دانلود رایگان مقاله تأثیر عمده تفکیک جزیی نیچ علوفه بر بروز بوتولیسم مرغی

Abstract

Avian botulism is a fatal disease of birds caused by ingestion of neurotoxins produced by Clostridium botulinum type C and is now recognized as the most common cause of death in waterbirds worldwide. Although tens of species have been reported to suffer from avian botulism, it remains unknown which ecological factors primarily determine inter-specific variation in the incidence of this disease. We hypothesized that an exposure of birds to botulin may largely depend on their foraging niche, as the toxin is available mostly at the sediment surface, especially during the carcass-maggot stage of botulism epizootics. To test this hypothesis we used capture-recapture methods to estimate mortality of two shorebird species differing in bill morphology and foraging niche, wood sandpiper Tringa glareola (short bill, surface-feeding) and common snipe Gallinago gallinago (long bill, deep probing), during a major avian type C botulism outbreak in central Poland. All the reported cases of shorebird mortality were attributed to botulism and we found large differences in daily survival rates of both species (0.87 and 0.99 in wood sandpipers and common snipe, respectively). Even assuming much shorter stopover duration of wood sandpipers, survival rate over the entire stopover period was estimated at 0.57 in the wood sandpiper and at 0.90 in the common snipe. To our knowledge, this is the first non-circumstantial evidence that relatively minor differentiation of foraging niche may have a major impact on the incidence of avian botulism in birds. Our data might also suggest that, on the evolutionary time scale, avian type C botulism may constitute a strong selective pressure acting on foraging niches of shorebirds, and possibly other waterbirds.

Discussion

Our capture-recapture study of two shorebird species, wood sandpiper and common snipe, revealed substantial differences in botulism-related mortality. Daily survival rates during the botulism epizootic were estimated at 0.87 in the wood sandpiper and 0.99 in the common snipe. Even assuming much shorter stopover duration of wood sandpipers (4.0 days vs. 16.7 days in the common snipe), survival rate over the entire stopover period was estimated at 0.57 in the wood sandpiper and at 0.90 in the common snipe. As both species used the same foraging habitat and, thus, were expected to have similar exposure to botulism neurotoxins released into the environment, we suggest that these striking differences in survival were most likely attributable to a differentiation of foraging niche via changes in bill morphology. The effect of foraging niche on the incidence of botulism in shorebirds has already been suggested based on some circumstantial observational evidence. Surveys of shorebirds affected by botulism in Canada showed a dominance of surface feeding species, indicating that they may be more prone to botulism than probers (reviewed in Adams et al. 2003). For example, two species of dowitchers Limnodromus, which are specifically adapted for deep probing, were clearly underrepresented in the mortality surveys during botulism outbreaks (Adams et al. 2003). By contrast, surface-feeding smallCalidrissandpipers have been reported to be most commonly affected by avian botulism, although the counts were not corrected for relative abundance (Strauman 1996; Peers 1998).