Long‐term effect of over‐supplementation 569
host plant resources only become fully available after other habitat elements have reached an appropriate condition, or that host plant abundance is not the factor limiting popula- tion abundance. The Apollo is sensitive to disturbance of the non-forest habitat structure, and hence even a slight in- crease in the number of shrubs in open grassland could make this habitat unattractive or even unsuitable (Fownes & Roland, 2002; Nakonieczny et al., 2007a; Matter et al., 2011). Experience from butterfly conservation practice indicates that habitat-oriented activities are crucial for effectiveness (New, 1991; Thomas, 1991; Pullin & Knight, 2001; Schultz and Crone, 2005; Adamski & Witkowski, 2007). The introduction of captive-reared individuals to sites where habitat is not yet in a suitable condition will not be effective. The Apollo recovery project initially assumed that because of the host plant’s rarity, butterfly abundance was the most appropriate measure of habitat quality (Witkowski et al., 1993). The host plant inventories (Witkowski et al., 1992) could have been misinterpreted be- cause the assumptions were oversimplified, but the break- down of the population after it had reached or exceeded the estimated carrying capacity suggests that estimation was reasonably reliable. Another question relates to the mechanisms potentially
responsible for the reduced carrying capacity. Earlier re- search suggested that overestimation of food plant abun- dance led to its excessive exploitation because too many captive-reared individuals were introduced to particular sites (Adamski & Witkowski, 2007; Adamski, 2016). This is corroborated indirectly by the fact that introduction effectiveness indices were lower at sites where many indi- viduals were introduced (Adamski & Witkowski, 2007). Moreover, the subpopulations in the eastern part of the me- tapopulation (Fig. 1b), not supplemented since the decrease in 2004, appeared to be more stable (Adamski, 2016): the shorter distances between subpopulations could have led to a higher migration ratio (Adamski, 2016). That this might be related to the host plant was supported by the ob- servation that a large proportion of fresh stonecrop shoots were consumed by young Apollo caterpillars early in the season (Olejniczak, 2011; P.Olejniczak,
pers.comm., 2008). The stonecrop plants, whose young shoots had been com- pletely eaten, developed new shoots from the rhizomes. Between consumption of the first shoots and the regrowth, the development of the Apollo’s larvae was presumably limited. These arguments and observations suggest that the re-
introduction of too many individuals is not only ineffective but can also lead to long-term negative consequences, such as a reduced carrying capacity. This does not imply, how- ever, that captive breeding has no role in butterfly recovery projects. It has been demonstrated that, in the absence of the maintenance or restoration of appropriate habitat quality, the introduction of captive-reared individuals is ineffective
(New, 1991; Thomas, 1991; Pullin & Knight, 2001; Schultz & Crone, 2005; Adamski &Witkowski, 2007). Nonetheless, in addition to obtaining large numbers of individuals for intro- duction into the wild, there are several conservation advan- tages of captive breeding. In this particular case, captive breeding combined with field studies enabled two threats to be addressed. The first was inbreeding in the wild population, which
was disrupted by introducing captive-bred individuals. In the early years of the captive breeding programme, Apollo butterflies from the larger population in the Slovak part of the Pieniny Mountains were incorporated. This decision was controversial, even though both populations were the same subspecies, P. apollo frankenbergeri, and there were reliable reports that until at least the early 1950s butterflies occasionally migrated between the two populations. After the introduction of the Slovak butterflies into the captive breeding programme, symptoms of genetic erosion (a high level of developmental mortality, Witkowski et al., 1993; wing deformations, and significant numbers of individuals incapable of stretching their wings upon emergence, Adamski & Witkowski, 1999b) decreased. In addition, the average individual fluctuating asymmetry (Adamski & Witkowski, 2002) was significantly reduced (Adamski &Witkowski, 1999a, 2007; Adamski, 2016). The second threat was that as a result of long-term iso-
lation, the Apollo butterflies in the Pieniny National Park no longer migrated between subpopulations (Adamski & Witkowski, 1999a, 2007). This changed after the introduction of individuals from Slovakia into the captive-breeding pro- gramme: individuals from the Slovak and Polish–Slovak breeding lines were significantly more likely to perform both long- and short-distance migrations (Adamski & Witkowski, 2007; Adamski, 2016). An additional benefit of captive breeding is that studies carried out in captivity have provided new information on Apollo butterfly biology and ecology, which may be of use in the conservation of this species (Adamski et al., 1999;Adamski, 2004; Nakonieczny &Kędziorski, 2005; Nakonieczny et al., 2007a,b; Łozowski et al., 2014). Ourmodelling confirms that if a butterfly population is to
be restored, habitat-orientedmeasures are crucial for achiev- ing long-term stability. On the other hand, the introduction of captive-reared individuals to habitatswhere the conditions are inappropriate is ineffective. Over-supplementation with introduced individuals can be counterproductive, as the habi- tat carrying capacity can be permanently reduced. Long-term studies of restored populations are crucial, as they facilitate the analysis of the time-lagged results of conservation mea- sures such as reintroduction.
Acknowledgements This study was financed from the statutory funds of the Institute of Nature Conservation, Polish Academy of Sciences. We thank the Management of the Pieniny National Park
Oryx, 2022, 56(4), 564–571 © The Author(s), 2022. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605321000296
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