Until recently, large apex consumers were ubiquitous across the globe and had been for millions of years. The loss of these animals may be humankind's most pervasive influence on nature. Although such losses are widely viewed as an ethical and aesthetic problem, recent research reveals extensive cascading effects of their disappearance in marine, terrestrial, and freshwater ecosystems worldwide. This empirical work supports long-standing theory about the role of top-down forcing in ecosystems but also highlights the unanticipated impacts of trophic cascades on processes as diverse as the dynamics of disease, wildfire, carbon sequestration, invasive species, and biogeochemical cycles. These findings emphasize the urgent need for interdisciplinary research to forecast the effects of trophic downgrading on process, function, and resilience in global ecosystems.

Background:The largest terrestrial species in the order Carnivora are wide-ranging and rare because of their positions at the top of food webs. They are some of the world's most admired mammals and, ironically, some of the most imperiled. Most have experienced substantial population declines and range contractions throughout the world during the past two centuries. Because of the high metabolic demands that come with endothermy and large body size, these carnivores often require large prey and expansive habitats. These food requirements and wide-ranging behavior often bring them into confl ict with humans and livestock. This, in addition to human intolerance, renders them vulnerable to extinction. Large carnivores face enormous threats that have caused massive declines in their populations and geographic ranges, including habitat loss and degradation, persecution, utilization, and depletion of prey. We highlight how these threats can affect the conservation status and ecological roles of this planet's 31 largest carnivores.

Among Earth's most stunning, yet imperiled, biological phenomena is long-distance migration (LDM). Although the understanding of how and why animals migrate may be of general interest, few sitespecific strategies have targeted ways in which to best retain such increasingly rare events. Contrasts among 29 terrestrial mammals from five continents representing 103 populations indicate that remnant long-distant migrants have poor long-term prospects. Nonetheless, in areas of low human density in the Western Hemisphere, five social and nongregarious species, all from the same region of the Rocky Mountains (U.S.A.), still experience the most accentuated of remaining New World LDMs south of central Canada. These movements occur in or adjacent to the Greater Yellowstone region, where about 75% of the migration routes for elk (Cervus elaphus), bison ( Bison bison), and North America's sole surviving endemic ungulate, pronghorn (Antilocapra americana), have already been lost. However, pronghorn still migrate up to 550 km (round-trip) annually. These extreme movements (1) necessitate use of historic, exceptionally narrow corridors (0.1-0.8 km wide) that have existed for at least 5800 years, (2) exceed travel distances of elephants ( Loxodonta africana) and zebras ( Equus burchelli), and (3) are on par with those of Asian chiru ( Pantholops hodgsoni) and African wildebeest (Connochaetes taurinus). Although conservation planners face uncertainty in situating reserves in the most biologically valued locations, the concordance between archaeological and current biological data on migration through specific corridors in these unprotected areas adjacent to the Yellowstone system highlights their retention value. It is highly likely that accelerated leasing of public lands for energy development in such regions will truncate such migrations. One landscape-level solution to conserving LDMs is the creation of a network of national migration corridors, an action in the Yellowstone region that would result in de facto protection for a multispecies complex. Tactics applied in this part of the world may not work in others, however, therefore reinforcing the value of site-specific field information on the past and current biological needs of migratory species. LaÚltima Milla: Como Sostener la Migración de Larga Distancia en Mamíferos Resumen: Entre los fenómenos biológicos más asombrosos, pero en peligro, de la Tierra está la migración de larga distancia (MLD). Aunque el entendimiento de cómo y porque migran los animales puede ser de interés general, pocas estrategias sitio-específicas han encontrado formas para retener tales eventos cada vez más raros. Los contrastes entre 29 mamíferos terrestres de cinco continentes que representar a 103 poblaciones indican que las MLD remanentes tienen perspectivas pobres a largo plazo. No obstante, enáreas con bajas densidades humanas en el Hemisferio Occidental, cinco especies sociales y no gregarias, todas de las misma región de las Montañas Rocallosas (E.U.A.) aun experimentan las MLD más acentuad...