1. Overview |
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- Evidence currently available suggests that the decrease in abundance
of some of these species is due to changes in breeding
productivity, whilst other species appear to have declined
primarily due to a fall in survival
rates.
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- The relative influence of both natural
(predation, disease and parasitism, climate) and human-related
factors (hunting, agricultural intensification) on the survival
rates of declining species are discussd.
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a. |
Predation may be responsible for the reduction
in survival rates and subsequent decline in abundance of some British
bird species, particularly waders and gamebirds, at a local scale.
However, there is little evidence to suggest that increased rates
of predation are responsible for the widespread population declines
observed for other species of conservation concern throughout the
UK. |
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b. |
The decline in grey partridge (Perdix perdix) abundance may
be due to a fall in survival rates resulting from an increased incidence
of infection by parasitic nematodes. Again, however, there is no evidence
to indicate that population declines of other species of conservation
concern are related to increases in levels of parasitism
or disease. |
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c. |
Climatic variables have been linked
to changes in both survival rates and population sizes. However, the
lack of a consistent trend towards an increase in the severity of
these conditions suggests that climatic changes are not directly responsible
for the widespread population declines of species of conservation
concern. |
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d.
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Previous studies suggest that, when unchecked, hunting
has the potential to significantly decrease the survival rates and
population sizes of raptors and wildfowl. Whilst there is evidence
that hen harrier (Circus cyaneus) populations may still be
limited in size by human persecution, hunting pressure on other species
has been reduced via regulation of shooting and legal protection.
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e.
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Decreases in food availability due to agricultural
intensification may be driving decreases in survival rates, and
therefore decreases in population sizes of some farmland bird species.
The implementation of land management practices which increase the
availability of food on agricultural land have led to a rapid increase
in the size of the UK cirl bunting (Emberiza cirlus) population. |
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