Interactions between nutrient availability and hydroperiod shape macroinvertebrate communities in Florida Everglades marshes

Hydroperiod and nutrient status are known to influence aquatic communities in wetlands, but their joint effects are not well explored. I sampled floating periphyton mat and flocculent detritus (floc) infaunal communities using 6-cm diameter cores at short- and long-hydroperiod and constantly inundated sites across a range of phosphorus (P) availability (total phosphorus in soil, floc and periphyton). Differences in community structure between periphyton and floc microhabitats were greater than any variation attributable to hydroperiod, P availability, or other spatial factors. Multivariate analyses indicated community structure of benthic-floc infauna was driven by hydroperiod, although crowding (no. g⁻¹ AFDM) of individual taxa showed no consistent responses to hydroperiod or P availability. In contrast, community structure of periphyton mat infauna was driven by P availability, while densities of mat infauna (no. m⁻²) were most influenced by hydroperiod (+correlations). Crowding of mat infauna increased significantly with P availability in short-hydroperiod marshes, but was constant across the P gradient in long-hydroperiod marshes. Increased abundance of floating-periphyton mat infauna with P availability at short-hydroperiod sites may result from a release from predation by small fish. Community structure and density were not different between long-hydroperiod and constantly inundated sites. These results have implications for the use of macroinvertebrates as indicators of water quality in wetlands and suggest the substrate sampled can influence interpretation of ecological responses observed in these communities.     

Microsite and elevational influences on early forest regeneration after catastrophic windthrow

Abstract. We compared vegetation establishment in 25 treefall pits and mounds along a hillside elevational gradient in a fourth-year catastrophic windthrow in eastern North America. Plant communities differed greatly between pits and mounds, with pit microsites having significantly greater species richness, total biomass, and total tree stem density. Species richness in pits and on mounds decreased with increasing elevation from the bottom of the hillside, although the effect of elevation on mound species richness was less than that of elevation on pit species richness. Biomass of Erechtites hieraciifolia decreased significantly, while that of Betula alleghaniensis increased significantly with elevation. However, total biomass of both pit and mound microsites was unrelated to elevation. Total stem density decreased with elevation in pits, but was unaffected by elevation on mounds. This study shows that both small-scale (microsite) effects and intermediate-scale effects influence the re-establishment of plant communities within this catastrophic windthrow. Consideration of both microsite and position along intermediate-scale gradients may allow more precise prediction of plant community composition and dynamics in recovery of disturbed areas.     

Community and population-level responses to disturbance in a sandflat community

The responses of soft sediment infauna were investigated in an intertidal sandflat to determine patterns of recolonization and succession at the community and population level. Experimental disturbance plots, 1 m², were initiated in August and sampled for 4.5 months along with ambient sediments. Sediment grain-size was used as a general indicator of the physical state of the disturbance patches, and grain-size distributions among disturbance and ambient patches became similar after ∼2.5 months. Recolonization varied among the dominant infaunal taxa. Densities of infauna that were most abundant in the habitat, primarily syllid polychaetes, did not recover to ambient levels until 3-4 months after disturbance, when ambient densities were falling to winter lows. Multivariate analysis indicated that community recovery occurred by the end of the study period after 4.5 months. Although community structure recovered by the end of the study, the population structure of the dominant species Parapionosyllis longicirrata remained significantly different among ambient and disturbed patches. On all sampling dates except one, disturbance patches had a higher number of larger individuals than ambient sediments. Previous studies have shown that bedload transport of juveniles and adults, and other processes, can cause recolonization to be relatively rapid on intertidal sandflats. However, our results indicate that recovery times may be on the order of months at large disturbance sizes. Therefore, rapid responses may occur primarily in the case of small-scale (<1 m²) disturbance patches. Secondly, recovery at the community level does not necessarily mean that population-level characteristics of species comprising the community have recovered. Population-level differences may be longer lasting than indicated by community level indicators of recovery.     

The dynamics of abundance and incidence of annual plant species during colonization in a desert

We studied colonization of annual plants in small-scale disturbances and undisturbed soil for four years in the northern Negev desert. The experiment consisted of 24 patches of I m²: eight undisturbed patches, eight 30 cm deep pits and eight 20 cm high mounds. Disturbance removed the seed bank from the pits and mounds. Rainfall was average during 1992 and 1993 (163.5 and 157.0 mm), very low in 1994 (97.5 mm) and very high in 1995 (283.0 mm). For all ca 100 species in the assemblage, we measured abundance, as the average local density in patches occupied by the species, and incidence, the proportion of patches occupied. Abundance and incidence were positively correlated in the four years of the study (R^2-= 0.71, 0.56. 0. 56 and 0.64). The relationship became steeper during the course of colonization. A minority of species colonized all patches rapidly and increased exponentially in abundance. These high incidence and abundance species responded little or not at all to variations in rainfall. Most of the species experienced frequent local extinctions and colonizations in few patches. These low abundance and low to medium incidence species responded strongly to rainfall by fluctuations in abundance. Differences per species between abundance and/or incidence in pits, mounds and matrix were evident in many species along the entire relationship. These were mostly in favor of pits. We propose that the positions and trajectories of the individual species in the abundance-incidence phase plane signify constraints on population growth along the abundance axis, and on colonization and population persistence along the incidence axis. Since species with lower incidence and abundance respond strongly to rainfall variation, we conclude that their local populations and spatial distribution are to a large extent site-limited. High-abundance and high-incidence species have few or weak constraints and experience density-independent population growth, which is only seed-limited. The causes of site limitation and its effect on population growth and on eotonization and extinction processes differ for different species. In a number of species, abundance and incidence may be limited by dispersal, in conjunction with dispersal mode and patch-specific seed capture. Based on our case study we suggest that the trajectories of the species in the abundance-incidence phase plane are a useful tool for investigating assemblage dynamics.     

Are Infaunal Predators Important in Structuring Marine Soft-Bottom Communities?

The role of infaunal predators in structuring marine soft-bottomcommunities was evaluated according to these predators': 1)effects on prey density based on manipulative field experiments,2) feeding rates, 3) effects on prey distribution, 4) effectson species diversity, and 5) interactions with their prey. Estimatesof feeding rates indicate that many predatory taxa have thepotential to reduce the size of prey populations and suggestthat nemerteans are likely to have a larger impact on infaunalabundances than polychaetes. Infaunal predators have been demonstratedto have a significant effect on infaunal densities and to affectthe spatial and temporal distribution of their prey. The effectsof these predators on species diversity apparently depend onthe predator and the diversity of the system. These conclusionsmay not be applicable to all soft-bottom habitats or all groupsof infaunal predators because they are based on studies of veryfew taxa conducted almost exclusively in intertidal, unvegetated,mud habitats. Additional studies are needed on the effects ofpredation by infauna on infaunal population dynamics and onthe mechanisms of interactions between predator and prey. Furtherinvestigation will probably reveal that different groups ofinfaunal predators play different roles in structuring soft-bottomcommunities.     

Effects of bannertail kangaroo rat mounds on small-scale plant community structure

The effects of bannertail kangaroo rat (Dipodomys spectabilis) mounds and associated soil-surface disturbance on plant species composition and diversity in the Chihuahuan Desert were examined with multivariate analysis. Kangaroo rat mounds created disturbance gaps and contributed to local species diversity by creating microhabitats that supported unique plant communities. These microhabitats supported populations of species that were relatively rare in surrounding areas. The diversity observed at the whole habitat level resulted from (1) local spatial heterogeneity, because the mounds offered microenvironments with distinctive nutrient, water, and light conditions; and (2) local patterning of disturbance, because the digging and traffic of the kangaroo rats maintained high levels of soil disturbance at and near the mounds. At a finer scale, species diversity was highest in the area immediately adjacent to active and inactive mounds, and was lower on both the highly disturbed soil of the mounds and in the relatively undisturbed area between mounds. Lowest species diversity occurred on inactive mounds. Annual plant biomass was much greater on mounds than in inter-mound areas. The results support the predictions that intermediate levels of disturbance and small-scale environmental heterogeneity contribute to supporting high species diversity.     

植物群落中物种小尺度空间结构研究

植物群落中, 物种小尺度空间结构影响着种群或群落的动态及有关的生态学过程。植物主要是和它同种或异种的邻近个体相互作用,植物个体周围的局部环境和大空间尺度下群落的平均水平是完全不同的。群落中的许多过程都影响小尺度空间结构的形成和动态,同样,局部空间结构反过来影响着植物的生长、更新和凋亡等重要过程。鉴于目前对小尺度空间结构进行的大量研究以及其重要性,有必要对其研究进展进行适当的总结,以期明确将来进一步的研究方向。该文以此为出发点,首先介绍了植物群落中物种小尺度空间结构产生的6个原因:1)生境的空间异质性;2)植物繁殖体的传播;3)植物之间的相互作用;4)生物环境(动物和微生物)的作用;5)外界干扰的作用;6)多因子综合作用。然后阐述了小尺度空间结构意义及对生物多样性、植物种群遗传学和恢复生态学研究的影响。最后对目前物种小尺度空间结构研究存在的几点问题及将来的研究方向作以下归纳:1)大尺度植被动态的研究应该整合小尺度空间结构的信息;2)不论从生物学还是生态学上来讲, 植被小尺度空间结构的研究应该把植物作为中心,确定适当的尺度和采取合理的空间统计方法;3)充分重视小尺度空间结构在退化生态系统恢复中的应用意义;4)注重从小尺度的局部格局研究入手对群落总体特征进行整合;5)植物群落动态研究中,物种小尺度空间结构与平均场假说相结合的必要性。     

Seed harvesting by a generalist consumer is context‐dependent: Interactive effects across multiple spatial scales

Granivore foraging decisions affect consumer success and determine the quantity and spatial pattern of seed survival. These decisions are influenced by environmental variation at spatial scales ranging from landscapes to local foraging patches. In a field experiment, the effects of seed patch variation across three spatial scales on seed removal by western harvester ants Pogonomyrmex occidentalis were evaluated. At the largest scale we assessed harvesting in different plant communities, at the intermediate scale we assessed harvesting at different distances from ant mounds, and at the smallest scale we assessed the effects of interactions among seed species in local seed neighborhoods on seed harvesting (i.e. resource–consumer interface). Selected seed species were presented alone (monospecific treatment) and in mixture with Bromus tectorum (cheatgrass; mixture treatment) at four distances from P. occidentalis mounds in adjacent intact sagebrush and non‐native cheatgrass‐dominated communities in the Great Basin, Utah, USA. Seed species differed in harvest, with B. tectorum being least preferred. Large and intermediate scale variation influenced harvest. More seeds were harvested in sagebrush than in cheatgrass‐dominated communities (largest scale), and the quantity of seed harvested varied with distance from mounds (intermediate‐scale), although the form of the distance effect differed between plant communities. At the smallest scale, seed neighborhood affected harvest, but the patterns differed among seed species considered. Ants harvested fewer seeds from mixed‐seed neighborhoods than from monospecific neighborhoods, suggesting context dependence and potential associational resistance. Further, the effects of plant community and distance from mound on seed harvest in mixtures differed from their effects in monospecific treatments. Beyond the local seed neighborhood, selection of seed resources is better understood by simultaneously evaluating removal at multiple scales. Associational effects provide a useful theoretical basis for better understanding harvester ant foraging decisions. These results demonstrate the importance of ecological context for seed removal, which has implications for seed pools, plant populations and communities.     

Spatial and temporal variations in nematode assemblages in tropical seagrass sediments

The changes in the community dynamics of infaunal nematodes associated with tropical Australian intertidal seagrasses at 4 estuarine sites were investigated through 3 seasons (autumn, winter and spring). Nematode densities were highest in winter in all but one of the sites, ranging from 1971 to 3084 inds./10 cm², with one site showing a highest density of 3411 inds./10 cm² in spring. Multiple regression revealed significant correlations between nematode density peaks and seasonal changes in temperature, salinity and surface seagrass cover. Non-metric Multi-Dimensional Scaling revealed that the communities were characterised by relatively low within-site spatial variability but relatively high temporal variability through the three seasons. This temporal variability was largely due to significant increases in abundance of epistrate-feeding species in winter and spring. An investigation of the dominant epistrate-feeding nematodes revealed that predominantly infaunal species were responsible for overall winter and spring density increases. This study provides further evidence of the role played by temperature in regulating tropical, intertidal meiobenthic communities but also indirectly provides evidence of micro-scale seagrass canopy effects (micro-algal supply and availability) that may further enhance the impacts of these larger-scale seasonal environmental changes on the infaunal nematode community.     

Local climate mediates spatial and temporal variation in carabid beetle communities in three forests in Mount Odaesan,Korea

1. Global environmental change can dramatically alter the composition of floral and faunal communities, and elucidating the mechanisms underlying this process is important for predicting its outcomes. Studies on global climate change have mostly focused on statistical summaries within wide spatial and temporal scales; less attention has been paid to variability in microclimates at narrower spatial and temporal scales. 2. The microclimate is the suite of climatic conditions measured in a local area. Environmental variables at the microclimatic scale can be critical for the ecology of organisms inhabiting each area. The effect of spatial and temporal changes in the microclimate on the ecology of carabid beetle communities in three sites on Mount Odaesan, Korea was examined. 3. Carabid beetle communities and quantified site‐specific environmental factors from measurements of air temperature, air humidity, light intensity and soil temperature over 5 years (2010–2015) were surveyed. 4. It was found that microclimatic variables and the patterns of temporal changes in carabid beetle communities differed between the three sites within the single mountain system. Microclimatic variables influencing temporal changes in beetle communities also differed between the sites. Therefore, it is suggested that variation in local microclimates affects spatial and temporal variation in carabid beetle communities at a local scale. 5. The present results demonstrate the importance of regular surveys of communities at local scales. Such surveys are expected to reveal an additional fraction of variation in communities and underlying processes that have been overlooked in studies of global community patterns and change.     

A multi-scale comparison of trait linkages to environmental and spatial variables in fish communities across a large freshwater lake

Species present in communities are affected by the prevailing environmental conditions, and the traits that these species display may be sensitive indicators of community responses to environmental change. However, interpretation of community responses may be confounded by environmental variation at different spatial scales. Using a hierarchical approach, we assessed the spatial and temporal variation of traits in coastal fish communities in Lake Huron over a 5-year time period (2001–2005) in response to biotic and abiotic environmental factors. The association of environmental and spatial variables with trophic, life-history, and thermal traits at two spatial scales (regional basin-scale, local site-scale) was quantified using multivariate statistics and variation partitioning. We defined these two scales (regional, local) on which to measure variation and then applied this measurement framework identically in all 5 study years. With this framework, we found that there was no change in the spatial scales of fish community traits over the course of the study, although there were small inter-annual shifts in the importance of regional basin- and local site-scale variables in determining community trait composition (e.g., life-history, trophic, and thermal). The overriding effects of regional-scale variables may be related to inter-annual variation in average summer temperature. Additionally, drivers of fish community traits were highly variable among study years, with some years dominated by environmental variation and others dominated by spatially structured variation. The influence of spatial factors on trait composition was dynamic, which suggests that spatial patterns in fish communities over large landscapes are transient. Air temperature and vegetation were significant variables in most years, underscoring the importance of future climate change and shoreline development as drivers of fish community structure. Overall, a trait-based hierarchical framework may be a useful conservation tool, as it highlights the multi-scaled interactive effect of variables over a large landscape.     

Small-scale patch dynamics after disturbance in litter ant communities

The dynamics of re-colonisation of disturbed patches may aid in the understanding of spatial variation of species richness. The present study experimentally tested the hypothesis that the variation of litter ant local species richness and composition is caused by the dynamics of re-colonisation after disturbances. We were particularly interested in whether the re-colonisation was by pre-existent species or species new to the patches, and whether the succession of species evidences the existence of dominance-controlled or founder-controlled communities. Litter patches of a forest remnant in Southeast Brazil were disturbed by removing most animals through litter drying, and litter samples were returned to the same sites from where they were removed. Ant species richness and composition were compared before and 2 months after the disturbance. Dissimilarity among disturbed and non-disturbed samples was compared to infer the succession model occurring after disturbance. Ant species richness did not recover after 2 months, and species composition of the disturbed samples showed more new colonisers than pre-existent species. Dissimilarity among samples in the disturbed plots was smaller than in the control plots, indicating a directional, or dominance-controlled, succession. The changes in species composition observed were caused by a decrease of some species, particularly predators, and an increase of species that are possibly opportunistic. Patches of litter are naturally disturbed in time and space, and evidence from the present paper indicates that succession occurring in these patches would lead to different species richness and compositions. Thus the dynamics of re-colonisation contributes to explaining the diversity of litter-dwelling ant communities at larger spatial and temporal scales. In each patch the succession seems to be directional, with opportunist species re-colonising preferentially empty plots. Therefore, these communities may attain a high diversity due to a small-scale patch dynamics model.     

Investigating patterns and processes of demographic variation: environmental correlates of pre-breeding survival in red-billed choughs Pyrrhocorax pyrrhocorax

1. Quantifying the pattern of temporal and spatial variation in demography, and identifying the factors that cause this variation, are essential steps towards understanding the structure and dynamics of any population. 2. One critical but understudied demographic rate is pre-breeding survival. We used long-term colour-ringing data to quantify temporal (among-year) and spatial (among-nest site) variation in pre-breeding survival in red-billed choughs (Pyrrhocorax pyrrhocorax) inhabiting Islay, Scotland, and identified environmental correlates of this variation. 3. Random-effects capture-mark-recapture models demonstrated substantial temporal and spatial process variance in first-year survival; survival from fledging to age 1 year varied markedly among choughs fledged in different years and fledged from different nest sites. Spatial variance exceeded temporal variance across choughs fledged from well-studied nest sites. 4. The best-supported models of temporal variation suggested that first-year survival was higher in years following high tipulid larvae abundance and when weather conditions favoured increased invertebrate productivity and/or availability to foraging choughs. These variables explained up to 80% of estimated temporal process variance. 5. The best-supported models of spatial variation suggested that first-year survival was higher in choughs fledged from nest sites that were further from exposed coasts and closer to flocking areas, and surrounded by better habitat and higher chough density. These variables explained up to 40% of estimated spatial process variance. 6. Importantly, spatio-temporal models indicated interactive effects of weather, tipulid abundance, local habitat and local chough density on first-year survival, suggesting that detrimental effects of poor weather and low tipulid abundance may be reduced in choughs fledged from nest sites surrounded by better foraging habitat and lower chough density. 7. These analyses demonstrate substantial temporal and small-scale spatial variation in pre-breeding survival, a key demographic rate, and indicate that this variation may reflect interactive effects of weather, prey abundance, habitat and geography. These patterns illustrate the value of holistic models of demographic variation, and indicate environmental factors that may limit the growth rate of Islay's protected chough population.     

Large grazers modify effects of aboveground-belowground interactions on small-scale plant community composition

Aboveground and belowground organisms influence plant community composition by local interactions, and their scale of impact may vary from millimeters belowground to kilometers aboveground. However, it still poorly understood how large grazers that select their forage on large spatial scales interact with small-scale aboveground–belowground interactions on plant community heterogeneity. Here, we investigate how cattle (Bos taurus) modify the effects of interactions between yellow meadow ants (Lasius flavus) and European brown hares (Lepus europaeus) on the formation of small-scale heterogeneity in vegetation composition. In the absence of cattle, hares selectively foraged on ant mounds, while under combined grazing by hares and cattle, vertebrate grazing pressure was similar on and off mounds. Ant mounds that were grazed by only hares had a different plant community composition compared to their surroundings: the cover of the grazing-intolerant grass Elytrigia atherica was reduced on ant mounds, whereas the relative cover of the more grazing-tolerant and palatable grass Festuca rubra was enhanced. Combined grazing by hares and cattle, resulted in homogenization of plant community composition on and off ant mounds, with high overall cover of F. rubra. We conclude that hares can respond to local ant–soil–vegetation interactions, because they are small, selective herbivores that make their foraging decisions on a local scale. This results in small-scale plant patches on mounds of yellow meadow ants. In the presence of cattle, which are less selective aboveground herbivores, local plant community patterns triggered by small-scale aboveground–belowground interactions can disappear. Therefore, cattle modify the consequences of aboveground–belowground interactions for small-scale plant community composition.     

Spatio-temporal dynamics of species richness in coastal fish communities

Determining patterns of change in species richness and the processes underlying the dynamics of biodiversity are of key interest within the field of ecology, but few studies have investigated the dynamics of vertebrate communities at a decadal temporal scale. Here, we report findings on the spatio-temporal variability in the richness and composition of fish communities along the Norwegian Skagerrak coast having been surveyed for more than half a century. Using statistical models incorporating non-detection and associated sampling variance, we estimate local species richness and changes in species composition allowing us to compute temporal variability in species richness. We tested whether temporal variation could be related to distance to the open sea and to local levels of pollution. Clear differences in mean species richness and temporal variability are observed between fjords that were and were not exposed to the effects of pollution. Altogether this indicates that the fjord is an appropriate scale for studying changes in coastal fish communities in space and time. The year-to-year rates of local extinction and turnover were found to be smaller than spatial differences in community composition. At the regional level, exposure to the open sea plays a homogenizing role, possibly due to coastal currents and advection.     

Spatial dynamics in model plant communities: what do we really know?

A variety of models have shown that spatial dynamics and small-scale endogenous heterogeneity (e.g., forest gaps or local resource depletion zones) can change the rate and outcome of competition in communities of plants or other sessile organisms. However, the theory appears complicated and hard to connect to real systems. We synthesize results from three different kinds of models: interacting particle systems, moment equations for spatial point processes, and metapopulation or patch models. Studies using all three frameworks agree that spatial dynamics need not enhance coexistence nor slow down dynamics; their effects depend on the underlying competitive interactions in the community. When similar species would coexist in a nonspatial habitat, endogenous spatial structure inhibits coexistence and slows dynamics. When a dominant species disperses poorly and the weaker species has higher fecundity or better dispersal, competition-colonization trade-offs enhance coexistence. Even when species have equal dispersal and per-generation fecundity, spatial successional niches where the weaker and faster-growing species can rapidly exploit ephemeral local resources can enhance coexistence. When interspecific competition is strong, spatial dynamics reduce founder control at large scales and short dispersal becomes advantageous. We describe a series of empirical tests to detect and distinguish among the suggested scenarios.     

Plant genetics shapes inquiline community structure across spatial scales

Recent research in community genetics has examined the effects of intraspecific genetic variation on species diversity in local communities. However, communities can be structured by a combination of both local and regional processes and to date, few community genetics studies have examined whether the effects of instraspecific genetic variation are consistent across levels of diversity. In this study, we ask whether host-plant genetic variation structures communities of arthropod inquilines within distinct habitat patches – rosette leaf galls on tall goldenrod ( Solidago altissima ). We found that genetic variation determined inquiline diversity at both local and regional spatial scales, but that trophic-level responses varied independently of one another. This result suggests that herbivores and predators likely respond to heritable plant traits at different spatial scales. Together, our results show that incorporating spatial scale is essential for predicting the effects of genetically variable traits on different trophic levels and levels of diversity within the communities that depend on host plants.     

Spatial and temporal variation of ecosystem properties at macroscales

Although spatial and temporal variation in ecological properties has been well‐studied, crucial knowledge gaps remain for studies conducted at macroscales and for ecosystem properties related to material and energy. We test four propositions of spatial and temporal variation in ecosystem properties within a macroscale (1000 km's) extent. We fit Bayesian hierarchical models to thousands of observations from over two decades to quantify four components of variation – spatial (local and regional) and temporal (local and coherent); and to model their drivers. We found strong support for three propositions: (1) spatial variation at local and regional scales are large and roughly equal, (2) annual temporal variation is mostly local rather than coherent, and, (3) spatial variation exceeds temporal variation. Our findings imply that predicting ecosystem responses to environmental changes at macroscales requires consideration of the dominant spatial signals at both local and regional scales that may overwhelm temporal signals.     

Temporal changes of local and regional processes in the assembly of herbivorous insect communities

Herbivorous insect communities are structured by multiple processes operating locally (e.g. bottom–up effects of plants) and regionally (e.g. dispersal limitation). Although the relative strength of these processes has been well documented, how it varies in time is less understood, especially in relation with the temporal dynamics of plant communities. If temporal turnover of local plant species composition is too rapid for insect community assembly to keep up with, bottom–up effects are expected to be weak. Here, in plant and herbivorous insect communities in Japanese grasslands, we studied how the relative importance of local (bottom–up effects of plants, structures of plant communities and top–down effects of predators) and regional (dispersal limitation) processes varies over the growing season. In addition, we tested the hypothesis that larger temporal turnover of plant species composition is related to the weaker bottom–up effects, that is, the lower explanation power of plant communities for insect communities. We found that, throughout the growing season, the insect species composition was mainly explained by local variables (plant species composition, vegetation height and predator abundance), and their explanation power was higher during later phases of the season (late summer). Furthermore, the variation not explained by plant species composition was correlated with the degree of temporal turnover of plants, suggesting that insect communities failed to track the temporal turnover of plant species. These results were pronounced when we focused on leaf sucker insects, whose host plant range is presumably more limited. We conclude that herbivorous insect communities are mainly regulated by local processes, especially bottom–up effects from plants, while stochasticity may have played a role in early phases of the season. Furthermore, we underscore the importance of considering relative time scale of community assembly and environmental shifts, especially in systems characterized by dynamic changes.     

The effects of chemical pollution on the bioturbation potential of estuarine intertidal mudflats

Bioturbation by benthic infauna has important implications for the fate of contaminants as well as for changes to the sediment structure, chemistry and transport characteristics. There is an extensive literature dealing with the influence of sedimentary variables on the structure and function of infaunal marine and estuarine organisms but less is known of the converse, the influence of biota on sedimentary structure. Although some work has been carried out regarding spatial and temporal patterns of bioturbation, little attention has been given to the effects of pollution. The paper gives a framework of animal sediment relationships in an intertidal environment and discusses the general role of macrofauna in structuring and modifying sedimentary features. A brief outline of the various techniques used for quantifying the degree of bioturbation is given and some of these techniques have then been used to demonstrate the effect of a petrochemical discharge on the bioturbation potential of intertidal communities in the Humber estuary, eastern England. These studies indicate an increase in bioturbation with increasing distance from the source of pollution, not only because of differences in abundance, animal size and depth of activity but also because of the difference in species composition between the communities. As a means of interpreting the responses, the species present have been broadly classified in terms of their feeding strategy and sediment modification potential. The paper concludes by discussing the potential impact, in terms of effect on sediment transport, of selectively removing the different guilds (by pollution). Received: 8 February 1999 / Received in revised form: 10 May 1999 / Accepted: 14 May 1999