Conservation strategy for the birds of the uttara kannada district.

Abstract

The forests of Uttara Kannada have had a long history of disturbance and transformation by humans. As they stand today, woody plant species numbers tend to fall with the declining rainfall gradient in a northeastern direction. The evergreen forests have the highest species richness. Since a broad vegetation type is structurally variable, very fine classification by species associations can lead to too many unrealistic divisions. Instead of a subjective classification of a broad type into degraded and well-preserved, an objective classification by structure is possible. Evergreen forests can be separated from the more disturbed and deciduous forests by taking the proportion of evergreen plants alone as an index. This is consistent with the classifications based on the abundance of thorny plants and also by ranking samples under several structural variables. Human interference reduces the diversity of woody plants in a primary vegetation type such as the evergreen forests. A complete transformation towards deciduous types or thorny open stands is also observed. The high-diversity evergreen forests of the district have experienced some disturbance in the past. That the least disturbed forests of the district are those disturbed to an intermediate level, and the most disturbed are excessively exploited, is open to further investigations.

Average similarity in species composition within a broad vegetation type is not more than 55%. This varies with the distance between the samples compared. Two randomly sampled localities within 5 km are expected to be only 35% similar in their species composition. The pattern of vertical layering is more consistent within the evergreen forests. Structural variation within a vegetation is influenced by the abundance and range of species of plants. The regeneration as witnessed today suggests that the evergreen forests are recovering from past disturbances. Only a few species of trees have sufficient recruits for a continued maintenance of their kind within a local stand. How the birds of the district respond to the vegetation structure will be the theme of the following chapters. The district shares a greater proportion of waterbirds than landbirds with the other biogeographic regions of the world.

Within the district, the richness of taxa increases from west to east in a northeasterly direction. Unlike the plants, the number of bird taxa increases with decreasing rainfall, reaching the highest densities of taxa in the zone of intermediate rainfall. The vegetation zones seem to delimit the ranges of taxa within the district. Five major zones were recognized by the density of taxa, viz., the coastal zone, the north evergreen zone, the disturbed/south evergreen zone, the moist deciduous zone, and the dry deciduous zone. The coastal zone and the dry deciduous zone share a lot of waterbirds despite being the farthest apart. The disturbed evergreen zone shares more taxa with the moist deciduous zone than with the evergreen zone. The secondary forests and heterogeneity of habitats resulting from greater human interference in this zone have favored invaders from the moist deciduous zone. The dry deciduous zone, also being more similar to the moist deciduous zone, suggests that the latter is transitional between the wetter and drier zones. The north evergreen zone has a set of rather unique taxa of birds, most of which are residents.

Summary and Conclusions The wide range of habitats available in the Uttara Kannada district are not equally utilized by the birds. Only a handful of bird taxa have restricted themselves to utilizing single habitats. The dominant habitat type acts as a source of birds for the other associated types. This is not, however, a one-way channel. Taxa from other habitats do opportunistically invade the dominant habitats as well. Frequently available habitats seem to be utilized by more birds, provided there is a larger source of taxa at close distance. Birds tend to utilize a variety of different locally available habitats over their entire geographical range. The size of a habitat does not appear to influence the number of taxa utilizing it. Small-sized habitats compensate for the lack of typical and interior taxa by adding edge taxa, thus having bird taxa numbers comparable with the more extensive ones. The moist deciduous forests are utilized by many birds as they are transitional between the major habitats such as the evergreen and dry deciduous forests. Small, structurally distinct, and isolated habitat patches such as secondary grasslands or a rocky cliff in the matrix of evergreen forests are also utilized by birds. Marshes are more productive, hence utilized by many taxa of birds.

The birds of Uttara Kannada distinguish two broad families of habitats, viz., the aquatic and terrestrial. Between the terrestrial habitats, birds seem to distinguish habitats by the levels of wetness. Measuring niche breadth, allowing for similarity between the habitats utilized, helps distinguish between taxa utilizing a few distinct habitats and those using a set of related habitats. Forest birds, which have a tendency to utilize a range of habitats and as a result appear to have a broader niche, are found to be just utilizing a set of closely related habitats. An analysis of niche breadth at the different levels of organization of birds suggests that the passerines have a comparatively broader niche than the non-passerines. At the dietary levels, the insectivores, herbivores, and carnivores have a broader niche than the omnivores. The resident birds and the migrant birds have equal niche breadths in the district. Waterbirds as a group are habitat specialists.

The assessment of a locality or habitat for conserving birds is a stepwise process. The identification of the appropriate attributes, criteria, and values is critical. For conservation assessment of localities/habitats in Uttara Kannada, the primary attribute chosen was the avifauna. The criteria chosen in this study were meant to assign values to birds by their geographic rarity, habitat specialization, taxonomic uniqueness, and degree of endangerment. Diversity or "species richness" taken alone does not help in assessing the localities/habitats, since the localities/habitats with the highest number of taxa are not always the ones with the most valuable taxa as well. For example, the evergreen forests have fewer taxa compared to the other terrestrial habitats. However, the average value of a taxon by geographic range outside the Malabar is the highest in evergreen forests. The sum of values of taxa in a locality is influenced very much by the number of taxa itself. Hence, it is recommended that the mean value of a taxon in a locality be used for assessing the localities. Ranking the localities under the different criteria makes identifying the more valuable localities simpler.

Identifying appropriate criteria for assessing localities by habitats has proved to be more difficult. Apart from vegetation structure, which influences bird diversity and assemblage of birds, no other criteria could be identified in this study. An assessment of vegetation quality using structural variables has proved useful for the evergreen forests of the district. It was found that the evergreen forests which rank high with respect to vegetation quality also rank high with respect to the birds. The extent of canopied vegetation in a locality positively influences the type of birds in that locality. Geographically rarer birds (outside the Malabar) are better represented in the samples from such localities. For the non-forest habitats, in lieu of other criteria, it is recommended that area be taken as a criterion for assessments.

It is safest to compare only localities with similar habitats if sample data alone is available while making conservation assessments. The number of taxa represented in a sample from a locality is biased by the size and type of habitat to some extent.