Two lowland stream riffles – linkages between physical habitats and macroinvertebrates across multiple spatial scales

The physical structure of two riffles in a lowland Danish stream was studied and its importance for the composition and density of the macroinvertebrate communities was evaluated. The two riffles were visually assessed to be very similar, but measurements revealed that they differed in overall hydraulic conditions, stability, substratum composition and consolidation. Differences affected abundance of both burrowing and surface dwelling macroinvertebrates. The unstable unconsolidated riffle had higher total macroinvertebrate abundance (4137 m⁻² vs. 1698 m⁻²), diptera abundance (2329 m⁻² vs. 386 m⁻²) and total estimated species richness (31.7 vs. 28.8) as well as lower evenness (0.77 vs. 0.83) than the compact riffle. Among samples within the unconsolidated riffle, variations in macroinvertebrate communities were related to differences in mean substratum particle size. Here a linear log–log relationship existed between macroinvertebrate abundance, the abundance of EPT taxa and the median particle size (r ² _total = 0.46, p = 0.002; r ² _EPT = 0.73, p < 0.001). No similar relationships were evident on the consolidated riffle. Moreover, macroinvertebrate communities on the unconsolidated riffle were dominated by species with a high colonising potential. Despite being assessed to the same morphological unit, physical variation between riffles was surprisingly high as the riffles differed substantially with respect to consolidation, substratum heterogeneity and overall hydraulic structure. Macroinvertebrate community structure and composition also differed between riffles despite being drawn from the same species pool. The findings address the question if we use the correct methods and parameters when assessing the macroinvertebrate communities at the scale of the morphological unit.     

Ecosystem services across the aquatic–terrestrial boundary: Linking ponds to pollination

Many small farmland ponds are built for nutrient retention, the conservation of biodiversity or both, yet they are relatively neglected habitats. For example, little is known about the potential for ponds to influence populations of beneficial terrestrial insects, deliver ecosystem services across the aquatic–terrestrial boundary and affect crop yield in insect-pollinated cash crops.We assessed whether the presence of a pond affects the abundance of pollinators and the quality and quantity of strawberry yield. We compared the abundance of pollinators and the quality and quantity of strawberries between habitats adjacent to the pond, semi-natural terrestrial habitat and field border without semi-natural vegetation (control habitat).We found significantly higher abundances of syrphids and bees next to ponds compared to control habitats. Also, syrphids were significantly more abundant at pond habitats compared to vegetation habitats and a similar tendency, although not significant, was found for the abundance of bees. The quantity and quality of strawberries was significantly higher near the vegetation and pond habitats compared to the control habitats.Our result supports the theory that the presence of semi-natural habitats, in the agricultural landscape benefits both public interest in biodiversity conservation and farmers’ interest in crop pollination. These benefits may also come from ponds as semi natural habitats. However, further studies are required to disentangle the effect of the pond per se and the effect of the associated terrestrial vegetation.     

Does spatial distribution of tree size account for spatial variation in soil respiration in a tropical forest?

We explored the relationship between soil processes, estimated through soil respiration (R _soil ), and the spatial variation in forest structure, assessed through the distribution of tree size, in order to understand the determinism of spatial variations in R _soil in a tropical forest. The influence of tree size was examined using an index (I _c ) calculated for each tree as a function of (1) the trunk cross section area and (2) the distance from the measurement point. We investigated the relationships between I _c and litterfall, root mass and R _soil , respectively. Strong significant relationships were found between I _c and both litterfall and root mass. R _soil showed a large range of variations over the 1-ha experimental plot, from 1.5 to 12.6 g_C m^?2 d^?1. The best relationship between I _c and R _soil only explained 17% of the spatial variation in R _soil . These results support the assumption that local spatial patterns in litter production and root mass depend on tree distribution in tropical forests. Our study also emphasizes the modest contribution of tree size distribution–which is mainly influenced by the presence of the biggest trees (among the large range size of the inventoried trees greater than 10 cm diameter at 1.30 m above ground level or at 0.5 m above the buttresses)–in explaining spatial variations in R _soil .     

Temporal consistency in the spatial pattern of seed predation across a forest – old field edge

Seed predation is an important factor in determining the rate of tree establishment in abandoned agricultural land. Edges, through altered habitat use by small mammals, may influence the spatial pattern of forest regeneration in these successional sites. To determine the spatial pattern of seed predation across a forest-old field edge, we used a grid that began 30 m inside the forest and extended 60 m into the old field. Seed stations were placed at regular 10-m intervals and were monitored for removal of Acer rubrum seed for 50 d. This design was repeated over four years (1995–1998). Small mammal trapping was conducted in the final year of the study to determine the spatial pattern of seed predators within the site. Removal rates varied among the four years of the study with years of high and low removal rates. However, the spatial pattern of seed removal rate was similar in all years. Final survival ranged from 0.7–15.5% of seeds, with lower final survival in years with faster rates of seed removal (1996 and 1998). Seed removal rates and rates of discovery were greatest at the forest edge and decreased with distance into the old field. The number of seeds surviving to the end of the experiment also varied across the edge gradient, with highest survival at greater distances into the old field in low predation years. Seed removal rate covaried with spatial pattern of Peromyscus leucopus captures within the site. Seed removal and discovery was also higher under the exotic shrub Rosa multiflora, which may have provided cover for foraging seed predators. These indirect effects of edges on plant communities can potentially alter the rate and spatial pattern of tree invasion into disturbed lands and illustrate the importance of understanding plant-animal interactions in the context of habitat fragmentation.     

Richness–productivity relationships between trophic levels in a detritus-based system: significance of abundance and trophic linkage

Most theoretical and empirical studies of productivity–species richness relationships fail to consider linkages among trophic levels. We quantified productivity–richness relationships in detritus-based, water-filled tree-hole communities for two trophic levels: invertebrate consumers and the protozoans on which they feed. By analogy to theory for biomass partitioning among trophic levels, we predicted that consumer control would result in richness of protozoans in the lower trophic level being unaffected by increases in productivity, whereas richness of invertebrate consumers would increase with productivity. Our data were consistent with this prediction: consumer richness increased linearly, but protozoan richness was unrelated to changes in productivity. The productivity–richness relationships for all taxa combined were not necessarily consistent with relationships within each trophic level. We used path analysis to investigate the mechanisms that may produce the observed responses of trophic levels to changes in productivity. We tested the importance of the direct effect of productivity on richness and the indirect effect of productivity mediated by effects on total abundance. For protozoans, only direct effects of productivity on richness were important, but both direct and indirect effects of productivity on richness were important for invertebrates. Protozoan richness was strongly affected by top-down impacts of abundance of invertebrates. These results are consistent with theory on biomass partitioning among trophic levels and suggest a strong link between richness and abundance within and between trophic levels. Understanding how trophic level interactions determine productivity–richness relationships will likely be necessary in order for us to achieve a comprehensive understanding of the determinants of diversity. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Facies stacking and extinctions across the Triassic–Jurassic boundary in a peritidal succession from western Sicily

An uppermost Triassic–lowermost Jurassic carbonate platform succession, which is 430 m thick, in northwestern Sicily is described with the aim to provide new data on the sedimentological and biological variations across the Triassic–Jurassic boundary in peritidal environments. The studied succession belonged to the rimmed carbonate shelf that developed during the Late Triassic along the margins of the Ionian Tethys. The peritidal sediments consist of meter-scale shallowing-upward cycles formed by subtidal, intertidal, and supratidal facies. Three main informal units are differentiated along the section on the basis of the variations recorded by the subtidal facies. The lower and middle units are attributed to the Rhaetian, on the basis of the common presence of the foraminifer Triasina hantkeni, associated with several benthic foraminifers, such as Aulotortus sinuosus and Auloconus permodiscoides. Megalodontids are particularly abundant and large in the lower unit, whereas they become rare in the middle unit and disappear in the upper unit. The last occurrence of T. hantkeni, along with the disappearance of the benthic foraminifer fauna, and the bloom of the calcareous alga Thaumatoporella parvovesiculifera is assumed as a proxy of the Rhaetian–Hettangian boundary. Recovery biota during the early Jurassic occurs about 20 m upward of the boundary zone, marked by the appearance of benthic foraminifers, such as Siphovalvulina sp. The observed biostratigraphic signature in the studied section is easily comparable to similar Tethyan sections already described from Italy, Greece, and Turkey; thus, it is believed that the faunistic turnover does not reflect local facies variations, in response to changes in the accommodation space of the platform, but regional changes in a more wide area of ocean Tethys.     

Nitrogen translocation between clonal mother and daughter trees at a grassland–forest boundary

There is abundant evidence from short-term experiments using herbs that nutrients can be translocated from mother ramets to daughter ramets, but there is little long-term evidence from woody plants. Here, we examine translocation in field populations of a clonal tree over two growing seasons. We applied ¹⁵N to mothers or daughters in clones of Populus tremuloides at the northern edge of the North American Great Plains, where mother ramets form closed-canopy stands on relatively nutrient-rich soils, and daughter ramets occur nearby in relatively nutrient-poor grasslands. Unlabeled daughters in clones with labeled mothers had δ¹⁵N values significantly greater than those in unlabeled clones, confirming translocation from mothers to daughters. However, unlabeled mothers in clones with labeled daughters also had δ¹⁵N values significantly greater than those in unlabeled clones, indicating translocation from daughters to mothers. Further, the total foliage accumulation of added ¹⁵N was significantly (c. 10×) greater in mothers than in daughters, suggesting that more N was translocated from daughters to mothers, than from mothers to daughters. Thus, ¹⁵N moved both from mothers to daughters and from daughters to mothers, with net flow toward mothers. Because long-lived woody ramets in the field face nutrient competition from other ramets, interspecific neighbors, and soil microbes, the environmental availability of nutrients for uptake may be low for both mother and daughter ramets, causing translocation within a clone to be toward larger ramets with greater demand.     

Crop–noncrop spillover: arable fields affect trophic interactions on wild plants in surrounding habitats

Ecosystem processes in agricultural landscapes are often triggered by resource availability in crop and noncrop habitats. We investigated how oilseed rape (OSR; Brassica napus, Brassicaceae) affects noncrop plants in managed systems and semi-natural habitat, using trophic interactions among wild mustard (Sinapis arvensis, Brassicaceae), rape pollen beetles (Meligethes aeneus, Nitidulidae) and their parasitoids (Tersilochus heterocerus, Ichneumonidae). We exposed wild mustard as phytometer plants in two cropland habitat types (wheat field, field margin) and three noncrop habitat types (fallow, grassland, wood margin) across eight landscapes along a gradient from simple to complex (quantified as % arable land). Both landscape and local factors affected the abundance of rape pollen beetles and parasitoids. Rape pollen beetle infestation and parasitism rates on these plants were lower in noncrop habitats and higher in wheat fields and field margins, whereas beetles and parasitoids responded differently to landscape scale parameters. We found the hypothesized spillover from OSR crop onto wild plants in surrounding habitats only for parasitoids, but not for pollen beetles. Parasitism rates were not related to landscape simplification, but benefited from increasing proportions of OSR. In contrast, rape pollen beetles benefited from simple landscape structures, presumably due to multi-annual population build-ups resulting from long-term OSR planting (as part of the crop rotation). In conclusion, we showed that spillover from cropland affects parasitism rates on related wild plants outside cropland, which has not been shown so far, but can be expected to be a widespread effect shaping noncrop food webs.     

Reef habitats and associated sessile-benthic and fish assemblages across a euphotic–mesophotic depth gradient in Isla Desecheo, Puerto Rico

Quantitative surveys of sessile benthos and fish populations associated with reef habitats across a 15–50 m depth gradient were performed by direct diver observations using rebreathers at Isla Desecheo, Puerto Rico. Statistically significant differences between depths were found for total live coral, total coral species, total benthic algae, total sponges and abiotic cover. Live coral cover was higher at the mid-shelf (20 m) and shelf-edge (25 m) stations, whereas benthic algae and sponges were the dominant sessile-benthic assemblage at mesophotic stations below 25 m. Marked shifts in the community structure of corals and benthic algae were observed across the depth gradient. A total of 119 diurnal, non-cryptic fish species were observed across the depth gradient, including 80 species distributed among 7,841 individuals counted within belt-transects. Fish species richness was positively correlated with live coral cover. However, the relationship between total fish abundance and live coral was weak. Abundance of several numerically dominant fish species varied independently from live coral cover and appeared to be more influenced by depth and/or habitat type. Statistically significant differences in the rank order of abundance of fish species at euphotic vs mesophotic stations were detected. A small assemblage of reef fishes that included the cherubfish, Centropyge argi, sunshine chromis, Chromis insolata, greenblotch parrotfish, Sparisoma atomarium, yellowcheek wrasse, Halichoeres cyanocephalus, sargassum triggerfish, Xanthichthys ringens, and the longsnout butterflyfish, Chaetodon aculeatus was most abundant or only present from stations deeper than 30 m, and thus appear to be indicator species of mesophotic habitats.     

Nutrient translocation in the xylem of poplar — diurnal variations and spatial distribution along the shoot axis

This investigation shows diurnal variations in the xylem sap composition of poplar (Populus tremula x P. alba). All major macronutrients reached a maximum concentration in the first half of the light period and decreased to the middle of the night. The relative abundance of the nutrients did not change during the day. The sap flow, which responded very fast to the environmental changes (2.2-fold increase within 10-20 min of illumination), reached a maximum value in the second half of the light period. Transpiration (and photosynthesis) was constant throughout the light phase. The calculated translocation rates displayed a maximum in the first half of the light period and therefore did not fit the time course of sap flow. During the night, translocation rates were 63-69% lower than the maximum. The regulation of nutrient translocation is discussed taking the active xylem loading into account. The axial distribution located the nitrate assimilation in younger leaves and storage of nitrate (and other macronutrients) in older leaves. Nitrate and potassium concentrations in the xylem sap did not change along the plant axis. However, the sap flow was greater in younger shoot sections than in older sections. We assume that the greater demand for nitrate in the younger shoot section was satisfied via an increased volume flow rather an increased nitrate concentration.     

The vascular epiphytes of a lowland forest in Panama—species composition and spatial structure

We conducted a comprehensive census of the vascular epiphytes in a lowland forest in Panama by means of a canopy crane. In 0.4 ha of ca. 40-m tall forest, 103 species of vascular epiphytes with 13,099 individuals were found. The orchids were the most important family both in species and individual numbers, accounting alone for >40% of all species and >50% of all individuals. There was a clear vertical segregation of species with a concentration at intermediate heights: more than 50% of all individuals were found between 15 m and 25 m above ground. Tree species identity, tree size and the position of a tree in the forest (“space”) all influenced species composition. However, none of the two environmental variables nor space alone explained more than 10% of the total variation in epiphyte assemblages in several canonical correspondence analyses. By far the largest proportion of the observed variation remained unexplained and is arguably due to mere chance. In the future, our results will be used as a baseline data-set for the direct observation of the long-term dynamics in a diverse epiphyte community.     

Litter in a first–order stream of a temperate deciduous forest (Margaraça Forest, central Portugal)

To evaluate the importance and fate of organic matter inputs in forested streams, we determined the litterfall inputs and the benthic coarse particulate organic matter (CPOM) in one headwater stream flowing through a mixed deciduous forest, during one year. Both vertical traps and the stream bottom were sampled monthly. The material collected was sorted into four main categories: leaves, fruits and flowers, twigs and debris. Litter production was 715 g m−2 y−1 and seasonal, with 73% of the annual total during October–December (autumn). Leaves comprised the largest litter component. Benthic organic matter was 1880 g m−2 y−1, and was also seasonal. Highest accumulation was attained in spring, and twigs and branches comprised the major component. This revised version was published online in July 2006 with corrections to the Cover Date.     

Factors affecting spatial and temporal distribution of Ostracoda assemblages in different macrophyte habitats of a shallow lake (Lake Fehér,Hungary)

Spatial and temporal distribution, abundance and habitat preferences of ostracod assemblages were studied in various habitats of Lake Fehér in Hungary. Of the 20 species Fabaeformiscandona hyalina and Cypridopsis hartwigi are new to Hungary. Significant spatial differences were recorded in the composition and abundance of the Ostracoda assemblages in the different habitats. The most diverse and abundant assemblages were found in the reed-belt, however, the mean abundance of the species differed in the different macrophyte communities of the reed-belt. The species richness and abundance were considerably lower in disturbed sites than in the other habitats. Most of the 20 species showed distinct habitat preferences and ecological tolerances. Especially the presence and coverage of the vegetation and the dissolved oxygen content influenced the distribution and abundance of the species.     

Self-organised spatial patterns and chaos in a ratio-dependent predator–prey system

Mechanisms and scenarios of pattern formation in predator–prey systems have been a focus of many studies recently as they are thought to mimic the processes of ecological patterning in real-world ecosystems. Considerable work has been done with regards to both Turing and non-Turing patterns where the latter often appears to be chaotic. In particular, spatiotemporal chaos remains a controversial issue as it can have important implications for population dynamics. Most of the results, however, were obtained in terms of ‘traditional’ predator–prey models where the per capita predation rate depends on the prey density only. A relatively new family of ratio-dependent predator–prey models remains less studied and still poorly understood, especially when space is taken into account explicitly, in spite of their apparent ecological relevance. In this paper, we consider spatiotemporal pattern formation in a ratio-dependent predator–prey system. We show that the system can develop patterns both inside and outside of the Turing parameter domain. Contrary to widespread opinion, we show that the interaction between two different type of instability, such as the Turing–Hopf bifurcation, does not necessarily lead to the onset of chaos; on the contrary, the emerging patterns remain stationary and almost regular. Spatiotemporal chaos can only be observed for parameters well inside the Turing–Hopf domain. We then investigate the relative importance of these two instability types on the onset of chaos and show that, in a ratio-dependent predator–prey system, the Hopf bifurcation is indeed essential for the onset of chaos whilst the Turing instability is not.     

Habitat structure, trophic structure and ecosystem function: interactive effects in a bromeliad–insect community

Although previous studies have shown that ecosystem functions are affected by either trophic structure or habitat structure, there has been little consideration of their combined effects. Such interactions may be particularly important in systems where habitat and trophic structure covary. I use the aquatic insects in bromeliads to examine the combined effects of trophic structure and habitat structure on a key ecosystem function: detrital processing. In Costa Rican bromeliads, trophic structure naturally covaries with both habitat complexity and habitat size, precluding any observational analysis of interactions between factors. I therefore designed mesocosms that allowed each factor to be manipulated separately. Increases in mesocosm complexity reduced predator (damselfly larva) efficiency, resulting in high detritivore abundances, indirectly increasing detrital processing rates. However, increased complexity also directly reduced the per capita foraging efficiency of the detritivores. Over short time periods, these trends effectively cancelled each other out in terms of detrital processing. Over longer time periods, more complex patterns emerged. Increases in mesocosm size also reduced both predator efficiency and detritivore efficiency, leading to no net effect on detrital processing. In many systems, ecosystem functions may be impacted by strong interactions between trophic structure and habitat structure, cautioning against examining either effect in isolation.     

Interactions between fish,grazing invertebrates and algae in a New Zealand stream: a trophic cascade mediated by fish-induced changes to grazer behaviour?

Experiments in laboratory stream channels compared the behaviour of Deleatidium mayfly nymphs in the absence of fish with that in the presence of either native common river galaxias (Galaxias vulgaris Stokell) or introduced brown trout (Salmo trutta L.). Galaxias present similar predation risks to prey during day and night but are more active at night. Whereas, trout present a higher predation risk during the day. Deleatidium maintained a fixed nocturnal drift periodicity that is characteristic of streams containing visually feeding fish regardless of the nature of the predation regime presented in the laboratory. However, the number on the substratum surface, and therefore able to graze algae, was lower when fish were present than when they were absent. The number was lower during the day in the presence of trout, when they present the highest predation risk, and lower during the night compared to the day in trials with galaxias when galaxias activity disturbs Deleatidium from the substratum. Increases in the probability of Deleatidium leaving a patch, reductions in the proportion of mayflies on high quality patches and reductions in the distance travelled from refuge also reflected variations in the predation regime. Similar differences in positioning were observed under the same predation regimes in in situ channels in the Shag River and these were associated with differences in algal biomass. Algal ash-free dry mass (AFDM) and chlorophyll a (chl a) were higher on the tops of cobbles when fish were present. Fish also affected the biomass and the distribution of algae on cobbles as AFDM and chl a were higher on the sides of cobbles from channels with trout compared to those with galaxias. Changes in grazing behaviour, caused by predator avoidance, are likely to have been responsible for differences in algal biomass because no significant differences were detected between treatments in the biomass of Deleatidium or of total invertebrates.     

?Microbialites in the Permian-Triassic boundary interval in central China: Structure, age and distribution

Summary A carbonate crust, with a principally digitate structure, caps latest Permian reef complexes in east Sichuan, China. The crust contains the conodontHindeodus parvus, confirming its age as earliest Triassic; it therefore closely postdates the end-Permian mass extinction, and is related to associated palaeoenvironmental change. Although its branches have lobate margins, and internal structures of radial fabrics and lobate fabrics in different specimens, an organic origin cannot be confirmed, because crust fabric is largely recrystallised. Therefore we apply the term ?microbialite to reflect uncertainty of its nature. The crust is co-eval (within theparvus Zone) with confirmed microbial biostromes and mounds in Guizhou Province, south of Sichuan. The sum of evidence, assembled by workers in several sites worldwide, indicates a sea-level rise occurred in the boundary interval, and this is corroborated by facies of the Sichuan crust. Abrupt appearance and disappearance of the crust, formed by precipitated carbonates, in east Sichuan, represents short-lived unusual post-extinction marine conditions which were switched abruptly on, then off. Microbial deposits overlying the P/T boundary in other locations in the Tethys Ocean (Iran and Japan) support the view that the unusual oceanic conditions had at least a regional distribution. Because the crust abruptly terminates, and is not succeeded by fossil-rich deposits, application of the disaster biota concept is inappropriate; an environmentally-driven control on carbonate precipitation is better supported by the evidence, whether or not it was biotically-mediated.