Habitat characteristics and odonate
communities at selected sites used byMortonagrion hirosei Asahina (Zygoptera: Coenagrionidae) in
Hong Kong
D.J. Stanton 1 & J.A. Allcock 2
1,2 Asia Ecological Consultants Ltd., 127
Commercial Centre, Palm Springs, Yuen Long, Hong Kong
Email: 1 davidstanton@asiaecol.com.hk(corresponding author), 2 jallcock@asiaecol.com.hk
Date of publication (online): 26 December 2011
Date
of publication (print): 26 December 2011
ISSN
0974-7907 (online) | 0974-7893 (print)
Editor: Albert
Orr
Manuscript details:
Ms
# o2891
Received
26 July 2011
Final
received 08 November 2011
Finally
accepted 17 November 2011
Citation: Stanton, D.J. & J.A. Allcock(2011). Habitat
characteristics and odonate communities at selected sites used by Mortonagrion hiroseiAsahina (Zygoptera: Coenagrionidae) in Hong Kong. Journal of Threatened Taxa 3(12):
2242–2252.
Copyright: © D.J. Stanton & J.A. Allcock 2011.
Creative Commons Attribution 3.0 Unported License. JoTT allows unrestricted use
of this article in any medium for non-profit purposes, reproduction and
distribution by providing adequate credit to the authors and the source of
publication.
Author Details: David J. Stanton and John A. Allcock are both professional
ecologists and are both associates at Asia Ecological Consultants Ltd. based in
Hong Kong. They conduct surveys and monitoring for a wide range of faunal
groups mostly in Hong Kong and also overseas.
Author Contribution: DJS
and JAA both participated in the design of the study, acquisition of data,
analysis and interpretation of data, and drafting of the manuscript. Both read
and approved the final manuscript. Both the authors have contributed
equally to this paper.
Acknowledgements: The authors would
like to thank Paul Leader and Graham Reels for their extensive comments and
constructive advice on early drafts of this report. We would like to also thank
three (anonymous) reviewers for providing critical reviews and greatly
improving this manuscript. We
would also like to thank Tony Nip for providing the local language abstract.
Abstract: Mortonagrion hirosei, a Near Threatened species, is a small
damselfly recorded from several isolated sites across its entire range in
eastern Asia. Previous research
has indicated a strong affinity for brackish wetlands, including reedbeds and
marshes, where potential predation or competition by other odonates is
reduced. Results from surveys
conducted in Hong Kong during 2009–2011 provide information on the
habitat at a number of sites occupied by M. hirosei and report on the presence of
populations in mangrove and mangrove-mosaic habitats as well as brackish marsh,
often in association with a diversity of other odonates. Information is also provided on two
previously unreported sites in Hong Kong. These new findings indicate that the
species uses a greater diversity of habitats than the odonate-poor Phragmites reedbeds
in which it has been well-studied in Japan, and consequently may be more
widespread than previously supposed. Given that coastal habitats are threatened throughout its range, it is
hoped this broader understanding of the species’ habitat requirements will
encourage others to explore other coastal sites and to aid in its conservation.
Keywords: Brackish wetlands, coastal habitat loss, Hong Kong,
mangrove, Mortonagrion
hirosei, odonates.
For
figures, images, tables -- click here
Introduction
Mortonagrion hirosei is a small damselfly (Images 1 & 2)
occurring in coastal areas of eastern Asia including Japan, Hong Kong, Taiwan
and Korea (Fig. 1). There is also
an inland record from Guangdong, China. It is currently listed as Near Threatened in the IUCN Red List of Threatened Species (Wilson & Reels 2011) because it is
known from only a very small number of sites and there has been a decline in
habitat area at the known Japanese sites. There is an ongoing loss of coastal habitats within the species’ known
and expected range in eastern Asia that presents the major threat to the
species’ survival.
Studies conducted in southern Japan found
that M. hirosei is extremely site-specific, mature adults
moving on average only 3.3m per day in the reedbed understorey; adults may not
move more than 100m in their entire life-time (Watanabe & Mimura 2003,
2004; Mimura & Watanabe 2006). The species is a weak flyer; it adopts a sit-and-wait tactic when
searching for prey or mates, and adults do not leave the site where they
emerge, remaining in the reed community throughout their entire adult life
(Watanabe & Mimura 2003, 2004). Given this low dispersive mobility, the species cannot easily disperse
to new locations and the long-term protection of the species relies upon the
protection of known locations and/or provision of suitable alternative habitat
within this small dispersal range.
Wilson & Reels (2011) recommend that a
more comprehensive understanding of habitat requirements is important for
conservation of the species. Preliminary observations by the authors in Hong Kong suggested that this
species actually uses a wider range of habitats than has been described at the
well-studied sites in Japan. This
study aims to document habitats present at a variety of sites in Hong Kong and
investigate whether sites used by M. hirosei generally have a depauperate odonate
community, as has previously been suggested. Such observations from Hong Kong may be applicable to
potential sites elsewhere in eastern Asia.
Distribution and
habitat selection at known sites
The species was
described in 1972 based on specimens collected in brackish Phragmites reedbeds in Hinuma, Ibraraki in
Japan. It was thought to be
endemic to Japan until another population was discovered at Mai Po Marshes
Nature Reserve in Hong Kong in 1991 (Reels 1994; Wilson 1994; Wilson 1995). This discovery provided additional
evidence that brackish reedbed is an important habitat for the species. Subsequently the species has since been
found at other locations in Hong Kong, including Sai Sha, Double Island, Hong Kong
Wetland Park (Tin Shui Wai), Luk Keng, Mai Po Marshes, Nam Chung, Sam A Tsuen
and Sha Po (Wilson 1997; Wilson 2004; Tam et al. 2008).
It was first recorded in
Taiwan in 2005, at a coastal wetland in Wu Gu near Taipei (Lin & Chen
2006), and has also been reported from Korea in paddy fields between 1997 and
2006 (Bang et al. 2009). Other
than Hong Kong and Korea, M. hirosei has not been recorded along the coast of
mainland eastern Asia, but it is likely that other populations remain
undiscovered in southern and eastern coastal China. There is also a single 2005 record from an inland site in
Gutoushan, Guangdong, China (Wilson & Reels 2011).
Phragmites reedbeds form an important component of
the habitat at the known sites on the east coast of Honshu in Japan, and
Watanabe et al. (2008) state that “Mortonagrion hirosei … inhabits the understory of dense reed
communities”. Conservation of the
species in Japan has therefore mainly involved the protection of brackish
reedbeds as well as the creation of additional brackish reedbed habitat. Watanabe (2007), Watanabe et al.
(2008), Iwata & Watanabe (2009) and Teramoto & Watanabe (2009) discuss
the colonisation of these created reedbeds by M. hirosei.
IUCN gives the main
habitat association as brackish marsh, including mangrove (Wilson & Reels
2011); the specific addition of mangrove in the IUCN assessment was based upon
our observations of M. hirosei in the Mai Po Marshes intertidal mangrove
in 2008 (G.T. Reels pers. comm.). At Luk Keng in Hong Kong, M. hirosei was reported to occur in water chestnut Eleocharis dulcis and Phragmites reeds, but was three times more abundant
in the latter habitat (Cheung 2008). The presence of an inland site in Guangdong, China has recently been
reported in Wilson & Reels (2011) but details of the habitats have not been
published at the time of writing.
Association with
other odonates
Because of its small size, M. hirosei is at risk of predation from other
odonate species in both adult and larval stages (Lin & Chen 2006; Matsu’ura
& Watanabe 2006; Iwata & Watanabe 2009). This risk may be aggravated by the adults’ weak flight and
bright coloration (Watanabe & Mimura 2003), thus the presence of certain
other odonate species might be expected to influence the presence and relative
abundance of M. hirosei at a given site. Indeed, Iwata & Watanabe (2009) have suggested that
predation pressure, particularly from Ischnura
senegalensis,
may be partly responsible for the importance of brackish reedbeds as a habitat,
as the reeds provide cover for adults and the brackish water is unsuitable
breeding habitat for most potential predators.
Methods
Observations of
habitat characteristics and odonate communities at selected sites in Hong Kong
The following paragraphs describe the
habitat characteristics and odonate assemblages at some of these locations,
including Mai Po Marshes, Sam A Tsuen and Sai Sha (Fig. 2). Similar details are also provided for
four locations which were not identified by Tam et al. (2008). These observations are based upon site visits conducted
between May 2009 and June 2011. The intention of these visits was to document the abundance of adult and
teneral M. hirosei, the habitat present (including plant species
present) and the odonate community at each location. All other odonates seen during survey were recorded. These species lists are not exhaustive
and it should be noted that other species may also be present. Any of the species recorded may prey on
or compete with M. hirosei, but to our knowledge predation has only
been observed by I. senegalensis (Watanabe 2007; Watanabe et al. 2008;
Iwata & Watanabe 2009) and Ceriagrion
auranticum (G.T.
Reels pers. comm.). No
observations of larvae were made. Visits did not follow predetermined
transects, and the frequency and duration of visits varied between locations
according to accessibility and the area covered. Maximum counts in the text are not directly comparable
between sites, but are included to give an impression of overall M. hirosei numbers at each site.
Nomenclature for odonates follows Wilson
(2004) and for vegetation, follows the arrangement used by the Hong Kong
Herbarium (http://www.hkherbarium.net/herbarium/frame.html).
Results
The following paragraphs describe the
findings at the six locations surveyed between May 2009 and June 2011 and
results are summarised in Table 1.
Mai Po Marshes
Nature Reserve - Gei
wai
Mai Po Marshes Nature Reserve (MPNR) (22029.7’N
& 1140 2.5’E) comprises a series of brackish “gei
wai”—ponds operated in a traditional manner for the culture of shrimps
(usually Metapenaeus ensis). Commercial shrimp culture no longer occurs within these gei wai, and the
pond operation is now managed by WWF-Hong Kong (WWF-HK) for the benefit of
wildlife. Gei wai are connected to the sea and are periodically
drained and flooded with sea water through sluice gates, creating brackish
water conditions. Most of the gei wai at MPNR contain Phragmites australis reedbeds and/or stands of mangroves. To the seaward side of the reserve is
an area of intertidal mangroves, adjoining extensive intertidal mudflats in
Deep Bay. MPNR is included in the
core of the Mai Po Inner Deep Bay Ramsar Site.
The first Hong Kong records of M. hirosei came from the MPNR gei wai in 1991, when
individuals were caught by Malaise traps set in one of the Phragmites reedbeds (Reels 1994). This study found M. hirosei to be one of the most numerous species
trapped in these Malaise traps, suggesting a high density in this habitat. The reedbeds used for Malaise trapping
were visited regularly during the 2009–10 visits, and no M. hirosei were recorded. Malaise trapping was also
conducted by WWF-HK at the same site in 2009–10, but M. hirosei was not recorded (Katherine K.S. Leung in
litt.). The nature of this reedbed
has changed since 1991; notably substrate levels have been lowered in much of
the reedbed to prevent colonisation of terrestrial vegetation. Other odonates observed in
2009–2010 at this brackish Phragmites reedbed included C. auranticum, Paracercion
melanotum, I. senegalensis, Anax parthenope, Brachythemis
contaminata, Brachydiplax chalybea, Orthetrum sabina,
Pantala flavescens and Tholymis tillarga.
Elsewhere in MPNR, the species has also
been recorded from vegetated bunds of both freshwater and brackish ponds and
intertidal mangroves (WWF-HK 2010). The only individual recorded in the gei wai of MPNR during the 2009–10 visits
was a single male seen in May 2010, on emergent vegetation composed of
individual Phragmites stems and mats of grasses on the edge of
a deep water channel. This
vegetation was not particularly tall or structurally dense and this individual
was in the open, perched on semi-submerged vegetation. Much of the potentially suitable
habitat in the gei wai cannot be easily accessed, though there is some limited
boardwalk access through some sections. No M. hirosei were observed but it is possible that
populations remain in the gei wai that were not visited in 2009–10.
Mai Po Marshes -
Intertidal Mangrove (Image 3)
A strip of mangroves (22029.7’N
& 11401.9’E) varying between 500 and 1000 m wide along the
northern boundary of the Reserve, separates the gei wai in MPNR from the open
intertidal mudflats of Inner Deep Bay. Dominant mangrove species include Kandelia obovata,
Acanthus ilicifolius, Aegiceras corniculatum, Avicennia marina, Bruguiera
gymnorrhiza, Excoecaria agallocha and Heritiera littoralis (WWF-HK 2006). This mangrove community continues around the landward side
of the intertidal mudflats in the whole of Inner Deep Bay, but is difficult to
access except through a network of floating and fixed boardwalks extending from
MPNR. Although the intertidal
mangroves are immediately adjacent to the gei wai at MPNR, these are treated
here as a different location because the habitat characteristics differ significantly
from those present in the gei wai. The low dispersive ability of M. hirosei means that individuals would not be able
to disperse far into the intertidal mangrove from known populations in the gei
wai.
Ecological surveys in the intertidal
mangroves during 2005–06 to assess the impacts of an extension to the
existing boardwalk access did not record M. hirosei (WWF-HK 2006). The species was first observed in this location by the
authors in 2008, and has also been recorded by WWF-HK (WWF-HK 2010). The maximum counts during the 2009-11
visits were 32 in July 2009 and 50 on 14 May 2011. Observations have been made on both rising and falling
tides, when bare mud is revealed at the base of mangroves.
The intertidal mangroves do not contain
any significant-sized stands of Phragmites reedbeds, but small stands (up to
approximately 2m2) of reeds are patchily located on the landward
side of the mangroves. Most observations of M. hirosei in the intertidal mangroves are distant
from any Phragmites reedbeds, with individuals observed up to
700m from the nearest reedbeds (located within the gei wai). This is seven times the estimated
maximum dispersal distance of an individual M. hirosei, suggesting that individuals seen cannot
have come from the Phragmites reedbeds. Within the intertidal mangrove, most
individuals were observed either at the interface between taller mangrove and
shrubbier A. ilicifolius, or where openings in the tall mangrove canopy
create areas of dappled sunlight, often perching on mangrove pneumatophores
(Image 4) or stems up to approximately 0.2m from the exposed substrate.
Individuals are not evenly distributed across the habitat but seem to be rather
patchily distributed, particularly in areas where uneven substrate ensures that
shallow water remains during low tide and where there is a higher density of
pneumatophores. Coupling has often
been observed in this habitat, indicating that egg laying and larval
development also take place in the intertidal mangrove system.
Numbers of other odonates are generally
low in these intertidal mangroves. Other odonate species recorded along these
boardwalks on the same dates as observations of M. hirosei included I. senegalensis, P. flavescens, O. sabina, Rhyothemis
variegata, Pseudothemis zonata and Trithemis aurora. Small numbers of I. senegalensis(no more than five on any visit) were recorded in similar habitats to M. hirosei,
but the other odonate species were observed in open areas along creeks or at
the edge of the taller mangrove stands.
Sai Sha
Small numbers of M. hirosei were recorded from a 200m section of
boulder-strewn coastline of the Sai Sha Peninsula (22026.0’N &
114015.7’E) between May and October 2009 and subsequently in 2010
and 2011. These individuals were
seen around shallow, brackish pools established in the shingle of the
supralittoral shore amongst the backshore vegetation (Image 5). The pools, no more than 0.2m deep, are
thought to have been created from a combination of exceptionally high tides,
sea spray, wet season rainfall and run-off from adjacent abandoned paddies. Vegetation surrounding the pools is
composed of shrub mangroves (K. obovata), coastal trees (Hibiscus tiliaceus) and grasses such as Zoysia matrella and Cynodon dactylon. Long-abandoned paddies on the landward side have developed into marsh
dominated by grassy vegetation but no M. hirosei were observed there. No Phragmites reeds are known to occur in the area.
This location is situated approximately 9km from the closest known M. hirosei population (at Sam A Tsuen in the
northeast New Territories).
Adult and teneral individuals of M. hirosei were regularly recorded during
2009–11 visits perched up to 0.2m from the ground on bare branches of low
shrubby mangroves or on the stems of grasses around the fringes of the brackish
pools. The maximum number of
individuals recorded was 27, including both adults and tenerals, during June
2011. Tandem pairs have also been
observed at this location. Other odonates were also regularly observed in the
coastal vegetation, including C. auranticum, Acisoma panorpoides, Orthetrum luzonicum and Orthetrum
poecilops. Orthetrum poecilops is listed by IUCN as Vulnerable (Wilson
2009) and is associated with brackish water habitats, especially mangroves.
Sam A Tsuen
Observations of M. hirosei at Sam A Tsuen (22030.9’N
& 114016.3’E) came from an area of brackish marsh derived from
abandoned paddies. This location was visited less frequently than the others
(three visits during 2009–11), and M. hirosei was observed only once in August 2009. Individuals of M. hirosei observed at the site were seen flying in
an area of grasses up to 0.1m tall, with scattered small individuals of the
mangrove-associated fern Acrostichum aureum. Other parts of the marsh contain extensive areas of grasses and sedges Cyperus spp. up to 0.3m high, but no M. hirosei were observed in this vegetation. Water within the marsh was mostly
shallow (less than 0.05m) during visits, with a deeper channel (0.3m) draining
through the marsh. The extent and
frequency of flooding at high tide is not known. A fairly open stand of
mangroves (including K. obovata, A. marina, E. agallocha and A. corniculatum) is present on the seaward side of the
marsh, approximately 100m from the location of the observations, but no Phragmites reeds are known at the site.
Other odonates recorded at the site were Agriocnemis femina, C. auranticum, I. senegalensis, A. panorpoides, Diplacodes
trivialis, Neurothemis tullia, O. sabina and P. flavescens; these species were present generally in
low numbers (up to five individuals of each species recorded), with the
exception of P. flavescens, which has been was seen in large
gatherings (up to 70 individuals) on the fringes of adjacent woodland during
surveys.
Tai O
In May 2009, another population of M. hirosei was discovered in Tai O, Lantau (22015.4’N
& 113051.9’E). This
is the first known record for Lantau Island and is located 29km southwest of
the nearest known population in Hong Kong. This is an area of former salt-pans which have been abandoned
and are now subject to regular tidal inundations. Vegetative succession on these salt-pans has created a
mosaic of microhabitats, including brackish marsh, deeper brackish pools and
mangrove (Image 6). Low numbers ofM. hirosei (up to two individuals) were recorded on
visits in 2009–10. Most
sightings were made in an area permanently under water of varying depths
(0.1–0.5m), often in the shade of larger mangrove specimens (A. ilicifolius, A. corniculatum,
A. marinaand K. obovata). Individuals observed were perched either on mangrove pneumatophores or
on grasses (e.g. Sporobolus virginicus), at about 0.2m from the water’s surface,
usually in the shade of a mangrove shrub. Whilst Phragmites is present in the area, it does not form
large stands or reedbeds, but is interspersed within the vegetation of the
brackish marsh, approximately 50m from the area where M. hirosei was recorded.
Nineteen other species of dragonfly were
recorded near the perching locations of M. hirosei including high numbers of I. senegalensis (100+ during each visit), the most
commonly occurring odonate at the site, along with moderate numbers of O. sabina and D. trivialis.
Yung Shue Au
The most recently discovered population ofM. hirosei is at Yung Shue Au (22032.7’N
& 114014.8’E) on the northeastern coastline of Hong Kong SAR in
July 2011. Seven individuals were
found at this site, occupying habitat similar to that observed at Sam A Tsuen,
with an area of short grass and sedge (up to 0.1m in height) which was
inundated at high tide (Image 7). This population is located approximately 3.9km from the nearest known
sites, at Sam A Tsuen and Luk Keng. Other odonate species present were C. auranticum, I. senegalensis, N. tullia and O. sabina.
Discussion and Conclusions
Habitat
requirements of M. hirosei
Until recently Phragmites reedbeds were considered to be the primary
habitat for this species. While Phragmites reedbed is an important habitat at some
Hong Kong sites, the observations on the 2009–11 surveys indicate that at
some locations in Hong Kong Phragmites is not an important component of the
habitat, and may not even be present. Habitats at most locations visited for this study were dominated by
grasses and/or mangroves, and the species was recorded in pure mangrove stands
in Inner Deep Bay, several hundred metres from the nearest reedbed or marsh
habitat. Considering the weak
flight of the species, the distance of these observations from the nearest
reedbed or marsh suggest that M. hirosei breeds in mangrove habitats in Hong
Kong. This conclusion is
strengthened by the observation of tandem pairs in the Mai Po Marshes
intertidal mangroves. Wilson &
Reels (2011) note that, despite the presence of adults, larvae have never been
recorded in mangrove habitats. Surveys for larvae or exuviae were not conducted at the sites studied,
however, and therefore successful breeding in mangrove habitats remains
unproven. Larval studies of mangrove and other habitats would provide further
understanding on the habitat requirements of this species.
Hong Kong is at the southern end of the
known global distribution of M. hirosei, and mangroves are to be found on much of
the intertidal soft shore. Mangrove diversity and abundance is much reduced in
more northern latitudes (Lee 2002) and in Japan mangroves are mostly limited to
the southernmost prefecture of Okinawa (ISME 2010). The known sites for M. hirosei are situated on the east of Honshu, where
mangrove does not occur. Reedbed may be the dominant vegetative community at
these latitudes, explaining the apparent preference of M. hirosei for this habitat. Thus latitudinal and climatic
differences may ultimately account for the difference in observed habitat
preferences between Hong Kong and Japan.
The most important factor influencing the
distribution of M. hirosei in Hong Kong appears to be the hydrology
of the site. All locations visited
at which M. hirosei was recorded contain shallow brackish
water, generally less than 0.2m deep. The site with deepest permanent water (at Tai O) contained water up to
0.5m deep. Deeper water may be
experienced at high tide in some locations (for example in the Mai Po Marshes
intertidal mangrove) but for most of the time the water does not reach this depth. No Hong Kong locations have yet been
discovered at which only fresh water is present; M. hirosei has been reported at MPNR on the bunds of
freshwater ponds (WWF-HK 2010), but the individuals involved may have strayed
from nearby gei wai.
Vegetation structure may also be important
in determining the suitability of a habitat. The species is known to use a
sit-and-wait strategy when searching for prey or mates, and Watanabe &
Mimura (2003, 2004) report that M. hirosei are often recorded perching at an average
height of about 20cm above the water or the ground surface within the reedbed
site studied. Similar behaviour was observed in all habitats during visits in
2009–11, with individuals observed perched on mangrove pneumatophores,
grass stems or the lower branches of individual mangrove plants at a similar
height.
Relationships with
other odonates
In Japan, M. hirosei occurs in dense, brackish reedbeds where
few other odonates occur (Watanabe & Mimura 2003) and is susceptible to
predation in both adult and larval form by other odonates, particularly I. senegalensis (Lin & Chen 2006; Watanabe &
Matsu’ura 2006; Watanabe et al. 2008, Iwata & Watanabe 2009).
At the Hong Kong locations visited during
this study, M. hirosei was sometimes present in habitats with a moderate
diversity and abundance of other odonates, both Zygoptera and Anisoptera, which
could be potential predators. These include I. senegalensis, which was observed at all locations in
high numbers. The 1991 study of Phragmites reedbed at Mai Po, which resulted in the
first records of M. hirosei in the territory, also recorded a total
of 10 odonate species from Malaise trapping (Reels 1994), and the true
abundance may have been underestimated in this study because Malaise trapping
tends to favour smaller, weaker flying odonates such as damselflies (Zygoptera)
and under-represent larger dragonflies (Anisoptera) (Glotzhober & Riggs
1998). Whilst it is acknowledged
that other faunal groups could potentially predate the various life stages of M. hirosei, this has not been investigated.
Although M. hirosei populations in Hong Kong are apparently
able to survive in the presence of potential predators, this predation may lead
to lower population densities. High densities of this species have been
reported in reedbeds in Japan, for example 22 adults per m2 as
reported by Teramato & Watanabe (2009) and comparable numbers by Watanabe
& Mimura (2003, 2004) and Watanabe et al. (2008). Although systematic studies of population densities were not
part of this study, it was clear that overall numbers at some Hong Kong sites
were significantly lower than those reported from Japan. Densities were lowest in habitats which
also supported a higher abundance of other odonate species which may indicate
that, while M. hirosei is tolerant of the presence of other
species, these may limit the overall population, either by predation as
suggested by Watanabe & Mimura (2003) or through competition for resources.
Potential
populations elsewhere in eastern Asia
One of the recommendations from IUCN is
that the coastal breeding sites of this damselfly should be protected (Wilson
2006; Wilson & Reels 2011). For this to occur, it is important that these sites are identified. With a known global distribution
including Japan, Taiwan, Korea, and southern China, it can be assumed that this
species is likely to be under-recorded along coastal areas of southern and
eastern China. These known sites
are separated by hundreds of kilometres of coastline or sea, and it would
appear unlikely that such a weak-flying species would be able to readily
colonise such widely-separated locations. When compared with Hong Kong and Japan, the Chinese coast has
historically had relatively few observers recording odonates and the small size
and biology of M. hirosei (including its weak flight and
sit-and-wait foraging strategy) make the species easy to overlook.
Misconceptions about its habitat requirements, especially the lack of awareness
of mangrove as a potential habitat in the southern part of its range, may also
have contributed to the species being overlooked at other sites in eastern
Asia. Searches of coastal marsh
and mangroves along the coastline of southern and eastern China may result in
the discovery of more populations. The presence of other odonates which may be potential predators or
competitors clearly does not preclude the survival of M. hirosei.
Conservation and
threats to global populations
Throughout eastern Asia, coastal habitats
are under extensive pressure for development and reclamation (Lin & Chen
2006; Wilson 2006; WWF-HK 2010). In Japan, the extent of reedbeds has been substantially reduced by river
alterations and the destruction of wetlands (Watanabe & Mimura 2003). Coastal habitat in China is similarly
being lost to city development, reclamation and dam construction (WWF-HK 2010),
and the same is happening in Korea (Cho 2007). For example, in coastal Shenzhen, adjacent to the species’
Hong Kong stronghold at Mai Po, there has been significant coastal habitat loss
in the past 20 years through development and reclamation (Ni & Qin
2003). Populations of M. hirosei throughout easterm Asia may be declining
and becoming increasingly isolated as coastal habitat is lost.
Habitat loss could also occur indirectly
through natural processes such as sedimentation. Mudflat levels in Inner Deep
Bay, where the species has its stronghold in Hong Kong, have risen by 0.3m
since the 1980s (WWF-HK 2006). Continued sedimentation may eventually prevent land from flooding at
high tide, resulting in loss of intertidal habitats. Local extinctions may result from highly localised events
such as extreme high tides washing through a small area of suitable habitat
(for example at Sai Sha, where apparently only a small population is present in
a narrow strip of habitat along the coast) or through changes in predation
pressure. The isolation of
populations as a result of coastal development, combined with the weak flight
ability of the species, may mean that suitable sites cannot be recolonised once
the population has been lost by stochastic events. At a larger scale, populations of a whole region may be at
risk from events such as the devastating tsunami that struck Japan during March
2011.
Many of the Hong Kong sites are isolated
by both developed areas and physical geography, and have few ecological
linkages suitable for a weak-flying species to exploit. Protection of known sites is therefore
important, so that these can ensure the continued survival of the species. Currently only four of the known
locations in Hong Kong are protected by legislation; at Mai Po Marshes Nature
Reserve (including the intertidal mangroves of Inner Deep Bay), Hong Kong
Wetland Park, Double Island and Luk Keng. Other sites are on land that may be at risk from development and/or
neglect. Suitable habitat management would also be beneficial in providing
increased habitat area or providing corridors to link populations. In Japan, research projects are being
conducted to look at reedbed re-establishment programmes and the maintainence
of dense reedbed communities in order to preserve this species, by reducing
habitat loss and fragmentation (Watanabe et al. 2008; Iwata & Watanabe
2009; Teramoto & Watanabe 2009). Research so far has shown that the species is able to readily colonise
newly created habitat if this is within the small dispersal range (Watanabe et
al. 2008; Iwata & Watanabe 2009; Teramoto & Watanabe 2009). Habitat management at Mai Po Marshes
and Hong Kong Wetland Park may permit the enhancement of habitats in these
locations, but at present no other known sites in Hong Kong receive active
habitat management.
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