Large-scale coexistence and small-scale segregation of key species on rocky shores

Biodiversity is defined by the scales at which organisms coexist. Coexistence at larger spatial scales may be underpinned by segregation at smaller spatial scales or temporal variability in behaviour and/or distribution. Limpets of the genus Patella are keystone grazers controlling the growth of macro-algae on many rocky shores. This study examined the distribution and abundance of P. vulgata and P. ulyssiponensis in relation to habitat and to each other at a range of spatial scales. Limpets were sampled on four shores on each of the east and west coasts of Ireland, in two different transects (10 m apart) at each of three shore heights in winter 2003 and summer 2004. P. vulgata and P. ulyssiponensis coexist around the coast of Ireland, but on shores where they co-occur, the pattern of coexistence begins to break down at the level of shore height: P. ulyssiponensis were more abundant on the low-shore than mid-shore, and P. vulgata were distributed evenly throughout the low and the mid-shore. At the level of habitat (pools vs. open rock), the two species exhibited a high degree of segregation: P. vulgata of all sizes were more common on open rock and P. ulyssiponensis of all sizes were more common in pools. The presence or absence of P. ulyssiponensis had no effect on the distribution of juvenile P. vulgata with respect to pools. Unexpectedly, however, a greater proportion of adult P. vulgata were found in pools at sites where P. ulyssiponensis were present in the pools than at sites where they were absent. The results of this study emphasize the need for multiple-scale analyses of the distribution and abundance of organisms to understand the organisation of natural assemblages. Handling editor: K. Martens     

Enhancement of habitat heterogeneity and species richness on rocky shores inundated by sand

Summary Many rocky shores are subject to periodic inundation by sand, which is often thought to reduce species richness by eliminating organisms intolerant of sand scour or sand smothering. However, regular disturbance (e.g. inundation) should promote richness by preventing the development of low diversity climax communities. A study of faunal richness on 10 regularly inundated shores showed that inundation does promote richness, but by increasing habitat heterogeneity. Some species are excluded from parts of the shore by sand, but because of the patchiness of sand deposits they are rarely excluded from the entire shore. Other species are found only on rocks associated with sand, while typically sandy shore animals occur in the sand deposits themselves. Total richness (281 species) was greater than for local noninundated shores and sandy beaches combined.     

Predicting impacts of climate-induced range expansion: an experimental framework and a test involving key grazers on temperate rocky shores

Climate change has strong potential to modify the structure and functioning of ecosystems, but experimental field studies into its effects are rare. On rocky shores, grazing limpets strongly affect ecosystem structure and their distribution in NW Europe is changing in response to climate change. Three limpet species co-occur in SW Britain ( Patella vulgata, Patella ulyssiponensis and Patella depressa ) on open rock and in pools. Shores in Ireland are similar, but currently lack P. depressa . It is anticipated that P. depressa will expand its range into Ireland as the climate warms, but we currently lack an empirical basis to predict the consequences of this change. Recent studies show that increasing abundance of P. depressa on British shores has been accompanied by a decline of P. vulgata suggesting interspecific competition. In this study, a new experimental framework was used to examine the potential for P. depressa to affect P. vulgata on Irish shores. P. vulgata was experimentally transplanted into enclosures on open rock and in pools in both Ireland and Britain. In pools, treatments also included transplanted P. ulyssiponensis to mimic natural assemblages. Growth and mortality of P. vulgata were measured over 6 months with no differences between Ireland and Britain. In Britain, P. vulgata caged in pools with transplanted P. depressa and P. ulyssiponensis showed reduced growth, compared with when caged in pools with P. ulyssiponensis alone. There was no effect of P. depressa on the growth rate of P. vulgata on open rock. Results indicate that if the range of P. depressa extends into Ireland, it would reduce the growth of P. vulgata where it co-occurs with P. ulyssiponensis in pools. The framework used here provides a field-based approach that could be used to examine the impacts of climate-induced range expansions on the structure and functioning of other ecosystems.     

Ecology of marine algae on rocky shores and subtidal reefs in temperate Australia

Experiments in intertidal and subtidal rocky marine habitats in temperate Australia have identified the effects of various biological and physical factors on algal assemblages. In intertidal habitats, these involve micro- and macro-algae and grazing by gastropods. In subtidal habitats, interactions among micro- and macro-algae, echinoids, gastropods, micro-invertebrates and sessile invertebrates have been studied. Experimental studies on physical disturbances of algal assemblages have focussed on the effects of desiccation and storms. Most studies have not considered more than one spatial or temporal scale. Few have been concerned with seasonal influences and fewer have been concerned with variation from year to year. Most of the work lacks applicability to biogeographic comparisons. More experimental work across a variety of spatial and temporal scales is required to determine significant biological and physical processes affecting structure of algal assemblages across broad areas of temperate Australia.     

Plant species coexistence at local scale in temperate swamp forest: test of habitat heterogeneity hypothesis

It has been suggested that a heterogeneous environment enhances species richness and allows for the coexistence of species. However, there is increasing evidence that environmental heterogeneity can have no effect or even a negative effect on plant species richness and plant coexistence at a local scale. We examined whether plant species richness increases with local heterogeneity in the water table depth, microtopography, pH and light availability in a swamp forest community at three local spatial scales (grain: 0.6, 1.2 and 11.4 m). We also used the variance partitioning approach to assess the relative contributions of niche-based and other spatial processes to species occurrence. We found that heterogeneity in microtopography and light availability positively correlated with species richness, in accordance with the habitat heterogeneity hypothesis. However, we recorded different heterogeneity-diversity relationships for particular functional species groups. An increase in the richness of bryophytes and woody plant species was generally related to habitat heterogeneity at all measured spatial scales, whereas a low impact on herbaceous species richness was recorded only at the 11.4 m scale. The distribution of herbaceous plants was primarily explained by other spatial processes, such as dispersal, in contrast to the occurrence of bryophytes, which was better explained by environmental factors. Our results suggest that both niche-based and other spatial processes are important determinants of the plant composition and species turnover at local spatial scales in swamp forests.     

The relative importance of habitat complexity and surface area in assessing biodiversity: Fractal application on rocky shores

Theoretical work predicts that complex habitats allow more species to co-exist in a given area. However, more field studies are still needed to clarify this relationship, especially in intertidal habitats. Furthermore, the potential separate effects of surface complexity and area on species richness and abundance have rarely been addressed. We tested the hypotheses that a more complex substratum or larger surface area will support a greater number of individuals and species of mobile macrofauna on three rocky shores in Hong Kong. Surface complexity, assessed by using fractals, was an important factor in species–area relationships. The number of species increased proportionally to habitat complexity and this relationship was homogeneous among different shores. Total abundance of animals, however, was more dependent on the available surface area. The slope of the size–frequency distribution of animals in samples taken on surfaces with different fractal dimensions (D) was significantly steeper with an increase in fractal dimension, showing that the relative abundance of small animals increased with surface complexity. Thus, surface complexity and area may be important in determining different aspects of the macrofaunal community structure on rocky shores. The resulting increase in surface area on more rough surfaces may introduce bias in density and species number assessments when two-dimensional sampling units (i.e., quadrats) are employed. It is necessary, therefore, to account for the surface complexity in the design and interpretation of the results of benthic studies. Using D as an index of surface complexity is very useful, but also involves some practical problems, e.g., surfaces may be anisotropic and different methods may give different estimates of D. Therefore, these different methods need to be calibrated before comparisons of D values between them are meaningful.     

Temporally explicit habitat ecology and the coexistence of species

Habitats may have dynamics that exist independently of the population densities of species occupying the habitat. For example, ephemeral habitat patches may disappear regardless of whether a particular species is present or not. Such habitat dynamics are frequently modelled by ignoring age-related variation in patch turnover rates. This can be thought of as a temporally implicit approach. An alternative, temporally explicit approach involves using age-structured models in order to describe variations in habitat dynamics. Simple models of coexistence between competing species show that temporally implicit models may be misleading where there is age-related variation in patch dynamics. Changing the shape of the patch survivorship function but not the average patch survivorship can result in mutual extinction, monocultures or coexistence of an inferior and a superior competitor. An explicit treatment of habitat demography may therefore offer improved predictive models and alternative landscape management strategies.     

The dynamics of coexistence: habitat sharing versus segregation patterns among three sympatric montane vipers

Contact zones of closely related and ecologically similar species constitute rare opportunities to study the evolutionary consequences of past speciation processes. They represent natural laboratories in which strong competition could lead to the exclusion of one species, or the various species may switch into distinct ecological niches. Alternatively, if reproductive isolation has not yet been achieved, they may hybridize. We elucidate the degree of taxon integrity by comparing genetics and habitat use of three similar‐sized congeneric viper species, Vipera ammodytes, Vipera aspis, and Vipera berus, of Nadiza Valley in western Slovenia. No hybridization was detected for either mitochondrial or nuclear genomes. Similarly, external intermediacy by a single prestudy viper (probably V. ammodytes × V. aspis) indicates that hybridization occasionally occurs, but should be very rare. Populations of the three related viperids are partially allopatric in Nadiza Valley, but they also coexist in a narrow contact zone in the montane grassland along the south‐exposed slope of Mount Stol (1673 m a.s.l.). Here, the three species that occupy areas in or near patches of rocky microhabitats (e.g. stone piles, slides, and walls) live in syntopy. However, fine‐scale measurements of structural components show partial habitat segregation, in which V. berus becomes more dominant at elevations above 1400 m and occupies mostly the mountain ridge and north‐exposed slopes of Mount Stol, V. aspis occurs below 1300 m and is the only species to inhabit stoneless patches of grass and bushes around 1000 m and lower, and V. ammodytes occurs at all elevations up to 1500 m, but is restricted to a rocky microhabitat. We suggest that a high degree of microstructure divergence, slightly different environmental niches, and a generally favourable habitat for all three viper species, keep the pressure for mis‐mating and hybridization low, although mechanisms such as reduced hybrid inferiority and temporal mating segregation cannot yet be excluded.     

Intertidal distribution and species composition of brachyuran crabs at two rocky shores in Central Portugal

The objectives of the present study are to describe and compare the brachyuran community of rocky shores within the Central Portuguese coast and to examine the zonation patterns of the most representative species. For this, randomly placed transects were surveyed to obtain crab counts according to microhabitat and intertidal level. Repeated sampling in two different shores during two different seasons provided spatial and temporal replication for zonation analyses. Seven species were registered: Pachygrapsus marmoratus, Eriphia verrucosa, Xantho incisus, Carcinus maenas, Necora puber, Pirimela denticulata and Pilumnus hirtellus. Species density rankings are the same at both localities, but the less exposed shore presents higher diversity. While most species are mainly confined to specific microhabitats in the lower level, P. marmoratus and E. verrucosa can exploit the whole intertidal range. Regardless of shore and season, E. verrucosa is more abundant in the lower intertidal levels, while no such zonation patterns were recorded for P. marmoratus. Initial predictions concerning the effect of wave exposure and temperature on the zonation of those species are not validated after analysing the factorial model proposed. Between-shore contrasts were found instead, with higher densities recorded in the more exposed locality for both species. Possible causes of the observed patterns are discussed.     

Mussel attachment on rocky shores: the effect of flow on byssus production

Mussels rely on a strong byssal attachment to persist in a range of habitats with differing rates of water flow. Recent studies, however, suggest that the ability of one mussel species to sense and respond adaptively to the flow in its environment is limited under even modest flow conditions because the process of byssal thread formation is disrupted. This study extends these findings to four mussel species, Mytilus trossulus, M. galloprovincialis, M. californianus, and Modiolus modiolus. Collectively, the response of byssal thread formation decreased with rates of flow above ～25 cm/s and the critical flow threshold was estimated to be <50 cm/s. How can mussels persist on shores where flow is an order of magnitude higher? Using a combination of techniques for measuring flow, velocity profiles were obtained above and within mussel aggregations in the laboratory and in the field. Flow was greatly reduced within mussel aggregations, ranging from 0.1% to 10% of free-stream velocity. These results suggest one key to the success of mussels in habitats with high rates of flow is the ability to form aggregations that ameliorate flows to a level that is conducive to byssal thread formation.     

Nesting habitat segregation between closely related terricolous sphecid species (Hymenoptera:Spheciformes): key role of soil physical characteristics

Microhabitat selection based on abiotic factors was examined in five species of digger wasps (Sphex funerarius, Ammophila heydeni, Ammophila pubescens, Ammophila sabulosa, Bembix tarsata), four of which are of high conservation interest. All the study sites were located in areas affected by anthropogenic disturbance (reclaimed brown-coal mining areas, and railway dykes) in Central Europe. The individual species responded differentially to the variability in each of the physical characteristics measured, which resulted in site-specific exclusion of individual sphecid species. The sphecids were found to discriminate between available microhabitats based on ground inclination (slope) and vegetation cover. All but A. heydeni segregated the microhabitats according to the content of gravel and sand particles. S. funerarius and B. tarsata were limited by their preferences of low soil compactness, whereas A. pubescens utilized highly compacted soils, which were predominant in the study area. Soil shear strength (cohesiveness) was driving the level of gregariousness of the sphecids. In B. tarsata, number of cells per burrow was found to be higher in soils with increased penetration resistance. Experimental increase in soil compactness led to the abandonment of nesting microhabitats in the subsequent nesting seasons. Besides the well-known biotic factors, such as specialization at different size or type of prey, variability in abiotic factors between individual microhabitats (e.g. patches of sand) was found to be both necessary and sufficient to allow the parallel presence of viable populations of several sphecid species.     

Effects of height on the shore and complexity of habitat on abundances of amphipods on rocky shores in New South Wales, Australia

Patterns of abundance of epifaunal crustaceans were compared between two common brown algae in intertidal rock pools on a shore near Sydney, New South Wales, Australia. Amphipods were significantly more abundant on Sargassum globulariaefolium (which was most common low on the shore) than on Hormosira banksii (more common at mid-shore levels). Experiments tested the hypotheses that height on the shore and structural complexity of the algae were the main factors influencing patterns of abundance of amphipods. Reciprocal transplants using defaunated plants and similar experiments using artificial plants allowed simultaneous tests of these hypotheses. The abundance of a common intertidal amphipod, Sunampithoe graxon was significantly smaller on Sargassum transferred from low to mid-tidal level of the shore than on any other treatments, indicating that height on the shore affects numbers of this species. There were no Sunampithoe on Hormosira even when transplanted to low shore areas, indicating that other factors, apart from the height on the shore, are also important for this amphipod. Another common amphipod, Hyale maroubrae was generally found in greatest abundances on Hormosira transplanted from mid- to low shore and on Sargassum low on the shore, again indicating the importance of height on the shore. There was no difference in abundance of amphipods on ‘complex’ and ‘simple’ artificial plants indicating that structural complexity (as defined in this study) did not influence these amphipods.     

Effect of competition on habitat utilization in two temperate climate gecko species

Competition over spatial niche utilisation is one of most common competitive interactions between species in sympatry. Moreover, competitive interactions may involve age classes, and can fluctuate temporally. Consequently, evasive strategies that enable co-existence are likely to be important in the evolution of species assemblages. Here we investigate a system of two co-existing species of temperate geckos with similar ecologies (the house gecko, Hemidactylus turcicus and the wall gecko, Tarentola mauritanica), providing an opportunity to study the effect of species interactions. Juveniles and adults of both species were investigated throughout their daily and annual cycle to explore the effect of inter- and intra-specific interactions on microhabitat use. The two species showed differences in habitat use for both age classes in sympatry. In sympatry, T. mauritanica uses more open habitats and is more active. In contrast, H. turcicus is found in more closed habitats, closer to the ground and to vegetation cover. In allopatry, H. turcicus was observed in more open habitats, closer to the ground, and to vegetation cover, when compared to the population in sympatry with T. mauritanica. These differences in habitat usage were significant for both age classes. Moreover, there were differences, both in sympatry and in allopatry, between age classes that were dependent on season. In conclusion, the presence of a competitor induces a spatial shift in individuals of both age classes of H. turcicus. Observed plasticity in habitat utilisation in both age classes of H. turcicus is used to argue for the invasive potential of this species.     

Abundance and habitat segregation in Mediterranean grassland species: the importance of seed weight

Abstract. We analysed the relationship between seed traits (weight, shape and dispersal structures) and the abundance and habitat segregation of Mediterranean grassland species. To take into account possible correlations with other plant traits, the study also includes 5 vegetative traits (growth form, plant longevity, clonality, onset of flowering and plant size) of commonly accepted functional importance. Data were recorded for 85 species from dehesa grasslands in central Spain. Species abundance was measured in upper (dry and less productive, high stress) and lower (moist and more productive, low stress) slope zones in the same area. Habitat segregation was estimated using an index based on the relative frequencies of species in upper and lower slope zones. Multiple regression models were fitted using species, as well as phylogenetically independent contrasts, as data points. Annual small‐seeded species without specialised dispersal structures are over‐represented in dehesa grasslands. Abundance was negatively related to seed weight in upper slope zones. None of the recorded plant traits were related to abundance in the lower slope zones. Habitat segregation was mainly related to seed weight, but also to some vegetative traits. Annual, early flowering and small‐seeded species were relatively more abundant in the upper than the lower slope zones. This pattern is independent of phylogeny. Our results suggest that in dry Mediterranean grasslands, abundance of many species is determined by dispersal (production of numerous small seeds) rather than by competitive ability.     

Intertidal zonation of animals and plants on rocky shores in the Bristol Channel and Severn Estuary–the northern shores

The vertical distribution of common species of macroalgae and fauna on rocky shores extending through the length of the estuary is described. In the west, the shores are fully marine and exposed to Atlantic waves. In an eastward direction, greater shelter and an increasing tidal range occur along a gradient of salinity and turbidity. The flora and fauna change along this gradient. No single biological feature signals a transition from the Bristol Channel to the Severn Estuary but four arbitrary regions are recognized: a 'marine' section eastwards to Swansea, a 'transitional' section between Swansea and Cardiff, an 'estuarine' section up to Newnham, and the tidal river Severn.     

Competition or niche segregation between Holopedium and Daphnia; empirical light on abiotic key parameters

A survey of 346 Norwegian lakes revealed some consistent patterns with regard to the occurrence of Holopedium gibberum versus Daphnia spp. Both biomass and frequency of Holopedium were negatively correlated with pH and Ca-concentration, and this species rarely occurred at pH > 8, or Ca-concentrations > 10 mg l^–1. On the contrary, the various Daphnia species, although showing different susceptibilities to fish predation and different preferences for lake productivity, were all positively correlated with Ca. Statistical analysis as well as laboratory experiments and biochemical considerations, suggest that Holopedium may be a superior competitor at very low Ca-concentrations, but is replaced by Daphnia at higher pH and increasing Ca-content. High pH in itself is possibly disadvantageous for Holopedium by preventing formation of the muco-polysaccaride mantle.     

Post-dispersal seed predation in Patagonia temperate forest depends on habitat patchiness and seed species

Plant Ecology - Post-dispersal seed predation is a key process regulating plant population dynamics and community composition. Because food preference (i.e., seed species selection) can interact...     

Disturbance and predation in an assemblage of herbivorous Diptera and algae on rocky shores

Summary Speculation about the effects of disturbance in marine benthic communities is often based on competition theory. Disturbances are thought to provision numerically depleted or competitively inferior species with resources associated with open substrate. However, disturbances that remove entire assemblages of sessile species also alter trophic structure, and thereby, influence the outcome of predator/prey relations. Aspects of community structure may be determined by patterns of disturbance and predation.The influence of disturbance and predation on the distribution and seasonality of blooms of ephemeral algae and associated Diptera was investigated with field experiments at several rocky beaches in central California. Blooms of ephemeral algae developed on high intertidal rock faces that were subject to severe seasonal disturbances caused by shifting sediment. These were subsequently colonized by the herbivorous larvae of several Diptera species for predictable periods each year. Other areas, without blooms, were not so disturbed.Experiments were done to determine if seasonal blooms were caused by seasonal disturbances that remove predators which otherwise might prevent the establishment of the Diptera/algae assemblage. The predators were crabs and limpets which eat both algae and larvae while foraging. Blooms of algae and larvae did not develop when limpets were transplanted to disturbed areas in periods between disturbances. Adjacent control areas did support blooms. Transplanted limpets did not survive periods of burial. When both limpets and crabs were excluded from treatment plots in undisturbed areas, blooms developed where they would not otherwise have occurred; controls remaied unchanged. Crabs and limpets differed in their effects on this assemblage. Crabs recruited quickly to the site of a bloom, but did not crop algal cover as closely, nor decrease larval density as much as the slowly recruiting limpets.The results suggest that disturbances favor blooms of some species by reducing predation. Severe localized disturbances increased the variability of the upper shore community by creating a patchwork of differing predator/prey abundances.