Coral Recruitment Shifts due to Sensitivity to Community Succession

By Patricia Albano, SRC intern

Environmental disturbances such as natural disasters, anthropogenic effects, and weather pattern changes have a significant impact on ecosystems. Following such disturbances, communities must adapt and rebuild through succession where they evolve to respond to changes. In this study, researchers Christopher Doropoulous, George Roff, Mart-Simone Visser, and Peter Mumby of the University of Queensland studied the positive and negative interactions that impact community succession in the wake of a disturbance and how these interactions differ along environmental gradients.

Figure 1. Soft Corals Found on Palau Reef Caption: These are examples of the soft corals (Nephtheidae) that inhabit reefs in Palau. These corals are important for the structure and function of the reefs. Source: Wikimedia Commons

Figure 1. Soft Corals Found on Palau Reef
These are examples of the soft corals (Nephtheidae) that inhabit reefs in Palau. These corals are important for the structure and function of the reefs.
Source: Wikimedia Commons

Succession in ecosystems usually follows 2 models: facilitation and inhibition (Connel and Slayter 1977). Facilitating organisms “set the stage” for environmental modifications, making the habitat more accommodating for the later-successional species. Inhibiting organisms are early arrivers that reserve space in the habitat for themselves and prevent the invasion of later-successional species (Connel and Slayter 1977). All species interaction includes two components: negative (competition, predation, inhibition) and positive (facilitation) (Paine1980). These two interaction types allowed the researchers to classify the changes they saw in the coral reef ecosystem study sites. This series of experiments investigates how early succession affects coral reef recovery after 2 subsequent typhoons in the island of Palau in the Western Pacific (Figure1) These 2 typhoons (occurring in December 2012 and November 2013) occurred after no typhoon disturbances for over 70 years. This study site was used to explore how the changes in species interactions after a disturbance affect succession in benthic communities with different environmental gradients and how these successional changes affect coral recruitment and recovery after a disturbance (Doropoulos et al. 2016).

The researchers analyzed the eastern barrier reef of the island that had significantly reduced abundances of juvenile corals. Six sites were chosen within to different wave environments: 3 at reefs with lower wave exposure and 3 at reefs with higher wave exposure (Figure 2). Variables accounted for include: grazing potential of herbivorous fish on available grazeable substrate and percent cover of algae groups in the ecosystems.

Figure 2: Six Reefs of the Study Site in Palau Caption: This map indicates where the 6 reefs used in the study are located along the Palau coastline. The pictures indicate differences in coral recruitment between caged (shielded from herbivorous fish) and uncaged (open to grazing) portions of the reef. Source: Doropoulous et al. 2016

Figure 2: Six Reefs of the Study Site in Palau
This map indicates where the 6 reefs used in the study are located along the Palau coastline. The pictures indicate differences in coral recruitment between caged (shielded from herbivorous fish) and uncaged (open to grazing) portions of the reef.
Source: Doropoulous et al. 2016

The researchers found that 3 patterns in environmental drivers influenced the ecosystem. First, a 70% reduction in fish grazing potential was found at the sites that had loss of the majority of live coral cover after the typhoons. Second, shifts in dominance from coral to microalgae occurred at 3 sites with medium wave exposure. Last, microalgae was more abundant in microhabitat crevice areas within both the medium and low wave exposure sites. Coral recruitment was also higher in these crevice areas within the reefs. The researchers came to the conclusion that when herbivorous fish are excluded from reef habitats, a gradual shit towards an algae dominated system occurred at both medium and low wave exposure locations. The results collectively showed that differences in interaction strengths along environmental gradients can lead to changes in the early succession of benthic life that can lead to the inhibition of system recovery after a disturbance such as a typhoon. This experiment revealed important information on how ecosystems recover after disturbances. With this knowledge, nations and conservation organizations can effectively manage their reef ecosystems following a disturbance such as a natural disaster or a weather change. This study also reveals how important it is for healthy reefs to exist in order for ecosystems to continue thriving and support a vast array of life.

Works cited:

Paine, R. T. 1980. Food webs: linkage, interaction strength and community infrastructure. The Journal of Animal Ecology 49: 667-685.

Connell, J. H., and R. O. Slatyer. 1977. Mechanisms of succession in natural communities and their role in community stability and organization. American naturalist:1119- 1144.

Re-evaluating the health of coral reef communities: baselines and evidence for human impacts across the central Pacific

By Shannon Moorhead, SRC Masters Student

In the past several decades, it has become clear to researchers that populations of reef-building corals have suffered significant declines worldwide. In the 1970s, coral covered on average,50% of benthic habitat (the sea floor) in the Caribbean; in the early 2000s, this was reduced to an average of 10%, with an estimated 80% decline in total cover throughout the Caribbean. Similar observations have been made in the Pacific, with an estimated decrease from 43% coral cover on average in the 1980s, to 22% cover on average in 2003. These declines are caused by a large variety of both global and local threats. Globally, increasing temperatures, ocean acidification, sea level rise, and disease outbreaks have caused mass coral mortality events and decreased rates of calcification – the rate at which coral grows its calcium carbonate skeleton. In addition to these worldwide stressors, corals face local threats such as overfishing of important grazers and predators, elevated nutrient levels, and increasing amounts of terrestrial sediment in coastal waters. These human impacts kill corals either directly or indirectly, by creating conditions that allow faster-growing algae species to thrive and overtake corals. In some cases, this algal growth can lead to a phase-shift: a change in ecosystem composition and function when macroalgae replaces corals as the dominant benthic cover. Because of this, the majority of studies done to assess reef health have focused only on percent cover of macroalgae and corals. However, recent research indicates that when coral cover declines it is rarely replaced solely by macroalgae and studies have shown that coral and macroalgae together only comprise 19-55% of the reef benthos, organisms that live on the sea floor.

Figure 1. (a) Theses images show hard coral and macroalgae, the two groups most often used to assess reef health. (b) These images show reef builders and fleshy algae, which can be used to assess reef-health with a more community-based approach.

Figure 1. (a) Theses images show hard coral and macroalgae, the two groups most often used to assess reef health. (b) These images show reef builders and fleshy algae, which can be used to assess reef-health with a more community-based approach.

Methods

In this study, Smith et al. also investigated percent cover of other members of the benthos: crustose coralline algae (CCA) and turf algae. Turf algae have a negative impact on coral cover, by growing over and smothering adult corals and preventing the settlement of larval corals. On the other hand, CCA, which produces calcium carbonate like corals, promotes reef resilience by stabilizing reef structure and creating a place for coral larvae to reside and grow, because there are many coral species whose larvae prefer to settle on some types of CCA. Smith et al. considered the cover of all four groups (coral, CCA, turf algae, and macroalgae) to compare central Pacific reef communities surrounding uninhabited islands with communities that surround populated islands and suffer from significant anthropogenic, or human-caused, stressors. Specifically, they examined whether cover of reef-builders (CCA and coral) and fleshy algae (turf and macroalgae) were inversely related, as well as whether the two groups were more common in the absence or presence of human populations.

Figure 2. (a) A map of the five island chains and 56 islands from which data were collected for this study; (b) 17 Hawaiian Islands, (c) 21 islands from the Line and Phoenix Islands, (d) six islands from American Samoa, and (e) 14 islands in the Mariana Archipelago. Stars represent inhabited islands while circles represent uninhabited islands.

Figure 2. (a) A map of the five island chains and 56 islands from which data were collected for this study; (b) 17 Hawaiian Islands, (c) 21 islands from the Line and Phoenix Islands, (d) six islands from American Samoa, and (e) 14 islands in the Mariana Archipelago. Stars represent inhabited islands while circles represent uninhabited islands.

Results

Smith et al. evaluated reef communities of 56 islands from five central Pacific island chains between 2002 and 2009 and acquired some surprising results. While coral cover was higher on uninhabited islands, there was not a significant difference between the two. Macroalgae cover varied by archipelago: while there was greater macroalgae cover on populated islands in the Line and Mariana Islands, the Hawaiian Islands and American Samoa had higher macroalgae cover on uninhabited islands. In addition, there was no significant relation seen between total coral and macroalgae cover, contradicting previous ideas that macroalgae directly replaced coral on degraded reefs. However, the average cover of reef-builders was significantly higher on uninhabited islands versus inhabited islands, while the average cover of fleshy algae was significantly higher on inhabited islands versus uninhabited. This result suggests that local anthropogenic stressors play a direct role in changes to the benthic community, and potentially reef health.

Outcomes

In this study, the authors suggest that a good indicator of reef health is net accretion, where the reef-building organisms are building calcium carbonate skeletons faster than they are being eroded. Because it appears that inhabited islands have a lower abundance of reef-building organisms, the reefs surrounding these islands are not as healthy as those near uninhabited islands and may have a harder time bouncing back from large-scale disturbances such as bleaching events and typhoons. Local management on inhabited islands should consider this when developing management strategies and work towards improving the resilience of their reef ecosystems. This research also demonstrates that percent coral and macroalgae cover are not always reliable indicators of reef health; instead management should take a more holistic approach and evaluate other members of the benthos when assessing reef health, in addition to measuring indicators of reef resilience such as coral growth and recruitment, which will help managers predict trends and changes in the structure of the benthic community of coral reefs.

References

Smith, J. E., Brainard, R., Carter, A., Grillo, S., Edwards, C., Harris, J., . . . Sandin, S. (2016). Re-evaluating the health of coral reef communities: baselines and evidence for human impacts across the central Pacific. Proceedings of the Royal Society of London B.

Trawling on seamounts: effects on corals

By Jennifer Dean,
Marine Conservation student

Have you ever wondered how your seafood is caught?  Many of the species commonly consumed by humans, such as cod, flounder, and shrimp, are caught by a method called bottom trawling (montereybayaquarium.org).  Bottom trawling is a large scale fishing method that has been used for many years, consisting of dragging a large net or nets along the seafloor to scoop up fish that live near the bottom.  Unfortunately, this method is not very ecosystem-friendly.  Most trawl nets include heavy wooden or metal frames to keep the nets open, and these frames drag along the bottom, creating troughs, re-suspending sediment, and damaging organisms.  Recently people have begun to realize just how devastating these impacts can be on seafloor habitats.  In fact, just this past September it was shown that bottom trawling severely alters the ocean floor through smoothing of the local topography (Puig et al. 2012).  One ocean habitat that has been heavily impacted by bottom trawling is that of seamounts.

Continue reading