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What effect does grazing management have on upland biodiversity?

Since 2002, The Macaulay Institute, along with a number of other organisations, has been investigating how livestock numbers on the Scottish hills affects the other plants and animals found there.

Study system

Comparison of grazing management regimes is being carried out in the Trossachs, at Glen Finglas, 23 miles northwest of Stirling.

We have established 24 fenced plots and each plot is about the size of five football pitches (3.3 hectares). We have stocked each plot in one of four ways - each with different numbers of sheep and cattle:

The plots were fenced off in 2002, and measurements of vegetation, invertebrates and birds were taken.

Sheep were then introduced in January 2003.

Each plot is divided up into grids, and records of the plants, invertebrates and birds are regularly taken at the same points. This way, we can accurately measure the biodiversity response over time due to changes in grazing management.

Grazing

As you might expect, the sward is tallest and densest when there has been no grazing allowed, and short and sparse at the maximum (commercial) stocking density.

When there is an intermediate number of grazing animals, a mosaic of short, preferred vegetation and long, less preferred vegetation patches build up.

This mosaic is at its most obvious when there is a mixed system of sheep and cattle grazing, as they tend to feed on different plants.

Preferred plants include blue fescue grass, star sedge, bog moss and heath bedstraw, whilst less preferred vegetation are species such as club rush, cross-leaved heath, soft rush and marsh bedstraw.

As you will see below, these patches of long, dense plants are great for invertebrates, and ultimately the birds that feed on them.

Bugs

Overall, we now find twice as many invertebrates in pastures that have less, or no-grazing on them when compared to areas with typical numbers of sheep.

This is because areas of tall plants are generally good for insects to safely lay their eggs in, including beetles and plant bugs which have increased in the plots with reduced or no-grazing.

Numbers of flies and moths haven’t changed in any of the plots, however the number of soil-living leatherjackets (the larvae of cranefly) have actually decreased in areas of tall vegetation.

This is probably because it is difficult for female craneflies to get into the soil to lay their eggs when the sward is very dense.

This further emphasises the importance to wildlife of both short and tall vegetation patches, as occurs in the moderately grazed system.

Spiders are doing particularly well in these patches because tall, dense vegetation is good for building their webs in.

When we sample for invertebrates, we typically find almost double the amount in pasture areas with reduced, or no-grazing than compared to the plots with a commercial stocking density of sheep.

Birds

All these extra invertebrates should mean more food for birds like meadow pipits and skylarks.

However, birds can only get to feed on the insects when they are in the short grass.

In the plots with both short and tall vegetation, insects often leave the long grass areas where they hatched from eggs. This is because there is less competition, less disturbance from grazing livestock, and more nutritious herbage for plant-feeding insects.

They are then much easier prey for the birds.

Birds also like the taller vegetation, as this is where they like to build their nests.

The result of this is that in plots with a mixture of  short and tall vegetation, there are not only more pipits nesting, but because of the extra food available, they lay eggs that are a third bigger than when there is either no grazing (only tall vegetation) or intensive grazing (only short vegetation)

We know from other studies that chicks from bigger eggs usually quickly grow up to be healthier, more successful adult birds.

Partners

 This work is funded by the Scottish Executive Environment and Rural Affairs Department. Our partners in this project are:                                                            

Scottish Agricultural College Centre for Ecology and Hydrology

RSPB (Royal Society for the Protection of Birds)

BioSS (Biomathematics and Statistics Scotland)

University of Aberdeen

University of Exeter in Cornwall


For more information, email: p.dennis@macaulay.ac.uk

 2007 The Macaulay Land Use Research Institute. All rights reserved.

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