Drivers of Bird Species Richness within Moist High-Altitude Grasslands in Eastern South Africa

Abstract

Moist high-altitude grasslands in South Africa are renowned for high avifaunal diversity and are priority areas for conservation. Conservation management of these areas conflicts with management for other uses, such as intensive livestock agriculture, which requires annual burning and leads to heavy grazing. Recently the area has become target for water storage schemes and renewable electricity energy projects. There is therefore an urgent need to investigate environmental factors and habitat factors that affect bird species richness in order to optimise management of those areas set aside for conservation. A particularly good opportunity to study these issues arose at Ingula in the eastern South African high-altitude grasslands. An area that had been subject to intense grazing was bought by the national power utility that constructed a pumped storage scheme on part of the land and set aside the rest for bird conservation. Since the new management took over in 2005 the area has been mostly annually burned with relatively little grazing. The new management seeks scientific advice on how to maintain avian species richness of the study area. We collected bird occurrence and vegetation data along random transects between 2006 and 2010 to monitor the impact of the new management, and to study the effect of the habitat changes on bird species richness. To achieve these, we convert bird transect data to presence only data to investigate how bird species richness were related to key transect vegetation attributes under this new grassland management. First we used generalised linear mixed models, to examine changes in vegetation grass height and cover and between burned and unburned habitats. Secondly, we examined how total bird species richness varied across seasons and years. And finally we investigated which habitat vegetation attributes were correlated with species richness of a group of grassland depended bird species only. Transects that were burned showed a larger decrease in vegetation cover compared to transects that were not burned. Grass height increased over time. Bird species richness was highest in summer compared to other seasons and increased over time. Overall bird species richness increased over the three summer surveys but species richness of birds that prefer heavily grazed habitat showed little change over the three years. Changes in bird species richness were best explained by the model with grass height for combined species richness of grassland depended birds but also for birds that prefer heavy grazing when treated alone. On one hand birds that prefer moderate grazing were best explained by a null model. However, overall bird species richness was better positively correlated to grass height than grass cover or dead grass. We conclude that frequent burning alone with relatively reduced grazing led to higher but less dense grass, which benefited some species and disadvantaged others. We suggest that management of this grassland use combination of fire and grazing and leave some areas unburned to accommodates birds of various habitat needs.

Fig 1. Comparisons of grass cover at Ingula over the three years of surveying.

Year is treated as fixed effect and transect as a random effect. The vertical black solid line represents the mean according to the best model (Table 1) and the histograms show the distribution of the raw data. The data consisted of the count out of nine squares in each sampling grid that fell on grass.

Fig 2. Comparison of influence of fire on grass cover along transects that were burned and those that were not burned at Ingula using summer data for three years (2006/7, 2007/08 and 2010/11).

Year is a treated as a fixed effect and transect as a random effect. Bold vertical lines show the estimated means according to the best model (Table 1).

Fig 3. Comparison of grass height during the three summers of survey at Ingula (2006/07, 2007/08 and 2010/11).

Year is treated as a fixed effect and transect as a random effect. Bold vertical lines represent the estimated means according to the best model (Table 2).

Fig 4. Comparison of Ingula bird species richness of all birds seen within 150m across the four seasons using transect data collected between 2006/07 to 2010/11.

The data come from three summers, two autumns, two winters and one spring survey and only half the total number of transects were surveyed during summer 2010/11. Vertical lines show the estimated means obtained from a generalized linear mixed effects model that treated season as a fixed effect and transect as a random effect.

Fig 5. Comparison of total bird species richness (combining birds that prefer heavy grazing with birds that prefer light grazing) at Ingula using data from summer surveys only.

Year is treated as a fixed effect and transect as a random effect. Bold bars represent the estimated means.

Fig 6. Comparison of bird species richness (birds that prefer heavy grazing only) at Ingula using data from summer surveys only.

Year is treated as a fixed effect and transect as a random effect. Bold bars represent the estimated means.

Fig 7. Comparison of bird species richness (birds that prefer moderate grazing only) at Ingula using data from three summer surveys.

Year is treated as a fixed effect and transect as a random effect. Bold bars represent the estimated means.

Fig 8. Response of birds species richness to grass height, cover and presence of dead grass along transect during the three summer surveys (2006/07, 2007/08 & 2010/11) at Ingula.

The lines show the best fitting linear relationships (from Models ‘Cover’, ‘Height’ and ‘Dead’ in Table 3.