Chapter 7
AN OVERVIEW OF
SPIDER DIVERSITY IN INDIA
Rajashekhar K P and Raghavendra N
Introduction
Due to an inherent drive for survival, the life forms have adapted and evolved. These adaptations are aimed at countering environmental challenges. As there can be alternate ways of adapting to a demand, life forms have evolved varied strategies. Various taxa arise due to such diverse approaches. Biodiversity refers to such variations in life forms. A group of individuals that have identical traits and are reproductively compatible constitute a species. Diversity also arises due to the fact that earth has a diverse geography. Based on geography, a classification of biomes has been formulated. The tropical humid rain forests host the most diverse and abundant biodiversity. Such tropical humid rain forests are seen confined to landmasses of the equatorial region. Due to unique geological formations and climate, some of the rain forests are also observed in south Asia in the Western Ghats of India, Northeast India and Myanmar. The climatic conditions here support a rich flora and consequently a diverse fauna.
Myers (1988) classified areas that are rich in endemic species and are threatened by human activity as “biodiversity hotspots”. These are high priority terrestrial eco-regions for conservation. Myers et al (2000) reclassified the hotspots and identified eight “hottest hotspots”. The Western Ghats/Sri Lanka region occupies the seventh place in this list. The recent “Global 200” classification by WWF ranks the Western Ghats at twentieth place among 200 ecologically sensitive regions. As the major threat to any biodiversity hotspot is the human intervention, Cincotta et al (2000) analysed the human population dynamics in biodiversity hotspots. The Western Ghats are the most threatened on this count as this region has the highest population density per sq. km (340 km-2) among the hotspots and a positive growth rate.
A quick glance at the biological diversity
reveals that arthropods are the most diverse group of organisms. The
product of evolution over millions of years has yielded a diversity
which is “biased”. It has generated a very diverse group of arthropods
and in particular, insects. Arthropods constitute 64.5% of the
described species as compared to plants (14.3%), fungi (4.2%) and
vertebrates (2.3%) (Anonymous 1995). The arachnids constitute the second
largest class (7%) of documented arthropods and it is estimated that
8.3% of arthropods are arachnids. Thus arachnids rank second among
arthropods. Arthropods comprise more than 900,000 described insect
species and about 34,000 described spiders. The order Araneae of class
arachnida consists of spiders. The suborders mesothelae and orthognatha
consist of primitive spiders, and the suborder labidognatha includes the
more recent spiders.
The significance of insects in ecology needs no emphasis. Spiders also have a very significant role to play in the ecology by being exclusively predatory (Wise, 1993) and thereby regulate insect populations. All spiders are venomous but only a few species are venomous enough to harm humans. However, the venom of some spiders is useful in study of neuromuscular and cardiac pharmacology. It is likely that spider silk will be the material of the future as its silk is the toughest material known. The gene for the silk of Nephila maculata has been cloned and the spinning technology needs to be perfected. The coloration of spiders is varied and is paralleled only by insects (Oxford and Gillespie 1998). Spiders may also serve as biocontrol agents (Raghavendra 2001). Inspite of several applied values mentioned above, spiders have received cursory attention. In conservation efforts, often “charismatic” species like birds and mammals draw most attention and ecological significant groups like spiders are often neglected. Only tarantulas are included in Appendix II of CITES.
Review of literature on spiders
Despite the paucity of interest in spider
fauna many good accounts on spider fauna are available and information
on internet does provide an outline of the spider fauna in various
regions. Conventionally, taxonomic accounts have been descriptive with
anatomical details and at best provide hand-drawn illustrations. For a
group like spiders, there is a need to document the fauna with colored
illustrations, which will help easy identification.
International status
The distribution and diversity of spiders has drawn attention of field
workers in different parts of the world. These are often restricted by
political boundaries and therefore often appear as studies on spider
fauna of individual countries. In an effort to popularize spider biology
and educate the general public many authors have put in their efforts.
Taylor (1999) provides a good and well-illustrated account of the
diversity, beauty and intricacies of spiders. Another recent
publication by the Discovery books (Anonymous 2000) illustrates spiders
and some of the families of spiders. A compendium of the spider fauna
of North America is provided in Vincent Roth’s Field Guide (2000), (Kaston,
1978). A general description of spiders from all over the world has
been provided by Preston-Mafham and Preston-Mafham (1986). The
distribution of spiders in Rice lands of South Asia has been well
recorded and illustrated by Barrion and Litsinger (1996). Information of
diversity of spiders of Australia, Southern United States and Mexico,
Europe and Canada (Website #1) is available on internet.
Indian literature
The knowledge on diversity and distribution of spiders in India is sparse as compared to other regions of the world listed above. The most comprehensive description yet on Indian spiders is by Tikader (1987). This Handbook does not provide the region in which the spider species listed is found. A brief account of spiders is also provided by Vijayalakshmi and Ahimaz (1993). The first detailed account of Indian spiders was provided by Pocock (1900) which lists 216 spider species under 17 families. Among the other early accounts is that of Gravely (1922). Tikader (1987) has listed 1066 under 43 families. Table 1 lists the number of spider species among major families found in India. Five of them namely, Lycosidae, Salticidae, Gnaphosidae, Thomisidae and Araneidae are the predominant ones. Each of these families constitutes about ten percent or more of the spider fauna recorded by Tikader (1987). Currently there are very few workers actively involved in surveying and recording Indian spiders (Ganeshkumar and Mohanasundram, 1998). Thus a pressing need exists to explore spider diversity in the country. The following example illustrates the shortcoming. The Canadian Biodiversity Survey has recorded about 1400 species of spiders in Canada (Website # 1). Canada is known for its cold climate and a relatively limited biodiversity. Added to that, it is a vast country and many times larger than India, which renders a survey at the national level, daunting. Under these circumstances 1400 species have been documented. In comparison India has a very rich diversity, smaller area to be surveyed, has a tropical climate with a biodiversity hotspot, has manpower to conduct biodiversity surveys, but the best account so far (Tikader, 1987) lists 1066 spider species.
Table 1. Families of
spiders with ten or more species occurring in India (Tikader 1987).
Spiders in Western Ghats
For reasons mentioned above, we have set about to survey and record the
distribution, diversity and biology of spiders. Western Ghats is one of
the regions rich in biodiversity and endemic species and an account of
spider fauna in this region is not available. Our expectation of
finding a rich diversity of spiders in the Western Ghats was an educated
guess. Taxonomy of spiders is an ambiguous area and consolidated
information is lacking. Our observations therefore arrive only at the
level of family or at best genus level. One of the impediments in
preservation of spiders is the fact that unlike insects, spiders cannot
be dry-mounted. The cuticle being soft, spiders tend to shrink and
disfigure on drying. Such specimens loose their coloration and hence
impede taxonomic studies. We therefore, photograph specimens either in
the field or as soon as they are brought to the laboratory. Among other
authors, keys for identification of spiders is provided by Tikader
(1987), Barrion and Litsinger ( 1995), Roth (1993). We have in our wet
collection a total of 200 species of spiders. Identification of these
is in progress. Based on our observations we have collated the eye
patterns of spiders (Fig. 1) which can also be used as a key for
identification.
The occurrence of major spider families and their relative abundance in
the central Western Ghats as revealed by our observations is listed
below. Our surveys at present are confined to arboreal spiders and does
not include epigeal and litter spiders. The major representation is from
the families Oxyopidae, Thomisidae, Araneidae and Salticidae. The
Western Ghats regions are characterized by heavy rains averaging 3000 mm
between June – August. We have also observed seasonal fluctuations in
relative abundance.
Table 2. Relative abundance of major
arboreal spider families in the central Western Ghats.
We took this as baseline data and extended our studies to observe whether the agricultural activities, which are an important factor in influencing the fauna do influence the spider fauna. Western Ghats are fragmented in many places due to agriculture including horticulture. As compared to the diversity of spiders shown above the garden crops arboreal spiders showed lesser diversity. The relative abundance of spiders among garden crops (Poornima, 2001) is as shown in Table 3.
Table 3.
Seasonal variation of spiders
The density of spider was high during the pre-monsoon season and
gradually decreased during monsoon. On some plants like Areca and
Ceasalpinia they were abundant in the month of December and their
number decreased by end of January. There was considerable variation in
the members of Araneidae during rainy season and winter. In
Areca plant Gasteracantha species are abundant, which
suggests that species prefer specfic habitats. Flowering plants such as
Nyctanthus have high density of spider belonging to Thomisdae.
The foothills of Western Ghats are often converted to monoculture
gardens like Cashew. In an study concentrating on diversity of spiders
on cashew, we collected 156 specimens and the major families of spiders
observed were: Araneidae, Salticidae, Thomisidae, Oxyopidae (Raghavendra,
2001). Except Araneidae the others are non web-building hunters. There
was also a seasonal fluctuation in their density with maximum diversity
in spring. This corresponds to the seasonal occurrence of pests on the
cashew crop (Devasahayam and Sunderaraju, 1987). Comparatively the
diversity of spiders is less in the garden crops. A reason for this
decline is use of pesticides. Crab-spiders (Thomosidae) are more in the
garden crops and they are lesser in pristine forest areas. Members of
Hersilidae, Heteropodidae, Clubionidae, Palpimonidae, Lycosidae,
Tetragnathidae, are absent in the garden. Thus by comparison it is
evident that conversion of pristine forests to agricultural land reduces
diversity of spiders.
Recommendations
1. Faunal diversity surveys and conservation efforts are often
concerned with charismatic species like higher vertebrates. We need to
pay attention to other faunal components also, like spiders because of
the significant ecological role the play.
2. In order to formulate conservation measures a baseline data about a
particular group is needed. For many of the arthropods such information
is lacking and therefore needs to be generated.
3. Deforestation selectively reduces diversity and density of fauna
like that of spiders. Additionally, use of pesticides may dramatically
influence their diversity. It would be worthwhile designing insecticides
such that they are less toxic to spiders.
4. Spiders appear to be promising candidates and can be used in
biological control of pests.
Spiders exhibit stunning morphological diversity and coloration. An ant-mimicking spider of the genus Myrmarachne belongs to salticidae. The salticid on top right panel shows brilliant coloration. The coloration of spiders comes both from colored hairs or pigments incorporated into the cuticle or placed below it. Orb-weaving Araneidae members have large abdomen and Oxyopidae are characterized by spiny hairs on the body. Oxyopidae (Lynx spiders) and Salticidae (Jumping spiders) members are hunters and do not spin webs.
References
Anonymous (1995). Global Biodiversity Assessment, Cambridge University Press, pp1140
Anonymous (2000). Insects &Spiders: An explore your world handbook. pp
192 Discovery Books London
Barrion A T and J A Litsinger (1995) Riceland spiders of south and southeast Asia, CAB International, Cambridge, UK.
Cincotta R P, Wisnewski J, Engelman R, (2000) Nature 404: 990-992
Devasahayam S and Sunderaraju D (1987) CPCRI Annual Report, 144-147
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Website1: http://www.biology.ualberta.ca/esc.hp/bsc/news18-1/spider.htm
Acknowledgement
Research funding support by the American Archeological Society, USA, to Raghavendra N is gratefully acknowledged.
Fig. 1. Variations in eye patterns cahracteristic to spiders belonging to different families