RESUMEN
Infection of wild red grouse Lagopus lagopus scotica by Cryptosporidium baileyi was first diagnosed in 2010. Within three years, signs of infection were reported from grouse on half of all grouse moors in northern England, bringing severe concerns of economic losses to grouse shooting. A total of 45,914 red grouse shot from 10 moors in northern England between 2013 and 2018 were visually screened for signs of respiratory cryptosporidiosis. Prevalence varied with age, being twice as high in juveniles (4.5%) as in adults (2.4%). It also varied nine-fold between moors and three-fold between years. Prevalence was highest in grouse shot later in the shooting season. Our results are consistent with the concept that disease incidence is highest in naïve juveniles that have previously not been exposed to infection, with prevalence dropping as birds develop immunity. We found no evidence of increased prevalence over time, and fears of escalated disease prevalence, bringing with it increased mortality and lowered productivity, that may have significant impacts on the economic viability of shoots, have not yet been realized. We recommend continued annual screening for clinical signs amongst shot birds, better hygiene associated with potential reservoirs of infection, and practices that both improve the detection and selective culling of diseased individuals and generally reduce overall grouse densities.
Asunto(s)
Enfermedades de las Aves/patología , Criptosporidiosis/patología , Galliformes/parasitología , Infecciones del Sistema Respiratorio/veterinaria , Factores de Edad , Animales , Enfermedades de las Aves/epidemiología , Enfermedades de las Aves/parasitología , Criptosporidiosis/epidemiología , Inglaterra/epidemiología , Prevalencia , Infecciones del Sistema Respiratorio/epidemiología , Infecciones del Sistema Respiratorio/parasitología , Infecciones del Sistema Respiratorio/patología , Factores de TiempoRESUMEN
Benzimidazole-based anthelmintics bound to grit (medicated grit) are annually prescribed on request by veterinary practices to grouse managers to control Trichostrongylus tenuis an intestinal parasite of red grouse Lagopus lagopus scotica Those prescribing medication typically do without knowledge of parasite loads and hence often prescribe when loads are low and unlikely to impact the host. Inappropriate use of anthelmintics in livestock has led to development of parasite resistance to anthelmintics. To encourage grouse managers to reduce anthelmintic use, the authors experimentally withdrew medication from parts of eight moors. The authors monitored parasite and grouse responses by counting eggs and adult worms and grouse mortality and breeding success. Rapid increases in parasite egg counts in early spring culminated in resuming medication at three wet, blanket-peat sites; one in the first spring and two in the second. Medication was restored, despite low parasite counts, at a fourth moor. On the remaining four moors, drier heaths in the east, parasite levels remained low, were not associated with grouse mortality, but breeding success was 16 per cent lower in years without medication. Better parasite monitoring by grouse managers and vets alike may reduce anthelmintic use, helping prevent drug resistance, but this may be off-set by reduced grouse productivity.
Asunto(s)
Antinematodos/administración & dosificación , Enfermedades de las Aves/prevención & control , Galliformes , Tricostrongiliasis/veterinaria , Trichostrongylus/efectos de los fármacos , Animales , Enfermedades de las Aves/parasitología , Inglaterra , Parasitosis Intestinales/parasitología , Parasitosis Intestinales/prevención & control , Parasitosis Intestinales/veterinaria , Escocia , Tricostrongiliasis/parasitología , Tricostrongiliasis/prevención & controlRESUMEN
1. Changes in the abundance of populations have always perplexed ecologists but long-term studies are revealing new insights into population dynamic processes. Long-term data are often derived from harvest records although many wild populations face high harvesting pressures leading to overharvesting and extinction. Additionally, harvest records used to describe population processes such as fluctuations in abundance and reproductive success often assume a random off-take. 2. Selective harvesting based on phenotypic characteristics occurs in many species (e.g. trophy hunting, fisheries) and has important implications for population dynamics, conservation and management. 3. In species with no marked morphological differences between the age and sex classes, such as the red grouse Lagopus lagopus scoticus during the shooting season, hunters cannot consciously select for a specific sex or age class during the shooting process but harvest records could still give a biased reflection of the population structure because of differences in behaviour between age and sex classes. 4. This study compared age and sex ratios in the bag with those in the population before shooting for red grouse at different points in the shooting season and different densities, which has rarely been tested before. 5. More young than old grouse were shot at large bag sizes and vice versa for small bag sizes than would be expected from the population composition before shooting. The susceptibility of old males to shooting compared to females increased with bag size and was high at the first time the area was shot but decreased with the number of times an area was harvested. 6. These findings stress that the assumption made in many studies that harvest records reflect the age and sex ratio of the population and therefore reflect productivity can be misleading. 7. In this paper, as in the literature, it is also shown that number of grouse shot reflects grouse density and therefore that hunting selectivity might influence population dynamics in a cyclic species. 8. The study is not only relevant for red grouse but applies to systems showing interactions between selective harvesting and wider ecological processes, such as age- and sex-related parasitism and territoriality, which may drive population fluctuations.
Asunto(s)
Passeriformes/fisiología , Dinámica Poblacional , Distribución por Edad , Animales , Femenino , Masculino , Razón de MasculinidadRESUMEN
While the concept of population growth rate has been of central importance in the development of the theory of population dynamics, few empirical studies consider the intrinsic growth rate in detail, let alone how it may vary within and between populations of the same species. In an attempt to link theory with data we take two approaches. First, we address the question 'what growth rate patterns does theory predict we should see in time-series?' The models make a number of predictions, which in general are supported by a comparative study between time-series of harvesting data from 352 red grouse populations. Variations in growth rate between grouse populations were associated with factors that reflected the quality and availability of the main food plant of the grouse. However, while these results support predictions from theory, they provide no clear insight into the mechanisms influencing reductions in population growth rate and regulation. In the second part of the paper, we consider the results of experiments, first at the individual level and then at the population level, to identify the important mechanisms influencing changes in individual productivity and population growth rate. The parasitic nematode Trichostrongylus tenuis is found to have an important influence on productivity, and when incorporated into models with their patterns of distribution between individuals has a destabilizing effect and generates negative growth rates. The hypothesis that negative growth rates at the population level were caused by parasites was demonstrated by a replicated population level experiment. With a sound and tested model framework we then explore the interaction with other natural enemies and show that in general they tend to stabilize variations in growth rate. Interestingly, the models show selective predators that remove heavily infected individuals can release the grouse from parasite-induced regulation and allow equilibrium populations to rise. By contrast, a tick-borne virus that killed chicks simply leads to a reduction in the equilibrium. When humans take grouse they do not appear to stabilize populations and this may be because many of the infective stages are available for infection before harvesting commences. In our opinion, an understanding of growth rates and population dynamics is best achieved through a mechanistic approach that includes a sound experimental approach with the development of models. Models can be tested further to explore how the community of predators and others interact with their prey.