Black Rat (Rattus rattus) Eradication from the San Jorge Islands ...

ICEG Technical Report: March 2002 1

Black Rat (Rattus rattus) Eradication from the San Jorge Islands, Mexico

C. Josh Donlan1*$, Héctor Avila-Villegas, Daniel Bercovich Ortega2, Noah Biavaschi1, Natasha Bodorff1, Rick Boyer2, Tosha Comendant1,3, Donald A. Croll1,3, Richard Cudney-Bueno2,4,

Ricardo Galván de la Rosa, Gregg R. Howald1, Luis Felipe Lozano-Román, Carlos Morales, Olegario Morales2, Zaid Morales-Gonzalez, Pete Raimondi3, Jose Angel Sanchez4, Diana

Steller3, Bernie R. Tershy1, Peggy Turk-Boyer2

1Island Conservation & Ecology Group University of California Long Marine Laboratories 100 Shaffer Road Santa Cruz, California 95060 USA 2Centro Intercultural de Estudios de Desiertos y Océanos Apdo. Postal #53 Puerto Penasco, Sonora, México 3Department of Ecology and Evolutionary Biology University of California Santa Cruz 100 Shaffer Road Santa Cruz, CA 95060 USA 4Department of Renewable Natural Resources University of Arizona Biological Sciences East Room # 104 Tucson, AZ 85721 USA 5Grupo de Ecología y Conservación de Islas A. C. Av. Del Puerto #375 interior 30, Frac. Playa Ensenada, Ensenada, Baja California, México * Authors’ names are in alphabetical order after first author $Correspondence: [email protected]; 831.459.1475

Suggested Citation:

Donlan, C. J. et al. 2002. Black Rat (Rattus rattus) eradication from the San Jorge Islands,

Mexico. Unpublished Report, Island Conservation and Ecology Group.

ICEG Technical Report: March 2002

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SUMMARY

Introduced commensal rats (Rattus spp.) are a major contributor to the extinction

and endangerment of island floras and faunas. The use of the toxin brodifacoum to

completely eradicate rats from islands is a powerful conservation tool. However,

brodifacoum is toxic to animals other than rats and on some islands its use may not be

feasible without prohibitively expensive mitigation. As part of a regional conservation

program, we experimentally tested brodifacoum and two less toxic rodenticides,

diphacinone and cholecalciferol, in a Rattus rattus eradication on the San Jorge Islands,

Mexico. All three rodenticides were successful in eradicating rats, suggesting that the less

toxic diphacinone and cholecalciferol may be valid alternatives to brodifacoum for some

island eradication programs. However, the choice of rodenticide must be balanced between

efficacy and the risks to non-target species. Applied field research on less toxic

rodenticides, as well as improving palatability of baits, is needed and may prove invaluable

in facilitating the prevention of extinctions and the restoration of increasingly complex

island ecosystems.

We expect this conservation action to have an important and lasting impact on

seabird and bat conservation in northwest Mexico. The San Jorge Islands are an important

seabird colony, including colonies of brown boobies (Sula leucogaster, 23,575 ± 6514 pairs),

two species of cormorants (Phalacrocorax auritius and P. penicillatus), Hermann’s gulls

(Larus heermanni), and red-billed tropicbirds (Phaethon aetherus). Some of these species

are known to suffer impacts from rat predation. Anecdotally, tropicbirds already appear to

have increased in number of nesting birds and the island is now a safe nesting site for the

extirpated Craveri’s murrelet (Synthliboramphus craveri). In addition, the rat eradication

of San Jorge Islands will likely benefit the Mexican endemic and endangered fishing bat

(Myotis vivesi), an island resident also known to be vulnerable to rat predation.

The future of the San Jorge Islands and their surrounding waters is an optimistic

one, as a collaborative effort between local users, scientists, NGO’s and the Mexican

government is emerging to better manage and conserve this island ecosystem. A long-term

working relationship between fishermen (the primary users of the islands) and a local


3

conservation NGO (CEDO) has led to the community-based establishment of San Jorge

Island as a temporary marine protected area for fishery resources. These actions, coupled

with future environmental education efforts to prevent reintroduction of exotic species will

likely lead to the long-term conservation value of the islands.


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RESUMEN

Las ratas comensales introducidas (Rattus spp.) son una de las mayores amenazas y

causa de la extinción de especies de flora y fauna de islas. El uso de los tóxicos a base de

brodifacoum para erradicar completamente a las ratas de las islas es una poderosa

herramienta de conservación. Sin embargo, brodifacoum es además tóxico a otras especies

de animales y en algunas islas su uso no es recomendado sin medidas de mitigación

extremadamente costosas. Como parte de un programa de conservación regional,

probamos de manera experimental brodifacoum y otros dos compuestos menos tóxicos,

difacinone y colecalciferol, en la erradicación de Rattus rattus en las Islas San Jorge,

México. Los tres rodenticidas fueron efectivos para erradicar las ratas, sugiriendo que los

menos tóxicos, difocinone y colecalciferol pueden ser unas alternativas validas al

brodifacoum para algunos programas de erradicación. Sin embargo, la decisión de elegir

un rodenticida debe ser balanceada entre efectividad y los riesgos que significa para otras

especies que pueden ser afectadas. Investigaciones de campo aplicadas sobre rodenticidas

menos tóxicos, así como el mejoramiento de palatividad de los cebos, es necesaria y podría

probar ser invaluable para prevenir la extinción de especies y la restauración de

ecosistemas cada vez mas complejos.

Esperamos que esta acción de conservación tenga un importante y duradero

impacto sobre la conservación de las aves marinas y murciélagos en el noroeste de México.

Las Islas San Jorge son una colonia importante de aves marinas, incluyendo colonias de

bobos café (Sula leucogaster, 23,575 ± 6514 pares), dos especies de cormoranes

(Phalacrocórax auritus y P. penicillatus), gaviotas ploma (Larus heermanni), y rabijunco de

pico rojo (Phaeton aetherus). Se sabe que algunas de estas especies sufren el impacto de

predación por ratas. Algunas anécdotas mencionan que el número de rabijuncos que

anidan aquí parecen haberse incrementado, y creemos que la isla es un lugar seguro para

la alcuela americana (Synthliboramphus craveri), que fue extirpado de esta isla. El

murciélago pescador, especie endémica de México y amenazado (Myotis vivesi) es una

especie residente de la isla y vulnerable a la predación por ratas.


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El futuro de las Islas San Jorge y las aguas que las rodean es alentador. Esto es

debido a los emergentes esfuerzos colaborativos entre los usuarios locales, la comunidad

cientifica, Organizaciones No-Gubernamentales, e instituciones del gobierno de Mexico

para un mejor manejo y la conservacion de este ecosistema insular. Una relacion de trabajo

de largo plazo entre los pescadores locales (los principales usuarios de las islas) y una

organizacion de conservacion local (Centro de Estudios de Desiertos y Oceanos / CEDO) ha

llevado a la comunidad al establecimiento de las Islas San Jorge como un area marina

protegida para los recursos pesqueros. Estas acciones, complementadas con educacion

ambiental dirigida a prevenir la reintroduccion de especies exoticas a las islas podran

llevar a la conservacion del valor de estas en el largo plazo.


INTRODUCTION

Commensal rats (Rattus spp.) introduced to

islands have contributed to a large

percentage of animal extinctions (Atkinson

1985; Ebenhard 1988; Groombridge et al.

1992). They are now found on over 90% of

the worlds island groups (Atkinson 1985),

where they continue to threaten insular

plants, invertebrates, reptiles, mammals, and

birds (Atkinson 1985; Daltry et al. 2001;

Daniel & Williams 1984; Herrera-Montalvo

& Flores-Martinez 2001; Palmer & Pons

1996; Stone et al. 1994; Sugihara 1997;

Towns et al. 2001). Until recently, it was

widely accepted that invasive rats were a

permanent part of these island ecosystems,

and management was limited to control

efforts. However, in the last 20 years,

techniques pioneered by New Zealand

conservationists have been developed to

eradicate invasive rats [Black (Rattus

rattus), Norway (Rattus norvegicus) and

Polynesian (Rattus exulans) rats] from

islands with the select use of rodenticides

(Taylor & Thomas 1989, 1993). These

techniques are powerful tools for preventing

extinctions and they have recently been

improved with the advent of new rodenticide

delivery techniques, such as aerial

Figure 1. Nest predation by introduced rats

broadcast. Using these techniques, invasive

rats have been removed from over 90 islands

worldwide, including most recently islands

in North America (Atkinson 2001; Donlan et

al. 2000; Dunlevy et al. 2000; Taylor et al.

2000; Towns & Ballantine 1993; U. S.

National Park Service 2000). As the science

of invasive rat eradication develops,

eradication programs are being conducted

on increasingly larger and more biologically

complex islands (e.g. Campbell Island, New

Zealand and Anacapa Island, USA; P.

McClelland, personal communication; U. S.

National Park Service, 2000), and the

techniques are being adopted for continental

control programs (Saunders & Norton

2001).


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The majority of invasive rat eradications

have been achieved using the second

generation anticoagulant brodifacoum (3-[3-

(4’-Bromo-[1,1’-biphenyl]-4-yl)-1,2,3,4-

tetrahydro-1-naphthalenyl]-4-hydroxy-2H-1-

benzopyran-2-one). Like other

anticoagulants, brodifacoum acts by

blocking the synthesis of the vitamin K

dependent clotting factors in the liver of

vertebrates (Hadler & Sahdbolt 1975). Death

results from uncontrolled bleeding after a

threshold level of the active ingredient

concentrates in the liver. Brodifacoum and

other second generation anticoagulants have

greater persistence and potency than some

other toxins used to kill rats and

consequently can cause death after a single

dose, a desirable characteristic for rat

eradications (Eason et al. 1994; Eason &

Spurr 1995).

Figure 2. Rat taking poisoned bait from a station

However, this greater persistence and

potency also increases the risk of primary

and secondary poisoning of non-target

animals (Eason & Spurr 1995).

Brodifacoum is toxic to all vertebrates to

varying degrees. Primary and secondary

poisoning from feeding on anticoagulant

killed rodents is well known and has been

demonstrated both in the lab (Newton et al.

1990; Townsend et al. 1981) and field

(Eason & Spurr 1995; Howald et al. 1999;

Joermann 1998). During rat eradications,

there are clear risks of non-target (1)

primary poisoning to herbivorous and

omnivorous birds by consumption of cereal-

based baits and (2) secondary poisoning to

avian predators and scavengers (Eason &

Spurr 1995; Howald et al. 1999). While less

known, insectivorous birds, bats, and lizards

may be also at risk to non-target poisoning

(Daniel & Williams 1984; Godfrey 1984;

Merton 1987) . In prior rodent eradication

campaigns, the risks of non-target poisoning

have been short-term and outweighed by the

long-term benefits of rat removal (Empson

& Miskelly 1999; Towns 1994), with native

species recovering quickly to pre-eradication

levels or higher (Davidson & Armstrong in


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press). However, invasive rats threaten

native species on a number of large

biologically diverse islands where primary

or secondary brodifacoum poisoning could

severely impact populations of native

species, and where effective mitigation may

be difficult and expensive. The use of less

persistent or less toxic rodenticides in island

eradication campaigns could help minimize

non-target poisoning risks. This would only

be an effective conservation strategy if these

alternative toxins are 100% efficacious

against invasive rats.

Diphacinone (2-(Diphenylacetyl)-1,3-

indandione), a first generation anticoagulant,

is similar to brodifacoum in toxicology and

pathology; however, it is virtually non-toxic

to birds, as well as much less persistent in

tissues (Buckle 1994). Cholecalciferol (9,10-

Seocholesta-5,7,10(19)-trein-3 betaol), also

known as Vitamin D3, is a subacute

rodenticide that causes mobilization of

calcium stores from bones to the

bloodstream; death results from

hypercalcaemia and calcification of the

blood vessels (Buckle 1994). Lab evidence

suggests that cholecalciferol is significantly

less toxic to birds than brodifacoum (Eason

et al. 1994). Diphacinone has recently been

used successfully to eradicate rats from

Buck Island (72 ha), Virgin Islands (G.

Witmer, personal communication). While

cholecalciferol has been used for rodent and

other exotic vertebrate control, it has never

been used for an island eradication program.

As part of a regional island conservation

program (Carabias-Lillo et al. 2000; Donlan

et al. 2000; Tershy et al. in press), we

removed black rats from the San Jorge

Islands, Mexico (Figs 3 and 4). Exploiting

the experimental opportunity of

conservation action on three adjacent islands

(sensu Donlan et al. in press), we used three

rodenticides: brodifacoum, diphacinone and

cholecalciferol, one on each of the islands.

Brodifacoum was used on the larger island,

while diphacinone and cholecalciferol where

used on adjacent, smaller islands. Here, we

(1) report on our conservation action on the

San Jorge Islands and (2) suggest and

provide field evidence that the rodenticides

diphacinone and cholecalciferol may be

feasible alternatives to brodifacoum in

certain island rat eradication programs

.


SAN JORGE ISLANDS: NATURAL

HISTORY

The San Jorge Islands (George’s Islands) are

located in the northern Gulf of California,

approximately 41 km from Puerto Penasco,

Sonora, Mexico (Figs 3 and 4). They are

northernmost rocky islands in the gulf and

are part of the Reserva de la Biosfera del

Alto Golfo and the Area de Proteccion de

Flora y Fauna Silvestre Islas del Golfo de

California. The island group consists of one

main island (c.14 ha), two smaller islands

(c.5 ha), and four associated islets. The

smaller islands are connected to the main

island during spring low tides by a narrow

isthmus. The islands are arid, steep and

rocky with no terrestrial plants. The

introduced plant, Chenopodium murale, was

reported in 1924 and no doubt was

introduced by guano workers; the population

has since gone extinct (Felger & Lowe

1976). There are no native nonvolant

mammals, reptiles, or nesting terrestrial

birds. The little-known fish eating bat

(Myotis vivesi, sometimes placed in its own

genus Pizonyx) is listed as endangered with

the Mexican government and vulnerable

under the IUCN. This Mexican endemic bat

has been recorded on San Jorge Island

(Reeder & Norris 1954), as well as many

other islands in the Gulf of California. This

endangered fish-eating bat is known to be

vulnerable to rat predation (Herrera-

Montalvo & Flores-Martinez 2001). We

observed a roosting bat on the north island

during the study.

Figure 3. San Jorge Island, northern Gulf of California

The islands are an important seabird colony

in the region (Everett & Anderson 1991;

Velarde & Anderson 1994). Reported

nesting species include nesting brown

boobies (Sula leucogaster), blue-footed

bobbies (Sula sula), double-crested

cormorants (Phalacrocorax auritius),

Heerman’s gulls (Larus heermanni), and

red-billed tropicbirds (Phaethon aetherus)

(Velarde & Anderson 1994). Mellink (2000)

discusses the breeding phenology of brown

boobies on the San Jorge Islands. Craveri’s


Figure 4. San Jorge Islands, Sonora, Mexico. Three rodenticides were used to eradicate black rats: brodifacoum from the main island and diphacinone and cholecalciferol from adjacent islands.


11

murrelets (Synthliboramphus craveri) have

been recorded historically (DeWeese

&Anderson 1976)and have likely been

extirpated due to rat predation as observed

elsewhere in the region (for the genus,

McChesney & Tershy 1998). Velarde and

Anderson (1994) report colony estimates for

some of the nesting seabird species (Table

1), but their methods and the accuracy of

these estimations are unclear.

Table 1. Nesting seabird species on the San Jorge Islands, Mexico Previous Colony

Estimates (indiv.) Estimates/Observations from

this Study Brown boobies (Sula leucogaster) 10,0001 23,575 ± 6,514 pairs

Blue-footed boobies (Sula sula) 2,0001 Not observed nesting, only

roosting Double-crested cormorants (Phalacrocorax auritius) 1,0001 Observed nesting

Brandt’s Cormorants (Phalacrocorax penicillatus) NR2 Nesting

Heerman’s gulls (Larus heermanni) 3001 Observed nesting

Red-billed tropicbirds (Phaethon aetherus) B3 Observed nesting, increasing?5

Craveri’s murrelets (Synthliboramphus craveri) E?4 Not observed

1 Velarde and Anderson 1994; 2 Not reported as a breeder; 3 Recorded as a breeder, AOU, 1983; 4 Possibly extirpated; 5 Anecdotally increasing after rat eradication, see text.

Figure 5. Nesting Brown Booby

Figure 6. Nesting double-crested cormorants


During all of our stays on the San Jorge

Islands, we made seabird observations. In

January 2001, we estimated the population

of nesting brown boobies. Using replicated

circular plots, we sampled the main island.

Assuming the nest densities are similar on

all three islands and somewhat homogenous,

and using the best available estimates for

island areas, we estimate 23,575 ± 6,514

nesting pairs (95% confidence interval; n = 70

plots). While we did not attempt population

estimates for any other nesting species, we made

some observations worth noting. During all

visits to the island, we did not observe nesting

blue-footed boobies, only individuals roosting.

We observed two species of nesting cormorants:

double-crested and Brandt’s cormorants

(Phalacrocorax penicillatus), the latter has

not been reported previously from the islands. In

January 2001, we observed approximately 50

Brandt’s cormorant nests with eggs on the north

island. We did not observe any Craveri’s

murrelets or storm petrels on the island. We did,

however, observe black (Oceanodroma

melania) and least storm-petrels (O.

microsoma) on the water in proximity to the

islands. Red-billed tropicbirds have been

recorded as nesters on the San Jorge Islands

(AOU 1983). Tropicbirds are known to be

vulnerable to nest predation by Rattus spp. and

predator control efforts in Puerto Rico have

increased tropicbird hatchling success

(Schaffner 1991; Schreiber & Schreiber 1993).

Further, red-billed tropicbirds are species of

special conservation concern, with less than

8000 pairs globally (Lee & Walsh-McGehee

2000). Anecdotally, we observed only 6

tropicbirds during the first month of the

eradication effort (August-September 2000) and

observed 9 active nests in January 2001. During

the final visit to the island (March 2002), we

observed encouraging evidence of a positive

response from rat removal, with 34 active

tropicbird nests (26 on the main island, 5 on the

south island, and 3 on the north island).

Figure 7. Red-billed Tropicbird

The San Jorge Islands are also known for

their rich marine resources including

commercially harvested black murex and

rock scallops and fish (groupers, snappers

and triggerfish). As with most of the Gulf’s

islands, only the land is officially recognized

as a reserve. The island and the surrounding


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waters are important sites for both

commercial fisheries and the ever increasing

tourist trade from Puerto Penasco. A related

project, Effects of rat eradication on the

intertidal ecology of Isla San Jorge:

development of a monitoring technique, is

currently underway. Predation by introduced

rats have been shown to have impacts on

intertidal communities (Navarrete & Castilla

1993). This collaboration between Pete

Raimondi and Diana Steller (UCSC), Centro

de Estudios de Desiertos y Océanos, and the

Island Conservation and Ecology Group is

investigating potential intertidal impacts of

introduced rats on the San Jorge Islands. Bi-

annual surveys of permanent intertidal plots

have been established on the islands to

compare to long-term monitoring data along

the coast at sites in Puerto Penasco. The

objective of this study is to establish

techniques for following intertidal

communities through time and compare the

trajectories of the communities on islands

with and without rats who forage in the

intertidal. During the 2002 year, islands in

northwest Mexico are being identified for

future rat eradications. Intertidal monitoring

stations will be set up on a subset of these

islands prior to eradication and will be

sampled through time to establish the

impacts of introduced rats on islands.

Black rats were introduced onto the San

Jorge Islands in the mid-1800s during guano

mining operations (Bowen 2000).

Expanding from the Pacific side of the Baja

peninsula, the American concession, the

Mexican Guano Company, mined guano on

four islands in the Gulf of California: Patos,

Rasa, San Pedro Martir and San Jorge.

Mining activity peaked on San Jorge circa

1861, with a record of three vessels stopping

at Guaymas bound for the Mauritius Islands

carrying more than a thousands tons of

guano (Bowen 2000). Mining ceased

between 1873-1875 and later resumed when

the English firm, the Gulf of California

Phosphate Company, acquired concessions

for both Rasa and San Jorge Islands in 1876.

The islands now receive little human

visitation for most of the year. However,

local fishermen use the nearshore waters

around the island and occasionally camp on

the island (Cudney-Bueno & Turk-Boyer

1998). Limited ecotourism (e.g. scuba

diving and sea lion watching) is also present.


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ERADICATION: METHODS AND

RESULTS

Two trips to the islands (November

1996, August 1997) confirmed their

presence through snap-trapping and removal

efforts began in August 2000. The timing of

the removal was selected to minimize

disturbance to nesting seabirds and

California sea lions (Zalophus

californianus). A bait-station approach was

adopted (Fig. 8; sensu Taylor & Thomas

1989). Bait stations were made from 50 cm-

lengths of 100 mm diameter plastic pipe and

placed on the islands in a 25 x 25 m grid.

Bait was hand-broadcasted in inaccessible

areas with steep cliffs. Bait was also hand-

broadcasted on the small associated islets.

Rats could enter the bait stations from open

sides and freely remove bait. Unbaited

stations were deployed two days prior to bait

deployment to allow rats to habituate to the

novel structures in their environment

(Taylor & Thomas 1989).

Three rodenticides were used in the

removal. Brodifacoum (50 ppm, Final®

Blox™ Bell Laboratories, Madison,

Wisconsin, USA) was used on the main

island, diphacinone (50 ppm, Ditrac® Blox™

Bell Laboratories, Madison) on the north

island, and cholecalciferol (750 ppm,

Quintox® Bell Laboratories) on the south

island (Fig. 1). Brodifacoum and

diphacinone bait were in 20 g extruded

cereal wax blocks, while cholecalciferol bait

was only available in cereal pellet form and

dispensed in 10 g packages. Stations were

monitored and bait replenished at regular

intervals for a month (August – September

2000). Subsequently, stations were checked

five times at irregular intervals over the next

two years (October 2000, January 2001,

April 2001, July 2001, February 2002). In

addition to monitoring bait uptake from the

bait stations, snap-traps and indicator blocks

(chew blocks) were used to monitor rat

presence just prior, during, and after the

eradication campaign.

Figure 8. Bait Station.


15

Rats occurred on all three islands prior to

eradication. Trap success on the islands on

days prior to poisoning was between 13 -

62% (Table 2). Rats began removing poison

from stations within days of baiting (Table

3). On the main island, brodifacoum uptake

peaked (66%) between 5 - 10 days and

ceased after 24 days (Table 3, Fig. 2). On

the south (cholecalciferol) and north

(diphacinone) islands, bait uptake ceased

around 10 days (Table 3, Fig. 2).

Cholecalciferol uptake peaked early (day 2 =

27%) and declined consistently; while,

diphacinone uptake peaked around day five

(Fig. 2).

Black rats were successfully eradicated from

all three islands. Monitoring of the islands

after the month of baiting showed no sign of

rat presence on any of the three islands.

During visits to the island over the next two

years (October 2000, January 2001, April

2001, July 2001, February 2002), we failed

to detect rats or rat sign on any of the islands

with both snap-trapping and indicator blocks

(Table 4).

Table 2. Snap-trap success just prior and 30 days following rat poisoning campaign. Percent trap success (# of traps nights).

Day of Eradication Operation Brodifacoum Island Diphacinone

Island Cholecalciferol

Island -2 62.5 (8) 50.0 (8) 36.0 (8) -1 13.3 (15) 41.6 (12) 50.0 (12) 0 20.0 (15) 3 0 (13) 6 9.0 (11) 7 0 (11)

10 0 (12) 15 0 (15) 16 0 (14) 0 (15) 0 (15) 21 0 (13) 0 (15) 0 (14) 25 0 (13)


16

Table 3. Activity of bait stations on the San Jorge Islands.

Brodifacoum Diphacinone Cholecalciferol Total number of bait stations 95 17 16 Percent of active bait stations 81 53 93 Mean lag time from baiting to activity (days) 4.1 1 3.3 2.6 1

Mean duration of activity (days) 8.5 2 4.9 2 7.4 2 Total amount of bait used (weight) 2063 (412 kg) 293 (59 kg) 241 (24 kg)

1Kruskal-Wallis ANOVA, Bonferroni, p < 0.001 2 Kruskal-Wallis ANOVA, Bonferroni, p < 0.04

Table 4. Monitoring effort on the San Jorge Islands. Monitoring included the use of snap-traps and indicator chew blocks (sweet potato, apples and wax blocks). After the initial poisoning effort (August – September 2000), we failed to detect any presence of rats. Trap nights (Indicator block nights).

Trap Nights (Indicator Nights) Brodifacoum Diphacinone Cholecalciferol

August – September 2000 107 (0) 45 (0) 64 (0) October 2000 40 (0) 20 (0) 20 (0) January 2001 36 (180) 10 (204) 10 (120) April 2001 50 (300) 30 (180) 29 (180) July 2001 23 (46) 10 (20) 5 (10) March 2002 84 (390) 48 (132) 12(120) Total Trap Nights 340 (916) 163 (536) 140 (430)


17

Figure 2. Bait uptake on the San Jorge Islands, Mexico.


DISCUSSION

All three of the rodenticides tested in this

study were 100% efficacious against

invasive black rats. We cannot rule out the

possibility that rats on the diphacinone and

cholecalciferol islands crossed over to the

brodifacoum island and consumed bait.

However, if this occurred, it likely only

affected a small number of rats whose home

ranges are in close proximity to adjacent

islands. On the larger brodifacoum island,

bait uptake and lag time to activity were

similar to other rat eradication campaigns,

showing a single pulse uptake event with a

lag time of a few days (Taylor et al. 2000;

Taylor & Thomas 1993). On the

diphacinone island, mean activity time was

less than the brodifacoum island (Table 3).

This is opposite of what might be expected

given that diphacinone is a multi-dose

anticoagulant. Two scenarios may account

for these observations. First, rats may have

been in low densities on the diphacinone

island and cached enough bait to result in

eventual mortality. Second, high rat

densities were present on the larger

brodifacoum island and there was selective

cohort killing, thus lengthening the mean

activity time of the bait stations. Rat cohorts

would have to wait for the previous

dominant cohort to die off before gaining

access to bait stations. On the

cholecalciferol island, bait uptake was

nearly immediate, peaked early, and ceased

around 10 days. This pattern is expected

given the high concentration (750 ppm) and

the acuteness of cholecalciferol (Buckle

1994).

Invasive rat eradication is only possible if

each individual rat makes the transition from

local food sources to bait containing

rodenticide. Rats can be neophobic and may

be hesitant to feed on a novel resource,

consuming small quantities at first (Barnett

1988). Thus, from an efficacy standpoint,

the bait should have the ability to kill the

target species after a single feeding and to

prevent the possibility of selecting for

individuals that avoid bait. Cholecalciferol

has the potential to induce bait shyness in a

population of rats because symptomatic

effects of poisoning can be felt after

ingestion of a sub-lethal dose (Prescott et al.

1992); however, it was successful in

eradicating black rats from this small c.5 ha

island. Brodifacoum and diphacinone both


19

cause a delayed onset of toxic symptoms

which minimizes the risk of bait shyness.

However, a major difference between the

two rodenticides is their metabolic

sensitivity. In the liver, both diphacinone

and brodifacoum bind to the vitamin-K

reductase enzyme impairing the production

of active clotting factors resulting in death

from internal hemorrhaging. Brodifacoum

tightly bounds to the enzyme and is

insensitive to metabolism, giving it the

ability to kill a rat after a single feeding.

Conversely, diphacinone fails to bind tightly

to the enzyme and hence is sensitive to

metabolism. Rats must feed on diphacinone

bait for seven to ten days before the

anticoagulant effect takes hold; ingestion

rate must exceed the rate of metabolism.

Despite the metabolic sensitivity, and hence

multi-dose requirement, of diphacinone, it

was successful in eradicating rats from the

north island (c.5 ha).

For successful island rat eradications, the

fundamental requirement is that every rat is

removed. The appropriate use of

rodenticides can eliminate 100% of an island

rat population (Taylor et al. 2000; Taylor &

Thomas 1989, 1993). Brodifacoum is one

of the most efficacious rat toxins and a

proven effective conservation tool.

Alternative toxins, such as diphacinone and

cholecalciferol used in this study, can reduce

the risks of primary and secondary

poisoning of non-target species; however,

their use increases the risk of failing to

completely eradicate rats due to the

metabolic sensitivity of diphacinone and the

potential bait shyness of cholecalciferol. As

we adopt ecosystem and food web

approaches to conservation and management

(sensu Power 2001; Zavaleta et al. 2001),

the choice of rodenticide must be balanced

between efficacy and the risks to non-target

species. The San Jorge islands are

depauperate with little alternate food sources

outside of seasonal seabirds and intertidal

invertebrates. The lack of year-round

abundant food resources may have played a

role in the success of diphacinone and

cholecalciferol. Nonetheless, these results

are encouraging and warrant further

experiments to test the use of toxins in

addition to brodifacoum that can be used to

successfully eradicate invasive rats from

islands. Applied field research on less toxic

rodenticides, as well as improving

palatability of baits, may prove invaluable in


20

facilitating the prevention of extinctions and

the restoration of increasingly complex

island ecosystems.

CONSERVATION GAIN We expect this conservation action to

have an important and lasting impact on

seabird and bat conservation in northwest

Mexico. The San Jorge Islands are an

important seabird colony, including colonies

of brown boobies (Sula leucogaster, 23,575 ±

6514 pairs), two species of cormorants

(Phalacrocorax auritius and P. penicillatus),

Hermann’s gulls (Larus heermanni), and red-

billed tropicbirds (Phaethon aetherus). Some

of these species are known to suffer impacts

from rat predation. Anecdotally, tropicbirds

already appear to have increased in number of

nesting birds and the island is now a safe

nesting site for the extirpated Craveri’s

murrelet (Synthliboramphus craveri).

In addition, the rat eradication of San Jorge

Islands will likely benefit the Mexican

endemic and endangered fishing bat (Myotis

vivesi), an island resident also known to be

vulnerable to rat predation.

The future of the San Jorge Islands and their

surrounding waters is an optimistic one, as a

collaborative effort between local users,

scientists, NGO’s and the Mexican

government is emerging to better manage and

conserve this island ecosystem. A long-term

working relationship between fishermen (the

primary users of the islands) and a local

conservation NGO (CEDO) has led to the

community-based establishment of San Jorge

Island as a temporary marine protected area

for fishery resources. These actions, coupled

with future environmental education efforts to

prevent reintroduction of exotic species will

likely lead to the long-term conservation value

of the islands.


ACKNOWLEDGEMENTS

This conservation action would not have been possible without the help of many. This

conservation is a result of a collaboration between the Island Conservation & Ecology Group,

Centro de Estudios de Desiertos y Océanos, Intercutural, and the Area de Proteccion de Flora y

Fauna Silvestre Islas del Golfo de California. CJD would like to thank and acknowledge all of

the authors for the help, support, sweat, and determination – that made this project a reality;

particularly the original “August in the Northern Gulf” team: Héctor Avila-Villegas, Natasha

Bodorff, Ricardo Galván de la Rosa, Luis Felipe Lozano-Román, and Jose Angel Sanchez. We

especially thank the fishermen of the Puerto Penasco community for the support, enthusiasm,

and logistic support. We also thank Ana Luisa Figueroa and Dick Spight for their support. This

work was greatly facilitated by the dedication to conservation of the Director and staff of the

Sonoran Office of the Reserva Islas del Golfo de California. Photographic credits go to T.

Comendant, G. Howald, G. Lasley, and R. Taylor, and. Funding was provided by Farallon Island

Foundation. This research was conducted under permit 4538 Secretaría del Medio Ambiente,

Recursos Naturales.


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Black Rat (Rattus rattus) Eradication from the San Jorge Islands ...

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