet (UV)-B (280–320 nm) or both UV-B and UV-A (320–400 nm) radiation of existing vegetation, with further water and fertilizer amendment treatments. Seven microarthropod species recovered in sufficient numbers for statistical analyses showed considerable spatial aggregation independent of treatment, a feature typical of many soil invertebrates. Analyses using negative binomial generalized linear modeling identified further significant and consistent treatment impacts on both individual species and species groups. Relative to controls, manipulations increasing temperature decreased numbers of microarthropods (particularly Collembola), as did exposure to near-ambient levels of UV radiation (separate significant effects for both UV-A and UV-B), while water amendment increased numbers. The impacts of temperature and water are consistent with our understanding of the importance of these two environmental variables and their interaction in Antarctic terrestrial ecosystems. The negative impact of UV (-A or -B) on arthropod heterotroph and detritivore populations in the Antarctic terrestrial food web is likely to be a secondary consequence of UV impact on vegetation characteristics. This is, again, consistent with general predictions of the impact of changing UV climate on ecosystem function.

Ecosystem Decay of Amazonian Forest Fragments: a 22‐Year Investigation

Abstract: We synthesized key findings from the Biological Dynamics of Forest Fragments Project, the world's largest and longest‐running experimental study of habitat fragmentation. Although initially designed to assess the influence of fragment area on Amazonian biotas, the project has yielded insights that go far beyond the original scope of the study. Results suggest that edge effects play a key role in fragment dynamics, that the matrix has a major influence on fragment connectivity and functioning, and that many Amazonian species avoid even small (<100‐m–wide) clearings. The effects of fragmentation are highly eclectic, altering species richness and abundances, species invasions, forest dynamics, the trophic structure of communities, and a variety of ecological and ecosystem processes. Moreover, forest fragmentation appears to interact synergistically with ecological changes such as hunting, fires, and logging, collectively posing an even greater threat to the rainforest biota.

Changes in trophic community structure of shore fishes at an industrial site in the Gulf of Aqaba, Red Sea

The semi-enclosed Gulf of Aqaba is under high pressure by urban and industrial pollu- tion, shipping and port activities as well as tourism. Off the Jordanian Red Sea coast, the trophic com- munity structure of shore fishes was determined on coral reefs in front of an industrial area (dis- turbed), in a marine reserve and site without industry or port activities (undisturbed), as well as in a seagrass-dominated bay. Planktivores were the most abundant feeding guild on coral reefs as well as in the seagrass-dominated bay. The relative abundance of feeding guilds other than planktivores seems to be strongly influenced by the benthic habitat. Multivariate analysis clearly separated dis- turbed from undisturbed sites, whereas univariate measures, such as species richness, diversity and evenness did not reveal any negative impact of disturbance. The disturbance of the coral reefs led to changes in the fish community through the reduction of total fish abundance by 50%, increased total abundance of herbivorous and detritivorous fishes, decreased total abundance of invertebrate- and fish-feeders, and increased relative abundance of planktivorous fishes.

Os grupos tróficos em Coleoptera

The beetles are a useful group for studies on trophic structure of communities, mainly in forested areas. These kind of studies are based on food habits of species groups. The different terms applied to nomminated these groups (trophic category, ecologic group, trophic group, guild, trophic guild) are discussed. The term trophic group, a natural unity, is proposed to form a group of species with the same food habits, not considering the trophic level. The guild, an artificial unity, is recognized as an important tool to group organisms group that use the same resources as defined by the investigator (Hawkins & MacMaiion, 1989; Simberloff & Dayan 1991; Wilson 1999). The known alimentary habits of Coleoptera are classified in five trophic groups - herbivores, algivores, fungivores, detrivores and carnivores. Within each trophic group, subgroups are recognized. The subgroup are in a lower hierarchic level and are defined by morphological (herbivores), taxonomic (fungivores) atributes of the food, by the food origin (detrivores) and by the way that the food is obtained (carnivores). A food resource diagram showing connections among the trophic groups, based on consumer-consumed (predator-prey) relations, is presented.

Effects of Sediment Contaminants and Environmental Gradients on Macrobenthic Community Trophic Structure in Gulf of Mexico Estuaries

Macrobenthic communities from estuaries throughout the northern Gulf of Mexico were studied to assess the influence of sediment contaminants and natural environmental factors on macrobenthic community trophic structure. Community trophic data were also used to evaluate whether results from laboratory sediment toxicity tests were effective indicators of site-specific differences in benthic trophic structure. A multiple regression model consisting of five composite factors (principal components) was used to distinguish the effects of sediment contaminants and environmental variables on benthic community trophic structure. This model explained 33.5% of the variation in macrobenthic trophic diversity (p<0.001), a variable derived from the distribution of taxas among nine original trophic categories. A significant negative relatinship was found between principal components reflecting concentrations of sediment contaminants and macrobenthic trophic diversity.

HERBIVORY AND PLANT SPECIES COEXISTENCE: COMMUNITY REGULATION BY AN OUTBREAKING PHYTOPHAGOUS INSECT

Most general theories proposed to explain the trophic structure of communities ignore the possibility that insect outbreaks can severely damage vegetation and reduce the abundance of dominant plant species over vast areas. Specialist chrysomelid beetles can irrupt and defoliate goldenrods (Solidago spp.), a group of widespread, long-lived, herbaceous perennials. We examined the long-term effects (10 yr) of suppressing insects, using insecticide in replicated plots on the structure and diversity of an old field dominated by the goldenrod, Solidago altissima. An outbreak of the chrysomelid beetle, Microrhopala vittata, that specializes on S. altissima, occurred during the experiment and persisted several years. Damage caused by this outbreak dramatically reduced the biomass, density, height, survivorship, and reproduction of S. altissima. Herbivore exclusion caused the formation of dense stands of goldenrods with a twofold increase in both standing crop biomass and litter. The understory in these dense stands had significantly lower plant abundance, species richness, flowering shoot production, and light levels; these conditions persisted for years following the outbreak. Thus, M. vittata functioned as a keystone species. Furthermore, insect herbivory indirectly increased the abundance of invading trees, thereby increasing the rate of succession, by speeding the transition of this old field to a tree-dominated stage. We conducted two follow-up experiments to test the hypothesis that insects altered community dynamics by their indirect effect on litter accumulation and light availability in the understory. In the first experiment, we tied back the canopy to increase light into the understory and removed litter in both the insecticide-treated and control plots. We found little effect of removing litter. By contrast, increasing understory light levels significantly increased understory forb abundance and species richness. In the second experiment, we placed rosettes of Hieracium pratense, the dominant understory forb, under nine levels of shade cloth, ranging from 95% shade to full sun. Flowering-shoot production was a linear function of light availability (r2 = 0.92; P < 0.0001). We concluded that insect herbivores indirectly promoted plant species richness and coexistence, primarily by augmenting light availability to suppressed understory species. Insect herbivory may often play a strong role in goldenrod stands, because outbreaks will likely occur at least once, if not more, during the period when goldenrods are dominant. Furthermore, our findings provide compelling evidence for two general mechanisms whereby insect herbivory promotes plant species diversity and coexistence. The first mechanism operates during outbreaks when insects act as keystone species. The second mechanism can operate at less than outbreak levels and occurs whenever insect damage augments light to a sufficient degree to enhance the fecundity of suppressed nonhost species. If this increase in fecundity increases recruitment of subordinate species, then insect herbivory will promote plant species coexistence and diversity. Our data suggest that there is a continuum in the influence of insect herbivory on plant communities from the more subtle, but important, effects of herbivory on the fecundity of nonhost species to the devastating influence of outbreaks. Also, our results demonstrate that long-term experiments are required to elucidate the role of insect herbivores. Finally, we propose that insect outbreaks are common enough in many community types, particularly forests, to warrant explicit consideration in theories of trophic regulation, particularly in terrestrial communities inhabited by long-lived plant species.

Studying insect diversity in the tropics.

Understanding the extent and causes of insect diversity in the humid tropics is one of the major challenges in modern ecology. We review some of the current approaches to this problem, and discuss how future progress may be made. Recent calculations that there may be more than 30 million species of insect on earth have focused attention on the magnitude of this problem and stimulated several new lines of research (although the true figure is now widely thought to be between five and ten million species). We discuss work based on insecticidal logging surveys; studies of herbivore and parasitoid specificity; macroecological approaches; and the construction of food webs. It is argued that progress in estimating insect diversity and in understanding insect community dynamics will be enhanced by building local inventories of species diversity, and in descriptive and experimental studies of the trophic structure of communities. As an illustration of work aimed at the last goal, we discuss the construction and analysis of quantitative host-parasitoid food webs, drawing on our work on leaf miner communities in Central America.

Organizing and understanding a winter’s seagrass foodweb network through effective trophic levels

Trophic structure of ecosystems is a unifying concept in ecology; however, the quantification of trophic level of individual components has not received the attention one might expect. Ecosystem network analysis provides a format to make several assessments of trophic structure of communities, including the effective trophic level (i.e. non-integer) of these components. We applied network analysis to a Halodule wrightii community in Goose Creek Bay, St. Marks National Wildlife Refuge, Florida, USA, during January and February 1994 where we sampled a wide variety of taxa. Unlike most applications of network analysis, the field sampling design was specific for network construction. From these data and literature values, we constructed and analyzed one of the most complex, highly articulated and site specific foodweb networks to be done. Care was taken to structure the network to reflect best the field data and ecology of populations within the requirements of analysis software. This involved establishing internally consistent rules of data manipulation and compartment aggregation. Special attention was paid to the microbial components of the food web. Consumer compartments comprised effective trophic levels from 2.0 (herbivore/detritivore) to 4.32 (where a level 4.0 represents ‘secondary carnivory’), and these values were used to organize data interpretation. The effective trophic levels of consumers tended to aggregate near integer values, but the spread from integer values increased with increasing level. Detritus and benthic microalgae acted as important sources of food in the extended diets of many consumers. ‘Bottom-up’ control appeared important through mixed trophic impact analysis, and the extent of positive impacts decreased with increasing trophic level. ‘Top-down’ control was limited to a few consumers with relatively large production or biomass relative to their trophic position. Overall, ordering results from various network analysis algorithms by effective trophic level proved useful in highlighting the potential influence of different taxa to trophodynamics. Although the calculation of effective trophic level has been available for some time, its application to the evaluation of other analyses has previously not received due consideration.

Can shallow- and deep-water chemoautotrophic and heterotrophic communities be discriminated in the fossil record?

The recognition of marine chemoautotrophic paleocommunities has added a new dimension to the interpretation of the fossil record. We compared the death assemblages formed by chemoautotrophic and heterotrophic molluscan faunas of the continental slope with their counterparts in shallow water and with a fossil chemoautotrophic analogue from the Tepee Buttes of the Pierre Shale (Cretaceous) to determine whether differences between them could be ascribed to differences in community structure or taphonomy. Evaluation of guild and tier structure and size-frequency distribution using abundance and energy flow (paleoproduction and paleoingestion) suggests that differences between inshore and offshore Recent heterotrophic assemblages reflect autecologic differences in the living communities forming them rather than differences in preservation. In contrast, the degree of similarity between chemoautotrophic assemblages is a function of their taphonomic and autecologic history. The differences observed amongst Recent chemoautotrophic assemblages issue from changes in the abundance and preservability of the small heterotrophic fauna, relatively poorly preserved offshore, and the presence of a large heterotrophic fauna, essentially restricted to shallow water. Taphonomic removal of the small heterotrophs (and juveniles of larger species) and the autecologic absence of large heterotrophs could produce an assemblage equivalent to that of the Cretaceous Tepee Buttes from any one of the Recent inshore or continental slope chemoautotrophic assemblages. Our analysis indicates that shelf communities represent the shallowest communities likely to be identified as fossil cold-seep faunas. Farther inshore, community attributes normally used to distinguish chemoautotrophic and heterotrophic faunas are no longer discriminative. Nearshore communities may have included a dominant chemoautotrophic guild, but recognition would rely solely on the identification of known chemoautotrophic taxa. Community trophic structure has classically been evaluated on the basis of numerical abundance and species composition. Comparing these chemoautotrophic and heterotrophic assemblages demonstrates the importance of evaluating community structure on criteria that more clearly document the way energy flowed through the community, namely paleoproduction and paleoingestion. Many of the unique structural attributes of these assemblages only became obvious using these measures.

Relationship between soft bottom macrofauna and polycyclic aromatic hydrocarbons (PAH) from smelter discharge in Norwegian fjords and coastal waters

Data from monitoring surveys were used to test the responses of soft bottom fauna in Norwegian fjords and coastal waters affected by effluents from aluminium and manganese alloy smelters to effluent discharges. Generally, polycyclic aromatic hydrocarbons (PAHs) have been considered the main threat to the biota in the receiving waters. Data from a total of 57 quantitative stations, of which 30 were simultaneously sampled for sedimentary PAHs, were analysed. Canonical Correspondence Analysis (CCA) was used to evaluate species patterns in relation to sediment PAH concentrations and natural environmental variables such as depth, sediment characteristics and riverine input. The PAHs varied from very high concentrations (760 pg g.' total PAH) at the most impacted sites to background levels in reference areas. Despite previous reports of little to no effects of the effluent discharges, the analyses identlf~ed significant species changes along PAH gradients, which could be associated with feeding modes. In particular, a number of carnivorous polychaetes had their dlstnbution ophma In the most affected areas. Based on species statistics calculated by CCA, response functions were fitted for the species on PAH and used to establish a generalised expression of trophic community structure in relation to PAH concentrations. At low PAH levels, the carnivores represented 15 to 20% of the specimens in the communities, but the proportion began to increase from about 10 pg g-' and exceeded 50% at 100 pg g-' total PAH. The fauna1 changes were most evident in shallow waters, possibly reflecting the effects of dissolved PAHs to which the species may react more directly than they do to sedlrnentary particle-associated PAHs.

Changes in the fish and fisheries ecology of a large man‐made lake in Tanzania, 1965–94

Abstract Nyumba ya Mungu Reservoir in northern Tanzania has a maximum surface area of 180 km2, an average depth of 6 m, and contains moderately saline, fertile water. The postimpoundment decade, 1965–74, was characterised by a spectacular rise and fall of a tilapiabased fishery. Since then a gill‐net fishery catching large numbers of large, periphyton‐grazing tilapias has given way to a mainly beach‐seine fishery producing large numbers of small, phytoplankton‐feeding Oreochromis esculentus (Graham). Yields remain higher than preimpoundment predictions but the size at maturity of all tilapia species has fallen and few larger than 15 cm TL are caught. These changes, and others in the fish community trophic structure, are chiefly explained by changes in the littoral environment and fishing pressures. Nyumba ya Mungu provides a case history of developments in the tilapia fishery of a man‐made river lake conducted on an essentially laissez‐faire basis.

Effects of hydropower peaking on epilithon and invertebrate community trophic structure

AbstractHydropower generation induces rapid and frequent fluctuations of hydrodynamic parameters in rivers downstream of hydroelectric impoundments. On the FontauliéGre river (ArdéGche basin, France), water releases come from the Loire basin and the flow varies from 1.3 to 20 m3/s for one reach and from 0.1 to 20 m3/s for another reach. A reference site upstream was chosen to compare the communities with two downstream impacted sites. These two sites enabled two different base flow situations to be studied simultaneously and also the effect of base flow enhancement at one site. Benthic and epilithic samples were collected at the three sites on three dates. Epilithon development was linked to hydraulic regimes, with a major increase in algal biomass, dominated by filamentous algae, linked to the minimum base flow level and to the duration of periods without peaks. The macroinvertebrate communities of impacted reaches were less diversified and more specialized, with predominance of one or two taxa, whereas communities in upstream reaches were more diverse. The different morphological units specificity was attenuated in hydropeaking situations. In the case of a very low base flow, the lentic units with very low velocities (less than 4 cm s−1) had different fauna. The downstream algal development explained part of the change in benthic community structure in trophic groups (i.e. dominance of scrapers) in the site with the higher base flow. When the base flow was too low, a trophic discordance was observed and excess algal biomass represented a limiting factor. Thus, this study indicated that base flow level and duration had important effects on epilithic development and on invertebrate trophic structure in terms of mean velocities and of number of days without peaks.

Infaunal Community Development of Artificially Established Salt Marshes in North Carolina

In recent years, artificial establishment of Spartina alterniflora marshes has become a common method for mitigating impacts to salt marsh systems. The vegetative component of artificially established salt marshes has been examined in several studies, but relatively little is known about the other aspects of these systems. This study was undertaken to investigate the infaunal community of artificially established salt marshes. Infauna were sampled from pairs of artificially established (AE) salt marshes and nearby natural marshes at six sites along the North Carolina coast. The AE marshes ranged in age from 1 yr to 17 yr. Total infaunal density, density of dominant taxa, and community trophic structure (proportions of subsurface-deposit feeders, surface-deposit and suspension feeders, and carnivores) were compared between the two types of marsh to assess infaunal community development in AE marshes. Overall, the two marsh types had similar component organisms and proportions of trophic groups, but total density and densities within trophic groupings were lower in the AE marshes. Soil organic matter content of the natural marshes was nearly twice that of the AE marshes, and is a possible cause for the higher infaunal densities observed in the natural marshes, Using the same three criteria, comparisons of the natural and AE marshes at each of the six locations revealed varying degrees of similarity. Similarity of each AE marsh to its natural marsh control appeared to be influenced by differences in environmental factors between locations more than by AE marsh age. Functional infaunal habitat convergence of an AE marsh with a natural marsh somewhere within its biogeographical region is probable, but success in duplicating the infaunal community of a particular natural marsh is contingent upon the developmental age of the natural marsh and the presence and interaction, of site-specific factors.

Comparisons of Community Structure Among Small Mammals of North-American and Australian Deserts

Data on the presence of small mammal species at 201 sites in North American deserts and 245 in arid Australia were used to quantify similarities and differences in community structure between the two continents. In all, 41 species of rodents were sampled in North America and 26 species, both rodents and marsupials, in Australia. On both continents, each species occurred throughout its geographic range with many other species and as a member of many different combinations. The number of species with which each species coexisted was positively correlated with its area of geographic range, although most species occurred at less than half the sites within their ranges. The median number of species at a site was three in North America and two in Australia, but the numbers of species per site on the two continents were not significantly different. Despite the relatively small numbers of species per site, numerous different combinations of species were observed (North America, 158; Australia, 108). Communities in North America were dominated by granivores and in Australia by omnivores; insectivores were also prominent in Australia. These results support conclusions of an earlier study of granivorous rodents in North America, in suggesting that there is substantial spatial variation in the composition of small mammal communities. The magnitude of this variation indicates that species and communities tend to be individualistic in spatial distribution, such that communities are not tightly restricted to particular specific combinations. There is evidence from North America that deterministic processes play a limited role in determining community structure; no such analyses have yet been undertaken for communities in arid Australia. Trophic structure of communities is markedly different on the two continents. Not all trophic differences can presently be explained, but they may be due to differences between the continents in temporal patterns of food supply.

Community Trophic Structure: The Roles of Diet, Body Size, and Activity Time in Vertebrate Predators

We examined trophic structure in an assemblage of 17 vertebrate predators - two snakes (Pituophis catenifer and Crotalus viridis), six hawks (Circus cyaneus, Buteo jamaicensis, B. regalis, Aquila chrysaetos, Falco sparverius, and F. mexicanus), six owls (Tyto alba, Otus kennicottii, Bubo virginianus, Speotyto cunicularia, Asio otus, and Aegolius acadicus), one passerine (Corvus corax), and two mammals (Canis latrans and Taxidea taxus). These predators fed collectively on prey in nine taxonomic classes. Mammalian prey were numerically most important in 15 diets and contributed the most biomass to all 17 diets. Mean prey mass ranged from 2.2 to 711 g, and correlated with predator mass. No significant differences were apparent in food-niche breadth or mean prey mass between diurnal and nocturnal predators, but dietary overlap among predators with the same diel activity was significantly greater than that among asynchronously-active predators. Mean diet overlap was significantly greater among nocturnal predators than among diurnal predators. Four feeding guilds were defined by dietary similarity. A ground squirrel-eating guild contained one snake, three hawks, and a mammal. A jackrabbit-eating guild consisted of a hawk and a mammal. An arthropod/mammal-eating guild was composed of an owl and the raven. Four owls, one hawk, and one snake formed a small-rodent guild. One owl and one hawk were not members of guilds. Mass ratios of adjacent-sized predators were greatest within guilds.