Urban green spaces represented by diverse habitat types ranging from the highly managed green infrastructures like green roofs to the remnant patches of urban forest are crucial for habitat connectivity and ecosystem service perspectives ( Milanovich et al. 2012; Aronson et al. 2017). They play a vital role to make towns and cities habitable area through the abatement of pollutants such as ozone, acting against the impacts of urban heat islands, moderating local climate, and enhancing thermal comfort (Klemm et al. 2015; Ballinas and Barradas 2016; Calfapietra et al. 2016; Jaganmohan et al. 2016; Livesley et al. 2016). Globally, as the rate of urbanization and biodiversity loss are analogous to each other, understanding the ecology of urban forests individually and within the network is essential for biodiversity conservation perspectives (Brook et al. 2008; Aronson et al. 2017). Due to the high density of people and the greater prevalence of the human-mediated mobility of the commodities, human-ecosystem interactions differ significantly in the city areas compared to other natural sites. This difference in interaction results in different patterns and processes of invasion in the urban context compared to natural sites (Gaertner et al. 2017).
Habitat patches within the cities are highly fragmented and heterogeneous, and abiotic and biotic environments are greatly altered, which affect local (alpha) or within-patch biodiversity (Faeth et al. 2012). Urban areas are often considered inhospitable for native species, especially sensitive ones, but the case is different for invasive species (Cadotte et al. 2017). As invasive species are opportunistic, the niches created due to the disappearance of the native species are occupied by invasive ones. As the urbanization proceeds, the non-native invasive species gradually replace the native species from the ecosystem of city areas (Cadotte et al. 2017). Urban areas are important in invasion research as they act both as entry points and dispersal locations for the invasive species (Gaertner et al. 2017; McLean et al. 2017). As the result, the diversity and abundance of the non-native invasive species are greater in city areas compared to their rural counterparts and other natural sites (Cadotte et al. 2017). Urban green space including the urban forests are the hotspots of biological invasion (Gaertner et al. 2017). As the urban green spaces are subject to high human pressure, they often consist of high human trails and other interactions which enhance edge and serve as propelling factor for opportunistic invasive species (McDonald and Urban 2006). However, the level of invasion and ecological success of invasive species are often regulated by context-specific factors (Dyderski and Jagodziński 2019).
Urban ecosystems may be one of the final frontiers of ecological exploration (Dolan et al. 2013). Mainstays of urban biodiversity are the urban forest patches, embedded in the matrix of urban infrastructure, remnants, or naturalized plantations. Though, urban ecological research is swirling in recent times (Lepczyk et al. 2017), limited information exists about the dynamics of urban forests in developing countries. Increased anthropogenic activities and increasing demands of ecosystem services have intensified both the stress and appeals of such areas. However, the invasion of such important areas as one of the most important threats has not been dealt with in detail. Invasive plant species bring a profound impact on the trophic structure of forest ecosystem (McCary et al. 2016). As developing countries have limited capacity to apply remedy measures against the problem of ecological invasion (Early et al. 2016), it is crucial to understand the factors that offer resistance of the ecosystem against invasion.
Rapidly urbanizing landscapes in developing nations with several urban forest patches provide an opportunity to deal with the ecological role of forest structure relating to the invasion and future vulnerability. In the face of growing challenges, understanding the factors which regulate the temporal and spatial dynamics is crucial for conserving urban biodiversity. Despite invasion being the subject of interest for more than six decades following the pioneering work of Elton (1958), we are yet to reach a common understanding of the factors responsible to drive the invasion of the species. In the case of urban areas, the problem is even severe as invasion science has yet to give significant attention to the invasion dynamics, patterns, and processes in the urban ecosystem (Gaertner et al. 2017).
There is a considerable debate on the factors associated with the invasion of the urban sites with invasive species. Native species richness can buffer the impact of invasive species when the sites are undisturbed but not the same when the sites are disturbed (Pinto and Ortega 2016). Understanding the role of forest dynamics and structural control on the spread of invasive species is essential to formulate the management plan (Baret et al. 2008). In the context of knowledge void regarding the implication of tree species richness, abundance, and forest structure on the regulation of invasion of the urban forest, this study aimed at generating information on two fronts: i) to assess the role site-level disturbances play on the invasion probability within the urban forests and ii) comprehend the importance of structural attributes of forests to provide defense against invasion. For this purpose, factors associated with the occurrence of invasive species in urban forests were explored employing a correlational approach followed by predictive modeling. Our study focuses on Kathmandu valley, Nepal is envisaged to fill the knowledge gap on the factors regulating the presence of invasive species in urban forest patches.