Remote monitoring of wader habitats

In a 2021 paper in Basic and Applied Ecology, Triin Kaasiku and colleagues use radar remote sensing to assess the continued suitability of large areas of Estonia for breeding waders. Although the focus of the paper is upon the performance of an agri-environment scheme, the results are of broader relevance, as conservation biologists seek to monitor vegetation growth and the encroachment of shrubs and trees, especially associated with a warming climate, afforestation and farmland abandonment.

Ground-nesting waders

Many species of waders breed in open habitats. Dense vegetation can remove feeding and nesting areas, and shrub and tree encroachment provides shelter for predators. Previous WaderTales blogs have discussed the perceived and actual predator effects of patches of woodland on breeding Lapwing populations and the difficulty of removing trees and predators that have been planted in peatland (Trees, predators & breeding waders). At a larger scale, vegetation growth, trees and shrubs are threatening breeding areas in arctic and subarctic regions, as discussed by Tómas Gunnarsson in Losing space for breeding waders. Locally, changes to land management, such as reduced sheep grazing and the abandonment of grouse moors in the British uplands, may have unintended consequences for breeding waders, including Curlew.

In Estonia’s coastal grasslands, horses are part of the grazing regime

Maintaining Estonia’s coastal grassland

Estonia’s farmed, coastal grasslands are semi-natural habitats that are vital for a range of species, particularly breeding waders. They also act as soft barriers that dissipate wave energy along coastlines and capture carbon. Across the European Union, the importance of these areas has been recognised, resulting in agri-environment schemes (AES) that preserve farm incomes while encouraging nature-friendly management methods that are associated with lower yields. 

Redshank nest in a clump of grass

Getting the balance right is tricky – for example, too much grazing can reduce heterogeneity and impacts nesting waders (see Big Foot and the Redshank nest), but too little grazing can mean that coastal marshes become rank, as tall grasses replace low-growing plant communities and scrub and trees start to encroach. In a previous paper – Managing coastal grasslands for an endangered wader species can give positive results only when expanding the area of open landscape – Triin Kaasiku, Riinu Rannap & Tanel Kaart emphasised the importance of open, wet grasslands for breeding Dunlin.

The main objective of coastal grassland management in Estonia, in addition to supporting local farming communities, has been to preserve the breeding habitat of several threatened wader species, particularly Dunlin, Ruff, Black-tailed Godwit and Redshank. Nine of the eleven species that breed in these areas have declined moderately or strongly since 1980. Population changes are believed to have been largely driven by habitat loss and increased predation. These problems are recognised around the whole Baltic coast, which is also experiencing more frequent summer storms and flooding. See Dunlin: tales from the Baltic.

Remote sensing

Cute Curlew chick

The Kaasiku team used freely-available European Space Agency Sentinel-1 Synthetic Aperture Radar (SAR) images to assess habitat suitability of coastal grasslands, as breeding sites for waders across Estonia. Although this technique has been used before on cut grasslands, it had not been tested on less uniform grazed areas. The aims of the project were to:

• Determine the ability of SAR satellite images to classify suitable and unsuitable areas for breeding waders.
• Compare the habitat suitability for breeding waders under different management schemes, across over 200 km² of Baltic coastal grassland habitat.
• Propose methods to improve coastal grassland management.

One of the key parts of the project was to be able to ground truth the SAR data, by visiting sites being managed in different ways (or not at all) in order to collect direct measurements and make observations relating to the suitability of habitat for breeding waders. Details of the methods used are given in the paper.

Research findings

Using SAR radar images, it was possible to correctly identify 88% of suitable habitat and 74% of unsuitable habitat, using a classification boundary that was created using a test set of the data.

Grazing maintains open habitats (left and centre) and restricts reed growth (right)

With renewed confidence, the research team was able to assess whether management prescriptions are actually delivering the desired habitats – for waders and for biodiversity. Their results are encouraging – these agri-environment schemes are broadly working:

• 43% of Estonia’s coastal grasslands are in a favourable state, in terms of vegetation height.
• The proportion of suitable habitat is higher (60%) in areas where basic-level conservation subsidies are paid.
• The highest proportion of suitable habitat (76%) is achieved in areas receiving a top-up conservation subsidy.
• Long-term grazing ensures better habitat quality: in areas where a basic subsidy has been paid for at least the last five years, 65% of the habitat was classified as suitable.
• The conclusions were tested by the breeding waders themselves – 98% of the 268 nests located by the team were found in areas classified as suitable.

Although the use of radar remote sensing provides a way of assessing the general suitability of grassland for nesting wader community, it does not provide information on the availability of species-specific microhabitats, finer-scale vegetation characteristics, or soil moisture, all of which are important to breeding waders.

Effectiveness of subsidies

Baltic Dunlin (schinzii) need help

In Estonia, the area of coastal grassland under conservation management has increased from 8,000 to 11,500 ha since the implementation of the AES scheme in 2001. Payments account for 40% of farmers’ incomes. While the payments ensure 60% of the habitat is managed suitably, the remaining 40% does not create habitat for breeding waders – the main target of the AES for coastal grasslands. The authors suggest that two decades of minimal oversight of the outputs of the scheme has led to sub-optimal delivery. This is unsurprising, given the cost of field visits to farms. Perhaps satellite imagery will help in the future, especially if used five years after the onset of conservation management, when the cumulative effects can be detected.  These assessments will also enable top-up payments to be focused upon areas where 75% of the shoreline is free of high vegetation.

The authors note that the current subsidies are not associated with prescribed stocking rates and suggest that this issue needs to be addressed. Using the right levels of grazing at the right time of year in the right places can make a big difference to breeding waders, as mentioned earlier and discussed in Dunlin – tales from the Baltic and the UK-focused blogs Big Foot and the Redshank nest and Redshank – the ‘warden of the marsh’. T

Nest-trampling rates seem low in Estonian coastal grazing habitat, suggesting that there may be scope to increase grazing pressure, especially if cattle are only released onto coastal grasslands in late May.

A broader context

Ruff nest in Estonia

While fencing out predators might be a good way to support breeding waders nesting at high densities in nature reserves or on intensively managed sites, landscape-level support is going to be needed if we are to conserve thinly distributed European species such as Curlew, Dunlin and Golden Plover. Across Europe, prescriptions are in place to adjust grazing levels so as to suit breeding waders but it is difficult to assess the effectiveness of these measures or even to judge whether management is taking place at the right scale and intensity. Triin Kaasiku and collegaues have demonstrated that remote sensing can be a cost-effective way to monitor the delivery of conservation measures indirectly, although they point out that this does not replace detailed monitoring of bird communities. This work has broader applicability, as we seek to measure global-scale habitat changes that will affect increasingly beleaguered wader populations, nesting in some of the most remote parts of the globe.

The paper at the heart of this blog is:

Radar remote sensing as a novel tool to assess the performance of an agri-environment scheme in coastal grasslands. Triin Kaasiku, Jaan Praks, Kaidi Jakobson & Riinu Rannap. Basic and Applied Ecology. July 2021.

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WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience. A catalogue of blogs is available HERE.