Invasive predators

Parental Behavior Linked to Declining Nest Success

New Red-billed Tropicbird data suggested declining nest survival rates may be more related to parental behavior than previously thought. A researcher from Utrecht University, along with STENAPA and CNSI staff analyzed nest characteristics, trap camera photos and historical data to better understand the driving factors to declining nest survival rates.

Seabird populations are facing unprecedented levels of threats which has resulted in nearly 70% loss of monitored seabird populations over the past fifty years.  This is faster than any other group of birds worldwide and can be linked to overfishing, habitat destruction, invasive species, increased storm events and changes in food availability.

Red-billed Tropicbird and chick. Photo credit: Michiel Boeken

Pilot Hill

The Red-billed Tropicbird are an iconic species for the northern Dutch Caribbean islands.  In fact, a small but globally significant colony of breeding pairs nests within the Boven Important Bird Areas of Sint Eustatius.  Unfortunately, this area has been experiencing high rates of nest failure over the past decade. New research, conducted by Utrecht University student Hailley Danielson-Owczynsky, analyzed a combination of previously collected information with new monitoring data from the Pilot Hill nesting area in an attempt to better understand what is causing these low success rates.

Nest Monitoring

Previously, high predation by cats, rats and crabs were blamed for declining egg survival rates.  This was partially linked to the success of predator control projects on Saba, where nest survival of one site was improved from 0 to 40% after two-seasons of cat removal. Additionally, when reviewing the 1.6 million photos taken by nest cameras between 2017 and 2022, 80.5% of monitored nests showed predator presence, leading managers to blame predators for declining numbers. Further analysis conducted by Danielson-Owczynsky found that before being consumed by predators, many of the eggs had already been left unattended by parents for a prolonged period of time.  This same study found that predators were much less likely to visit a nest when a parent was nearby. Therefore, egg failure is more likely attributed to parent behavior prior to its predation.  It is important to note that invasive predators are still a concern, these results merely give an additional factor for conservation managers to consider.

Difficult foraging conditions for Tropicbird parents could lead to decreased nest attendance.  Red-billed Tropicbird’s diet is comprised primarily of flying-fish; however, overfishing has led to a decrease in overall fish populations requiring parents to forge over greater distances for longer periods of time.

Red-billed tropicbird. Photo source: Christian König

Implications

Pelagic seabirds, such as the Red-billed Tropicbird possess many unique traits, such as long lifespans, slow development and low reproductive rates, often attributed to their limited and sporadic food supply.  Given their long-life spans and the fact that they forage across wide ranges, these birds have been used as indicators for overall marine ecosystem health.  Therefore, the rapid decline in local populations gives key insight and rising concern for overall marine environmental conditions.  Understanding the magnitude of threats facing these and other related species will be key in safeguarding a strong and resilient marine environment moving forward.

 

Report your sightings

Species reports by local communities and tourists are invaluable for nature conservation efforts to help increase public awareness and overall species protection.

You can report your Red-billed Tropicbird sightings and photos on the website www.Observation.org or download the free apps (iPhone (iObs) & Android (ObsMapp)). You can also send your information to research@DCNAnature.org for support. An automatic species recognition tool is in development, so stay tuned for updates.

 

To learn more, you can find the full report by using the link below.

More info in the Dutch Caribbean Biodiversity Database

 

 

Published in BioNews 56.

Date
2022
Data type
Media
Theme
Research and monitoring
Geographic location
St. Eustatius
Author

Density-dependent colonization and natural disturbance limit the effectiveness of invasive lionfish culling efforts

Culling can be an effective management tool for reducing populations of invasive species to levels that minimize ecological effects. However, culling is labour-intensive, costly, and may have unintended ecological consequences. In the Caribbean, culling is widely used to control invasive Indo-Pacific lionfish, Pterois volitans and P. miles, but the effectiveness of infrequent culling in terms of reducing lionfish abundance and halting native prey decline is unclear. In a 21-month-long field experiment on natural reefs, we found that culling effectiveness changed after the passage of a hurricane part-way through the experiment. Before the hurricane, infrequent culling resulted in substantial reductions in lionfish density (60–79%, on average, albeit with large uncertainty) and slight increases in native prey species richness, but was insufficient to stem the decline in native prey biomass. Culling every 3 months (i.e., quarterly) and every 6 months (i.e., biannually) had similar effects on lionfish density and native prey fishes because of high rates of lionfish colonization among reefs. After the hurricane, lionfish densities were greater on all culled reefs compared to non-culled reefs, and prey biomass declined by 92%, and species richness by 71%, on biannually culled reefs. The two culling frequencies we examined therefore seem to offer a poor trade-off between the demonstrated conservation gains that can be achieved with frequent culling and the economy of time and money realized by infrequent culling. Moreover, stochastic events such as hurricanes can drastically limit the effectiveness of culling efforts.

Date
2017
Data type
Other resources
Theme
Research and monitoring

To eat or be eaten: Consumer induced behavior in variegated feather duster worms (Bispira variegata)

Predator-prey interactions are a driving force for many identifying features of ecosystems. Predation pressure not only strongly influences population structure and distribution but also influences prey behavior. Predator avoidance behavior may play a large part in self-defense. In marine systems, there are many cases of organisms which are not capable of changing physical location as a predator avoidance technique (e.g.,, sponges, corals). In this study, Bispira variegata was used as an indicator for consumer impact on behavior in a benthic, sessile organism. Primarily, population structure (i.e. size, density) was assessed. Paired clipped and unclipped populations were used to quantify an increase of potential predators based on exposure and the impact that they may have on worm response. In order to determine the impact of predation pressure, worms were observed and video recorded to assess the distance from simulated predation that caused worms to react. There was a mean density of 12 worms m-2. The population was variably distributed with no bias towards the large nor the small end of the size range represented. There was significant difference in the number of potential predators which entered the clipped plots (p = 0.005). For the predator simulation, there was extreme variation on the distance from the simulated predator that triggered a reaction, specifically retraction. The results of this study can be used as a stepping stone for understanding the effect of predation pressure on benthic, sessile organisms by showing the impact of three-dimensional habitat as well as the behavioral reactions to simulated predation.

This student research was retrieved from Physis: Journal of Marine Science IV (Fall 2008)19: 35-39 from CIEE Bonaire.

Date
2008
Data type
Other resources
Theme
Research and monitoring
Geographic location
Bonaire
Author

Anti-predator responses of herbivorous ocean surgeonfish, Acanthurus bahianus, to invasive lionfish, Pterois volitans

Since their introduction into the western Atlantic, lionfish (Pterois volitans) have become a major threat to the coral-reef ecosystems. Lionfish have proven to be formidable predators and it has been demonstrated that they have to potential to reduce recruitment of native fishes and even contribute to phase shifts. It is known that lionfish are capable of hunting and catching many species of native fishes, however little is known about the anti- predator responses of those prey fish. This study examined the antipredator responses of herbivorous ocean surgeonfish to lionfish. The ocean surgeonfish were visually exposed to a lionfish and their resulting behaviors were analyzed to determine if the fish exhibited an anti-predator response to an invasive predator. The naïve prey hypothesis predicts that prey will not react to predators if they do not share an evolutionary history with those predators, therefore it was predicted that juvenile ocean surgeonfish would not exhibit anti-predator behaviors in response to lionfish. The results of this study were inconclusive due to the absence of a positive control, but they indicate that ocean surgeonfish do not exhibit an anti-predator response to lionfish. If this study were to be successfully repeated with a positive control, the results may help to enhance our understanding of the ecological effects lionfish are having on the coral-reef ecosystem and will help us to mitigate those effects.

This student research was retrieved from Physis: Journal of Marine Science XV (Spring 2014)19: 21-29 from CIEE Bonaire.

Date
2014
Data type
Other resources
Theme
Research and monitoring
Geographic location
Bonaire
Author

What Doesn’t Kill You Makes You Wary? Effect of Repeated Culling on the Behaviour of an Invasive Predator

Abstract:

As a result of being hunted, animals often alter their behaviour in ways that make future encounters with predators less likely. When hunting is carried out for conservation, for example to control invasive species, these behavioural changes can inadvertently impede the success of future efforts. We examined the effects of repeated culling by spearing on the behaviour of invasive predatory lionfish (Pterois volitans/miles) on Bahamian coral reef patches. We compared the extent of concealment and activity levels of lionfish at dawn and midday on 16 coral reef patches off Eleuthera, The Bahamas. Eight of the patches had been subjected to regular daytime removals of lionfish by spearing for two years. We also estimated the distance at which lionfish became alert to slowly approaching divers on culled and unculled reef patches. Lionfish on culled reefs were less active and hid deeper within the reef during the day than lionfish on patches where no culling had occurred. There were no differences at dawn when removals do not take place. Lionfish on culled reefs also adopted an alert posture at a greater distance from divers than lionfish on unculled reefs. More crepuscular activity likely leads to greater encounter rates by lionfish with more native fish species because the abundance of reef fish outside of shelters typically peaks at dawn and dusk. Hiding deeper within the reef could also make remaining lionfish less likely to be encountered and more difficult to catch by spearfishers during culling efforts. Shifts in the behaviour of hunted invasive animals might be common and they have implications both for the impact of invasive species and for the design and success of invasive control programs. 

Date
2014
Data type
Scientific article
Theme
Research and monitoring
Journal

BioNews 13 - February 2014

This month’s issue profiles several scientific discoveries with new implications for conservation in the Dutch Caribbean. On St. Eustatius, the first evidence of a rat stealing a seabird egg from a nest was captured with the use of camera traps – a component of STENAPA’s Red-billed Tropicbird conservation work. On Curaçao, a research team from Naturalis Biodiversity Center in the Netherlands carried out dedicated surveys on crabs, shrimps and molluscs living in association with corals and other invertebrate hosts. The study yields several new species records for Curaçao, new symbiotic associations and possibly even species new to science.

Caribbean-wide, lionfish removal efforts have been a major component of conservation action in regards to this invasive predator. New evidence suggests this could be a permanent, but successful solution in controlling and limiting the impacts of the Lionfish. Additionally, the lionfish control website (www. lionfishcontrol.org), originally set up by STINAPA Bonaire and DCNA to support and monitor lionfish control efforts has been expanded to include Aruba, Curaçao and St. Maarten.

Content:

  • ‘Breaking News’: Rats confirmed stealing eggs on St. Eustatius!
  • New Biodiversity Records for Curaçao
  • Lionfish Control Efforts Show Success
  • Overview of Research and Monitoring Efforts
  • Calendar of Upcoming Events, Meetings and Workshops
Date
2014
Data type
Media
Theme
Governance
Legislation
Geographic location
Aruba
Bonaire
Curacao
Saba
Saba bank
St. Eustatius
St. Maarten
Author

BioNews 10 - November 2013

This month’s issue highlights the Saba Bank Expedition that took place from 19 to 27 October. As a follow up to the Saba Bank survey in 2011, an international team of experts investigated the ecological functionality of the Bank, focussing on corals, fish, genetics, nutrients and dissolved organic carbon. The first glance at the results appears promising with coral cover increasing and algae cover decreasing, but final results will be published by next year.

The publication of a study on the island of Saba shows that invasive predators, such as feral cats, can have a large impact on breeding colonies of Red-billed Tropicbirds. Cat densities were investigated and varied largely between elevation levels. In some parts of the island, overall cat health turned out to be very poor. This already influenced the decision to stop releasing spayed/neutered cats back into the wild. Hopefully this will aid in decreasing the overall feral cat population and save the beautiful Red- billed Tropicbird, which has become an iconic symbol for the island of Saba.

Other content:

Date
2013
Data type
Media
Theme
Education and outreach
Research and monitoring
Geographic location
Aruba
Bonaire
Saba
Saba bank
St. Eustatius
St. Maarten
Author

Predation threats to the Red- billed Tropicbird breeding colony of Saba: focus on cats

Summary:

Feral domestic cats (Felis catus) are recognized as one of the most devastating alien predator species in the world and are a major threat to nesting colonies of the Red-billed Tropicbird (Phaethon aethereus), on Saba island, Dutch Caribbean. Cats and rats are both known to impact nesting seabirds and hence are both potential threats to the tropicbird on Saba. However, whereas the tropicbird has coexisted with rats for centuries, cats have only recently become a problem (since about 2000). Several studies from the region suggest that the tropicbird may be less-vulnerable to rats but cats have been unequivocally implicated in the depredation of tropicbird nests on Saba (unpublished data, Michiel Boeken). In this study we collected baseline data on cat and rat distribution, and cat diet and health. We also conducted 83 questionnaire interviews with Saba residents to assess their views on cats, rats, tropicbirds and the acceptability of different management options.

Two methods were used to assess cat density distribution. We used baited camera traps (73 successful 2-night deployments divided among 4 habitat categories) as an index of relative density. We also used scat densities (collected from 15,474 meters of transect from eleven trails) from which to extrapolate and compare relative cat densities in different habitats. For the study of diet we collected and analysed a total of 94 cat scats and studied the intestinal contents of 13 sacrificed feral cats.

Both scat densities and camera trap recordings showed large and statistically significant differences in cat density on trails between habitat zones. Cat densities were lowest in the lush forest habitat found at higher elevations (mean: 4 cats/km2) on the island and highest at lower coastal elevations. Densities were particularly high (mean: 286 cats/km2) in the small area surrounding the island landfill where food, consisting of human refuse and garbage was abundant. These densities are for areas along hiking- and goat-trails. These are actively selected by cats. Therefore, densities along trails cannot be simply extrapolated to the rest of the island.

While at the landfill daily incineration of garbage takes place, un-incinerated garbage is left open overnight almost every day. This provides feral cats with an ample food source. Construction of a vermin- proof, concrete overnight storage pen could greatly reduce food availability to cats and rats. Dry woodland and coastal scrub trail habitat had intermediate cat densities (respectively, 107 and 166, cat/km2). Rat density as documented using camera traps was highest in the forest habitat where food, water and shelter for rats was particularly abundant. Rat and cat density were markedly inversely related. Cats were concentrated at lower elevations and in more open areas where tropicbirds principally nest.

Diet analysis showed the feral cats on Saba are an opportunistic predator-scavenger consuming various categories of prey (mammals, birds, reptiles, and insects) as well as anthropogenic waste. Food composition of the feral cats of Saba differed significantly between habitats. Overall, rats were the most important food species of the cat, followed by reptiles and birds. However at the landfill where cat population densities were highest, birds and rats were strongly reduced in the diet, while garbage and reptiles were of greatly increased importance. Scats collected in the forest zone suggested a lower importance of rats in the diet of cats than at lower arid elevations. The cat scats collected in the urban environment had no prey species represented and were apparently all from well-fed house cats. The opportunistic and flexible food habits documented for cats in this study allows them to easily switch to seasonally abundant prey (for instance during the seabird breeding seasons).

Preliminary veterinary assessments on cats removed from the landfill showed the animals to be in overall poor health. This suggest that releasing neutered cats back into the wild without any further supporting care may be much less humane than typically assumed. Based on these results and taking into account the welfare concerns of the tropicbirds preyed upon by cats, the Saba Foundation for Prevention of Cruelty to Animals (SFPCA) has decided to discontinue its practice of releasing neutered unwanted cats into the wild (Trap-Neuter-Release, or TNR).

Public views regarding cats, tropicbirds and management options as assessed using a simple questionnaire and 83 questionnaire returns were obtained. Around 30% of the participants owned cats of which only about 5% were not neutered. A significant majority of respondents (66%) believed feral cats on Saba are an environmental problem. In this there was no difference between natives and expat residents. Most (44%) believed that cats were principally a problem in being a threat to wildlife. Diseases and parasites was quoted as the second-most perceived problem with feral cats (30%). Feral cats and rats scored as the two most important perceived threats to the tropicbird (respectively, 65% and 70%). When asked “how many saved tropicbirds do you think justifies the death of one cat”, 38% of Sabans and 22% of expats did not answer the question. Of those that did answer, 36% of Sabans and 48% of expats, valued the life of a tropicbird more than that of a single cat. The remainder considered cats somewhat more important than one tropicbirds but only few (5% natives, 18% expats) considered feral cats more important than the combined sum of all their tropicbird prey. Between 70-80% of respondents thought registration, neutering and removal of cats from breeding colonies was a good idea. When asked if euthanization would be acceptable to them, a significant majority (80%) found it to be an acceptable method for use in cat control. Finally, 43% even thought that total eradication of all cats (domestic and feral) from the island would be a good idea. Of the participants upwards of 80% stated that rats were also an environmental problem and more measures to control rats are supported by 75% of those interviewed. Awareness and willingness of the Saban resident population towards measures against cats and rats are clearly high. This means that there exists a wide management scope to implement measures with which to address these problems.

For more than 15 years government rat control has used brodifacoum as the main rodenticide, but rats remain a widespread and unrelenting problem on Saba. This suggests that the rats might well have become partly resistant to this anticoagulant toxin and that the time has come to alternate to a different rodenticide. Alternating use of rodenticides is the internationally recommended practice for rat control. It is already being practiced on nearby St. Eustatius where rat problems are much less acute than on Saba (but where feral fruit trees are also less abundant and landfill practices are also less favourable to vermin).

Key management recommendations:

  • Upgrade the 2004 Saba “Island Ordinance on Identification and Registration of Livestock and Domestic Animals” to prohibit the importation and keeping of unneutered cats.
  • Capacitate the SFPCA to enforce the mandatory registration of domestic animals.
  • Construct a vermin-proof concrete overnight pen at the Saba landfill (for secure storage of the garbage that might not be incinerated the same day).
  • From now on euthanize all unwanted and uncared-for stray and feral nuisance cats.
  • Judiciously use humane euthanization to address the acute overpopulation of cats in particularly sensitive areas (such as the mapped seabird colonies) as this is quite acceptable to Saban residents.
  • Start use of a new alternative rodenticide, following the example from St. Eustatius.
  • Management measures need to be accompanied by an (inter)active and effective communication plan to keep public support levels high.

Key recommendations for research:

  • Investigate the role of rats as predators, not only with respect to seabirds but also in the forested zone where they are most abundant and may seriously impact native forest species.
  • Assess the effect of cat removal on tropicbird breeding success and on rat population density, as well as the broader predator-prey relationships on the island. 
Date
2014
Data type
Research report
Theme
Research and monitoring
Report number
C011/14
Geographic location
Saba

Interactions between invasive mammals and their effects on Red-billed Tropicbird (Phaethon aethereus) nesting productivity

Abstract:

Red-billed Tropicbirds (Phaethon aethereus) are cavity-nesting seabirds with important nesting colonies on Saba and Sint Eustatius in the Dutch Caribbean. Breeding productivity has been steadily declining and nest failure is primarily attributed to the presence of invasive mammalian predators. Camera traps were deployed to determine the extent of invasive species presence and impact. Cats, rats, and goats as well as native land crabs and lizards were present on both islands and actively visiting nest sites. The positive relationship between goat visitation and cat visitation suggests that goats may be facilitating cat predation by increasing visibility and accessibility to nests. Disturbance from goats may be causing nest abandonment by breeding adults, which warrants further study. A significant proportion of nests surveyed (n=106) were failures (n=71). It can be inferred that invasive mammalian presence at those sites is contributing to nest failure. There is not enough evidence to draw any conclusions on the possible impact of native predators. Further and more extensive monitoring is recommended to better quantify impacts and invasive species behaviour for eradication programme design. 

Date
2013
Data type
Research report
Theme
Research and monitoring
Geographic location
Saba
Author