Eradicating introduced mammals from a forested tropical island

  • Published source details Rodríguez C., Torres R. & Drummond H. (2006) Eradicating introduced mammals from a forested tropical island. Biological Conservation, 130, 98-105.


Oceanic island faunas have suffered degradation due to human activities, particularly through the accidental or deliberate introduction of mammals. Conservation managers, particularly in countries such as New Zealand, have eradicated introduced mammals from some islands in order to reduce predation on native, and often endangered, fauna (e.g. see Cases: 106, 157, 174 and 242). In 1930, domestic cats Felis catus and black ship rats Rattus rattus were introduced by fisherman to Isla Isabel off the Pacific coast of Mexico. By 1991, Isla Isabel had one of the densest insular populations of feral cats in the world (113/km²). Through scat analysis, approximately 50% of cat diet consisted of marine birds, and 24% native reptiles. Although attempts to eradicate black ship rats Rattus rattus on Isla Isabel were unsuccessful (see Case 323), methodologies from New Zealand were successfully adapted and used to eradicate cats from this tropical island, as summarised in this case study.

Study area: Isla Isabelle is a 194 ha, uninhabited, densely forested island located in the Pacific Ocean 30 km from the coast of the state of Nayarit (Mexico). The island has variable topography with steep cliffs, rocky slopes and flatter areas covered with relatively dense tropical forest. It was gazetted as a National Park in 1981 because of its importance for seabird. Eight species nest in large colonies: blue-footed booby Sula nebouxii, brown booby S.leucogaster, brown pelican Pelecanus occidentalis, magnificent frigatebird Fregata magnificens, red-billed tropicbird Phaethon aethereus, Heermann's gull Larus heermanni, sooty tern Sterna fuscata, brown noddy Anous stolidus. There is also a small colony of red-footed booby Sula sula.

The island comprises three major habitat zones:

1) A central forested zone, dominated by two tree species: garlic pear Crataeva tapia and the tree-spurge Euphorbia schlechandalii.

2) North-eastern forest, grassland and rock slabs, with introduced plants, notably sugar cane Saccharum officinarum and bananas Musa paradisiaca.

3) Southern grassland and forest with introduced sugar cane, banana, pineapple Ananas comosus and lime Citrus aurantifolia.

Cat eradication: The cat eradication programme began on 21 October 1995, capitalising on the time of year when both bird nesting activity and the availability of alternative food in the form of fish scraps from fisher's camps was minimal. Over nine days, 94 trails were cut over walkable areas of the island (cliffs were excluded). The trails ran parallel to each other at 20 m intervals. Three techniques were used to eradicate cats: poisoning, trapping and hunting with firearms.

On the 8 November 1995 and during the next eight days, fresh fish laced with poison (4 ml of a 0.05% solution of 1080, injected into 16 g cubes of locally captured marine fish) was laid out at bait stations. Bait stations were set along every other trail and at 40 m intervals, with a total of 613 bait stations in all. Similar-sized lumps of chicken flesh or canned tuna was used as an alternative bait after the intial 9-days. Fresh fish and chicken baits were suspended 40 cm above the ground from a tree limb using nylon fishing line. Lumps of canned tuna (too friable to hang) were put on plastic plates set on boulders. In treeless areas, all baits were placed on small boulders. Baits were laid daily between 17:00 and 20:00 hrs and any unconsumed baits were removed and buried next day between 05:00 and 07:00 hrs.

After the initial eight days, trapping and hunting was also carried out for three years until all cats had been captured or killed. Leg-hold Victor 1.5 traps were set beside paths, in small clearings and outside potential dens. Five attractants were used: 1) a 5 cm³ (25 g) cube of raw fish soaked in fish oil nailed to a tree trunk 40–50 cm abovethe trap; 2) 200 g of 'Nine Lives' commercial cat food smeared over a tree trunk with the accompanying oil sprinkled around the trap; 3) a 5 × 5 cm cloth bag of catnip tied to a tree trunk; (4) sand impregnated with the urine and faeces of domestic cats of another population; 5) vaseline containing mountain lion Puma concolor faeces and commercial mountain lion attractant smeared or sprinkled around the lower trunks of trees or surfaces of boulders. Trap surroundings were often modified to funnel cats toward them. Traps were checked twice daily, at 07:00–09:00 and 15:00–18:00 hrs.

Hunting was carried out by two experienced local hunters along trails after dark and before dawn (20:00–01:00 and 04:00–06:00 hrs). Throughout the programme a diversity of techniques, baits and attractants were used and frequently varied for novelty, favouring techniques that had recently worked well. Monitoring of cats was carried out by direct observation, presence of footprints & scats, bait consumption and fishermen's reports.

Assessing benefits to island fauna: Every year (between 1991 to 2004), 30 days after the start of the nesting season in October, the mortality of breeding sooty tern adults (the primary cat prey species) was estimated. The total number of adults nesting was estimated through recording all nests with eggs or chicks in a series of 5 × 5 m quadrats at 10 m intervals with in a sample area about 25% the size of the total nesting area. Counts were converted into a yearly estimate of the total number of adults nesting in the area. Tern mortality was estimated through tern corpse counts (mostly just pairs of wings) within and up to 50 m from the nesting colony as soon as breeding had ended and the birds had left the island. Other non-quantitative estimates were made from observations between December to July to estimate impacts of cat eradication on other island fauna.

Cat eradication: Most cats died through poisoning in the first eight days of the eradication programme, inferred from the steep decline in bait consumption (35% of baits were consumed on the first day, by the third consumption was much reduced and remaining low thereafter, with only 7% of cubes consumed on day 8). The total number of fish baits consumed (or missing) in the first eight days of baiting in zones I, II and III was 1,935, 243 and 204 baits, respectively. Although the majority of cats were killed in this initial period, poisoning, trapping (using various attractants) and hunting over the subsequent three years was needed to ensure removal of the last few individuals. By the fourth year (1998) there were no signs of cats were evident.

Benefits to fauna: The proportion of terns found dead at the end of the nesting seasons dropped from between 22-33% (prior to cat eradication) to 5% (post cat eradication), and remained below 2% through 2004. Monitoring indicated that the eradication resulted in the mortality of terns falling permanently by more than 80%. Results also suggest that although only the release of sooty terns from cat predation has been quantified, conservation benefits for native reptiles such as the black iguana Ctenosaura pectinata, and the previously rare Clark's spiny lizard Sceloporus clarkia are also apparent, these species being observed more frequently.

Conclusions: This is the first eradication of cats from a tropical island in Mexico. It resulted in substantial benefits to sooty terns and other fauna such as the island's reptiles. Despite the reputed reluctance of cats to try new food sources, they readily consumed baits that were unfamiliar foods (chicken and tuna) or familiar foods in unfamiliar presentations (fish scraps hanging from branches).

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