V. P. Upadhyay, P. K. Mishra and J. R. Sahu
PEER REVIEWED
Mangrove swamps, some of the most productive wetlands on the planet, are under intense pressure from development, population spread and pollution. This report evaluates species density and distribution in a national reserve using standard ecological sampling methodology.
PEER REVIEWED
Mangrove forests are extraordinarily diverse
coastal communities anchored by salt-tolerant plants along certain tropical
seacoasts. Their distinctive aerial roots help to trap sediment, prevent
shoreline erosion, and provide habitat for a variety of sea life. Their unique
mode of viviparous reproduction (producing seeds that germinate before becoming
detached from the parent plant) allows for the rapid dissemination of viable
young plants. Mangrove swamps or forests are among the most productive wetlands
on the planet. They are also under intense pressure from development,
population spread and pollution. The mangroves of Bhitarkanika National
Park, in the Kendrapara district of Orissa, India
(located at approximately 20°40’N, 87°00’E) make up about 700 of the 2500 sq.
km of mangroves on India’s east coast. This mangrove forest is
home to one of the largest nesting colonies of sea birds in India, including a
variety of egrets, cormorants and herons, as well as an enormous diversity of
fish, shrimp, prawn, crabs and other larger marine animals including saltwater
crocodiles and sea turtles. The following report evaluates species density and
relative distribution amongst four sites within the Bhitarkanika reserve, using
standard ecological sampling methodology.
Mangrove forests are evergreen estuarine and open
systems which receive nutrients, fresh water and sediments from terrestrial
environments. Mangroves vary both in their salinity tolerance and the degree to
which salinity may be necessary to maintain their growth and competitive
dominance (6). Mangroves grow throughout the tropics wherever the average
monthly minimum temperature is 200C (4). The ecological importance
of these ecosystems for maintaining marine life, their high productivity and
role in supplying organic material to coastal marine ecosystems have been
demonstrated in many studies (12, 14, 17). The mangroves import nutrients,
fresh water and sediments from the terrestrial environment and outflow organic
matter and water to the marine and estuarine environment. Coastal mangroves
perform regulatory functions by reducing coastal erosion and flooding,
supplying nutrients, and retarding runoff. They provide vital shelter belts to
protect inland homesteads, agriculture, livestock, and other properties located
in near-shore environments. Mangroves ensure improved fish catch to coastal
communities and thus have great socioeconomic value (1). The mangrove areas
provide habitat for spawning and nursery ground for various marine species like
fishes, shellfishes, crustaceans and other invertebrates (9, 15, 21). India has
deltaic, estuarine, backwater, sheltered and insular bay types of mangrove
formations (3). About 56.7% of the total mangrove area of India is found along
the east coast, 23.5% on the west coast and 19.8% in the Andaman and Nicobar
islands (11). Mangroves are spread over an area of 214 sq. km in Orissa (11).
There is an increase of 4 sq. km compared to a 1997 assessment (10, 11) with an
increase reported in Bhadrak and Kendrapara districts. However, there is an
overall decline in quality and quantity of mangrove cover in Orissa mainly due
to shoreline changes, settlements, sediment loading and conversion for
agriculture and aquaculture (21).
We carried out a phytosociological study in the Bhitarkanika
Mangrove Sanctuary located on the eastern coast of the state of Orissa, India
at 4 sites (i.e. Dangmal, Bhitarkanika, Thakurdia and Kakranasi blocks [Figure
1]). At each site, 30 quadrats of 10 m x 10 m were sampled randomly, thus
totaling 120 quadrats with a total area of 12,000 m2. Excoecaria agallocha
was generally the densest species at all sites (Table 1). In Dangmal Block
20 species from 14 families were recorded. Heritiera fomes had the
highest density followed by Excoecaria agallocha and Cynometra ramiflora,
with Xylocarpus molluccensis and Brownlowia tersa showing the
lowest density. Bhitarkanika Block encompasses a total of 24 tree species
belonging to 13 families and has a similar trend of species density. However, Avicennia
alba and Thespesia populnea exhibited lowest density at this site.
Sixteen species were found from 11 families in Thakurdia Block. Excoecaria agallocha
showed the highest density, followed by Ceriops decandra and Lumnitzera
racemosa. Xylocarpus granatum and Kandelia candel were
observed having the lowest density. In Kakranasi Block 17 tree species from 10
families were recorded. Excoecaria agallocha and C. decandra
exhibited highest density. B. gymnorrhiza , R. apiculata, S. apetala and
X. mekongensis showed lowest density. These mangrove areas and species
provide diverse ecological and socioeconomic services to humans and the coastal
environment. Several species are used for fuel wood, fodder, and timber
purposes and also have medicinal values. The Heritiera species is known
for the durability and strength of its wood. Avicennia is used as
fodder. Indigenous medicines are prepared from Bruguiera gymnorrhiza
(diarrhea and to control blood pressure), Rhizophora mucronata (angina),
Acanthus ilicifolius (asthma, rheumatism), Lumnitzera racemosa
(herpes and itches), Cynometra ramiflora and Excoecaria agallocha
(Leprosy). Further, these plants are also used to treat various other disorders
like headache, abdominal troubles, skin diseases, etc. (21).
(Click to enlarge)
Figure 1: Location of study sites (in bold) in Bhitarkanika
Mangrove Sanctuary, on the eastern coast of the state of Orissa, India
Among the 4 study sites, the species number is highest
in Bhitarkanika block. This block, along with Dangmal, is a core area of
wildlife sanctuary. Cerbera manghas is found only in Bhitarkanika and Dangmal
forest blocks with a density of 43/ha in Bhitarkanika and 17/ha in Dangmal
block. E. agallocha is a dominant species at all sites. This species is
a characteristic mangrove associate occurring in the back mangal areas
(landward zone) (20) and has highest density at Thakurdia block among all
species and sites. This species could be a primary mangrove element at the Thakurdia
block due to peculiar topography and a higher elevation. Although tidal
amplitude in the nearby Baunsagada River is high (2-5 m) most areas do not get regular tidal inundation. It is possible that this forest block has been
through the usual successional stages of early colonizers (Porteresia coarctata,
Myriostachya wighitiana) having a characteristic assemblage of associated
species such as Salvadora persica, Caesalpinia crista, etc.
Bhitarkanika has the highest species diversity among
all study sites (5.33 species per quadrat). Statistical analysis to find the
correlation between species diversity and density established that there is a
strong positive relationship between the above parameters across all sites with
a value of coefficient of determination above 81% (r =0.911-0.932; p<0.001).
Species richness is higher (> 30 to 55 species) at latitudes between 0 and
200 (N or S) and at longitudes between 70 and 1350E (7). It
is highest in the Indo West Pacific and declines relatively smoothly from the
peak species richness of 1000 E longitude (8). No records are
available regarding the changes in the species composition in Bhitarkanika
areas. The main reason for changes in species composition is reduction in the
periodicity and quantity of fresh water reaching the mangrove environment (18).
This system is still not much affected by such factors. The floral components,
forest structure, and biomass of the mangrove wetlands are governed at the
micro level by soil and water salinity and particularly by the salinity of the
water that is present in the pores of the soil (5). The presence and luxuriant
growth of salt-marsh succulent species such as Suaeda spp. are
indicative of an increase in salinity (13) and a degrading mangrove site.
However, such luxuriant growth of Suaeda is not reported from all the
forest blocks in Bhitarkanika and therefore it could be said that an increase
in salinity is not a stress factor in the area.
The Biodiversity Conservation Prioritisation Project
(BCPP) India and Zoo Outreach Organisation, India through the CAMP
(Conservation Assessment and Management Plan) noted 12 (20%) Indian mangrove
species Critically Endangered, 40 Endangered (66%), 5 (8%) Vulnerable, 1 at
Lower Risk near Threatened (Salicornia brachiata), 1 under the category
of Lower Risk Least Concern (Acrostichum aureum) and 1 species could not
be evaluated (16). The IUCN criteria were used for assessing the status of the
species. We also categorized the mangrove species of Bhitarkanika following the
above criteria.
Species of Bhitarkanika that are Critically Endangered
are: A. alba, A. marina, B. gymnorrhiza. Species that are
listed as Endangered are: A. ilicifolius, A. rotundifolia, A. corniculatum,
A. cucullata, A. officinalis, B. tersa, C. manghas, C. decandra, H. fomes, K. candel,
L. racemosa, M. wighitiana, P. paludosa, S. apetala, S. caeseolaris, T. troupii,
X. granatum, X. mekongensis, and X. molluccensis. Species that are
identified as Vulnerable are: E. agallocha, P. coarctata, and R. mucronata.
The vegetation of Bhitarkanika and the adjacent Mahanadi
delta is diverse and seems to be among the richest of the world. The estuarine
regions of Bhitarkanika do not have species like Sonneratia griffithii,
which is available in the Sundarbans area of west Bengal. H. fomes, S.
griffithii and Aegialitis rotundifolia are endemic to the coastal
part of South Asia (2). Although the structural attributes of mangrove
vegetation of Bhitarkanika studied during the present investigation do not
indicate conspicuous signs of degradation, a conservation and management plan
should take note of differences in species number and absence of some species
from some sites to develop regeneration strategies. Further, the above
classification has established that more species are in the Endangered
category, thus requiring special management intervention to increase the
species population in this mangrove ecosystem. Research on the reproductive
life history of these species, including the knowledge of hydrodynamic
functions of the ecosystem, is immediately needed and will be useful to develop
scientific conservation and management plans for mangroves of Bhitarkanika
Sanctuary.
Table 1: Number of plants per hectare at the
study sites
|
Species
|
Dangmal
|
Bhitarkanika
|
Thakurdia
|
Kakranasi
|
Average for the area
|
Excoecaria
agallocha
|
1568
|
1772
|
5814
|
5188
|
3586
|
Heritiera
fomes
|
3267
|
3080
|
833
|
866
|
2012
|
Cynometra
ramiflora
|
1199
|
759
|
|
|
979
|
Phoenix
paludosa
|
70
|
23
|
160
|
|
84
|
Hibiscus
tiliaceous
|
260
|
83
|
|
30
|
124
|
Pongamia
pinnata
|
180
|
67
|
|
|
124
|
Avicennia
officinalis
|
80
|
196
|
50
|
1212
|
385
|
Sonneratia
apetala
|
50
|
236
|
13
|
17
|
79
|
Kandelia
candel
|
27
|
53
|
10
|
|
30
|
Amoora
cucullata
|
40
|
123
|
|
|
82
|
Rhizophora
mucronata
|
40
|
63
|
30
|
40
|
43
|
Hertiera
littoralis
|
47
|
|
|
|
47
|
Cerebra
manghas
|
17
|
43
|
|
|
30
|
Xylocarpus
granatum
|
37
|
73
|
10
|
13
|
33
|
Aegiceras
corniculatum
|
176
|
363
|
1868
|
150
|
639
|
Bruguiera
gymnorrhiza
|
70
|
60
|
|
7
|
46
|
Tamarix
troupii
|
20
|
|
|
|
20
|
Ceriops
decandra
|
30
|
103
|
4222
|
3750
|
2026
|
Xylocarpus
molluccensis
|
3
|
40
|
|
|
22
|
Brownlowia
tersa
|
7
|
13
|
|
|
10
|
Intsia
bijuga
|
|
27
|
|
|
27
|
Sonneratia
caeseolaris
|
|
73
|
17
|
1012
|
367
|
Avicennia
alba
|
|
10
|
726
|
969
|
568
|
Thespesia
populnea
|
|
10
|
17
|
7
|
13
|
Rhizophora
apiculata
|
|
20
|
|
10
|
15
|
Xylocarpus
mekongensis
|
|
33
|
|
10
|
22
|
Aegialitis
rotundifolia
|
|
|
203
|
107
|
155
|
Lumnitzera
racemosa
|
|
|
1875
|
150
|
1014
|
Avicennia
marina
|
|
|
246
|
|
246
|
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