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A strategy for Galapagos weeds
A. Tye, M. C. Soria, and M. R. Gardener
Charles Darwin Research Station, Galapagos. Postal address: CDRS, AP 17-01-3891, Quito,
Ecuador. E-mail: [email protected]
Abstract Galapagos has a native vascular flora of some 500 species, 60 more species that are doubtfully native, and
more than 600 introduced species. Introduced species are the most serious problem facing the native biota. The worst
invasive plants are trees and other woody species, vines and grasses, and most of them were introduced deliberately.
Many have invaded the Galapagos National Park and are also invasive in agricultural zones. A strategy for tackling the
weeds problem includes prevention, control, eradication and restoration, the research required to develop and prioritise
these management actions, and development of a legal framework for their implementation. Given limited resources for
control, a risk assessment system for prioritising problem species and key sites is essential and is being developed. It
will evaluate both species that are already present, and proposed introductions. A quarantine system for Galapagos has
been designed and implementation commenced. Quarantine is essential if the balance between introduction and eradication is to be tipped towards the latter. Research includes investigations of the ecology and distribution of introduced
plants, to determine factors (such as reproduction and dispersal rates and longevity of plants and their soil seedbanks)
essential for the design of successful management programmes. Research on control techniques is also essential, since
many Galapagos invasives are useful species that have not been subject to control elsewhere. Restoration research is
beginning, focussing on methods of control combined with active restoration, such as seeding with native species.
Invasive plants have only recently been widely recognised as high priority in Galapagos, and the first projects investigating the ecology of serious weeds are now yielding results. Control trials are leading to the development and adoption
of effective field methods. Attempts have begun to eradicate species with still-small populations, but which are known
as invasive elsewhere. A pilot project is also beginning, to assess the feasibility of eradicating a well-established invasive tree species. These measures, aside from their scientific and conservation value, also act as confidence builders,
demonstrating to the public and land managers both the dangers of introduced species and the possibilities for their
control and eradication.
Resumen Galápagos cuenta con una flora vascular nativa de algunas 500 especies, más 60 especies que son dudosamente
nativas y más de 600 especies introducidas adicionales. Los organismos introducidos constituyen el problema más
grave que enfrenta la biota nativa. Las peores plantas invasoras son árboles y otras especies leñosas, trepadoras y
pastos, y la mayoría de las especies que están causando o podrían causar problemas fueron introducidas a propósito.
Muchas de estas especies han invadido el Parque Nacional Galápagos, y son igualmente agresivas en las zonas agrícolas.
Una estrategia para enfrentar este problema incluye prevención, control, erradicación y restauración, las investigaciones
necesarias para desarrollar y priorizar estas acciones de manejo, y el desarrollo de un marco legal para su implementación.
El primer paso es desarrollar medidas para priorizar los problemas, tomando en cuenta los recursos limitados para el
control. Un sistema de evaluación de riesgos se está desarrollando, para evaluar tanto especies que ya se dan en las islas
y para introducciones propuestas. Este último forma parte de la prevención: un sistema de cuarentena para Galápagos
ha sido diseñado, y su implementación iniciado. La cuarentena puede reducir pero jamás parar las introducciones, pero
es necesario para cambiar el equilibrio entre la introducción y la erradicación. El control y erradicación tienen dos
componentes: investigación y manejo. Las investigaciones de la ecología y distribución de las plantas introducidas nos
permiten determinar los factores necesarios para diseñar programas de control y erradicación que sean exitosos, tales
como tasas de reproducción y dispersión, longevidad de plantas y semillas etc. La investigación para desarrollar nuevos
métodos de control también se necesita, por lo que muchas especies invasivas en Galápagos son especies útiles y no han
sido sujetos del control en otras partes. La investigación para la restauración ya empieza, con su enfoque en combinar
acciones de restauración positivos en combinación con el control, tales como siembra de especies nativas. El programa
de plantas introducidas en Galápagos esta creciendo rápidamente, por lo que las plantas invasoras han sido solo recién
ampliamente reconocidas como de alta prioridad. Los primeros proyectos para investigar la ecología de las malezas
más graves ya son produciendo resultados. Los ensayos de control llevan al desarrollo y uso de métodos de campo
eficientes. Se han iniciado intentos para erradicar especies aun representadas únicamente por pequeñas poblaciones,
pero las cuales están conocidas como invasoras graves en otros lugares. También ha comenzado un proyecto piloto para
evaluar la factibilidad de erradicar un árbol invasora bien establecido. Estas medidas, aparte de su valor científico y
para la conservación, además pueden aumentar la confianza y cambiar la opinión general sobre el peligro de las especies
introducidas y la factibilidad de su control.
Keywords Environmental weeds; strategic planning; Galapagos; prevention; control; eradication; restoration;
research; islands.
Pages 336-341 In Veitch, C. R. and Clout, M. N. (eds.). Turning the tide: the eradication of invasive species. IUCN SSC Invasive
Species Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK.
Tye et al: A strategy for Galapagos weeds
THE PROBLEM
Galapagos is an isolated oceanic archipelago of volcanic
islands lying 1000 km west of Ecuador, straddling the equator (Fig. 1). The date of discovery of the archipelago is
usually recognised as 1535. The islands were uninhabited
at that time, and no evidence proving earlier human presence has been found (Slevin 1959; Hickman 1985). The
first visitors after discovery were mainly buccaneers, passing sailors, whalers and sealers (Hickman 1985). Settlement began on Floreana Island in the early 1800s, but Santa
Cruz Island was only settled as late as the 1920s (Slevin
1959; Schofield 1989).
The pirates and whalers deliberately or accidentally introduced some alien species, including goats, rats and, probably, insects and plants. Even before permanent settlement,
Floreana had large areas dominated by introduced plants
such as Citrus spp. (Slevin 1959; Hamann 1984). The rapidly increasing settled population, growing at 8% per year
in the 1990s through both immigration and births, has been
accompanied by an enormous number of new introductions of alien plants and animals (Mauchamp 1997). Although agricultural development began at the time of settlement, the process has been uneven, leading to different
rates of introduction of alien species. Floreana has the longest history of the presence of a large introduced flora, while
agriculture on Santa Cruz was minimal until about 1960
(Moll 1990).
The Galapagos National Park forms 96.4% of the land
area of Galapagos; inhabited areas (urban and agricultural
zones, military bases and airports) make up the rest. Alien
plants that have escaped from cultivation are mostly found
on the five inhabited islands, especially the four with agricultural and urban zones (Floreana, Isabela, San Cristóbal,
Santa Cruz: Fig. 1); the fifth, Baltra Islet off the north coast
of Santa Cruz, is a military base and civil airport. There
are also a number of deliberately-introduced species on
Santiago, which was formerly inhabited. Aliens that were
introduced accidentally often have a much wider distribution in the archipelago but are mostly less problematic than
deliberately introduced species. Most introduced plant
Fig. 1 Galapagos archipelago.
species are found in the more humid, higher altitudes of
the four larger inhabited islands, and the settled areas are
the major source of invasion into the Galapagos National
Park.
The Galapagos islands support a native vascular flora of
about 500 species, with an additional 60 doubtfully native
species, principally pantropical ruderals, which may have
arrived naturally or may have been introduced by the earliest human visitors to the islands. In this paper, “introduced” and “alien” are used interchangeably to mean introduced deliberately or accidentally due to the actions of
man. “Naturalised” means reproducing in natural or seminatural habitat without the further assistance of man (beyond habitat disturbance). “Invasive” means invading natural (undisturbed) habitats. “Weed” means a naturalised
species.
Porter (1822) mentioned the first alien species (pumpkins
Cucurbita sp. and potatoes Solanum tuberosum), which
were introduced about 1807. Numbers increased slowly
until the 1960s, although true numbers are unknown, since
earlier references (especially Wiggins and Porter 1971;
Porter 1984) took into account only naturalised species.
The list continued to increase, reaching 438 in 1995
(Mauchamp 1997) and over 600 by November 2000 (Database of the Galapagos Flora, Charles Darwin Research
Station). The minimum detection rate has thus been more
than 10 per year in the last 30 years (Fig. 2), and more
than 120 during 2000. However, the recent apparent rate
of increase is obviously affected by increased interest in
recent years in the introduction process, as well as increased
sampling effort and individual research projects, and the
inclusion of cultivated, non-naturalised species in more
recent lists (Tye 2001b).
The principal threat to the terrestrial biota of Galapagos is
introduced species (Loope et al. 1988). Most (c. 75%) of
the alien plant species were introduced deliberately as useful plants, for their ornamental, agricultural, medicinal or
timber value, although some were introduced accidentally.
An even higher percentage of the worst invaders was introduced deliberately (see Mauchamp 1997). Some 45%
of introduced plant species have naturalised (A. Tye unpublished data).
Fig. 2 Rate of increase in numbers of introduced plant species recorded in Galapagos
since their discovery in 1535 (Tye 2001b).
337
Turning the tide: the eradication of invasive species
Most introduced plant species have not significantly affected the ecological equilibrium of the islands. However,
at least 37 species (Table 1) have invaded large areas and/
or appear to be adversely affecting the natural ecosystem
to a degree more than simply occupying space within an
existing community (e.g. altering community composition
or threatening a native species), or (in a few cases) are
naturalised but with limited distributions and known to be
extremely serious invasives in other parts of the world. At
Table 1. Invasive species in Galapagos that are
known or suspected to be causing significant
ecological change, including in natural areas.
Family
Species
Agavaceae
Furcraea hexapetala (Jacq.) Urb.
Aristolochiaceae Aristolochia odoratissima L.
Asteraceae
Pseudelephantopus spicatus
(B. Juss. ex Aubl.) C.F. Baker
Bombacaceae
Ochroma pyramidale
(Cav. ex Lam.) Urb.
Boraginaceae
Cordia alliodora (Ruiz & Pav.) Oken
Caesalpiniaceae Caesalpinia bonduc (L.) Roxb.
Senna obtusifolia
(L.) H.S. Irwin & Barneby
Capparidaceae
Cleome viscosa L.
Crassulaceae
Bryophyllum pinnatum (Lam.) Oken
Cucurbitaceae
Cucumis dipsaceus Ehrenb. ex Spach
Euphorbiaceae
Dalechampia scandens L.
Ricinus communis L.
Lamiaceae
Hyptis cf. atrorubens Poit.
Lauraceae
Persea americana Mill.
Meliaceae
Cedrela odorata L.
Mimosaceae
Leucaena leucocephala (Lam.) de Wit
Myrtaceae
Psidium guajava L.
Syzygium jambos (L.) Alston
Syzygium malaccense
(L.) Merr. & L.M. Perry
Passifloraceae
Passiflora edulis Sims.
Poaceae
Digitaria decumbens Stent
Melinis minutiflora P. Beauv.
Panicum maximum Jacq.
Pennisetum clandestinum Hochst.
ex Chiov.
Pennisetum purpureum Schumach.
Urochloa brizantha
(Hochst. ex A.Rich) R.D.Webster
Urochloa mutica
(Forssk.) T.Q. Nguyen
Rosaceae
Rubus niveus Thunb.
Rubiaceae
Cinchona pubescens Vahl
Rutaceae
Citrus aurantiifolia
(Christm.) Swingle
Citrus limetta Risso
Citrus limon (L.) Bur.f.
Solanaceae
Cestrum auriculatum L’H¾r.
Datura stramonium L.
Solanum lycopersicum L.
Ulmaceae
Trema micrantha (L.) Blume
Verbenaceae
Lantana camara L.
338
least 80 more species have naturalised and are still uncommon but are known or suspected to have invasive tendencies. Another 140 or so have naturalised but do not
appear to be causing obvious ecological damage, while at
least another 100 species present in the islands, which have
not yet escaped from cultivation, are known to be seriously invasive in other parts of the world. The rest of the
introduced species are present in cultivation; some may
become invasive. Lawesson (1990) lists only eight serious invasives, while Mauchamp (1997) lists 11, omitting
one of those included by Lawesson. The 37 species in
Table 1 include these 12, the increase largely caused by a
re-evaluation of the threat posed by certain species (four
species listed by Mauchamp 1997 as potential invaders
are included in Table 1), as well as a few recently-added
species.
There have been few rigorous studies of the effects of the
invasions, but some species have caused drastic habitat
changes, forming monospecific stands, shading out or otherwise replacing native vegetation communities, or preventing seedling regeneration by forming impenetrable
carpets. Where detailed studies have been made, dramatic
community changes have been revealed (Jäger 1999). The
worst effects seem to be caused by woody species, especially trees such as Psidium guajava, Cedrela odorata and
Cinchona pubescens, and bushes that form impenetrable
thickets, such as Lantana camara and Rubus spp. Many
vines and grasses are also causing serious problems.
Although most of the serious invaders in Galapagos were
introduced deliberately as useful plants, most of the species that are problematic in the National Park are also causing problems for farmers, and this includes cultivated species as well as accidental introductions. To a large extent,
therefore, there is little conflict regarding priorities for
control or eradication of invasives. However, this is not
true in every case. For species such as Cinchona pubescens
(which produces no economic yield of quinine or wood),
all parties wish to see the pest eradicated, whereas
Pennisetum purpureum is a serious invader of the National
Park but is a valued pasture grass and its removal from
agricultural areas would be politically and economically
difficult. In such cases, an eradication strategy would have
to include replacement by a non-invasive substitute.
THE STRATEGY
The strategy for dealing with invasive plants comprises
five levels of action: (1) prevention, (2) control and eradication, (3) restoration, (4) the research required to prioritise
and develop appropriate techniques to carry out these actions, and (5) development of a legal framework for their
effective implementation.
Research
Prioritisation
Given limited resources for control, an essential first step
is prioritising the problems. A risk assessment system is
Tye et al: A strategy for Galapagos weeds
used to evaluate both species that are already present in
the islands and proposed introductions. Permitted lists for
import of species and products have already been drafted,
based on a preliminary risk assessment procedure, while
all introduced plant species have been subject to a subjective risk assessment. A formal risk assessment system is
currently being developed (Tye 2001a). Risk assessment
and prioritisation will also be applied to sites. This is already done in a subjective manner, but a more formal system based on conservation value is being developed.
Risk assessment is the major tool used to decide whether
an introduced species should be merely controlled, or
whether an eradication attempt could be considered. The
decision depends on the plant’s inherent biological characteristics (including growth form, reproductive strategies,
dispersal mechanisms, reproductive rate, age at first reproduction, and longevity), its current distribution, abundance, rate of spread, ecological and other impacts, susceptibility to treatment (visibility, availability of control
techniques cheap and effective enough to enable eradication to be contemplated with available resources), and urgency of action required.
Ecology and distribution
The success of a risk assessment system depends on sound
scientific data about the distribution, biology, and ecology
of the introduced species. These factors are also essential
to the design of control or eradication programmes. Research currently in progress or planned for the next five
years includes complete surveys of introduced plants in
the agricultural and urban zones of the four major inhabited islands, ecological studies of the life cycle of the worst
invaders and their effects on native vegetation, and gathering of information on the biology of introduced species
from sources outside Galapagos. So far, ecological and
distributional studies have focussed on Cinchona
pubescens (Jäger 1999), Psidium guajava and Rubus spp.
Control trials
Research on control techniques is also essential, since many
Galapagos invasives are useful species and have not been
subject to control elsewhere. We need effective methods
of killing the invaders that are both as cheap as possible
and are appropriate for use within a national park, causing
minimal damage to the native vegetation. Limitations are
imposed by the unavailability of some herbicides in Ecuador and by the difficulties of obtaining, using, and servicing sophisticated application equipment. The trials take
these limitations into account, although where a herbicide
is not locally available (but seems by far the most effective), trials are undertaken along with negotiations with
potential suppliers and the Ecuadorian authorities in order
to try to ensure that the required materials become available for use within the archipelago. Research includes trials of manual methods, herbicides, and herbicide application techniques. Trials so far have included 15 of the worst
invasive plant species. Biological control has never been
applied in Galapagos and is controversial because of the
potential dangers of introducing yet another organism, but
laboratory research is currently under way in preparation
for what may become the first biocontrol field trial, for
cottony cushion scale insect Icerya purchasii. Should this
first instance prove successful, the climate of opinion
should improve, and trials for the biocontrol of weeds,
including Cinchona pubescens, Lantana camara, and Rubus spp.,may be considered.
Given the pressure for control of the worst invaders, trial
results are often applied to control and eradication campaigns before completion of trial monitoring. In such cases
the control action is also monitored to enable better assessment of the efficacy of the techniques. Trial results
are also fed into the weed risk assessment system, since
availability of suitable techniques is an important factor in
risk assessment, particularly in helping to decide whether
a plant species should be controlled or whether an eradication attempt could be considered.
Restoration research
Restoration research is just beginning in Galapagos. The
initial focus is on methods of control combined with active restoration, such as seeding with native species. Only
one project to investigate the possibility of assisted restoration has so far been undertaken, examining restoration
of native Scalesia forest following control of elephant grass
Pennisetum purpureum. In other cases, monitoring of selected control sites is being undertaken, to identify when
positive intervention to restore native vegetation communities following control is needed, and to design such
projects. Two examples include monitoring of Miconia
Zone vegetation following control of Rubus niveus and
other introduced species at the unique highland crater-lake
site of El Junco on San Cristóbal Island, and monitoring
of regeneration following control of Cedrela odorata in
Transition Zone forest on Santa Cruz Island. Given the
limited resources available for introduced plant control in
Galapagos in the foreseeable future, it is unlikely that major
restoration projects will be able to be considered for some
time.
Management
Prevention
Prevention is the first stage of management action: a quarantine system for Galapagos has been designed and its
implementation commenced (Zapata et al. 2000). Quarantine can slow but never stop introductions, although it is
essential if the balance between introduction and eradication is to be tipped towards the latter. The quarantine system will eventually implement primary control outside of
Galapagos, at the mainland port and airports from which
traffic reaches Galapagos. Secondary quarantine control
is already in place on the islands, and an early warning
and rapid response strategy is being planned for species
that evade these controls. The system deals also with transport between the islands, given the importance of interisland differences in biodiversity, with many single-island
endemic species.
339
Turning the tide: the eradication of invasive species
Control and eradication
Eradication is considered for plants at both ends of the
invasion scale in Galapagos. One priority group comprises
plants that are known to be invasive in other parts of the
world but are present in Galapagos in very small
populations and are not regarded as indispensable by the
local community. At the other end of the scale are species
that are seriously invasive already in Galapagos, but for
which an assessment indicates that eradication might be
feasible. We have two successful examples of eradication
of plants in the first group and projects in progress to eradicate more, but are only beginning to consider eradicating
plants in the second group.
Among the first group, plants known to be invasive but
still present in small populations in Galapagos, two species are considered eradicated from Galapagos. Echinopsis
pachanoi (Cactaceae) was known from a single garden
plant on Santa Cruz Island, which was cut down and burned
in the 1980s. The other species was tropical kudzu Pueraria
phaseoloides (Fabaceae), which is widely grown in mainland Ecuador for ground cover in orchards and as a forage
plant and soil improver. It was introduced by one farmer
in 1996 and planted in a single pasture field. The plant
was spotted by a CDRS botanist, and after the potential
danger of the species was explained to the farmer (it is a
close relative of kudzu Pueraria lobata, one of the worst
invaders of the south-east U.S.A), he agreed to allow it to
be eliminated. The plot was treated with glyphosate and
monitored regularly, with new growth spot treated. No
plants have been seen since 1997 although monitoring
continued until 2001 (further details in Soria et al. 2002).
Although considered eradicated, we still lack introduced
species inventory data on the agricultural and/or urban
zones of three of the four inhabited islands, so there may
yet be other plants of these species present there.
In the past, apart from these two isolated cases, eradication has not been attempted. Instead, funds have been directed as a priority towards control of the most serious
invaders in selected important sites. However, during the
past year we have begun to attempt eradication on a larger
scale. A project began in 2001, with sufficient funding to
permit the selection and attempted eradication of 30 plant
species still present in small populations. Eradication in
some cases will be from the entire archipelago and in other
cases only from selected islands. Eradication attempts have
already begun with the following species:
L Citharexylum gentryi, a timber tree present in two parts
of the agricultural zone of Santa Cruz.
L Rubus megalococcus and R. glaucus, each present in a
single population on Santa Cruz.
L Rubus niveus from the island of Isabela. This species is
already widespread and beyond current eradication capability on Santa Cruz and San Cristobal islands, but is
present in only a small area on Isabela.
L Rubus adenotrichos, present in single localities on
Isabela and Santa Cruz.
340
These cases are discussed in more detail by Soria et al.
(2002). The remaining priority plants to make up the initial target 30 species will include other timber trees and
ornamental plants. A preliminary list of some 60 potential
targets has been drawn up based on the basic criteria of
limited distribution and known invasive tendencies, and
the final list of 30 will be selected following a risk assessment based on a full range of criteria including feasibility
of eradication.
The second priority group of species for eradication comprises serious invaders that are widespread but whose characteristics suggest that eradication might still be feasible.
The pilot species for this work is Cinchona pubescens, a
small tree that has become the greatest single threat to the
native highland vegetation of Santa Cruz. The principal
characteristics suggesting that it might be feasible to control it include that it is conspicuous, not regarded by the
local community as a useful species but on the contrary
widely recognised among conservation workers and local
people as a serious pest, and it is present on only one island. In addition, whereas previously it had been difficult
or impossible to control Cinchona by acceptable herbicides, safe and effective chemical techniques for killing it,
using picloram for cut stump treatment and picloram-methyl metsulphuron mixtures for hack-and-squirt treatment,
have recently been identified by research in Galapagos.
These herbicide treatments for larger trees, combined with
manual removal of seedlings and saplings, could eventually permit complete eradication. This project is still in
the planning stage, with larger-scale control trials under
way and an ecological study in progress that is designed
to provide information on factors affecting the speed and
likelihood of eradication, such as density, distribution,
seedbank longevity and dispersal ability. These studies will
lead to better cost estimates, which are needed for the ultimate decision on whether to attempt eradication.
Control is undertaken where eradication is not currently
considered feasible but where the plant is considered to
pose a significant conservation risk. Such plants include
most of the most serious invaders, such as Rubus niveus,
Lantana camara, vines such as Passiflora edulis, and very
widespread invasive trees such as Psidium guajava and
Cedrela odorata. Virtually every widespread introduced
plant that is not a tree also falls into this category, as do
useful (indispensable to local people) but invasive species
for which no effective substitute species has yet been identified. Some potentially invasive garden ornamentals are
also considered not yet eradicable, since they are so popular.
Restoration
So far, no active restoration has been undertaken following invasive plant control. Monitoring of selected control
sites will provide data that will be used to identify needs
for intervention to restore native vegetation communities
and to design such projects.
Tye et al: A strategy for Galapagos weeds
Legal framework
A comprehensive “Special Law for Galapagos Province”
was passed by the Ecuadorian Government in 1998. It
covers all aspects of conservation and development in the
province (which includes the entire archipelago), from
immigration and waste disposal to natural resource use,
conservation of rare species, and control of pests. Regulations for the implementation of the new law are currently
being drawn up, including sections on quarantine and introduced species management. Weed risk assessment will
be written into these regulations, as will requirements for
planning for the control or eradication of introduced plants.
The regulations will place legal obligations on various
public and private bodies, as well as on individual landowners, to undertake specified actions in regard to declared
weed species or new introduced plants, including monitoring, reporting, controlling, and eradicating. Such regulations and obligations are especially essential for the success of eradication efforts, where all populations of the
species, whether on public or private land, must be treated.
THE FUTURE
Recent advances in weed control in Galapagos have included the development of an objective risk assessment
system, the first properly designed, implemented and monitored control trials, the first comprehensive projects to investigate the ecology of serious weeds, the first strategic
eradication programme, the first attempt to eradicate a wellestablished invader, and the approval of the first comprehensive legislation to deal with the problem of environmental weeds. All of these developments have taken place
in the last five years, and serious attempts to eradicate invasive species from the archipelago have begun to look
promising. These activities, aside from their scientific and
conservation value, should act as valuable confidencebuilders, changing the climate of opinion about the dangers of introduced species and the feasibility of their control and eradication. If successful, we could be about to
experience the first recorded decrease in the number of
introduced plant species in Galapagos since their discovery in 1535.
ACKNOWLEDGMENTS
We thank Dick Veitch for encouraging this paper, Heinke
Jäger and Susan Timmins for comments on a draft, and
André Mauchamp for data used in his 1997 article.
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