Our experiment on P. mannii’s habitat use makes clear that the species’ invasion across Central Europe was promoted by urbanization; unlike two congeneric species, we observed this butterfly exclusively in urban environments. This exploitation of ecological opportunity associated with human presence resembles patterns of range expansion documented in other invasive species (Davis et al. 2014; Greig et al. 2017; Marques et al. 2020; Thawley and Kolbe 2020; Rivest and Kharouba 2021). Recognizing that P. mannii is occasionally reported from (semi)natural habitats within its newly colonized range, the highly consistent association with urban environments in our and other investigations (Ziegler 2009; Herrmann 2010; Geier 2016), and the high density of such habitat across much of invaded Central Europe, suggest that most of P. mannii’s global population may now well live synanthropically in towns and cities. Since representatives of the genus Pieris generally dominate urban butterfly communities numerically, P. mannii must qualify as the most common butterfly overall at least in some urban localities within the invaded range (see also Ziegler 2009; Herrmann 2010; and Rivest and Kharouba 2021 for extremely high abundances in another invasive butterfly). This may be true especially for larger cities, given our indication that P. mannii abundances tend to scale positively with a proxy of city size.
Although our investigation focused on a relatively small region colonized early during P. mannii’s range expansion across Europe (the species has currently progressed c. 700 km further north; Wiemers et al. 2020b), the particular expansion mode, relying strongly on urbanized environments, is likely to be representative of the butterfly’s invasion as a whole. The reason is that records from more recently colonized regions further north also come mostly from urban areas (Herrmann 2010; Von Scholley-Pfab and Pfab 2017; Vantieghem 2018). This butterfly thus provides a striking example of an invasive species enriching urban faunas locally, but at the same time making urban biodiversity more homogeneous at a broad spatial scale (Rahel 2002; Sax and Gaines 2003; McKinney 2006; Winter et al. 2009; Piano et al. 2020).
Our work raises the question how urban environments facilitate the establishment of P. mannii. The answer must partly be related to the larval host plants (see also Dexheimer and Despland 2023). Based on field observations, this butterfly has been speculated to have broadened its host plant repertoire within the invaded range (Geier 2016; Köhler 2021). However, direct field observations indicate that host shifts are unlikely to be a crucial requirement for this butterfly’s expansion success: at least two plant species know as hosts and wide-spread in the original native range of the species occur commonly, and are consumed by P. mannii larvae, in the newly colonized urbanized environments (Fig. S1) (Ziegler 2009; Herrmann 2010; Geier 2016; von Scholley-Pfab and Pfab 2017; BD and SR, personal observations). Urban habitats may thus generally lie within the species’ original foraging niche. However, equating host plant availability with butterfly habitat suitability may ignore more subtle requirements along the whole life cycle (Dennis et al. 2003; Vanreusel and Van Dyck 2007). Indeed, adult P. mannii appear to prefer sloping, rocky habitats – one reason why this butterfly occurs only locally in its original range (Ziegler and Eitschberger 1999). This structural habitat preference might be coupled with some form of home range fidelity, as indicated by our recapture data revealing a lower dispersal tendency in P. mannii than in its sister species P. rapae. Within our urban study sites, qualitative observations during field work (DB, SR) also suggest that the species frequently flies along vertical structures such as walls, escarpments and hedgerows. Although the latter needs formal examination, we speculate that cities not only offer adequate food resources, but may also structurally resemble natural rocky localities typically inhabited by the species (Ziegler 2009). P. mannii’s establishment in urban environments despite relatively narrow ecological requirements (Ziegler and Eitschberger 1999) emphasizes the importance of fortuitous pre-adaptation to such environments (McKinney 2006; Kowarik 2010; Cadotte et al. 2017), and highlights that successful urban colonizers need not necessarily be generalists (McIntyre 2000).
What is the ecological impact of P. mannii’s invasion? Since P. mannii is largely synanthropic within its novel range and its larvae here feed on non-native host plant species, direct resource competition with native fauna should not be a concern. However, the butterfly’s expansion may have less obvious consequences. One example might be apparent competition with other Lepidoptera, mediated by shared parasites. Within its invaded range, P. mannii is attacked in the pupal stage by a pteromalid parasitoid wasp (Pteromalus puparum) that also parasitizes a wide range of other lepidopterans (Graham 1969). The parasitation rate of P. mannii can be extremely high, sometimes approaching 100% (Jürgen Hensle, personal communications; see Tajagi 1987 for similarly high parasitation rates in P. mannii’s sister species), which may be one possible explanation for the dramatic drop in P. mannii densities observed during the summer months at site 21 (Fig. 4, top). (Another possible explanation is that the butterfly undergoes partial heat diapause or quiescence – July and early August 2022 were dry and hot, with daily temperature maxima mostly > 30°C; DB, SR and NM, personal observations). At least within urban areas, the high abundance of P. mannii might thus expose other Lepidoptera species to elevated parasitation pressure and hence greater mortality.
Another possible consequence of the invasion concerns the loss of ecological and genetic diversity within P. mannii itself. The invasion of this butterfly likely started from a relatively restricted area in south-eastern France (Ziegler 2009), and it is conceivable that the rapid spread of a range-expansive lineage may genetically homogenize native local populations, especially small localized populations at the edge of the original range (Kromer 1963; SBN 1991). This possibility is to be investigated by population genomic analyses including individual samples from the original and newly invaded range. In any event, following to what extent P. mannii remains primarily synanthropic or expands into natural habitats within the newly colonized range is relevant to both conservation and our understanding of invasion dynamics (Cadotte et al. 2017; Borden and Flory 2021).
An important question also concerns to what extent P. mannii’s range expansion may progress northward. Responses of butterflies to global warming suggest that the number of generations may be a crucial determinant of range expansion success, with multiple generation per year allowing the flexible accommodation of the life cycle to changing season length (Macgregor et al. 2019). In our study of P. mannii’s phenology, we identified three well-separated initial generations from March to early June. These early generations are likely reliably inferred despite being based on partly sparse observation data. The reason is that in the subsequent year (2023) at the same site, unidentified Pieris first appeared in mid-March (see also Schurian and Siegel 2016 and Wiemers 2016 for reports of P. mannii appearing in March in the invaded range), fresh P. mannii were recorded around the April-May transition, and a striking rise in abundance occurred in mid-June (D. Berner, personal observation) – qualitative observations very closely matching those from 2022 (Fig. 4). Between these three initial generations and a relatively well-separated last generation in September, neither our age distribution nor density data resolved the number of summer generations. However, at 23–26°C constant temperature indoors, P. mannii originating from site 21 completes a full life cycle (larval hatch to oviposition) within around 30 d (D. Berner, personal observation; see also Geier 2016). We thus conclude that in the study region, P. mannii can complete six generations per year, and hence that the range expansion by this butterfly is unlikely to be constrained by season length; provided that other ecological requirements including host plant availability are satisfied, the species may expand further northward by adjusting the number of generations per season.
To summarize, we document that within our study region, range-expansive P. mannii butterflies are strongly synathropic, indicating that the invasion of this species across Central Europe was facilitated by urbanization. Exciting opportunities for future research include genetically resolving the origin of the invasive P. mannii population, exploring potential adaptations to the expansive and/or urban life style, and investigating how expansive P. mannii interact with native conspecific populations and with other organisms within and outside urban environments.