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Evolving agricultural policies have influenced management practices within agroecosystems, impacting available habitats for
many species of wildlife. Enhancing wildlife habitat has become an explicit objective of existing agricultural policy. Thus,
there is renewed focus on field borders and the use of shelterbelt agroforestry systems to achieve conservation goals in the
Midwest. Two Representative Farms – a 283-ha dryland and 510-ha irrigated farm were created in Saunders County, Nebraska.
The Habitat Analysis and Modeling System (HAMS) was used to describe the composition and spatial pattern of the existing farms
and surrounding landscape, as well as for the landscapes surrounding selected Breeding Bird Survey (BBS) routes. Simulated
land use changes resulting from the implementation of two shelterbelt scenarios, Agricultural and Wildlife, were incorporated
on each Representative Farm and surrounding landscape. Landscape variables which influence breeding bird species richness
and community composition as determined from BBS routes were measured on simulated farm landscapes. A more heterogeneous landscape
results from implementing either scenario. The percent total woods was a significant determinant of bird species richness
on the BBS routes and was important in influencing bird communities at the farm- and landscape-level. Other landscape metrics
which influenced the bird community composition on BBS routes were woody edge percentages and edge density values. Policies
promoting shelterbelts create edge habitats which ultimately favor birds within the Forest-edge/generalist guild while bird
species in need of conservation such as grassland-field species would potentially be negatively affected.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
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Squyres SW Aharonson O Clark BC Cohen BA Crumpler L de Souza PA Farrand WH Gellert R Grant J Grotzinger JP Haldemann AF Johnson JR Klingelhöfer G Lewis KW Li R McCoy T McEwen AS McSween HY Ming DW Moore JM Morris RV Parker TJ Rice JW Ruff S Schmidt M Schröder C Soderblom LA Yen A 《Science (New York, N.Y.)》2007,316(5825):738-742
Home Plate is a layered plateau in Gusev crater on Mars. It is composed of clastic rocks of moderately altered alkali basalt composition, enriched in some highly volatile elements. A coarsegrained lower unit lies under a finer-grained upper unit. Textural observations indicate that the lower strata were emplaced in an explosive event, and geochemical considerations favor an explosive volcanic origin over an impact origin. The lower unit likely represents accumulation of pyroclastic materials, whereas the upper unit may represent eolian reworking of the same pyroclastic materials. 相似文献