Influence of host plant diversity on the survival and abundance of certain shoreline insects of a tropical lake of Bhopal, India.
Dr. Asma Ali (Ecologist)
ABSTRACT: (1) Plant-insect interaction is a vital component of community structure in any aquatic ecology, particularly with respect to predation, competition and resource spiraling. Availability of aquatic plants can potentially influence every aspect of the life of marginal invertebrate communities of lakes, such as their life cycle, choice of habitat and behavior.
(2) To investigate the importance of host plant diversity for marginal insects of a fresh water body called as Upper Lake, Bhopal, India, population densities of the marginal insects of order, Hemiptera, Coleoptera, Odonata, Orthoptera, Diptera and Strepsiptera were recorded during two years of study period, at four different sampling stations of the large man made tropical Lake.
(3) The data of the study suggest that, most of the insect species had significantly higher survival rate when they were feeding on several host plants, as compared to those, who fed on specific plant species. In the present study it was observed that an increase of plant matter in a subsurface ecosystem indicated significantly high abundance and diversity of aquatic as well as terrestrial insect population.
(4) The present data also demonstrate that loss of shoreline vegetations may reduce the productivity of both aquatic and terrestrial insects, possibly by adversely affecting the amount of allochthonous material entering the shoreline along with microclimate conditions. Although data on plant quality were not available in this study, population data were analyzed to determine the effect and interrelationship of plant insect population growth.
Key words: Plant-insects interactions, shoreline vegetations, insect’s communities, host plant, population abundance and diversity.
1.
INTRODUCTION:
In recent years, there has been a considerable interest in how aquatic species diversity influences ecosystem functions. Various experimental designs have been used to investigate this question in laboratory micro and meso-cosms as well as in field plots with different numbers of plant species (Hector et al., 1999, Boyero 2002, Briers and Biggs 2005) in which diversity has been manipulated at a variety of trophic levels. Consequently the trophic ecology of aquatic invertebrates has received considerable attention from stream ecologists, however in temperate regions of the Northern Hemisphere (Hynes 1970, Merrit and Cummins 1984, Carehino et al. 2003) and to a lesser extent, the Southern Hemisphere (Chessman 1986, Yule, 1996, Death & Zimmermann 2005, Clarke et al. 2008). On the other hand such species diversity influences and their trophic significance in tropical large water bodies are wanting.
Shoreline insects in relation to shoreline macro-vegetations in the ecotone between the lake and land.
Such transitional habitats are often ignored because of the different background of aquatic and terrestrial ecologist. However, these ecotones are important for species and process that encounter them as boundaries.
Thus the equally challenging and interesting area compelled us to undertake the present study where macrophytes and other shoreline vegetations and their population abundance in relation to the certain shoreline insect’s growth are very interesting and indicating the rich potential with dense vegetation zone. It has been attempted to document an association between both less vegetated and dense vegetated shoreline zones and their associated shoreline insect fauna in one of the largest man made tropical lakes of central India called as Upper Lake. The purpose of the study was firstly to determine the presence of supralittoral vegetation, affecting the diversity and composition of insect communities in shoreline ecotone and secondly to identify the nature of such associations between the vegetation and insect communities.
2. MATERIAL AND METHODS:
2.1 Description of study area:
The Upper Lake is located in Bhopal city, the capital of Madhya Pradesh, the largest state of India. Constructed on earthen dam across the river Kolans in the 11th century created this lake, the Upper Lake has water spread area of 30.72 sq.km at FTL. The storage capacity is 101.6 million cu.m, the maximum and mean depth being 11.7 and 6 m. respectively. The Upper Lake is under a massive conservation, restoration and management project funded by Overseas Economic Cooperative Fund (OECF) Japan to protect it from environmental degradation not only due to its natural aesthetic value and rich biodiversity, but also since it is the main source of potable water.
Selection of the sampling sites of the Upper Lake chiefly was done on the basis of weeds and consequent biomass sampling. Sampling was done at four sampling sites of Upper Lake viz. Bhadbhada, Van-vihar (National Park), Pump-house and Bairagarh designated as SI, SII, SIII and SIV. Among all the sampling stations, Van-vihar (SII) and Bairagarh (SIV) have dense vegetated shoreline habitats, whereas Bhadbhada weir (SI) is generally more disturbed than other three sampling sites, primarily because of its use as a camping site.
Upper Lake is divided into perennial water, covered, marshy and submerged cum dry zones .Due to shallow nature of the last zone the lake becomes exposed from post monsoon period to summer season, therefore the Lake supports mainly three types of vegetations consisting of more than 100 terrestrial or marshy plants and 34 aquatic plant species. The aquatic species have been categorized as floating, submerged and emergent forms. Selection of the sampling sites of the Upper Lake chiefly was done on the basis of weeds and consequent biomass sampling.
2.2. Sampling of marginal insects:
To investigate the species richness and abundance of marginal hemipteran insects as well as their plant interaction a belt quadrate method (Dicks et al., 2003) was applied to the transects. The quadrates were used at the start to the end of the sampling season. Sampling stations were studied for insects in each season i.e. summer, winter and rainy. Sampling walks started between 10 to 12 am, the period of relatively high visitation by insects in the order Hemiptera were collected, and after each sampling, one specimen of each species was brought back to the laboratory to confirm identification using standard texts.
The size, number and distribution of samples to be taken in an area was decided upon before hand in order to make sure that the sampling is representative typifying the study area as a whole. The quadrates used for sampling, must be of such dimensions that all the species, which occur in that community is fully represented.
2.3. Water analysis:
Sampling for water quality parameters and aquatic insects were carrying out at monthly intervals, covering dry and rainy seasons. Air and water temperatures were recorded with a thermometer, whereas pH, total alkalinity, dissolved oxygen, total hardness, calcium hardness and nutrients were determined according to APHA (2003) methods. Monthly rainfall, humidity and ambient temperature data were obtained from the meteorological station Bhopal, India.
2.4. Collection and Identification of macrophytes:
As primary producers, macrophytes represent an important food resource, and they also play a significant role in the oxygen balance and nutrient cycle of many watercourses. In view of their varying requirements, the composition of macrophyte species in a water body makes it possible to draw conclusions about the local chemical and physical conditions. Species that prefer low nutrient concentrations, in particular, have become much less prevalent. Also in decline are species that are dependent on natural riverbanks with sandy and gravelly substrates. Aquatic plants are especially sensitive to changes (increases) in nutrient concentrations (notably phosphorus and ammonium) and to organic pollutants. Samples of shoreline macrophytes were collected from different types of habitat like lake margin and identified by his guidelines given by Needham and Needham (1962) and Haslam (1978).
2.5.Statistical analysis:
The data were analyzed by SPSS (Statistical Package for Social Services Version 10). Descriptive statistics such as probability, frequency, percentage and mean values were used.
RESULTS:
During the ecological analysis of the Upper Lake in relation to diversity of insects group and shoreline vegetations in the present study, interesting observations have been made .At all the sampling stations of Upper Lake 35 plant species have been found to be present in varying numbers .Out of these species 15 dominant plant species such as Ipomoea aquatica, Ipomoea fistulosa, Ceratophyllum demersum, Hydrilla verticillata ,Najas minor, Potamogeton pectinatus, Potamogeton crispus, Potamogeton nodusus ,Eichhornia crassipes, Leersia hexandra, Azolla pinnata, Lemna minor, Utricularia flexuosa and Alternanthera were conspicuous throughout the study period and remaining plant species such as Ageratum conyxoides, Kirgenalia reticulata, Trapa bispinosa and Cyperrus torundus were observed as a sub-dominant or rare species.
Shoreline vegetations of Upper Lake has been divided on the basis of their occurrence i.e. emergent, submerged, rooted and free floating forms, where Stations II and IV had a dence shoreline vegetations as compare to station I and III. Interestingly approximately 83 species of shoreline insects belonging to different orders were recorded in varying population numbers, ranging from 0.6 to14.6 sps/m² throughout the study period. Out of these insects species five insects orders were found to be present dominantly such as Coleoptera, Hemiptera, Odonata, Orthoptera and Lepidoptera. According to their dominance ,the station wise order has been found to be SII >SIV > SIII >SI. Seasonally it was observed that maximum vegetation was present during summer (18 to 70 species /20 m²) and rainy (10 to 52 species/20m²) seasons, whereas winter season had less dense population (8 to 46 species/20 m²) abundance of shoreline vegetations (Table 1).
The relationship between marginal insects and macrophytes were observed significantly positive for Ipomoea aquatic and Ipomoea fistulosa (P<0.001, r =0.89), Potamogeton pectinatus (P <0.001, r =0.90), Eichhornia crassipes (P < 0.001, r =0.90) for sub-merged, rooted macrophyte with porvopotamidae growth form, such as Hydrilla verticillata (P < 0.001, r = 0.90) and for common shoreline macrophyte, Leersia hexandra (P< 0.001, r =0.89). Two macrophyte species of Ipomoea, such as Ipomoea aquatica and Ipomoea fistulosa, three species of Potamogeton such as P. pectinatus, P. crispus, P. nodosus and the member of Pontedriaceae such as Eichhornia crassipes constituted 36.5±2.7, 29.5±2.4 and 23.9±2.7% respectively of total abundant vegetation during the study period.
In the present investigation it was observed that all three species of Potamogeton were found abundantly at station II (40 to 68 species/20m²) and IV (30 to 70 species /20 m²) and were observed less abundant at stations I and III being 10 to 20 species /20m². Interestingly it was also observed that some species of shoreline Coleopterans such as Dineutus indicus,Dineutus unidentatus and Colymbetus fuscus inhabitated on Potamogeton pectinatus, therefore at stations II and IV these Coleopterans were recorded in maximum population density (32 to 46 species/20m²) as compared to stations I and III(18 to 28 species/20m²Table 1)
Eichhornia crassipes was a dominant macrophyte belongs to family Pontedriaceae, germinates after onset of rains, with high temperature range (30-32ºC) and also good nutrients and soft mud of the littoral zone provided a favorable growth of E. crassipes. Interestingly insect population densities on E. crassipes were found to be significantly higher than on all other plant species. Most of the Dipterans (Culiseta longiareolata, Dolicocephala irrorata, Syrphus vitripennis) some Odonatens (Lestes orientalis, Ischnaru elegans, Libellula quadrimaculata) Coleopterans (Colymbetes fuscus, Dineutatus indicus, Dineutus unidentatus) and Hemipterans (Belostoma indicum, Corixa varicunda, Aradus acutus) were recorded maximum upon E. crassipes because of its high presence during most part of the year, in all sampling stations of Upper Lake (Fig.2).
On an average analysis of the three seasons, it was observed that maximum number of dominant shoreline vegetation was recorded in summer because of the fact that light strikes in the Lake surface more directly in summer, than in winter, permitting greater light penetration, which was more favorable for the growth of this type of aquatic vegetation. Consequently maximum (18 to 70 species/20m²) shoreline insects of Upper Lake were also observed in summer season, optimum temperature was found to induce the growth and development of insects coupled with the dense population of shoreline vegetation gave support, shelter, food and breeding place to shoreline insects, making a positive symbiotic influence of vegetation as well as insect population (Table 2).
DISCUSSION:
For insects that feed on plants, variations in plant quality can have a profound effect on both levels of competition and mortality from natural enemies (Lill et al. 2002, Helms et al., 2004). Moreover, insects, by comparison with plants, have more specific other needs, such as for food, mating, nesting or oviposition and avoidance of death. Removal of shoreline vegetations may have cascading effects on both the aquatic and terrestrial components of the shoreline ecotone. There is increasing evidence that altering freshwater transitional ecotones affects the structure and function of the aquatic ecosystem they border .For example, reductions in leaf litter have been shown to limit the abundance of detritivores, thereby reducing population size of both aquatic and terrestrial insect prey available for fresh water fish.
In the present study it was observed that several aquatic macrophytes such as Eichhornia crassipes, Leersia hexandra and Potamogeton pectinatus confined themselves to shallow euphotic zones of the Upper Lake, which provided shelter to a large number of insects such as Belostoma indicum, Dineutus indicus, Dineutus unidentatus and Colymbetus fuscus Similarly riparian vegetation provided temperature and micro-climate, habitat complexity and prey refuges which increased the abundance of food and nutrient inputs as well Clear cutting of vegetations at Upper Lake margins altered the composition of these invertebrates communities through changes in either food choice and habitat. Riparian vegetation provided temperature and micro-climate, habitat.
Across all taxa, Arthropods included several insects, their richness and abundance was higher in the dense vegetated site (stations II and IV) than at the less vegetated sites (stations I and III) of Upper Lake shorelines. Shoreline insects of Upper Lake were several times more abundant on an average at the dense vegetated sites as opposed to the less vegetated sites.
This suggested that shoreline vegetations provided important functions for both aquatic and terrestrial insects associated with the marginal ecotone. Although the extent of association varied with different macrophytes within different season, at the same station and also between different stations for all the orders of insects found in the present study. These variations between insects and their host plants, was mainly due to the changes in food and shelter requirements, fish predation and the life cycle of individual groups.
Season wise high abundance of marginal insects of 18-70 species/20m² was recorded in summer due to high temperature, which ranged 28 to 36.4ºC. This single factor induced photosynthetic activities of marginal vegetations. There was another factor of high insect abundance in summer, which may be due to phloem sap of grasses and forbs, which is generally accepted to be more nutritious in summer. Current results are similar to the findings of Douglas (1993) and Giulio and Edwards (2003) who have reported that high temperature range of 30 to 35 species/20m² affects on vegetation’s phloem sap and make them more nutritious, this attractive abundance of insect population.
Several species of Hemiptera such as G.lacustris inhabited on free floating vegetations of E. crassipes, whereas C. varicunda and R. varipes were found to colonise on submerged macrophytes such as V. spiralis and H. verticillata. Another abundant Hemiptera of Upper Lake B. indicum was found to be lotic and lentic in habitat and were observed on submerged macrophytes, H verticillata. Similar pattern of Hemipteran abundance has also been observed by several earlier workers such as Hilsenhoff (1977), Merrit and Cummins (1984), Erman (1984), Dobson (1994) and recently by Clarke et al. (2008) who have reported that peak of population abundance of aquatic insects coincided with the density of higher aquatic vegetation. In the present investigation it has been observed that most of marginal insects were specialized to their host plants where as some species had variable host plants.
An interesting observation regarding variability of vegetation was seen in present study that Heteropteran insect survival rate was found higher when they feed on variable plants as compared to those who feed on only specific plant species, i.e. N.maculata, T. tendipes, C. longiareolata, B. indicum, R. varipes and N. viridula had more than one species of host plants such as H. verticillata, V. spiralis, E. crassipes and L. hexandra and they showed high survival rate as well as high population abundance. These results are in fairly good corroboration with the findings of Hunter (2001), Helms et al. (2004), Death & Zimmermann (2005) and Barnett & Beisner (2007) who have reported that survival of aquatic insects was dependent on their feeding habits and variation in host plant abundance.
As evident from the several reports it may be concluded that almost all insects species of Upper Lake are affected by the presence of shoreline macrophytes. In the present study it was also observed that an increase of plant matter in a subsurface ecosystem induced both the abundance and diversity of aquatic as well as terrestrial population. Much as terrestrial insects such as Nemobious fasciatus, Atractomorpha crenulata, Chorthippus paralleus, Nezara viridula and Grillotalpa africana were primary consumers of plant matter so do aquatic insects such as Notonecta maculata, Hydrometra stagnorum, Belostoma indicum, Dineutus indicus and Gerris lacustris accounted for a large amount of the break down of plant material in subsurface ecosystem.
Ramanuk and Levings (2003) Bahuguna et al. (2004) and Gallego et al. (2010) also suggested that insects are much dependent on the presence and composition of vegetations. Our results suggest that loss of shoreline vegetation may reduce the productivity of both aquatic and terrestrial insects, possibly by adversely affecting the amount of allochthonous material entering the shoreline and microclimate conditions, thus lake vegetations considerably influences survival and abundance of shoreline insects.
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