The experimental manipulation of large marine herbivores (urchins, gastropods and fish) has repeatedly demonstrated their strong influence on the abundance and composition of benthic primary producers. However, the effects of smaller herbivores (amphipods, isopods and small gastropods) on community structure are not as well understood. We used a cageless technique (a slow-release insecticide) to exclude amphipods from seagrass meadows to test for their effects on epiphyte and seagrass biomass. Lower amphipod densities in a Posidonia sinuosa meadow after 7 wk were associated with 25% higher epiphyte biomass -evidence of a strong influence by amphipods on seagrass epiphytes. In an Amphibolis spp. meadow, lower amphipod densities showed a non-significant trend to increase leaf epiphytes, but did not affect stem epiphyte biomass. Effects of amphipod exclusion on seagrass biomass were not detected in either meadow. Our results indicate that natural densities of amphipods can reduce epiphyte biomass on seagrasses, but that their impacts may vary across seagrass meadows. A challenge for future research is to identify the conditions under which small marine herbivores are likely to influence the biomass of primary producers.
Coral reefs have evolved over millennia to survive disturbances. Yet, in just a few decades chronic local pressures and the climate catastrophe have accelerated so quickly that most coral reefs are now threatened. Rising ocean temperatures and recurrent bleaching pose the biggest threat, affecting even remote and well-managed reefs on global scales. We illustrate how coral bleaching is altering reefs by contrasting the dynamics of adjacent reef systems over more than two decades. Both reef systems sit near the edge of northwest Australia's continental shelf, have escaped chronic local pressures and are regularly affected by tropical storms and cyclones. The Scott reef system has experienced multiple bleaching events, including mass bleaching in 1998 and 2016, from which it is unlikely to fully recover. The Rowley Shoals has maintained a high cover and diversity of corals and has not yet been impacted by mass bleaching. We show how the dynamics of both reef systems were driven by a combination of local environment, exposure to disturbances and coral life history traits, and consider future shifts in community structure with ongoing climate change. We then demonstrate how applying knowledge of community dynamics at local scales can aid management strategies to slow the degradation of coral reefs until carbon emissions and other human impacts are properly managed.
Indirect effects of fishing extending 2 or more trophic levels have been observed in a range of ecosystems, but they are not ubiquitously present, and identifying the circumstances in which they occur (or do not occur) is a key challenge for ecologists. We compared a fully protected sanctuary (no fishing allowed) with 2 other types of management zones (one in which all types of fishing were allowed and another in which only recreational line fishing was allowed) at Rottnest Island, Australia. We predicted that fishing would result in lower abundance of predatory fish in the 2 fished zones. We further predicted that, if indirect effects of fishing are important, there would be lower predation on sea urchins and turbinid gastropods, higher abundance of these herbivorous invertebrates and lower biomass of macroalgae in both types of fished zones. The abundance of predatory fish and predation on the sea urchin Heliocidaris erythrogramma were higher in the sanctuary than in fished zones. Predation on the sea urchin Centrostephanus tenuispinus was higher and densities of H. erythrogramma were lower in the sanctuary, but the magnitude of the difference in these measures (and, therefore, the results of statistical tests) was variable among years. However, predation on turbinid gastropods, densities of C. tenuispinus and turbinid gastropods, and the biomass of macroalgae did not differ significantly among management zones. The pattern of higher abundance of predatory fish and higher rates at which prey were consumed in the sanctuary provide evidence for the indirect effects of fishing on the abundance of H. erythrogramma. However, there was no evidence that these indirect effects extend to a second species of sea urchin, or to turbinid gastropods, and no evidence that the effects lead to broad changes in the state of the ecosystem.
Marine reserves that prohibit fishing often result in greater densities of individuals and more species than adjacent fished areas. However, simple conclusions about their effects on species richness are confounded, because more species are expected to occur wherever there are more individuals. Here, there is an important distinction between the number of species per sampling unit (species density), and species richness measured as the number of species per given number of individuals. When conservation of species richness is an important goal, analyses need to discriminate between the alternative explanations for differences in the number of species. We used rarefaction to test whether species richness was higher in two 'no-take' marine reserves after controlling for differences in the density of individuals. We surveyed each reserve in three different years. There was a higher density of individuals and species in each reserve than in adjacent fished areas. However, rarefaction analyses indicated that effects on species richness were weak after controlling for the number of individuals: slightly higher species richness was recorded inside each reserve in one of three surveys, but the difference was small, and was apparent only when the maximum number of individuals was approached. Our results therefore indicate that patterns in species density were not reflected by patterns in species richness-the application of rarefaction methods is needed to determine the responses of species richness to protection elsewhere. The distinction between species density and species richness will not be important in all situations, but when it is important, inferences about species richness cannot be reliably deduced from measurements of species density.
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