Human interference and
avifaunal diversity of two wetlands of Jalpaiguri, West Bengal, India
Tanmay Datta
Department of Zoology, Ananda Chandra College, Jalpaiguri, West
Bengal 735101, India
Email: tdatta1963@gmail.com
Date of publication (online): 26 December 2011
Date of publication (print): 26 December 2011
ISSN 0974-7907 (online) | 0974-7893 (print)
Editor: Rajiv S. Kalsi
Manuscript details:
Ms # o2739
Received 28 March 2011
Final received 18 October 2011
Finally accepted 28 October 2011
Citation: Datta, T. (2011). Human interference and
avifaunal diversity of two wetlands of Jalpaiguri, West Bengal, India. Journal
of Threatened Taxa 3(12): 2253–2262.
Copyright: © Tanmay Datta 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: Associate Professor in Zoology of Ananda Chandra College,
Jalpaiguri, West Bengal. Presently working on wetland
ecology and biodiversity; specially on diversity of zooplanktons, fishes and
water birds.
Acknowledgements: This study was
financially supported by the University Grants Commission, India. I am thankful to the Department of
Zoology, Ananda Chandra College, for providing all sorts of infrastructural
support. I also thank Dr. Amal Kumar Patra, Mr. Santanu Ghosh Dastidar and Mr.
Suman Senupta who helped me in various ways during field studies and laboratory
works.
Abstract: Avifaunal diversity and abundance were studied in two wetlands
of Jalpaiguri District, West Bengal, India, in relation to eight wetland
characteristics supposedly directly or indirectly affected by human
activities. Although the climatic
and geophysical conditions of both the wetlands are almost similar, a total of
80 bird species were recorded from one wetland and the other supported only 42
species. The relationship between
habitat characteristics and community structure varied throughout the year,
suggesting that the birds respond differently to one or other habitat
characteristic depending on the season. Larger wetland size supported higher bird diversity and abundance as far
as resident and local migrants are concerned. Winter migrant density and diversity, however, reached
higher values in structurally more heterogeneous wetlands having fewer
submerged aquatic vegetation. All
these habitat characteristics become highly influenced by intense agricultural
practices in the wetland with fewer bird diversity and density.
Keywords: Habitat heterogeneity, human interference, Jalpaiguri,
submerged aquatic vegetation, waterbirds, wetlands.
For figures, tables -- click here
Introduction
Although wetlands are one of the most productive ecosystems and
most severely affected habitats next to tropical forests, they are being
neglected in densely populated countries like India. In the last century, over 50% of wetlands in the world have
been lost, and the remaining wetlands have been degraded to different degrees
because of the adverse influence of human activities (Fraser & Keddy 2005).
Wetlands harbour a large number of threatened birds, in addition
to a variety of wildlife and are vital to their conservation. At least 20% of the threatened bird
species inhabit wetlands in the Asiatic region whichis far more than the 10% of the globally threatened brids (Kumar et al.
2005). Out of 310 Indian wetland
birds, 107 species are winter migrants (Kumar et al. 2005). Migratory waterfowls are one of the
most remarkable components of global biodiversity (Li & Mundkur 2004). Waterbirds are not only the most prominent
groups which attract people to wetlands, but also are good bioindicators and
useful models for studying a variety of environmental problems (Urfi et al.
2005).
The wetlands of South Asia are facing tremendous anthropogenic
pressure, which can greatly influence the structure of the bird community
(BirdLife International 2003). The
loss of waterbird habitats through direct and indirect human interferences has
led to a decline in several waterbird populations. Therefore, it is vital to understand the underlying causes
for the decline in populations and to control these trends in order to prevent
the loss of key components of the biodiversity of wetland habitats. In this study, the diversity and
richness of waterbirds of two almost similar wetlands were analyzed, to
identify the consequences of direct and indirect human interferences.
Study
Area
Both the study sites (Gajoldoba Beel and Domohani Beel) are
perennial cut-off meanders by the left side of Teesta River in Jalpaiguri
District of West Bengal, India. Gajoldoba Beel (26.763897N & 88.597498E) with an area of about
148ha is situated by the side of the Gajoldoba barrage and about 26km
upstream to Domohani Beel (26.569688N & 88.765644E) having an area of about
52ha. The Gajoldoba
Beel is managed by the state-owned Teesta Barrage Division, Odlabari,while the Domohani Beel is privately owned. The average rainfall of this region is about 3160mm and the
average temperature ranges from 32.80C (max) to 6.90C
(min).
The Gajoldoba Beel is connected with the river Teesta,therefore, its water level fluctuates in synchrony with the river. The region experiences about 78%
rainfall during the monsoon (June to September) and only 0.98% rainfall during
winter (December to February), however, Gajoldoba Beel
experiences the highest water level during the winter season because during
that period most of the gates of the barrage remain closed. Domohani Beel, on the other hand,
becomes connected with the river Teesta only during the period of the monsoon
and the water level in this wetland fluctuates with the normal hydrological
cycle of the region.
The flora of both the wetlands is typical of this region; but
Domohani Beel is infested with more pollution tolerant aquatic plants. There is no floating vegetation in
about 50% of the area of Gajoldoba Beel, however, all parts of Domohani Beel is
infested with floating vegetation like Eichornia crassipes, Trapa natans,
Wolffia arrhiza, Nymphea odorata, Nymphea pubescens, Nymphoides cristatum,
Jussiaea repens, Neptunia natans, Hygrophila polysperma, etc. Prominent floating hydrophytes at
Gajoldoba Beel are Nymphea odorata, Nymphoides cristatum, Spirodela polyrrhiza and few patches of Eichornia crassipes. Among suspended and submerged vegetation Ceratophyllum
demersum, Utricularia flexuosa, and Hydrilla verticillata were found in both the
wetlands but Vallisneria spiralis was found only at Gajoldoba. Emergent vegetations were predominant
in many parts of Gajoldoba Beel, which were not so common in most parts of
Domohani Beel. The most notable
emergent hydrophytes were Ammania baccifera, Cyperus corymbosus, Cyperus cephalotes,
Limnophila indica, Scirpus articulatus, Potamogeton
nodosus, and Potamogeton
pectinatus. Typha latifolia was found only in Gajoldoba Beel but not in Domohani; similarly
wetland grasses like Phragmites were common in Domohani Beel but were totally absent in
Gajoldoba.
Methods
Both the wetlands were surveyed twice a month from March 2009 to
August 2010. To estimate the number of individuals of each species and to
record all sorts of birds and human activities more than 200 hours were spent
in each wetland from dawn to dusk.
Each wetland was divided into three zones (viz. G1, G2, G3 for
Gajoldoba and D1, D2, D3 for Domohani) for convenience of study considering the
physical boundaries (mainly spurs of embankment), vegetation characteristics,
bird species and human activities. The presence of humans was documented separately in each zone by
instantaneous sampling during the morning (at about 0730hr) and afternoon (at
about 1630hr) when such activities touch the highest magnitude. Direct human interference was measured
in terms of average number of persons present in a one-hour duration in a
particular zone. Besides getting
data about direct human interference from direct observations, on site queries
were made to several people to learn about the types and magnitude of indirect
human interference.
Major impacts of human interference in wetlands were
eutrophication and conversion of land. To measure these effects, six parameters, namely, water phosphate
content, percentage of floating vegetation (mostly water hyacinth), relative
abundance of submerged aquatic vegetation (in terms of percentage of submerged
aquatic vegetation present in a unit area of water), depth of water (average
value of various records of depths measured about the center of the zone),
total water covered area and heterogeneity of the zones (in terms of
differential topographical and vegetation characteristics and human use; e.g.
deep/shallow/no water zones, with floating/submerged/emergent vegetations, with
cultivated/noncultivated areas, etc.) were recorded periodically. Also data regarding magnitude of
grazing (in terms of the average number of cattle present) were collected to
predict the impact of human interference.
Bird counts were done between sunrise and 1000hr and between
1500hr and sunset, using binoculars (Olympus 10×50). On each day of observations, surveys
began from vantage points, from where most of the surface area and edge were
visible, and bird species were identified and counted (Bibby et al. 2000). Hidden and cryptic birds were flushed
by walking around the perimeter and identified. Additionally, a walk was undertaken through the emergent
vegetation zone and inaccessible parts of the wetlands were accessed by boat to
count all the birds seen or heard within the wetlands. Species were identified using Grimmett
et al. (1998), Kazmierczak & van Perlo (2000) and Kumar et al. (2005).
For every zone of both wetlands, nesting status of each species
was determined. A species was
considered nesting if its nest, eggs or young were found; and a probable nester
if it displayed behaviour consistent with nesting and there were suitable
nesting sites available. Status of
the migratory birds was ascertained as per the available literature (Ali &
Ripley 1988; Grimmett et al. 1998; Kumar et al. 2005).
The Pearson’s Correlation Coefficient (rp) was used for
the simple relationship analyses between the variables. Data, which departed from normal distribution were logarithmically transformed. Forward stepwise multiple regression
analysis was done for each period, using premonsoon (April–May), Monsoon
(July–August) and Winter (December–January),
with the number of birds as dependent variable and the characteristics of the
wetlands having simple significant relationship with the number of species in
the wetlands as independent variables.
Results
Bird assemblages
In the two wetlands, a total of 86 bird species were recorded
(Appendix 1). Eighty species were
recorded at Gajoldoba Beel, and 42 species were recorded at Domohani Beel. Out of the 80 species recorded at
Gajoldoba, 44 species were exclusive to this wetland. Of these 44 exclusive
species, 32 species (Anatidae being dominant) were winter migrants or passage
migrants, one summer migrant and 11 were residents or local migrants. Out of 42 species recorded from
Domohani Beel, only six species were exclusive to this wetland. Of these six species, only one (Vanellus cinereus) was a winter migrant and
the remaining five were residents or local migrants. Winter migrating duck avoided Domohani Beel. During this study period only twice,
for very short periods, wintering ducks (total eight in number) were found to
settle at Domohani Beel. Most of
the winter migrants at the Domohani Beel were shorebirds (mainly wagtails,
sandpipers and plovers). The
density (number per unit area) of winter migrants at Gajlodoba Beel was
significantly higher than at Domohani Beel (Table 1). However, population density of resident or local migrants
(in pre-monsoon and winter season) and nesting bird density were significantly
higher at Domohani Beel. Only the
resident/local migratory birds used these wetlands for breeding and other
purposes during the monsoon period and their density was not significantly
different (Table 1) at the two wetlands.
Types and magnitude of direct human
interferences
Local people used both the wetlands for various purposes (Table 2)
for their livelihood, fishing being most common activity. At Gajoldoba Beel the type and
magnitude of human use remained almost the same through all the seasons. At Domohani Beel also the fishing
activity was high during monsoon and winter. However, during the drier parts of the year intense
agricultural work (in almost 80% area) was observed. During winter, a considerable number of tourists and picnic
parties visited Gajoldoba, however, the birds were indifferent to such
disturbances. Hunting of birds
(with indigenous weapons) was reported only twice during winter at Domohani. During January to March, most parts of
the Domohani Beel are used for ‘Rabi’ (Boro) rice cultivation. For that purpose, the land was cleared
and leveled, ridges and embankments were built up, cultivated land was flooded
with the remaining wetland water, and insecticides were sprayed indiscriminately. As a result, the areas with intense
agricultural activities seemed as having no vegetative or topographical
heterogeneity (Fig. 1).
Relationships between bird assemblages and
human influenced wetland variables
Out of eight parameters only five were significantly correlated
with the number of waterbird species (Table 3). Direct human interference, grazing, and phosphorus content
in the covered area and the percentage of submerged aquatic vegetation were significantly
correlated with bird species numbers in all the seasons. Average depth of wetland and habitat
heterogeneity were significant in the premonsoon and winter seasons but not
during the monsoon period. Floating
vegetation percentage was significantly correlated with waterbird species
numbers only during the monsoon period.
The five significantly correlated parameters were entered in
forward stepwise multiple regression analysis separately for the resident /
local migratory birds (for all seasons), winter migrants (for winter period),
and nesting birds (Table 4). In
winter, the number of migratory birds was best predicted by habitat
heterogeneity. Habitat
heterogeneity was also the major characteristic that best predicted the
wintering migratory duck assemblage. However, wintering migratory bird density was best predicted by the
percentage of submerged aquatic vegetation. The number of resident or local migratory
birds was best predicted by the total water covered area and also it was
positively related to the presence of submerged vegetation. However, average
depth of the water body and the presence of floating vegetation had a negative
impact on the number of resident or local migrants through all the seasons
(Table 4). The nesting bird population was best predicted by the percentage of
floating vegetation covered area and the total water covered area.
Discussion
Although the climatic and geophysical conditions of these two
wetlands are almost identical, there is a considerable difference in waterbird
diversity. Winter migrants,
particularly the wintering ducks, are not attracted to Domohani Beel. However, residents and local migrants
use both the wetlands with almost the same zest. As far as type and magnitude of human interferences are
concerned, both the wetlands face almost similar problems during the monsoon
and pre-monsoon periods. In
winter, the boro cultivation, which is practised intensively only at Domohani
Beel and not attracting any migratory duck species, opens up the scope of
exploring a possible relationship between the absence of wintering ducks and ‘boro’
cultivation.
As the results suggest, direct human interferences do not impose
any real threat to the birdlife of these two wetlands. Possibly general awareness of the
people of this region and the surveillance of the Gajoldoba barrage authority have restricted people from doing any harm to the birds. However, intense agricultural
activities have changed the wetland habitat variables at Domohani Beel and that
in turn has influenced the bird life.
There exists a strong positive correlation between habitat size
and species diversity that consistently corresponds with results of other
studies in a variety of environments (Sillen & Solbreck 1977; Brown &
Dinsmore 1986; Opdam 1991; Andren 1994; Turner 1996; Babbitt 2000; Paracuellos
& Telleria 2004; Gonzalez-Gajardo et al. 2009). The only other wetland characteristic, which is
significantly correlated with waterbird species diversity during all the
seasons, is submerged aquatic vegetation. Interestingly submerged aquatic vegetation percentage was positively
correlated with avifauna diversity in the monsoon period but there exists
significant negative correlation between these two parameters during winter and
premonsoon periods. During the
monsoon period, mostly nondiving wading and dabbling birds are found in this
region and these birds prefer foraging for submerged vegetation in shallow
water, even when food is in abundance in deeper water (Holm & Clausen
2006). Wintering birds, however,
preferred to forage in water with less submerged vegetation.
As expected, a number of variables were associated with the
densities of waterbirds during the monsoon period. Water covered area is the most important criterion that
dictates bird number positively but the average depth of the water body has a
negative impact on bird numbers. Many studies have indicated that water depth affects waterbird diversity
(Velasquez 1992; Elphick & Oring 1998; Colwell & Taft 2000; Isola et
al. 2002; Darnell & Smith 2004). Non-diving waterbirds generally prefer to forage in shallow water. As the wading and dabbling birds are
the dominant waterbird groups in most regions worldwide, the greatest waterbird
diversity and density generally occur at a relatively shallow water depth
(Elphick & Oring 1998, 2003; Colwell & Taft 2000; Isola et al. 2002;
Taft et al. 2002). Foraging in
shallow water is also beneficial in terms of higher net energy intake (Kushlan
1978; Guillemain & Fritz 2002; Nolet et al. 2002; Sustainable Ecosystems
Institute 2007). During the
monsoon, when submerged areas are abundant, the greatest concentration of
waterbirds is expected in shallow wetlands like Domohani Beel.
Only resident or local migratory birds nested in both the wetlands
during the monsoon and the predominance of floating vegetation in the preferred
nesting zone supports the views of many other studies that advocate the
importance of floating vegetation in the breeding success of many waterbirds
(Owen & Black 1990; Froneman et al. 2001; Sánchez-Zapata et al. 2005). Plenty of water hyacinth dominated
floating vegetation at Domohani Beel possibly attributes to the higher nesting
density in comparison to the Gajoldoba Beel. Lower nesting density at Gajoldoba Beel may also be due to
higher water level fluctuation. During
the monsoon (nesting season) periodically most of the gates of the barrage
remain open resulting in huge fluctuations of water level. In fact the lowest water level at
Gajoldoba Beel was recorded during the monsoon period. Water level fluctuations often create “ecological
traps” and are detrimental for breeding birds (Kaminski et al. 2006). Many studies have shown that the brood
densities of waterbirds are greater on wetlands with stable water levels than
on seasonally flooded wetlands (e.g., Ogden 1991; Connor & Gabor 2006).
In this study habitat heterogeneity was found to be the key component
to attract winter migrants and more specifically the wintering ducks. Many studies have demonstrated the
importance of habitat heterogeneity in wetland bird richness and abundance
(Svingen & Anderson 1998; Edwards & Otis 1999; Fairbairn & Dinsmore
2001; Riffel et al. 2001; Zárate-Ovando et al. 2008; Gonzalez-Gajardo et al.
2009). At present Domohani Beel
does not attract the wintering ducks possibly because of its loss of habitat
heterogeneity. Intense
agricultural practices during drier parts of the season make it impossible to
maintain structural heterogeneity, both in terms of vegetative heterogeneity
and topographical heterogeneity. Thick submerged aquatic vegetation also appeared as a deterrent factor
for winter migrants. Predominance
of such vegetation at Domohani Beel possibly came as an artifact of
agricultural eutrophication. The
run-off from agricultural land enters the wetland causing an increase in the
nutrient concentrations of soil and water. The most evident results of the nutrient input are the
replacement of the primary native species with hypertrophy tolerant
species. This in turn alters the
ecosystem considerably. Nutrient-enriched water bodies thus get choked with excessive
growths of aquatic macrophytes (Roelofs 1983; Wright 2009).
In summary, species richness and bird abundance was fundamentally
affected by attributes of the size of the water coveredarea, particularly in case of local migrants and resident birds. Diversity and abundance of wintering
migratory birds appears to be affected more by habitat heterogeneity, and
preponderance of submerged aquatic vegetation played a negative role in this
regard. Loss of habitat
heterogeneity and predominance of submerged aquatic vegetation in turn appears
to be an artifact of agricultural practices. Thus agricultural practices at Domohani Beel are supposed to
be the main cause of avoidance by wintering migratory birds. However, local migrants and resident
birds are still thriving and breeding successfully in both the wetlands, which
indicate that the level of alteration and eutrophication borne out of
agricultural practices have not impaired the birdlife totally at Domohani Beel
and also it advocates the adaptability of local birds.
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