Discussion

The degree of nestedness was significant in these datasets (T = 16.41°C, p = 4.43e-31, matrix fill 14%), with nearly 40% of species were idiosyncratic species. Compared to temperate diatoms showing much higher nestedness⁽¹⁴⁾ (T = 28.4°C), the tropical diatoms assemblages in the present study was colder, which is attributed by the similar environmental condition of the sampled sites, which determines the species assemblages.

The idiosyncrasy is in greater level in aquatic organisms when compared to the terrestrial organisms, which may be due to high dispersal ability of aquatic organisms⁽²⁵⁾. This would probably increase matrix temperature as indicated in stream fish community⁽³²⁾ and lake molluscs⁽⁴¹⁾.  Hence, relatively high degree of idiosyncrasy observed in the present study, is not an exception in the aquatic ecosystem particularly in eukaryotic assemblages. It is also possible that smaller the organism, higher is the temperature, which is due to greater dispersal ability and adaptive radiation or simply due to more number of species compared to the higher organisms. The variation in matrix temperature can also be due to ecological differences between island-like systems and streams^(45,46).

Furthermore, a promising and more appropriate justification for such difference in matrix temperature are related to the habitat quality and differences among sites^(47,15). Hylander et al.,⁽⁴⁸⁾ have revealed that nested habitat quality have a strong influence on species pattern: high quality sites also contain more tolerant and ubiquitous species, which also occur in low quality sites. Likewise, a study on fungi by Berglund and Jonsson⁽⁴⁹⁾ has shown that habitat quality was the major factor determining nestedness in substrate-specific fungi. Many of our sites have been influenced by the local water quality, which is a major determining factor of diatom assemblages. It is also important to note Allan’s⁽⁵⁰⁾ avowal on stream ecosystem heterogeneity that streams within a drainage system is highly connected and dynamic systems, where changes in environmental conditions occur on a temporal basis.

Species composition of diatoms in tropical streams of Central Western Ghats is significantly correlated with environmental distance across geographical extent. This is seemingly in agreement with the findings of some previous studies in stream biota (e.g.,(51,52) in which environmental distance was identified as the key habitat feature in structuring of stream biota assemblages. However, relationship of diatoms with environmental and geographical variables in boreal stream seems different as the assemblages were related to environmental and geographical distance⁽¹⁴⁾. The disagreement is probably due to differences in the scale of investigations, as evident from the regional scope of the present study, which included a small portion in a biogeographic zone of 1,60,000 sq. km. The current pattern suggests that at regional level of Western Ghats, that local niche-based control rather than the effect of dispersal limitations on the species composition of epilithic diatoms in streams. One possible reason for the variation in the diatoms assemblages across sites in the same biogeographic zone is local environmental factors such as water quality, which in turn heavily altered by wide range of human activities from land cover change to stream diversions. Western Ghats region, like other parts of the tropics, is undergoing rapid transformation. The deforestation rate is high and forests are being transformed into agriculture and monoculture plantations. Hydroelectric projects, mining, and extraction of forest products are also altering the landscape⁽⁵³⁾. In precise, the stream biota are the first one which get affected by the landscape level changes due to contaminated runoff, which is reported earlier through fishes⁽⁵⁴⁾ and aquatic invertebrates⁽⁵⁵⁾. The altered quality of water could be a favoring factor for wide occurrence of the idiosyncratic species. Many studies have been carried out since late 1960’s on the stream water quality and land cover linkage shows that catchment land-use is a major stressor on stream ecosystems, especially in agricultural regions. For example, agricultural land-use in the catchment can significantly modify both water chemistry⁽⁵⁶⁾ and physical habitat conditions⁽⁵⁷⁾ which eventually decrease biological integrity in these streams⁽⁵⁸⁾. Diatom community structure is believed to be strongly controlled by local environmental factors, especially by water nutrient concentrations and ionic composition in both tropical and temperate streams^{(59,60,61,62)}. Heino and Muotka⁽⁴¹⁾ and McAbendroth et al.,⁽⁶³⁾ observed that the habitat suitability as one of the principal determinants of nestedness pattern.

Among the environmental variables analyzed in this study PCA identifies electrical conductivity, alkalinity, chlorides, calcium, sodium, potassium, magnesium, phosphates, nitrates and dissolved oxygen as the major parameters which decides the principal components. All the above mentioned parameters are altered by the human activities such as agriculture and organic run-off from the human settlements.