Draft National Wind Farm Development Guidelines - July 2010

the incidence of wildlife collisions with traffic, major new road developments are not
routinely required to predict the number of fauna collisions that might occur once they are
operational nor to quantify the effects such developments might have on fauna
populations.
The effects of wind farms on birds and bats should be properly assessed in order to
determine the real impacts they may have. Negative effects on birds and bats can be
caused by:
• Direct loss of habitat.
• Reduction in habitat suitability resulting from aversion to the presence of turbines,
associated infrastructure and other disturbance caused by the wind farm.
• Injuries and fatalities resulting from interactions with turbine structures.
It should be noted that there are two distinct values that can be placed on any impact on
birds or bats. Both may be valid but it is vital to have clarity about the two concepts and to
distinguish them.
The first is the purely biological effect on the functioning of a bird or bat population. The
second is the level of community and political ‘acceptability’ of the impact. The latter may
be subjective and indefinable and some iconic species clearly rate higher than others in
public perception. This measure is a reality and should not be ignored in decision-making.
However, objective evaluation should principally be concerned with the first measure
which relates to the persistence of populations. Whatever the cause, all individuals die and
thus impacts should be measured in terms of the effect they may have on the functioning
of populations and any consequent change in the conservation status of bird or bat taxa.
This is clearly of most relevance to threatened species.
A principle of ecology is that, all other things being equal, naturally functioning populations
maintain equilibrium through regular turn-over as individuals that die are replaced by those
that are born and survive to adulthood. The population size is regulated by a host of
natural factors, such as availability of food and nest-sites, numbers of predators, etc. Such
naturally functioning populations are limited only by such factors and if their influences
remain reasonably constant then the population can be expected to continue to replace
itself and remain relatively stable. Such a population is considered to be densitydependent
and, so long as appropriate resources are available, the death of any
individual is of no consequence to the population as it will be replaced by another. Some
fluctuations due to minor environmental variables are normal and usually do not affect the
long-term stability of secure populations. Where this equilibrium is upset, such as by the
sudden increase or decrease in numbers of a key predator, a population may decline or
increase despite the influence of other resources. Ultimately, it will then either reach the
point of extinction or of a new equilibrium in an altered ecological state.
Despite this underlying ecological principle, density-dependence, or –independence is
difficult to demonstrate in wild populations. In part, this is often an issue of geographic
scale. Populations can usually be studied only at a local level in which local fluctuations
may mask the stability or otherwise of the entire broader population.
Any impacts on bird and bat populations should be assessed from this ecological
perspective. Alienation or direct loss of habitat can be expected to reduce the local
population in proportion to the loss. However, if resources required by the population are
not lost but some individuals die due to collisions, this may not necessarily have an impact
on the population because deaths due to the collisions may simply replace deaths due to
other causes and the individuals lost may be replaced by births or increased survivorship of
other individuals. On the other hand, an artificially increased mortality rate could be
deleterious and deaths may be of much greater importance for populations already in
decline due to other causes. They may also influence local populations if particular age- or
sex-classes are more susceptible to wind farm mortalities than are other classes. This could
occur if particular components of the population (e.g. breeding males or females, first-year
juveniles, etc.) behave in ways differently from others.
Draft National Wind Farm Development Guidelines – 2 July 2010 Page 115