The above image is by Daniel Piraino (creative commons license)
By Lauren Alleman & Sasha Wright
We are in an extinction crisis. Global loss of biodiversity is nearly 4 orders of magnitude greater than at any point in measured history. Not surprisingly, one in four imperiled species lives in a metro area, and the patterns of intensifying urbanization will continue to threaten their existence.
By 2050, there will be over 9 billion people on this planet competing with wildlife (and each other) for space and resources, and 70% percent of these people will live in cities.
Intrinsic value aside, biodiversity conservation in a city is important because biodiversity can regulate disease vectors (think mosquitoes and West Nile), improve ecosystem stability during natural disasters, and increase ecosystem services (e.g., primary productivity).
In New York City, a metro area with 8 million people living in a 469-square mile area, decision makers and resource managers are in the middle of a conversation: how can we preserve biodiversity in a place like this? Creating conditions that allow for animals to use cities as permanent habitat, or allow them to disperse through the city as they migrate, is complicated. The task will require defining target species of concern, their home range and resource needs – as well as the scale of conservation efforts and the types of green “interventions” that can be made to existing open spaces or to the built environment. Finally, managing for biodiversity may result in unintended consequences – for example, do we really want coyote-wolves running around Manhattan?
The importance of patch size – a winged case study
New York City contains over 80,000 acres of open space, divided into hundreds of thousands of discrete parcels bounded on all sides by a matrix of built infrastructure.
And despite the city being so fragmented, one of the most comprehensive and long-term biodiversity datasets for New York City tell us that large patches in the city are supporting high bird diversity; Jamaica Bay Wildlife Refuge and Central Park hotspots have recorded over 262 and 312 species, respectively. But massive urban parks like Jamaica Bay and Central Park only tell part of the story.
The size, complexity, and quality of a patch has predicted species richness of birds, plants, and butterflies in a number of urban biodiversity studies in the past (Oliver et al. 2011; Beninde et al. 2015; Concepcion et al. 2015). Structural complexity is correlated with greater bird diversity and abundance in a number of habitats, including urban forest patches. Native plant density may also be important for insectivorous bird species because of plant-insect evolutionary relationships. Some birds found in New York City are migrating along the Atlantic Flyway, whereas others are breeding in the city and using the habitat in unique ways depending on the quality of forage in particular patches. If a single habitat patch doesn’t provide enough food or structure, resident birds must travel between tall buildings and across busy roads to access what they need.
For better or worse, birds are also using small, isolated patches in densely populated neighborhoods like Bryant Park, a 9-acre parcel consisting of mostly lawn with some tree canopy. Citizen science data tells us that 121 species of birds have been recorded in Bryant Park between 2002-2016. Troubling, however, is a recent study by NYC Audubon that recorded 14 different species of birds that were victims of bird-building collisions during a single fall migration season in Bryant Park. This “window group” included Northern Parulas, Blackpoll Warblers and Common Yellowthroats. These types of data may be the key for designing an urban biodiversity management plan. Are these our target species of most concern? Where should we look for others?
Importantly, birds are also a uniquely well-monitored species: citizen scientists, NGO’s, and governmental agencies have compiled an enormous amount of data on both migratory and resident birds and how they use NYC as habitat. But how should we start identifying sensitive species when we are talking about some of our less charismatic neighbors… say the rats, raptors, and bats of our urban habitats.
Living in isolation – the importance of species life history traits
Different organisms use the landscape in incredibly different ways. Birds may be able to disperse over long distances, while squirrels may live out their entire life in a single city park. The size of an ideal patch, and the distance between ideal patches, can vary widely depending on the species you are assessing. Not surprisingly, improving habitat size and condition for birds would not guarantee that all other urban wildlife would benefit.
For example, recent work by Su et al. (2015) in PLoS One found that urban patch area and connectivity did not have a positive influence on insect species richness in Beijing – the eighth largest city in the world.
In fact, smaller patches had higher per-area measures of species richness and the local connectivity between patches did not improve diversity. The authors speculated that predators may not key into populations of prey when they are very isolated and that perhaps, for several orders of the most common insects sampled (Hemiptera, Diptera, and Hymenoptera), fragmentation is not necessarily a bad thing.
It’s possible that fragmentation isn’t a bad thing for birds, either. And for essentially the same reasons. Addressing patch size by improving connectivity could reduce the risks of moving from patch to patch, but it could also improve access for predators like raccoons, skunks, and feral cats that could increase the odds of bird predation (and, incidentally, human-wildlife conflict). There is also evidence that increased connectivity could accelerate the transportation of invasive species, especially those that require abiotic dispersal. These unintended consequences don’t suggest that biodiversity conservation in cities is a bad idea;
it’s just important to recognize that every decision will likely solve some problems, while introducing others. We just have to decide which problems we are going to be okay with.
Compelling urbanites to value biodiversity and conservation is no small task, so we should focus on creating connectivity for flagship species that will inspire and connect New Yorkers to nature. We should also select species that we can realistically manage for – species that have small home ranges and are residents in the metro area. After the New York City conservation community sorts through what it wants to conserve, we will have the opportunity to make recommendations in two major areas. First, what types of improvements are necessary in existing green spaces? And second, what types of green interventions (bioswales, green roofs, street trees, floating wetlands, etc.) can be used to convert the built environment into more biodiversity-rich habitat?
The last major consideration is the human dimension – temperatures are expected to increase by 4-7.5 degrees Fahrenheit by the 2080s and this could increase heat-related morbidity of underserved human populations. Green infrastructure has the potential to be situated in places that benefit both people and wildlife; this will require coordination between different landowners and jurisdictions, as well as conservation professionals from both the private and public sector, to collaborate on the creation of new habitats that serve ALL of the organisms living within New York City – often in unexpected places.
Lauren Alleman is an urban ecologist with The Nature Conservancy’s New York City Program. Her work is focused on restoring habitat quality and resilience in New York’s open space, through on-the-ground ecological restoration projects, and through conceptual planning exercises such as connectivity conservation that inspired this blog. All views expressed are her own.
This PLOS Ecology Community post reflects the views solely of its authors, which are not necessarily shared by PLOS.