Health and Sustainable Agriculture
Editors: Leif Norrgren and Jeffrey M. Levengood
Ecology and Animal Health
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Stewardship of Biodiversity
Introduction
Use of land for agriculture had led to the loss of over one third of the globe’s natural habitat. And in the next 100 years, habitat loss owing to conversion for food, fiber and fuel production may reach 60% (Wade et al., 2007). Agricultural conversion coupled with increasing urbanization/suburbanization will place unprecedented strains on natural ecosystems and their constituent biota (Robertson and Swinton, 2005).
One inevitable result of conversion to agriculture is habitat loss and fragmentation. The direct effects of each on ecological integrity and biodiversity have been docu- mented thoroughly for nearly all biogeographic regions (for example review papers, see Andren, 1994; Aguilar et al., 2006; Cushman, 2006). The magnitude of change varies with the ecological context and history of land-use, but there is little uncertainty that agricultural conversion can cause profound loss of biodiversity and ecosystem function (Best et al., 2001; Wade et al., 2007). Typically, only remnants of natural habitat are retained and the spe- cies that flourish are commensal with human dominated landscapes.
Human needs for land and natural resources will only increase; therefore, the challenge to conservation biolo- gists and land managers is identify not only the problems associated with agricultural conversion, but also the solu- tions that will minimize local and regional losses of biodi- versity (Kleijn and Sutherland, 2003). Increasingly, these solutions are to be found in the disciplines of ecological restoration and landscape ecology. The objective of this chapter is to briefly review the effects of agricultural con- version on biodiversity and to review the effectiveness of selected restoration practices with a focus on avian com- munities and populations.
Habitat Fragmentation in Agricultural Landscapes
Habitat fragmentation has two elements; habitat loss and isolation of the remaining tracts. The effects of fragmen- tation on biodiversity and the integrity of ecosystems are largely determined by the size of habitat tracts, the spa- tial arrangement and connectedness of these tracts, and
Maintaining and
Restoring Avian Habitat in Agricultural Landscapes
Jeffrey D. Brawn
University of Illinois at Urbana-Champaign, Urbana, IL, USA
CASE STUDY USA
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the landscape matrix in which these tracts reside. The mobility and size of an organism determine the relevant spatial and temporal scales at which these factors exert influence. Birds, for example, are comparatively mobile and vagile; therefore, their population processes can op- erate at the local to regional spatial scale (Robinson et al., 1995). At more local scales, the habitat-matrix interface is especially important for avian productivity since “edge effects” can have profound influence on the nesting suc- cess of resident populations (Brawn and Robinson, 1996).
Larger tracts of habitat in a landscape matrix that is not inhospitable generally support more diverse and viable communities.
In North America, much research on the habitat frag- mentation has been conducted in the Midwest where the land’s great value for agricultural production with inten- sive row cropping has led to chronic fragmentation and habitat loss (Donovan et al., 1997). For forest birds, the source-sink dynamic among spatially structured subpop- ulations appears to be especially applicable. Small habitat patches in a matrix of agriculture function as population sinks and, plausibly, ecological traps. The productivity of bird populations in small forest fragments is so low, owing to nest predation and brood parasitism, that many species likely persist only because of emigration from less frag- mented and more productive source regions (Robinson et al., 1995; Brawn and Robinson, 1996). Similar find- ings have been reported in European landscapes as well (Baillie et al., 2000). The effects of agricultural conver- sion in North America have been most serious for species that inhabit grassland or prairie ecosystems. As a result, grassland bird communities are among the most threat- ened (Herkert, 1994).
Solutions for Habitat Loss and Fragmentation
Natural habitat in agricultural landscapes comes about by preservation or restoration. Much research has been devoted to assessing how to maximize the value of rem- nant or restored natural habitat. In the U.S., government sponsored programs to set aside agricultural land (e.g., the Conservation Reserve Programs) for soil, water and
wildlife conservation have motivated research on the value of filter strips or conservation buffers for wildlife.
These buffers are often created between rivers or streams and production fields. A study of 33 filter strips in Iowa found that many were too narrow to afford protec- tion from nest predators characteristic of edge habitats.
Notwithstanding, the authors concluded that the strips were of value to certain species of management concern (Heningson and Best, 2005).
Research on avian conservation and agriculture in the United Kingdom, where the government has committed to reverse the declines of birds by 2020, also indicate that linear habitats such as hedgerows and field margins can enhance the local diversity of birds, but the primary prob- lem is management practices in fields themselves. In a more general study of the effects of environmentally sen- sitive farming practices on biodiversity in five European countries, Kleijn et al. (2006) concluded that the primary benefit was to species that are relatively common.
The function of linear habitats as corridors for enhanc- ing habitat connectivity has been assessed for a diverse suite of habitats and species (Chetkiewicz et al., 2006).
Generalizations about the value of these corridors for movements are controversial. The use and value of cor- ridors depends largely on the resource requirements, behavior, and perceptual abilities of the organism under consideration (Baguette and Van Dyck, 2007). For highly mobile organisms such as birds, the value of movement corridors may be different than for species than cannot easily bridge unfavorable habitats. Notwithstanding, the value of corridors appears positive for terrestrial organ- isms such as butterflies and large mammals (Davros et al., 2006).
Restoration efforts in agricultural landscapes have proven successful in attracting and retaining unique com- munities and populations of birds – especially in distur- bance mediated ecosystems such as grasslands and sa- vannas (Fletcher and Koford, 2002; Brawn et al., 2001;
Brawn, 2006). With managed application of ecological disturbance, such as the use of prescribed fire, otherwise poor habitats can be restored relatively quickly. The size of restored tracts can be an important determinant of the diversity of constituent animal communities in grassland and forest species. In contrast, communities of birds as- sociated within disturbance-mediated habitat and ecosys-
Stewardship of Biodiversity
tems may be relatively area insensitive. For example, in North America, the size of successional habitats such as scrublands and oak savannas account for little variation in the diversity of birds (Brawn, 2006). Such “area insen- sitivity” enables land managers to consider the restora- tion and management of certain habitats on a relatively small scale in landscapes dominated by production agri- culture and where the restoration of large tracts is simply infeasible.
Research Needs and Future Directions
In a comprehensive review, Wade et al. (2007) recom- mend a complex research agenda for establishing ef- fective and sustainable management practices that will maintain some semblance of biodiversity where agricul- tural production is (or will be) demanding of land and other resources. A list of sample research questions and themes is provided in Table 3.1. Nearly all management options will require region-specific biological monitor- ing and significant support from governments to offer incentives to landowners. Fundamental questions for new issues such as biofuel production include the effect of alternative crops and feedstocks on biodiversity and
the potential of native grasslands for biofuels production.
Though the issue is not new, the role of controlling farm pests with biological agents such as insectivorous birds is receiving renewed interest (e.g. Van Bael et al., 2008) and merits additional attention. Growth in the resources needed to meet human demands will require innovative incentives and solutions for maintaining local and region- al biodiversity. Close and mutual communication among researchers, managers and policy makers is essential.
What are the implications of increased emphasis on cellulosic or food crop biofuel production? Do certain options provide better habitat for birds than traditional row crops?
What is the role of birds in integrated pest management? What landscape configurations enhance the effectiveness of birds as predators of crop pests?
Do conservation buffers in agricultural landscapes support viable populations of birds?
What size should restoration units be to attract a viable grassland bird community? What effect does the surrounding landscape matrix have on the community. Are certain birds of conservation concern area-insensitive?
What can be done to minimise the probability of nest predation on birds in agricultural landscapes?
Are patch connectivity and corridors necessary for highly mobile species such as birds?
What is the most cost-effective procedure for monitoring the suc- cess of restoration efforts in agricultural landscapes?
Table 3.1. Selected research questions and themes for the management and conservation of birds in Agricultural Landscapes.
Chapter 3
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Chapter 4
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Chapter 5
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