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1 23
Journal of Coastal Conservation
Planning and Management
ISSN 1400-0350
Volume 21
Number 1
J Coast Conserv (2017) 21:35-45
DOI 10.1007/s11852-016-0470-8
Long-term trend of the waterbird
community breeding in a heavily man-
modified coastal lagoon: the case of the
important bird area “Lagoon of Venice”
Francesco Scarton
1 23
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Long-term trend of the waterbird community breeding
in a heavily man-modified coastal lagoon: the case
of the important bird area BLagoon of Venice^
Francesco Scarton
1
Received: 19 June 2016 /Revised: 16 October 2016 /Accepted: 9 November 2016 /Published online: 14 November 2016
#Springer Science+Business Media Dordrecht 2016
Abstract The aim of the paper is to examine the temporal
and spatial changes observed over a 25-year period in the
waterbird community nesting in the largest coastal lagoon
around the Mediterranean; to examine driving factors for
the observed changes; to address the most urgent conser-
vation actions. Published sources and field surveys made
between March and July were used to assess number of
breeding pairs of the commonest waterbirds in 1990–
1992, 2000–02 and 2012–14. The breeding waterbird
community exhibited several changes in its structure, with
an overall positive trend; the number of species increased
from 14 to 25 and the mean yearly abundance increased
from 6155 to 14,008 pairs. The diversity (H′) increased
slightly, whereas similarity indices and nMDS ordination
both highlighted clear differences between 1990 and 1992
and 2012–2014 communities. The increase in richness
and abundance were mostly due to the immigration of
birds from nearby wetlands, to the partial recovery of
lagoon ecological conditions since the end of the 1980s
and to the occurrence of suitable man-made habitats, such
as fish farms, dredge islands and a constructed wetland.
The fraction of the population nesting at artificial sites
and fish farms increased from 50% in 1990–1992 till
80% in 2012–2014, highlighting the importance of artifi-
cial breeding sites in costal lagoons. At the opposite nat-
ural nesting habitats, such as saltmarshes and beaches are
losing importance for breeding waterbirds, thus requiring
urgent conservation measures.
Keywords Seabirds .Wad er s .Mediterranean .Saltmarshes .
Man-made sites
Introduction
Long-term studies of animal populations have often been
considered to be of paramount importance in understand-
ing the frequency, duration and amplitude of variations in
ecological systems (O’Connor 1991; Sergeant et al. 2012).
For instance, only long-term studies on the distribution
and density of animal populations highlighted the effects
of climate change (Clutton-Brock and Sheldon 2010). In
particular, monitoring bird populations is important if the
species concerned are relevant for conservation and the
sites on which they depend must be managed or preserved
effectively (Atkinson et al. 2006); as pointed out by
Tou re nq e t al . (2000), management decisions based on
short-term studies might not reflect the biology of the spe-
cies involved. Waterbird species are considered to be in-
dicators of the quality and the importance of wetlands (Ma
et al. 2009; Sadoul 1997; Farinos et al. 2013); in particu-
lar, changes in the structure and function of waterbird
communities have been linked to changes in the biological
integrity of wetlands (Farinos et al. 2013). Spatial (at a
single wetland scale) and temporal (mid- to long-term)
variations can offer insight into variations induced by
environmental stressors.
In Mediterranean coastal wetlands, examples of long-term
surveys of the whole waterbird community are very rare, with
the notable exceptions of the Camargue (Sadoul et al. 1996)
and the Mar Menor coastal lagoon (Farinos and Robledano
2010). In Italy, some long-term studies have targeted selected
groups (Brichetti and Foschi 2006; Scarton and Valle 2015),
*Francesco Scarton
scarton@selc.it
1
SELC soc. coop, Via dell’Elettricità 3/d, 30175 Marghera
(VE), Venezia, Italy
J Coast Conserv (2017) 21:35–45
DOI 10.1007/s11852-016-0470-8
Author's personal copy
or the whole waterbird community, but for shorter periods
(Boldreghini and Dall’Alpi 2008; Redolfi De Zan et al. 2011).
The lagoon of Venice is situated at the northern corner of
the Mediterranean sea and for centuries, it has been profound-
ly shaped by anthropic activities. Until the 1970s, the lagoon
has suffered from reclamation, subsidence induced by water
extraction and water and sediment pollution (Seminara et al.
2011; Newton et al. 2014; Brigolin et al. 2014). About 12
million tourists visit the town of Venice annually, which is
located in the centre of the lagoon, whereas several million
more people visit the beaches of the Cavallino peninsula
(UNESCO 2011). Professional fishing and clam harvesting
are spread throughout the lagoon, whereas hunting takes place
from September to the end of January. Some of these pressures
have slowed down in recent decades; for instance, nutrient
concentrations in water and sediments are lower today than
in the past and are still decreasing, whereas water anoxia is no
longer observed (Solidoro et al. 2010). Other pressures, such
as morphological changes are nevertheless still evident:
saltmarsh erosion for instance, has continued to occur even
in more recent years (Sarretta et al. 2010).
The objective of this study were the following: 1) to exam-
ine the temporal and spatial changes observed over a 25-year
period in the breeding waterbird community; 2) to examine
potential driving factors for the observed changes, 3) to
address the most urgent conservation actions for those breed-
ing sites which are in danger.
Study area
The Venice Lagoon is the largest coastal lagoon in the
Mediterranean area; it covers an area of 55,000 ha along
the Adriatic Sea, with its centre at 45°26’N, 12°19’E
(Fig. 1). Two barrier islands and one peninsula separate
the lagoon from the sea. A large part of the lagoon consists
of an open water body about 37,000 ha in size with shallow
bottoms, deep channels and tidal flats; fish farms occupy
an additional 10,000 ha along the borders. Permanently
emerged islands (5000 ha) or tidally inundated saltmarshes
(3800 ha) comprise the remaining area. The mean depth of
the lagoon is 1.1 m and the mean tidal range of 0.6 m is one
of the highest observed in the whole Mediterranean; the
climate is temperate, with a mean annual temperature of
14.5 °C and the mean rainfall is 800 mm per year (Solidoro
et al. 2010). The Venice lagoon is the most important wa-
terbird wintering site in Italy (Zenatello et al. 2014), prob-
ably the most important in the whole Mediterranean. In
mid-January 2011–2015, about 368,000 waterbirds were
counted on average, with nine species regularly exceeding
1% of their biogeographical populations (Basso and Bon
Fig. 1 The lagoon of Venice,
with limits of the IBA BLagoon of
Ve n ic e ^superimposed.
Saltmarshes are shown
in dark grey
36 Scarton F.
Author's personal copy
2015). As a breeding site, the lagoon hosts significant frac-
tions (i.e., >1% of the Italian populations reported in
Nardelli et al. 2015) of several species, such as sandwich
tern Thalasseus sandvicensis, redshank Tringa totanus,
oystercatcher Haematopus ostralegus, Kentish plover
Charadrius alexandrinus. Despite its well-recognised im-
portance for waterbirds, only a very small part of the la-
goon, named Valle Averto and 500 ha in extent, was des-
ignated in 1989 as a Ramsar site. Since 1989, the whole
lagoon, including a small area on the adjacent mainland
and the littoral strip, has been listed as an Important Bird
Area (Heath et al. 2000). In 2007, the whole lagoon was
declared a Special Protection Area (IT 3250046 Laguna di
Venezia), according to the Birds Directive 2009/147/EC.
Several wildlife reserves, which were set up by local au-
thorities or non-governmental organisations and where
hunting is permanently banned, are scattered throughout
the lagoon. In this study, the limits of the IBA Laguna di
Venezia were mainly used as the study-area borders; the
only difference was the inclusion of two small areas out-
side, but adjacent to, the borders, as they have hosted im-
portant colonies of waterbirds for many years.
Several lagoon habitats are used by waterbirds to nest: 1)
the littoral strip: beaches and low-elevation dunes are located
along the two islands and the peninsula. Most ofthese beaches
are heavily disturbed by sunbathers and other visitors in the
spring and summer but they still are the nesting habitats of
Kentish plover, oystercatcher and little tern Sternula albifrons.
2) Saltmarshes, which are regularly flooded during mean high
tides. The dominant plant species include Sarcocornia
fruticosa, Salicornia veneta, Limonium narbonense and
Halimione portulacoides. Small to large colonies of seabirds
(little tern,sandwich tern,common tern S. hirundo)and
waders (black-winged stilt H. himantopus, redshank) also oc-
cur here. 3) Artificial sites: these include dredge islands, a
treatment wetland site (constructed wetland), and two large
islands created in the 1960s. Since 1985, about 120 dredge
islands have been built in the lagoon, with a mean surface area
of 11.2 ha and a total area of about 1300 ha (Scarton et al.
2013a). These sites are slightly more elevated above sea level
than the nearby saltmarshes. Dredge islands consist of a con-
tainment cell formed using wooden piles around the exterior;
the shallow water inside is then filled with sediments originat-
ing from the regular dredging of lagoon channels or inlets.
From almost a bare surface, several phases lead to an almost
continuous coverage of halophytes; naturally formed and
man-made shallow tidal ponds and creeks have a different
extent and complexity at each site (Scarton 2005; Scarton
and Montanari 2015). A large (110 ha) artificial island created
in the 1960s was recently transformed into a treatment wet-
land site, where large reedbeds and shallow water basins host
a rich avifauna. Two large artificial islands (1100 ha in total)
also built in the 1960s were abandoned before being used for
any purpose and now are largely covered with a diverse veg-
etation, from halophytes to trees, and contain several freshwa-
ter shallow ponds. Overall, these artificial sites host several
thousand nesting pairs each year, mostly of the yellow-legged
gull Larus michahellis, little tern,sandwich tern,avocet
R. avosetta, redshank and oystercatcher.4) Small islands:
apart from Venice and a few other major islands, the lagoon
includes about 30 small islands that were used by man in
previous centuries, but now most of these are completely
abandoned. A lush vegetation consisting of bushes and trees
usually covers most of these islands. Over the last 15 years, a
few of these have been occupied by colonies of herons (grey
heron Ardea cinerea, little egret Egretta garzetta, cattle egret
Bubulcus ibis) and cormorants (pigmy cormorant Microcarbo
pygmeus and cormorant Phalacrocorax carbo). 5) Fish farms:
the fish farms are also artificial sites, but these are considered
here separately, due to their peculiar environmental character-
istics. They encompass about 10,000 ha along the borders of
the lagoon and are used for extensive fish rearing, but espe-
cially for waterfowl hunting. For this last purpose, the man-
agement activities on the fish farms include the tight regula-
tion of water levels, the building of islets for bird preening and
resting and food provision to the waterfowl during the colder
winter months. The fish farms consist of a mosaic of open
bodies of brackish waters, saltmarsh islets and reedbeds; pe-
rennially emerged areas include small woodlots and arable
fields. The breeding species include yellow-legged gull, little
tern, black-winged stilt, common tern and seven species of
colonial herons.
Methods
In long-term analyses of waterbird communities, the orig-
inal data often come from a variety of sources, including
surveys, partial counts and estimates. This is particularly
true for the Venice lagoon, where comprehensive and si-
multaneous counts of all the breeding waterbirds have nev-
er been performed to date. Nevertheless, surveys dealing
with herons, gulls, terns and waders have been made sev-
eral times since the 1980s; I choose three periods (1990–
1992; 2000–2002; 2012–2014) for which the available da-
ta are more detailed (Amato et al. 1994; Valle et al. 1996;
Mezzavilla and Scarton 2002; Scarton et al. 2005,2013b;
Scarton and Valle 2015). Data for the period 2012–2014
were collected by the author and colleagues through fre-
quent field surveys made each year from May to July or, in
a few cases, are personal estimates based on opportunistic
observations. All the waterbirds nesting in at least one of
the above-mentioned periods were considered; neverthe-
less lack of detailed data did not allow the inclusion of
three other regularly nesting species such as mallard Anas
Long-term trend of a waterbird breeding community 37
Author's personal copy
platyrhynchos, little grebe Tachybaptus ruficollis and
common moorhen Gallinula chloropus.
For each period, I calculated the minimum and maximum
number of pairs for each species, but only the mean values were
used in successive elaborations, to reduce the importance of
occasional spikes, which are relatively common in waterbird
populations. Several community metrics were then calculated
for each period: richness (number of species), abundance (num-
berofpairs),Shannon–Wiener diversity, Sørensen and Bray-
Curtis similarity indices as in Magurran (2004). The trends
were calculated as (C-A/A)*100, where C = abundance in
2012–2014, A = abundance in 1998–2000. Species with differ-
ences >20% were considered to be increasing, those with dif-
ferences from +20% to −20% to be stable and the remaining
ones to be decreasing (as in Deceuninck 2001). To allow further
analysis, the 26 species considered were grouped into seven
trophic guilds, chosen according to the literature (Boldreghini
and Dall’Alpi 2008;Liordos2010) and personal knowledge: 1)
benthos feeders: oystercatcher, black-winged stilt, little ringed
plover C. dubius, Kentish plover, redshank; 2) dabbling phy-
tophagous: mute swan Cygnus olor,greylagAnser anser;3)
diving hyctiophagous: little tern, sandwich tern, common tern;
4) generalists: cattle egret, black-headed gull Chroicocephalus
ridibundus, mediterranean gull L. melanocephalus, yellow-
legged gull,gull-billed tern Gelochelidon nilotica; 5) pursuit
ichthyophagous:cormorant,pigmy cormorant;6) scythers:
shelduck Tadorna tadorna, greater flamingo Phoenicopterus
roseus, avocet;7) wading ichthyophagous: night heron
Nycticorax nycticorax, squacco heron Ardeola ralloides, little
egret,great white egret Ardea alba, grey heron, purple heron
Ardea purpurea.
The conservation status of each species refers to the Italian
Red List of Breeding Birds (Peronace et al. 2012); the
European (EU 27) Red List of Birds (BirdLife International
2015); the Species of European Concern assessment (Birdlife
International 2004) and the inclusion in Annex 1 of the EU
Birds Directive. A conservation value index (CVI) was calcu-
lated giving the following score to each species: 1) Birds
Directive, Annex 1: 1 if species was included, 0 if not; 2)
SPEC: 6 if SPEC 1, 4 if SPEC 2, 2 if SPEC 1, 0 for Non-
SPEC; 3) Italian Red List: 6 if Endangered; 4 if Vulnerable; 2 if
Near Threatened; 0 if in the lowest categories. The mean num-
ber of pairs in each period (log
10
(x+1) transformed) for a given
species was then multiplied by the respective conservation
value obtained and the results were summed to obtain a con-
servation value index for that period. Chi-square analysis was
used to test the hypothesis that the distribution of species
among guild/habitat types did not differ among the three pe-
riods; non-metric multidimensional scaling (nMDS) was used
for the ordination of species and periods, using abundance data
and the Bray–Curtis similarity (Sinha et al. 2011). Numerical
and statistical analysis were performed using PAST version 2.9
(Hammer et al. 2001) and EstimateS v. 9.1 (Colwell 2013).
Results
Between 1990 and 2014, the number of breeding species in-
creased from 14 to 25, and the mean yearly abundance from
6155 to 14,008 pairs (Table 1). The diversity (H′)decreased
slightly between 1990 and 1992 and 2000–2002 but then in-
creased to a maximum; the two similarity indices indicate that
the last two periods were more similar to each other than to the
first period (Table 2). This is also shown by the nMDS ordi-
nation biplot (Fig. 2); the two last periods are both well sepa-
rated from the first period. The same figure depicts one cluster
of species whose abundance is linked with the 2012–2014
period (upper right quadrant) and a second cluster linked with
the 2000–2002 period (lower right quadrant).
The composition and structure of the breeding bird com-
munities showed several changes throughout the years
(Fig. 3). In 1990–1992, the three most abundant species were
the yellow-legged gull, common tern and little egret, overall
60.9% of the total breeding pairs; in 2000–2002, the species in
the same ranks were the yellow-legged gull, redshank and
little egret (66.9%), whereas in 2012–2014 they were respec-
tively yellow-legged gull, redshank and flamingo (54.6%).
Conservation values of the nesting populations increased over
time. Among the breeding species that nested in 1990–1992,
nine were listed in Annex 1 of the EU Birds Directive,and this
increased to 12 in 2000–2002 and 14 in 2012–2014. In 2012–
2014, five of the nesting species were Threatened (either
Critically Endangered or Vulnerable) according to the Italian
Red List. The highest values of the CVI belong to pigmy
cormorant, Kentish plover, little tern and sandwich tern
(Table 1). The CVI for each three-year period (taking into
account the mean number of nesting pairs) increased from
95.3 to 138.9 and then to 168.2.
Only 14 species out of 26 nested in all the three periods; an
additional seven species nested for the first time in the lagoon
between 1993 and 2002, and four more species nested for the
first time ever between 2003 and 2014. Thus, between 1990
and 2014, eleven species settled for the first time in the study
area, while oystercatcher began to nest again in the lagoon
after about a century of absence (Scarton et al. 1998). The
population trends between 1990 and 1992 and 2012–2014
appear to be species-specific, despite being positive in most
cases (Table 1). Eleven species were new breeders and an
additional eight species were increasing. Only one species
was stable, five showed a decreasing trend and one bred only
in the central period. The highest increase was shown by av-
ocet, yellow-legged gull and little tern, whereas the most neg-
ative trends were shown by three heron species (night heron,
cattle egret and little egret).
Figure 4shows the absolute and relative abundance for
each guild, with statistically significant differences among pe-
riods (Chi-square: 4481.5, d.f. = 8; P< 0.0001). All the guilds,
apart from the wading ichthyophagous, showed an increase in
38 Scarton F.
Author's personal copy
absolute abundance; the highest increases were shown by the
generalists, mostly composed by the yellow-legged gull, and
the scythers, which in turn were mostly represented by the
greater flamingo, in the last period. In 1990–1992, the com-
munity was dominated by wading ichthyophagous and diving
ichthyophagous species (51.4% in total), but about twenty
years later, the community was dominated by generalists and
diving ichthyophagous species (55.3%).
The distribution of breeding pairs among the habitat types
changed significantly over the years (Chi-square: 2183.5;
d.f. = 8, P< 0.0001). For saltmarshes, a loss in terms of the
percentage of breeding birds was evident, whereas the amount
of pairs almost doubled at artificial sites (from 20% to 40%)
and remainedstable in fish farms; the percentage of waterbirds
nesting along the littoral strip is nowadays close to 0 (Fig. 5).
Therefore, 80.1% of the whole nesting population nest nowa-
days at artificial sites, which are either managed (fish farms
and the treatment wetland site) or unmanaged (dredge islands
and the two large artificial islands).
Discussion and conclusions
It is well known that considering only one parameter, i.e., the
number of species or abundance of individuals, to analyse
changes in biodiversity might be misleading, since these indi-
cators can sometimes increase, even if the environmental
Tabl e 2 Diversity in the three periods and similarity values
1990–1992 2000–2002 2012–2014
Shannon H′2.07 2.03 2.38
Sørensen Bray–Curtis
1990–1992 vs 2000–2002 0.77 0.64
1990–1992 vs 2012–2014 0.71 0.43
2000–2002 vs 2012–2014 0.89 0.74
Tabl e 1 Minimum, maximum and mean (no. of pairs) values for the study periods
Scientific names 1990–1992 Mean 2000–2002 Mean 2012–2014 Mean Difference between 2012–14
and 1990–92 (%)
CVI
Cygnus olor 60 90 75 100 120 110 110 130 120 60.0 0
Anser anser 0 0 0 1 3 2 30 40 35 New breeder 0
Tadorna tadorna 3 4 3.5 35 54 44.5 70 100 85 2328.6 4
Phalacrocorax carbo 0 0 0 102 102 102 338 373 355.5 New breeder 0
Microcarbo pygmeus 0 0 0 13 13 13 312 337 324.5 New breeder 9
Nycticorax nycticorax 300 310 305 139 151 145 48 58 53 -82.6 7
Ardeola ralloides 60 60 60 31 36 33.5 4 5 4.5 -92.5 3
Bubulcus ibis 0 0 0 3 4 3.5 50 60 55 New breeder 0
Egretta garzetta 1100 1100 1100 1000 1495 1247.5 378 438 408 -62.9 1
Ardea alba 000 465 000 0.0 3
Ardea cinerea 0 0 0 110 130 120 292 313 302.5 New breeder 0
Ardea purpurea 70 215 142.5 410 460 435 108 143 125.5 -11.9 3
Phoenicopterus roseus 000 000 029891494.5Newbreeder 3
Haematopus ostralegus 0 0 0 11 20 15.5 120 165 142.5 * New breeder 2
Himantopus himantopus 200 250 225 310 589 449.5 400 600 500 122.2 1
Recurvirostra avosetta 40 60 50 18 136 77 500 600 550 1000.0 1
Charadrius dubius 000 000 343.5Newbreeder 2
Charadrius alexandrinus 139 151 145 80 120 100 64 140 102 -29.7 9
Tringa totanus 980 1030 1005 1500 1600 1550 1500 1800 1650 64.2 4
Chroicocephalus ridibundus 110 160 135 120 140 130 150 180 165 22.2 0
Larus melanocephalus 0 0 0 0 0 0 200 300 250 New breeder 1
Larus michahellis 1100 1600 1350 3800 4800 4300 4000 5000 4500 233.3 0
Sternula albifrons 198 320 259 262 468 365 690 1670 1180 355.6 9
Gelochelidon nilotica 000 000 153 Newbreeder 5
Thalasseus sandvicensis 0 0 0 550 686 618 625 1528 1076.5 New breeder 9
Sterna hirundo 1200 1400 1300 483 1005 744 330 715 522.5 -59.8 1
*Had already nested before 1990–92. CVI = conservation value index, see text for details
Long-term trend of a waterbird breeding community 39
Author's personal copy
quality decreases. This occurs, for example, when local spe-
cies are replaced by non-native species (Ma et al. 2009;Sax
and Gaines 2003). In addition to choosing a set of metrics
instead of just one, it has also been recommended that the
importance of the species that structure the community should
be considered, using the conservation value (Humphries et al.
1995). All the chosen parameters indicate that the breeding
waterbird community of the Venice lagoon shows a positive
trend from 1990 until 2014. In addition to a remarkable in-
crease in the number of species, their absolute abundance as
the H′diversity of the community has also increased; the
number of species of conservation value at an Italian or
European scale has also increased. In 2012–2014, the water-
bird community was thus richer, larger and more diverse that it
was in 1990–1992. The two periods share only 14 species out
of 25, and their respective communities show several differ-
ences in composition.
The driving factors for the changes observed in the com-
munity might have acted either locally or in a wider area, or a
combination of both. For instance, negative trends in winter-
ing and breeding waterbird populations have been linked to
large environmental transformations, such as wetland recla-
mation, to an increase in water pollution or hunting pressure,
or to large engineering works (Burton et al. 1996;Kingsford
and Thomas 2004;Maetal.2009;Yangetal.2011). At the
opposite, setting new protected areas, imposing hunting bans
or creating new habitats have often led to a local increase in
abundance and/or richness at several coastal sites (Perco and
Perco 1992; Fox and Madsen 1997; Melvin and Webb 1998;
Garaita and Arizaga 2015). The increase in the Venice lagoon
breeding waterbird communities represents a different scenar-
io. During the 25-year study period, no new protected areas
were declared and neither were hunting activities stopped or
drastically reduced throughout the lagoon. The only local
Fig. 2 Non-metric multi-
dimensional scaling (nMDS;
stress value = 0.08) ordination
plots based on pair abundance and
showing the relationships be-
tween species and periods
Fig. 3 Graph of rank abundances
in the three periods
40 Scarton F.
Author's personal copy
factor that provided new extensive and safe nesting habitats
was without doubt the creation of man-made habitats, such as
dredge islands or a large treatment wetland.
Different environmental pressures are still acting in the
lagoon, although some of these, such as water pollution, have
been showing signs of amelioration in the last two decades
and the recovery of the ecological state of the lagoon, from its
very poor condition at the beginning of the 1980s, has been
recently discussed (Solidoro et al. 2010). The pressures that
continue to exist include the erosion of saltmarshes, a massive
tourist presence and leisure and commercial boat traffic. All
these do not seem to affect the entire breeding waterbird com-
munity, although they might explain the observed changes in
habitat use. Again, large engineering works such as the ongo-
ing buildingof mobile gates at the three lagoon inlets (Newton
et al. 2014), did not lead to changes in the breeding waterbird
communities. More subtle negative effects, such as contami-
nation with toxic substances (see Borghesi et al. 2011 for
greater flamingoes contaminated with mercury) have been
observed in the lagoon of Venice, but their effects on the
breeding population are unknown. The largest and most pos-
itive impacts were therefore due to extrinsic factors, in partic-
ular, to the arrival of new breeding species. Between 1993 and
2014, 11 species of waterbirds nested for the first time, a figure
that is probably higher than in the previous fifty years. This
was the case for the grey heron and great white egret (the first
nesting record for both species was in 1993), sandwich tern
(1995), mediterranean gull (1996), pigmy cormorant (1997),
cormorant (1997), greylag goose (introduced in 1998), cattle
egret (2000), little ringed plover (2004), gull-billed tern
(2008) and flamingo (2008). None of these is an exotic or
invasive species; the sacred ibis Threskiornis aethiopicus is
observed with increased frequency in the lagoon, but has nev-
er nested to date. The colonisation of the lagoon of Venice by
the new species was due to their increase on a larger scale, i.e.,
at an Italian or Mediterranean scale (Galewski et al. 2011;
Nardelli et al. 2015) and as a consequence, they took advan-
tage of suitable nesting sites and trophic resources available in
Fig. 4 Absolute (number of
pairs) and relative (% of the
yearly total) abundance in the
three periods according to the
trophic guild
Long-term trend of a waterbird breeding community 41
Author's personal copy
the lagoon. The importance of the settlement of new breeding
species has been examined at other large Mediterranean wet-
lands, such as the Camargue (Sadoul 1997)andtheEbroDelta
(Almaraz and Oro 2011). The structure of the waterbird com-
munity has also changed: Scarton and Valle (2015) noted that
the seabird community of the lagoon is now more Bpelagic^
than in the past, due to the strong increase in the abundance of
sandwich tern, a species that feeds mainly in marine waters up
to 15–20 km from the colony. The guild that showed the
largest increase from 1990 to 1992 and 2012–2014 is never-
theless that of scythers, due to the massive occurrence of
greater flamingoes in recent years. The largest guild has al-
ways been that of Bgeneralists^, which mostly contains the
yellow-legged gull. This species showed a strong increase in
the lagoon of Venice between the 1980s and the early 2000s,
together with the occupation of rooftops in the historical city
centre, but subsequently, it slowed down. The negative effect,
direct or indirect, of the yellow-legged gull on other species of
higher conservation value has been reviewed in detail for the
Mediterranean area by Vidal et al. (1998). As already ob-
served by Oro and Martínez-Abraín (2007) in their study area,
the data for the Venice lagoon indicate that the increase in the
abundance of the yellow-legged gull does not necessarily
cause a decrease in that of other species breeding in the same
wetland, if it is large enough to accommodate alternative
nesting sites. In the lagoon of Venice, several artificial sites
support yellow-legged gull colonies and their presence ex-
cludes most other species from occupying the same locations
(pers. obs.); however, hundreds of alternative sites exist and
tens of them are occupied by other waterbirds. No field data
are available about predation of eggs and chicks made by
yellow-legged gulls, which on the other side could reduce
the reproductive success of the other species.
In addition to the change in the waterbird community
throughout the years, dramatic changes have also occurred
in the use of the different nesting habitats by waterbirds.
Saltmarshes hosted about 40% of the breeding pairs in
1990–1992, the number of which decreased to 20% in
2012–2014. Most of the little tern and avocet populations,
which used to nest almost exclusively in saltmarshes, moved
to dredge islands within a few years; other species, such as the
redshank and black-winged stilt only partially moved to the
new sites. This shift was likely an adaptive response to the
observed increase in the local mean sea level, which was about
18 cm between 1989 and 2013 (see details in Scarton and
Va l l e 2015). This increase caused more frequent flooding of
the saltmarshes during the nesting seasons and subsequent
breeding failures (pers. obs.). The saltmarshes of the Venice
Fig. 5 Absolute (number of
pairs) and relative (% of the
yearly total) abundance in the
three periods according to the
nesting habitat
42 Scarton F.
Author's personal copy
lagoon host few breeding species, but most of these are of high
conservation value; for instance, the largest Mediterranean
population of redshank nests here (Hale et al. 2005). The
progressive decrease in the populations breeding at these in-
tertidal sites is a cause for concern and requires urgent action
of active habitat conservation.
Another effect observed over the years was the almost
complete abandonment of the littoral strip by the little tern
and the strong reduction in the Kentish plover population
breeding in the same habitat. Both species moved to
dredge islands within about a decade and the primary
cause is undoubtedly the increase in human pressures on
beaches and dunes, especially between May and August;
predation by the hooded crow Corvus cornix,andtheferal
cat Felis silvestris catus is also a cause for concern. In
recent years, campaigns dedicated to the protection of
Kentish plover nests have been employed in the littoral
strip of the Venetian lagoon by NGOs and these have
been relatively effective in protecting the small nesting
population surviving there.
Among the majority of the 26 species considered here that
increased in number between 1990 and 1992 and 2012–2014,
some might cause conflict with selected stakeholder catego-
ries, particularly fish farmers. Apart from the long-debated
example of cormorants, more recent criticisms have been
raised against the presence of hundreds (mute swans) or thou-
sands (greater flamingos) birds in the fish farms of the Venice
lagoon. These species have been held responsible by local fish
farmers for interfering with game species, such as ducks,
through disturbances due to territorial behaviour, competition
for trophic resources or increase in water turbidity. At least
one case of greater flamingo harassment in a fish farm just
before the nesting season is known, and illegal shooting at
mute swans has been reported but never proven. Both species
have been referred to in the scientific literature as ecosystem
engineering species, since they affect resource availability for
other birds; a detailed study performed in the Camargue
showed that they can have both positive and negative effects
on submerged vegetation (Gayet et al. 2012), whereas in some
French fish ponds, no negative interactions were observed
among mute swans and other Anatidae (Gayet et al. 2011).
Detailed field studies are thus required, to elucidate the true
impact of these two species in the Venetian fish farms.
The few declining species included three heron species: the
night heron, the squacco heron and the little egret. The first
two species declined markedly throughout Italy during the last
10–15 years, whereas the third species decreased only in
northern Italy (Nardelli et al. 2015). An opposite trend was
shown by two other species of herons; the grey heron and the
cattle egret, which colonised the lagoon from the 1990s and
are still increasing in number, as in the rest of Italy. Thus, the
situation observed in the lagoon of Venice reflects the situation
on a broader scale.
The data presented above show that a large, heavily man-
modified lagoon can still host a diverse waterbird community
that has been increasing over the last 25 years. This is due to a
combination of factors: 1) its large extent, at least by European
standards, which allows several nesting habitats to coexist and
thus, favours bird diversity; 2) the abundant trophic resources,
from brackish and marine ichthyofauna to benthic organisms,
which have at least partially recovered in the last two decades;
3) the increasing trend shown by several waterbird popula-
tions, already described in the recent literature for the western
Mediterranean; 4) the occurrence of large privately owned fish
farms, where disturbance to the birds during the nesting sea-
son is low, with the notable exception of some conflictual
species; 5) the occurrence of other man-made habitats, such
as dredge islands, where seabirds and waders have found suit-
able nesting sites.
The long-term analysis of the breeding waterbird commu-
nity has clearly shown that the two natural habitats –the
saltmarshes and the beach-dune complex –have been decreas-
ing in importance as breeding sites for waterbirds. In the la-
goon of Venice, protection of the saltmarshes from erosion has
been occasionally performed to date using light structures,
such as fascines (Barausse et al. 2015), or heavier structures,
such as poles or gabions if the saltmarshes were situated along
channels frequently used by boats (Seminara et al. 2011). To
mitigate the effects of rising sea levels, piles of straw or shell
fragments have also been used in the past to provide seabirds
with suitable nesting sites in saltmarshes that are less prone to
flooding; these measures have been effective, but were not
repeated in subsequent years. Much more widespread activi-
ties should be undertaken in the future to effectively protect
widespread saltmarsh areas.
The protection of the few beach-breeding pairs appears to
be more difficult. Nowadays, only very small areas of beaches
and dunes along the Venice lagoon islands and peninsula can
be realistically devoted to nesting Kentish plovers and little
terns, given the intensive and widespread presence of visitors.
Nevertheless, the importance of even small-scale actions is
high, as they increase the perception of the conservation needs
and raise awareness among the public.
Finally, the data presented here highlight the increasing
importance of managed and unmanaged man-made sites for
breeding waterbirds. Nevertheless, unmanaged sites, such as
dredge islands, will probably decrease in importance in the
future, due to natural processes such as vegetation over-
growth. Urgent simple but effective management activities,
such as regular vegetation cutting, are thus required, to pre-
serve the importance of these artificial sites for breeding
waterbirds.
Acknowledgements Many friends and colleagues helped to perform
the fieldwork throughout two decades: R. Valle, M. Baldin, P. Bertoldo,
S. Borella, E. Checchin, D. Smania. Others provided unpublished
Long-term trend of a waterbird breeding community 43
Author's personal copy
information: M. Bon, A. Sartori, L. Sattin, E. Stival. Part of the data for
2013–2014 years were collected for the CORILA-Consorzio per la
Gestione del Centro di Coordinamento delle Attività di Ricerca inerenti
il Sistema Lagunare di Venezia (BStudio B.6.72- B10 e B.11. Attività di
rilevamento per il monitoraggio degli effetti prodotti dalla costruzione
delle opere mobili alle bocche di porto^), on behalf of the
Provveditorato Interregionale per le Opere Pubbliche per il Triveneto -
Ministero dei Lavori Pubblici (Italian Ministry of Public Works). The
assistance of C. Dabalà (CORILA) is warmly acknowledged.
Compliance with ethical standards
Conflicts of interest As stated in the Acknowledgments, the study was
made in the framework of a wider monitoring project funded to my
company (SELC soc. coop., Venice) by CORILA on behalf of
Provveditorato Interregionale per le Opere Pubbliche per il Triveneto -
Ministero dei Lavori Pubblici (Italian Ministry of Public Works).
Research involving human participants and/or animals My study
did not involve humane participants and/or Animals.
Informed consent Not applicable to my study.
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