Linking intraspecific variability in trophic and functional niches along an environmental gradient

1. Intraspecific trophic variability has important ecological and evolutionary implications, and is driven by multiple interacting factors. Functional traits and environmental conditions are important in mediating the trophic niche of individuals because they determine their ability to consume certain prey, their energetic requirements, and resource availability. In this study, we aimed at investigating the interacting effects of functional traits and environmental conditions on several attributes of trophic niche in natural populations.
2. Here, we quantified intraspecific variability in the trophic niche of 12 riverine populations of European minnow (Phoxinus phoxinus) using stable isotope analyses. Functional traits (i.e. morpho-anatomical traits) and environmental conditions (i.e. upstream–downstream gradient, forest cover) were quantified to identify the determinants of (1) trophic position and resource origin, (2) trophic niche size, and (3) trophic differentiation (β-diversity) among populations.
3. We demonstrated that trophic position and resource origin covaried with functional traits related to body size and locomotion performance, and that the strength and shape of these relationships varied according to local environmental conditions. The trophic niche size also differed among populations, although no determinant was identified. Finally, trophic β-diversity was correlated to environmental differentiation among sites.
4. Overall, the determinants of intraspecific variability in trophic niche appeared highly context-dependent, and related to the interactions between functional traits and environmental conditions. Because populations are currently facing important environmental changes, understanding this context-dependency is important for predicting food web structure and ecosystem dynamics in a changing world.

     

Temporal variability in fish assemblages from disturbed and undisturbed streams

Fish assemblages were sampled by electrofishingover a two- to ten-year period in undisturbedand anthropogenically disturbed South Carolinacoastal plain streams. Jaccard similarity,Bray–Curtis similarity, and Spearman rankcorrelations among samples collected from thesame sites over time were significantly greaterat undisturbed sites than at disturbed sites,suggesting greater fish assemblage persistenceand stability at the undisturbed sites. TheIndex of Biotic Integrity (IBI) also exhibitedsignificantly less variation over time atundisturbed sites than at disturbed sites.Physical habitat structure changed more overtime at disturbed sites than at undisturbedsites, and this variability was directlyrelated to temporal variability in fishassemblage structure. Comparisons betweenmultiple and single pass electrofishing samplessuggested that only a small proportion of thetemporal variability observed at the studysites was caused by inefficient sampling.Assessment of temporal variation in fishassemblage structure can serve as an indicatorof environmental disturbance and facilitate thedistinction of substantive ecological changefrom normal background variation.     

Hierarchy,spatial configuration,and nutrient cycling in a desert stream

Abstract Recent studies of nutrient cycling in Sycamore Creek in Arizona, USA, suggest that a thorough understanding requires a spatially explicit, hierarchical approach. Physical configuration determines the path that water follows as it moves downstream. Water follows flowpaths through surface stream components, the hyporheic zone beneath the surface stream, and the parafluvial (sand bar) zone. Characteristic biogeochemical processes in these subsystems alter nitrogen (N) species in transport, in part as a function of available concentrations of N species. At several hierarchical levels, substrate materials are an important determinant of nitrogen dynamics in desert streams. Sand is present in bars of variable size and shape, each of which can be considered a unit, interacting with the surface stream. Groups of these stream-sandbar units form a higher level, the reach. At the next higher scale, sandy reaches (runs) alternate with riffles. Where flowpaths converge, rates of N transformation are high and, as a result, change in concentration is a non-linear function of flowpath length. Disturbance by flash floods alters sandbar configuration. Between floods, the interaction of subsurface and surface flowpaths shapes configuration in each, thus a self-organizing element of spatial structure exists. Sandy runs are dominated by subsurface processes and are likely to be net nitrifiers while riffles are dominated by surface flow and are nitrogen fixers. Whether a stream ecosystem retains nitrogen, or transports it to downstream recipient systems, or is a net emitter of gaseous forms of N, depends upon the dynamics of a spatial mosaic of interacting elements. An understanding of the net effect of this mosaic requires a spatially explicit, hierarchical, multi-scale approach.     

Size-mass relationships of stream invertebrates in a northern Spain stream

We show the regression equations that relate the head width of 10 freshwater insect species or the length of the first thoracic segment of 2 amphipods with their dry mass in 3 reaches along a small stream in northern Spain. Spatial differences in size–mass relationships were found. Organisms stored in alcohol showed lower dry mass than those specimens that were dried immediately after collection. The use of both size measures for dry mass estimation in field samples is discussed. It is recommended that the size–mass relationships be constructed using non-preserved organisms. Furthermore, great care is required when such allometric equations are taken from the literature.     

Winter feeding: an adaptive strategy broadening the niche of the central mudminnow,Umbra limi

Synopsis Mudminnows, Umbra limi, were euryphagic carnivores during the ice-free period consuming invertebrates of terrestrial and aquatic origins. In winter, diversity and amount of food eaten were reduced. Age 0 fish consumed aquatic invertebrates as did males aged 1 and 2+ but females aged 1 and 2+ consumed mainly fishes. In the laboratory mudminnows were able to capture small fish at temperatures of 12.6° C and less. Their rates of digestion of fish prey varied directly with temperature and estimates of time for 100%, gastric evacuation ranged from 161 h at 1.1° C to 26 h at 12.6° C. The ability to forage actively during winter and to digest food relatively rapidly at cold temperatures are seen as mechanisms to broaden the niche along dimensions of resources utilized and time, thereby increasing the chances of survival and reproduction in an environment with high temporal heterogeneity.     

Changes in chironomid assemblage composition in two Mediterranean mountain streams over a period of extreme hydrological conditions

An opportunity to test Coffman's (1989) proposal that ecological heterogeneity is one of the main factors for chironomid species richness occurred when prolonged drought in southern Spain was broken by heavy rain. Chironomid assemblage composition was studied in two Mediterranean mountain first-order streams, one permanent and the other temporary. Samples were collected in the permanent stream over extreme hydrological conditions: 1991–95 with relatively low and stable flow, and 1996–97 with relatively high and unstable flow. The temporary stream flowed, and therefore was sampled, only during the second period. In the permanent stream, spate events resulted in a notable increase in species richness due to the arrival of 32 species and to the permanence of 81% of the species resident over the prespate period. This relatively high percentage suggests that chironomids, as a group, have important resilience properties. However, some species disappeared from the permanent stream after the spates and the considerable decrease in abundance of other species, shows that, at the species level, resilience may vary greatly within the Chironomidae. Most of the species that appeared in the temporary stream were the same as those of the permanent stream during the fast flow period despite the great ecological differences between the two streams under study, suggesting the key role of discharge in structuring chironomid assemblages. The outstanding exception was the fugitive species Orthocladius calvus, the most abundant species in the temporary stream during the initial successional stages but never collected in the permanent stream. This revised version was published online in July 2006 with corrections to the Cover Date.     

Composition,Microdistribution and Food of the Macroinvertebrate Fauna Inhabiting Wood in Low‐Order Mountain Streams in Central Europe

The macroinvertebrate community inhabiting woody debris in low‐order mountain streams was investigated using hand collections and emergence traps. With respect to dry mass, Amphipoda, Plecoptera and Diptera are the most important taxa. From the gut content analyses it was found that the microhabitat is colonized by xylophagous species as well as taxa of other feeding types. Regarding the whole community, there is no correlation between the abundance of specimens and density of wood, tree species, bark cover, consistency class, and surface structure. However, single species do show preferences; surface structure is the most important factor determining community composition.     

Female‐biased size dimorphism in a diapausing caddisfly,Mesophylax aspersus: effect of fecundity and natural and sexual selection

1. The effect of mating success, female fecundity and survival probability associated with intra‐sex variation in body size was studied in Mesophylax aspersus, a caddisfly species with female‐biased sexual size dimorphism, which inhabits temporary streams and aestivates in caves. Adults of this species do not feed and females have to mature eggs during aestivation. 2. Thus, females of larger size should have a fitness advantage because they can harbour more energy reserves that could influence fecundity and probability of survival until reproduction. In contrast, males of smaller size might have competitive advantages over others in mating success. 3. These hypotheses were tested by comparing the sex ratio and body size of individuals captured before and after the aestivation period. The associations between body size and female fecundity, and between mating success and body size of males, were explored under laboratory conditions. 4. During the aestivation period, the sex ratio changed from 1 : 1 to male biased (4 : 1), and a directional selection on body size was detected for females but not for males. Moreover, larger clutches were laid by females of larger size. Finally, differences in mating success between small and large males were not detected. These results suggest that natural selection (i.e. the differential mortality of females associated with body size) together with possible fecundity advantages, are important factors responsible of the sexual size dimorphism of M. aspersus. 5. These results highlight the importance of taking into account mechanisms other than those traditionally used to explain sexual dimorphism. Natural selection acting on sources of variation, such as survival, may be as important as fecundity and sexual selection in driving the evolution of sexual size dimorphism.     

The cooler the better: Increased aquatic hyphomycete diversity in subtropical streams along a neotropical latitudinal gradient

Aquatic hyphomycetes are microbial decomposers in freshwater environments that, together with detritivores, play an essential role in the functioning of low-order streams. Here, we evaluated aquatic hyphomycetes communities associated with decomposing leaves of Nectandra megapotamica, a common Neotropical riparian tree, along a subtropical-tropical latitudinal gradient. Two forest streams located in subtropical regions and 3 in tropical regions were selected. We identified 29 species of aquatic hyphomycetes, 22 (75.8%) in subtropical streams and 15 (51.7%) in tropical streams. We also found a higher fungal biomass in subtropical streams. However, the amounts of leaf mass loss did not differ between regions, but the values were higher in summer than in winter. High temperature, pH and electrical conductivity values, as well as low dissolved oxygen levels, negatively affected spore production. These results suggest that the subtropical-tropical gradient is an important predictor of aquatic hyphomycete diversity; however, the observed species had different sensitivities to local environmental factors.     

Effects of episodic acidification on macroinvertebrate assemblages in Swiss Alpine streams

1. Despite long‐standing ecotoxicological evidence that episodes of acidification in streams are important biologically, there is still uncertainty about their effects on invertebrate communities. We surveyed 20 streams in an acid sensitive Alpine area (Canton Ticino, Switzerland), where episodes are driven by snowmelt in spring and by rainstorms at other times of the year. Samples of water and macroinvertebrates were collected in pre‐event conditions (winter and summer) and during periods of high flow (spring and autumn). 2. Using pH, [Ca²⁺] and [Alⁿ⁺], streams were clustered into six acid–base groups that were either well buffered (groups 4–6), soft‐water with stable pH (group 3), or poorly buffered with low pH at high flow (groups 1 and 2). 3. Severe episodes occurred during snowmelt, when the group 1 streams became acidic with pH down to 5.0 and [Alⁿ⁺] up to 140 μg L^?1. pH declined to 6.2 in streams of group 2, but remained > 6.6 in groups 3–6. 4. Detrended canonical correspondence analysis showed that the streams sensitive to episodes (groups 1 and 2) had different invertebrate assemblages from well‐buffered sites (groups 4 and 5) or soft‐water stable streams (group 3), with faunal differences largest following spring snowmelt. Empididae, Isoperla rivulorum, Rhithrogena spp. and Baetis spp. were scarce in streams sensitive to episodes (groups 1 and 2). By contrast, Amphinemura sulcicollis was scarcer in hard‐water streams (groups 4–6). Taxonomic richness was lower in the episodic streams of group 1 than in other streams. 5. Together, these results indicate clear biological differences between acid‐sensitive streams with similar low‐flow chemistry but contrasting episode chemistry. Severe episodes of acidification appear to affect macroinvertebrate assemblages in streams in the southern Swiss Alps.