Introduction

In an era of human impact on natural ecosystem, a major challenge for ecologists is to understand the structure and dynamics of biological communities in relation to environmental variability⁽¹⁾. Community diversity and the population abundance of a particular group of species are controlled by immediate environment, landscape conditions, inter and intra species interactions, historical events and evolutionary processes^(2,3,4,5). Appreciation of the phenomena of community is not only important from basic science of ecology, but is also fundamental to conservation biology⁽⁶⁾. Although attempts to understand the spatial patterns have been documented in many studies of plant and animal diversity, it has not been attempted in microbial species. This is a serious omission given that microorganisms constitute much of the biodiversity on earth⁽⁷⁾ and have vital functional roles in biogeochemical cycles as well as ecosystem functioning^(8,9). With limited information on macro ecological patterns of micro organisms, theories were built and tested, but lack facts⁽¹⁰⁾. However, Martiny et al.,⁽¹¹⁾ concludes that the microbial diversity is partly decided by the environment and the processes that generate and maintain biogeographic patterns in macroorganisms could operate in the microbial world.

Diatoms (Division: Bacilliariophyta), one of the largest groups of microorganisms, are one of the most successful groups of photosynthetic eukaryotic micro-organisms. They occur in almost all wet/damp places with a diverse range of habitats across the continents. Diatoms grow as single cells, or form simple filaments or colonies. They form the base of aquatic food webs in marine and freshwater habitats. Diatom species are very particular about the physical and chemistry parameters of water such as pH, nutrients, salinity, temperature and water current in which they live. As a result, diatoms are used extensively in environmental assessment and monitoring across the globe. Like many other microorganisms diatoms are least studied organism, particularly from tropical regions. Recently, there are few studies from temperate region analyzing the spatial patterns^(12,13) and assemblages’ patterns⁽¹⁴⁾ of diatoms. These studies emphasise the importance of dispersal and migration in structuring diatom communities at regional to global scales with giving consideration to the regional diatom pool and habitat availability.

Considering the meager understanding of species assemblage patterns of tropical streams, we attempt to study the nestedness pattern in stream diatom assemblages of Central Western Ghats. There are many methods available to measure patterns in the species assemblages; nestedness being one among them and is very popular for a wide range of taxa. Nestedness is a measure of order in an ecological system, referring to the order in which the number of species is related to area or other factors. The more a system is "nested" the more it is organized. In general, the species assemblages are associated with species–area relationship known as the ‘nested subsets’ pattern⁽¹⁵⁾. The nested subset pattern arises because species differ in their distributions across space. Some species use a wider range of resources or persist across a wider range of habitats than others^(16,17,18). Generally, species that use a wide range of resources or tolerate a variety of abiotic conditions can establish more populations in more places than comparable species with relatively narrow niches^(19,20). However the factors affecting the degree to which species are nested or idiosyncratic are poorly understood.

Analyzing the nestedness pattern using nestedness calculator was proposed by Patterson and Atmar⁽²¹⁾, which further lead to an increase in interest in this analysis (Referred in 339 Research papers as on 31 October 2009; ISI database search). It works with a combination of a thermodynamic measure of order and a Monte-Carlo simulation⁽²²⁾. Detailed account of nestedness calculator and its application is available in Puyravaud et al.,⁽²³⁾. Many studies have revealed that variations in species assemblages can reflect nested distribution patterns^{(24,25,26,27,15,28,29,30,31,14,32)}. Majority of these studies conclude that nestedness pattern exist due to selective extinction by which species disappear from different habitats in roughly the same order⁽³³⁾. However, Cook and Quin⁽²⁰⁾ proposed that nested pattern develops in an ecosystem due to differential colonization abilities of species.

Identifying the nested assemblage patterns of freshwater diatoms in streams of Central Western Ghats and to understand the possible drivers for such pattern are the primary objectives of this paper.