BIMONTHLY BULLETIN of the CAYMAN ISLANDS DEPARTMENT of ENVIRONMENT‘S TERRESTRIAL RESOURCES UNIT
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BIMONTHLY BULLETIN of the CAYMAN ISLANDS DEPARTMENT of ENVIRONMENT‘S TERRESTRIAL RESOURCES UNIT @ Jane Haakonsson Sister Islands Rock Iguana Survey 2019 By Vaughn Bodden The Sister Islands Rock Iguana (Cyclura Distance sampling is a method for nubila caymanensis) is critically estimating density within a defined area endangered due to threats such as which does not assume that detection is habitat loss, invasive mammal perfect within that area, relaxing the predation, and mortality from traffic assumption of a traditional plot count along roads (Goetz & Burton, 2012). survey. The main aim of distance sampling is to try to estimate a detection function which is how the The last survey of the Sister Islands Rock detection probability declines with Iguana (SIRI) population in Little distance from a transect line or point Cayman, completed in 2015, estimated (Buckland et al., 2001). The data to the population size to be 2,915 iguanas estimate the detection function is (95% confidence interval: 1,966 – 4,135) obtained from recording the distance of (Rivera-Milán et al., 2015). However, as every iguana from a transect or point. threats to the SIRI population have This decline in detection probability is increased since 2015, updated used to determine what is the effective estimates were needed to inform the area surveyed (i.e. at what distance is management of the species. the likelihood of seeing an iguana too From November 13 – 22, 2019, the small). Then, the counts along with the Terrestrial Resources Unit along with detection function are used to DoE intern Tanja Laaser and two Little determine density of the surveyed area, Cayman volunteers (Greg McTaggart & which can be extrapolated out evenly Tamara Doyle), conducted a distance (or based on habitat types) to the entire sampling survey across the island to get habitable area on the island to get an the data needed for this assessment. estimate of the total population size. Adult un-beaded SIRI and Vaughn Bodden scouting for iguanas with range-finding binoculars on Little Cayman. Photos by Jane Haakonsson. Flicker Bulletin # 46 – FEB / MAR 2020 Using the method described above, the A hatchling radio tracking study found population size of SIRI in Little Cayman that two out of 18 tagged hatchlings is now estimated to be 1,786 (95% (with known fates) were lost due to cat confidence interval: 1,274 – 2,576) (Felis catus) predation (Moss et al., iguanas. This indicates a substantial 2019), while confirmed reports from decline from the 2015 and 2014 surveys residents of Little Cayman indicate a (see figure 1 below). constant loss of hatchlings to cat Although, there is some overlap in the predation. 95% confidence intervals, the declining trend should hold and is supported by external evidence of high traffic mortality and feral cat predation. Iguanas are attracted to roads for thermoregulation and social interaction which makes them particularly vulnerable to vehicle traffic. Traffic mortality reports from the last two years indicate that at least 40 iguanas per year were struck and killed by vehicles. Many more road mortalities, particularly hatchling and yearling sized iguanas, likely go unseen due to being scavenged by land crabs and night herons. Figure 1: SIRI population size comparison between 2014, 2015 and 2019 on Little Cayman. Image by Mark Pothier documenting predation event by feral cat, also on Little Cayman. Flicker Bulletin # 46 – FEB / MAR 2020 Apparent low recruitment levels in the In an attempt to account for the effects population are supported by the of roads and habitat type on density, we observed age class distributions from added relevant covariates to the model each survey (see figure 2 below). which resulted in a much lower population estimate of 886 iguanas In 2019 hatchlings accounted for only (95% confidence interval: 144 – 2,473), 4% of our sample while in 2014 & 2015 compared to when density is they accounted for 19.4% of the extrapolated evenly across the entire sample. The future of the SIRI study area. However, the addition of population in Little Cayman is at severe these covariates on abundance risk if recruitment does not recover. introduced uncertainty in the model and resulted in poor goodness-of-fit test statistics so the results should be Another cause for concern is that the interpreted with caution. The large 95% estimated population size is likely confidence intervals more realistically biased due to the even density represents our (lack of) knowledge of extrapolation across the survey area how many animals are using habitat in and road bias in the survey design. Most the interior which has limited transects were placed on roads due to representation in the current survey accessibility and, because iguanas are design due to accessibility and time attracted to roads, the estimated constraints. In the next Little Cayman density obtained from the model is survey for SIRI our aim is to increase the likely biased high (Buckland et al., number of off-road transects to help 2001). address this unknown. Figure 2: Comparison of age structure across the three surveys (2014, 2015 and 2019). What is worrying is not only the low number of hatchlings (yellow) but also subadults (grey). This indicates low recruitment in the last couple of years. Flicker Bulletin # 46 – FEB / MAR 2020 Combined, these threats have and will References continue to cause an unsustainable Buckland, S. T., D. R. Anderson, K. P. Burnham, J. decline in the SIRI population. L. Laake, D. L. Borchers, and L. 5 Thomas. 2001. Introduction to distance The total reproductive output is being sampling. Oxford University Press, New York, reduced by high breeding adult NY, USA. mortality and recruitment is being reduced by high hatchling mortality. The Goetz, M. & Burton, F.J. 2012. Cyclura nubila key recommendations from this ssp. caymanensis. The IUCN Red List of assessment are: Threatened Species 2012: e.T6043A3100030. http://dx.doi.org/10.2305/IUCN.UK.2012.RLTS.T 6043A3100030.en. Downloaded on 07 January (a) Control the feral cat population on 2020. Little Cayman; Moss, J. B, Gerber, G. P., Goetz, M., Haakonsson, J. E., Harvey, J. C., Laaser, T., Welch, M. E. (b) Promote responsible pet cat (2019). Contrasting patterns of movement husbandry (i.e. keep pet cats across life stages in an insular iguana indoors); population. Journal of Herpetology. Rivera-Milán, F., Haakonsson, J., Harvey, J. 2015. (c) Use current estimates to highlight Little Cayman Rock Iguana Survey Dec 2015. the need for enforcement of road Cayman Islands. safety laws; (d) Promote safe driving practices with consideration for rock iguanas. The same threats are also impacting the Cayman Brac SIRI population. There are management actions required on both islands to mitigate these threats and safeguard this unique species from extinction. The current population size of SIRI on Cayman Brac is unknown but appears to be much smaller than that of the Little Cayman population size. We plan to conduct an assessment for the Cayman Brac population in November 2020. Stay tuned for an update on those findings. Adult SIRI foot and tail drag print on Little Cayman. Photo by Jane Haakonsson. Flicker Bulletin # 46 – FEB / MAR 2020 The 2020 Caribbean Waterbird Census There is still a chance to participate in The CWC is a programme led by the 11th annual Caribbean Waterbird BirdsCaribbean. This multi-partner, Census (CWC)! Join hundreds of birders region-wide programme spans many all over the Caribbean in tracking down communities, individuals and Caribbean waterbirds near you this weekend. organisations. The goal is to enhance our understanding of waterbirds and More than 185 species of waterbirds their range, status and abundance reside in, or migrate to, the Caribbean. throughout the Caribbean in order to Whether endemic and/or globally better conserve and manage these birds threatened or migratory, all species of as well as their habitats. waterbirds are the object for study during this time. Waterbirds can be Waterbirds inhabit areas that are seabirds, wading birds, waterfowl or increasingly important to human shorebirds –you are sure to know a few civilization. Wetlands, such as right away; Magnificent frigatebirds mangroves, sandy beaches, salt ponds (Fregata magnificens), West Indian and tidal flats protect us from storms whistling ducks (Dendrocygna arborea) and floods. In Cayman, such habitats and perhaps Blue-winged teal (Anas provide nurseries for local fish stock, discors) (see front page image by Nicole they support our economy through Martin). tourism and they store and sequent carbon from the atmosphere to name but a few ecosystem services. West Indian whistling duck (left) and juvenile frigatebird (right). Juveniles have white heads, adult males are entirely black while females have a white chest. Photos by Jane Haakonsson. Flicker Bulletin # 46 – FEB / MAR 2020 Participating in the CWC is easy. For a criteria needed for protection as a detailed description of logging your Western Hemisphere Shorebird Reserve records please visit here. You need to Network (WHSRN) site of Regional create an eBird account (click here) if Importance as a direct result of CWC you haven’t got one already and it is efforts. important that you log your observations as a ‘CWC count’ (see Conversely, it is also important to assess screenshot below). if existing protected areas are still suitable for waterbirds and if The CWC is designed to encompass management actions are sufficient, or different levels of complexity which perhaps need revision. When means anyone can get involved attempting to understand population implementing the programme and trends, especially in a small island contribute to the overall monitoring. environment where we still have a This is important as wetlands have chance to save some of our very limited decreased by 64% globally since 1900. resource of wetlands, knowledge Effects of development, degradation, generated by programmes such as the pollution and rising sea-levels render CWC is invaluable.