DISCUSSION
Effects of artificial ecological islands on macroinvertebrates
The macroinvertebrate community in the park was mainly composed of
Mollusca, Palaemon and Lnsecta, possibly because the plant
community was mainly composed of Phragmites australis ,Typha orientalis and Myriophyllum . The substrate was
mostly silt that was rich in humus, which meets the requirements of some
species for dissolved oxygen and organic debris. The park’s artificially
controlled water flow keeps constant, providing excellent conditions for
slow-moving species, such as mollusks, to thrive (Zuo et al. 2016; Chen
et al. 2014). With the regulation of farmland around the wetland park,
it is forbidden to dump waste water into the park, which makes the water
quality of the park relatively cleaner, and hydrophyte growth is thick.
The abundances of Palaemon (Exopalaemon) modestus , Ephemerptera,
Lestidae, Dytiscidae and other aquatic insects such as Nepidae,
Belostomatidae and Haliplidae that are suitable for living in aquatic
plants also increased. The macroinvertebrate species were abundant and
dense, but the biomass was low, which was related to their geographical
environment and the short amount of time that had passed since the
farmland was converted to wetland. Regarding the increasing trend in the
macroinvertebrate diversity, consistent with the works of Du and Lu, the
diversity index showed an increasing trend with the extension of time
since the construction of conservation engineering projects (Du et al.
2011; Lu et al. 2013). Over time, the ecological effects of these
projects become evident.
We found evidence that the islands could affect the composition of
macroinvertebrates. First, on the one hand, the islands changed the
water depth, causing the islands to have a certain slope. The light and
temperature at the bottom of the islands also changed, further
influencing the distribution of sediment, primary productivity and
hydrophytes and increasing the niche range of the plant species. At the
same time, island creation increased the living area, provided diverse
habitats for feeding and reproduction and created concealment conditions
for macroinvertebrates. The living requirements of different species
were met due to the different water levels, water temperatures and light
levels (Wen et al. 2016; Carvalho et al. 2012; Freitas et al. 2011). On
the other hand, most species live on the sediment surface, which is rich
in oxygen and organic matter, and changes in water depth affect the
amount of organic carbon, phytoplankton and sedimentary organic matter
(Chen et al. 1975), which is conducive to the adjustment of community
structure (Zuo et al. 2016; Chen et al. 2014; Du et al. 2011).
Therefore, the establishment of islands increased the water depth
gradient diversity, which significantly changed the community structure
and distribution pattern of the macrobenthos.
Second, it is generally believed that macroinvertebrate species richness
differs in different bottom environments (Lu et al. 2007); the community
in pebble substrates was richer than that in fine sand substrates (Xie
et al. 2007). Therefore, when building the PSI, the park’s designers
chose larger stones as the covering, rather than fine sand or small
stones, which ensured the stability of the matrix to a certain extent.
In this study, the SSIs were surrounded with many hydrophytes, such asPhragmites australis , Typha and floating grass, and the
bottom had loose debris that was rich in organic matter; these
conditions provided a place for the macroinvertebrates to eat, breed and
evade predators and made the substrate stable, reducing the impact of
water level changes on the macroinvertebrates (Duan et al. 2007).
Therefore, compared with the PSIs, with less vegetation and humus, the
SSIs are more suitable for survival.
Third, the biomasses of the dominant species, such as Typha andPhragmites australis , on the island increase each year, and as a
result, only single species of hydrophytes exist. Phragmites
australis are shallow-rooted scattered plants that have a strong
ability to secrete oxygen from their roots, and these conditions can
meet the respiratory needs of mollusks, such as Gastropoda, which
require high dissolved oxygen levels. However, due to their high growth
density, the stems and leaves do not decompose easily (Zuo et al. 2016),
and the amount of organic detritus they produce is relatively small;
therefore, the abundance and diversity of all macroinvertebrate species
were not as high as those on the islands that were constructed
relatively later.
Finally, plants are more likely to survive on the soil island (which has
a high abundance), but as the soil island ages, the plants tend to
become more homogeneous, while the stone island has been scoured by
water for a long time, which makes it less stony and more conducive to
plant survival. For example, SIMPER analysis showed that the three most
contributing species on the island, which was built in 2011, werePhragmites australis (50.63%), Scutellaria scordifolia(16.01%) and Inula japonica (8.38%). However, the top three
contributing plants on the soil island built in 2013 were Carex
bohemica (20.62%), Calamagrostis epigeios (19.15%) andPhragmites australis (15.93%). Due to mowing a year ago, soil
island J has the highest plant biodiversity, species and abundance
compared with other soil islands. The number of macroinvertebrate
species was negatively correlated with plant abundance (r = -0.689,P = 0.04), as well as, ageing of islands leads to a loss of
attractiveness for plant and birds (Scarton et al. 2013). Therefore, it
is recommended that wetland parks conduct reed-cutting work regularly to
promote the increasing diversification of hydrophytes.
Effects of artificial ecological islands on waterfowl
In addition to providing habitat and increasing the abundance of
macroinvertebrates, artificial islands are also important breeding
grounds for waterfowl. Macroinvertebrates provide an important food
source for waterfowl, especially during the breeding season when birds
lay eggs and broods. This may be due to the particularly high demand for
protein during egg development of waterfowl (Joyner 1980; Murkin and
Kadlek 1986).
Patra described the interactions occurring among macrophytes,
macroinvertebrates and waterfowl in freshwater wetlands as a complex
interdependency (Patra. et al., 2010). The low-water areas of the
islands may have increased the temperature of the water and/or the light
penetrating the water column, thereby promoting the growth of aquatic
plants and increasing the food source for birds.(van den Berg et al.,
1997;Zimmer et al., 2000).The increased density of aquatic vegetation
will provide a carbohydrate-rich food source for waterfowl, which is
important for the gathering and migration of waterfowl in the fall
(Baschuk 2010; Baldassare and Bolen 2006). At the same time, the higher
density of aquatic vegetation will also increase the number of habitats
available for invertebrates, which may increase the abundance of
invertebrates and further increase the abundance of bird food (van den
Berg et al. 1997; Hornung and Foote 2006). The dynamic changes of bird
communities were indirectly influenced by macroinvertebrates through the
impact of vegetation decomposition on vegetation habitats (Wilson
1990;Schneider et al., 1981;Backwell et al., 1998). Macroinvertebrates
indirectly influence the dynamic change of bird community by decomposing
vegetation to change vegetation habitat. (Wilson 1990; Schneider et al.
1981; Backwell et al. 1998; Patra. et al., 2010). In the present study,
changes in the aquatic substrate conditions directly impacted the
macroinvertebrates and aquatic macrophytes and initiated taxonomic
changes in the waterfowl assemblages.
The structure and composition of the vegetation within the wetlands also
appear to influence the distribution of waterbirds and their use of the
wetlands (Desrochers and Ankney 1986; Rehm and Baldassarre
2007).Vegetation species such as Typha spp. and Phragmites australis are
preferred by walking marsh birds because they provide dense cover and
residual vegetation that allows the birds to move along the water
surface (Baschuk 2010). For example, Botaurus commonly forage along the
vegetation/water interface, concealing themselves in the vegetation and
ambushing passing prey in the open water (Lor 2007; Rehm and Baldassarre
2007). The construction of artificial islands has created more
vegetation/water edge that have increased the number of available
foraging sites, reducing interspecies competition at these sites and
allowing birds to proliferate in the wetlands.
Vegetation species such as Typh and Phragmites australis can
reach up to 2.5 m in height in summer, providing deep, over-water
nesting habitat for waterfowl species such as some of Anatidae, as well
as marsh birds such as Podicipedidae and Fulica atra (Welling et al.
1988; Murkin et al 1997). The interspersion of emergent vegetation
provides concealment during foraging. In addition, Cyperaceae on islands
provide dry, upland habitat for nesting, which may increase the amount
of upland nesting sites (Duebbert and Lokemon 1977, Swanson and Duebbert
1989). A large number of aquatic plants provide runways for birds to fly
(Hua et al. 2009) and also play an important role in providing visual
isolation between waterfowl breeding pairs during the breeding season.
However, Typha was avoided by dabblers and divers, perhaps due to high
stem densities and large amounts of residual litter. Dense emergent
vegetation can hinder the movement of waterfowl and may also hinder
their entry. Therefore, waterfowl may have avoided Typha as it did not
provide favourable cover for thermal protection or nesting (Baschuk
2010). To sum up, it is suggested that the park should regularly control
and manage the vegetation on the island.
Habitat selection by birds is strongly related to the food distribution,
water depth and food availability. Water depth is the most important
factor that limits the use of waterfowl habitat and affects the
composition of nesting and thermal cover vegetation. Water depth limits
the feeding behavior and energy consumption of waterbirds, which affects
the availability and availability of food and determines whether the
habitat can be used (Ma et al. 2010, Murkin et al. 1997). For example,
the length of the waders’ beaks and legs limits the gate’s range in the
shallows. (Nolet et al. 2002). Waterbirds with the same water depth and
the same feeding habits need to reduce spatial niche overlap by
utilizing habitats with different water depths, while the construction
of artificial islands increases the types of available habitats (Zhang
et al. 2014). Most waterfowl used the shallow water habitats most;
Ardeidae, Charadriidae and Scolopacidae, due to their morphological
characteristics and foraging strategies, were limited to feeding in the
shallow water habitats, such as shallow water areas at the edges of
islands that were less than 20 cm deep (Shao et al. 2016). It may also
be related to the higher frequency of food (such as hydrophytes,
zooplankton, fish and other invertebrates) in shallow waters (Xia et al.
2010). Anatidae, Podicipedidae and Rallidae
generally
feed on seeds, fish, and other foods, mostly use deep water areas. The
preference for deeper water by the Aythya ferina andPodiceps cristatus was expected as these species require deep
water to allow mobility during foraging and escape (Baschuk et al.
2012). During the whole observation period, the open water area changed
within a small range, and the water depth was maintained at
approximately 2-3.5 m. Because Fulica atra individuals that did
not breed during the year also clustered on the water surface, the
observed numbers remained high. The density of wading birds was
negatively correlated with the water depth, while the density of
swimming birds was positively correlated with the water depth (Baschuk
et al. 2012). The change in water depth is an important factor for the
formation of niche differentiation and the stable coexistence of
waterbird communities (Shao et al. 2016). However, in this study area,
the deepest water depth was only approximately 3.5 m. Due to topographic
characteristics, the water depth did not change significantly, and the
difference in the distribution of the bird habitats was not obvious.
The habitat selection by the waterbirds showed a ”nesting pattern” in
the spatial gradient. Wader’s quantity of foraging was strongly related
to water depth: if there was not enough water or the substrate was too
dry, it was not easy to dig for food; furthermore, if the water level
was too shallow, there were fewer large fish and more small fish, and
the deep water area had more fish, but they were difficult to obtain
(Zhang et al. 2014). For example, universal species (Fulica atraand Anatidae) usually choose deep water habitats, while obligate species
(Charadriidae and Scolopacidae) choose shallow water habitats. The
shallow water areas of artificial islands can solve this problem,
providing more diverse water depth gradients for different birds to
forage and inhabit, making them more competitive. Shallow water at the
edge of the island with water depths of 10-20 cm can provide a series of
water gradients that attract relatively more birds and promote bird
diversity and richness (Colwell et al. 2000). An increase in surface
area can increase the diversity of swimming birds, and water depth
variation affects the diversity of waders (Hua et al. 2009). Island
construction changed the original single topography of the wide open
water and then changed the water depth, that is, the bird diversity
increased by increasing the water depth gradient and increasing the
available habitats for obligate species. By increasing the water depth
heterogeneity of the habitat, the micro-habitat diversity increased to
accommodate waterbirds with different ecological niches.
Whether a habitat is suitable for bird migration and reproduction
determines the distribution level of birds. During the construction of
water level management, the park provides a mosaic of deep and shallow
wetlands, staggers the water level of the wetland complex, and creates a
diverse range of habitats available in relatively close proximity.
Artificial islands could be used as shallow wetland habitat to promote
the use by dabblers, whereas the open-water area could be used as deep
water habitat to promote the use by divers and marsh birds, thus
creating a diverse wetland habitat. Wetland diversity will provide wide
range of available habitats to waterfowl and help and promote avian
biodiversity. The wetland park can play a positive role in protecting
the important passage of bird migration in northeast Asia.
Unfortunately, the number of visitors to the wetland park has been
increasing for three years, which has disturbed the normal habitat of
waterbirds. Therefore, the number and species of waterbirds in this
study showed a downward trend. We also recommend that wetland parks
separate tourist areas from bird breeding areas to reduce disturbance to
birds. This study was conducted only in wetlands in China, and we did
not study whether the increase in bird species was due to individual
migration from adjacent sites or other factors. We also did not
investigate whether the increase in birds came from other adjacent
wetlands or was due to increased breeding rates. These need further
study in the future. But the results clearly show that the island is
useful in increasing the biodiversity of waterfowl and
macroinvertebrates. It is not difficult to predict that over time; the
ecological benefits of artificial wetland islands will become more and
more prominent, which can provide reference for wetland restoration work
around the world.