DISCUSSION
The impact of invasive plants on native species cannot be generalised.
With the increase in invasion time, some native species will gradually
adapt to invasive plants, where they activity, forage, or build
habitats, more native birds will also find use of invasive plants
(Schirmel et al., 2016; Blackburn et al., 2011; Graves, 2019). Different
birds have different adaptive abilities, and a few birds with flexible
behaviours and a wide range of diets can adapt and take advantage of new
invasive habitats faster and better (Dylewski et al., 2019). After
Smooth Cordgrass invaded from the east coast to the west coast of the
United States, as time went on, birds, such as the Clapper RailRallus crepitans , Merida Wren Cistothrus meridae and Song
Sparrow Melospiza melodia , also began entering and reproduction
in invasive habitats (Nordby et al., 2009; Delach, 2006; Lampert et al.,
2014). In Chongming Dongtan, Shanghai, China, the invasion of Smooth
Cordgrass attracted the nonnative species of Marsh GrassbirdHelopsaltes pryeri to enter and nest in it (Ma et al., 2011). It
has also been recorded in Yancheng, Jiangsu, that the native bird, Plain
Prinia Prinia inornata , adapts to and utilises
Smooth Cordgrass and enters it
and sings during the breeding season (Chen et al., 2022). However,
invasive plants may also be potential ecological traps. Although native
birds have entered Smooth Cordgrass to reproduce, their nests are more
likely to be destroyed by mammals, and the reproductive success rate is
significantly reduced (Nordby et al., 2009; Fisher et al., 2011; Hao et
al., 2015). Moreover, the courtship calls of native bird entered Smooth
Cordgrass have changed, which greatly reduces the courtship success rate
(Chen et al., 2022).
Studies and reports have shown that the Reed Parrotbill is a bird that
is highly dependent on reed wetlands for survival, and all stages of its
life cycle are completed in reeds (Xiong et al., 2013). It prefers the
use of a mixture of old, new, and dense reeds (Boulord et al., 2011).
During the whole overwintering period, we found that the Reed Parrotbill
entered Smooth Cordgrass flocking and chirping. There used to be a large
area of reeds in that area, and the invasive Smooth Cordgrass is located
on the other side of the unsurfaced road near the coast. On the other
hand, Smooth Cordgrass may have invaded at least 40 years, dead and new
Smooth Cordgrass are mixed. Dense
vegetation can also support behaviours such as standing and singing.
We conjecture that the original
reed habitat was destroyed by human interference and the Reed Parrotbill
forced to enter the Smooth Cordgrass. The Reed Parrotbill in this area
passivily entered Smooth Cordgrass, but initiatively evolution behaviors
to adapt the Smooth Cordgrass. It can be determined that Smooth
Cordgrass provide an important overwintering place for the Reed
Parrotbill. At present, we have not found the nests of the Smooth
Cordgrass, but the native species of the Vinous-throated Parrotbill has
been nesting in the Smooth Cordgrass (the same family with the Reed
Parrotbill, and they both rely on reeds to nest) (Chen, 2020). We
speculate that if the Smooth Cordgrass continue to exist and there is no
suitable reed habitat, the Reed Parrotbill will also nest in Smooth
Cordgrass. However, it still needs further observation and research.
At present, the Chinese government mainly takes physical measures, such
as manual uprooting, mowing, shading, flooding, and burning,
supplemented by chemical measures to suppress the spread of Smooth
Cordgrass to completely remove it (Xie et al., 2019; Tang et al., 2021).
According to the experiment in the removal project area, the species and
number of birds increased significantly after Smooth Cordgrass was
completely removed, but it had a greater negative impact on macrobenthos
(Ma et al., 2017). In addition, the use of physical methods has the
disadvantage of requiring multiple removals, the cost is relatively
high, and it easily causes secondary intrusions (Zhao et al., 2022).
However, simple removal of chronic invasive plants may sometimes not be
an appropriate management measure. On the east coast of the United
States, with an increased history of invasive Smooth Cordgrass and
hybridisation with native Spartina, the Clapper Rail has become
acclimated to the new Spartina and has become increasingly dependent on
this habitat. The removal and management of Smooth Cordgrass has even
seriously threatened the normal reproduction of this species (Lampert et
al., 2014). The discovery that this specific bird Reed Parrotbill
flocking and singing in the invasive Smooth Cordgrass suggests that
coastal birds in China may be repeating what happened in North America,
and simple removal may not be suitable for the long-term invasion of
Smooth Cordgrass. We need more intensive and comprehensive research to
find suitable ways to govern Smooth Cordgrass.