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Western Australia’s coral reefs have largely escaped the chronic pressures affecting other reefs around the world, but are regularly affected by seasonal storms and cyclones, and increasingly by heat stress and coral bleaching. Reef systems north of 18°S have been impacted by heat stress and coral bleaching during strong El Nino phases and those further south during strong La Nina phases. Cumulative heat stress and the extent of bleaching throughout the northern reefs in 2016 were higher than at any other time on record. To assess the changing regime of disturbance to reef systems across Western Australia (WA), we linked their site-specific exposure to damaging waves and heat stress since 1990 with mean changes in coral cover. Since 2010, there has been a noticeable increase in heat stress and coral bleaching across WA. Over half the reef systems have been severely impacted by coral bleaching since 2010, which was further compounded by cyclones at some reefs. For most (75%) reef systems with long-term data (5–26 yrs), mean coral cover is currently at (or near) the lowest on record and a full recovery is unlikely if disturbances continue to intensify with climate change. However, some reefs have not yet experienced severe bleaching and their coral cover has remained relatively stable or increased in recent years. Additionally, within all reef systems the condition of communities and their exposure to disturbances varied spatially. Identifying the communities least susceptible to future disturbances and linking them through networks of protected areas, based on patterns of larval connectivity, are important research and management priorities in coming years while the causes of climate change are addressed.

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.

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.

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 fish- ing 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 pre- dation 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. ery- throgramma 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.