Wednesday, 9 April 2014

Cattle grazing and raptors in a Peruvian protected area


Posted by Renzo Piana and Stuart Marsden

Renzo finished his PhD with us a couple of years ago on the ecology and conservation of diurnal raptors in the Tumbesian region of northern Peru. Here we talk about the latest publication from his PhD which looks at the influence of cattle grazing pressure on raptors.





Uncontrolled cattle grazing is frequent in protected areas across the tropics, but its effect on habitat structure and biodiversity is poorly known. From April to December in years 2008 and 2009, I surveyed raptors presence and vegetation structure in 70 one km² square plots randomly positioned in North West Biosphere Reserve (NWBR), a 120,000 ha protected area that includes the Cerros de Amotape National Park, Tumbes National Reserve and buffer areas in the core of the Tumbesian Endemic Zone. The NWBR holds many Tumbesian endemic bird species and at least 36 diurnal raptor species (50% of all those occurring in Peru), including the ‘Endangered’ Gray-backed Hawk, are present here.

Map of the Tumbesian Endemic Zone (adapted from Best and 
Kessler 1995) and detail of study area (in the circle).

Given the amount of forest conversion into cattle pastures, particularly in the Tumbes National Reserve, I estimated cattle density in the protected areas - this was done by counting the density of cattle dung, estimating the number of dungs produced by an individual cow, and the decay rate of dungs. In Ecuador, conversion of Tumbesian forests into cattle pastures and agricultural areas have seriously reduced forested areas west of the Andes. It is estimated that less than 10% of these forest remain (Dodson and Gentry 1991). Besides direct forest removal, trampling of forest floor and consumption of palatable plant species close to ground level affects understory, which in turn, might impact community structure of wildlife species, including those that are preferred prey for raptors.


Cattle grazing and its impact in the Tumbes National Reserve (Photos: Renzo Piana).
To examine this, we used generalized additive models (GAMs) to examine the precise relationships between cattle grazing intensity, vegetation structure, and raptor occurrence and richness across the NWBR. Cattle grazing was widespread and intensity was negatively correlated with average canopy height and percentage of vegetation cover at 5-15 m. 


In the NWBR, presence of some range-restricted species like the Gray-backed Hawk (left) and Black Hawk-Eagle (right) decreased with increasing cattle density (Photos: Renzo Piana).

Raptors were influenced by cattle density but they were probably more strongly influenced by canopy and sub-canopy characteristics. We also found that raptor species responded differently to increasing cattle ‘density’: presence of species that hunted or searched for food in open habitats (i. g. Short-tailed Hawk, King Vulture) or preferred edge habitats (Harris´s Hawk) increased with cattle density, while presence of range restricted species that hunted from perches (Black Hawk-Eagle), and declining species (Gray-backed Hawk) decreased. 

Abundance of species that occurred in open or edge habitats such as the King Vulture (left) and Harris´s Hawk (right) increased with cattle density (Photos: Renzo Piana).

Moderate cattle densities (20-60 cows per sq km) may actually benefit some raptor species and help to maintain the high raptor diversity in the study area. However, given the high rates of forest that already have been cleared, Tumbes National Reserve authorities should act to reduce cattle densities in some areas, which can exceed 120 cows per sq km, and that seriously affect some key raptor species. Additionally, cattle grazing inside the Cerros de Amotape National Park should be strictly controlled.


References

Dodson, C.H., and A.W. Gentry. 1991. Biological extinction in western Ecuador. Annals of the Missouri Botanical Garden 78: 273-295.


Best, B., and M. Kessler. 1995. Biodiversity and conservation in Tumbesian Ecuador and Peru. BirdLife International, Cambridge
.


Renzo's PhD was funded by the Peregrine Fund



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