Between‐year variations in Little Bustard Tetrax tetrax population densities are influenced by agricultural intensification and rainfall

Abstract

The Little Bustard is suffering a widespread population decline mainly due to agricultural intensification. This study evaluates the effects of intensification level, habitat availability and rainfall on the population dynamics of this species. The population density of males was monitored for 7 years (2002–2008) at 184 points located within three sites with contrasting levels of agricultural intensification in southern Portugal. Densities decreased along the intensification gradient from 8.2 to 2.3 males / km2. Overall, there was an approximately 50% population decline during the period 2002–2008, driven by a decline observed in one of the less intensive sites, whereas in the other two sites densities remained fairly constant. Yearly variations in male densities were influenced by intensification level, amount of grassland habitat and rainfall patterns. Thus, agricultural intensification is having a negative effect on population densities of this threatened species, particularly through the loss of grasslands (fallow fields and pastures) suitable for displaying males. The results also suggest a positive impact of rainfall on male densities, although this is more likely in grasslands within less intensive agricultural regions of poorer soil quality, where higher breeding male densities occur. Grassland habitat quality, driven by both climate and human management, probably plays a major role in the population dynamics of this threatened steppe bird in its strongholds.

EU protected area network did not prevent a country wide population decline in a threatened grassland bird

“…Such was the case of a national SPA survey in 2010. Castro Verde, the most important breeding area for the species, that has also been counted every year since 2002 ( Delgado & Moreira, 2010 ; A Delgado, pers. comm., 2017).…”

Mosaic-Level Inference of the Impact of Land Cover Changes in Agricultural Landscapes on Biodiversity: A Case-Study with a Threatened Grassland Bird

“…Firstly, we used 1∶25000 scale maps and field checks to identify the available road network that crossed each site. Following standard procedures to count little bustard males [12] , [18] , [26] , points were then placed along the whole network of accessible non-paved roads crossing each sampling site, with the distance between survey points being set at a minimum of 600 m, to avoid double counts, and with the additional constraint that each point was at least 300 m from the site boundaries, villages and farmsteads, to minimise potential disturbance effects on little bustards. Because of differences in site area and road network density, the number of survey points per site ranged from 16 to 72 (mean = 29.1, total number of points = 2326).…”

“…Roads were travelled by car, and the total number of males detected within 250 m of each survey point (an area of 19.6 ha) during 5 minutes was recorded. This radius was selected because it is the distance at which any calling male is most likely to be detected, and this survey method has been widely used in other studies [16] , [25] , [26] . GPS point coordinates and a map (or aerial photographs) overlaid with the boundary of the search circles facilitated the assessment of the survey area and the bird counts.…”

Changes in grassland management and linear infrastructures associated to the decline of an endangered bird population

“…The region is home to the large majority of the national breeding population of the species 72 . Bird density was estimated following a standardized protocol targeting male birds, as females have a cryptic behavior and are hard to detect 34 , 73 , 74 . A network of point counts defined along non-paved roads, 600 m from each other and from paved roads or inhabited houses, was used to survey birds in each area, covering an average density of approximately 1.0 points/km 2 per survey (range 0.47–3.00).…”