Plant invasions in the rivers of the Iberian Peninsula, south-western Europe: A review

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Plant invasions in the rivers of the Iberian Peninsulasouth-western Europe A reviewF C F Aguiara amp M T Ferreiraa

a Centro de Estudos Florestais Instituto Superior de Agronomia Universidade de LisboaTapada da Ajuda P-1349-017 Lisboa PortugalAccepted author version posted online 01 Nov 2013Published online 25 Nov 2013

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Plant Biosystems 2013 Vol 147 No 4 1107ndash1119 httpdxdoiorg101080112635042013861539

PLANT INVASION IN SOUTHERN EUROPE

Plant invasions in the rivers of the Iberian Peninsula south-western Europe A review

F C F AGUIAR amp M T FERREIRA

Centro de Estudos Florestais Instituto Superior de Agronomia Universidade de Lisboa Tapada da Ajuda P-1349-017 Lisboa Portugal

Abstract Aquatic and riparian ecosystems are known to be highly vulnerable to invasive alien species (IAS) especially when subjected to human-induced disturbances In the last three decades we have witnessed a growing increase in plant invasions in Portugal and Spain (Iberian Peninsula south-western Europe) with very detrimental economic social and ecological effects Some of these species such as the giant reed (Arundo donax L) and the water hyacinth (Eichhornia crassipes (Mart) Solms-Laub) number among the worldrsquos worst weeds We present an appraisal of this invasive alien river flora and the most problematic aquatic weeds We review various aspects of invasion ecology including spatial and temporal patterns of invasion species invasiveness species traits of invasive weeds and relationships between human disturbance in rivers and surrounding areas and invasibility and contextualize them in overall state-of-the-art termsWe also acknowledge the use of IAS as bioindicators of the ecological quality of rivers wetlands and riparian zones Remote-sensing tools and Geographic Information Systems for detecting and monitoring IAS in Iberian rivers are presented

Keywords Alien flora aquatic weeds exotic species Iberian Peninsula Mediterranean region rivers Portugal Spain

Abbreviations IAS invasive alien species

Introduction concern) which usually adversely affect native

biodiversity andor ecosystem functioning ldquoInvasive

Alien species (synonyms exotic introduced non- alien speciesrdquo (IAS) refers to alien plant taxa that are

native non-indigenous) are organisms whose pre- usually recognized as aquatic weeds

sence beyond their natural range is due to intentional Rivers wetlands and riparian habitats have

or accidental introduction as a result of human repeatedly been documented as being among the

activity Alien plants can become ldquoinvasiverdquo ndash ie ecosystems that are most threatened by invasive

naturalized plants that produce reproductive off- weeds (Hood amp Naiman 2000 Vila et al 2007

spring often in very large numbers ndash at considerable Arianoutsou et al 2010 Pysek et al 2010) and offer

distances from the parent plants and thus have the some of the most impressive examples of native

potential to spread over a large area (Richardson species displacement and ecosystem disruption

et al 2000 Pysek et al 2004) Most invasive plants Blooms of Azolla filiculoides Lam (azola) stream

do not have detrimental effects (economic environ- clogging by Eichhornia crassipes (Mart) Solms-Laub

mental or both) but some become weeds whose (water hyacinth) and Myriophyllum aquaticum (Vell) controleradication is mandatory Following termi- Verdc (parrot feather) and extensive bank strings of

nology suggested by Pysek et al (2004) and adopted Arundo donax L (giant reed) are some of the most

in a similar study by Celesti-Grapow et al (2010) we documented aquatic plant invasion events in the

use the terms ldquoaquatic weedsrdquo and ldquoinvasive weedsrdquo Iberian Peninsula south-western Europe (Carrapico

for both alien plant taxa that invade freshwater et al 1996 Moreira et al 1999a Pereira et al

ecosystems and cause economic losses and 2001 Sanz-Elorza et al 2004 Aguiar et al 2007

ldquoenvironmental weedsrdquo (ie species of environmental Laranjeira amp Nadais 2008 Ruiz Tellez et al 2008)

Correspondence F C F Aguiar Centro de Estudos Florestais Instituto Superior de Agronomia Universidade de Lisboa Tapada da Ajuda P-1349-017

Lisboa Portugal Tel thorn 351 213 653 380 Fax thorn 351 213 653 338 Email fraguiarisaulisboapt

q 2013 Societa Botanica Italiana

1108 F C F Aguiar and M T Ferreira

Figure 1 Invasion of water hyacinth in Lezıria Grande de Vila Franca de Xira near Lisbon Portugal (Photo I Moreira) giant reed invasions

in western coastal rivers of Portugal (Photo FC Aguiar) ldquoazola bloomrdquo in Guadiana River south-eastern Portugal (Photo MT Ferreira)

(Figure 1) In addition the Millennium Ecosystem

Assessment (2005) concluded that the impact on

biodiversity of invasive species in inland waters is

currently greatly increasing

Besides the ecological threats to biodiversity the

abundance of alien species affects many ldquosupportshy

ingrdquo ldquoprovisioningrdquo ldquoregulatingrdquo and ldquoculturalrdquo

ecosystem services and interferes with human wellshy

being (Vila et al 2009) In particular for river and

other freshwater ecosystems (eg lakes and ponds)

invasive weeds greatly modify community composhy

sition alter habitats and refugia for fish and other

biotic elements degrade water quality by blocking

photosynthesis and change nutrient dynamics creatshy

ing a cascade effect for other organisms They lead to

flooding and drainage problems obstruct drinking

irrigation and hydroelectric power water pipes cause

difficulties for fishing interfere with both navigation

and recreational activities such as ecotourism and

alter the aesthetic values and public perception of

landscapes Aquatic invasive weeds have huge

economic impacts worldwide (Pimentel et al 2000)

and some rank at the top of many ldquoblacklistsrdquo of worst

invasive organisms (Lowe et al 2000 Vila et al 2001

GEIB 2006 2011 Vila et al 2009) What is more

many of the emerging invasive alien plants that the

European and Mediterranean Plant Protection

Organization have detected for Mediterranean

countries are aquatic species (Brunel et al 2010)

In fluvial ecosystems the flow-facilitated disshy

persal of seeds and propagules increases the spread of

IAS These ecosystems are converted into invasion

conduits and sources of propagules plants and seeds

for downstream sections and drainage and irrigation

channels The spread of IAS in rivers is recognized as

usually related with anthropogenic and hydrological

disturbances (Hood amp Naiman 2000 Richardson

et al 2007) Schnitzler et al (2007) investigated the

establishment of alien species in European riparian

forests and concluded that the most invaded riparian

forests were generally at lower latitudes with

intermediate levels of disturbance

Despite the early herbarium cites and the

chorological and taxonomic works published in the

early and mid-twentieth century (eg Sampaio 1909

Garcia 1947 Bolos amp Masclans 1955 Lawalree

1964 Bolos 1967 Tutin et al 1968) studies on

aquatic plant invasions really began in the 1980s

after the spread of water hyacinth in the Tagus river

and drainage and irrigation channels in Portugal

(Moreira et al 1983 Figueiredo et al 1984 Moreira

et al 1989) and the occurrence of Azolla filiculoides in Spain (Ruis de Clavijo et al 1984)

Following these pioneering studies a lot of

research on invasion ecology (eg Ferreira et al

1998 Ferreira amp Moreira 1999 Sobrino et al 2002

Aguiar et al 2005 2006a Cobo et al 2010) species

bioecology and its control (eg Catarino et al 1997

Moreira et al 1999b 1999c Andreu amp Vila 2007

2009) relationships between plant traits and species

invasibility (Bernez et al 2006) correlations with

climate change (eg Sobrino et al 2001) and

ecological quality assessments (Ferreira et al 2005

Aguiar et al 2009) have been carried out in the

Iberian Peninsula In Spain concern about the

diagnosis and management of aquatic plant invasions

resulted in the publication of lists and atlases of

invasive species for that country (Sanz-Elorza et al

2001 2004 Andreu et al 2009) as well as in reports

and studies regarding the various Spanish Adminisshy

trative regions (eg Campos amp Herrera 1997 2008

2009 Dana et al 2003 De la Torre 2003 Garcıa-

Murillo et al 2004 Romero Zarco 2004 CA

Andalucıa 2004ndash2006 Capdevila Arguelles et al

2006 Romero 2007 Rivas-Rodrıguez et al 2010)

Recently remote-sensing approaches using high

spatial resolution images and multispectral data have

been explored as a tool for detecting and monitoring

invasive weeds (Dıaz-Delgado et al 2008 Ruız

Tellez et al 2008 Aguiar et al 2011 Fernandes et al

2011 2013)

This paper reviews and contextualizes most of the

studies related with the ecology of aquatic and

riparian IAS in the Iberian Peninsula (Portugal and

Spain) south-western Europe We have chiefly

focused on four main aspects (i) alien flora and

aquatic invasive weeds in Iberia ndash spatial and

temporal patterns of invasion (ii) relationships

between human disturbances and aquatic plant

invasions (iii) using IAS to evaluate the ecological

quality of rivers and (iv) tools for the early detection

and monitoring of IAS in Iberian rivers

IAS in Iberian river systems

According to Almeida and Freitas (2006) in less than

a decade the alien vascular taxa (terrestrial and

aquatic species) in continental Portugal increased by

c 13 to 564 vascular species or c 17 of total

Portuguese flora Likewise for Spanish alien flora

(inland Balearic and Canary Islands North African

and islets squares) Sanz-Elorza et al (2004) reported

937 alien taxa corresponding to around 12 of total

Spanish flora while Del Monte and Aguado (2003)

estimated that 170 species were introduced into

peninsular Spain between 1975 and 2002 at a rate of

almost 6 speciesyear ndash a figure that increased to at

least 10 speciesyear between 2002 and 2005 (Valdes

2004 ndash2006) The Spanish regions with higher

numbers of alien species are located in the south

and east ndash Andalucıa Valencia and Cataluna In NE

Spain the coastal areas displayed the greatest capacity

to accommodate IAS (Sanz-Elorza et al 2002) given

the predominance of urban areas and rivers there In

Portugal the western coastal rivers and northern

regions possess the highest representations of IAS

(Aguiar amp Ferreira 2012)

However the contribution of alien taxa to the

total flora in Iberian streams with a Mediterranean

climate is low (Tabacchi et al 1996 Aguiar et al

2006a) compared with the Mediterranean streams in

South Africa western North America and southshy

western France where the proportion of alien taxa

ranges from 20 to 30 (Planty-Tabacchi et al

1996 Hood amp Naiman 2000) Aguiar et al (2007)

reported 4 ndash 10 of alien species in central and

southern Portuguese basins and Tabacchi et al

(1996) documented 6ndash8 of alien species for southshy

eastern Spain The regions with a marked Meditershy

ranean climate displayed the lowest figures ndash 4 for

the Guadiana river basin (Ferreira et al 2002) 4

for the Sado river basin and 6 for the Algarve river

basin Nonetheless a wide-ranging 2004 ndash2005

floristic survey that included up to 400 stream

reaches across continental Portugal revealed that

more than 80 of the surveyed sites presented alien

species (Aguiar amp Ferreira 2012) This survey was

conducted for the implementation of the Water

Framework Directive (WFD Directive 200060CE

European Council 2000) in Portugal and sampling

design requirements meant that it did not include

rivers with a high degree of invasion by aquatic

weeds However it was observed that alien plants

were also widespread in near-natural river stretches

(87 of the sites) but were mostly casual species

Plant invasions in Iberian rivers a review 1109

(82 were found in 5 of the surveyed area) and

numbered only a few individuals usually with

negligible surface cover

Duarte et al (2004) listed 139 alien taxa

associated with aquatic and riparian habitats in

continental Portugal with only 8 being truly aquatic

species (hydrophytes) and 2 amphibious (heloshy

phytes) while the others were hygrophytes such as

Salix pound sepulchralis Simonk or Cortaderia selloana (Schult amp Schult F) Asch amp Graebn Some are

weeds that grow in rice fields and irrigation crops

and in certain situations can colonize slow-flowing

rivers and marshes (Vasconcelos et al 1999)

Examples include Ammannia pound coccinea Rottboell Lindernia dubia (L) Pennell and Echinochloa oryzishycola (Vasinger) Vasinger Herbaceous ruderals such

as Conyza spp Erigeron karvinskianus DC among

many others are also frequent in riparian and

wetland habitats In the alien woody species group

Acacia dealbata Link Acacia longifolia (Andrews) Willd Acacia melanoxylon R Br Ailanthus altissima (Miller) Swingle and Robinia pseudoacacia L are frequently found in riverbank surveys and alter

important riparian communities of Spain and

Portugal such as those of mesoeutrophic rivers

described by Costa et al (2011)

The Aguiar et alrsquos (1996) survey of the

perception of the aquatic weeds in Portuguese

freshwaters by public and private entities with

responsibility for water resource management

revealed that on average 44 had serious problems

with aquatic weeds in their area of jurisdiction

Around 35 of these were related with IAS namely

water hyacinth parrot feather giant reed knotgrass

and azola The proportion of problems with IAS in

western central regions was higher than in other

regions with IAS being responsible for c 47 of the

invasions in rivers reservoirs ponds and drainage

channels Seven of the 14 freshwater nature reserves

in Portugal have been severely invaded by Eichhornia crassipes Myriophyllum aquaticum Azolla filiculoides and Arundo donax (Aguiar et al 2006b) For

instance the Paul de Boquilobo UNESCO-MAB

Biosphere Reserve a freshwater wetland surrounded

by marshes has been threatened by extensive floating

mats of water hyacinth since the 1970s (Moreira

et al 1999c) Another example is the Pateira de

Fermentelos which is one of the largest natural

freshwater lagoons in the Iberian Peninsula and is

integrated into the Ria de Aveiro Special Protection

Area (PTZPE0004) where the water hyacinth has

spread over more than 50 of the lagoon surface

(Laranjeira amp Nadais 2008)

Portuguese legislation (Executive Law no 56599

of 21 December 1999 published in Series I-A of

Diario da Republica no 295 issued by the Ministry of

the Environment MA) listed around 400 alien plant

species (Gimnospermae Angiospermae and Pteridoshyphyta) but only around 10 were recognized as

species that are dangerously invasive of aquatic

habitats and wetlands (Almeida amp Freitas 2002

Duarte et al 2004) Similarly Spanish Royal Decree

no 16282011 of 14 November 2011 (published in

Boletın Oficial del Estado of 12 December 2011

issued by the Ministry of the Environment and the

Rural and Marine Environments MMAMRM)

listed around 190 IAS in continental Spain and

islands of which around 15 are invasive weeds of

freshwater ecosystems (truly aquatic species and

emergent species) out of the 53 included in Annex I

(List of Alien Invasive Species ndash Flora ldquoCatalogo

Espanol de Especies Exo ticas Invasorasrdquo) that are

considered as plant invasive species in all habitats In

its TOP 20 list of alien organisms in Spain (GEIB

2006) the nationwide organization Specialist Group

on Biological Invasions (GEIB) listed 2 invasive

aquatic weeds ndash Azolla filiculoides and Eichhornia crassipes ndash and 4 IAS of various habitats including riverbanks ndash Acacia dealbata Ailanthus altissima

Cortaderia selloana and Robinia pseudoacacia Table I presents a list of invasive weeds in the

rivers and on riverbanks in Portugal and Spain based

on available scientific articles reports (eg Curco

1996 Conesa amp Sanz-Elorza 1998 Sanz-Elorza et al

2004 Garcıa-Murillo et al 2007) publications by

public and non-governmental organizations (eg

EPPO and GEIB) legislation (MA 1999

MMAMRM 2011) and expert knowledge All the

listed species present serious impacts on the

environment human activities andor ecosystem

services in Iberian rivers (Portugal andor Spain)

together with difficulties for the control and manageshy

ment of invasions usually posing large demands in

economic terms Our aim was to include all major

invasive weeds in the rivers of the Iberian Peninsula

especially the aquatic plant invasive species (hydroshy

phytes) However this short species list is not

exhaustive and the perception per region can differ

from the perception of the species that are generally

problematic at the Iberian level It thus does not

dispense reading the consulted documents and the

species classification in the annexes of the Spanish

and Portuguese laws

Aquatic plant invasions in this region involve a

number of strong competitors which are very

detrimental but usually restricted to areas with

particular abiotic and historical features that favour

an ecosystemrsquos invasibility (Salvo 1990 Aguiar et al

2001 GEIB 2006 Brunel 2009) A good example is

the water hyacinth in the Guadiana river (Spanish

part) Annual control costs were 67 Me in 2009 (Andreu amp Villa 2009) and 21 Me in 2012 (EPPO

2013)While very detrimental some of these species

such as Elodea canadensis Michx and Eryngium

pandanifolium in Portugal or Salvinia natans (L) Allioni and Pistia stratiotes L in Spain are in fact geographically limited or located in temporal

windows as in the case of Azolla filiculoides in Portugal However some species that are invasive of

riverbanks and wetlands ndash namely the Acacia species Arundo donax and Paspalum paspalodes ndash are wideshyspread in Iberia (Sanz-Elorza et al 2004 Aguiar

et al 2005 2006b)

Species invasiveness and ecosystem invasibility in Iberian rivers

The management of invasive species and the

prevention of invasions in new ecosystems have

been supported by three types of approach (i) a

species-centred approach which deals with the

speciesrsquo potential to invade ndash invasiveness (ii) an

ecosystem-centred approach which is devoted to

drivers of the invasibility of ecosystems habitats and

regions and (iii) invasion pathways and vector

management

Rejmanek and Richardson (1996) concluded that

very often the same attributes allow a species to both

spread in its native range and invade new territories

With regard to species invasiveness in rivers some

studies have addressed particular species attributes

or traits of successful invaders including their

physiological genetic and demographic features

dispersal strategies (eg Kean amp Barlow 2000) and

life-history traits For instance Caffrey et al (2010)

found that key traits of the aquatic weed Lagorosiphon major (Ridley) Moss include its very rapid growth

rate its ability to create dense light-excluding

canopy layers on the water surface and its ability to

vegetatively disperse over a wide area In a global

approach Bernez et al (2006) studied the attributes

of native and alien aquatic invasive species and

analysed their occurrence and abundance in more

than 400 locations in central and southern Portugal

Although it was not possible to draw the profile of the

ldquoideal invaderrdquo the most important traits were

identified and were mostly related with reproduction

dispersal growth and strategies for resource acquishy

sition The studied IAS were preferentially dispersed

via plant fragments (stems and rhizomes dispersed by

floods) possessed high propagule viability and

presented high phenotypic plasticity or competitively

superior genotypes (eg Eichhornia crassipes Paspashylum paspalodes and Arundo donax) Lambdon et al

(2008) offer a similar vision for plant invasions in

Mediterranean islands They observed that alien

invasions occurred across a wide range of habitats

and that IAS did not share predictable traits

Schnitzler et al (2007) reviewed the diversity of

attributes in IAS in riparian forests and mentioned

that IAS often present larger growth forms 3ndash4m

I Someofthemajorinvasiveweedsin

fluvialsystem

softheIberianPen

insula

(alphabetic

order)

Year of thefi rst report

Species

Family

Origin

Portugal

Habitat

Introduction

Invasive plant

Acacia

dealbata

Fabaceae

SEAustraliaTasm

Before

PortugalSpain

TOP20List

Acacia

melanoxylon

Fabaceae

SEAustraliaTasm

PortugalSpain

RBr in A

Acacia

longifolia

Fabaceae

Australia

Portugal Spain

ndrews) W

Acer negundo L

Aceraceae

merica

Mid-nineteenth

century

PortugalSpain

(Guatemala up to C

anada)

Ailanthus altissima

aroubaceae

TaiwanC

Earlynineteenth

century

PortugalSpain

TOP20List

(Miller)

Swingle

KoreaPen

insula

Amaranthus hybridus L

Amaranthaceae

merica

Eighteen

th cen

Portugal Spain

Arundo donax L

Poaceae

C()andS A

Before

PortugalSpain

Azolla

filiculoides

Azollaceae

NandS A

merica

PortugalSpain

TOP20List

Buddleja

davidii

Franch

Buddlejaceae

CChinaTibet

Cortaderia

selloana

Poaceae

SAmerica(B

razilArgen

Mid-nineteenth

century

orncult

PortugalSpain

TOP20List

amp JH

ultes)

ChileandUruguay)

Aschersamp G

Datura

stramonium

Solanaceae

America

Portugal Spain

Egeria

Planch

Hydroch

aritaceae

merica

Restricted area

Eichhornia

crassipes

Pontederiaceae

TropicalAmerica

Before

PortugalSpain

A2List(E

PPO 2008)

(Mart)Solm

s-Laub

TOP20List

Elaeagnus angustifolia

Elaeagnaceae

Eurasia

id-nineteenth cen

Elodea

canadensis M

Hydroch

aritaceae

NAmerica

Before 2000

Portugal Spain

Restricted area

Eryngium

pandanifolium

Apiaceae

merica

Portugal

Restricted area

Chamamp

ondegoriver

basin)

Fallopia

baldschuanica

Polygonaceae

Eurasia

(Regel) J H

Heteranthera

limosa

Pontederiaceae

America

Restrictedarea

Heteranthera

reniformis

Pontederiaceae

America

Restricted area

amp Pavo

Heteranthera

rotundifolia

Pontederiaceae

NAmerica

Restricted area

(Kunth) Grisebach

Ludwigia

grandiflora

Onagraceae

merica

A2 List

(Michx)Greuteramp Burdet

Ludwigia

peploides

Species

Family

Origin

Portugal

Habitat

Introduction

Invasive plant

Myriophyllum

aquaticum

Haloragaceae

merica

Portugal

(Vell)Verdc

Paspalum

paspalodes

Poaceae

Tropical America

Portugal Spain

Phytolacca

americana

Phytolaccaceae

NAmerica

Eighteen

th cen

Portugal Spain

Pistia

stratiotes

Araceae

Tropical regions

Restricted area

Reynoutria

japonica

Polygonacae

Eurasia

orn crop

Portugal Spain

Robinia

pseudoacacia

Fabaceae

NAmerica

Eighteen

thcentury

PortugalSpain

TOP20List

Salvinia

molesta

Salviniaceae

merica

Tradescantiafluminensis

Commelinaceae

merica

Nineteenth cen

Portugal

Velloso

Xanthium

spinosum

Asteraceae

merica

Notes Typeofintroductionornasornamen

talcropcropproductioncu

ltivated(formed

icinalpurposeswindbreaks)aqutaquarium

accacciden

talHabitatrivbriverbanksaq

aquatic (X)IA

Portugalwithldquoinvasive statusrdquo ndash

inisteriodoAmbiente 1999) (V)IA

withldquoinvasive statusrdquoAnnex

I (S) specieswithinvasive potentialAnnex II(M

inisterio

edio A

mbiente yM

edio R

ural yM

arino 2011)

for Fallopia spp and more than 9m for Arundo donax These invasive weeds can occur as clonal perennials

forming dense impenetrable stands Competitive

attributes include mycorrhizal transfer of carbon

from native plants and hybridization

Pysek and Richardson (2007) confirmed that

ldquotraits matterrdquo but caution is necessary because

IAS had been introduced at different times and are

found at different stages of naturalizationinvasion

Some traits can be valuable at a given stage and in a

particular habitat but may be neutral or even

unfavourable in another phase of the invasion

process or in different environmental settings

A recent review of the naturalization of alien species

and their ecological determinants concluded that

many other factors besides traits play a key role in

determining invasive success with an importance

that differs along the introduction ndash naturalization ndash

invasion continuum (Richardson amp Pysek 2012)

It is widely recognized in numerous invasion

ecology studies that ldquodisturbances matterrdquo but

disturbance effects in invasion processes are complex

(Lepart amp Debussche 1991) This is particularly

evident in fluvial ecosystems that simultaneously

suffer the effect of manifold pressures these

ecosystems are often subjected to continuous stress

(eg vegetation removal and diffuse pollution)

permanent hydrological alterations (eg regulation

and water diversion) and long-term direct disturbshy

ances (eg channelization widening linearization

and dredging of fluvial channels) Both direct and

indirect pressures alter hydromorphological and

habitat features and frequently lead to an overturning

of native species assemblages

A good example of the effects of hydromorphoshy

logical disturbance in Mediterranean Europe is given

in the works of Aguiar et al (2001) and Barbosa et al

(2006) who analysed the effects of the regularization

works in Mondego ndash the largest entirely Portuguese

river The hydrological regime before the intervenshy

tions was very irregular with discharges that could be

1m3s for several days in a year and flood

discharges of up to 3000m3s In the 1980s the

river was subjected to flow regulation channelizashy

tion inclusion of groynes and riprap and two-stage

bank reinforcement The aquatic and riparian

vegetation in the channelized stream and the non-

impaired river reaches was compared Inner banks

along the channelized reaches were dominated by

Eryngium pandanifolium a swamp-dwelling plant

from the South-American subtropical zone and by

Paspalum paspalodes an American IAS The alien

hydrophytes Azolla filiculoides and Myriophyllum aquaticum were also largely disseminated in the

river channel In the understory below the riparian

canopies of the first terrace dense stands of the

South-American species Tradescantia fluminensisTable I ndash continued

Velloso were established whereas the second terrace

was dominated by the woody Australian species

Acacia dealbata Link Two main reasons were

suggested for the channelized riverrsquos higher suscepshy

tibility to plant invasion in relation to non-regulated

reaches First new environmental conditions that

were favourable to invasion were attained after

channelization including a reduction in the intensity

and frequency of scouring floods and an increase in

the fine sediment and nutrient load Second the

disturbance caused by the regulation works led to an

increase in propagule pressure and a higher dispersal

rate and residence time of the alien species that were

already present before the interventions Vicente

et al (2010) used alien plant species distribution in a

northern region of Portugal to explore the reasons

why some habitats are more invasible than others by

fitting models that relate alien species richness to

various environmental predictors Overall alien

invasions were influenced first by climate and then

determined by various anthropogenic pressures such

as those caused by land-use management Cooper

et al (2013) reviewed the effects of land use in

Mediterranean streams and observed as a general

pattern that the number and relative proportion of

alien plant species increase with intensive land use in

the catchment and particularly correlate with urban

development In the same region Santos et al

(2011) used a stochastic dynamic methodology to

predict the response of plant invasive richness to

anthropogenic disturbance and found that invasive

richness trends were clearly associated with ongoing

socio-economic changes related with growing urbanshy

ization Nutrient enrichment and organic pollution

of rivers and streams via agriculture and effluents

from livestock production are frequently responsible

even considered key elements in upholding ecosystem

services (Thuiller et al 2007 Walther et al 2009)

Aguiar and Ferreira (2012) offered a global

visualization of the most invasible Portuguese river

types The central and southern calcareous rivers in

the coastal areas of Portugal were the most invaded

whereas the small-sized and highly seasonal rivers in

mountainous areas were less invasible (Figure 2)

Rivers in the coastal areas in Portugal were among

the most disturbed over time and plant invasions

were related with impacts of both urbanization and

the intensive agricultural systems settled in the high-

quality soils of their floodplains Aguiar et al (2006a)

used a nested sampling design to study patterns of

invasion in a Mediterranean river in the semi-arid

region of continental Iberia and revealed that alien

species richness and cover displayed contrasting

patterns in relation to a humidity gradient Whereas

the number of alien (and native) species increased

from the aquatic system to upland areas an increase

in invasive alien plant cover was observed towards the

aquatic system with a notably higher cover in

inundated areas (Figure 3 Aguiar et al 2006a) Alien

plant cover was consistently related with the increase

for dense populations of many IAS not only in rivers 2

and streams but also in wetlands marshes ponds

lagoons and reservoirs Morphological disturbances 1 of streams and banks such as channelization

linearization and bank reinforcement severely alter 20 habitats and thus mediate an ecosystemrsquos invasibility

(Ferreira amp Moreira 1995 Hood amp Naiman 2000

Aguiar et al 2001)

In addition climate change is known to have

multiple and complex effects on the number and

severity of plant invasions in freshwater ecosystems

by altering species introduction pathways and

ecosystem properties (Lee et al 2008 Rahel amp

Olden 2008 Heino et al 2009) Higher water

temperatures increased salinity altered rainfall and

streamflow patterns are some of the expected

consequences of climate changes in freshwaters

However these novel environmental conditions can

favour or prejudice invasive species in a dynamic

context of shifting species ranges and changing

communities and in certain situations aliens can be

0 M L N N-S S

River types

Figure 2 Patterns of richness and cover for alien species in five

regions of Portugal (river types) N northern rivers L coastal

rivers M mountainous rivers N-S transitional rivers northndash

south S southern rivers Means ^ SD are represented (after

Aguiar amp Ferreira 2012)

Alien species Native species

FF IF RF

Landform

Figure 3 Patterns of richness and cover for alien and native species

at the three valley locations FF frequently flooded (river channel)

IF infrequently flooded RF rarely flooded Means ^ 095 confidence intervals are represented Columns with different

letters indicate significant differences at p 001 using Tukey post hoc comparison tests between valley locations within each

group of species (native or alien) (adapted from Aguiar et al

2006a)

in fine particulate matter in the substrate and

nutrient inputs (Figure 4)

Assessment of ecological quality of rivers and IAS

Many assessment methods currently use the

expression of community attributes (eg dominance

cover frequency and richness) of alien plant species

to evaluate and monitor the ecological status of

surface water bodies riparian areas and wetlands

Duarte et al (2007) used field data from Portuguese

rivers to assess the relationship with disturbance of

more than 300 attributes of native and alien

components of Portuguese river flora and highshy

lighted the potential of alien species and community

traits for the evaluation of the ecological quality of

rivers The WFD requires all Member States to

develop assessment methods to evaluate and monitor

the ecological status of surface water bodies in order

to achieve good ecological status by the year 2015

Assessment methods classify an ecosystem according

to its deviation from a desired near-natural ecological

condition and various metrics that were developed

and evaluated for the implementation of the WFD

include measures of alien plant invasions or use alien

species in their scoring systems Macrophyte-based

indices have very often included the proportion or

abundance of alien plant species as a metric (ie a

trait that is responsive to disturbance) for assessment

methods For instance Aguiar et al (2009) develshy

oped the riparian vegetation index (RVI) for

evaluating and monitoring the ecological river quality

of Portuguese rivers RVI is a structural-based

multimetric index and uses functional patterns and

compositional attributes of the overall fluvial

vegetation response to multiple types of disturbance

Around 300 candidate plant metrics from various

metric categories (eg life cycle and propagation

nutritional resources and vegetation structure) were

pre-screened and 32 were subjected to final data

treatment Statistical analysis validated 13 plant

(scoring) metrics for the final index inasmuch as

they do reflect the multiple dimensions of the

biological ecosystem and of disturbance Scoring

metrics were estimated or evaluated using surface

cover proportion or number of species of functional

groups (eg aliens nythrophyllous and ruderals)

species attributes (eg life form and reproduction

strategies) and indicator taxa Metrics were transshy

formed to dimensionless numbers for aggregation

and combined in an additive index

Dodkins et al (2012) developed a species scoring

system for the evaluation of the water quality of

Portuguese rivers Aquatic species predominantly

responded to nutrient enrichment (r 2 frac14 0602 p 00001) and alien species accordingly returned lower scores representing a low water quality status

For example Elodea canadensis and Arundo donax scored ldquo1rdquo and Myriophyllum aquaticum scored ldquo3rdquo (range 1 ndash 10) whereas most native bryophytes and

species from oligotrophic locations such as Philonotis fontana (Hedw) Brid and Sphagnum spp displayed the highest scores (9ndash 10) Similarly aliens scored

lower in the metric used as a national assessment

method for the implementation of the WFD in the

UK and Ireland (Willby et al 2009) France

Belgium Italy and Croatia (Haury et al 2006) as

well as in other studies in Mediterranean regions

(eg Manolaki amp Papastergiadou 2012) Integrity

Biotic Indices developed to assess the ecological

quality of Iberian rivers have also used the proportion

andor abundance of alien species Examples include

the riparian quality index (Gonzalez del Tanago amp

Garcıa de Jalon 2006 Gonzalez del Tanago et al

2006) the Iberian multimetric plant index

(IMPI Ferreira et al 2005) the riparian forest

quality index (Munne et al 2003) and the RVI

ExoticFF IF RF cover ()

(m) 18

100 16

7012 60

10ndash2 ndash2 2 6 10 14 18 22 26 30 0

Distance from water (m)

Figure 4 Kriging map of alien species cover and fine organic particulate matter in the Guadiana river Proportions of sediment load are

labelled and overlaid on the shaded contour map of percentage of alien cover The midpoint of the 90 2m pound 2m sampling plots is indicated

(thorn) Landforms are noted as follows FF frequently flooded (river channel) IF infrequently flooded (inner banks) RF rarely flooded

(upland ndash 25ndash30m from river channel)

(Aguiar et al 2009) In particular for semi-arid rivers

in the southern region of Portugal the IMPI used the

abundance of the Arundo donax ndash a very detrimental

IAS ndash and the overall alien species richness whereas

the RVI ndash a structural and functional metric for

Portuguese rivers ndash used both the proportion of alien

species in the overall community and their

abundance

Tools for detecting and monitoring IAS in Iberian rivers

Invasions by alien species have also been recognized

as a demanding issue in the conservation and

management of surface waters The ability to detect

invasions can be determinant to the success of

control measures as new introductions of alien

species in rivers and riparian habitats are regularly

noticed (Almeida amp Freitas 2006 Venturella et al

2012) Among the most promising tools in this field

are Geographic Information Systems (GIS) and

remote-sensing approaches to the early detection of

newly invaded areas which are used to prioritize

which areas to manage and to monitor ecosystems

that have already been invaded or subjected to

control measures Dıaz-Delgado et al (2008) used

satellite imagery and hyperspectral data to map the

spread of azola in the Donana protected area

(Southern Spain) and Ruız Tellez et al (2008)

used GIS and physicochemical data from Portuguese

and Spanish locations to model the potential

distribution of azola and predict the risk of

infestation by water hyacinth in the Guadiana river

basin Fernandes et al (2011) used landscape

metrics to characterize the structure of riparian

woodlands in tributaries of the Tagus river central

Portugal and as an additional outcome observed

that areas invaded by the giant reed could be

remotely assessed thanks to their peculiar spatial

structure Giant reed stands were characterized by

having elongated shapes with low edge complexity in

opposition to the riparian woodlands which are

frequently composed of complex and meandering

patches and highly interspersed A study on the

spectral discrimination of giant reed from surroundshy

ing vegetation was performed in the Tagus river

basin It concluded that giant reed patches were

spectrally separable from the adjacent vegetation in

both the vegetative and the senescent periods with

the exception of the native morphospecies Phragmites australis (Cav) Trin ex Steud during the vegetative

period (Fernandes et al 2013) An interesting result

was that giant reed stands that regenerated after

mechanical cutting displayed the highest spectral

separability values when compared with the giant

reed without control measures Ongoing studies by

the same authors are attempting to refine the

detection of giant reed invasions by adding spectral

and textural information to the spatial structural

attributes using an object-based classification

approach to develop an effective automated method

for mapping giant reed stands

Other techniques for identifying potential invashy

sive species and preventing new introductions were

experimented with in Mediterranean areas of

Spain These included the Weed Risk Assessment

(Pheloung et al 1999) ndash an assessment scheme that

was initially developed for Australia but could serve

as an early-warning mechanism for casual species

with the potential to become invasive in the

Mediterranean Basin (Gasso et al 2010) and the

Risk Assessment for Central Europe (Weber amp Gut

2004) Andreu and Vila (2009) used both systems

and observed that in general the aquatic plants

displayed the highest scores corresponding to the

highest invasion potential In addition top-down

approaches combining landscape levels and small-

scale monitoring have been explored (see Aguiar

et al 2011 Croce et al 2012) as an ecological

support for management and planning decisions

concerning plant invasions Given that they offer the

advantage of reducing the costs of management and

restoration planning they may also help prioritize

areas for restoration

Concluding remarks

There is increasing awareness of the threats posed by

and the major economic social and ecological

impacts of invasive weeds in the rivers and riparian

ecosystems of the Iberian Peninsula as well as of the

need to both manage existing plant invasions and

prevent the introduction of problematic invasive

weeds This study gives a general overview of the IAS

and major invasive aquatic weeds in the rivers and

riparian habitats of the Iberian Peninsula supported

by an extensive bibliographic review Numerous

documents including reports legislative documents

scientific articles and websites and grey literature

(ie papers published in regional and local literature

and not indexed in international databases) upgrade

our knowledge of the actual situation with regard to

plant invasions patterns of IAS their causes and

some management efforts designed to control

aquatic invasive weeds However we detected a

number of divergences between the diverse sources

and the views provided by the legislative documents

with the absence of some species from blacklists

provided by regional administrations and local

authorities and vice versa We nonetheless encounshy

tered a general consensus that the reduction of

human-induced disturbance in rivers and surroundshy

ing ecosystems and the control of pathways of

introduction and vector management are extremely

important to both the control of aquatic plant

invasions and the prevention of new invasions and reshy

introductions Past experiences such as the invasive

spread of water hyacinth in the 1980s (Tagus river

Portugal) and knowledge about species ecology and

the causes of ecosystem invasibility can all help

managers in similar situations and in the early

detection of invasive species Remote-sensing

approaches the use of GIS and optimized field

assessments are important tools for the actual

management and monitoring of plant invasions

However we are aware that there is a limited ability

to make predictive statements about the relationship

between the occurrence of a multitude of invasive

species and plant invasion phenomena and we are

still seeing blooms of azola in the transnational

Guadiana river as well as the failure of the long-term

control of water hyacinth in the entirely Spanish

section of the river While invasive species traits are

very diverse knowledge about them can still help

predict the potential of IAS that are not naturalized

or can be introduced from eg the aquarium trade or

gardening through a comparison with other species

that actually cause plant invasions Here organizshy

ations such as EPPO groups of specialists and

researchers can make an important contribution to

the prevention of plant invasions by targeting species

and processes that underlie a successful invasion

Acknowledgements

The authors thank the Portuguese Environmental

Agency APA (ex-Water Institute INAG IP)

Portugal for supplying data for Figure 2 and

Anto nio Albuquerque for help with the fieldwork in

the Guadiana river who provided data for Figure 4

The authors are also indebted to three anonymous

reviewers for comments on the manuscript that

much improved the original version This work is a

tribute to Professor Ilidio Moreira who passed away

in 2011

Funding

The first author was supported by a post-doctoral

grant (SFRH BPD293332006) from the Founshy

dation for Science and Technology (FCT) Portugal

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Abstract