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Limnetica, 29 (2): x-xx (2011)
Limnetica, 36 (1): 15-27 (2017). DOI: 10.23818/limn.36.02
c
Asociación Ibérica de Limnología, Madrid. Spain. ISSN: 0213-8409
Colocasia esculenta (L.) Schott (Araceae), an expanding invasive
species of aquatic ecosystems in the Iberian Peninsula: new records
and risk assessment
Elías D. Dana1,∗, Juan García-de-Lomas2, Filip Verloove3,∗, David García-Ocaña4, Vanesa
Gámez5, Juan Alcaraz6and José Miguel Ortiz7
1Grupo de Investigación, Transferencia I+D en Recursos Naturales, Universidad de Almería (Almería, Spain).
2Grupo de Investigación Estructura y Dinámica de Ecosistemas Acuáticos, Universidad de Cádiz (Cádiz, Spain).
3Botanic Garden of Meise, Nieuwelaan 38, B-1860 Meise (Belgium).
4Calle Delfín, 4 1oB izquierda.18015, Granada (Spain).
5Calle El Valle, 1. 18813 Cuevas del Campo, Granada (Spain).
6Casa Forestal del monte dunas de Tarifa, CP. 11380, Tarifa (Spain).
7Oficina Administrativa Agentes de Medio Ambiente. Plaza de Andalucía 22, 11350-Castellar de la Frontera,
Cádiz (Spain).
∗Corresponding author: eliasdana.ecology@gmail.com and filip.verloove@plantentuinmeise.be
2
Received: 16/12/15 Accepted: 15/12/16
ABSTRACT
Colocasia esculenta (L.) Schott (Araceae), an expanding invasive species of aquatic ecosystems in the Iberian Peninsula:
new records and risk assessment
Colocasia esculenta (L.) Schott, Araceae, is becoming an invasive plant in Spain (Iberian Peninsula). Four newly invaded
localities are presented in this study, and its population status, habitat and climatic features in Spain are presented. The species
has colonised some localities in south Portugal. To characterise the species’ climatic tolerance, its world distribution was
reviewed, and the climate types of other invaded areas worldwide were identified. Global data show that the species has the
potential to colonise aquatic ecosystems under a wide variety of climate types. Finally, based on two different procedures,
risk assessments were conducted for the Iberian Peninsula and for continental Europe. Both suggested a high ecological risk
associated with this species. Caution is called for in terms of its use outside of its native distribution range. It is proposed that
this species should be considered as invasive and risky to European and Iberian water bodies and should be legally banned in
Europe.
Key words: Plant invasion, climate, risk assessment, Köppen-Geiger, Spain, Portugal, European Union.
RESUMEN
Colocasia esculenta (L.) Schott (Araceae), una invasora en expansión por los ecosistemas acuáticos ibéricos: nuevos
registros y análisis de riesgos
Colocasia esculenta (L.) Schott, Araceae, se está convirtiendo en una especie invasora en España (península ibérica). En
este estudio se da cuenta de cuatro nuevas localidades invadidas, para las que se describen el estado de las poblaciones, las
características del hábitat invadidoy los rasgos climáticos de las localidades invadidas en España. La especie ha colonizado
algunas localidades del Sur de Portugal. Se revisa su distribución mundial y los parámetros climáticos de las áreas invadidas a
fin de caracterizar su tolerancia climática. El conjunto de datos muestra que la especie tiene amplia capacidad para colonizar
ecosistemas acuáticos de agua bajo un amplio rango de condiciones climáticas. Finalmente, empleando como contraste dos
métodos diferentes, se realiza un análisis del riesgo de invasión a dos escalas, Península Ibérica y Europa. En ambos casos
los resultados alertan del alto riesgo ecológico asociado a la especie. Se hace una llamada a la precaución sobre su uso
fuera de su rango nativo, y se propone que la especie debería ser considerada como invasora y de riesgo para los humedales
europeos e ibéricos, y por ello, legalmente prohibida en Europa.
Palabras clave: Invasión por plantas, clima, análisis de riesgos, Köppen-Geiger, España, Portugal, Unión Europea.
16 Dana et al.
INTRODUCTION
Colocasia esculenta (L.) Schott, Araceae (taro,
elephant ear or cocoyam), is an emergent,
perennial, semi-aquatic herbaceous Asian plant
species (Plucknett, 1976). Within its native
range, C. esculenta grows in tropical areas with
high rainfall (1800-2500 mm/year) and tempera-
tures in the range of 25-35 ◦C. It occurs mainly in
wetlands with low salinity levels (<5 mM NaCl),
although it can also be found in dry lowland
environments (Fujimoto, 2009). Taro was one of
the earliest crop plants in the Solomon Islands
and New Guinea, where it has been grown for
more than 10,000 years (Loy et al., 1992). Cu-
rrently, C. esculenta is commonly cultivated in
many warm and tropical areas, and it is the fifth
most consumed root vegetable worldwide (Mace
& Godwin, 2002). This species has also been tra-
ditionally cultivated outside the tropics, including
in some Mediterranean areas such as Portugal,
the Canary Islands and Madeira. Some au-
thors (García-Sánchez et al., 2008; García-Sán-
chez, 2013) consider that the species was likely
grown in the Iberian Peninsula in al-Andalus
both as a crop, frequently associated with banana
(Musa sp.) and sugarcane (Saccharum offici-
narum) farming, and as an ornamental plant in
orchards. Indeed, ornamental use is also frequent
in warm areas of the world (Wirth et al., 2004).
C. esculenta has shown invasive behaviour
in several warm and temperate areas of the
world. Extensive stands of elephant ear alter the
vegetation composition, structure and dynamics
of riparian plant communities. Many cultivars
are well adapted to saline conditions, low water
availability and/or seasonally flooded soils and
are also widely cultivated throughout tropical and
subtropical regions (Onwueme, 1999). By virtue
of its corms, or vegetative fragments, it may
become invasive along irrigation channels and
surrounding lands, riversides and lakes, where
it can modify the vegetation composition and
structure as well as the dynamics of riparian plant
communities (Cufodontis, 1953-1972; Kunkel,
1975; Wester, 1992; Visser et al., 1999; FLEPPC,
2000; Tye, 2001; Brown & Brooks, 2003; García-
Camacho & Quintanar, 2003; Henderson, 2007;
Atkins & Williamson, 2008; Silva et al., 2008;
Ferrer-Gallego et al., 2015). C. esculenta is also
an allelopathic species (Pardales et al., 1992).
These undesired ecological consequences have
led to the implementation of management ac-
tions to control invasive populations (Brown
& Brooks, 2003; Atkins & Williamson, 2008).
However, little attention has been paid to this
species as an invasive plant in the Iberian Penin-
sula. García-de-Lomas et al. (2012) reported the
details of the reproduction, spreading patterns
and dynamics of an invasion record in southern
Spain, whose origin could be associated with pri-
vate ornamental use of the species. In this area,
the plant grew as dense stands along rivulets and
showed evidence of rapid downstream spread by
means of satellite (coloniser fragment) individ-
uals. These authors also recommended further
studies on its potential invasion in freshwater
wetlands at broader geographical scales. More
recently, Ferrer-Gallego et al. (2015) reported
two invaded wetlands in eastern Spain, where
control actions are being implemented.
In the present work, we provide four novel
field records for taro, review the climatic char-
acteristics of other invaded locations worldwide
and conduct risk analyses for this species in con-
tinental Europe.
MATERIALS AND METHODS
Our findings of new naturalised populations of
C. esculenta located in the south of Spain are
incidental. These findings led us to review the
species’ status in the wild in the Iberian Penin-
sula. For each new population found, the follow-
ing data are provided: UTM projected coordi-
nates (Datum ETRS89/TM30), date of observa-
tion, habitat, accompanying species, size of the
area invaded and climatic parameters (average
annual rainfall; average, minimum and maximum
mean temperature and minimum temperature of
the coolest month). The climatic data and climate
type of each locality were retrieved from climate-
data.org, an online database built from a model
containing more than 220 million data points col-
lected between 1982 and 2012 and a resolution of
Limnetica, 36 (1): 15-27 (2017)
Colocasia esculenta, an expanding invasive species of aquatic ecosystems... 17
30 arcseconds (0.0083 decimal degrees), or ap-
proximately one square km. Xenotype was as-
signed following Korna´
s (1990) and Sanz-Elorza
et al. (2004) according to the habitat features and
population state.
Records showing the status of C. esculenta
as naturalised or invasive were reviewed in pub-
lished papers and main flora databases such as
GBIF (http://www.gbif.org), CABI (http://www.
cabi.org/isc), and Discover Life (http://www.dis
coverlife.org). Records that did not unequivo-
cally refer to naturalised populations were dis-
carded.
To evaluate the climatic amplitude of the
species, we followed two independent proce-
dures. The first procedure used ‘Climatch’ online
software (available at http://data.daff.gov.au:80
80/Climatch/climatch.jsp). The underlying algo-
rithm produces a classification of localities
based on the similarity level shown by climatic
features between the source localities (i.e., the
climatic station nearest to the invaded locality in
the Iberian Peninsula) and the target localities
(in this work, the rest of the localities with
climatic stations). The resolution of this model
is lower than that required by the risk analysis
questionnaire developed by Garcia-de-Lomas et
al. (2014) –1 km2– since it classifies similarities
between points (locations of climatic stations).
‘Closest Euclidean match’ was chosen as the
algorithm to calculate the ‘climate distance’
between the input sites and each target site
across the climate variables used in the analysis
(Elmore & Richman 2001; Crombie et al., 2008).
All climatic variables recorded were used in the
analysis to gather the maximum environmental
variability. Fourteen stations located in the
surroundings of clearly naturalised populations
of C. esculenta were used as the source region.
Seventy-six stations covering a territory extent
of ca. 472.510 km2were employed as the target
region for comparison. The results obtained
were contrasted with those retrieved using the
closest standard score as a distance measure.
Since they were almost identical, only the results
obtained with the Euclidean metric are presented
in this work. This procedure was applied only to
predictions at the scale of the Iberian Peninsula
in order to reduce potential interferences caused
by heterogeneity among the floristic records pub-
lished worldwide in a so widely spread species
and to reduce the influence of errors that could
be introduced by the analyser’s selection criteria
of so many climatic stations as source data.
The second procedure for climate analysis
was based on inspecting those regions depicted
by their climatic similarity in the World Map
of the Köppen-Geiger climate classification
(Kottek et al., 2006). In this system, the first
letter describes the main climate classes, namely
tropical (A), arid (B), warm-temperate (C),
cold-temperate (D) and cold climates (E). The
second letter accounts for precipitation regimes:
hot (h) and cool (k). The third letter refers to
the temperature classes: hot summer (a), warm
summer (b), dry, short summer (c) and very cold
winter (d). This second analysis was applied to
European countries. First, we identified the cli-
mate types of the regions in which C. esculenta
has become invasive in the world. The European
areas where those climate-types are represented
were depicted using the general world map
prepared by Wilkerson & Wilkerson (2010). The
final map, freely available to users, has a spatial
resolution of 0.5 degree, which seems more than
sufficient to conduct a visual inspection of the
distribution ranges of the climatic conditions
required by the species across large territories.
Finally, with all the information gathered,
a risk analysis assessment was conducted fol-
lowing two different procedures (Gordon et al.,
2010; García-de-Lomas et al., 2014) to evaluate
the invasion risk in the Iberian Peninsula and
in the continental part of the European Union
as a whole. The use of these two different
approaches helps to inspect the robustness of
the results obtained. The first method is based
on the Australian Weed Risk Assessment, WRA
(Pheloung, 1995). The WRA scheme follows
a quantitative scoring method, which adds the
scores obtained from 49 questions on biogeog-
raphy, biology/ecology, and traits contributing
to invasiveness. Depending on the answer, each
question is awarded between 3 and 5 points
(mostly1to1),andthefinal WRA score is the
sum of the points for all answered questions.
Limnetica, 36 (1): 15-27 (2017)
18 Dana et al.
This final score, ranging potentially from −14
(benign taxa) to 29 (maximum risk), leads to
one of three outcomes: the species is accepted
for introduction (< 1 total points), rejected (> 6
points), or recommended for further evaluation
of invasive potential (1–6 points). The type
of response for each criterion is simple (yes,
no; low-intermediate-high) but rather prone to
subjectivity. As a consequence, we followed the
particular instructions to fill the questionnaire
developed by Gordon et al. (2010).
The second method, proposed by García-de-
Lomas et al. (2014), follows a semi-quantitative
scoring system. A total of 19 criteria were consid-
ered, including biological features and potential
impacts as well as two specific questions related
to the potential impacts on economy and human
health. The questions were grouped into three
categories (critical, key and secondary). Thus, if
any of the critical questions has been answered
with the option “a”, the output is “high risk”.
If more than three key questions have been an-
swered with the option “a”, the output is “high
risk”. Each question receives generally from 0
to 5 points. If all questions are answered, the
final sum varies between -14 and 63. To facili-
tate easier interpretation, this output value is then
transformed onto a 0-100 scale, so that the fi-
nal output is “low risk” (total sum < 54 points)
or high risk (total sum ≥54 points). Therefore,
Figure 1. Published records of Colocasia esculenta (L.) Schott in the Iberian Peninsula. Empty squares show records indicated
in the ancient literature, which could not be confirmed in this study, likely due to the heavy environmental modifications caused
by humans in those areas. Records from Portugal must be considered at present as ephemerophytes escaped from cultivated lands.
The remaining dots correspond to naturalised populations. Registros publicados de Colocasia esculenta (L.) Schott en la península
ibérica. Los cuadrados vacíos muestran registros indicados por literatura antigua, que no pudieron ser confirmados en este estudio,
probablemente por las profundas modificaciones ambientales provocadaspor el ser humano en esas áreas. Los registros de Portugal
deben ser considerados, por ahora, como efemerófitos escapados de cultivos adyacentes. El resto de registros corresponde a las
poblaciones naturalizadas.
Limnetica, 36 (1): 15-27 (2017)
Colocasia esculenta, an expanding invasive species of aquatic ecosystems... 19
only two outputs are possible (“high risk”, “low
risk”). This method reduces the time of evalua-
tion since the evaluator rarely needs to answer
all the questions. The possibility to answer “un-
known” is always included as a measure of uncer-
tainty. As a precautionary approach, if all ques-
tions were answered as “unknown”, the output
is “high risk” (it is assumed that if there is no
available information on one species, the species
should not be introduced within a new territory).
This protocol also requires the inclusion of the
source of information used to ensure objectivity
and transparency. A synthesis of the relevant in-
formation for risk analyses is provided in Sup-
plementary Information (see Tables S1, S2, avail-
able at www.limnetica.com).
Vouchers for some of the new records in
Spain were deposited in the MGC herbarium
(the herbarium of the University of Malaga).
RESULTS AND DISCUSSION
Newly invaded localities
Four new records of Colocasia esculenta are re-
ported here. The maximum distance between the
newly detected populations is 135 km. The new
records included a variety of habitats colonised,
including stream banks and artificial wetlands.
•Cádiz, Punta Paloma (Parque Natural del
Estrecho, Tarifa), 48 m.a.s.l. Coordinates:
254401/3995053. Date: 19/06/2015. Length
invaded: 25 m. Mean cover density = 100%.
Habitat and main species: temporal stream
under pinewood (Pinus pinea L., Rubus ulmi-
folius Schott, Smilax aspera L.). Xenotype:
naturalised as hemiagriophyte. Voucher: MGC
81003, Legit.: Juan Alcaraz. Det.: E.D. Dana.
•Cádiz, Jimena de la Frontera, in temporal
stream joining Hozgarganta River, on the
path named Pasada de Alcalá, near coun-
cil’s Ethnobotanical Garden (Parque Natural
Los Alcornocales), 92 m.a.s.l. Coordinates:
279613/4035216. Date: 15/10/2013. Length
invaded: 150 m. Mean cover density = 100%.
Habitat and main species: temporal Mediter-
ranean stream connected to Hozgarganta River
in open ground with Rubus ulmifolius Schott.
Xenotype: naturalised as hemiagriophyte. Vou-
cher: MGC 80806 and 81213, Legit.: D. Gar-
cía Ocaña, V. Gámez & J.M. Ortiz. Det.: E.D.
Dana.
•Sevilla, La Puebla del Rio (Cañada de los
Pájaros, Reserva Natural Concertada), 10
m.a.s.l. Coordinates: 222509/4126114. Date:
18/10/2014. Length invaded: 30 m. Mean
cover = 50%. Habitat and main species: ar-
tificially swamped soils with other invasive
species, Eichhornia crassipes (Mart.) Solms
1883, Cyperus sp. Xenotype: epoecophyte.
Field observation on private land, no voucher
available (obs. J. García-de-Lomas).
•Sevilla, Utrera (Paraje Natural Brazo del Este),
20 m.a.s.l. Coordinates: 229542/4113475.
Date: 23/11/2015. Length invaded: 3 m. Mean
cover = 100%. Habitat and main species:
marshland, Phragmites australis (Cav.) Trin.
ex Steud. Xenotype: holoagriophyte. Field
observation, no voucher available (obs. J.A.
Barragán and E.D. Dana).
In Spain, there are a few additional reports
of naturalised patches of C. esculenta in the
XIXth century in Málaga and Cádiz provinces
(Fig. 1), but, as geographic coordinates are too
vague, its persistence cannot currently be con-
firmed. Boissier (1839) found it near Churriana
(30SUF65) and in Alhaurinejo (30SUF55) and
Alhaurín (30SUF45) (“In humidiusculis regionis
calida e ferè spontanea”). Additionally, Pérez-
Lara (1886) recorded the species in Algeciras
(Barranco del Quejigo, 30STF80) and in an un-
specified location between Jimena de la Frontera
and Alcalá (30STF83). Subsequent references
have never confirmed the species in these or
other localities nearby (e.g., Casimiro-Soriguer
& Pérez-Latorre, 2008). This may be due to the
increased urbanisation in Málaga province where
the species was initially located (Moreira, 2011),
whereas the localities reported in Cádiz province
are too imprecise to be located. Other sources
have reported naturalised populations of C. es-
Limnetica, 36 (1): 15-27 (2017)
20 Dana et al.
culenta in seminatural wetlands in Valencia and
Castellón (Ferrer-Gallego et al., 2015) and Tar-
ragona (Balada, 1993; Royo, 2006; Curcó, 2007).
It must be highlighted that the four new Span-
ish localities presented in this work are located
within protected areas. Of great importance is the
new patch found in Sevilla (Brazo del Este, Nat-
ural Area). It appears to be isolated and several
kilometres away from the nearest known popula-
tion (García-de-Lomas et al., 2012). In less than
four years, it has colonised a new distant point,
whichseemstoconfirm the prediction by García-
de-Lomas et al. (2012) that those water bodies
located downstream of an invaded course are un-
der high risk of being invaded over short periods
of time.
The status of this species in mainland Portu-
gal is unclear. ‘Taro’ has not been recorded as an
established invasive species in continental Portu-
gal by the most recent and comprehensive studies
(Sequeira et al., 2011; Almeida & Freitas, 2012).
However, it shows clear invasive behaviour and
Table 1. Main climatic features of localities in which Colocasia esculenta (L.) Schott has been found in the Iberian Peninsula.
Climate type according to Köppen-Geiger’s system, updated by Kottek et al. (2006). Records from Portuguese localities must be
considered at present as ephemerophytes escaped from cultivated lands. Principales parámetros climáticos de las localidades de las
que se conoce la presencia de Colocasia esculenta (L.) Schott. La designación del tipo climático sigue a Köppen-Geiger’s system,
actualizado por Kottek et al. (2006). * Los registros de las localidades portuguesas deben considerarse, por ahora, como escapes
procedentes de cultivos próximos.
LOCATION
Altitude (meters above sea level)
Average annual T (oC)
Mean T of coolest month (oC)
Mean T of warmest month (oC)
Average of minimum Ts of coolest month (oC)
Averageof max. Ts of warmest month (oC)
Annual variation of average T (oC)
Total annual rainfall (average, in mm)
Average minimum precipitation (mm) of driest month
Averageprecipitation (mm) of wettest month
Difference in precipitation (mm) between the driest
month and the wettest month
Type of climate
SPAIN
TARIFA 52 17.2 11.8 23.4 7.9 27.7 11.6 834 0 140 146 Csa
JIMENA 24 17.6 12.0 24.3 8.1 34.5 12.3 739 1 130 129 Csa
DOS HERMANAS 47 18.1 10.3 27.1 5.8 35 16.8 591 1 89 88 Csa
LA PUEBLA DEL RÍO 28 18.3 10.4 27.1 6.0 34.6 16.0 574 1 87 86 Csa
GANDÍA 26 18.0 11.2 26.1 6.4 30.9 14.9 497 7 83 76 BSk
CASTELLÓN DE LA PLANA 34 17.0 10.1 24.7 6.1 28.8 14.6 434 13 68 55 BSk
TARRAGONA 53 17.0 9.5 25.0 5.4 29.9 15.5 536 13 82 69 Csa
PORTUGAL*
MONCHIQUE 439 14.8 9.6 21.3 5.8 27.3 11.7 624 2 98 96 Csb
ORTIGA 132 16.6 10.2 24.0 6.7 30.7 13.8 759 6 109 103 Csa
BARREIRO 8 17.0 11.6 23.0 8.5 28.3 11.4 678 3 103 100 Csa
Limnetica, 36 (1): 15-27 (2017)
Colocasia esculenta, an expanding invasive species of aquatic ecosystems... 21
associated impacts on the Portuguese islands of
Madeira and Azores (Silva et al., 2008). On the
other hand, several records exist for mainland
Portugal in the databases. The most interesting
information is presented in the GBIF database,
which identifies three localities based on per-
sonal observations by André Carapeto in 2013
in Monchique (Faro) and Barreiro (Setúbal) (So-
ciedade Portuguesa de Botânica, 2016). How-
ever, the information provided by the original ref-
erence does not allow these populations to be
considered established. At least the references for
Monchique seem to be escapes from nearby cul-
tivations (Paulo Alves, personal communication).
The CABI database contains a reference (http:
//www.cabi.org/isc/datasheet/17221), but this re-
cord in fact points to undefined general com-
ments made by the EPPO database (2014) regard-
ing its presence on Madeira and Azores. Therefo-
re, this record is not useful for this work. Finally,
another record is presented by the database Dis-
cover Life (http://www.discoverlife.org/mp/20q),
which, in fact, points to a sequence deposit at
GenBank. No information is provided about the
origin of the sample, which led us to discard the
reference. We are aware of escaped individuals
in the Monchique area (P. Alves, personal com-
munication). Therefore, in view of their potential
interest, we would suggest specific future assess-
ment of the populations observed in Portugal by
the Sociedade Portuguesa de Botânica (2016).
Finally, it is important to note that since the
streams of the Iberian Peninsula have not been
extensively sampled in search of this species and
that since all the records published here corre-
spond to incidental findings, it seems reasonable
to assume that the presence of C. esculenta as
naturalised in the Iberian Peninsula could be, to
some extent, underestimated.
Habitat and climate features
In Spain, C. esculenta invades small temporary
streams, irrigation channels, inland wetlands and
large rivers, such as the Ebro (Balada, 1993;
Ferrer-Gallego et al., 2015). The southernmost
populations of this species are established as
hemiagriophytes and holoagriophytes in water-
courses. In all cases, the origin of the naturalised
populations seems to be associated with escapes
from nearby gardens.
The current distribution of C. esculenta in
Spain shows a wide range of environmental pa-
rameters such as the differences in precipitation
values between the driest and the wettest month
(55-146 mm, Table 1), average temperatures
(11.6-16.8 ◦C), mean annual rainfall values
(434-834 mm) and minimum temperatures of
the coolest month (5.4-8.1 ◦C). The reasons
underlying the species’ tolerance to this variation
in climatic parameters may correspond to its
growth form (as a geophyte, dormant organs are
protected underground during the coldest months
of the year) and habitat types colonised, since
watercourses and wetlands may compensate for
variations in rainfall among years and buffer
temperature extremes at the micro-site level.
According to the climate classification of
Köppen-Geiger (Kottek et al., 2006), the invaded
areas in the Iberian Peninsula are associated
with the climate subtypes ‘Csa’ and ‘Csb’ (warm
temperature, dry and hot –‘Csa’– or dry and
warm –‘Csb’– summer) in the central and south
of the Iberian Peninsula and ‘Bsk’ (arid, dry
summer, cold arid) in the east of Spain (Kottek
et al., 2006). The ‘Csa’ type is present in the
Mediterranean part of Europe and in southern
Portugal, Turkey and eastern Mediterranean
countries, as well as in the coastal territories of
Morocco, Tunisia and Algeria, parts of Libya,
and some areas of the Irano-Turanian regions,
India, south and south-western Australia, and
California. ‘Csb’ may be interpreted as a transi-
tion towards subtypes with more mild summers.
The ‘Bsk’ climate type is mainly represented in
more continental areas and appears in several
countries of central Asia, south Australia, south
Argentina, central and north Mexico and the
central inner states of the USA.
Table 2 shows the climate type correspon-
dence to invaded areas worldwide. It is obvious
that the species may appear in the wild under a
large number of climatic conditions: equatorial
savannah with dry winter (‘Aw’), the aforemen-
tioned warm temperate climate with dry summer
(with hot or warm summer, ‘Csa’ and ‘Csb’, re-
Limnetica, 36 (1): 15-27 (2017)
22 Dana et al.
Figure 2. Graphical results of similarity climate analyses conducted for the Iberian Peninsula through ‘Climatch’. Analyses were
produced using all default variables and ‘closest Euclidean match’ as a distance measure for 14 source localities with climate stations
in the surroundings of the clearly established populations of C. esculenta and 76 target localities (territory covered ca. 472.510 km2).
The higher the match class, the closer the match of climates between source and target sites (0, minimum matching, 10, maximum
matching). Resultados gráficos de los análisis de semejanza climática realizados para la península ibérica usando ‘Climatch’. Los
análisis se realizaron usando todas las variables por defecto y el ‘Ajuste Euclídeo más próximo’ como medida de distancia para 14
localidades fuente con estaciones climáticas en las cercanías de poblaciones establecidas de C. esculenta, y 76 localidades objetivo
(cobertura territorial aproximada 472.510 km2). El nivel de coincidencia climática entre localidades fuente y objetivo aumenta con
la clase de corte (mínima coincidencia en 0 y máxima coincidencia en 10).
Table 2. Climate types of invaded areas worldwide, with type of climate according to Köppen-Geiger’s system, updated by Kottek et
al. (2006). See text for an explanation of climate classes and abbreviations used. *Records from Portuguese continental localities must
be considered at present as ephemerophytes escaped from cultivated lands. Tipos climáticosde áreas invadidas en diversaspartes del
mundo, según la clasificación climática de Köppen-Geiger, actualizada por Kottek et al. (2006). Véase el texto para una explicación
sobre las clases climáticas y las abreviaturas empleadas. * Los registros de las localidades portuguesas deben considerarse, por
ahora, como escapes procedentes de cultivos próximos.
Country Region Types of climates Reference
Australia North Aw, BSh NCRIS (2015)
East Cfa, Cfb NCRIS (2015)
South-east Cfb NCRIS (2015)
South-west (Perth area) Csa NCRIS (2015)
New Zealand Northern Island Cfb NCRIS (2015)
USA Texas (San Antonio, Onalaska, Houston) Cfa, Cfb Moran & Yang (2012)
Florida (almost all counties) Cfa, Aw EDDMapS (2015)
Portugal
South and Central mainland Portugal*,
Madeira, Azores Csa, Csb, Cfa, Cfb
*Sociedad Portuguesa de Botânica
(2006) –mainland–;
Silva et al. (2008) –Atlantic Islands–
Spain
Canary Islands Csb, Bsh Kunkel (1975)
South coastal areas Csa This work
Northeast (Ebro river, coastal area) Csa Balada (1993)
Guadalquivir River Basin Csa García-de-Lomas et al. (2012)
East (coastal provinces) BSk Ferrer-Gallego et al. (2015)
Limnetica, 36 (1): 15-27 (2017)
Colocasia esculenta, an expanding invasive species of aquatic ecosystems... 23
Figure 3. Europe and neighbouring territories with Köppen-Geiger type-climates compatible with C. esculenta requirements. The
map has been adapted from that provided by Wilkerson & Wilkerson (2010), which is based on the original map published by
Kottek et al. (2006). Legend letters indicate climate types and sub-types according to Kottek et al. (2006). See text for a description.
Regiones europeas (y países del entorno) con tipos climáticos de Köppen-Geiger compatibles con los requerimientos mostrados por
C. esculenta. El mapa ha sido adaptado del provisto por Wilkerson & Wilkerson (2010), que está basado en el publicado por Kottek
et al. (2016). Las letras de la leyenda indican los tipos y subtipos climáticos de acuerdo con Kottek et al. (2006). Véase el texto para
una descripción.
spectively), a warm and fully humid temperate
climate (with hot or warm summer, ‘Cfa’ and
‘Cfb’, respectively) and a steppe climate (hot
or cold arid, ‘BSh’ and ‘BSk’ subtypes, respec-
tively). It is important to highlight that ‘Csa’,
‘Csb’ and ‘Cfb’ are the subtypes most repre-
sented in Europe, both in inland and coastal Eu-
rope (Kottek et al., 2006).
Risk analysis and final considerations
The results of the analysis conducted with ‘Cli-
match’ (Fig. 2) demonstrated that a large part of
the Iberian Peninsula shares climatic conditions
with those present in the already invaded regions.
Thus, the Iberian Peninsula represents climate
conditions suitable for the establishment of C. es-
culenta.
The comparison of the Köppen-Geiger cli-
mate types present in Europe to those regions
worldwide in which C. esculenta behaves as a
clear invader allows us to conclude that the vast
majority of European countries are under cli-
matic conditions that are suitable for the species:
Bsh, Bsk, Csa, Csb, Cfa, and Cfb (Kottek et al.,
2006). Figure 3, which is based on the map pre-
pared by Wilkerson & Wilkerson (2010), shows
the distribution and extent of each of these six
climate types in Europe. Therefore, it can be
concluded that the majority of European coun-
tries show climate features compatible with the
species’ requirements.
According to Rubel & Kottek (2010), as a re-
sult of climate change, the areas characterised
by the ‘Csa’ climate type are predicted to de-
cline and to be replaced by the ‘Bsk’ type to
an important extent in Spain and in some other
areas in Europe by 2070-2100. The ‘Csa’ type
climate is also predicted to cover a large part
of north-western France that is now covered by
Limnetica, 36 (1): 15-27 (2017)
24 Dana et al.
the ‘Cfb’ climate type. These authors also found
that, assuming an A1FI emission scenario for the
period 2076-2100, projections will result in an
increased coverage of 31.82% of ‘B’ climates
(+2.68%) and 15.20% of ‘C’ climates (+0.53%)
of the global land area. It must be highlighted that
1% corresponds to an area of 1.43 ·106km2.
In the risk analyses conducted, C. esculenta
was classified as a species showing high risk
for wetlands and riversides, regardless of the
procedure employed and the geographical scale
considered. Using the assessment system pro-
posed by García-de-Lomas et al. (2014), the total
score obtained for the species was 73.2 points
out of 100. The procedure presented by Pheloung
(1995) and Gordon et al. (2010) yielded a similar
result, i.e., that the use of this species should be
prohibited, either when applied to the Iberian
Peninsula or to continental Europe, with a total
score of 9 points out of 29. Both the biological
features shown by C. esculenta and its potential
impact on the ecosystems invaded (e.g., ability
to form monospecific dense stands) were respon-
sible for the high risk of invasion. In agreement
with Regulation (EU) no. 1143/2014 of the
European Parliament and of the Council of 22
October 2014 on the prevention and management
of the introduction and spread of invasive alien
species, a synthesis of the relevant information
for risk analysis is provided in Supplementary
Information (Tables S1 and S2, available at www.
limnetica.com).
Urbanisation of the surroundings of natural
and semi-natural areas has been increasing in the
Iberian Peninsula since 1960 (Gormsen & Klein,
1986). Given the variety of climate types that are
adequate for the species, it seems that an increase
in the use of C. esculenta may represent an in-
creased risk of invasion of the rivers and wetlands
of the Iberian Peninsula. Invasion by this species
would be a threat added to others such as pollu-
tion, destruction of native plant communities and
flow reduction. It must be taken into account that
the influence of general climate conditions at
broad scales may be locally moderated in rivers
and wetlands, a fact that could increase the effec-
tive potential area for this species. It also seems
reasonable to extrapolate these results, at least to
other European countries.
Consequently, it seems reasonable to legally
regulate the use of this species in Europe. Spain
and Portugal have specific legislation on invasive
taxa. Therefore, it is proposed to include C. es-
culenta within the list of invasive alien species
banned for use in the Iberian Peninsula. In ar-
eas where the species is an ancient crop, such
as in the Canary Islands and Azores, with impor-
tant areas of cultivated lands –called ‘ñameras’–
, banning is not a feasible nor realistic option,
and hence preventive measures focused on con-
trolling escaped individuals could be promoted.
Spanish and Portuguese public administra-
tions are also recommended to remove all the
stands cited in literature to prevent a greater
degree of invasion. The efficacy of different
control methods (manual removal, application
of herbicides (glyphosate), mechanical cutting,
a combination of mechanical cutting followed
by application of glyphosate to cut petioles
and shadowing) has been tested (Atkins & Wi-
lliamson, 2008; Ferrer-Gallego et al., 2015). The
effectiveness of manual removal is moderate
(Atkins & Williamson, 2008; Ferrer-Gallego et
al., 2015). Moreover, physical contact may cause
dermatitis because this species contains calcium
oxalate (Franceschi & Nakata, 2005; Oscarsson
& Savage, 2007). In addition, manual removal
may result in the multiplication and spread of the
species from stolons and corms (Ferrer-Gallego
et al., 2015). The use of herbicides in aquatic
environments is not generally allowed and may
be harmful to non-target aquatic fauna and flora.
A promising alternative method is the overshad-
owing of plant stands by anti-grass double-layer
opaque shade cloths (Ferrer-Gallego et al.,
2015). This method avoids handling the speci-
mens directly, and by depriving them of light, the
plants end up withering and rotting. However,
this method is suggested only on banks with
heavily invaded monospecific stands and in areas
with aggregated and discontinuous distributions
of C. esculenta. However, when banks are
colonised by other species (such as trees or
shrubs), the use of this technique might not be
Limnetica, 36 (1): 15-27 (2017)
Colocasia esculenta, an expanding invasive species of aquatic ecosystems... 25
advised because of potential damage to native
communities.
ACKNOWLEDGEMENTS
Thanks are due to Tobias Schwarz for advice
on the use of the climate-data projections con-
tained in www.climate-data.org and to Belinda
Mitterdorfer (Plant Biosecurity, Australian Gov-
ernment Department of Agriculture and Water
Resources) for sharing information on the Aus-
tralian Weed Risk Assessment protocol. Finally,
Paulo Alves (Centro de Investigação em Bio-
diversidade e Recursos Genéticos, Universidade
do Porto) is thanked for providing data about
the Portuguese records. Our gratitude also goes
to Alfonso Barragán for alerting us regarding a
new population in Sevilla. Special thanks go to
two anonymous referees and the associate editor,
whose constructive comments considerably im-
proved an earlier version of the manuscript, es-
pecially in the section related to climate analyses.
We are grateful to the MGC staff for helping us
with vouchers handling.
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