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A Microscopic Worm Produces More Babies in Response to Parasite Attack

Blog written by Victoria L. Pike. Read the full paper here.

Virtually all organisms are being constantly attacked by parasites. Hosts can deal with it in a variety of ways, but they ideally want to limit the damage caused by the parasite. One strategy that can limit damage to host reproductive success is ‘fecundity compensation’ (Minchella, 1985; Vale and Little, 2012), a response to infection exhibited by species across the animal kingdom including fish, snails, planktonic crustaceans and mice. Fecundity compensation involves an increase in the number of offspring produced when hosts are exposed to parasites, allowing the host to counteract any reduction in lifespan caused by infection.  

Despite the widespread occurrence of fecundity compensation, understanding how hosts can manipulate their reproduction in response to the threat of parasites remains largely unknown. Previous work has shown that potential links between other damage limiting responses (e.g., physically evading pathogens), stress responses, and the immune response (Gleichsner et al., 2016; Schulenburg and Ewbank, 2007).

In our study, we investigated whether fecundity compensation was linked with the stress hosts experience when faced with parasite infection. We used a microscopic worm, Caenorhabditis elegans, as the host in our experiment and a parasitic bacterium, Staphylococcus aureus, that produces toxins that destroy gut cells when inside the worm. This animalwas a good choice of host for this experiment as it has a rapid life cycle which it completes in around 3.5 days (Hope, 1999), meaning that experiments can be replicated quickly. Additionally, the worm’s diet is made up of microbes, so it can be easily infected by eating the parasite. Moreover, mutants of this animal have been made which allow us to knock down or ramp up the host stress response relative to a wild-type worm.  

Image of Caenorhabditis elegans on a lawn of Escherichia coli food.Image by Anke Kloock.

In our study, we used three different mutants of the wormwith either a normal, suppressed or heightened stress response to explore links between stress response and fecundity compensation upon parasite attack. We also maintained control groups of each worm mutants only given food to eat, not parasites. We then counted the number of offspring mutant and wild-type worms produced once exposed to parasites or only fed food.

Our results showed that fecundity compensation is linked to host stress in C. elegans worms. Once they survived parasite exposure, both wild-type worms and those with the heightened stress response, increased their reproductive output in the presence of the parasite. Worms with a suppressed stress and dampened immune response, however, did not produce more offspring relative to the control after parasite exposure.

These results have helped to reveal a valuable system for future research on fecundity compensation. The worm’s rapid lifecycle makes it suitable for experimental evolution approaches, as experiments can take place in large populations and over multiple generations quickly. Using this model, one could thus investigate the conditions under which fecundity compensation might evolve and be maintained. Biologists can also further investigate the mechanism of fecundity compensation in the worms. We have uncovered a potential mechanism linking fecundity compensation to stress, but more experiments with a greater diversity of mutants, targeting different pathways of the stress response, could help to clarify this mechanism. Furthermore, we only conducted our experiment with one type of parasite – it would be interesting to see whether fecundity compensation varies with different parasites that vary in the harm they cause.

Parasites are everywhere. With global change increasing the contact organisms have with infections (Altizer et al., 2013; Harvell, 2002) and ramping up the environmental stress hosts can experience (Gleichsner et al., 2016), it is crucial we understand the ways parasites can alter host reproduction. In our study, we have uncovered a link between enhanced offspring production and the host’s stress response. This research provides a springboard for further investigation into how fecundity compensation works.

References:

Altizer, S., Ostfeld, R.S., Johnson, P.T.J., Kutz, S., Harvell, C.D., 2013. Climate Change and Infectious Diseases: From Evidence to a Predictive Framework. Science 341, 514. https://doi.org/10.1126/science.1239401

Gleichsner, A.M., Cleveland, J.A., Minchella, D.J., 2016. One stimulus—Two responses: Host and parasite life-history variation in response to environmental stress. Evolution 70, 2640–2646. https://doi.org/10.1111/evo.13061

Harvell, C.D., 2002. Climate Warming and Disease Risks for Terrestrial and Marine Biota. Science 296, 2158–2162. https://doi.org/10.1126/science.1063699

Hope, I.A., 1999. C. elegans : a practical approach. Oxford University Press.

Minchella, D.J., 1985. Host life-history variation in response to parasitism. Parasitology 90, 205. https://doi.org/10.1017/S0031182000049143

Schulenburg, H., Ewbank, J.J., 2007. The genetics of pathogen avoidance in Caenorhabditis elegans. Molecular Microbiology 66, 563–570. https://doi.org/10.1111/j.1365-2958.2007.05946.x

Vale, P.F., Little, T.J., 2012. Fecundity compensation and tolerance to a sterilizing pathogen in Daphnia. Journal of Evolutionary Biology 25, 1888–1896. https://doi.org/10.1111/j.1420-9101.2012.02579.x

On the search for rabbits: where citizen science meets the scientist’s needs

Blog written by Emilie Roy-Dufresne. Read the full paper here.

Where are they and where could they go? Those are the main two questions asked by conservation scientists when it comes to invasive species. Originally from other regions and introduced accidentally or intentionally for recreation and commercial purposes, introduced species often migrate into new areas where environmental conditions differ from their native range. When these new environmental conditions are favourable, introduced species can thrive in their new habitats and become invasive. In most cases, invasive species upset the ecological balance of their new host ecosystems, resulting in the extinction of native species and a myriad of socio-economic impacts. Understanding how new environmental conditions can affect an invasive species’ population dynamics and distribution are critical so that resources can be deployed to the right time and place to support effective management and environmental protection.

Many scientific approaches are used to characterise invasive species’ distributions and their interaction with their new host environments, with monitoring and field experiments the most commonly understood and applied. While field studies are invaluable resources of detailed ecological information regard invasive species, their application at large scale can be difficult as they are very time consuming and costly, leading to delays in efficient and effective management actions. An alternative, pro-active approach is to use statistical models known as correlative-SDMs (short for Species Distribution Models), to predict where in a new habitat an invasive species may establish and persist. These sites can then be targeted by landscape managers for control or eradication programs.

Correlative-SDMs are powerful and flexible methods, as they only require known presence and absence locations of the species, and information on the environmental conditions recorded at these sites (e.g. maximum and minimum temperature, rainfall, index of green vegetation, etc.). This information is statistically analysed to generate maps of ‘environmental niche preference’ for the species – that is, areas where the species is likely to establish and persist. A regional comparison can then be used to identify regions with the highest potential risk of being invaded.

Correlative-SDMs are heavily reliant on good data to make good predictions. The best models are built from data covering the entire range of environmental conditions suitable for any given species. While collecting these data is not as complex as directly monitoring a species abundance, it remains challenging because invasive species can be widely distributed in their non-native habitat, and so data may be missing from regions which have not been surveyed due to logistical or financial constraints. One way to overcome this data limitation is to supplement the data collected by experts with data collected by volunteer citizen scientists (e.g. through phone apps device).



European rabbit (Oryctolagus cuniculus), Freemantle, 2015. Photographer: Michael Graham

In our study, we used the case of the European rabbit (Oryctolagus cuniculus) in Australia to explore the advantages and disadvantages associated with the use of citizen science data within correlative-SDMs. The European rabbit (Oryctolagus cuniculus) was introduced into Australia in 1788. Rabbits are now considered a significant pest of agricultural and environmental ecosystems, being listed as a Key Threatening Process to Australian ecosystems and biodiversity in 1999. They compete with native fauna and local livestock by overgrazing both native and introduced plants, which can lead to soil erosion. In the past 50 years, the presence and abundance of rabbits have been monitored through extended management programs led by expert scientists across Australia (Roy-Dufresne et al., 2019). In 2009, a citizen science app was developed as a vigilance program for the rabbit (Feral Scan Data, 2016). This allowed us the opportunity to investigate the pros and cons associated with the use of citizen science data within correlative-SDMs, by comparing the models’ performance when expert and citizen science data were used separately or together.

Front page of the Rabbit Feral Scan website, 29/9/19

We found that there were massive advantages to using both expert and citizen science datasets when formulating correlative-SDMs, with great improvements in model performance. Addition of the citizen science dataset doubled the spatial coverage of expert-only derived occurrence data used to build our models – adding an additional total area equivalent to a third of the total landmass of Australia. On top of this boost in data coverage, citizen science data also provided new and critical information on the environmental conditions associated with the ecological niche of rabbits in Australia. Together, these insights drastically improved model performance and reliability, and highlighted the invaluable value-add of using citizen science when studying and managing invasive species.

Rabbits at Quinyambie Station, NE Australia. Photographer: Peter Bird

Citizen science can therefore be a crucial value-adding component to the development and implementation of more effective monitoring programs for invasive species on a national scale. Our work shows that collaborations between experts and citizen scientists can be a valuable tool, when addressing data deficits associated with low levels of monitoring in areas which are difficult and costly to access. Although it remains important that fieldwork led by experts persists, especially when evaluating the accuracy of data collected by citizen scientists and the level to which the invasive species impact their host habitats, directing the activities of citizen scientists towards areas with missing information can provide an effective solution to collect data quickly and cheaply so that management decisions can occur on a more productive and effective timescale. Frameworks and networks such as the one we used can easily be extended to further other collaborative actions. For example, the collection of dead rabbit carcasses by citizen scientists can help scientists to survey more widely in order to understand diseases dynamics in rabbit populations across Australia, data which can then be used to understand and improve the effectiveness of biological control programs across the country.

Rabbit warren, characterised by heavy grazing and soil erosion, Barossa Valley, 2015

In an attempt to improve the data accessibility in science, we published our complete datasets in an open online publication (Roy-Dufresne et al., 2019).

References:

Feral Scan Data. (2016). https://www.feralscan.org.au/

Roy-Dufresne, E., Lurgi, M., Brown, S. C., Wells, K., Cooke, B., Mutze, G., … Fordham, D. A. (2019). The Australian National Rabbit Database: 50 yr of population monitoring of an invasive species. Ecology, 100, e02750.

A new approach for ecosystem-scale manipulations of bird abundance and species richness

Blog written by Chelsea L. Wood. Read the full paper here.

For generations, ecologists have relied on manipulative experiments to explore the dynamics of ecological communities. Some of the most influential studies in the ecology canon are experimental manipulations – think Bob Paine’s experimental exclusion of the keystone predator Pisaster ochraeceous, Stephen Carpenter’s whole-lake manipulation of nutrients, Dan Simberloff and EO Wilson’s island biogeography experiments in the Florida Keys, or Gene Likens’ forest-clearance experiments at Hubbard Brook. Correlational and comparative studies have their place for detecting and exploring the generality of patterns, but experimental manipulations are needed to understand the causal relationships that underlie ecological patterns.

But despite the value of manipulative experiments, they can be exceedingly difficult to execute, particularly when the focal community contains large-bodied, vagile species. For example, several studies point to the ecological importance of birds as predators, nutrient importers, hosts, seed dispersers, pollinators, and scavengers. But progress in understanding the ecological role of birds at the ecosystem level has been hampered by the difficulty of performing experimental manipulations of bird abundance across large spatial extents.

Our paper, recently published in Ecology and Evolution, presents a new method for experimentally increasing the abundance and richness of birds at the scale of entire aquatic ecosystems, with minimal cost, risk to wildlife, and need for maintenance. This approach involves the use of attractants that encourage birds to use a particular site, instead of deterrents that discourage birds from using that site or physically preventing their access to the site (approaches whose efficacy often attenuates over time). Our approach was effective at increasing the abundance and species richness of water‐associated birds at central California ponds.

Figure 1. Map of study sites in the East Bay region of central California. Eight experimental ponds were located in Joseph D. Grant County Park (circles) and eight were located on San Felipe Ranch (triangles). Of these, eight were randomly assigned to the bird attractant treatment (blue) and eight were randomly assigned to the control treatment (red)

We worked at 16 small ponds located on two adjoining properties in the East Bay area of central California (Figure 1). To attract birds to attractant‐treatment sites, we added perching habitat, nesting habitat, two mallard duck decoys (one male, one female), and one floating platform to each pond (Figure 2). We then assessed bird abundance by monitoring ponds with trail cameras. We compared the change in bird species richness and abundance from before the manipulation (i.e., 2014) to two years after manipulation (i.e., 2017) in control versus attractant treatments (a before–after–control–impact or BACI design).

Figure 2. Attractant manipulations installed at Glorious Pond, Joseph D. Grant County Park. bb = bird nesting boxes, fp = floating platforms, yellow arrows indicate added perching habitat

We found that our bird-attractant treatments augmented both bird species diversity and bird abundance. Bird species richness declined over time in both treatments, probably due to the effects of a prolonged drought that affected California during the time period of our experiment, but the decline in bird richness was less pronounced in the attractant compared to control treatments. Total bird abundance (across all species) increased in the attractant treatment while it declined in the control treatment. The bird species in which the attractant treatment had the most positive influence on abundance were American Robins, Black Phoebes, California Quail, Western Kingbirds, unidentified passerines, raptors, and waterbirds; together, these taxa accounted for 83% of total bird detections.

Our results suggest that simple, inexpensive modifications to existing pond habitat can produce a substantial change in bird abundance and richness – providing a way forward for field experiments that effectively quantify the ecological role of birds. It is especially notable that the manipulations were effective two years after their implementation; this allows for experiments with long temporal durations, a key feature for assessing ecological processes that occur on long time scales.

The manipulations we implemented were inexpensive, easily maintained, and unobtrusive. We estimate that our attractant treatments cost approximately US$103 per pond ($60 for wood duck box, $25 for generic bird box, $2 for fence posts to mount bird boxes, $6 for duck decoys, $10 for materials to construct floating platform), and required fewer than two person‐hours to install. In addition to their low cost, our manipulations were durable and easily maintained: despite the presence of large mammals (e.g., deer, pigs, coyote, cows) that might trample or otherwise compromise attractants, we observed no negative wildlife interactions. Manipulations required very minimal maintenance; we checked on ponds once per year and spent ~15 person‐minutes per pond per year re‐positioning floating platforms or duck decoys, supplementing shoreline perching habitat, or (for only one pond over the two‐year experiment) re‐mounting a fallen bird box. Importantly, the manipulations were unobtrusive and inconspicuous. This low visibility minimizes the chance that the treatments will be noticed by human visitors, reducing the likelihood of vandalism, theft, and objections by neighbors, park users, landowners, or land managers concerned about the aesthetic value of ponds. In fact, one of the land managers we worked with was enthusiastic about these manipulations, which she hoped would contribute to the conservation value of wetlands under her stewardship. The low cost, ease of maintenance, inconspicuousness, and conservation benefits of our approach allowed us to maximize the size and number of manipulated ponds, increasing statistical power and biological realism.

There are numerous potential applications of our approach to manipulating bird abundance and richness. We plan to use this method to perform a large‐scale, long‐term bird manipulation experiment in central California ponds. Our aim is to quantify the effect of increases in local bird abundance and richness on the composition of pond communities, and particularly on the transmission of parasites within ponds. Birds play a variety of roles in these pond ecosystems: as dispersers of parasites, predators of hosts, and hosts for vectors and the pathogens they transmit. Manipulative experiments are therefore necessary to disentangle the potential effects of change in bird biodiversity on disease processes and to discover the net effect of bird biodiversity loss on the prevalence of disease in ponds. Our method of bird augmentation might also be useful for scientists working on other questions about the ecological roles of birds, or in other ecosystems. Most bird manipulation experiments to date have investigated the role of birds as predators using bird deterrence, and bird exclusion is a suitable approach for assessing the impacts of bird predation on community composition at small spatial scales. However, because our approach can be deployed across larger spatial scales than traditional caged or netted bird exclosures, it can also be used to investigate processes that occur at large spatial scales: for example, nutrient export/import, seed dispersal, and scavenging/decomposition. Our approach could also be easily adapted to augment birds across large plots in other relatively open ecosystems—for example, grasslands, meadows, open woodlands, tundra, marshes, wetlands, dunes, and beaches.

The vanishing ghost butterfly and a faint hope for their continued existence in urban forests fragments

Blog written by Antonio C. de Andrade & Matthew Adams. Read the full paper here.

There is a large, white butterfly (Morpho epistrophous nikolajewna) that still flies within the Brazilian Atlantic forest; once common, but now vanishing. Unfortunately, we only have a vague idea of what could be driving this species into oblivion.

The Brazilian Atlantic forest was at the front line of the havoc wrought by the early Europeans centuries ago. By fire and axe, as so elegantly and poignantly described by Dean (1995), the forest was converted to a small patchwork of forests that today is mostly immersed in sugarcane fields. Yet forest fragmentation is not the sole culprit for the demise of the ghost butterfly. There are other agents at work, some obvious (e.g. pesticides and logging) and other not so obvious (subtle local microclimate alteration and by-products of agricultural practices). Our paper details a history of loss, but also hope, which was found in an unexpected forest sanctuary surrounded by roads, buildings and all the urban chaos that can stress wildlife.

Sugarcane burning. Photo by Craig Elliot, FreeImages

The deleterious effects of human activities are greatly augmented in cities. The best example, and perhaps the most emblematic, is vehicular pollution. Thus, we were quite amazed to find a thriving population of ghost butterfly in an urban forest fragment, which are often subject to profound additional environmental stresses compared to rural ones.

The side effects of anthropogenic activities can be scary, yet it is difficult to comprehend their magnitude and how exactly they affect the biodiversity. For example, a group of amateur German entomologists and scientists (Hallmann et al. 2017) reported an alarming 76% decline in the biomass of flying insects over a period of 27 years in protected areas. They could not pinpoint the precise driving forces responsible for this decline, although suggesting agricultural intensification (e.g. pesticide use and habitat loss) as a plausible cause.

It was disappointing, but also important, to find out that many suitable areas failed to maintain populations of this butterfly. Why is the ghost butterfly absent from larger and well-preserved rural forests, while occurring in an urban forest fragment? This is puzzling. The most striking difference we found between these fragments is that all rural fragments we surveyed are surrounded by a matrix of sugarcane that is burned before harvesting, and perhaps during the ghost butterfly’s development they could be subjected to higher particulate pollution in rural fragments.

Pre-harvest burning of sugarcane is a widespread practice – the crops are burned in order to facilitate the harvesting process. The resulting smoke contains harmful gases and tiny particles. Insects can be sensitive to changes in air quality due to the direct way the air enters cells inside their body; they breathe via spiracles, valve-like openings on the outside of their exoskeleton. These openings connect to internal tracheal tubes that form a branching network that reach every part of the insect’s body. We think the effects of the pre-harvesting burning might have a negative effect in the ghost butterfly population, and may have previously been neglected.

It is noteworthy that the proportion of silicon dioxide (SiO2) in the sugarcane ashes is quite high (Le Blond et al. 2010) and could act as insecticide on herbivores (Edwards & Schwartz, 1981). The deposition of particulate, with a high amount of silica, on leaves inhibits feeding, which probably occurs via  physical action by wearing down the mandibles due to the abrasiveness of ashes/dust. Ashes could also cause the impairment of the spiracular function leading to respiratory stress (Elizalde 2014).

Dorsal and ventral views of male ghost butterfly. Photo by Andre V. Lucci Freitas.

The wider implications that the pre-harvest burning of sugarcane might have an impact on biodiversity remains speculative, yet plausible. The presence of a population of butterflies in one place, versus absence at several other places, does not provide unequivocal support for the assumed drivers of rarity. Indeed, the full extent of the effect of smoke pollution still needs to be investigated. Unambiguous evidence of the mechanism we suggest would require some experimental manipulation e.g.  transplants of caterpillars/adults to fragments surrounded by pasture, or experimentally smoking caged individuals using sugar cane residue.

We suggest that the long-life cycle of the ghost butterfly makes it especially sensitive to the sugarcane burning pollution. In this sense, it is interesting to highlight that the endangered M. menelaus eberti occurs in one larger rural fragment – Gargau. This species has multiple generations per year (Andre V. Lucci Freitas, personal communication).

Of course, there could be a number of other hypotheses for causal agents of decline, but in these cases, one would need to show change in abundance through time, and the causal agent of decline. Monitoring ghost butterfly population in urban areas, and comparing these to populations in rural forest remnants is essential to provide a foundation for the assessment of suitable habitats and elucidate the key drivers affecting population dynamics of the ghost butterfly within the Brazilian Atlantic forest.

Our paper shows the importance of native urban forest remnants for conservation i.e. unexpected sanctuaries given the pollution found in cities. Whether urban pollution will drive these populations to evolve, or disappear into oblivion, remains unknown. Human activities have a profound effect on biodiversity and our study serves as a warning regarding the many, often subtle, ways that these activities can cause rarity of once common species.

References

Dean, W. (1995) With broadax and firebrand: the destruction of the Brazilian Atlantic forest. University Press of California.

Edwards, J.S. & Schwartz, L.M. (1981) Mount St. Helens ash: a natural insecticide. Canadian Journal of Zoology 59, 714–715.

Elizalde, L. (2014) Volcanism and arthropods: a review. Ecologia Austral 24, 3–16.

Hallmann, C.A. et al. (2017) More than 75% decline over 27 years in total flying insect biomass in protected areas. PLoS ONE 12, eo185809.

Le Blond, J.S. et al. (2010) Generation of crystalline silica from sugarcane burning. Journal of Environmental Monitoring 12, 1459-70

The role of reef sharks in structuring predatory fish communities after recovery from illegal fishing

Blog written by Mark Meekan & Conrad Speed. Read the full paper here.

Predators structure prey communities both through consumption and by altering the behaviours of their prey. Prey that is wary of a predator is less likely to use a risky habitat and will spend more time being vigilant, which can be costly as it can mean that there is less time for foraging and reproduction. Such influences of predators on prey communities are well documented in terrestrial and some aquatic ecosystems but remain largely unstudied on coral reefs.

Although sharks are the most obvious and ubiquitous large predator in reef environments, research into their effects on fish communities is both limited and controversial. Because sharks can move over large distances and are slow to reproduce, experimental studies that seek to examine the role of sharks as structuring agents by excluding or removing them from reefs are fraught with logistic and ethical difficulties. However, due to fishing, populations of reef sharks in many areas have been in steep decline.

Some researchers have used this situation as a “natural experiment” to compare fish communities of reefs where sharks have been removed to communities on other reefs where populations remain intact. Such studies suggest that sharks may affect the abundance, trophic role and morphology of mesopredatory fishes at lower or equivalent positions in the food chain.

These conclusions are controversial, because fish communities can also differ between reefs due to habitat quality, productivity and the history of natural and anthropogenic disturbances. To avoid these issues, our study used a different approach. Instead of comparing communities between reefs, we looked at how fish communities have changed on the same reef where the enforcement of a no-take Marine Reserve and cessation of illegal fishing has allowed reef shark and fish communities to recover through time. As the reader might imagine, given the current state of the world’s oceans, such opportunities are very rare. 

We worked at Ashmore Reef, a remote atoll located hundreds of kilometers northwest of Australia. The reef is a no-take marine reserve that has had near-fulltime protection of its borders by Australian Customs and Border Force since 2008. Enforcement was essential due to the numbers of vessels fishing illegally in the area, many of which targeted sharks for their fins.

Our earlier work (Speed, Cappo and Meekan 2018) has already shown that shark numbers at Ashmore Reef have recovered at a remarkable rate – up to six times the pace predicted by demographic models. Today, shark abundances on Ashmore are 4.6 times the numbers we recorded before protected status was enforced. But what happened to the rest of the predatory fish community? To find out, we compared data collected by baited remote underwater video stations (BRUVS) in 2004 (prior to enforcement of the reserve), with a more recent BRUVS survey in 2016, which occurred after eight years of strict protection of the reef in 2008. The 2016 survey was conducted as part of the Global FinPrint Project (https://globalfinprint.org/).

The illegal fishing that targeted sharks also caught other large predatory fishes, so it is not surprising that these also recovered once fishing was stopped. In comparison to 2004, large mesopredatory fishes (> 100 cm in length) – species such as the big snappers and large trevallies (jacks) – increased in numbers by a factor of 2.3. The story was similar for medium-sized mesopredators (50 – 100 cm in length) which were mostly coral trout, snappers and emperors. These increased in abundance by a factor of 1.5. However, outcomes were very different for small mesopredators (< 50 cm in length). These species declined in abundance by a factor of 2.5 between 2004 and 2016. Today, abundances and size structures of the shark and fish faunas of Ashmore Reef are comparable to those of other “pristine” reefs in the region nearby, suggesting that fish and shark communities have largely recovered from exploitation.

The changes in abundance of mesopredators at Ashmore Reef we recorded are consistent with theories and observations of how large predators structure trophic pyramids. When large predators are removed, smaller mesopredators become abundant, a phenomenon known as “mesopredator release”. Conversely, when populations of large predators recover, abundances of smaller mesopredators are suppressed. It is difficult to separate the relative contributions of reef sharks and the other large mesopredators to our result. However, it is interesting that we found only a small change over time in the numbers of smaller mesopredators in the near-reef habitats (areas of sand and rubble adjacent to the coral reef) we sampled with BRUVS. In this habitat there was a much smaller increase in shark numbers, but an increase in the abundance of larger mesopredators (by a factor of 2.3) similar to the one that occurred on the reef. This implies that abundances of reef sharks were more influential than large mesopredators in determining changes in numbers of smaller mesopredators.

Why did these changes in numbers of small mesopredators occur? The obvious answer is that predation by larger predators was a key driver, but the story is not likely to be so simple. For example, we found some evidence that certain species of smaller mesopredators moved to different habitats when predation pressure changed. Although total numbers of the spot-cheek emperor (Lethrinus rubrioperculatus) did not vary between surveys, the species occurred in highest numbers in reef habitat in 2004, when there were few sharks and large mesopredators. In 2016, the same species occurred in highest abundances in the near-reef habitat, where there were fewer sharks.

Overall, our results are consistent with earlier studies in NW Australia that used spatial comparisons among reefs with and without sharks to examine the role of these predators in fish communities. These studies also found evidence for mesopredator release in smaller size categories of reef fishes. Although the mechanism(s) underlying the changes we observed in some parts of the fish communities remain to be documented, our study does provide a very positive message for managers and researchers working toward the conservation of coral reef ecosystems. The predatory fish communities of coral reefs, which are invariably the primary targets of exploitation and are so often over-fished in many places around the tropics, can recover at an unexpectedly fast pace. Our study shows that well-enforced marine reserves could play a key role in making this happen.

References

Speed CW, Cappo M, Meekan MG. 2018. Evidence for rapid recovery of shark populations within a coral reef marine protected area. Biological Conservation 220:308-319.

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