Ecological processes and the distribution of green spaces
These analyses can be interpreted as urban greenspaces being part of an “archipelago” of patches in a continuous ocean of infrastructure, susceptible to be colonized from relatively nearby parcels that are a reservoir of local biodiversity. Frequency analysis under this vision makes obvious the differences between cities. MXC functions as an archipelago of medium islands fed by a large green space in the south (=mainland), and a vast sea of buildings without any green space. NYC functions more like a small group of “mainland” centers with medium-size islands in an archipelago spread in the vast city territory. Island biogeography theory suggests these differences must have consequences in the distribution and survival capacities of species of both cities. The green “mainland” in both cities can harbor many species, including those whose niches require more area. The frequency analysis and the map suggest that the mainland at the south part of MXC is still capable to harbor most of the 83 native species of mammals (Guevara-López, Botello, & Aranda, 2016), particularly large ones such as the puma (Puma concolor), grey fox (Urocyon cinereoargenteus), deer (Odocoileus virginianus), teporingo rabbit (Romerolagus diazi), cacomixtle (Bassariscus sumichrasti), and opossum (Didelphis virginiana) (García, Lozano, Ortiz, & Monroy, 2014).
The data from NYC present several medium-sized “mainlands,” capable of hosting many species (Gargiullo, 2007; Kieran, 1982). In NYC, green spaces are closely positioned to waterways such as Hudson and East Rivers, and Newark, New York, and Jamaica Bays. These have high biodiversity; Jamaica Bay is a vast wildlife preserve (Handel et al., 2016; Stalter & Lamont, 2002), and New York Harbor is the site of large-scale bird and fish migrations (U.S. Fish and Wildlife Service, 1997). These fringing habitats facilitate many plant and animal species dispersing and having access to the green spaces inland.
The distribution and size of green spaces modify their colonization capability, including barriers, such as highways, that must be navigated by species like big cats (Vickers et al., 2015). The number of green spaces in each hexagon of both cities is highly heterogeneous. Most green spaces have another green neighbor within 200m, although there are also many areas within 400m. However, in MXC, there are vast areas in the northeast without any green space and consequently a reduced possibility of colonization. In NYC, there are two extensive infrastructure belts between Queens and Brooklyn that may reduce the migration from the south to the north there.
Both cities need planning for more hexagons with larger and closer green areas, particularly where they are lacking. This would be possible by increasing the number of areas that would increase habitat heterogeneity (Chang & Lee, 2016; Gaston, Ávila-Jiménez, & Edmondson, 2013). This is important in NYC, where there is no large mainland analogue, making smaller areas responsible for sustaining the city's diversity. High quality within an urban park, especially larger ones, is critical for biodiversity support.
The movement patterns of seeds are known from studies in open or forested areas (Howe & Smallwood, 1982). Dispersal patterns of species common in urban areas also are known. For example, oak (Quercus) species are common in New York and Mexico City. Oak seeds can be carried up to 2km by jays (Darley-Hill & Johnson, 1981). These broader movements help link tree populations widely separated across urban matrices (Lundberg, Andersson, Cleary, & Elmqvist, 2008; Lundberg & Moberg, 2003). However, the seed shadow is short, 15-20m, for most bird-dispersed species (Hoppes, 1988; Howe & Smallwood, 1982). A 2km distance would move a diaspore among many urban parcels in our study, but 20m would typically land on the pavement, not a recruitment site. There is wide variation for wind-dispersal plants, but most seeds fall close, less than 100m to the mother plant (e.g., Vittoz and Engler 2007). The less-common broadly dispersing seeds are critical for starting new populations and genetic mixing, but we have few studies of how tall and dense urban structures block movement. Human actions (fragmentation, logging) interfere with dispersal dynamics in non-urban areas, but the interplay may be even more intense in the built hurdles of cities (Markl et al., 2012).
Differences in urban dispersal capacities also occur for animals. Within NYC, movement and genetic relatedness studies of coyotes (Canis latrans) have shown rapid spread in very dense areas and that animals across many city parks are closely related (Henger et al., 2020; Nagy, Koestner, Clemente, & Weckel, 2016). In MXC, mammals such as cacomixtles and opossums are common in grey areas. The mammals use house roofs and trees to reach even small gardens, which become habitats.
Birds have greater dispersal capability than terrestrial organisms. Birds cross built areas in Sapporo at a high frequency; the urban matrix was not a barrier (Shimazaki et al., 2016), to reach the green spaces. In a broad review of birds and other taxa in urban parks (Nielsen, van den Bosch, Maruthaveeran, & van den Bosch, 2014b), the microhabitat heterogeneity and quality of habitat within the parks were most decisive in improving biodiversity. It is possible to watch herons, ducks, and coots flying to small 10m2 ponds in the south part of MXC, but rarely in the north (pers. obs.). Distribution and number of bird species in NYC are positively correlated with green space area, although the shape and isolation of patches were not significant to the number of bird species (La Sorte, Aronson, Lepczyk, & Horton, 2020). In MXC, the 355 recognized species of birds (Melendez-Herrera, Gómez de Silva, & Ortega-Álvarez, 2006) can share green spaces habitats regardless of their native, exotic or migratory condition (Ramírez-Cruz et al., 2019), but the diversity increases in areas where there is canopy (Ortega-Álvarez & MacGregor-Fors, 2009). This supports the conclusion that larger urban green spaces are essential for maintaining high bird biodiversity.
The invertebrate biodiversity is essential for urban food web structure. In NYC, bee species diversity in small urban gardens is significant, 54 species, although this is smaller than surveys in the larger NYC urban parks (Kevin C. Matteson, Ascher, & Langellotto, 2008). In MXC at least 269 species of bees have been identified (Cano-Santana & Romero-Mata, 2016). Bee diversity was also positively correlated with area of the plots and presence of wild, “unmanaged” plant species (Kevin C. Matteson & Langellotto, 2010), but floral resources and bee diversity varied across space and time in NYC and MXC vegetated urban areas (Domínguez-Alvarez & Cano-Santana, 2008; K. C. Matteson, Grace, & Minor, 2013). These studies are mirrored by other, wider insect studies in urban areas (Harrison & Winfree, 2015; Winfree, Bartomeus, & Cariveau, 2011). Small urban fragments have high insect β-diversity (Tscharntke, Steffan-Dewenter, Kruess, & Thies, 2002) with arthropods that respond positively to vegetated area, but patch isolation was less important (Turrini & Knop, 2015). Together these several insect-focused studies show that insect diversity can be high in urban centers and patch quality is important. Consequently, design improvements within parks may have significant value compared to purchasing new green spaces (Nielsen et al., 2014a).
Advancing ecological structure in urban planning
These two cities are old, where urbanization patterns reflect historical process. In both NYC and MXC, there is a clear pattern of areas with a substantial number of parks and other areas where fast urbanization, not always planned, did not consider or leave urban green areas. These hardscape areas generate landscape grey holes difficult for many species to cross. On the contrary, in both cities, some medium-size urban green spaces are related to historical processes within each city; that is the case for Central Park and the Brooklyn Botanical Garden in NYC and Chapultepec Park and Xochimilco in MXC. This shows the serendipitous way that cultural needs can support biodiversity concerns. Future urban planning initiatives can mirror this convergence by considering design decisions that advance the ecological needs as well as the social goals of a development (Le Roux et al., 2014).
To continue to rebuild our historical cities’ design using these two goals, we need to understand which information, data sets, and institutional changes are needed. The spatial analyses we report illustrates base maps upon which biodiversity improvements can be molded. The quality of existing patches must be studied to determine what ecological improvements are feasible. Data on current biodiversity, movement, and practical ecological targets are needed to set priorities for ecological designers (e.g., Alvey 2006; Saura and Rubio 2010). Which species can survive in current, stressful urban conditions and with projected climate shifts? Which species can persist in existing spatial arrangements of green spaces and which will need some type of corridor, management change, or additional patches to maintain their population structure (e.g.,(Angold et al., 2006; Threlfall, Williams, Hahs, & Livesley, 2016).
In addition to increasing the number of green spaces, new management concepts and training for existing green spaces may be needed to maintain or improve ecological function (Palazzo & Steiner, 2014). Our mapping of the green archipelago in these old cities is a quantitative start towards new design efforts to advance urban biodiversity and its many values. Work within historical cities can then be expanded for ecological improvements of regional metropolitan areas (Forman, 2008), as has been done for the NYC region (Flores, Pickett, Zipperer, & Poyat, 1998; Lewis, Nordenson, & Seavitt, 2019).
Cities are dynamic areas in which both structures and regulations can change in the future. The life span of a patch and addition of new habitats varies with local regulations and development pressures (Le Roux et al., 2014). Additionally, the economic factors important in urban design can be incorporated into decisions on ecological corridor construction and improvement (Peng, Zhao, & Liu, 2017). Because change in an urban green patch's presence or surroundings occurs regularly, species equilibrium may not always occur, a metapopulation effect. Green space bridges can assist sustainability in both cities. "Stepping stone" areas are of value (Ignatieva, Stewart, & Meurk, 2011; Andersson & Colding, 2014) in both cities and may be easier to create than continuous corridors. Ecological connections have been used in cities (LaPoint, Balkenhol, Hale, Sadler, & van der Ree, 2015), and these case studies can be a foundation for future plans in the cities analyzed here. The study’s hexagons also help to prioritize paths for ecological restoration, for example, in the mentioned grey belts between Queens and Brooklyn in NYC and the North East area in MXC (Zambrano, Aronson, & Fernandez, 2019). These areas must be a priority for integrating green spaces by generating corridors for species to use, knowing that the behavior of the dispersers will determine the pattern and tempo of movement.
New rules and practices can be encouraged. For example, new architectural designs exist to add animal microhabitats into the facades of big buildings (Weisser & Hauck, 2017). Glass facades can be modified to be less dangerous to bird movements. “Green streets” can increase biodiversity movement (Mason, Moorman, Hess, & Sinclair, 2007), as well as increase engineering value (stormwater absorption, mitigation of heat). The spatial patterns of existing urban biodiversity are diverse (Rastandeh, Brown, & Pedersen Zari, 2017), and many different local solutions are possible.
Urban design decisions must include small-scale green spaces that accommodate to improve urban biodiversity, such as bioswales and infrastructure that comprise "green streets" (Beatley, 2017; Kemp, 2006). The known biodiversity in cities can be mirrored by a diversity of design decisions to break down the "grey-green dichotomy" that is the caricature of recent urban planning (Parker, 2015).
Historical cities’ constraints challenge improvement of urban biodiversity. It is better to have an ecological presence in decision making early in planning and design (Dunnett & Hitchmough, 2004). New city designs may be a more accessible template upon which to create biodiverse urban centers. Patterns that will allow sustainability of urban biodiversity have been explored theoretically and by site analyses (e.g., (Beninde, Veith, & Hochkirch, 2015; Ikin, Knight, Lindenmayer, Fischer, & Manning, 2012; Markl et al., 2012; Weisser & Hauck, 2017). New conceptual areas for improving our efforts are continually being posed (Angold et al., 2006; Lepczyk et al., 2017; Müller, Werner, & Kelcey, 2010). There is a big opportunity to improve new urban spatial structures based on theoretical underpinnings from landscape ecology. Closer collaboration between ecologists and the design professions is paramount.
There are short-term actions to advance urban biodiversity, but our perspective must have a longer view, even in a time of rapid climate change (Nilon et al., 2017). Damschen et al. (2019) showed that biodiversity among reconnected habitat fragments increased after 18 years. The value of the connections still had not reached an asymptote. In both NYC and MXC, much of the array of green spaces have existed for well over a hundred years. This hexagon-based analysis supports decisions and policies for the addition of green areas with particular sizes, distances, and locations. New design decisions must demonstrate short term progress to secure public and political approval, but the ecological time scale of change is long and must outweigh the typical emphasis on annual or short-term goals.