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Conservation of Biodiversity within Canadian Agricultural Landscapes: Integrating Habitat for Wildlife PIERRE MINEAU and ALISON MCLAUGHLIN Canadian Wildlife Service National Wildlife Research Center 100 Blvd. Gamelin Hull, Qu6bec Canada, K1A 0H3

Industrialized agriculture currently .substitutes many of the ecological functions of soil micro-organisms, macroinvertebrates, wild plants, and vertebrate animals with high cost inputs of pesticides and fertilizers. Enhanced biological diversity potentially offers agricultural producers a means of reducing the cost of their production. Conservation of biodiversity in agricultural landscapes may be greatly enhanced by the adoption of certain crop rnanagement practices, such as reduced pesticide usage or measures to prevent soil erosion. Still, the vast monocultures comprising the crop area in many Canadian agricultural landscapes are often of limited conservation value, thus the inclusion of appropriate wildlife habitat in and around arable lands is a fundamental prerequisite for the integration of wild species within agricultural landscapes. This review of current literature considers the potential for non-crop areas within agricultural landscapes to be reservoirs of agronomically beneficial organisms including plants, invertebrates, and vertebrate species. Non-crop habitats adjacent to crop land have been identified as significant for the maintenance of plant species diversity, for the conservation of beneficial pollinating and predatory insects, and as essential habitat for birds. A key component for enhancement of biodiversity is the reintroduction of landscape heterogeneity by (1) protection and enhancement of key non-crop areas, (2) smaller fields and farms, and (3) a greater mixture of crops, through rotation, intercropping and regional diversification. The benefits of increased biodiversity within arable lands are reviewed for various species groups. In the Canadian context, any serious attempt to derive significant agronomic benefit from increased biodiversity will require considerable changes in the agricultural programs and policies which shape mainstream industrialized agriculture. The problems of crop depredation by vertebrate species, weed and insect competition, which still represent significant impediments to the creation and proper management of wildlife habitat, are also discussed. Abstract

Journal of Agricultural and Environmental Ethics 1996, 9(2), 93-113

Pierre Mineau and Alison McLaughlin

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Keywords: biodiversity, wildlife habitat, agriculture, hedgerows, field margins.

Introduction Canadian biologicaldiversity,or biodiversity,comprises the vast array of living organisms which have evolved and become an integralpart of the Canadian landscape. The Canadian environment may not be as species-richas an equivalent area in the tropics but many of the species or genetic forms to be found within it are uniquely adapted and are therefore of great national and international conservation interest. While it is difficultto put a price on biodiversity,Canada draws a significant proportion of its material wealth both directly and indirectly from biological entities. The National Accounts and Environment division at Statistics Canada has begun the process of developing "natural-resource accounts" for several of Canada's natural resources-a key step towards incorporating the value of biologicallysignificant resources within politicallyinfluential statistics. The drastic reduction in biodiversitycaused by the activitiesof industrialized agriculture is one example of the way in which undervalued natural assets (biodiversity) have been depleted in favour of maximizing valuable commodities (crops). In failingto account for the ecological (and even agronomic) contributions of non-crop plants and animals (assuming this could be measured), agricultural policies have generally favoured increased efficienciesin the form of pesticideand inorganic fertilizer usage, monocropping, large uniform fields,and conversion to agricultural use of non-crop lands (such as wetlands) situated within agricultural landscapes. While industrial agriculture has been extraordinarily productive in terms of output, it has also been extraordinarily destructive in terms of natural habitat and biodiversity. Intensificationof agriculture has contributed to the vulnerability of natural plant communities (Romo, 1991) and has increased the isolationof natural or semi-natural habitats (Merriam, 1978; O'Connor and Shrubb, 1986; Wilcox, 1980; Mader, 1988). M a n y agricultural landscapes are characterized by a relative paucity of wild plant and animal species, although the species which are present may be abundant (Moore, 1977). For many groups of species, especiallythose which spend alltheir time above ground, itisthe hedgerows, shelterbelts,fieldmargins and other non-crop lands which provide the necessary habitat in agricultural landscapes. Therefore, the enhancement and preservation of these non-crop habitats is often the focus of conservation effortsin agriculture. It is highly unlikely that we can restore, into agricultural landscapes, the full complement of habitats and species native to each agricultural area. For example, an isolatedwoodlot in the Carolinian ecozone, regardless of itssize and levelof protection, is unlikely to ever again support the full complement of species typical of the pristine Carolinian forest.Similarly, an agriculturalhedgerow in the same ecozone can offer only a very truncated listof Carolinian species. Because of conflicts between humans and some species (e.g.,crop pests, large predators), it may not always be desirable to maintain all species in a given agriculturalecosystem (Gall and

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Orians, 1992). It is important for society, and ecologists in particular, to clarify and clearly articulate our goals with respect to biodiversity. The emphasis in this review is on conservation of habitat and species groups rather than on particular species. The main reason for this is outlined in the Canada Study submitted to United Nations Environment Program (UNEP) prior to the drafting of the Convention on Biodiversity (Mosquin and Whiting, 1992); namely, our species-specific knowledge is poor for most of our native fauna and flora. It is clear that it is inefficient and foolhardy to be managing on a species by species level and to forever be reacting to the threat of single species extinction. Single species management may encourage actions or polices that negatively impact on other species within the system; the costs of single species management are also quite prohibitive.

Integration of Biodiversity and Agriculture It could be argued that keeping a limited landbase totally devoted to intensive, industrialized agriculture, while leaving other areas for conservation purposes has some appeal. There is an ongoing debate as to whether those concerned with the conservation of biodiversity should encourage a reduced-input agriculture, if it means having to devote a larger land area to cultivation rather than a more chemical and energy-intensive form of agriculture over a more limited land base. There are, however, problems with relying on the intensification of agriculture to free up more land for conservation: (1) The expansion of intensive agriculture has taken place without the benefit of land use planning to ensure that a suitable quantity of representative natural habitat was set aside for conservation. Over half of the original wetlands in southern Canada have been lost including 70% of wetlands in southern Ontario (Keating, 1989). In western Canada, there is less than 13% of short grass prairie, 19% of mixed grass prairie, 16% of aspen parkland and almost none of the tall grass prairie remaining in its native state (Millar, 1986). The loss of prairie habitat accounts for a disproportionately high number of endangered species in Canada; one half of Canada's endangered and threatened birds and mammals share the prairies (Holroyd, 1991). (2) Certain mobile organisms, such as migrating birds, cannot be restricted to the geographic boundaries of a protected area, and will continue to disperse into agricultural land. Isolated patches of protected areas cannot provide habitat over the full geographic range of these organisms and conflicts between industrial agriculture and wildlife (such as pesticide poisonings) will continue to occur on agricultural lands. (3) Conversely, fertilizers and farm chemicals, both massively employed in industrial agriculture do not respect boundaries and can easily move away from areas of intensive agriculture to affect protected areas. (4) In areas where the economic stakes are significant, it may be difficult to secure protected areas or to maintain the level of protection required to conserve biodiversity. It is uncertain how many protected areas might remain, and where, in a few

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hundred years as the h u m a n population expands and places increasing pressure on the land. (5) Last but not the least, there is growing consensus that there are limits to industrial agriculture. A number of indicators point to serious problems on our agricultural lands including loss of soil,as well as reduced soiland water quality (World Commission on Environment and Development, 1987; Eijsackers and Quispel, 1988; National Academy of Sciences, 1989). It is therefore clear, to these authors at least,that the resolution of current conflictsbetween agriculture and biodiversity will involve more than simply growing more food from a more restrictedland base. It is also clearthat a system of protected areas alone is inadequate to preserve biodiversity;this suggests a need to integrate wildlifein the agriculturallandscape. A network of protected areas can be an invaluable tool where this is not feasible (i.e.for interior forest species and large m a m mals), but it is not a substitute for the integration of species which can livecompatibly with agriculture (i.e.edge species and small vertebrates). Agricultural lands have the potential to play a much more positiverole in the maintenance of biodiversity,especiallyin those areas where there are competing, more disruptive land uses such as urbanization. The integration of some wild species within agriculturallandscapes helps to keep c o m m o n species common, and possibly helps to preserve a few rare or endangered species also. Our main challenge in the area of conservation has been, and will continue to be, to find the right compromise between our demands from agricultural lands and those of its native flora and fauna.

Biodiversity and Sustainable Agriculture Preserving the quantity and quality of soilsis one of the main objectives of our current efforts to make agriculture more sustainable and the object of much government funding and intervention under Canada's Green Plan (Dunn, 1993).While current effortsto protect the agricultural resource base may have a positive influence on environmental quality and, by extension, on the wild biota in agricultural landscapes, this is no guarantee that biodiversity is being preserved. The conservation of biodiversity in landscapes dominated by agriculture involves more than ensuring that our agricultural system is sustainable in terms of production. It is, however, essential to achieve agricultural sustainability (i.e.the conservation of soil and water) ifwe hope to ensure the conservation of biodiversity in and around agricultural lands. If agricultural landscapes are to be effective reservoirs for the conservation of some species, agricultural practices should not offset any potential benefits to wildlife. Objectives for conservation of biodiversity and sustainable agriculture should therefore be developed in parallel (Paoletti et al.,1992). The challenge will be to set biodiversity objectives for agricultural areas which are clearly enunciated and buttressed by the necessary research, and to subsequently incorporate such objectives into programs targeted at increasing agricultural sustainability.Producers and land managers will need to be convinced that these biodiversityobjectives are worthwhile.

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A good example ofjoint interestbetween conservationists and agriculturalistsis the preservation of agricultural lands in the lower Fraser delta in the face of massive urban sprawl from the Greater Vancouver area. The delta is seen as vitalto the long-term viabilityof several migratory bird populations such as American widgeon (Anas americana). An initiativeunder the North American Waterfowl Management Plan entitled"Greenfields"encourages farmers to plant winter cover crops (through cost-sharing arrangements) and adopt other soil conservation strategies to help maintain soil quality and integrity and also improve wintering habitat for birds (Duynstee, 1992).

Agricultural Benefits from Increased Biodiversity It is usual to see the argument that genetic biodiversityshould be protected because the germ plasm thus conserved is the raw material we may require for future improvements to our crop species.Although a very valid concern, this is not the type of benefit we wish to explore in this paper. Here, we would like to consider briefly the possibilitythat agriculture may benefit directly from localbiodiversity conservation and enhancement. Several types of organisms, wild plants, macro-invertebrates and birds have been investigated for their direct agronomic contributions.

Plants A high plant diversity in crops is obviously not desirable as evidenced by the continued use of herbicides to eliminate and control unwanted weed species. On the other hand, Altieri (Altieri and Whitcomb, 1979; Altieri and Letourneau, 1982; Altieri, 1991) cites numerous examples of positive effects of weeds on crops for biological control of insect pests. In some cases weeds may even be planted deliberately with crops as a trap for insect pests (Firbank, 1993; Hokkanen, 1991) or to contribute to long-term yield by increasing soil fertility (Weiner, 1990). At least one Canadian weed is actually incorporated into an alternative agricultural system; black medick (Medicago lupulina) has recently been found to be an excellent rotational plant that enriches the soil while discouraging other weeds (Benzing-Purdie et al., 1991). As early as 1950, Cocannouer recounted how some broadleaf weeds performed valuable services in fields: they bring minerals from the subsoil to the topsoil, break up the soil for crop roots and some soil animals, prevent minerals and nutrients from being blown or leached away by storing them, increase soil humidity, and in some cases may be good indicators of soil condition (see also Hill and Ramsay, 1977; Whitehurst, 1988; Day, 1992). There have been recommendations that weeds should be managed rather than indiscriminately controlled or eradicated (Whitehurst, 1988), and there have been attempts to change growers' attitudes towards weeds (Dekker and Anderson, 1981). Weed management, rather than eradication, may offer Canadian growers substantial savings on the cost of herbicides. Hurle (1988) has noted that economic and period thresholds of weed population can be defined. Unfortunately, in the prairies (the area where the highest proportion of the land in crop or fallowed is treated an-

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nually with herbicides), two of the most heavily used herbicides, triallateand trifluralin,are both applied pre-plant or pre-emergence, before the extent of the weed competition can be assessed. This offerslittleopportunity for a more integrated weed control strategy. Similarly, the trend toward application of pre-plant herbicides in the fall as slow-release granules, would appear to be in conflict with ecologically sound weed control strategies.

Macro-invertebrates The high input, monoculture approach which has traditionally characterized annual crops is associated with an exceptionally low diversity of macro-invertebrates. Insect species that comprise the "biodiversity" of agricultural systems tend to include a disproportionately high number of widespread, mobile "weedy"taxa and a correspondingly diminished number of less widespread insects (Holloway and Stork, 1991). Diminished numbers of insects and other arthropods in annual cropping systems is of concern to agronomists: Diminishing arthropod biodiversity is correlated with the loss of natural pest control (Altieri, 1987; Altieri and Schmidt, 1985; Altieri and Whitcomb, 1979; Altieri and Letourneau, 1982; Bugg, 1992; Bugg et al., 1987; Kevan, 1986; Russell, 1989). Reduction in insect pollinators, especially bumblebees, results in the loss of pollination as a fundamental ecosystem process (Kevan et al., 1990a, 1990b; Kevan and Plowright, 1989; Corbet et al., 1991; Williams, 1986). One such example is the reduction in Manitoba alfalfa seed production from 1000 kg/ha to 150 kg/ha as field size increased and leaf-cutting bee habitat was removed during the 1940s and early 1950s (Stephen, 1955; Dr. P. Kevan, University of Guelph, pers. comm.). There is evidence that diminished macro-invertebrate populations can significantly affect decomposition rates (Morris et al., 1991) and may jeopardize community stability (Holloway and Stork, 1991; Waage, 1993; Pimentel et al., 1992). Several studies have discussed the practical aspects of enhancing the diversity of predators, parasitoids or pollinators in row and field crops through the enhancement of adjacent plant and habitat heterogeneity (Thomas and Wratten, 1988; Dennis and F r y , 1992; Altieri and Letourneau, 1982; Kevan et al., 1991). There is a need for Canadian research which examines the links between habitat composition and the abundance and diversity of insects and other invertebrates. Most orchard crops, such as apples, pears and tart cherries, also require insect pollination. MacKenzie and Winston (1984) found that native pollinators increased species' richness and abundance with increasing distance from commercial berry farms, which was attributed to the deleterious effects of pesticides, habitat destruction, and competition from managed honey bees (Apis meUifera). Canadian orchard growers appear to be increasingly aware of the drawbacks of insecticide usage, particularly of the negative impact on pollinator populations, and many are undertaking economic assessments of pest pressure prior to making a decision on pesticide spraying. Insect pest management, rather than eradication, has been efficacious for orchard growers and offers substantial savings on the cost of insecticides. Insect pest management in row and field crops may offer similar benefits.

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Birds The relationship between birds and crops has long been the subject of intense interest. Yet, for a long period, the science dwelled not on the crop depredation problem (see the next section), but rather on the potential for birds to control insect pests in crops and forests. The historical and biological basis for what has been termed "economic ornithology" has recently been researched under contract to the Canadian Wildlife Service (Evenden, 1993a, 1993b) and is the subject of an upcoming review (Kirk, Evenden and Mineau, in press). McFarlane (1976) concluded that birds may help bring down local outbreaks of insect pests (although much of that evidence is anecdotal-e.g., McAtee, 1922) but that their main benefit is to have a longer-term depression effect on agricultural pests when these are at low to moderate densities. Like insect predators and parasitoids, they may reduce the need to apply pesticides leading to a lower frequency of application. There are two very well documented situations where birds have been found to provide economically important control of insect p e s t s - b o t h concern woodpeckers. The first is the control of overwintering corn borer larvae (Barber, 1925, 1942; Wall and Whitcomb, 1964; Black et al., 1970; 1971; Floyd et al., 1971) and the second, control of coddling moths in orchards of Nova Scotia (MacLellan, 1958, 1971). Recent research has also shown that bird predation is important in limiting grasshoppers in range (Fowler et al., 1991) and cutworms in cereal fields, even in the context of today's intensive agricultural practices. In order for birds to be able to exert some control over insect pests, the right conditions must be present. As pointed out by some of the early investigators, birds cannot be expected to provide control if there is no habitat nearby to fill their other requirements. Birds tend to be more active on the borders of fields (Best et al., 1990) and it is probable that, the larger the field, the lesser the impact of bird predation on crop pests.

Conflicts between Agriculture and Vertebrate Wildlife In the context of benefits and disadvantages stemming from a higher level of biodiversity on the farm, it is important to consider that vertebrate wildlife, including birds, can at times enter into direct conflict with agriculture. A few species of birds such as American crows (Corvus brachyrhynchos), blackbirds [mostly red-winged blackbirds] (Agelaiusphoeniceus) and common grackles (Quiscalus quiscula)] and passenger pigeons (Ectopistes migratorius) as well as a few mammals, such as whitetailed deer (Odocoileus virginianus), racoons (Procyon lotor) and grey squirrels (Sciurus carolinensis) especially are thought to have had a major influence on the beginnines of agriculture in North America (Jones, 1974). North American Indians were said to be skilled in bird-scaring techniques and kept a constant vigil over their cornfields. Passenger pigeons caused the most problems to early agriculture by eating grain at seeding or descending in large flocks into the ripening grain fields. The first underground seeder was apparently built in response to farmer's requests to make seed grain unavailable to the birds (Jones, 1974). According to Jones (1974), grey squirrels were the most destructive mammalian pest in Pennsylvania and neigh-

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bouring States resulting in extensive (and costly) bounties being instituted. Whether or not concerns are always warranted, the issue of crop loss to vertebrate wildlife represents a significant impediment to the creation and preservation of wildlife habitat in agricultural landscapes and the problem needs to be addressed. Rodenhouse et al. (1994) estimated that only 10 of 215 migrant landbirds are reported to cause significant damage to crops over a wide geographical area; albeit some of the unwanted species are very abundant. The red-winged blackbird is by far the most important. In both agriculture and forestry, we have elevated certain insect species to the rank of major pest by providing them with large areas of uninterrupted feeding opportunities. The same argument can be made with respect to birds. Stone et al. (1973) reported that crop damage from blackbirds is highest where crop and landscape diversity are lowest and, therefore, where the birds have the fewest sources of alternate food; landscape simplification to corn monoeulture has meant an increased reliance on the crop on the part of blackbirds. Despite the propensity of that species to take corn borers during the breeding season, this probably does not compensate for the damage done to ripening corn (Bendell and Weatherhead, 1982). In western Canada, the most publicized situation is the impact of waterfowl on swathed grain left to dry in the fields before it is combined. An extensive crop compensation and insurance program is available to growers. The most recent analysis, however, found that correlations between mallard (the most abundant duck species and the one most implicated in damage claims-Anas platyrhynchos) or goose abundance and crop damage were weak, whereas those between damage and harvest delay were consistently much stronger (Clark et al., 1993). The abundance of mallards or geese was not found to contribute substantially to the magnitude of crop damage, even after accounting for the effects of harvest chronology. As outlined earlier, there is evidence that many bird species can be useful to agronomic interests. A single historical reference will suffice to remind us of the dangers of being too quick in using the "pest" label to describe any one species. In the case of blackbirds, Benjamin Franklin had the following to say in 1749: In New England, they once thought blackbirds useless and mischievous to the corn. They made efforts to destroy them. The consequence was, the blackbirds were diminished; but a kind of worm, which devoured their grass, and which the blackbirds used to feed on, increased prodigiously... They wished again for the blackbirds. (Cited in Glacken, 1967) According to Jones (1974) the loss of hay following the partial eradication of blackbirds was such that it had to be imported from other areas.

Herbaceous F i e l d M a r g i n s a s Habitat for Beneficial Organisms Uncropped herbaceous wildlife strips, with no fertilizer, lime, and limited pesticide usage, and cultivation, have been found to provide high diversities and numbers of

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plants and invertebrates. These include both rare biota, and beneficial predatory arthropod species which have been found to disperse into the crop (Hawthorne, 1993). Although crops do support a variety of insects, most of which are common and abundant, most evidence indicates that field margins have the best potential to enhance species richness and may even help to restore predator-prey relationships. Phytophagous insect populations generally seem to recover faster after disturbance events, as compared to predatory insect populations, thus, it appears that stable, complex field margins seem to favour natural predator control. Morris and Webb (1987) have reviewed the importance of field margins for the conservation of insects in the United Kingdom. American and European studies have shown that vegetation in areas adjacent to the crops can provide food and habitat essential for the development of predators (Thomas and Wratten, 1988; Dennis and Fry, 1992) parasitoids (Altieri and Letourneau, 1982), and of other invertebrates of conservation interest such as butterflies (Dover, 1991). Altieri and Todd (1981) demonstrated that predators were more abundant in the edges of soybean fields adjacent to pea fields or weedy tracts than in soybean edges adjacent to vegetation free fields. In their study of herbaceous field margins, LagerlSf and Wallin (1993) found that field margins with a naturally diverse flora harboured the highest abundance and diversity of above-ground arthropods, and that dense monotypic stands of ceuchgrass (Agropyron repens), which is considered to be a serious weed species, provided an important refuge for predatory insects. Field boundaries around cereal fields in the U.IC have been found to harbour over 1000 aphid-eating insects per square meter (i.e. ground beetles, rove beetles, spiders, hoverflies) in the winter, and have thus been identified as important habitat areas for predatory insects (Thomas, 1989, 1991; Chiverton, 1991). However, not all weed-insect associations are beneficial to the farmer's interest. It was shown, for instance, that the abundance of Carrot rust fly (Psila rosea) in non-crop borders could be explained by the presence of stinging nettle (Urtica dioica) (Wainhouse and Caker, 1981). In the Canadian context, it is important to emphasize the role of field margins, headlands or turn-rows, fence-lines, road, rail, and utility fights of way, public lands, and so forth as overwintering habitats for a wide diversity of invertebrates. Few predatory insects survive the Canadian winter in open cultivated fields and densities of these insects in the spring depend on available refuges (Doane, 1981). Much of the mammal and bird wildlife in intensively farmed parts of Canada probably depends in turn on the activities of insects in pollinating wild plants which provide winter and spring sustenance to herbivores. H e d g e r o w s a n d S h e l t e r b e l t s as H a b i t a t f o r B e n e f i c i a l O r g a n i s m s Not all species of wildlife benefit from hedgerows and shelterbelts; however, the creation of these "linear" habitat features offers promise as a compromise between the need for the creation of non-crop habitat in low diversity agricultural landscapes and the needs of the farmer for cost-effective measures to prevent soil loss and conserve moisture without excessive losses of crop area. Hedgerows consisting of tall trees planted near roads, smaller trees behind these and small shrubs planted

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nearest fields reduce wind erosion of soils and liftwind away from fields without harmful eddies (Myers et al., 1984). The horizontal extent of wind protection is generally proportional to windbreak height, while m a x i m u m wind reductions are closely related to the porosity of the windbreak (Heisler and Dewalle, 1988). The construction, maintenance and preservation of hedgerows for conservation purposes is well established in the U.K. and the value of hedgerows recognized in both the scientificand lay literature (Game Conservancy, 1987-1992; Way and Greig-Smith, 1987, Bryson, 1993). Several British groups are active in raising consciousness and assisting private landowners with design and management of hedgerows (e.g.,G a m e Conservancy Review - The Advisory Service, 1989). Whereas North-American windbreaks planted in the 1930s and '40s tended to be wide, the modern trend is towards narrower plantings (Tibke, 1988). While some narrow plantings may be sufficientfor reducing wind erosion, they are often insuffident for providing habitat for vertebrate species. Wider fencerows are believed to be more attractive to vertebrates because they are more heterogeneous in vegetative structure and composition than narrow ones (Petrides, 1942; Shalaway, 1979; Rodenhouse and Best, 1983); birds have been the subject of most hedgerow habitat studies. Stauffer and Best (1980) reported higher numbers of breeding bird species in Iowa in wider than narrower habitats. In a three-season study of hedgerows in Iowa, Best (1983) found that bird species diversity was greatest in fencerows with continuous trees and shrubs (48 species) rather than in fencerows with scattered trees and shrubs (38 species) or in herbaceous fencerows (12 species).Other studies of fencerow use by birds note similar trends: More birds are found in woody fencerows than in those containing only herbaceous vegetation (Dambach, 1945; Linehan, 1957; Shaiaway, 1979; Beyer et al.,1989). Rodenhouse et al. (1994) provide a review of the North American literature on the suitabilityof edge habitat. The relationship between the vertical complexity and other attributes of hedgerows and their suitability to birds has been most extensively studied in the U.K. (O'Connor and Shrubb, 1986; O'Connor, 1993). The relationship between bird species diversity and the complexity of the hedgerow is so predictable that it can be plotted as a linear regression. Based on the long-standing common bird census in Britain, these authors proceeded to provide recipes for enhancing bird populations in hedgerows: Generally, bird populations are larger and more diverse in wide versus narrow hedgerows strips, in undisturbed versus disturbed (e.g., grazed) hedgerows, in treed and woody hedgerows rather than herbaceous buffers, and in riparian rather than non-riparian habitats. The type of tree species planted in a windbreak also has a considerable bearing on their year-round effectiveness (Tibke, 1988), as well as the capacity of the hedgerow to provide habitat. For example, the endangered Loggerhead Shrike appears to have had mixed success in adapting to the exotic plantings of shclterbelts and hedgerows in Alberta (Smith, 1991). The "appropriateness" of the hedgerow tree species in a biodiversity framework has rarely been considered and exotic species which may not contribute to habitat requirements are still being recommended for hedgerow plantings (Canada-Saskatchewan Agreement on Soil Conservation, 1993). The use of native Canadian species displaced within Canada may also present con-

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servation problems. O n the other hand, certain exotic species are recommended for shelterbelt plantings because they provide good habitat for wildlife (Prairie Farm Rehabilitation Administration, 1990). There is a need for ecologicallysound policy on the type of tree and shrub species which should be used in wildlife habitat plantings. Before this policy can be formulated, however, there is a general need for Canadian research into the links between floralhabitat and faunal biodiversity in hedgerows. As reviewed by Rodenhouse et al. (1994),reproductive success of songbirds in agriculturalcropland is generally thought to be very low. Factors that may contribute to nest failure are: cultivationpractices,application of pesticides,enhanced predation and nest parasitism opportunities for Brown-headed cowbirds (Molothrus ater). The same can be said of waterfowl nesting in agricultural landscapes (Greenwood and Harvey, 1982). There is a real concern on the part of conservationists that narrow linear habitats such as hedgerows may be population sinks maintained by the dispersal of animals from more suitable habitat (Holt, 1985; Pulliam, 1988). Small m a m m a l studies by Rails and Ballou (1992) in Kansas revealed that certain species such as voles (Microtus sp.) and cotton rats (Sigmodon sp.)persist in greater numbers on largo patches of prime habitat, while other species such as deer mice (Peromyscus maniculatus) are found in larger numbers on smaller patches; apparently this difference was due to interspecificcompetition combined with the increased ability of some species to exploit less suitable habitat. Rails and Ballou (1992) concluded that sink populations might retard species extinctions from fragmented landscapes, but that they were vulnerable to stochasticprocesses. After considering the evidence for Britain, O'Connor and Shrubb (1986) and O'Connor (1987) concluded that hedgerows stillexerted a positive influence on birds. They concluded that hedgerows, in alllikelihood,do not attract woodland edge-nesting birds out of more suitable habitat but rather act as overflow areas. Currently, there are no available data to confirm or dispute these findings in the Canadian context. In summary, hedgerows in Canada have been typicallyseen as serving the exclusive functions of reducing soilloss and entrapping snow. Canadian ecologistslag far behind in their abilityto make specificrecommendations to farmers and landowners that would benefit wildlifewhile achieving goals of reduced soil loss and moisture trapping. There is a need for further research which will provide a base of knowledge for Canadian hedgerow management in the context of biodiversity conservation.

Managing Habitat for the Conservation of Biodiversity The considerable variation in plant species richness, lifespan,origin,and weed status among different farmland habitats is beginning to be known for Canada, but many plant communities stillneed to be documented (Achuff, 1991). If we are to manage non-crop habitats to conserve biodiversity,there is a need for Canadian research to improve knowledge of ecosystem composition and to identify plant populations thought to be at risk (Thomasson, 1991; Allen, 1991). Keddy et al. (1993) showed that many species of agricultural landscapes were unique to a particular

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habitat, particularly woodlands, woody fencerows and roadsides. Boutin et al. (1994) found that many species inventoried in woodlots, woodland edges and hedgerows in the Great Lakes-St. Lawrence ecozone were species typically characteristic of the original forests which grew there. Several rare plant species were also identified in ditches, woodlots and hedgerows. Based on their preliminary results, these authors concluded that maintaining plant species diversity in farmland requires increasing the richness of habitat types, especially non-crop habitats. Several recent publications on reduced-tillage or no-till farming have advocated a very aggressive control of weeds in field margins, either through herbicide use, burning or mowing in mid-summer (e.g., Manitoba-North Dakota Zero Tillage Farmers Association, 1991). Such actions might largely negate the value of hedgerows or fieldborders for the enhancement of biodiversity. Research is needed to see whether such management of weeds in field borders is necessary and which species in particular need to be controlled in this fashion. In Great Britain, management techniques are being promoted to ensure that margins will not be foci of weed infestation for the crop (Game Conservancy, 1987-1992). Research has shown that problems arise when plowing takes place too close to the field edge, broad spectrum herbicides are sprayed or are allowed to drift onto margins, and fertilizer is allowed to move into field edges. All of these actions favour annual species and are detrimental to the establishment and maintenance of perennial broad-leaved species. Specific herbicides are even being developed to selectively remove only those species where control in the field margins is advisable on agronomic grounds while promoting the development of "good weeds" (Game Conservancy, 1991). Clearly, hedgerows and field margins can provide habitat for agriculturally beneficial predatory insects and birds known to feed on crop pests. The efficacy of predators in suppressing pest species is dependant, however, upon their ability to disperse from field margins into the crop. The size of many cultivated fields in Canada, such as the giant wheat monocultures in the Prairies, is clearly prohibitive to the dispersal of beneficial insects. European agricultural models have succeeded in demonstrating a significant level of natural predator control within croplands, partly because of a greater abundance of field margin habitat, and partly because of the relatively smaller scale of the cropped area. In Canada, there has been a tendency to regard field margins as reservoirs of pest plants and animals, obstacles for machinery, and wasted land which would be better converted to crop. Any serious attempt to incorporate natural predator control within Canadian agriculture~means a significant shift in attitudes, reduction in field size, increased crop diversification through rotation, intercropping and regional diversification, and the introduction (or reintroduction) of hedgerows and field margins.

Agricultural Policy and Conservation of Biodiversity Today's agricultural systems are intensively manipulated; the fields are vast and, natural habitats within agricultural landscapes greatly reduced in extent. The ecological roles of plants and animals have largely been displaced by the introduction of pesticides and inorganic fertilizers. In this context, it is difficult to gain a clear

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view of any potential role for an increased biodiversity of non-crop plants, invertebrates and vertebrates in agriculture. Due to the ever-increasing cost of inputs, such as fertilizer, pesticides, fuel and land itself, growers have now run into serious debt (Myers, 1984). The response has primarily been to increase these inputs and attempt to reap even greater harvests, rather than seeking out an alternate system of agriculture requiring less costly technology. On the other hand, the steadily increasing costs of production, concern for personal and family health, environmental consciousness, and a growing awareness of various feasible options to reduce inputs has persuaded some producers to change at least some of their agricultural methods. The lack of flexibility in current agricultural policies and programs (i.e. quota and insurance programs), and the institutionalized pressure to conform to conventional agricultural practice, has tended to work at cross purposes with conservation while presenting numerous infrastructural obstacles for growers willing to adopt methods of low-input farming (Hill and MacRae, 1992; see review in Mineau and McLaughlin, 1994). Canadian farmers willing to retain wildlife habitat on their land must also work against the pressure of agricultural programs and policies which provide many incentives to convert natural habitat to cultivated acreage, and very few incentives to conserve. The protection of biodiversity within agricultural landscapes is a worthwhile objective as a wildlife conservation strategy, and, importantly, can be adapted to benefit agriculture also. Harmonization of agriculture with conservation interests will, however, require some major agricultural policy changes. Agricultural and environmental policy makers will need to consider carefully the means for integration of biodiversity within agricultural lands, and the relative merits of conventional versus alternative agricultural methods for meeting their goals. Programs and policies must then be adjusted in a consistent manner to encourage this objective. It has been suggested that, given the historical loss of wildlife habitat within agricultural areas in Canada, and the erosion of key habitat types such as wetlands, all remaining wildlife habitat within agricultural landscapes must now be conserved and enhanced (Benzing-Purdie et al., 1991). Recent policy recommendations certainly emphasize habitat protection (e.g., Federal-Provincial Agriculture Committee on Environmental Sustainability, 1990; Government of Canada, 1991) although it is not yet clear how landowners will be given the incentive to protect key wildlife habitats on their land or more generally be rewarded for environmentally-sensitive stewardship. Agricultural areas of Canada at highest risk for the loss ofhiodiversity are largely under private ownership. JUstifications for the maintenance and enhancement of biodiversity will have to appeal to the landowner directly. The landowner will need to be assured that re-establishing some habitat heterogeneity (e.g., planting hedgerows) will not be detrimental to the crop or, that if there is a cost associated with a habitat modification, that it can be measured so that adequate fiscal incentives can be put in place. Ideally, landowners who do not "maximize" crop production through intensive agriculture would not be penalized, while landowners who maintain wildlife habitat and use environmentally sound methods of cropping would be recompensed for providing valuable ecological services.

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Conclusion Agriculture has repeatedly been identifiedas one of the largest contributors to the loss of biodiversity worldwide. This is because of the large land area devoted to this activityas well as the high degree of physical manipulation and inputs of pesticides and fertilizersinherent in our current way of farming. Habitat loss and fragmentation resulting from intensive agriculture have been identified as primary reasons for the loss of biodiversity in Canada and elsewhere (Rubec, Turner and Wiken, 1992). The best agriculturallands often overlap with the most productive or critical habitats for wildlife(Rubec et al.,1992). The problem with accelerating agricultural intensification is often the cumulative loss of habitat. Ratti and Scott (1991) have remarked that, where fields are ploughed to the immediate margin of roads and streams, steep hillsidesare tilledand idle land is burned, mowed, or sprayed with non-specific herbicides, even the most c o m m o n farmland species of birds and mammals are virtuallyeliminated. Similarly,Sugden and Beyersbergen (1984) found that over half of their 101 65-ha agricultural plots in Saskatchewan parklands had little or no potential for wildlifeproduction. The vast monocultures comprising the crop area in many Canadian agricultural landscapes are often of limited conservation value, thus most of the conservation emphasis in agriculturalsystems centres on remaining non-crop habitat at the edges. In recent years, agriculturalproducers and conservationists have mutually benefited from the introduction of soil and water conservation measures under the general umbrella of "sustainable agriculture". Overwhelmingly, agronomists have found that conservation efforts on the farm are truly benefiting individual landowners. Non-crop habitat management for increased biodiversity is a worthwhile conservation effort in its own right, which can also potentially benefit agriculture. Hedgerows and field margins can provide refuge for pollinating and predatory insects, helpful soil arthropods, and birds which feed on crop pests. The degree to which any of the benefits from enhanced biodiversity are actually realized, depends largely on the willingness of growers to modify their current agricultural practices. Agricultural policy-makers must clarify their objectives with regards to biodiversity in agricultural landscapes and provide the incentives for change. Approaches to agricultural management, non-crop habitat on farmland, and biodiversity more generally, will depend on farm economy, on government policy, and on agricultural producers as well as general public awareness of biodiversity values of farmland.

Acknowledgements This paper was produced as part of a scientificassessment of Canadian biodiversity issues conducted by Environment Canada (Mineau and McLaughlin, 1994). The provision of information relatingto insect pollinatorsby Professor P. Kevan and to soil fauna by Drs. G. McLeod and A. Tomlin is gratefullyacknowledged. Input on plants and hedgerows was also received from Drs. C. Boutin and K. Freemark while M. Evenden provided the historical information relating to birds in agroecosystems. The authors are further indebted to a number of other individuals and institutions

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for their constructive comments and criticism: Hugh Boyd, Tony Diamond and Pare Krannitz, Canadian Wildlife Service; numerous staff of the Prairie Farm Rehabilitation Administration (PFRA), the Environment Bureau, and the Research Branch of Agriculture and Agi-Food Canada. The views expressed here are those of the authors and do not necessarily reflect current Canadian policy or directions.

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