Biodiversity and use of bioacoustics in biodiversity studies

Dr Rajkumari Ashalata / Dr Laishangbam Sanjitkumar *

Birds as seen in winter season in a Paddy field in Imphal in Oct 2012 :: Pix - Jinendra Maibam

About biodiversity

Our green planet called Earth is endowed with a rich variety of life forms, ranging from the unicellular primary producers to the complexly built higher plants and animals. These living organisms are independent with one another but delicately linked with each other by complex laws of nature.

It is all too easy to ignore our dependence on the environmental resource bases - the water, topsoil, vegetation, biodiversity, climate, etc. So, we are over exploiting this environmental resource base with impunity to an extent that is leaving a severely impoverished planet. In 1997 Costanza and others/co-workers gave estimation of the economic value of our environmental supports (goods and services) at $33 trillion worldwide per year, which is larger than the global economy of $29 trillion at that period/time.

In short, global natural product is more valuable than global national product. These very valuable natural resources have been considerably depleted during the last few decades because of over exploitation and unsustainable use. This has led some to believe that we are heading for another mass extinction.

Though extinction has always been a part of evolution since life began 3.5 billion years ago, the rate of species formation used to exceed the rate of species extinction. But the recent massive reduction in the number of life forms may surpass even the 'mass extinction', which occurred during a few periods in evolutionary history. The activities (over exploitation and unsustainable use of environmental resources) of Homo sapiens are one of the main causes of the substantial loss of Earth's valuable resources.

Over the last three centuries, more than 400 known animal species and unknown number of plant species have become extinct. The conservation of these species is inextricably linked with the survival of the human race. Thus, conservation of biodiversity has recently taken centre stage on the global (environmental) platform and biodiversity has become a 'household name'.

Biological diversity or biodiversity or biotic diversity refers to the variety and variability among living organisms and the ecological complexes in which they occur. It can be quantified in two ways: species diversity and species richness. Diversity can be defined as the number of different items and their relative frequency. For biological diversity, these different items are organized at many levels, ranging from landscape level to the chemical structures that are the molecular basis of heredity. Thus the term encompasses different ecosystems, species, landscapes (from top of the mountain to the deep ocean), genes and their relative abundance. Bioacoustics in Biodiversity Studies and use of birds as indicator species:

Birds are more detected in the field by their song and call notes than by visual observation. Song and call notes in birds perform a variety of functions such as territory establishment and its maintenance through advertisement and mate acquisition, synchronization of reproductive behavior, mate guarding, mate recognition, parent offspring recognition and neighbor stranger discrimination.

Vocal signals in birds can be classified into song and calls. Some birds do not deliver song at all; they use only calls to communicate their messages. A song is generally made up of a number of distinct sections, called phrases and each phrase consists of a series of similar or dissimilar units (called notes or elements) that occur together in a particular pattern.

An element is a continuous sound, preceded and followed by a silent gap. A call may be composed of a single element/note.

Birds living in dense vegetation such as rain forests or thick reed beds characteristically have loud and persistent voices as a means of keeping in touch with each other. The vegetation not only obstructs vision but also absorbs sound. Birds living in areas of dense foliage sang lower pitched songs whereas those in more open forests sang higher pitched songs. In areas of dense vegetation, natural selection appears to concentrate sound energy in song in lower frequencies to increase their carrying power.

Song experiments are useful in testing whether populations with different songs are different species. The experiments involve playing tape recordings of songs to males in the field. Males respond with song, flight approach, or attack to their own song, but not to the songs of other species. However, it is a fact that not all birds with similar songs are conspecifics.

Many birds which have nocturnal habits are more often detected by their calls. Relying on only song and call notes of birds in avian survey work has certain limitations also. Acoustic signal in birds only indicate the presence or absence of certain birds.

It is often very difficult to assess the abundance of certain species of bird by their song or call notes alone. Thus acoustic signals in bird are more useful in studies on bird species richness rather than bird species diversity.