Invasion of the Portuguese dune ecosystems by the exotic species Acacia longifolia (Andrews) Willd.: Effects at the community level

Abstract

Pristine ecosystems are becoming rare along the Portuguese coastline because the sand dunes have been invaded by several exotic plant species, particularly Acacia species from Australia. At the beginning of the 20 th century, a few Acacia species, e.g. Acacia longifolia (Andrews)Willd., were deliberately introduced into Portugal for dune stabilization. The invasion process is strongly correlated with the occurrence of fires, so there are long periods of stability interspersed with rapid increases in abundance of the exotic Acacia species. In this study, three sites, S.Jacinto, Palheirão and Quiaios, were monitored along the Portuguese coast to determine the impact of the invasions by A.longifolia. At each site, 12 permanent plots were established, six with and six without A. longifolia. The plots were monitored from winter 1998 to autumn 1999, and species numbers and plant cover were recorded. Shannon diversity indices and dominance-diversity curves confirmed that A. longifolia is changing the community composition and structure of the dune ecosystems promoting monospecific communities, decreasing species richness and increasing plant total cover.

Full text

Invasion of the Portuguese dune ecosystems by the
exotic species Acacia longifolia (Andrews) Willd.:
effects at the community level.
Hélia Marchante1*; Elizabete Marchante2 and Helena Freitas2
1. Escola Superior Agrária de Coimbra, Bencanta, 3040-316 Coimbra, Portugal 2.
Departamento de Botânica, Faculdade de Ciências e Tecnologia, Universidade
de Coimbra, 3000 Coimbra, Portugal; e-mail: * hmarchante@mail.esac.pt
Abstract
Pristine ecosystems are becoming rare along the Portuguese coastline because the sand dunes have
been invaded by several exotic plant species, particularly Acacia species from Australia. At the
beginning of the 20th century, a few Acacia species, e.g. Acacia longifolia (Andrews)Willd., were
deliberately introduced into Portugal for dune stabilization. The invasion process is strongly correlated
with the occurrence of fires, so there are long periods of stability interspersed with rapid increases in
abundance of the exotic Acacia species.
In this study, three sites, S.Jacinto, Palheirão and Quiaios, were monitored along the Portuguese
coast to determine the impact of the invasions by A.longifolia. At each site, 12 permanent plots were
established, six with and six without A. longifolia. The plots were monitored from winter 1998 to
autumn 1999, and species numbers and plant cover were recorded. Shannon diversity indices and
dominance-diversity curves confirmed that A. longifolia is changing the community composition and
structure of the dune ecosystems promoting monospecific communities, decreasing species richness
and increasing plant total cover.
Introduction
Ecosystem structure and functioning is seriously affected by invasive species, which
pose one of the most substantial threats to the Earth’s biodiversity (Cronk and Fuller
1995, Chapin et al. 2000). Biological invasions lead to “ecological homo-
genisation” of the world and contribute considerably to global change (D’Antonio
and Vitousek 1992, Lodge 1993, Arroyo et al. 2000, Mooney and Hobbs 2000).1
Coastal ecosystems are important barriers against the advance of the ocean. Sand
dunes in particular are vulnerable to disturbance through natural and human
pressure (Hanson and Lindh 1993, Carter 1995). The stability of dune ecosystems
relies on their natural high diversity of native plant species, which bind the sand and
minimize the effects of erosion (van der Putten and Peters 1995). Along the
Portuguese coast, pristine dune systems are becoming increasingly rare, with native
plant species being replaced by several invasive exotic species, mainly Acacia
Miller species from Australia (Alves et al. 1998).
Plant Invasions: Ecological Threats and Management Solutions, pp. 75-85
Edited by L.E.Child, J.H. Brock, G.Brundu, K. Prach, P.Pysek, P.M. Wade, M. Williamson
© Backhuys Publishers, Leiden, The Netherlands

76 Hélia Marchante et al.
The genus Acacia includes about 1200 species, the majority native to Australia and
Southern Africa (Whibley 1980). It belongs to the subfamily Mimosoidea, which
comprises several of the worst invasive plant species in the world (Cronk and Fuller
1995). The genus Acacia is absent from the Portuguese native flora (Franco 1971,
Castroviejo et al. 1999).
The invasive success of Acacia species is attributable mainly to high growth
rates of the plants, prolific seed production, with high longevity in the soil and high
dispersal efficiency (Cronk and Fuller 1995), stimulation of seed germination by
fire (Ross 1975, Taylor et al. 1985, Crawley 1997), and the absence of natural
enemies in areas where the plants have been introduced (Callaway and Aschehoug
2000). The ability to fix nitrogen (Leguminosae) has enabled the acacias to invade
nutrient-poor environments.
Since the beginning of the 20th century, 13 Acacia species have been introduced
to Portugal (Castroviejo et al. 1999). Of these A.melanoxylon R.Br. in W.T.Aiton,
A.longifolia (Andrews) Willd. and A.saligna (Labill) H.L.Wendl. were utilized for
dune stabilization in coastal ecosystems (Rei 1924, Neto 1993). At present,
A.dealbata Link, A.melanoxylon and A.longifolia are the most prolific invaders in
Portugal, especially in conservation areas where the climate is comparable with the
native range of the plants. Invasions by Acacia species are strongly correlated with
fire events when the accumulated Acacia seeds in the soil are stimulated to
germinate and the rapidly-growing young acacia plants smother seedlings of native
species.
The aim of this work was to assess the ecological impact of the invasions of
Acacia species in some Portuguese dune ecosystems.
Methods
Study sites
Three study sites were selected and monitored along the Portuguese coast, all
located in the central-northern regions-S.Jacinto Natural Reserve (SJ), Beach of
Palheirão (BP) and Beach of Quiaios (BQ) (Figure 1). At present, SJ is classified as
a Natural Reserve, and BP and BQ are Natura 2000 sites. All three sites have high
conservation potential (Neto 1993, Martins 1999). Despite its classification, the BQ
site is extremely disturbed by tourist pressure.
The experimental study was performed landward of the first dune, where the
characteristic dune vegetation consists of a low cover, open structure and just a few
nano/microphanerophytes with a role similar to A.longifolia, (Corema album
(L.)D.Don, common in all studied places, Cistus salvifolius L., present in just 3
plots BQ and 2 plots BP and Pinus pinaster Aiton, present in only 2 plots in both
BP and BQ), whose frequency increases with the distance from the sea. At the time
of the study, the three sites had patches of vegetation that was uninvaded and
patches that had been invaded to a greater or lesser extent by A.longifolia . All three
sites had patches that supported a dense over-story of A.longifolia (Marchante
2001), but patches with 100% A.longifolia cover were not included in the surveys.

Invasion of the Portuguese dune ecosystem by the exotic species Acacia longifolia 77
Fig. 1. Position of the study sites on the Portuguese central-northern coast: S.Jacinto Natural
Reserve (SJ), Beach of Palheirão (BP) and Beach of Quiaios (BQ).
Nevertheless, monospecific A.longifolia stands are very frequent, becoming more
and more common as far as the distances to the sea increases. Cartographic
information confirms the increase of monospecific Acacia patches over time.
Although geographically isolated, all three study sites had similar fire histories
(the most recent fires happened seven years before the surveys in SJ and nine years
before the surveys in BQ and BP) and species composition and vegetation struc-
tures were relatively similar. The introduction of Acacia species in the early parts of
the 20th century used different methodologies but always aimed at dune stabiliza-

78 Hélia Marchante et al.
tion. In fact, A.longifolia promotes dune fixation but spreads after fire events and it
does not respond to sand mobility as, for example, the marram grass (Ammophila
arenaria (L.) Link subsp. arundinacea H.Lindb.fil.). In SJ, only A.longifolia was
introduced while in BP and BQ others species (namely A.melanoxylon) were
planted along with A.longifolia (Rei 1924). However, nowadays A.longifolia is the
dominant species and the only one present in the experimental plots. Despite its
introduction about 100 years ago, it was only after the fire in the beginning of 1990
that A.longifolia started to show invasive behaviour in the studied places, with
remarkable increases in the biomass in a short period of time.
Experimental design and data analysis
In December 1998, twelve permanent plots of 25m2 were demarcated in an area of
approximately 3000m2, six with A.longifolia and six without A.longifolia . Quadrat
selection was made in a semi-randomised stratified pattern (Goldsmith 1996), to
include areas with and without A.longifolia. The plots were monitored seasonally
from winter 1998 to autumn 1999. For each quadrat sampled, species number and
percentage canopy cover of each species were recorded. For each community,
Shannon diversity indices and Pielou evenness indices (Magurran 1988) were
calculated and dominance-diversity curves (Whittaker 1972, Kent and Coker 1992)
were computed. Parameters were used to compare invaded and uninvaded quadrats.
The data were analysed using a 2-way ANOVA to test if the species richness and
cover remained constant after invasion, with season considered as the second factor
(Zar 1996). Significant effects (P<0.05) were then tested using the Newman-Keuls
multiple comparison test. All six replicates were gathered in a composite plot, and a
t test proposed by Hutcheson in 1970 (see Magurran 1988, Zar 1996) was used to
compare the Shannon diversity indices in invaded and uninvaded quadrats.
A.longifolia was always treated as just one more species.
Results
Species richness
In general, species richness was higher in uninvaded plots (Figure 2), and in two
sites was significantly dependent on the presence of A.longifolia, SJ (F(1,40)= 12.998;
P = 0.0009) and BP (F(1,40)= 15.36; P= 0.0003). However, significant differences
were only detected in spring (Figure 2).
Cover
A.longifolia presence accounted for highly significant increases in total plant cover
at all three sites during all seasons (SJ F(1,40)=147.86; P<0.0001; BP F(1,40)=200.51;
P<0.0001 and BQ F(1,40)=206.83; P<0.0001) (Figure 3).

Invasion of the Portuguese dune ecosystem by the exotic species Acacia longifolia 79
0
3
6
9
12
15
18
1 2 3 4
SJ 1 2 3 4
BP 1 2 3 4
BQ
species richness
aaa
aaa
a
aa
a
a
aa
a
a
b
a
aaaa
aa
b
invaded
uninvaded
Fi
g. 2. Species richness (mean+SE; n=6) of invaded and uninvaded plots, at all sites SJ=
S.Jacinto, BP = Palheirão, BQ = Quiaios, along four seasons 1 = winter, 2 = spring, 3 =
summer, 4 = autumn. Columns with different letters are significantly different from each
other at P<0,05 (Newmans-keuls Test).
0
20
40
60
80
100
120
140
1 2 3 4
SJ 1 2 3 4
BP 1 2 3 4
BQ
species cover (%)
a
a
b
b
a
a
b
b
b
a
a
a
b
a
b
b
a
aa
b
a
b
b
b
invaded
uninvaded
Fig. 3. Species total cover (mean+SE; n=6) of invaded and uninvaded plots, at all sites SJ =
S.Jacinto, BP = Palheirão, BQ = Quiaios, along four seasons 1 = winter, 2 = spring, 3 =
summer, 4 = autumn. Columns with different letters are significantly different from each
other at P<0,05 (Newmans-keuls Test).
Shannon Diversity Index and Pielou Evenness Index
Diversity measures for invaded and uninvaded plots produced different Shannon
indices and evenness at all three sites, for all seasons (Table 1). In each situation,
uninvaded plots had significantly higher diversity than A.longifolia invaded plots
(Hutcheson, P < 0,001 for all situations but two seasons in BQ and one in SJ).
In all invaded plots evenness showed lower values (0,471, Autumn SJ, to 0,602,
Summer BQ) when compared with uninvaded plots (0,639, Summer SJ, to 0,834,
Autumn BQ), as a result of higher dominance of A.longifolia.

80 Hélia Marchante et al.
Table 1. Diversity (H’) and evenness (E) indexes in A.longifolia invaded and uninvaded plots at all
sites and all seasons. Data are calculated for composite plots. Hutcheson t test was used to test for
Shannon index differences among invaded and uninvaded plots in each site (* = P < 0,005; ** = P <
0,001).
Winter Spring Summer Autumn
S.Jacinto
JacintoS.jacinto
Shannon index H’ Invaded plots 1.365 1.321 1.283 1.207
Uninvaded
plots
2.149 2.210 1.882 1.936
Hutcheson t value 4.198** 4.759** 3.328* 4.041**
Pielou i ndex E Invaded plots 0.517 0.488 0.486 0.471
Uninvaded
plots
0.717 0.695 0.639 0.658
Palheirão
Shannon index H’ Invaded plots 1.550 1.670 1.553 1.589
Uninvaded
plots
2.117 2.337 2.134 2.258
Hutcheson t value 3.356** 3.985** 3.556** 4.115**
Pielou index E Invaded plots 0.547 0.567 0.548 0.540
Uninvaded
plots
0.782 0.780 0.788 0.814
Quiaios
Shannon index H’ Invaded plots 1.782 1.900 1.834 1.840
Uninvaded
plots
2.208 2.501 2.323 2.455
Hutcheson t value 2.473* 3.663** 3.199* 4.092**
Pielou index E Invaded plots 0.569 0.590 0.602 0.587
Uninvaded
plots
0.737 0.759 0.789 0.834
Dominance-diversity curves
Only spring curves are shown because that was the season when the plant
communities had most annual and biennial species. All the species computed in
dominance-diversity curves can be accessed in Appendix 1.
The dominance-diversity curves clearly showed that:
1) In SJ (Figure 4), BP (Figure 5) and BQ (Figure 6) invaded quadrats were
dominated by a single species, A.longifolia (about 60% cover in BP and SJ
and 45% in BQ). In these invaded communities, species with lower cover
never reach more than 10% (in SJ) – 15% (in BP and BQ).
2) In SJ, in uninvaded quadrats, one species is partially dominant (Ammophila
arenaria subsp. arundinacea, with about 35% cover) and Helichrysum
italicum (Roth) G.Don fil. subsp. picardi (Boiss.&Reuter) Franco and
Crucianella maritima L. species have similar covering areas, 15-25%. In
BP and BQ in uninvaded quadrats 3-4 species co-dominate (Ammophila
arenaria subsp. arundinacea, Corema album, Helichrysum italicum and
Crucianella maritima), each one responsible for 10 -15% cover area.
3) All three sites showed higher evenness in uninvaded communities.
4) Especially in BP and BQ, a higher number of species with extremely low
cover was observed in invaded plots.
5) Species richness is higher in uninvaded plots, particularly in SJ.

Invasion of the Portuguese dune ecosystem by the exotic species Acacia longifolia 81
SJ
0,0001
0,001
0,01
0,1
11357911 13 15 17 19 21 23 25
species sequence
abundance (log)
com Acacia
sem Acacia
invaded
uninvaded
A.longifolia
Fig. 4. Dominance-diversity curves showing the diversity of uninvaded and Acacia invaded
communities in SJ, in spring. Abundances (logarithmic scale), on the ordinate, are plotted against
species number in the sequence from most to least abundant, on the abscissa.
BP
0,0001
0,001
0,01
0,1
1
1357911 13 15 17 19 21 23
species sequence
abundance (log)
com Acacia
sem Acacia
invaded
uninvaded
invaded
uninvaded
A.longifolia
Fig. 5. Dominance-diversity curves showing the diversity of uninvaded and Acacia invaded
communities in BP, in spring. Abundances (logarithmic scale), on the ordinate, are plotted against
species number in the sequence from most to least abundant, on the abscissa.
BQ
0,0001
0,001
0,01
0,1
1
1357911 13 15 17 19 21 23 25 27 29
species sequence
abundance (log)
com Acacia
sem Acacia
invaded
uninvaded
A.longifolia
Fig. 6. Dominance-diversity curves showing the diversity of uninvaded and Acacia invaded
communities in BQ, in spring. Abundances (logarithmic scale), on the ordinate, are plotted against
species number in the sequence from most to least abundant, on the abscissa.

82 Hélia Marchante et al.
Discussion
Monitoring the three communities invaded by A.longifolia was necessary to assess
the species impact at several levels. In SJ, BP and BQ, A.longifolia has apparently
caused a decline of other species in all seasons (Figure 2), but differences are not
always significant. In BP and BQ, a significant difference among invaded and
uninvaded plots was observed in spring, due to the sprouting of annual and biennial
species only in uninvaded plots. A.longifolia is thus preventing the germination of
therophytes and hemicryptophytes in spring. Plant species present in the three sites
are quite different (Appendix 1) which can be partially responsible for the between-
site observed differences.
In BP and BQ, the dominance-diversity curves describing communities invaded
by A.longifolia revealed high diversity then SJ but some species with extremely low
abundances; the presence of one single specimen was quantified in species richness
but its very low representativity can be an evidence of being threatened. The
presence of A.longifolia is thus depressing the abundance of some species, as has
been shown for other invasive Acacia species in the South African fynbos
(Richardson et al. 1992, Holmes and Cowling 1997a).
Increasing A.longifolia cover (Figure 3) can promote shade, thus affecting
native species. A.longifolia has a high growth rate when compared with native
species, and this fact together with its abundant seed bank and strong regeneration
response after fire (Taylor et al.1985, Jeffery et al. 1988), may explain its increase
in cover. In fact, by increasing its cover Acacia takes advantage of light, controls
shade to other species and get space otherwise available to other species. This is
particularly important when the invasive species establish hes dense
populations (Weber 2000), as was observed in many areas of the systems studied.
Dune species have small leaf area and low growth habits. This normally allows
relatively high light levels at soil level. Species naturally adapted to these conditions
do not easily tolerate low light (Smith and Huston 1989), and A.longifolia, by
causing shade, threatens their survival. Plant species in dunes develop strategies that
focus on survival rather than domination (Carter 1995). With the presence of
A.longifolia, these systems are being replaced and the dominance of one single
species is prevailing.
A.longifolia decreased plant diversity in all sites studied. Reducing biodiversity
is one of the main effects of invasive species (Binggeli 1996, Williamson 1999,
Chapin et al. 2000), as was already found with other species of Acacia invading the
South African fynbos (Holmes and Cowling 1997a, b).
The presence of A.longifolia is accompanied by a substantial accumulation of
dormant seeds in the soil. With the high frequency of fires in Portugal, these seeds
will be stimulated to germinate, resulting in the expansion of A.longifolia
populations and the further loss of native species.

Invasion of the Portuguese dune ecosystem by the exotic species Acacia longifolia 83
Conclusions
A.longifolia is invading the studied Portuguese dune ecosystems, decreasing
biological diversity and promoting monospecific communities. As a microphanero-
phyte, a different life form compared to the majority of the native species present in
the studied communities, A. longifolia is causing structural changes in these com-
munities. Plant communities in dunes, naturally characterized by an open structure
and low cover, are gradually being transformed into Acacia woodlands, with arbo-
real structure, higher cover and low species number. The presence of this invasive is
decreasing species richness.
The results of this work emphasize the need to prioritise areas for management
and control measures in the invaded Portuguese dune systems, as well as the
identification of specific effects that should be mitigated in future programs for the
rehabilitation of invaded systems.
Acknowledgement
We are grateful to John Hoffman and Ilídio Moreira for valuable comments on the
manuscript, and to Filipe Campelo for valuable help in fieldwork.
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Invasion of the Portuguese dune ecosystem by the exotic species Acacia longifolia 85
Appendix 1.
The species present in each studied system which were represented in dominance
diversity curves are sequenced from the dominant to the rarest (left to right end of
the curves). Nomenclature follows Franco (1971-1984), Franco and Afonso (1994-
1998) and Coutinho (1939).
Figure 4: SJ invaded plots - Acacia longifolia, Helichrysum italicum subsp. picardi, Carpobrotus
edulis (L.) N.E.Br. in Phillips, Ammophila arenaria subsp. arundinacea, Corema album, Crucianella
maritima, Iberis procumbens Lange subsp. procumbens, Antirrhinum majus L. subsp. cirrhigerum
(Ficalho) Franco, Calystegia soldanella (L.)R.Br., Conyza canadensis (L.)Cronq., Otanthus maritimus
(L.) Hoffmanns.& link., Euphorbia paralias L., Poaceae (seedlings), Leontodon taraxacoides
(Vill.)Mérat subsp. taraxacoides, Pancratium maritimum L.; SJ uninvaded plots: Ammophila
arenaria subsp. arundinacea, Helichrysum italicum subsp. picardi, Crucianella maritima, Corema
album, Carpobrotus edulis, Silene littorea Brot., Iberis procumbens subsp. procumbens, Sedum
sediforme (Jacq.)Pau, Otanthus maritimus, Leontodon taraxacoides subsp. taraxacoides, Calystegia
soldanella, Pancratium maritimum, Euphorbia paralias, Corynephorus canescens (L.)Beauv.,
Herniaria ciliolata Melderis subsp. ciliolata, Conyza canadensis, Senecio gallicus Vill., Acacia
longifolia, Linaria caesia (Pers.)Chav. subsp. decumbens (Lange)Laínz, Polycarpon tetraphyllum (L.)
L., Poaceae (seedlings), Eryngium maritimum L., Antirrhinum majus subsp. cirrhigerum, Medicago
marina L.
Figure 5: BP invaded plots - Acacia longifolia, Carpobrotus edulis, Corema album, Crucianella
maritima, Ammophila arenaria subsp. arundinacea, Helichrysum italicum subsp. picardi, Pinus
pinaster, Sedum sediforme, Cistus salvifolius, Antirrhinum majus subsp. cirrhigenum, Corynephorus
canescens, Medicago marina, Pancratium maritimum, Calystegia soldanella, Tuberaria guttata (L.)
Raf. Fourr, Polycarpon tetraphyllum, Reichardia gaditana (Willk.)Coutinho, Vulpia membranacea
(L.)Dumont, Cyperus capittatus Vandelli; BP uninvaded plots: Crucianella maritima, Ammophila
arenaria subsp. arundinacea, Helichrysum italicum subsp. picardi, Corema album, Corynephorus
canescens, Carpobrotus edulis, Sedum sediforme, Antirrhinum majus subsp. cirrhigenum, Policarpon
tetraphyllum, Tuberaria guttata, Calystegia soldanella, Cyperus capittatus, Vulpia membranacea,
Herniaria ciliolata subsp. ciliolata, Erodium cicutarium (L.)L'Hér., Medicago marina, Pancratium
maritimum, Reichardia gaditana, Leontodon taraxacoides subsp. taraxacoides, Lagurus ovatus L.
Figure 6: BQ invaded plots - Acacia longifolia, Corema album, Pinus pinaster, Carpobrotus edulis,
Artemisia campestris subsp. maritima, Cistus salvifolius, Sedum sediforme, Helichrysum italicum
subsp. picardi, Rubia peregrina L., Seseli tortuosum L., Tuberaria guttata, Antirrhinum majus subsp.
cirrhigenum, Crucianella maritima, Medicago marina, Pancratium maritimum, Sedum album L.,
Ammophila arenaria subsp. arundinacea, Compositae (seedlings), Calystegia soldanella,
Corynephorus canescens, Poaceae (seedlings), Briza maxima L., Silene littorea, Verbascum litigiosum
Samp., Euphorbia portlandica L.; BQ uninvaded plots: Corema album, Carpobrotus edulis, Sedum
sediforme, Crucianella maritima, Seseli tortuosum, Corynephorus canescens, Helichrysum italicum
subsp. picardi, Artemisia campestris subsp. maritima, Sedum album, Ammophila arenaria subsp.
arundinacea, Medicago marina, Pancratium maritimum, Cistus salvifolius, Tuberaria guttata, Silene
littorea, Cyperus capittatus, Lagurus ovatus, Cerastium glomeratum Thuill., Compositae (seedlings),
Antirrhinum majus subsp. cirrhigenum, Calystegia soldanella, Verbascum litigiosum, Euphorbia
portlandica, Rumex bucephalophorus subsp. hispanicus (Steinh.)Rech. Fil, Medicago littoralis Loisel,
Senecio gallicus Vill., Plantago coronopus L.