The natural exclusion of red deer from large boulder grazing refugia and the consequences for saxicolous bryophyte and lichen ecology

Large boulder grazing refugia permitted comparison of saxicolous bryophyte and lichen assemblages with those boulder tops accessible to red deer (Cervus elaphus) on a sporting estate in northwest Scotland. Plant succession was predicted to occur unchecked by grazing on the tops of these large boulders with cascading effects on bryophytes and lichens—assuming boulders had been in place over the same time period. Fifty pairs of boulders (one ≥2 m and the other accessible to red deer) were selected at random from various locations below north-facing crags. Percentage cover of each bryophyte and lichen species was estimated from three randomly placed quadrats on each boulder top. Due consideration was given to the influence of island biogeography theory in subsequent model simplification. Mean shrub cover and height, leaf-litter, bryophyte cover and bryophyte species richness were significantly higher within quadrats on large boulder tops that naturally excluded red deer. Lichen cover and lichen species richness were significantly higher on boulder tops accessible to red deer. Lichen cover was in a significant negative relationship with bryophyte cover, shrub cover and litter cover. Bryophyte cover showed a significant positive relationship with shrub height but there was an optimum shrub cover. Natural exclusion of red deer from the tops of large boulders has facilitated plant succession. The results suggest that grazing arrests the lithosere on boulder tops accessible to red deer at an early plagioclimax favouring saxicolous lichens. The results are relevant to situations where red deer might be excluded from boulder fields that hold lichen assemblages of conservation value.     

Restoring Intertidal Boulder‐Fields as Habitat for “Specialist” and “Generalist” Animals

Restoration is important in urban areas where habitat destruction is greatest. It incorporates many levels of intervention, with creation of new habitat the most extreme form. Most research on habitat creation has been terrestrial, or in marine habitats dominated by large structuring biota, such as mangroves. Intertidal boulder‐fields in urban areas are vulnerable to disturbances and habitat loss, which adversely affect numerous habitat specialists. This study describes experiments in which quarried stones were used to create new habitat outside natural boulder‐fields as a practical approach to restoring habitat. Colonization by specialist fauna and by common algae and invertebrates was measured for a year after deployment. Despite sessile assemblages on new boulders differing from those on natural boulders, common and rare animals rapidly colonized the new habitat. There was no clear succession, but colonization was variable and patchy at all scales examined, although diversities and abundances of some species in this novel habitat matched those of natural boulders within a few months. Rare and common animals generally colonized the new habitat as adults moving in from surrounding areas. Creating new boulder‐fields using quarried rocks is a successful approach to restoration and conservation of fauna where natural boulder‐fields are threatened.     

Impacts of the emerald ash borer (EAB) eradication and tree mortality: potential for a secondary spread of invasive plant species

Since the discovery of the emerald ash borer in 2002, eradication efforts have been implemented in an attempt to eliminate or contain the spread of this invasive beetle. The eradication protocol called for the removal of every ash tree within a 0.8 km radius around an infested tree. In 2005 this study was established to identify environmental changes attributed to the eradication program and measure subsequent shifts in forest community composition and structure. We conducted this study in Ohio and compared areas that received the eradication treatment (ash trees cut down), to areas that were left uncut, (ash still standing). The goal of this project was to identify how the plant community is responding in these two areas. The eradication protocol accelerated the formation and size of gaps within the forest and thus increased the duration and intensity of light penetrating through to the forest floor. In addition, the use of track vehicles for removal of cut trees resulted in significant soil compaction. The resultant plant community had greater species diversity (H′). When specific species composition differences were compared, an increase in the establishment of invasive plant species was detected in areas that received eradication efforts compared to those that did not. Invasive species accounted for 18.7% of the total herbaceous cover in this highly disturbed environment which included Cirsium arvense, Rhamnus cathartica and 2 species of Lonicera. In contrast, invasive species accounted for <1% of the total herbaceous cover in the undisturbed uncut areas.     

Seaweed succession on artificial reefs on different bottom substrata

Artificial rest reefs were set on sandy and rocky bottoms at 5–10 m depth along the coast of southern Japan. Mature thalli ofSargassum, Gelidium and other seaweds were transported from other coastal areas, packed in mesh bags and attached to the reefs to start the beds. After one year, the seaweed flora on the reef on a sandy bottom consisted of more than 20 species, includingSargassum spp. andGelidium amansii, which are important animal food species. Coralline algae were the dominants on the rocky bottom reefs. The lower biomass on reefs on the rocky bottom was due to grazing by urchins. The same number of species was present in the first and second years on reefs on sandy bottoms, but there were moreSargassum thalli the second year.Maximum algal biomass of the artificial reef in May of the second year was 9998 g wet wt m^–2 in sandy areas, 441 g wet wt m^–2 in boulder areas and 228 g wet wt m^–2 in rocky areas. Reefs on rocky bottoms continued to be covered by coralline algae and several species ofCodium andDictyota.     

Neither connectivity nor genetic diversity matter in the conservation of a rare fern and a moss on insular erratic boulders

Erratic boulders provide habitat for rock-dwelling species and contribute to the biodiversity of landscapes. In the calcareous Swiss lowlands, siliceous erratic boulders are exclusive habitat islands for the regionally critically endangered fern Asplenium septentrionale, about 20 bryophyte species and numerous lichens. Focusing on island biogeographical processes, we analysed the conservation genomics of A. septentrionale and the moss Hedwigia ciliata on insular erratic boulders in the Swiss lowlands and the adjacent “mainland” in siliceous mountains. We genotyped both species using double digest restriction associated DNA sequencing (ddRAD). For the tetraploid A. septentrionale, abundant identical multilocus genotypes within populations suggested prevalent intragametophytic selfing, and six out of eight boulder populations consisting of a single multilocus genotype each indicated single spore founder events. The genetic structure of A. septentrionale mainland populations coincided with Pleistocene glacial refugia. Four genetic lineages of H. ciliata were identified, and populations consisting of a single multilocus genotype were less common than in A. septentrionale. For both taxa, multilocus genotype diversity on boulders was lower than in mainland populations. The absence of common genetic groups among boulder populations, and the absence of isolation by distance patterns, suggested colonisation of boulders through independent long-distance dispersal events. Successful boulder colonisation of A. septentrionale seems to be rare, while colonisation by H. ciliata appears to be more frequent. We conclude that pivotal principles of conservation biology, such as connectivity and genetic diversity, are of less importance for the studied cryptogams on insular erratic boulders because of long-distance dispersal, intragametophytic selfing and polyploidy.

     

Estimating nitrogen budgets of Typha angustifolia by considering the regrowth shoot productivity and nitrogen content after harvesting aerial organs in different growing seasons

A dynamic model that includes regrowth after harvesting aerial shoots of an emergent macrophyte, Typha angustifolia L., was applied to evaluate the nitrogen (N) budget and the N uptake by the plant from sediment in Shibakawa Pond, Japan. Under natural conditions (control/uncut stands), the analysis showed that the annual uptake of N from sediment was 26.6 gN/m² and harvesting Typha shoots at their growth peak removed 29.0 gN/m² from the system (142 days in summer). Harvesting in winter after weathering of leaves removed only 13.9 gN/m². To evaluate the N budget considering regrowth shoot characteristics, three sets of harvesting experiments were done on 16 May, 8 July, and 5 August 2003. Our study revealed that May, July, and August harvesting removed 9.4, 21.9, and 16.3 gN/m², respectively. Further, combining the first harvesting from spring to summer and the second harvesting in autumn (before the start of senescence of regrowth shoots), the annual total N removals in stands cut in May and autumn and July and autumn were 34.7 and 36.0 gN/m², respectively—higher than that in stands cut in August and autumn (22.2 gN/m²) or that in uncut stands (13.9 gN/m²). At the same time, the amounts stored in rhizomes by stands cut in May and autumn, July and autumn, and August and autumn were 9.1, 8.4, and 4.4 gN/m², respectively, lower than that in uncut stands (18.8 gN/m²). Our results suggest that summer harvesting, especially in July to August, improves N removal efficiency and decreases the translocation of N from primary shoots to rhizomes, which is important for the sustainable management of Typha-dominated wetlands. Combined summer and autumn harvests further increase the removal efficiency but drastically reduce the storage of N. This might be useful when we need to control the plants properly.     

A preliminary investigation of diversity,abundance, and distributional patterns of chitons in intertidal boulder fields of differing rock type in South Australia

The rock type of hard substrata marine habitats can affect numerous benthic invertebrates, but little is known of the effects on molluscan assemblages, for example, the chitons often found under intertidal boulders. We compared chiton assemblage composition, abundance, species richness, and patterns of frequency distribution in 10 boulder fields containing either hard metamorphic/igneous boulders or soft limestone boulders in two geographical areas in South Australia. Similar species richness occurred in both types of boulder fields, but hard rock boulder fields had greater overall abundances, because of particularly large abundances of some common species. Differences in abundances of common species also resulted in significantly different assemblages occurring between the boulder field types. Some species appeared aggregated among boulders, but this pattern was variable between boulders in differing areas and of differing rock type. In one area, a common species had variable aggregation that caused frequency distributions to differ significantly between boulders of different rock types. These results indicate that rock type needs to be considered when investigating ecological patterns and processes involving specialist molluscs such as under-boulder chitons and to ensure comprehensive marine reserve planning for protecting rare invertebrates in rocky intertidal reefs.     

Experimental effects of kelp canopies on subtidal coralline algae

Plants are often grouped as canopy species or understorey species because it is thought that that these sets of taxa interact in predictable ways. Mensurative experiments in southern Australia demonstrated that the percentage cover of encrusting coralline algae was greater, and articulated (branching) coralline algae less, on boulders under a canopy of dense kelp (>7 plants per m²), Ecklonia radiata, than on boulders without kelp. Experimental clearances of kelp and reciprocal transplants of boulders between patches of E. radiata and patches without kelp showed that canopies maintained and facilitated the growth of encrusting coralline algae and reduced the cover of articulated coralline algae. Potential artefacts associated with clearing kelp and transplanting boulders were not detected when tested with a series of translocation controls. These results reject the model that the co‐occurrence of E. radiata and encrusting corallines is just an assemblage of plants caused by spatial and temporal coincidence. Instead, they support the model that kelp facilitates the growth and survival of understorey algae.     

Habitat preferences as patch selection of larval and emerging chironomids (Diptera) in a gravel brook

Four common species of larval and emerging chironomids were investigated during 1981 and between 1984/1985 in an alpine gravel brook. The emergence patterns ofN. dubius, O. rivulorum andH. ornaticollis were significantly affected by fluctuations of the water level, while rises in water temperature tended to trigger the peaks in emergence of the same species and ofO. frigidus. N. dubius exhibited its main emergence activity over areas of bed sediment which had a lower mean water velocity or were discontinuously flooded. On the other hand, the imagines ofO. frigidus, O. rivulorum andH. ornaticollis reached peak densities in zones of the main current channel in the Seebach. The larvae of these species inhabited horizontally and vertically different areas of bed sediment depending on the developmental stage. Early instars ofN. dubius andH. ornaticollis preferred zones near the banks and reached their maximum densities in sediment depths of 10 to 20 cm and 20 to 30 cm, respectively. In contrast, the larvae ofO. frigidus andO. rivulorum reached their highest densities in the first 10 cm of the sediments with significant preferences for the zones of stronger water flow.     

Diet,food preference,and algal availability for fishes and crabs on intertidal reef communities in southern California

Synopsis Herbivory by wide-ranging fishes is common over tropical reefs, but rare in temperate latitudes where the effects of herbivorous fishes are thought to be minimal. Along the west coast of North America, herbivory by fishes on nearshore reefs is largely restricted to a few members of the Kyphosidae, distributed south of Pt. Conception. This paper presents information on natural diets and results from feeding choice experiments for two abundant kyphosids from intertidal habitats in San Diego, California —Girella nigricans andHermosilla azurea, and similar data for the lined shore crab,Pachygrapsus crassipes, which also forages over intertidal reefs. These results are compared with the availability of algae in intertidal habitats measured during summer and winter, on both disturbed and undisturbed habitats. The diets of juveniles ofG. nigricans andH. azurea collected from nearshore habitats were dominated by animal prey (mainly amphipods), but adults of these fishes, andP. crassipes, consumed algae nearly exclusively, with 26, 10, and 14 taxa of algae identified fromG. nigricans, H. azurea, andP. crassipes, respectively. Algae with sheet-like morphologies (e.g.Ulva sp.,Enteromorpha sp., members of the Delesseriaceae) were the principal algae in the diets of the fishes, and calcareous algae (e.g.Corallina sp.,Lithothrix aspergillum) and sheet-like algae (Enteromorpha sp.) comprised the greatest identifiable portion of the shore crab's diet. Feeding choice experiments indicated that the fishes preferred filamentous algae (e.g.Centroceras clavulatum, Polysiphonia sp.,Chondria californica) and sheet-like algae (e.g.Enteromorpha sp.,Ulva sp.,Cryptopleura crispa) over other algal morphologies, whereas the shore crab chose jointed calcareous algae (e.g.Lithothrix aspergillum, Corallina vancouveriensis, Jania sp.) most frequently. The diets and preferences for algae by the fishes were generally most similar to the assemblage of algae available in early successional (disturbed) habitats during summer when sheet-like and filamentous algae are abundant. The shore crab exhibited the opposite trend with a diet more similar to late successional (undisturbed) habitats.     

Changes in the coral reefs of San Blas,Caribbean Panama: 1983 to 1990

Between 1983–1990 large changes in abundances of corals and macroalgae occurred on shallow (1–5m) lagoonal reefs in the San Blas Islands of Panama. In 1983 these reefs were dominated by the vertical plate forms of the coral generaAgaricia andMillepora. By 1990 we observed the following major changes: (1) loss of approximately one-half of the initial live coral cover, primarily during 1983–1986, and almost completely due to a decline in the abundance of Agaricia. Corals only occupied 12–26% of the reef area by 1990. (2) Macroalgae (mostlyDictyota andHalimeda) increased from 2% cover in 1983 to 28% cover in 1990. (3) Microalgal cover increased two to ten-fold between 1983 and 1986, then declined to 50% greater than the initial values by 1990. There are at least three contributors to these changes in the benthic community: (1) a coral bleaching event in 1983; which disproportionately affectedAgaricia; (2) the mass mortality ofDiadema antillarum in 1983, which led to decreases in grazing pressure on algae; and (3) possible increases in sediment and nutrient loads due to runoff from deforested mountainsides. Temporal patterns and observations of interactions suggest that the decrease inDiadema herbivory is a major factor in this shift in coral and algal populations.     

Spatio-temporal distribution patterns of the invasive macroalga Sargassum muticum within a Danish Sargassum-bed

Sargassum muticum was first observed in Scandinavia in Limfjorden (Denmark) in 1984, where it is now the most abundant and conspicuous macroalga. Despite the ecological importance of Sargassum, few studies have described seasonal patterns within Scandinavian Sargassum beds. We quantified the dynamics of macroalgae among years and seasons along a depth transect through a typical Sargassum bed in Limfjorden. The annual investigations (summer transects 1989–1999) showed a gradual increase in the dominance of Sargassum, especially at the 2–4-m depth interval. Significant seasonal dynamics in macroalgal abundance and assemblage structure were observed in this depth interval; the mean cover of Sargassum varied from ca. 5% (autumn and winter) to 25% (mid-summer). In comparison, encrusting algae had high and relatively stable covers throughout the year (ca. 20%). Other perennial macroalgae had low mean covers (<2%) characterized by a few patches of higher abundances. Except from a spring bloom, filamentous algae had low covers throughout the year. Within this relatively uniform bed, Sargassum abundance was positively related to boulders >10 cm in diameter and species richness was negatively correlated to depth and stones <10 cm in diameter, and non-correlated to other algal form-groups or grazer densities. Thus, in Limfjorden, the distribution of Sargassum is determined by large- (>6 m) and small-scale (<1 m) depth differences where low light limits Sargassum at depth, physical disturbance and sediment stress limits Sargasum in shallow waters, and the presence of stable boulder substratum facilitate Sargassum. Competition for space from other macroalgae and herbivory are probably of minor importance.     

Vegetation regeneration on blanket mire after mechanized peat-cutting

• 1 Blanket mire in Northern Ireland is an ecologically threatened habitat in which land use for hand peat‐cutting, forestry and agriculture has had a major influence. A recent land use change is the introduction of tractor‐powered peat‐harvesting. In this paper, the effect of machine peat‐cutting on ombrotrophic blanket mire vegetation is assessed from a regional sample of cut and uncut plots.
• 2 Principal components analysis identified water‐table depth and grazing intensity as major factors influencing the species composition of uncut mire. A key variable affecting the composition of machine‐cut mire across the drainage gradient was the number of times cut, with multiple annual cutting causing progressive decreases in acrotelm depth, catotelm bulk density and plant cover. Ericaceous species and Sphagnum spp. were particularly sensitive to cutting, with Eriophorum angustifolium and Campylopus introflexus characteristic of multiple‐cut sites.
• 3 Redundancy analysis, with number of times cut partialled out, showed that recovery time accounted for a significant amount of variance in vegetation composition. Species that significantly increased in abundance with recovery time were Sphagnum spp., Odontoschisma sphagni , Erica tetralix and Drosera rotundifolia.
• 4 Sites cut frequently, or which were grazed, recovered more slowly. Recovery from cutting was partly dependent on the post‐cutting structure of the mire surface and the species that survive cutting. The rate of recovery on sites cut once, then abandoned, is relatively rapid compared with multiple‐cut sites where species colonization is constrained by bare compacted peat.

     

Recent increase of filamentous algae in shallow Swedish bays: Effects on the community structure of epibenthic fauna and fish

A summary is presented of estimates of distribution and growth of filamentous algae and its effect on the structure and functioning of epibenthic fauna and fish communities in shallow bays on the Swedish west coast. As a consequence of coastal eutrophication vegetation cover has gradually increased during the last decade, and during 1990's most bays in the Skagerrak-Kattegat area were variously covered with filamentous algae during spring and summer (May–July). In some areas filamentous algae (mainlyCladophora andEnteromorpha) completely covered the bottom. In field studies it was demonstrated that increased cover and dominance of filamentous algae result in structural changes of the epibenthic fauna community. Field studies showed that species richness and biomass of epibenthic fauna increased in a sandy bay with a moderate increase (30 to 50%) of filamentous algae cover. At higher cover (90%), biomass of epibenthic fauna was reduced, however, to the same level as for the sandy habitat, although the dominant epibenthic species were different. Heavy growth of epiphytic filamentous algae on eelgrass resulted in reduced biomass and a shift in the species composition of the epibenthic fauna community. Fish assemblage structure was also related to changes in vegetation. In eelgrass beds, fish species numbers were reduced with increasing cover of epiphytic filamentous algae, and at rocky bottoms with kelp algae (dominated byFucus), fish biomass decreased with increasing cover of attached filamentous algae. Further, foraging efficiency of juvenile cod and settling success of plaice were reduced as a response to increasing dominance of filamentous algae.     

Microbial populations associated with the surface of the brown algaAscophyllum nodosum

The microorganisms on the surface of the brown algaAscophyllum nodosum, collected from an intertidal area in Nahant, Massachusetts, were examined using scanning electron microscopy. Differences in the microbial populations on the holdfast, internodal regions of the stipe, and the apical tips were apparent. The populations ranged from a lawn of end-attached bacteria above the holdfast to microcolonies of yeast cells near the apical tips. The greatest diversity of microorganisms was noted in the internodal region representing the fourth year of growth where a dense lawn of end-attached bacteria was overlaid by filamentous bacteria, pennate diatoms, and filamentous blue-green algae. A simple procedure was developed to estimate the number of bacteria on the surface of the seaweed using the scanning electron microscope. The observed distribution of epiphytes may be explained in terms of the age of the algal surface, differences in light intensity, and the differential secretion of tannin by various parts ofAscophyllum.     

Ecological impact of marine plant harvesting in the northwest Atlantic: a review

The ecological impact of marine plant harvesting is related to the intensity of exploitation, the harvesting technique, and the vulnerability of the species or habitat to perturbation. In eastern Canada information was available on four levels of impact: long-term changes in the target species and direct loss or damage to non-target species, direct or indirect impact on the habitat or community, indirect effects of changes in habitat or community structure, and trophic level impact. Near monoculture stands of Chondrus crispus have associated with them up to 36 animals species and 19 major species of algae that are vulnerable to removal as by-catch. Indirect effects of changes in macrophyte cover were not observed in fish species utilization of Ascophyllum nodosum beds on rising tides. Subtidal areas devoid of all macrophyte cover had lower levels of the preferred foods for Homarus americanus than kelp-covered areas; however, barren grounds are not created by macrophyte exploitation rates of 20% to 80% in eastern Canada. Long-term harvesting has altered the population structure and population ecology of C. crispus and A. nodosum in some areas. In general both target species and associated communities are resistant to perturbation.     

An appraisal of the fitness consequences of forest disturbance for wildlife using habitat selection theory

Isodar theory can help to unveil the fitness consequences of habitat disturbance for wildlife through an evaluation of adaptive habitat selection using patterns of animal abundance in adjacent habitats. By incorporating measures of disturbance intensity or variations in resource availability into fitness-density functions, we can evaluate the functional form of isodars expected under different disturbance-fitness relationships. Using this framework, we investigated how a gradient of forest harvesting disturbance and differences in resource availability influenced habitat quality for snowshoe hares (Lepus americanus) and red-backed voles (Myodes gapperi) using pairs of logged and uncut boreal forest. Isodars for both species had positive intercepts, indicating reductions to maximum potential fitness in logged stands. Habitat selection by hares depended on both conspecific density and differences in canopy cover between harvested and uncut stands. Fitness-density curves for hares in logged stands were predicted to shift from diverging to converging with those in uncut forest across a gradient of high to low disturbance intensity. Selection for uncut forests thus became less pronounced with increasing population size at low levels of logging disturbance. Voles responded to differences in moss cover between habitats which reflected moisture availability. Lower moss cover in harvested stands either reduced maximum potential fitness or increased the relative rate of decline in fitness with density. Differences in vole densities between harvested and uncut stands were predicted, however, to diminish as populations increased. Our findings underscore the importance of accounting for density-dependent behaviors when evaluating how changing habitat conditions influence animal distribution.     

Effects of interactions between algae and grazing gastropods on the structure of a low-shore intertidal algal community

Summary At low levels on shores in New South Wales, foliose algae are abundant and often occupy all substrata; microalgal grazing gastropods are rare or absent. At higher levels, foliose algae are sparse or absent and grazing gastropods are abundant. Hypotheses for the causes of the lower vertical limits of distribution of these grazers include the effects of increased predation or the deleterious physiological effects of increased period of submergence at lower levels on the shore. Alternatively, the presence of the algae, because they occupy space and deprive the grazers of substratum for feeding, may prevent the downward movement, or survival of the grazers at low levels. Under the first two of these hypotheses, algae are able to colonize and grow in low-shore areas as an indirect result of factors which remove grazers. Under the third hypothesis, the algae are directly responsible for the lack of grazers.Experimental clearings of the low-shore algae and introductions of the mid-shore limpets Cellana tramoserica and Siphonaria denticulata were used to test these hypotheses. C. tramoserica grazes microalgae and removes them from the substratum, preventing colonization. S. denticulata, in contrust, crops the algae, leaving a visible cover of algae on the substratum, which can grow rapidly. Because of its method of feeding, S. denticulata had no measurable impact on the rates of colonization, nor on the dry weights of algae, compared with those of ungrazed areas. C. tramoserica could keep cleared areas tree from foliose algae, but only when the limpets were mainfained in great density (10 per 900 cm²). They were less effective where wave-action was greater.Neither species of limpets could survive when placed onto beds of mature algae, because they had no substratum on which to cling and were swept away by the waves. C. tramoserica did not invade clearings below their lower limit of distribution where they had to move over a bed of foliose algae. Few C. tramoserica moved directly downshore into cleared areas. When placed on bare rock within low-shore beds of algae of different ages, S. denticulata remained amongst the algae and maintained their tissue-weights. Few C. tramoserica remained in areas with well-developed algae, compared with areas having sparse algal growth. Those Cellana which remained amongst well-developed algae lost weight, whereas limpets in areas with less algal growth mammtained their weights. In experimental cages in low-shore beds of algae, where the limpets were inaccessible to potential predators, C. tramoserica lost weight and died. On cleared areas they survived for many weeks, but lost weight and died as algae grew and covered the substratum. In the absence of predation, the micro-algal grazer C. tramoserica could not survive in lowshore areas because algae grew too fast and occupied the substratum, making it inaccessible for the limpets to graze; the algae, once grown beyond small sporelings, are not a suitable food-source for C. tramoserica, and the loss of weight and death of these limpets is attributable to starvation.The lower limit of distribution of C. tramoserica is not due to the direct effects of physical factors associated with prolonged submersion, nor to the impact of predators, but is apparently determined by the presence of rapidly growing, extensive beds of foliose algae at low levels on the shore. The cause of the limit of distribution of S. denticulata is not yet known and predation may prove to be important. Removal of S. denticulata from low-shore algal beds would not, however, affect the domination of substrata by algae. Grazing by S. denticulata at very great density had no effect on algal cover nor weight. In the intertidal community studied, the persistence of a low-shore algal zone, bounded above by abundant grazers is not influenced by the activities of predators, but is a direct result of interactions between the grazers and the algae.     

Modification of the physical environment by an <Emphasis Type="Italic">Ecklonia radiata</Emphasis> (Laminariales) canopy and implications for associated foliose algae

Macroalgal canopies modify their surrounding environment and thereby influence the structure of associated algal assemblages. Canopies can modify many factors that can be hard to separate and, consequently, the importance of individual factors often remains unknown. Experiments were carried out to test the hypotheses that Ecklonia radiata canopies modify light, sediment cover and water motion, and that each of these physical factors separately influence the assemblage of associated foliose algae. We measured light, sediment cover and water motion across six naturally occurring E. radiata densities and found a reduction in light and sediment cover as kelp density increased. The outcome for water motion was inconclusive. We also manipulated each of these three factors, while controlling for the two others, to determine the separate effects of light, sediment cover and water motion on the assemblage of foliose algae. Reduction in light had a strong effect on the foliose assemblage, reducing species richness and biomass. Reduction in sediment cover and water motion did not cause separate effects at the level of the assemblage, but the biomass of individual species of foliose algae indicated both positive and negative effects. We conclude that E. radiata canopies at Marmion, Western Australia, modify at least two factors of their physical environment, light and sediment cover. However, only light is modified to an extent where it has effects at the assemblage-level because, in contrast to the effects of sediment cover and water motion, the direction of responses are consistent among individual species of algae.     

Patterns of distribution and abundance of chitons of the genus Ischnochiton in intertidal boulder fields

Abstract Chitons of the genus Ischnochiton are found predominantly on the undersurfaces of boulders, compared with other intertidal or subtidal habitats. They therefore appear to be habitat‐specialists at this scale. This, combined with the fact that boulder fields are relatively sparse compared with other intertidal habitats, makes these animals vulnerable to natural and anthropogenic disturbances. In addition, many species of Ischnochiton are relatively rare and appear to have very patchy abundances, making them likely to have very specific requirements for habitat. We need to understand the habitat requirements in order to manage, conserve and restore disturbed habitats. The present study was carried out at three intertidal boulder fields separated along approximately 200 km of the coast of New South Wales, Australia, centred around Sydney. The boulder fields were representative of those found in this region. The boulders were made of different materials: shale in the north, sandstone in Sydney and quartzitic sandstone in the south. Some boulders in each boulder field were covered by up to 0.4 m of water during low tide. The study showed that the seven species examined were overdispersed among boulders in each of these three intertidal boulder fields. Most boulders did not have associated chitons, but there were very large abundances on a very small number of boulders. Chitons were also overdispersed among boulders that they occupied. These patterns were consistent among shores and among species, even though patterns of abundance were extremely different among different species. These species appear therefore to show specific requirements for habitat at a small spatial scale, using only a small proportion of potential patches of habitat (i.e. boulders) in any place. Extremely patchy patterns of dispersion can be caused by variation in patterns of recruitment, mortality or behavioural responses to habitat or other species. Before performing experiments to investigate such processes, it is useful to test hypotheses of association with habitat using mensurative experiments to identify environmental correlates that might explain the observed patterns. In the present study, sizes of boulders and the associated sessile and mobile assemblages were proposed as mechanisms that could affect dispersion of chitons among boulders. None of these factors, however, showed strong associations with abundances of chitons. The lack of support of these models rules out some features of habitat to which species of Ischnochiton might respond, thereby precluding manipulative experiments involving these features, which are unlikely to be involved in the very patchy patterns of dispersion of species of Ischnochiton.