COMMUNITY STRUCTURE ANd POPULATION ABUNDANCE OF CERTAIN MARGINAL INSECTS: PREDICTING THE POTENTIAL IMPACTS OF CLIMATE CHANGE IN A TROPICAL MAN MADE LAKE.
Dr. Asma Ali ( Ecologist )
Shoreline insects are a vital component of food web of aquatic as well as terrestrial ecosystems, which also act as pollution indicators and are more or less associated with water. Climatic factors are expected to have an important effect on marginal insect survivorship and reproductive success, since temperature, humidity and rainfall have acted as a forcing function with respect to understanding the insect abundance and population dynamics.
No significant work has been done in the tropical large man made lakes, hence in the present work the population abundance of marginal hemipterans has been examined from shorelines of Upper Lake Bhopal, a large man made lake with reference to the potential effects of climate change on insect community.
The abundance and community structure of marginal insects especially hemipterans showed strong evidence of impact of temperature (ambient as well as water) in comparison to abundance values which demonstrated high significance (P<0.05) between all the sampling stations.
The analysis indicates that community structure may be fairly resilient to climate change. The displacement and local extinction of species, especially the species that are found at only one sampling station however, may lead to significant changes in community composition
INTRODUCTION:
Temperature is one of the most important factors that influence the development rate of shoreline insects. Among insects, the relationship of developmental rate to constant temperature tends to be nonlinear. Development does not occur below a low temperature until an optimum is reached. Numerous studies determined an appropriate developmental rate function for phonological or population models that can be used under a variety of conditions to predict important events in the insect’s abundance for control strategies (Briere et al., 1999, Voldovinos et al., 2007, Adandedjan et al., 2010, Shin-Ya et al., 2010, Murray et al., 2012, Sangle et al., 2015). Over the coming century, shifting climate zone is going to affect the distribution and abundance of many invertebrate aquatic species, potentially leading to changes in the structure and composition of species and communities. Environmental gradients are a useful tool for understanding the role of current climate in structuring insect communities and have been used as a surrogate for predicting responses to future climate change. Little is known about factors that currently determine the abundance and distribution of most invertebrate species, in tropical lakes of India, thus restricting our ability to predict how these species, and the communities they live in, will respond to a rapidly changing climate in the coming period.
In the case of marginal insect communities, the effects of factors such as host plant chemistry, phylogeny, and architecture are also known to be important determinants of community composition (Walther 2002, Boyero 2002, Briers & Biggs 2003 and Heino 2009). Further, many comparisons of community structure between tropical and temperate regions have been limited, making generalizations difficult.
However in tropical regions the distribution of marginal insects, which depends on the ecological balance between various climatic factors such as temperature, humidity and rainfall, gets greatly modified by the changes in humidity and vice-versa (Voldovinos et al., 2007, Adandedjan et al., 2010, Shin-Ya et al., 2010).
For the past fifteen years, an increasing number of studies have been focused and published on biodiversity. This is principally due to the fact that the world's flora and fauna are disappearing at rates greater than the historical mass extinction events (Root et al., 2003). As suggested by Shin-Ya et al. (2010), there is an 18% to 35% risk of species level extinction resulting from climate changes in the year 2050. Moreover, other processes such as agricultural expansion, for example, in response to an increasing demand for food have a negative impact on biodiversity as a result of habitat destruction (Tilman et al., 2001).
In the present study, our objective was to study the insect abundance of shorelines of a large tropical lake, of central India, that can describe the nonlinear relationship of developmental rate to temperature for several insect species and provide an upper and lower estimated developmental threshold. The aim of this study was to examine the role of climate in determining the structure and composition of a shoreline insect community of hemiptera.
This study focused on temperature as the main variable, while holding other variables relatively constant. It is interesting to observe that most of the reports incorporating seasonal interference in the community structure and population dynamics of the hemipterans are from temperate and semi tropical regions. However this appears to be the first report from a tropical large man made lake exhibiting seasonal influences on the community structure and population density of hemipterans.
(i) 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.
(ii) 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., 2002) 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.
Some help in insect identification were taken from the scientist of Indian Agriculture Research Institute (I.A.R.I.) New Delhi and Zoological Survey of India (Z.S.I.), Jabalpur. The method for analysis of insect abundance and their frequency was followed as the method of Knapp (1984).
iii) 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.
(iv) Collection and Identification of macrophytes:
Aquatic macrophytes are an important component of many watercourses, providing a structures and habitat for fish and invertebrates, offering protection against currents and predators, and forming a substrate for the deposition of eggs. 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. 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 habitats like lake margin and identified by his guidelines given by Needham and Needham (1962) and Haslam (1978).
(v) Statistical analysis:
The data was analyzed by SPSS (Statistical Package for Social Services Version 10). Descriptive statistics such as probability, frequency, percentage and mean values were used.
RESULTS:
The present study was conducted on the four sampling sites SI, SII, SIII, SIV of shorelines of Upper Lake, Bhopal. It is noteworthy to mention here that all the four sampling stations have significant variations in their location, distance, and physicochemical and biological parameters. The climate of Bhopal is known to be relatively moderate and dry except in the monsoon season, indicating seasonal rhythms of weather. The year is divided into three seasons viz. summer, monsoon and winter. Major part of Bhopal district lies on Malwa Plateau, where extremes of temperature are not very much marked and the climate is generally pleasant.
During the two years of study period the maximum temperature of Bhopal was noted as 40.2ºC in the month of May and minimum ambient temperature of 10.2ºC was observed in January . Humidity of Bhopal ranged between 12.8 to 93.19%. Maximum of 93.19% was observed in August and minimum of 12.8% was observed in April, whereas maximum rainfall of 21.67 mm was observed in July and minimum of .0003mm was in the month of May. Station wise maximum water temperature of shorelines of Upper Lake was recorded at station IV being 36.4ºC in the month of May , due to high intensity of solar radiation and significantly high rate of photosynthesis by macrophytes. On the other hand minimum water temperature of shorelines was recorded at station I being 18.5ºC in the month of December.
In the present study, at all the sampling stations of Upper Lake, more than 15 species of marginal hemipterans have been found to be present in varying population densities ranging from 6-60 species/20 m². Out of many species recorded in varying numbers, 5 dominant species of hemiptera such as Gerris lacustris, Hydrometra stagnorum, Belostoma indicum, Notonecta maculata, Corixa varicunda and Ranatra varipes were conspicuous throughout the study period, which ranged between 10-58 species/20m². Garris lacustris and Hydrometra stagnorum were found to be the most dominant species of hemiptera, observed maximally (24-48 species/20m²) in the months of summer, when the water and ambient temperature reached its high peak of 31.2 to 36.4ºC and 35.5 to 41.08 ºC respectively .
R. Varipes, C. varicunda, N. maculata and B. indicum were the next dominant species of hemiptera, which were observed at all the sampling stations, but found most abundant (32-68 species/20²) at stations II and IV due to the favorable weather conditions and availability of their prey. In summer seasons, density of these species was found maximum (38-68 species/20m²), whereas minimum densities were observed in the rainy (18-38species/ 20m²) and winter (10-28 species/20m²) seasons respectively. These data show that high temperature regimes in summer season were responsible for enhanced growth of these marginal hemipterans. In and around the Upper Lake, apart from dominant hemiptera, there were five subdominant species, which were observed and present in different numbers in all the three seasons. The subdominant hemipterans had maximum population of Nezara viridula, Atheas exiguas and Reduvious personatus which ranged between 18-50 species/20m²; whereas Eusarcocoris ventralis and Eusarcocoris guttiger were found to be totally absent in summer season may be due to high temperature, which inhibited their growth and development.
An interesting feature that was recorded during the two years of study period was regarding the shift in population of hemipterans, observed in sampling stations of Upper Lake. During the first year i.e. 2004 in summer season G. lacustris were the dominant species of lake margins, which showed population frequency of 38-42 species/20m² when the water temperature was recorded between 26-34.4ºC. However in the next year G. lacustris became subdominant and H.stagnorum and N. maculata species became dominant species in summer season (population frequency 42-48 species/20m², 48-68 species/20m²respectively) when water temperature was recorded between 28-36.4ºC. Similarly E. ventralis and A. exiguas were observed in monsoon season, when water temperature ranged between 26-34.4ºC, these hemipterans were totally absent in the next monsoon season of study period, when temperature observed 30-39ºC. Thus it appears that temperature extremes directly affects the hemipteran abundance, within the exposed shore zone, generally increased in the summer season.
In the present study the data also demonstrate that during the rainy season, additional occurrence of Charisterus antennator, Andrallus spinidens and Dysdercus cingulatus were recorded at shorelines of Upper Lake, because these hemipterans required low temperature ranges being 22.4-26.6ºC for their significant high growth. These data clearly demonstrate that seasonal variations in temperature, humidity and rainfall induced significant changes in the growth of hemipterans along with other biotic factors in a complex interplay of cascading factors.
DISCUSSION:
Temperature, humidity and rainfall are important physical parameters of any ecological study, which regulate and maintain many physiological activities in the living forms. The temperature is an important factor indicating the quality of water, influencing the aquatic life and concentration of dissolved gases and chemical solutes as well. With regard to the temperature of various sampling stations of Upper Lake, difference of 3.4ºC was found at station IV and minimal of 33ºC at station I. Interestingly changes in temperature regime coincided with the marked variations in the population levels of all insects of the order hemiptera studied. Aquatic temperature does not have the same range as air temperature and insect species usually show a definite restriction of water of a certain temperature range.
In the present study, season wise high temperature was recorded in summer months, which ranged 28-36.4ºC. The speed of development and activity of different insect species of order hemiptera have been found to be regulated by ambient as well as water temperature, humidity and rainfall. For hemipteran insects, these climatic factors can be undoubtedly correlated with growth and development, as it was found to be suitable for round the year development of few hemipterans such as Notonecta maculata, Belostoma indicum, Tetrix subulata, Corixa varicunda and Ranatra varipes. The egg incubation period, nymphal development, survivorship and longevity of shorelines hemipterans were influenced by climatic changes. Hatching of egg required a definite temperature; hence it influenced the developmental period of insects. Therefore maximum population abundance (3.1-3.3 species/20m²) of hemiptera such as N. maculata, G.laucustris and H. stagnorum was observed in summer and monsoon seasons, which gave favorable climate to hemipteran development. The present data are well corroborated with the earlier reports of Nebeker (1971), Ward & Standford (1983) and Careghino et al. (2003), who have stated that increased temperature accelerated the emergence of aquatic insects where high peak of population abundance was observed in summer. Similarly the recent findings of Voldovinos et al. (2007), Murray et al. (2012), Raza et al. (2014) and Sangle et al. ( 2015) also support the present data, that temperature and other climatic factors induced the growth and maturation of related fauna of the system, especially insects and mollusks.
The data of the present study further indicate that, these insects of order hemiptera, exhibited interdependence with temperature and shoreline vegetations as well and showed their high abundance in definite temperature regime dependent upon season, these observations are fully corroborated with the findings of Ward and Standford (1983), Reilly et al. (2003) and Briers and Biggs (2005) who have reported that optimum degree of temperature and abundant occurrence of marginal aquatic macrophytes such as Eicchornia crassipes, Ipomoea fistulosa, Jussiaea repence, Potamogeton pectinatus and Vallisnaria spiralis have much effect on the occurrence and peak abundance of several shoreline insects. Season wise high frequency of marginal insects of 32-68 species/20m² was recorded in summer season due to high temperature, which ranged between 28-36.4ºC, which significantly induced photosynthetic activities of shoreline vegetations. There is 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. Nitrogen contents of plant food can be crucial factor for the development and reproduction of herbivores. Stem, fruits and flowers of plants changed considerably both seasonally and during the course of plant development.
As protein contents of marginal plants are known to vary during the season, many insect species of different orders were found to be more abundant in summer. These results are quite similar to the findings of Douglas (1993) and Giulio& Edwards (2003), who have reported that high temperature (30-35ºC) affects vegetation phloem saps and make them more nutritious, which seems to be attracting more insects as in the present work.
As evident from the reports of Ewers and Didham (2006), Taki and Kewan (2007) and Clarke et al. (2008), it has been observed that there is a broad correlation between climatic factors viz. temperature, humidity and rainfall on the occurrence and development of shoreline insects. On comparing population abundance of shoreline hemipterans at different sampling stations of Upper Lake, maximum abundance was observed in summer. This may attributed to the favorable weather conditions, balance nutrient levels and of course optimum temperature for development, growth and maturation, therefore occurrence of shoreline insect was found to be dependent on the climatic factors, which accelerated seasonally. It is interesting to observe that most of the reports incorporating seasonal interference in the community structure and population dynamics of the hemipterans are from temperate and semi tropical regions. However this appears to be the first report from a tropical large man made lake exhibiting seasonal influences on the community structure and population density of hemipterans.
Conclusion:
Climate change is one of the most crucial and influential ecological problems of our age; therefore large numbers of investigations are required to deal with this problem, which is increasing permanently. Climatic changes and variability can influence aquatic ecosystem in a very sensitive way, so the research of the possible effects of climate change on aquatic ecosystem means an indispensable task. In the present study it has been observed that the optimum degree of temperature of 28-36ºC and abundant occurrence of marginal aquatic macrophytes have much affects on the occurrence and peak abundance of marginal hemipterans of Upper Lake. In the present work it was also observed that climatic factors such as temperature, humidity and rainfall accelerated the dominance of hemipterans and showed shift in population frequency each year. Since we do not have the chance to reverse global warming and climate change phenomena, the only thing that needs to be done is to minimize the foreseen harms in the future. To this end, mankind needs to understand the global warming problem and cooperate on an international level using aquatic model studies of invertebrate and vertebrate species preferably in large tropical water bodies, which are on the brink of eutrophication and extinction. Upper Lake of Bhopal the largest man made lake built by Raja Bhoj in AD 1100 is such an example, which needs to be of major concern from conservational aspects.
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