Estimating Inter-annual Variability in Project Take for Rare Events

Compliance for incidental take permits (ITPs) requires monitoring to demonstrate that the number of fatalities identified are in compliance with estimated take limits established in an Eagle Conservation Plan or Habitat... [ view full abstract ]

Compliance for incidental take permits (ITPs) requires monitoring to demonstrate that the number of fatalities identified are in compliance with estimated take limits established in an Eagle Conservation Plan or Habitat Conservation Plan. The duration of an ITP often ranges from 5 to 30 years; and understanding the potential sources and effects of annual variability in take estimates is important for identifying authorized take limits and for determining the amount of monitoring required to have confidence in the measurement of project impacts. If take is likely to be stable, one or two years of monitoring may effectively measure the take rate. However, if inter-annual variation is high or operational measures are implemented that are expected to change the fatality rate, longer or additional periods of monitoring will be required to confidently estimate project take.
Inter-annual variation in take can be driven by random natural temporal variation, landscape-scale variables such as temporal trends in population size, or operational changes (e.g., low wind speed curtailment [LWSC] or deterrents). It is important to distinguish between random natural temporal variation and the other two sources that drive change in the take rate. In analyzing data where the estimated rates of take change over time, it is important to have both a trigger for identifying when a change has occurred and a metric for measuring change. Triggers and metrics may vary depending on the cause of the change and the data available for estimating the change.
Hawaiian hoary bat take at the Auwahi Wind Farm is presented as a case study where the long-term monitoring strategy utilizes pulsed standardized monitoring periods. The monitoring pulses are timed to provide measures of estimated take over the life of the 25-year permit. Standardized monitoring allows for the identification of temporal changes resulting from landscape-scale changes such as population size and for the measurement of the effectiveness of implementation of LWSC. There is strong evidence that LWSC is effective in reducing take of migratory tree-roosting bats, and we assume that implementation of LWSC is a source of change in the estimated take rate. Therefore, the implementation of LWSC alone serves as the trigger for incorporating a change in estimated take rate into the estimation of project take. The measurement of a perceived change in take is quantified by comparing the median value of annual take from the baseline to the changed condition.
Longer ITP terms and the need for adaptive management of operations require approaches to evaluating whether operational changes or temporal changes in the estimated take rate are occurring over time. Long term measurements of estimated take require quantification of these temporal changes. Because the take of listed species is typically infrequent at wind farms, approaches using metrics based on small sample sizes such as the case study presented here or surrogate measures may be required.