Black Rat Rattus rattus Eradication from the San Jorge, Sonora, Mexico

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Black Rat (Rattus rattus) Eradication from the San Jorge Islands, Mexico
Technical Report · January 2002
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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
1
Island Conservation & Ecology Group
University of California Long Marine Laboratories
100 Shaffer Road
Santa Cruz, California 95060 USA
2
Centro Intercultural de Estudios de Desiertos y Océanos
Apdo. Postal #53
Puerto Penasco, Sonora, México
3
Department of Ecology and Evolutionary Biology
University of California Santa Cruz
100 Shaffer Road
Santa Cruz, CA 95060 USA
4
Department of Renewable Natural Resources
University of Arizona
Biological Sciences East Room # 104
Tucson, AZ 85721 USA
5
Grupo 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: jdonlan@islandconservation.org; 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 Unpublished Report, Island Conservation and Ecology Group.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
1
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
ICEG Technical Report: March 2002
2
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.
ICEG Technical Report: March 2002
3
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.
ICEG Technical Report: March 2002
4
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.
ICEG Technical Report: March 2002
5
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
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).
improved with the advent of new rodenticide
delivery techniques, such as aerial
ICEG Technical Report: March 2002
6
The majority of invasive rat eradications
However, this greater persistence and
have been achieved using the second
potency also increases the risk of primary
generation anticoagulant brodifacoum (3-[3-
and secondary poisoning of non-target
(4’-Bromo-[1,1’-biphenyl]-4-yl)-1,2,3,4-
animals (Eason & Spurr 1995).
tetrahydro-1-naphthalenyl]-4-hydroxy-2H-1benzopyran-2-one). Like other
Brodifacoum is toxic to all vertebrates to
anticoagulants, brodifacoum acts by
varying degrees. Primary and secondary
blocking the synthesis of the vitamin K
poisoning from feeding on anticoagulant
dependent clotting factors in the liver of
killed rodents is well known and has been
vertebrates (Hadler & Sahdbolt 1975). Death
demonstrated both in the lab (Newton et al.
results from uncontrolled bleeding after a
1990; Townsend et al. 1981) and field
threshold level of the active ingredient
(Eason & Spurr 1995; Howald et al. 1999;
concentrates in the liver. Brodifacoum and
Joermann 1998). During rat eradications,
other second generation anticoagulants have
there are clear risks of non-target (1)
greater persistence and potency than some
primary poisoning to herbivorous and
other toxins used to kill rats and
omnivorous birds by consumption of cereal-
consequently can cause death after a single
based baits and (2) secondary poisoning to
dose, a desirable characteristic for rat
avian predators and scavengers (Eason &
eradications (Eason et al. 1994; Eason &
Spurr 1995; Howald et al. 1999). While less
Spurr 1995).
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
Figure 2. Rat taking poisoned bait from a station
species recovering quickly to pre-eradication
levels or higher (Davidson & Armstrong in
ICEG Technical Report: March 2002
7
press). However, invasive rats threaten
less toxic to birds than brodifacoum (Eason
native species on a number of large
et al. 1994). Diphacinone has recently been
biologically diverse islands where primary
used successfully to eradicate rats from
or secondary brodifacoum poisoning could
Buck Island (72 ha), Virgin Islands (G.
severely impact populations of native
Witmer, personal communication). While
species, and where effective mitigation may
cholecalciferol has been used for rodent and
be difficult and expensive. The use of less
other exotic vertebrate control, it has never
persistent or less toxic rodenticides in island
been used for an island eradication program.
eradication campaigns could help minimize
non-target poisoning risks. This would only
As part of a regional island conservation
be an effective conservation strategy if these
program (Carabias-Lillo et al. 2000; Donlan
alternative toxins are 100% efficacious
et al. 2000; Tershy et al. in press), we
against invasive rats.
removed black rats from the San Jorge
Islands, Mexico (Figs 3 and 4). Exploiting
Diphacinone (2-(Diphenylacetyl)-1,3-
the experimental opportunity of
indandione), a first generation anticoagulant,
conservation action on three adjacent islands
is similar to brodifacoum in toxicology and
(sensu Donlan et al. in press), we used three
pathology; however, it is virtually non-toxic
rodenticides: brodifacoum, diphacinone and
to birds, as well as much less persistent in
cholecalciferol, one on each of the islands.
tissues (Buckle 1994). Cholecalciferol (9,10-
Brodifacoum was used on the larger island,
Seocholesta-5,7,10(19)-trein-3 betaol), also
while diphacinone and cholecalciferol where
known as Vitamin D3, is a subacute
used on adjacent, smaller islands. Here, we
rodenticide that causes mobilization of
(1) report on our conservation action on the
calcium stores from bones to the
San Jorge Islands and (2) suggest and
bloodstream; death results from
provide field evidence that the rodenticides
hypercalcaemia and calcification of the
diphacinone and cholecalciferol may be
blood vessels (Buckle 1994). Lab evidence
feasible alternatives to brodifacoum in
suggests that cholecalciferol is significantly
certain island rat eradication programs
.
ICEG Technical Report: March 2002
8
SAN JORGE ISLANDS: NATURAL
has been recorded on San Jorge Island
HISTORY
(Reeder & Norris 1954), as well as many
other islands in the Gulf of California. This
The San Jorge Islands (George’s Islands) are
endangered fish-eating bat is known to be
located in the northern Gulf of California,
vulnerable to rat predation (Herrera-
approximately 41 km from Puerto Penasco,
Montalvo & Flores-Martinez 2001). We
Sonora, Mexico (Figs 3 and 4). They are
observed a roosting bat on the north island
northernmost rocky islands in the gulf and
during the study.
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
Figure 3. San Jorge Island, northern Gulf of
California
rocky with no terrestrial plants. The
The islands are an important seabird colony
introduced plant, Chenopodium murale, was
in the region (Everett & Anderson 1991;
reported in 1924 and no doubt was
Velarde & Anderson 1994). Reported
introduced by guano workers; the population
nesting species include nesting brown
has since gone extinct (Felger & Lowe
boobies (Sula leucogaster), blue-footed
1976). There are no native nonvolant
bobbies (Sula sula), double-crested
mammals, reptiles, or nesting terrestrial
cormorants (Phalacrocorax auritius),
birds. The little-known fish eating bat
Heerman’s gulls (Larus heermanni), and
(Myotis vivesi, sometimes placed in its own
red-billed tropicbirds (Phaethon aetherus)
genus Pizonyx) is listed as endangered with
(Velarde & Anderson 1994). Mellink (2000)
the Mexican government and vulnerable
discusses the breeding phenology of brown
under the IUCN. This Mexican endemic bat
boobies on the San Jorge Islands. Craveri’s
ICEG Technical Report: March 2002
9
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.
ICEG Technical Report: March 2002
10
murrelets (Synthliboramphus craveri) have
McChesney & Tershy 1998). Velarde and
been recorded historically (DeWeese
Anderson (1994) report colony estimates for
&Anderson 1976)and have likely been
some of the nesting seabird species (Table
extirpated due to rat predation as observed
1), but their methods and the accuracy of
elsewhere in the region (for the genus,
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
10,0001
23,575 ± 6,514 pairs
(Sula leucogaster)
Blue-footed boobies
Not observed nesting, only
2,0001
(Sula sula)
roosting
Double-crested cormorants
1
1,000
Observed nesting
(Phalacrocorax auritius)
Brandt’s Cormorants
NR2
Nesting
(Phalacrocorax penicillatus)
Heerman’s gulls
3001
Observed nesting
(Larus heermanni)
Red-billed tropicbirds
B3
Observed nesting, increasing?5
(Phaethon aetherus)
Craveri’s murrelets
E?4
Not observed
(Synthliboramphus craveri)
1
2
3
Velarde and Anderson 1994; Not reported as a breeder; Recorded as a breeder, AOU, 1983;
4
5
Possibly extirpated; Anecdotally increasing after rat eradication, see text.
Figure 5. Nesting Brown Booby
Figure 6. Nesting double-crested
cormorants
ICEG Technical Report: March 2002
11
During all of our stays on the San Jorge
increased tropicbird hatchling success
Islands, we made seabird observations. In
(Schaffner 1991; Schreiber & Schreiber 1993).
January 2001, we estimated the population
Further, red-billed tropicbirds are species of
of nesting brown boobies. Using replicated
special conservation concern, with less than
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
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
island areas, we estimate 23,575 ± 6,514
the final visit to the island (March 2002), we
nesting pairs (95% confidence interval; n = 70
observed encouraging evidence of a positive
plots). While we did not attempt population
response from rat removal, with 34 active
estimates for any other nesting species, we made
tropicbird nests (26 on the main island, 5 on the
some observations worth noting. During all
south island, and 3 on the north island).
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,
Figure 7. Red-billed Tropicbird
however, observe black (Oceanodroma
melania) and least storm-petrels (O.
The San Jorge Islands are also known for
microsoma) on the water in proximity to the
their rich marine resources including
islands. Red-billed tropicbirds have been
commercially harvested black murex and
recorded as nesters on the San Jorge Islands
rock scallops and fish (groupers, snappers
(AOU 1983). Tropicbirds are known to be
and triggerfish). As with most of the Gulf’s
vulnerable to nest predation by Rattus spp. and
islands, only the land is officially recognized
predator control efforts in Puerto Rico have
as a reserve. The island and the surrounding
ICEG Technical Report: March 2002
12
waters are important sites for both
sampled through time to establish the
commercial fisheries and the ever increasing
impacts of introduced rats on islands.
tourist trade from Puerto Penasco. A related
project, Effects of rat eradication on the
Black rats were introduced onto the San
intertidal ecology of Isla San Jorge:
Jorge Islands in the mid-1800s during guano
development of a monitoring technique, is
mining operations (Bowen 2000).
currently underway. Predation by introduced
Expanding from the Pacific side of the Baja
rats have been shown to have impacts on
peninsula, the American concession, the
intertidal communities (Navarrete & Castilla
Mexican Guano Company, mined guano on
1993). This collaboration between Pete
four islands in the Gulf of California: Patos,
Raimondi and Diana Steller (UCSC), Centro
Rasa, San Pedro Martir and San Jorge.
de Estudios de Desiertos y Océanos, and the
Mining activity peaked on San Jorge circa
Island Conservation and Ecology Group is
1861, with a record of three vessels stopping
investigating potential intertidal impacts of
at Guaymas bound for the Mauritius Islands
introduced rats on the San Jorge Islands. Bi-
carrying more than a thousands tons of
annual surveys of permanent intertidal plots
guano (Bowen 2000). Mining ceased
have been established on the islands to
between 1873-1875 and later resumed when
compare to long-term monitoring data along
the English firm, the Gulf of California
the coast at sites in Puerto Penasco. The
Phosphate Company, acquired concessions
objective of this study is to establish
for both Rasa and San Jorge Islands in 1876.
techniques for following intertidal
The islands now receive little human
communities through time and compare the
visitation for most of the year. However,
trajectories of the communities on islands
local fishermen use the nearshore waters
with and without rats who forage in the
around the island and occasionally camp on
intertidal. During the 2002 year, islands in
the island (Cudney-Bueno & Turk-Boyer
northwest Mexico are being identified for
1998). Limited ecotourism (e.g. scuba
future rat eradications. Intertidal monitoring
diving and sea lion watching) is also present.
stations will be set up on a subset of these
islands prior to eradication and will be
ICEG Technical Report: March 2002
13
ERADICATION: METHODS AND
Bell Laboratories, Madison) on the north
RESULTS
island, and cholecalciferol (750 ppm,
Two trips to the islands (November
Quintox® Bell Laboratories) on the south
1996, August 1997) confirmed their
island (Fig. 1). Brodifacoum and
presence through snap-trapping and removal
diphacinone bait were in 20 g extruded
efforts began in August 2000. The timing of
cereal wax blocks, while cholecalciferol bait
the removal was selected to minimize
was only available in cereal pellet form and
disturbance to nesting seabirds and
dispensed in 10 g packages. Stations were
California sea lions (Zalophus
monitored and bait replenished at regular
californianus). A bait-station approach was
intervals for a month (August – September
adopted (Fig. 8; sensu Taylor & Thomas
2000). Subsequently, stations were checked
1989). Bait stations were made from 50 cm-
five times at irregular intervals over the next
lengths of 100 mm diameter plastic pipe and
two years (October 2000, January 2001,
placed on the islands in a 25 x 25 m grid.
April 2001, July 2001, February 2002). In
Bait was hand-broadcasted in inaccessible
addition to monitoring bait uptake from the
areas with steep cliffs. Bait was also hand-
bait stations, snap-traps and indicator blocks
broadcasted on the small associated islets.
(chew blocks) were used to monitor rat
Rats could enter the bait stations from open
presence just prior, during, and after the
sides and freely remove bait. Unbaited
eradication campaign.
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™
ICEG Technical Report: March 2002
Figure 8. Bait Station.
14
Rats occurred on all three islands prior to
diphacinone uptake peaked around day five
eradication. Trap success on the islands on
(Fig. 2).
days prior to poisoning was between 13 62% (Table 2). Rats began removing poison
Black rats were successfully eradicated from
from stations within days of baiting (Table
all three islands. Monitoring of the islands
3). On the main island, brodifacoum uptake
after the month of baiting showed no sign of
peaked (66%) between 5 - 10 days and
rat presence on any of the three islands.
ceased after 24 days (Table 3, Fig. 2). On
During visits to the island over the next two
the south (cholecalciferol) and north
years (October 2000, January 2001, April
(diphacinone) islands, bait uptake ceased
2001, July 2001, February 2002), we failed
around 10 days (Table 3, Fig. 2).
to detect rats or rat sign on any of the islands
Cholecalciferol uptake peaked early (day 2 =
with both snap-trapping and indicator blocks
27%) and declined consistently; while,
(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
-2
-1
0
3
6
7
10
15
16
21
25
Brodifacoum Island
62.5 (8)
13.3 (15)
20.0 (15)
Diphacinone
Island
50.0 (8)
41.6 (12)
Cholecalciferol
Island
36.0 (8)
50.0 (12)
0 (13)
9.0 (11)
0 (11)
0 (12)
0 (15)
0 (14)
0 (13)
0 (13)
0 (15)
0 (15)
ICEG Technical Report: March 2002
0 (15)
0 (14)
15
Table 3. Activity of bait stations on the San Jorge Islands.
Brodifacoum
Diphacinone
Cholecalciferol
95
81
17
53
16
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
2063 (412 kg)
293 (59 kg)
241 (24 kg)
Total number of bait stations
Percent of active bait stations
Total amount of bait used (weight)
1
2
Kruskal-Wallis ANOVA, Bonferroni, p < 0.001
Kruskal-Wallis ANOVA, Bonferroni, p < 0.04
Table 4. Monitoring effort on the San Jorge Islands. Monitoring included the use of snaptraps 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
October 2000
107 (0)
40 (0)
45 (0)
20 (0)
64 (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)
ICEG Technical Report: March 2002
16
Figure 2. Bait uptake on the San Jorge Islands, Mexico.
ICEG Technical Report: March 2002
17
activity time of the bait stations. Rat cohorts
DISCUSSION
would have to wait for the previous
All three of the rodenticides tested in this
dominant cohort to die off before gaining
study were 100% efficacious against
access to bait stations. On the
invasive black rats. We cannot rule out the
cholecalciferol island, bait uptake was
possibility that rats on the diphacinone and
nearly immediate, peaked early, and ceased
cholecalciferol islands crossed over to the
around 10 days. This pattern is expected
brodifacoum island and consumed bait.
given the high concentration (750 ppm) and
However, if this occurred, it likely only
the acuteness of cholecalciferol (Buckle
affected a small number of rats whose home
1994).
ranges are in close proximity to adjacent
islands. On the larger brodifacoum island,
Invasive rat eradication is only possible if
bait uptake and lag time to activity were
each individual rat makes the transition from
similar to other rat eradication campaigns,
local food sources to bait containing
showing a single pulse uptake event with a
rodenticide. Rats can be neophobic and may
lag time of a few days (Taylor et al. 2000;
be hesitant to feed on a novel resource,
Taylor & Thomas 1993). On the
consuming small quantities at first (Barnett
diphacinone island, mean activity time was
1988). Thus, from an efficacy standpoint,
less than the brodifacoum island (Table 3).
the bait should have the ability to kill the
This is opposite of what might be expected
target species after a single feeding and to
given that diphacinone is a multi-dose
prevent the possibility of selecting for
anticoagulant. Two scenarios may account
individuals that avoid bait. Cholecalciferol
for these observations. First, rats may have
has the potential to induce bait shyness in a
been in low densities on the diphacinone
population of rats because symptomatic
island and cached enough bait to result in
effects of poisoning can be felt after
eventual mortality. Second, high rat
ingestion of a sub-lethal dose (Prescott et al.
densities were present on the larger
1992); however, it was successful in
brodifacoum island and there was selective
eradicating black rats from this small c.5 ha
cohort killing, thus lengthening the mean
island. Brodifacoum and diphacinone both
ICEG Technical Report: March 2002
18
cause a delayed onset of toxic symptoms
of the most efficacious rat toxins and a
which minimizes the risk of bait shyness.
proven effective conservation tool.
However, a major difference between the
Alternative toxins, such as diphacinone and
two rodenticides is their metabolic
cholecalciferol used in this study, can reduce
sensitivity. In the liver, both diphacinone
the risks of primary and secondary
and brodifacoum bind to the vitamin-K
poisoning of non-target species; however,
reductase enzyme impairing the production
their use increases the risk of failing to
of active clotting factors resulting in death
completely eradicate rats due to the
from internal hemorrhaging. Brodifacoum
metabolic sensitivity of diphacinone and the
tightly bounds to the enzyme and is
potential bait shyness of cholecalciferol. As
insensitive to metabolism, giving it the
we adopt ecosystem and food web
ability to kill a rat after a single feeding.
approaches to conservation and management
Conversely, diphacinone fails to bind tightly
(sensu Power 2001; Zavaleta et al. 2001),
to the enzyme and hence is sensitive to
the choice of rodenticide must be balanced
metabolism. Rats must feed on diphacinone
between efficacy and the risks to non-target
bait for seven to ten days before the
species. The San Jorge islands are
anticoagulant effect takes hold; ingestion
depauperate with little alternate food sources
rate must exceed the rate of metabolism.
outside of seasonal seabirds and intertidal
Despite the metabolic sensitivity, and hence
invertebrates. The lack of year-round
multi-dose requirement, of diphacinone, it
abundant food resources may have played a
was successful in eradicating rats from the
role in the success of diphacinone and
north island (c.5 ha).
cholecalciferol. Nonetheless, these results
are encouraging and warrant further
For successful island rat eradications, the
experiments to test the use of toxins in
fundamental requirement is that every rat is
addition to brodifacoum that can be used to
removed. The appropriate use of
successfully eradicate invasive rats from
rodenticides can eliminate 100% of an island
islands. Applied field research on less toxic
rat population (Taylor et al. 2000; Taylor &
rodenticides, as well as improving
Thomas 1989, 1993). Brodifacoum is one
palatability of baits, may prove invaluable in
ICEG Technical Report: March 2002
19
facilitating the prevention of extinctions and
In addition, the rat eradication of San Jorge
the restoration of increasingly complex
Islands will likely benefit the Mexican
island ecosystems.
endemic and endangered fishing bat (Myotis
vivesi), an island resident also known to be
vulnerable to rat predation.
CONSERVATION GAIN
We expect this conservation action to
have an important and lasting impact on
The future of the San Jorge Islands and their
seabird and bat conservation in northwest
surrounding waters is an optimistic one, as a
Mexico. The San Jorge Islands are an
collaborative effort between local users,
important seabird colony, including colonies
scientists, NGO’s and the Mexican
of brown boobies (Sula leucogaster, 23,575 ±
government is emerging to better manage and
6514 pairs), two species of cormorants
conserve this island ecosystem. A long-term
(Phalacrocorax auritius and P. penicillatus),
working relationship between fishermen (the
Hermann’s gulls (Larus heermanni), and red-
primary users of the islands) and a local
billed tropicbirds (Phaethon aetherus). Some
conservation NGO (CEDO) has led to the
of these species are known to suffer impacts
community-based establishment of San Jorge
from rat predation. Anecdotally, tropicbirds
Island as a temporary marine protected area
already appear to have increased in number of
for fishery resources. These actions, coupled
nesting birds and the island is now a safe
with future environmental education efforts to
nesting site for the extirpated Craveri’s
prevent reintroduction of exotic species will
murrelet (Synthliboramphus craveri).
likely lead to the long-term conservation value
of the islands.
ICEG Technical Report: March 2002
20
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.
ICEG Technical Report: March 2002
21
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