To examine the interaction networks of metacommunities of bird, reptile, frog, and butterfly among the parks in the urban areas, we applied the nestedness analysis of WNODA to measure and observe the order/disorder of the community ecological patterns in habitats facing fragmentation or isolation conditions (Atmar and Patterson 1993). Results showed significant nestedness of all the metacommunities among these parks, implying the importance of parks in the urban ecosystems as “island-like” habitats governing the assemblage networks and ecological community interactions of bird, reptile, frog, and butterfly. Given the selective colonization and extinction pressure caused by the respective environmental properties of each habitat, the assemblage networks are prone to develop nested structure with unequal biogeographical patterns of the species distribution and abundance (Atmar and Patterson 1993; MacArthur and Wilson 2001). As a result, when the system of these ecological islands become stable, the ubiquity of nestedness shows (Thebault and Fontaine 2010).
Through the analysis of WNODA, the size of the species-habitat matrix representing the nested structure can be standardized allowing unbiased comparison of different communities (Pinheiro et al. 2019). Based on the nestedness calculated from WNODA with native species only, reptiles and frogs showed stronger nested structure than birds and butterflies. We thought that this result may indicate greater influence of habitat fragmentation on the biological groups with lower mobility being more restricted for their movements (Habel et al. 2019). This necessitates the conservation strategy of green spaces during land development for those being threatened or struggling to survive, such as the lower mobility biological groups, in the highly fragmented urban environments. However, with consideration of both the native plus the exotic species in the WNODA calculation, results appeared a declined nestedness of all the four selected metacommunities showing a more disordered interaction networks of the associated assemblages among these parks (Čeplová et al. 2017). We argue that the calculated WNODA with exotic species included may be a result of randomness, rather than a nested structure in the parks of Taipei. This randomness implies that the niches of the exotic species are more general than those inhabitant native species in terms of the adaptation to the urban environments (Atmar and Patterson 1993; Čeplová et al. 2017). As such, for effective biodiversity conservation in urban areas, it is recommended to focus on restricting the spread out of the exotic species and regulating the transaction or trade. If an exotic species has been found to form prolific population in urban areas, it should be considered to lower the risk to the survival of native species by conducting effective control or removal on the exotic species to prevent their territory development or spread out (Gaertner et al. 2016).
Given the importance of parks in sustaining the interaction networks among different metacommunities from the nestedness analysis, we cooperated the Spearman’s correlation test to explore the critical environmental factors affecting the metacommunities to provide science-based suggestions of crucial elements and proper designs for landscape planning and improvement (Ulrich et al. 2009). Results found that various types of land use had different effects on the species incidence of birds, reptiles, frogs, and butterflies. In particular, areas of trees and shrubs within parks were found to be positively correlated with the species incidence of native species of birds, reptiles, and butterflies. This reveals that the design of multi-layered environment could potentially benefit biodiversity conservation for providing diverse habitats for different species, enhancing the robustness of the ecological interaction networks (Oliveira and Scheffers 2019).
In addition, area of parks exhibited a positive correlation with the species incidence of birds and butterflies significantly. According to the correlation matrix, we observed a negative correlation of “area of parks” to the boundary ratio of parks, and road or building density around parks (Fig 3). It demonstrates that a larger park with lower boundary and isolation may help reduce the edge effects and maintain diverse natural habitats, which is beneficial to the richness and abundance of birds and butterflies. It also strengthens the knowledge ground for preservation of large green spaces to produce various niches to accommodate different species for biodiversity conservation. Surprisingly, habitat heterogeneity, as a transformed index, did not show significant correlations to birds, reptiles, frogs, and butterflies. It might be due to the scale of the parks in Taipei being relatively small, so that the associated correlation coefficients of the habitat heterogeneity were relatively minor in comparison with other environmental factors. This implies the more direct impacts of the habitat features on shaping the distribution and survival of species (Ulrich 2009), yet the environmental factors interacted with the habitat heterogeneity can together determine the nestedness of communities (Schouten et al. 2007; Rico-Silva et al. 2021). This warns the irreplaceable role of the measurements or observations to be more directly linked to the reality or the in-situ conditions, as well as the use of the transformed index to be taken as a supplementary and/or a more comprehensive addition.
Moreover, our results found that birds were negatively correlated with distance to the neighbouring parks, and positively correlated to distance to the nearest forests and rivers, indicating the importance of the connection between the parks and natural areas as habitats to the bird metacommunities. Studies found that loss of connection could potentially impact the networks and resilience of metacommunities, increasing the probability of extinction (Alexandre et al. 2008). Interestingly, the incidence of birds, either native or exotic, were found to remain a positive correlation with distance to the nearest forests and rivers significantly. This situation may be caused by the habits of the generalist species accounted for the majority of total weighted abundance, such as Eurasian Tree Sparrow (Passer montanus) accounted for 33.7% to prejudice the arrangements of the abundance. Nonetheless, since the ecological requirements for the generalists of birds, often the omnivorous, granivorous, or cavity nesting birds, are more easily fulfilled in urban areas (Simberloff and Cox 1987), it may represent that indirect impact of urbanization on the richness of native birds with narrow niches (Chace and Walsh 2006). As a result, improvement on the connection between habitats, such like the urgent advocate of green corridor developments (Simberloff and Cox 1987), increasing the natural designs, planting native nectars and fruits between parks and green space (Akif et al. 2020), and preservation of existing green space, can help stabilize the interaction networks and increase the re-colonization rates of native species. These strategies can uphold the decreasing biodiversity (Alexandre et al. 2008), in the highly urbanized environment, especially for native birds.
When including the exotic species, we observed that the significance level of some correlations shifted. In terms of land use types within each park, the factor of “area of trees” turned into non-significant, while “area of shrubs” remains its positive and significant correlation. Besides, with the inclusion of the exotic bird species, “density of buildings around the parks” became less negative and not significantly related, reflecting the species-specific adaptability of the exotic birds in the urban areas and their utilization of the roads and buildings. If the thrived population of exotic bird species outcompete the native bird species, it may cause the loss of biodiversity (Chace and Walsh 2006). Similarly, the species incidence of native reptiles was found to be significantly correlated to “area of trees” and “average NDVI within each park”, expressing the need of native reptiles, such as Swinhoe's Japalure and Hekou's Gecko (Gekko hokouensis), for higher quality of green vegetation and large arbors. However, once the exotic reptile species were included, the correlation coefficient between species incidence and “area of parks” became higher and the correlation to “average NDVI within each park” decreased. These changes may be caused by the dominant exotic species of red-eared sliders, an aquatic species that have often been abandoned in large parks, and have been found to establish their population there (Zhang et al. 2020). In this regard, these results reinforce the need to control the introduction of the exotic species for native species protection. Also, it is vital to preserve and plant native vegetation to conserve the connection or provide niche for native species as another strategy for biodiversity conservation.
In conclusion, metacommunities of bird, reptile, frog, and butterfly showed significant nestedness among the 16 parks in Taipei, indicating strong interaction networks of the metacommunities among these habitats (Atmar and Patterson 1993). As a result, the existence of parks plays a vital role for sustaining these metacommunities (Almeida and Ulrich 2011). In addition, land development may affect the biological groups with lower mobility more seriously, such that in our analysis reptiles and frogs were found to be more easily threatened by habitat fragmentation (Habel et al. 2019). To reduce the impact of urbanization, and help retain healthy interactions and connections of metacommunities among the habitats, we suggest to conserve green space with large areas and construct the green corridors (Alexandre et al. 2008). In the parks, it is recommended to increase the area of shrubs and trees, and create crucial elements such as increase the vertical complexity with large and multi-layered trees for various mobility level species (Oliveira and Scheffers 2019). As for the exotic species, control the introduction is important preventive work for native biodiversity conservation in urban areas. We believe that these strategies are essential to construct wildlife-inclusive cities and conserve the community ecology in urban areas.