Predation

One of the most deleterious invasive introduced predators worldwide is the domestic cat which has been found responsible for many island extinctions worldwide. Cats can live off both natural prey and garbage and can be a particularly serious threat to ground-nesting bird populations. Saba is an important location for the Red-billed Tropicbird, Phaethon aethereus and feral cats are thought to be the main reason for the low breeding success in the southern coastal colonies of this bird.

To make proper decisions in invasive predator management, information is needed on the effects of cat removal on the tropicbird breeding success and the possible resulting increase in egg predation by rats in the case of any “mesopredator release effects”. In this study we collected the first season of data needed to assess the effect of cat removal on the breeding success of the tropicbird on Saba. Two tropicbird nesting colonies (Great Level and Tent) were monitored in terms of egg and chick predation, under different net cat-removal intensities and the resulting survival was compared to survival prior to cat removal (as documented elsewhere).

In total, Saba Conservation Foundation removed 19 cats from the entire study area, of which eleven adult cats were removed from the Great Level colony and only six adult cats and two kittens were removed from the Tent colony. The gut contents of the 17 of these 19 feral cats consisted of natural prey (grasshoppers, rats, chickens, anoles and crickets), bait placed in the trap or even plant material. In the previous season 18 cats had already been removed (12 trapped, 6 shot) from the Great Level area.

During the period of September 2013 to May 2014, 46 occupied tropicbird nests were monitored, 27 at Tent, 15 at Great Level and 4 at Fort Bay. Fort Bay was not used in the data analysis. Egg-laying was documented in 34 of these nests. Observed egg failures were due to a variety of causes such as failure to hatch , broken eggs, including the breaking of an egg by an adult, and the disappearance of the whole nest due to heavy rainfall. Egg survival did not show a significant difference between the two colonies. In total 23 chicks were born, of which at least 15 died. Chick survival did show a significant difference between the two colonies, whereas prior to cat removal both had had zero chick survival. The breeding success of the tropicbirds and percentage of chicks fledged did appear to increase encouragingly in the breeding colony where cats had been more intensely culled (Great Level; 28 of initial 35 adult cats removed during two trapping seasons). The success on Great Level is notable, because in the breeding season of 2011/2012 the breeding success had been zero percent for several years.

Around the Tent colony only six adult cats were removed this season (total of 7 removed during two trapping seasons), which was insufficient to effectively increase breeding success in the tropicbird. A comparison of camera-trap densities showed that effective cat density at Tent by the end of trapping remained 4-5 times higher than at Great Level where 28 of the initial 35 adult cats had been removed. In total four black rats were observed on the camera traps but only appeared to be scavenging and no active egg predation was observed. These preliminary results suggest that cat removal seems to improve fledgling survival at no appreciable expense in terms of egg predation and that risks of any hypothetical “mesopredator release effects” are limited. Due to the low sample sizes in this first season, and natural fluctuations in breeding success which are normal in seabirds, clearly happenstance or other causative factors could equally explain the results obtained. Therefore, more definitive conclusions will depend on a more extensive and multi-year effort. 

Key recommendations:

• Continue with and expand feral cat removal from the main tropicbird nesting colonies.
• Simultaneously monitor nesting success and fledgling survival to develop a more robust data set over a longer time-frame. With an expanded sample size, the benefits in terms of net fledgling survival and any risks of potential “mesopredator release effects” can be more firmly assessed.
• Many cats were documented to be wary of traps. Trapping was also very labour-intensive and entailed both trapping and handling stress. For these reasons additional, more effective yet humane methods (such as predator baiting or shooting) should be used. These methods have proven to be key to effective control of invasive predators worldwide.
• As long as legislation and control of cat importation, keeping and sterilization remain less than strictly implemented and failsafe solutions remain wanting, we recommend to focus removal efforts towards key tropicbird nesting colonies shortly before or during the main nesting season each year.

  This research was funded as part of the Wageningen University BO research program (BO-11-011. 05- 029) under project number 4308701028 (A Debrot, PI). 

Reef fish have a pelagic larval stage and settle onto the reef before transitioning to their juvenile or adult morphologies. Settlement can be dangerous for new recruits and mortality is highest during the first one to two days after settlement. Experiments were conducted to determine the effects of predation and habitat depth on reef fish recruitment. Standard habitat units (SHUs) were created from pieces of Millepora skeleton. Two treatments were created using SHUs. The first contained an SHU placed on the substrate that was open to predation (NC). The second treatment contained an SHU in a wire cage to exclude predators (FC). Two replicates of each treatment were placed in two meters of water and at six meters. New recruits were surveyed twice a week for six weeks (n=11 surveys). SHUs were cleared of recruits and algal growth was removed after each census. Overall recruitment was greater in two meters than six meters of water. Recruitment was also greater in FC treatments than in NC treatments at both two and six meters. Trends in recruit density should not have been observed since recruit censuses were taken as replicates. However, recruitment increased over the course of the experiment, which coincided with the lunar cycle. Seven species of fish were observed over the course of the experiment but only two of these species, the wrasse, Halichoeres bivittatus, and the razorfish, Xyrichtys splendens, were observed at six meters. One species, the pufferfish Canthigaster rostrata, was only observed once.

This student research was retrieved from Physis: Journal of Marine Science XII (Fall 2012)19: 1-8 from CIEE Bonaire.

The burglar alarm hypothesis states that dinoflagellate bioluminescence is used to avoid predation from copepods by taking an altruistic approach. This study aims to test the effectiveness of the in-situ dinoflagellate defense mechanism in the waters of the western coast of Bonaire, (Dutch Caribbean). According to the burglar alarm hypothesis, areas of higher bioluminescence should have lower abundances of copepods, however there is little evidence supporting this. To analyze the effectiveness of the burglar alarm hypothesis, horizontal tows were used to collect plankton samples at four depths: the surface, 10, 20, and 30 feet. Dinoflagellate bioluminescent display frequency was recorded immediately after collection in a dark room. Copepods and Dinoflagellates were then identified under a compound microscope and abundance was analyzed. Two-way ANOVA tests showed significant relationships between the decreased ratio of copepod to dinoflagellate abundance associated with increased bioluminescence frequency and increased bioluminescence frequency in relation to increased depth. No significant difference was found in bioluminescent displays in comparison with various weather conditions. This study shows that the burglar alarm has positive effects in maintaining dinoflagellate populations. The environment at depth allows for more dinoflagellate bioluminescent displays to occur than at the surface. Implications of light pollution may cause variations in bioluminescence at shallower depths, lessening the effectiveness of the luminescent defense mechanism of dinoflagellates. Comparisons of biolumi-nescence between the areas exposed to light pollution and areas sheltered from ambient light can be used to analyze how the primary trophic level will change with increasing human impacts.

This student research was retrieved from Physis: Journal of Marine Science XIII (Spring 2013)19: 45-52 from CIEE Bonaire.

Populations of Diadema antillarum have had low densities ever since its mass mortality event in 1983. A slow population density increase has resulted from fertilization complications due to extensive distance between individuals. The relationships between D. antillarum and their competitors and predators as a cause for the lack of population recovery has not been directly studied. The correlation of D. antillarum density with the abundance of predators, competitors, and microalgae, was studied to determine additional possible explanations for the low density of individuals. There were three dives during the day at six sites. The day dives included observational fish counts and transects and quadrates to assess percent algae cover in a 10 m2 . While the night dives include observation counts of all of the urchins in the 10 m2 . No increase was found in Diadema antillarum density compared with a study in 2009 (0.005 individuals per m 2 ). No significant correlation was determined between D. antillarum density and predator density. A weak, positive correlation between competitor density and D. antillarum density was determined. In contrast, a strong, positive correlation between percent algae cover and D. antillarum density was found This study revealed additional pressures on D. antillarum population (e.g. competitors, percent algae cover), which could account for the slow recovery of local D. antillarum population in Bonaire.

This student research was retrieved from Physis: Journal of Marine Science XIV (Fall 2013)19: 25-32 from CIEE Bonaire.

Hermodice carunculata, commonly known as the Bearded Fireworm, is a corallivorous Polychaete found throughout the Atlantic Ocean and the Caribbean and is noted for its fluorescence. Studies have found that the highest abundance of H. carunculata is in water shallower than 1 m. The present study observed the habitat, size, and fluorescent patterns of H. carunculata in the intertidal zone of Yellow Submarine dive site on Bonaire. Three transects were laid at 55 cm and 110 cm deep, at 20 and 50 minutes after sunset. Additionally, fireworms were caught in wire traps to be more closely observed in the laboratory under a dissecting microscope. There was no significant difference between the depth (110 cm or 55 cm) and the size (less than or greater than 6 cm), nor was there a difference in abundance between the two time periods of data collection (20 minutes and 50 minutes after sunset). Furthermore, there was no significant difference between the fluorescent pattern (GREEN, GOB, OOB, or ROB) and the substrate (algae, coral, rubble, rock, or sand) the individual was found on, or fluorescent pattern and size. There was, however, a significant difference in density of fireworms per square meter over the five-week study period. Fireworm predation can have a large impact on the health of corals. This paper aims to increase the understanding of H. carunculata, so that the corals can be better protected, and the interaction between these two organisms can be better understood.

This student research was retrieved from Physis: Journal of Marine Science XVIII (Fall 2015)19: 54-60 from CIEE Bonaire.

Globally, scleractinian coral populations are declining, and to fully understand this decline it is important to study potential coral stressors in-situ. One particularly interesting means of studying stressor effects is fluorescence in corals. Till now fluorescence research has focused primarily on laboratory studies. These experiments cannot fully account for real world effects of stressors such as disease, predation or competition on corals fluorescent patterns in nature. The purpose of this study was to develop a means of in-situ observation to study how coral are using fluorescent proteins in nature. Five sample organisms were used for each of the three categories of stress, and one group of healthy corals were used as control, UV photographs of each were then taken on a weekly basis. Visual trends across the photographs were analyzed for gradients in both red and green fluorescence using Photoshop. From this we detected patterns on predated and competing corals as well as significant gradients in both diseased and healthy corals. Healthy coral results indicated issues in light dispersal across coral colonies necessitating a reworking of the methodology for clearer results. However the presence of discernable trend lines across all other categories supports that this methodology could still be effective for future monitoring efforts. RFP and GFP associated proteins are good candidates for indicating the health of threatened coral reefs due to their ease of use and associations with important coral functions making the methodology discussed here significant in allowing their use.

This student research was retrieved from Physis: Journal of Marine Science XIX (Spring 2016)19: 64-73 from CIEE Bonaire.

Various marine organisms are known to consciously select specific types of habitat that provide maximum shelter from potential predators. Reef fish such as Haemulon chrysargyreum (smallmouth grunts) are commonly seen congregating in groups around the coral structures in Bonaire. Observing schooling fish can provide pertinent information on the refuge provided by structurally complex and diverse ecosystems. This study assessed the habitat preference of H. chrysargyreum based on species of coral, complexity of sites, and substrate type. Levels of phosphate, nitrate, and ammonia were also analyzed in the areas occupied by shoaling H. chrysargyreum to see if they provide a significant input of nutrients into the coral reef ecosystem. The results of this study demonstrate that H. chrysargyreum prefer areas of medium to high complexity accompanied with a soft substrate (sand, rubble) and an overhanging structure. Nutrient level analysis was inconclusive and, therefore, requires further studies. This research sought to identify certain species of coral and structures that are used by H. chrysargyreum for habitation. Such knowledge can aid conservation efforts by honing in on specific areas that schooling fish utilize for shoaling and feeding. Additionally, data from this study provided preliminary assessment for future studies on the potential nutrient input of H. chrysargyreum to the marine ecosystem.

This student research was retrieved from Physis: Journal of Marine Science XIX (Spring 2016)19: 52-63 from CIEE Bonaire.

Abstract:

Species invasions have a range of negative effects on recipient ecosystems, and many occur at a scale and magnitude that preclude complete eradication. When complete extirpation is unlikely with available management resources, an effective strategy may be to suppress invasive populations below levels predicted to cause undesirable ecological change. We illustrate this approach by developing and testing targets for the control of invasive Indo-Pacific lionfish (Pterois volitans and P. miles) on Western Atlantic coral reefs. We first developed a size-structured simulation model of predation by lionfish on native fish communities, which we used to predict threshold densities of lionfish beyond which native fish biomass should decline. We then tested our predictions by experimentally manipulating lionfish densities above or below reef-specific thresholds, and monitoring the consequences for native fish populations on 24 Bahamian patch reefs over 18 months. We found that reducing lionfish below predicted threshold densities effectively protected native fish community biomass from predation-induced declines. Reductions in density of 75- 95%, depending on the reef, were required to suppress lionfish below levels predicted to over-consume prey. On reefs where lionfish were kept below threshold densities, native prey fish biomass increased by 50-70%. Gains in small (<6cm) size classes of native fishes translated into lagged increases in larger size classes over time. The biomass of larger individuals (>15cm total length), including ecologically important grazers and economically important fisheries species, had increased by 10-65% by the end of the experiment. 

Crucially, similar gains in prey fish biomass were realized on reefs subjected to partial and full removal of lionfish, but partial removals took 30% less time to implement. By contrast, the biomass of small native fishes declined by more than 50% on all reefs with lionfish densities exceeding reef-specific thresholds. Large inter-reef variation in the biomass of prey fishes at the outset of the study, which influences the threshold density of lionfish, means that we could not identify a single rule-of-thumb for guiding control efforts. However, our model provides a method for setting reef-specific targets for population control using local monitoring data. Our work is the first to demonstrate that for ongoing invasions, suppressing invaders below densities that cause environmental harm can have a similar effect, in terms of protecting the native ecosystem on a local scale, to achieving complete eradication.