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Most studies investigating demographics focus on the breeding portion of the population for multiple reasons. Recruitment must follow reproduction, and hence, the breeding component of populations is the obvious place to start when assessing population status. Second, it is fairly easy to monitor individuals during their reproductive cycle, particularly those that initiate, maintain and defend territories and/or nests, or are conspicuous for other reasons, for example, mate attraction. However, by focusing attention only on the breeding population, researchers may neglect an important demographic component influencing species or population trajectory (Penteriani, Ferrer & Delgado, 2011). In species where individuals are long-lived and either require multiple years to achieve reproductive maturity or breeding territories and nest sites are limited, sub-adults or non-territorial ‘floaters’ may reside elsewhere geographically, away from breeding individuals and therefore, undocumented during census work. For example, to reproduce and defend territories is costly (e.g. Hanssen et al., 2005; Harshman & Zera, 2007). Therefore, it may benefit individuals who are not defending territories or maintaining a pair bond to focus on meeting life-history requirements elsewhere or possess movement patterns that may elude researchers when conducting field counts.

Accurate appraisal of population size, including non-breeding individuals, however, is crucial for conservation planning (Hunt, 1998; Newton, 1998; Penteriani et al., 2011), and coupled with finite resources for conservation, researchers are often limited by their ability to monitor an entire population to assess overall population stability. Rudnick et al. (2005, 2008) provided a relatively simple approach using non-invasive ‘genetic tags’ to assess population size, assuming that the species under study possess behavior where non-territorial individuals roost communally allowing systematic tissue collection. By collecting all molted feathers (n=1800+) below multiple communal roost sites of the Eastern imperial eagle Aquila heliaca, then genotyping each feather using a panel of polymorphic microsatellite loci to identify unique individuals similar to DNA-based profiling used in forensics, the researchers were able to provide a minimum estimate of non-breeding individual abundance within the Naurzum Zapovednik in north-central Kazakhstan. When compared with the genotypes of the breeding pairs on known territories (∼40) in the study area, the researchers were able to assess the relative frequency of breeders to non-breeders in the population. Their findings showed that the non-breeding population size determined by traditional census methods conducted by direct observation was greatly underestimated by an order of magnitude and was actually well over three times the size of the breeding population (280 and ∼70 individuals, respectively; Rudnick et al., 2008). In any situation focused on population management, such dramatic bias would easily introduce error and possibly misdirect efforts originally intended to preserve or improve long-term population viability. This is certainly the case for the Eastern imperial eagle population in Naurzum Zapovednik as reported by Katzner et al. (2011) where discrepancies between direct observational census estimates and those based on genetic tags at communal roosting sites (Rudnick et al., 2008) led to very different results on modeled population estimates.

What is not known, or at least not reported in Katzner et al. (2011), is the extent to which Eastern imperial eagles exist immediately outside of the study area, and thereby inhabit additional breeding territories that could also be used by non-breeders once available or unoccupied. To what extent would this additional information influence the model results given by Katzner et al. (2011)? For example, as reported by Rudnick et al. (2008), despite the lack of population differentiation observed among sampled non-territory individuals based on a genetic clustering method implemented in the program Structure (Pritchard, Stephens & Donnelly, 2000), the estimate of FIS was positive, albeit low, but significant suggesting either possible inbreeding or a Wahlund effect where genetically divergent subpopulations are unknowingly included in a single population sample, that is, non-breeders. The latter scenario resulting in a positive FIS value is more likely given the extent to which breeding territories are limited and the species' potential for dispersal or low natal philopatry reported for the study area (Rudnick et al., 2008).

The lack of genetic differentiation observed within the non-breeding portion of the population as assessed by the method Structure may be explained by sampling effects. For example, it is not known to what extent non-breeders within the study area originate from multiple subpopulations. The mathematical model used by Structure clusters individuals into Hardy–Weinberg/linkage equilibrium populations (Pritchard et al., 2000). If communal roosting in this species is non-discriminatory with respect to natal origin, the power to detect differentiation may be reduced if few individuals from multiple subpopulations were sampled or if low levels of differentiation existed between subpopulations (Manel Berthier & Luikart, 2002; Latch et al., 2006; Waples & Gaggiotti 2006; see also Kalinowski, 2010). Further, it would be interesting to know if any genetic differentiation between the breeding and non-breeding portions of the sampled population exists, as Rudnick et al. (2008) reported results based only on non-breeding individuals.

Nonetheless, the Naurzum Zapovednik appears to serve as an important resource for Eastern imperial eagles in Kazakhstan, regardless of reproductive status, but caution is warranted before making any generalizations with regard to demographic inferences obtained from this study population. The habitat for example must provide resources for breeding pairs and their offspring, but also for the much larger sized non-breeding portion of the population. Therefore, any demographic appraisal obtained from this population (e.g. fledgling success, nest site turnover) may not necessarily reflect those of other populations of Eastern imperial eagles. Therefore, to what extent is the Naurzum Zapovednik different from other geographic areas in this species' breeding distribution with respect to conditions that may promote or allow communal roosting behavior (e.g. Beauchamp, 1999; Bijleveld et al., 2010)? Similarly, it would be interesting to know how common roosting behavior is in general, throughout the species' distribution or if it varies across years even within the Naurzum Zapovednik? It was not reported, or it may not be known, if the high number of non-breeders within the reserve in 2004 is consistent across multiple years or throughout the duration of the breeding season, since the majority of non-breeding feather samples were obtained in a single year over a 15-day period (Rudnick et al., 2008). Although a much smaller number of feather samples were collected in 2003 (n=144) compared with 2004 (n=1822), preventing accurate comparisons between years due to different sampling strategies (Katzner et al., 2011), similar numbers of molted feathers to that in 2004 were collected but not genotyped in both 2005 and 2006 (Rudnick et al., 2008) suggesting that the number of non-breeding individuals may be as high in those years too. More work is certainly recommended to assess the extent to which these results reflect either short- or long-term patterns within this population, and overall connectivity over a much larger geographic distribution within Kazakhstan and neighboring countries. In so doing, these data will provide further support concerning possible factors influencing cryptic population size in Eastern imperial eagles and its potential conservation implications. Whether similar discrepancies exist in other species between non-breeder census sizes based on direct field observation and that obtained by exhaustive genotyping certainly deserves further study.

References

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  2. References
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