Browsing by Author "Thompson, Richard C."
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- Enhancing stocks of the exploited limpet Patella candei d'Orbigny via modifications in coastal engineeringPublication . Martins, Gustavo M.; Thompson, Richard C.; Neto, Ana I.; Hawkins, Stephen J.; Jenkins, Stuart R.It is widely recognised that microhabitats are important for a variety of marine organisms, yet this knowledge has rarely been applied in the construction of engineered structures as a means of enhancing biodiversity or populations of species at risk. Here we examined the influence of microhabitats on the distribution and survival of the exploited limpet Patella candei on natural shores before determining the effect of introducing such habitats to an artificial seawall. On natural shores individuals were associated with pits (a natural feature of volcanic rocky shores). Animals inhabiting pits showed reduced mortality and were smaller than those on open rock. Microhabitat utilisation was similar over the vertical range of distribution of P. candei. Following observation of natural patterns, we applied this knowledge by experimentally drilling pits at varying densities and sizes in a seawall that had been constructed with simple topographical complexity. Overall, the number of animals increased in areas with experimentally increased microhabitat area. There was evidence that this was the result of immigration (larger animals) but also of increased recruitment. This study demonstrates one cost-effective way of conciliating the need to protect our coastlines while promoting the conservation and stock enhancement of over-exploited species.
- Exploitation of intertidal grazers as a driver of community divergencePublication . Martins, Gustavo M.; Thompson, Richard C.; Neto, Ana I.; Hawkins, Stephen J.; Jenkins, Stuart R.1. The possibility that different assemblages of species may represent alternative stable states has been the subject of much theoretical and empirical work. Alternative stable states may in theory arise froma perturbation of sufficient magnitude that pushes an assemblage from one stable equilibrium point to another. Overfishing is one such disturbance that can lead to cascading community level effects. Yet, whether these different assemblages represent alternative stable states or are the consequence of chronic disturbance from fishing is still a matter of debate. Understanding the mechanisms that drive community stability is fundamental if we are to assess the consequences of anthropogenic impacts on the structure and function of ecosystems to better inform management of disturbed habitats. 2. To investigate the extent to which present-day community state is stable versus being maintained by chronic exploitation, we manipulated the time and intensity of physical disturbance and grazing by limpets in a system where over-harvesting of limpets has led to a regional-scale shift in community structure to one in which algal turfs have replaced barnacles as the primary space occupier in the mid-intertidal. 3. After a 1-year period since disturbance was applied, assemblages in disturbed areas were significantly different from undisturbed areas, but the timing of disturbance and its intensity had little effect on the outcome of succession. Undisturbed areas were highly resistant to new colonization and persisted unchanged throughout the study period. 4. Manipulation of limpet abundance in disturbed patches showed that, where present, limpets successfully prevented the recolonization of space by algal turfs. Moreover, there was evidence that grazing by limpets at the turf ⁄open-rock boundary effectively pushed the turfs back, extending the area of open-rock. 5. Synthesis and applications. Our findings provide evidence that in this system the dominance by algal turfs does not represent an alternative stable state but that chronic exploitation of limpets leads to the persistence of this community. Conservation strategies aimed at protecting or enhancing limpet abundances (e.g. no-take marine reserves) should allow the gradual restoration of this community to its pre-disturbed state.
- Exploitation of keystone species: population and community-level effectsPublication . Martins, Gustavo M.; Thompson, Richard C.; Hawkins, Stephen J.; Neto, Ana I.; Jenkins, Stuart R.
- The exploitation of patellid limpets in the Azores.Publication . Martins, Gustavo M.; Jenkins, Stuart R.; Hawkins, Stephen J.; Neto, Ana I.; Thompson, Richard C.
- Exploitation of rocky intertidal grazers : population status and potential impacts on community structure and functioningPublication . Martins, Gustavo M.; Jenkins, Stuart R.; Hawkins, Stephen J.; Neto, Ana I.; Thompson, Richard C.A wide range of anthropogenic activities are impacting the ecology of coastal areas. Exploitation of marine resources is one such activity, which, through cascading trophic effects, can have influences well beyond that of the target species. We investigated the mid-rocky-shore community structure of the Azores archipelago, a seldom-studied habitat, where there is a local tradition of exploiting limpets, the main intertidal grazers. The limpet population structure differed among islands, and there was an inverse relationship between the abundance of larger limpets and the human population per coastal perimeter, but not the associated catch data. At small scales of resolution (quadrats), there was a negative relationship between the cover of algae and limpets and a positive relationship between barnacles and limpets. These relationships were also apparent at the larger scale of islands as a function of the gradient of exploitation. Our results show how natural habitat fragmentation may be useful where the experimental testing of a hypothesis is not possible, and provide evidence for the trophic cascading effects of limpet exploitation at landscape scales.
- Factors limiting the establishment of canopy-forming algae on artificial structuresPublication . Cacabelos, Eva; Martins, Gustavo M.; Thompson, Richard C.; Prestes, Afonso L.; Azevedo, José M. N.; Neto, Ana I.Macroalgal canopies are important ecosystem engineers, contributing to coastal productivity and supporting a rich assemblage of associated flora and fauna. However, they are often absent from infrastructures such as coastal defences and there has been a worldwide decline in their distribution in urbanised coastal areas. The macroalga Fucus spiralis is the only high-shore canopy forming species present in the Azores. It is widely distributed in the archipelago but is never found on coastal infrastructures. Here we evaluate factors that may potentially limit its establishment on artificial structures. A number of observational and manipulative experiments were used to test the hypotheses that: (i) limited-dispersal ability limits the colonisation of new plants onto artificial structures, (ii) vertical substratum slope negatively influences the survivorship of recruits, and (iii) vertical substratum slope also negatively influences the survivorship and fitness of adults. Results showed that the limited dispersal from adult plants may be a more important factor than slope in limiting the species ability to colonise coastal infrastructures, since the vertical substratum slope does not affect its fitness or survivorship.
- Illegal harvesting affects the success of fishing closure areasPublication . Martins, Gustavo M.; Jenkins, Stuart R.; Hawkins, Stephen J.; Neto, Ana I.; Medeiros, André; Thompson, Richard C.There is clear evidence that marine reserves can be used as effective tools to foster the recovery of disturbed ecosystems. In the Azores, intense exploitation of the patellid limpets Patella candei and P. aspera has led to a rapid decline in their populations and subsequent collapse of the fishery in 1985. In 1993, legislation was passed to protect limpets, including the establishment of limpet protected zones (LPZs) where harvesting was completely prohibited. Outside LPZs, a seasonal fishing closure prohibited the harvesting of limpets from October to May. Here we examine the effect of such measures 16 years after they were put into practice. In each of the 3 years examined, limpet density, biomass and size were generally similar both inside and outside the LPZs. In addition, there were clear signs of exploitation as most individual limpets inside the LPZ were smaller than the legal catch size suggesting that illegal harvesting was taking place. Observations confirmed that illegal harvesting of limpets was common both inside and outside LPZs. Lack of enforcement of regulations is therefore a likely reason for the failure of legislation to protect limpet populations and facilitate stock recovery.
- Long-term modifications of coastal defences enhance marine biodiversityPublication . Martins, Gustavo M.; Jenkins, Stuart R.; Neto, Ana I.; Hawkins, Stephen J.; Thompson, Richard C.Realization that hard coastal infrastructures support lower biodiversity than natural habitats has prompted a wealth of research seeking to identify design enhancements offering ecological benefits. Some studies showed that artificial structures could be modified to increase levels of diversity. Most studies, however, only considered the short-term ecological effects of such modifications, even though reliance on results from short-term studies may lead to serious misjudgements in conservation. In this study, a sevenyear experiment examined how the addition of small pits to otherwise featureless seawalls may enhance the stocks of a highly-exploited limpet. Modified areas of the seawall supported enhanced stocks of limpets seven years after the addition of pits. Modified areas of the seawall also supported a community that differed in the abundance of littorinids, barnacles andmacroalgae compared to the controls. Responses to different treatments (numbers and size of pits) were speciesspecific and, while some species responded directly to differences among treatments, others might have responded indirectly via changes in the distribution of competing species. This type of habitat enhancement can have positive long-lasting effects on the ecology of urban seascapes.Understanding of species interactions could be used to develop a rule-based approach to enhance biodiversity.
- Microhabitat provision influences small-scale patterns of distribution in Patella candei.Publication . Martins, Gustavo M.; Thompson, Richard C.; Neto, Ana I.; Hawkins, Stephen J.; Jenkins, Stuart R.
- Population and community level effects of exploitation of rocky intertidal grazers.Publication . Martins, Gustavo M.; Neto, Ana I.; Hawkins, Stephen J.; Thompson, Richard C.; Jenkins, Stuart R.