Seasonal variation in epifaunal communities associated with giant kelp (Macrocystis pyrifera) at an upwelling‐dominated site

Kelp forests are highly productive and species‐rich benthic ecosystems in temperate regions that provide biogenic habitat for numerous associated species. Diverse epifaunal communities inhabit kelp sporophytes and are subject to variations in the physical environment and to changes experienced by the kelp habitat itself. We assessed seasonal variations in epifaunal invertebrate communities inhabiting giant kelps, Macrocystis pyrifera, and their effects on this seaweed. Six seasonal samplings were conducted over a year at an upwelling‐dominated site in northern‐central Chile where physical conditions are known to fluctuate temporally. More than 30 taxa were identified, among which peracarid crustaceans stood out in both diversity and abundance. Species richness and abundance differed among sporophyte sections (holdfast and fronds) and throughout the year. The frond community was dominated by two grazers (the amphipod Peramphithoe femorata and the isopod Amphoroidea typa), while suspension feeders, grazers, and omnivores (the amphipod Aora typica, the isopod Limnoria quadripunctata, and polychaetes) dominated the holdfasts. Abundances of the dominant species fluctuated throughout the year but patterns of variation differed among species. The most abundant grazer (P. femorata) had highest densities in summer, while the less abundant grazer (A. typa) reached its peak densities in winter. Interestingly, the area of kelp damaged by grazers was highest in autumn and early winter, suggesting that grazing impacts accumulate during periods of low kelp growth, which can thus be considered as ‘vestiges of herbivory past.’ Among the factors determining the observed seasonal patterns, strong variability of environmental conditions, reproductive cycles of associated fauna, and predation by fishes vary in importance. Our results suggest that during spring and early summer, bottom‐up processes shape the community structure of organisms inhabiting large perennial seaweeds, whereas during late summer and autumn, top‐down processes are more important.     

Selectivity and competitive interactions between two benthic invertebrate grazers (Asellus aquaticus and Potamopyrgus antipodarum): an experimental study using 13C- and 15N-labelled diatoms

1. Tracer experiments with two diatoms labelled with ¹³C (Nitzschia palea) and ¹⁵N (Fragilaria crotonensis), were conducted to investigate feeding selectivity and interspecific competition between the grazers Asellus aquaticus (Isopoda, Crustacea) and Potamopyrgus antipodarum (Hydrobiidae, Gastropoda). Conventional methods, such as cell counts and estimated biovolume, were used first to detect feeding preferences within the different grazer treatments. 2. The results revealed a significant decline in algal biovolume in all grazer treatments and no indications of active selectivity were observed. In contrast to conventional methods, measurements based on isotope signatures showed strong differences in tracer uptake, thus indicating different degrees of assimilation and digestion by the two grazers. 3. The selectivity index Q, which provides information on the uptake ratio of ¹³C to ¹⁵N, showed a significant time effect for both grazer species and a significant difference between single‐ and mixed‐grazer treatments for P. antipodarum. Thus, this technique enabled the direct quantification of the uptake by grazers and, therefore, served as an ideal tool for the detection of passive selectivity. 4. Our results indicate a shift in feeding preferences related to between‐species competition and a potential divergence of trophic niches when species coexist.     

Influence of sedimentation in the absence of macrograzers on recruitment of an annual population of Macrocystis pyrifera in Metri Bay,Chile

The effects of sedimentation and substrate orientation on algal and sessile invertebrate assemblages were tested on an annual population of Macrocystis pyrifera in Metri Bay, southern Chile. In the laboratory, M. pyrifera zoospores were seeded on Crepipatella fecunda shells, the primary substrate for M. pyrifera in this system. The seeded shells were deployed at Metri Bay inside cages and were orientated vertically and horizontally under two sedimentation regimes (bottom and suspended). Due to differences in grazer accessibility and the species present between the sedimentation treatments, grazers (>1 cm) were excluded. We followed sporophyte development of M. pyrifera and the natural recruitment of other algal and invertebrate species. Sedimentation rates were significantly higher in the cages attached to the bottom compared to suspended cages (P < 0.001). In total M. pyrifera and three additional algal genera were detected and all algal recruits showed significantly greater recruitment on the horizontally orientated substrate compared to the vertical substrate. Macrocystis pyrifera sporophytes were present only on the horizontal, suspended (less sedimentation) treatment. In contrast, Ulva and Ectocarpus spp. also occurred in the horizontal, high sediment treatment. Invertebrate recruitment (amphipods, barnacles and spirorbids) dominated the vertically oriented shells regardless of sedimentation. Results indicate that high sedimentation negatively affected the development of M. pyrifera sporophytes while other opportunistic species were able to recruit under these conditions.     

Increases of Chamber Height and Base Diameter Have Contrasting Effects on Grazing Rate of Two Cladoceran Species: Implications for Microcosm Studies

Aquatic microcosm studies often increase either chamber height or base diameter (to increase water volume) to test spatial ecology theories such as “scale” effects on ecological processes, but it is unclear whether the increase of chamber height or base diameter have the same effect on the processes, i.e., whether the effect of the shape of three-dimensional spaces is significant. We orthogonally manipulated chamber height and base diameter and determined swimming activity, average swimming velocity and grazing rates of the cladocerans Daphnia magna and Moina micrura (on two algae Scenedesmus quadricauda and Chlorella vulgaris; leading to four aquatic algae-cladoceran systems in total) under different microcosm conditions. Across all the four aquatic systems, increasing chamber height at a given base diameter significantly decreased the duration and velocity of horizontal swimming, and it tended to increase the duration but decrease the velocity of vertical swimming. These collectively led to decreases in both average swimming velocity and grazing rate of the cladocerans in the tall chambers (at a given base diameter), in accordance with the positive relationship between average swimming velocity and grazing rate. In contrast, an increase of base diameter at a given chamber height showed contrasting effects on the above parameters. Consistently, at a given chamber volume increasing ratio of chamber height to base diameter decreased the average swimming velocity and grazing rate across all the aquatic systems. In general, increasing chamber depth and base diameter may exert contrasting effects on zooplankton behavior and thus phytoplankton-zooplankton interactions. We suggest that spatial shape plays an important role in determining ecological process and thus should be considered in a theoretical framework of spatial ecology and also the physical setting of aquatic microcosm experiments.     

Niche partitioning between close relatives suggests trade‐offs between adaptation to local environments and competition

Niche partitioning among close relatives may reflect trade‐offs underlying species divergence and coexistence (e.g., between stress tolerance and competitive ability). We quantified the effects of habitat and congeneric species interactions on fitness for two closely related herbaceous plant species, Mimulus guttatus and Mimulus laciniatus, in three common habitat types within their sympatric range. Drought stress strongly reduced survival of M. guttatus in fast‐drying seeps occupied by M. laciniatus, suggesting that divergent habitat adaptation maintains this niche boundary. However, neither seedling performance nor congeneric competition explained the absence of M. laciniatus from shady streams where M. guttatus thrives. M. laciniatus may be excluded from this habitat by competition with other species in the community or mature M. guttatus. Species performance and competitive ability were similar in sympatric meadows where plant community stature and the growing season length are intermediate between seeps and streams. Stochastic effects (e.g., dispersal among habitats or temporal variation) may contribute to coexistence in this habitat. Habitat adaptation, species interactions, and stochastic mechanisms influence sympatric distributions for these recently diverged species.     

Large Intercontinental Differentiation of Moina Micrura (Crustacea: Anomopoda): One Less Cosmopolitan Cladoceran?

Moina micrura Kurz, 1874 (Crustacea: Anomopoda), commonly regarded as a cosmopolitan cladoceran species, can be found almost all over the world except for arctic and cold-temperate regions. M. micrura has been recorded from virtually all types of limnetic habitats, including temperate permanent ponds, lakes, ephemeral desert pools, and tropical brackish fishponds. Its high morphological and ecological variability has so far been considered a result of intraspecific diversity. We tested the hypothesis that genetically isolated groups exist within M. micrura with crossing experiment and DNA sequence analysis. We compared two clones originating from populations from Central Europe (area of the type locality) and Australia. As there was extremely low production of hybrid eggs, the results of the crossing experiment strongly suggest the existence of reproductive isolation. The sequence divergence of mitochondrial gene for 12S rRNA (7.9%) was comparable to intraspecific differences within Daphnia pulex group. The sequence divergence of the clones of Moina macrocopa from Europe and Central Africa was much lower (1.4%). Our results suggest that at least two sibling species exist within M. micrura complex, which is in agreement with other data questioning the cosmopolitanism of various cladoceran species.     

A taxonomic revision and species delimitation of the genus Purpuraria Enderlein, 1929 (Orthoptera: Pamphagidae) using an integrative approach

Recent studies on the endemic Canarian genus Purpuraria have shown that the taxonomy of its only recognized species (P. erna) is probably erroneous. In this study, an integrative revision of the genus is performed, based on a large number of specimens and geographical sampling. As a result, (1) the diagnostic characters at the genus level are re‐described, (2) Purpuraria magna n. sp. based on morphological, morphometric and genetic data is described and (3) the taxonomic status of a formerly described subspecies is clarified. Intraspecific and interspecific morphometric differences have been found, indicating that the genus is undergoing a process of morphological diversification. Nevertheless, the possibility of interspecific mating between individuals of the two species is suggested, because no significant differences have been found between their respective calling songs. Genetic analyses using mitochondrial and nuclear DNA sequences suggest that P. erna and P. magna are recent species with evidences of secondary contact episodes in the past.     

Grazer species effects on epilithon nutrient composition

1. Field and laboratory experiments were conducted to investigate the excretion stoichiometry of nitrogen (N) and phosphorus (P) of two benthic macroinvertebrate grazers, the crayfish Orconectes propinquus and the snail Elimia livescens, that differ in body stoichiometry (mean body molar N : P 18 and 28, respectively). Crayfish excretion had a significantly higher ammonium : soluble reactive phosphorus (SRP) ratio in the laboratory and in three natural streams than did snails, as predicted by ecological stoichiometry theory. 2. In greenhouse recirculating artificial streams, treatments consisting of crayfish, snails, or no grazers were used to examine responses in dissolved nutrient concentrations and epilithon nutrient composition and limitation. SRP concentrations depended upon the grazer species, with the snail treatment having a higher SRP concentration than other treatments (P < 0.05). Dissolved inorganic N was not affected by grazers, but appeared to be rapidly incorporated in epilithon. 3. Epilithon N content was dependent upon the grazer species present, with the crayfish treatment having a significantly higher N content than other treatments (P = 0.001). No grazer species effects on epilithon P content were found. However, both grazer treatments had significantly lower epilithon P content than the no‐grazer treatment. 4. Traditionally, studies have focused on how grazer‐induced structural changes to epilithon can alter epilithon nutrient dynamics, but this structural mechanism could not solely explain differences in epilithon nutrient contents and ratios in the present study. Our results rather suggest that benthic grazers can alter epilithon nutrient composition and limitation via nutrient excretion. Consequently, macroinvertebrate grazers may serve as ‘nutrient pumps’ that partly regulate the availability of nutrients to algae in stream ecosystems.     

Top‐down and bottom‐up control of periphyton by benthivorous fish and light supply in two streams

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The influence of moss on grazers in high‐altitude streams: food,refuge or both?

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Ecological genetics of sediment browsing behaviour in a planktonic crustacean

Zooplankton can display complex habitat selection behaviours that influence the way they interact with their environments. Some species, although primarily pelagic, can exploit sediment‐borne particles as a food source or use sediments as a refuge from pelagic predation. However, this strategy may increase the exposure to other risks such as benthic predation and infection from sediment‐borne parasite transmission stages. The evolution of habitat selection behaviour in these species is thus expected to be influenced by multiple and possibly contrasting selective forces. Here, we study the browsing behaviour of the water flea Daphnia magna on bottom sediments. First, we demonstrated genetic variation for sediment browsing among D. magna genotypes from natural populations sampled across a broad geographic range. Next, we used an F2 recombinant panel to perform a QTL analysis and identified three regions in the D. magna genome contributing to variation in browsing behaviour. We also analysed the correlation between our data and previously published data on the phototactic behaviour of genotypes from the same F2 panel. Clonal means of the two behavioral traits were not correlated, suggesting that they may evolve independently. Browsing behaviour is likely to be a relevant component of habitat selection in D. magna, and its study may help to incorporate the interactions with the sediment into eco‐evolutionary models of this key freshwater species.     

Consistency in a marine algal‐grazer interaction over multiple scales

Coralline algae are conspicuous members of many marine assemblages, especially those characterized by intense grazing pressure. We explored whether articulated species, especially Corallina vancouveriensis, depend on grazing invertebrates to both establish and flourish in an exposed rocky intertidal setting, and whether this plant–grazer relationship varied over more than three orders of magnitude (≈100–>300,000 μm). Three experimental manipulations, supplemented by observations on recruitment, demonstrated that (i) C. vancouveriensis failed to recover rapidly from disturbed areas when grazers were experimentally excluded; (ii) recruitment occurred in the presence of grazers; (iii) increasing surface texture of molded surfaces enhanced coralline recruitment more when grazers were present; and (iv) settlement occurred predominately in microtopographical low areas of a molded surface, whereas a competitively superior fleshy red alga tended to recruit to high areas. These results confirm that coralline algal establishment and persistence are enhanced by grazers and reveal that this relationship is consistent over a range of biologically relevant scales.     

COLONY SIZE OF PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE) AS INFLUENCED BY ZOOPLANKTON GRAZERS1

The haptophyte Phaeocystis antarctica G. Karst. is a dominant phytoplankton species in the Ross Sea, Antarctica, and exists as solitary cells and mucilaginous colonies that differ by several orders of magnitude in size. Recent studies with Phaeocystis globosa suggest that colony formation and enlargement are defense mechanisms against small grazers. To test if a similar grazer‐induced morphological response exists in P. antarctica, we conducted incubation experiments during the austral summer using natural P. antarctica and zooplankton assemblages. Dialysis bags that allowed exchange of dissolved chemicals were used to separate P. antarctica and zooplankton during incubations. Geometric mean colony size decreased by 35% in the control, but increased by 30% in the presence of grazers (even without physical contact) over the 15 d incubation. The estimated colonial‐to‐solitary cell carbon ratio was significantly higher in the grazing treatment. These results suggest that P. antarctica colonies would grow larger in the presence of indigenous zooplankton and skew the carbon partitioning significantly toward the colonial phase. While these observations show that the colony size of P. antarctica was affected by a chemical signal related to grazers, the detailed nature and ecological significance of this signal remain unknown.     

Contrasting trophic‐cascade effects driven by variation in morphology of the perches used by a larval damselfly

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Competition and facilitation within and between a snail and a mayfly larva and the effect on the grazing process

We studied the competitive effects within and between two taxonomically distant freshwater herbivores, a snail and a mayfly, common in Swedish lakes, Lymnaea peregra and Cloeon dipterum, respectively, and their effect on grazing in a laboratory experiment. The experimental set-up consisted of 2-l aquaria, each containing a periphyton covered tile. Intra- and interspecific effects were tested by increasing the density of one species at a time in four different treatments, (1) snails (intraspecific treatment), (2) mayflies (intraspecific treatment), (3) mixed-snails (interspecific treatments, snails kept constant) and (4) mixed-mayflies (interspecific treatments, mayflies kept constant). Intraspecific competition affected both snails and mayflies negatively, i.e. increasing mortality with increasing con-specific density. Furthermore, there was a decrease in snail growth with increasing snail density. In the mixed-species treatments both species changed their microhabitat use indicating interspecific competition. Despite this, we also found a positive effect of mayfly density on snail growth, most likely due to indirect commensalism. No density-dependent effect of grazing on periphyton was found, probably due to interference competition between grazers. However, there was a significant difference in periphyton biomass, due to species composition of grazers. Irrespective of their densities, if they co-existed, the two grazer species decreased the periphyton biomass significantly compared with both single-species treatments. We considered this as a joint action of facilitation and interaction. Our results suggest that competition can be an important structuring factor in macroinvertebrate communities and that species composition can be significant for ecosystem processes within lentic environments.     

Phylogeography,phylogeny, and taxonomical revision of the Midday jird (Meriones meridianus) species complex from Dzungaria

The Midday jird Meriones meridianussensu lato is a widely distributed and highly variable species complex, whose taxonomy is still controversial despite several genetic and morphological studies. Dzungaria is a crucial region for understanding the phylogeny and phylogeography of this species complex. In this work, the genetic variation of M. meridianus in Dzungaria and adjacent areas was studied based on mitochondrial (Cytb) and nuclear (BRCA1 and IRBP) genes. To compare morphological and genetic differentiation among lineages, craniological variation of the genotyped specimens was examined. Moreover, environmental data were used to estimate the most probable dispersal routes of M. meridianus clades in the studied area. Three clades of M. meridianus with interspecific level of differentiation were found and treated as distinct species: M. psammophilus (Milne-Edwards, 1871); M. meridianus (Pallas, 1773); and M. penicilliger (Heptner, 1933). The distribution ranges of all three species overlap in Dzungaria. According to the molecular data, M. psammophilus and M. meridianus are sister taxa that diverged ca. 500 kya, while M. penicilliger separated from the common ancestor of the other two lineages ca. 800 kya. These three species have significantly different cranial measurements. Moreover, traces of hybridization between M. psammophilus and M. meridianus were detected, but not between M. penicilliger and the other two species. Finally, the dispersal scenarios for M. psammophilus, M. meridianus, and M. penicilliger are herein discussed. It is suggested that M. meridianusand M. penicilligerindependently colonized Dzungaria from the west via Dzungarian gates.     

Evolutionary history of the scorpionfly Dicerapanorpa magna (Mecoptera,Panorpidae)

Climate changes can have fundamental impacts on the distributional patterns of montane species, and range shifts frequently lead to allopatric divergence followed by the establishment of secondary contact zones. Many European and North American organisms have retreated to southern refugia during glacial periods and colonized northward during postglacial periods, but little is known about the evolutionary response of cold‐adapted insects to Pleistocene climate changes in eastern Asia. The scorpionfly Dicerapanorpa magna (Chou), with cold temperate habitat preference and weak dispersal ability, provides a good model system to explore how climate changes have influenced the distribution and divergence of cold‐adapted insects in eastern Asia. This study reconstructed the demographic dynamics and evolutionary history of D. magna with phylogeographic approaches, and predicted the species’ suitable areas under the Last Glacial Maximum (LGM) and current scenarios with the ecological niche modelling analysis. The mitochondrial cytochrome c oxidase subunit I resolved three phylogenetic lineages in D. magna dating back to Pleistocene, corresponding well with the geographically isolated Qinling, Bashan and Minshan Mountains. The ecological niche modelling recovered the suitable habitats for D. magna were the Qinling and Bashan Mountains under LGM and current conditions. The three lineages of D. magna might be in a process of incipient speciation, and likely derived their current distribution from separate glacial origins, followed by vicariance and divergence.     

Mapping potential habitats of threatened plant species in a moist tall grassland using hyperspectral imagery

We examined the capability of hyperspectral imagery to map habitat types of under-storey plants in a moist tall grassland dominated by Phragmites australis and Miscanthus sacchariflorus, using hyperspectral remotely-sensed shoot densities of the two grasses. Our procedure (1) grouped the species using multivariate analysis and discriminated habitat types (species groups) based on P. australis and M. sacchariflorus shoot densities, (2) used estimated shoot densities from hyperspectral data to draw a habitat type map, and (3) analyzed the association of threatened species with habitat types. Our identification of four habitat types, using cluster analysis of the vegetation survey coverage data, was based on P. australis and M. sacchariflorus shoot density ratios and had an overall accuracy of 77.1% (kappa coefficient = 0.71). Linear regression models based on hyperspectral imagery band data had good accuracy in estimating P. australis and M. sacchariflorus shoot densities (adjusted R ² = 0.686 and 0.708, respectively). These results enabled us to map under-storey plant habitat types to an approximate prediction accuracy of 0.537. Among the eight threatened species we examined, four exhibited a significantly biased distribution among habitat types, indicating species-specific habitat use. These results suggest that this procedure can provide useful information on the status of potential habitats of threatened species.     

Detecting signatures of competition from observational data: a combined approach using DNA barcoding,diversity partitioning and checkerboards at small spatial scales

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Effects of gastropod grazers on recruitment and succession of an estuarine assemblage: a multivariate and univariate approach

Grazers have been shown to affect assemblages of species in many habitats. Here we studied the effects of the gastropod grazers, Austrocochlea porcata and Bembicium auratum, on intertidal estuarine assemblages in a sheltered bay in New South Wales, Australia. We examined the effects of gastropods on individual species and on the assemblage as a whole. The multivariate response was compared with data on succession in these assemblages to estimate potential effects of grazers on succession. The experiment was repeated several times to determine the generality of grazer effects in the light of possible variation in the timing or intensity of recruitment. There were different responses of individual species to the presence of grazers. Grazers reduced the abundance of ephemeral algal species, bryozoans, copepods, insect larvae and Balanus spp. barnacles. They had a positive effect on oysters and spirorbids and no effect on the barnacles Elminius covertus and Hexaminius spp. These effects were consistent through time. Multivariate analyses confirmed that grazers caused significant changes to whole assemblages and that these effects were far-reaching and not only caused by changes to algal species. The removal of grazers appeared to neither speed up nor slow down succession, but rather caused a completely different assemblage to develop. Apparent important mechanisms affecting the composition of animal species when grazers were removed included accumulation of sediments and detritus and pre-emption of space by algae. Received: 13 May 1996 / Accepted: 1 September 1996