The influence of site, season and day of the week on exploitation of rocky intertidal biota in central Transkei, South Africa

Temporal and spatial variations in the pattern and intensity of exploitative use of rocky shores in the densely populated central Transkei region of the south-east coast of South Africa are described. Surveys of intertidal exploiters and their catches were conducted simultaneously on five randomly selected weekdays and weekends per season between June 1995 and June 1996 at Umtata Mouth, Mngcibe and Lwandile. Three-way ANOVAs were used to assess the influence of site, season and day of the week (weekday or weekend) on the density of the three major groups of exploiters: shellfish-gatherers, seaweed-harvesters and bait-collectors, and on the offtake by shellfish-gatherers. The densities of shellfish-gatherers or seaweed-harvesters did not differ between sites, but there were marked differences in the density of bait-collectors and the quantity of organisms removed by shellfish-gatherers. The numbers in all exploiter groups were lowest in winter, because of factors such as inaccessibility caused by rough seas, poor quality of the preferred shellfish and a clash with such agricultural activities as harvesting of maize. The densities of adult shellfish-gatherers and shellfish offtake were both significantly higher on weekdays than at weekends. However, more children gathered shellfish during the weekends and in summer, perhaps attributable to the closure of schools.Keywords: exploitation, rocky shores, temporal/spatial variation, TranskeiAfrican Journal of Marine Science 2002, 24: 57–6

Patchiness and Co-Existence of Indigenous and Invasive Mussels at Small Spatial Scales: The Interaction of Facilitation and Competition

Ecological theory predicts that two species with similar requirements will fail to show long-term co-existence in situations where shared resources are limiting, especially at spatial scales that are small relative to the size of the organisms. Two species of intertidal mussels, the indigenous Perna perna and the invasive Mytilus galloprovincialis, form mixed beds on the south coast of South Africa in a situation that has been stable for several generations of these species, even though these populations are often limited by the availability of space. We examined the spatial structure of these species where they co-exist at small spatial scales in the absence of apparent environmental heterogeneity at two sites, testing: whether conspecific aggregation of mussels can occur (using spatial Monte-Carlo tests); the degree of patchiness (using Korcak B patchiness exponent), and whether there was a relationship between percent cover and patchiness. We found that under certain circumstances there is non-random conspecific aggregation, but that in other circumstances there may be random distribution (i.e. the two species are mixed), so that spatial patterns are context-dependent. The relative cover of the species differed between sites, and within each site, the species with higher cover showed low Korcak B values (indicating low patchiness, i.e. the existence of fewer, larger patches), while the less abundant species showed the reverse, i.e. high patchiness. This relationship did not hold for either species within sites. We conclude that co-existence between these mussels is possible, even at small spatial scales because each species is an ecological engineer and, while they have been shown to compete for space, this is preceded by initial facilitation. We suggest that a patchy pattern of co-existence is possible because of a balance between direct (competitive) and indirect (facilitative) interactions

A review on macrobenthic trophic relationships along subtropical sandy shores in southernmost Brazil

Main trophic relationships that occur along the exposed sandy shores in southernmost Brazil (,336 S) are established taking into account several biological compartments operating at morphodynamically distinct environments. Beaches are predominantly of the intermediate type but some stretches of coastline are truly dissipative (Cassino Beach) or tending-to-reflective (Concheiros Beach), presenting thus diverse biological compartments and inhabitant macrobenthic assemblages. Dense concentrations of the surf-zone diatom Asterionellopsis glacialis are responsible - at least for the intermediate shorelines - for the most year-round primary production, sustaining several consumers up to tertiary level. Among them, bivalves Amarilladesma mactroides, Donax hanleyanus and the hippid crab Emerita brasiliensis can account for more than 95% of all the surf-zone secondary production, in addition to the elevated biomass of the suspension-feeder polychaete Spio gaucha and the scavenger isopod Excirolana armata. Crabs, whelks, carnivorous polychaetes, seabirds and surf-zone fishes may also be present and occupy superior trophic levels depending on the beach morphodynamics. Based on the high species richness, abundance and the role of macrobenthic fauna in transferring matter and energy to seabirds and the surf-zone fish assemblages, we address this important issue on the Southwestern Atlantic ecology. Conservation efforts should be implemented for the southernmost Brazilian sandy shores, at least for those non urbanized areas.As principais relac¸o˜ es tro´ ficas observadas ao longo da costa arenosa exposta do extremo sul do Brasil (,336 S) sa˜o estabelecidas levando-se em considerac¸a˜o os va´ rios compartimentos biolo´ gicos que operam em ambientes morfodinamicamente distintos. Estas praias sa˜o predominantemente do tipo intermedia´ rio, embora alguns trechos sejam dissipativos (Praia do Cassino) ou apresentem tendeˆncias reflectivas (Praia dos Concheiros), possuindo compartimentos biolo´ gicos diversificados e associac¸o˜es macrobentoˆnicas residentes. Densas concentrac¸o˜ es da diatoma´cea Asterionellopsis glacialis sa˜o responsa´ veis - pelo menos para as praias intermedia´ rias - por grande parte da produtividade prima´ ria anual, sustentando va´ rios consumidores ate´ o nı´vel tercia´ rio. Entre estes, os bivalves Amarilladesma mactroides, Donax hanleyanus e o crusta´ceo Emerita brasiliensis sa˜o responsa´ veis por mais de 95% de toda a produc¸a˜o secunda´ ria da zona de surfe, em adic¸a˜o a` elevada biomassa do poliqueta suspensı´voro Spio gaucha e do iso´pode necro´fago Excirolana armata. Caranguejos, gastro´ podes, poliquetas carnı´voros, aves marinhas e peixes da zona de surfe tambe´m podem estar presentes, ocupando nı´veis tro´ ficos superiores dependendo da morfodinaˆmica praial. Baseado na alta riqueza de espe´cies, na abundaˆncia e no papel da macrofauna bentoˆ nica em transferir mate´ria e energia para as aves marinhas e as assemble´ias de peixes da zona de surfe, esta importante questa˜o ecolo´ gica do Atlaˆntico Sudoeste foi investigada. Esforc¸os de conservac¸a˜o devem ser implementados nas praias arenosas do extremo sul do Brasil, pelo menos naquelas a´ reas na˜o urbanizadas

Ecological traps in shallow coastal waters—Potential effect of heat-waves in tropical and temperate organisms

Mortality of fish has been reported in tide pools during warm days. That means that tide pools are potential ecological traps for coastal organisms, which happen when environmental changes cause maladaptive habitat selection. Heat-waves are predicted to increase in intensity, duration and frequency, making it relevant to investigate the role of tide pools as traps for coastal organisms. However, heat waves can also lead to acclimatization. If organisms undergo acclimatization prior to being trapped in tide pools, their survival chances may increase. Common tide pool species (46 species in total) were collected at a tropical and a temperate area and their upper thermal limits estimated. They were maintained for 10 days at their mean summer sea surface temperature +3°C, mimicking a heat-wave. Their upper thermal limits were estimated again, after this acclimation period, to calculate each species' acclimation response. The upper thermal limits of the organisms were compared to the temperatures attained by tide pool waters to investigate if 1) tide pools could be considered ecological traps and 2) if the increase in upper thermal limits elicited by the acclimation period could make the organisms less vulnerable to this threat. Tropical tide pools were found to be ecological traps for an important number of common coastal species, given that they can attain temperatures higher than the upper thermal limits of most of those species. Tide pools are not ecological traps in temperate zones. Tropical species have higher thermal limits than temperate species, but lower acclimation response, that does not allow them to survive the maximum habitat temperature of tropical tide pools. This way, tropical coastal organisms seem to be, not only more vulnerable to climate warming per se, but also to an increase in the ecological trap effect of tide pools