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.     

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.     

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.     

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.     

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.     

A novel theory to explain species diversity in landscapes: positive frequency dependence and habitat suitability

Theories to explain the diversity of species have required that individual species occupy unique niches and/or vary in their response to environmental factors. Positive interactions within a species, although common in communities, have not been thought to maintain species diversity because in non-spatial models the more abundant species always outcompetes the rarer species. Here, we show, using a stochastic spatial model, that positive intraspecific interactions such as those caused by positive frequency dependence and/or priority effects, can maintain species diversity if interactions between individuals are primarily local and the habitat contains areas that cannot be colonized by any species, such as boulders or other physical obstructions. When intraspecific interactions are primarily neutral, species diversity will eventually erode to a single species. When the landscape is homogeneous (i.e. does not contain areas that cannot be colonized by any species), the presence of strong intraspecific interactions will not maintain diversity.     

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.     

Predator diversity and density affect levels of predation upon strongly interactive species in temperate rocky reefs

Indirect effects of predators in the classic trophic cascade theory involve the effects of basal species (e.g. primary producers) mediated by predation upon strongly interactive consumers (e.g. grazers). The diversity and density of predators, and the way in which they interact, determine whether and how the effects of different predators on prey combine. Intraguild predation, for instance, was observed to dampen the effects of predators on prey in many ecosystems. In marine systems, species at high trophic levels are particularly susceptible to extinction (at least functionally). The loss of such species, which is mainly attributed to human activities (mostly fishing), is presently decreasing the diversity of marine predators in many areas of the world. Experimental studies that manipulate predator diversity and investigate the effects of this on strongly interactive consumers (i.e. those potentially capable of causing community-wide effects) in marine systems are scant, especially in the rocky sublittoral. I established an experiment that utilised cage enclosures to test whether the diversity and density of fish predators (two sea breams and two wrasses) would affect predation upon juvenile and adult sea urchins, the most important grazers in Mediterranean sublittoral rocky reefs. Changes in species identity (with sea breams producing major effects) and density of predators affected predation upon sea urchins more than changes in species richness per se. Predation upon adult sea urchins decreased in the presence of multiple predators, probably due to interference competition between sea breams and wrasses. This study suggests that factors that influence both fish predator diversity and density in Mediterranean rocky reefs (e.g. fishing and climate change) may have the potential to affect the predators' ability to control sea urchin population density, with possible repercussions for the whole benthic community structure.     

Competition and stoichiometry: coexistence of two predators on one prey

The competitive exclusion principle (CEP) states that no equilibrium is possible if n species exploit fewer than n resources. This principle does not appear to hold in nature, where high biodiversity is commonly observed, even in seemingly homogenous habitats. Although various mechanisms, such as spatial heterogeneity or chaotic fluctuations, have been proposed to explain this coexistence, none of them invalidates this principle. Here we evaluate whether principles of ecological stoichiometry can contribute to the stable maintenance of biodiverse communities. Stoichiometric analysis recognizes that each organism is a mixture of multiple chemical elements such as carbon (C), nitrogen (N), and phosphorus (P) that are present in various proportions in organisms. We incorporate these principles into a standard predator-prey model to analyze competition between two predators on one autotrophic prey. The model tracks two essential elements, C and P, in each species. We show that a stable equilibrium is possible with two predators on this single prey. At this equilibrium both predators can be limited by the P content of the prey. The analysis suggests that chemical heterogeneity within and among species provides new mechanisms that can support species coexistence and that may be important in maintaining biodiversity.     

Projecting present and future habitat suitability of ship-mediated aquatic invasive species in the Canadian Arctic

A rise in Arctic shipping activity resulting from global warming and resource exploitation is expected to increase the likelihood of aquatic invasive species (AIS) introductions in the region. In this context, the potential threat of future AIS incursions at a Canadian Arctic regional scale was examined. Habitat suitability under current environmental conditions and future climate change scenarios was projected for a subset of eight potential invaders ranked as having a high risk of establishment in the Canadian Arctic based on dispersal pathways/donor regions, biological attributes and invasion history: (1) Amphibalanus improvisus, (2) Botrylloides violaceus, (3) Caprella mutica, (4) Carcinus maenas, (5) Littorina littorea, (6) Membranipora membranacea, (7) Mya arenaria and (8) Paralithodes camtschaticus. Habitat modelling was performed using MaxEnt based on globally known native and non-native occurrence records and environmental ranges for these species. Results showed that under current environmental conditions the habitat is suitable in certain regions of the Canadian Arctic such as the Hudson Complex and Beaufort Sea for L. littorea, M. arenaria and P. camtschaticus. Under a future climate change scenario, all species showed poleward gains in habitat suitability with at least some regions of the Canadian Arctic projected to be suitable for the complete suite of species modelled. The use of these models is helpful in understanding potential future AIS incursions as a result of climate change and shipping at large spatial scales. These approaches can aid in the identification of high risk regions and species to allow for more focused AIS monitoring and research efforts in response to climate change.