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This is the eighth story in a series of articles on frugivory and seed dispersal, leading up to the FSD 2020 symposium in March.

The relationship between urban spaces and animal seed dispersal is a circular one. The disturbed landscape and anthropogenic presence within cities alter both animal abundance and animal behaviour, thus impacting animal seed dispersal. Similarly, animal seed dispersal stimulates habitat regeneration and determines plant community composition within urban areas, thereby defining a city’s landscape.

Like other natural habitats dominated by human activity, cities too suffer from fragmentation, defaunation and the introduction of non-native species. However, the dynamics leading to these are very different in urban spaces, thus giving us a varied perspective on how such processes lead to altered seed dispersal patterns.

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Raccoons prefer woodlands but are attracted to human food waste in urban areas, making them potential seed transporters between urban habitats. This raccoon was seen in an urban garden in Fort Lauderdale, Florida. Photograph: Susana Candusso

Green habitats within cities are extremely diverse, ranging from natural urban forests to rooftop gardens, flowerbeds and even cracks in building walls. The seed movement between these habitats is determined by the landscape around them (i.e. railway networks, ravines), how well animals can move through them, and how much the animal behaviour is affected by human activity.

It can be difficult to generalise the effects of the urban landscape on animal seed dispersal since each specific urban infrastructure will have a species-specific effect on seed dispersal patterns. The same urban infrastructure can have opposite effects on two different disperser species. For example, roads may limit the movement of small animals such as rodents which are scatter hoarders, isolating green areas from seed influx, but they will also act as movement corridors for bigger animals such as carnivores travelling along roadsides. Even birds such as the Purple-throated Euphonia following mistletoes on artificially planted trees along roads promote seed exchange between green areas. 

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Blue-crowned Conure Parrots, native to South America, are now densely present in urban areas of Southwestern Florida. Many think parrots are seed predators, but there is evidence to believe they are important seed dispersers. Photograph: Susana Candusso

The most outstanding aspect of the relationship between urban spaces and animal seed dispersers is how dispersers determine the regeneration rates of these habitats by creating connectivity between urban green areas and surrounding natural areas. For example, in Stockholm, European Jays recruit an estimated 33,000 saplings of oaks per year within the Stockholm National park. Thrushes (Turdus spp.) in Brazil create a seed flux between urban forests and gardens by using both habitats for different behaviours, such as feeding and breeding, and travelling back and forth. As a determinant for habitat regeneration, knowledge on how seed disperser behaviour and their seed dispersal patterns are influenced by the urban environment could be used to manage the urban landscape and surrounding natural areas.

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American Robins seasonally migrate between Florida and the northern states looking for optimal living temperature and potentially take seeds from urban areas along their route. Their habitats also include forest areas, so just like squirrels, they also maintain seed flux between forests and urban areas. Photograph: Susana Candusso

It is important to note that the connectivity created by animal seed dispersers can also be detrimental if dispersers are carrying alien species into natural forests. Both animal and plant alien species can further disrupt seed dispersal processes by displacing native seed dispersers or by competing for seed dispersers. These events have mostly been documented in urban ant seed dispersal so far. For example, the invasive Argentine Ants in Melbourne, Australia, which displaced the native ant dispersers, reduced seed dispersal of the dispersed plants by removing fewer seeds and by burying seeds closer to parent plants.

Research into urban seed dispersal is still at its nascent stage and research has been mainly on birds and ants, and focused on one animal-plant interaction within urban forests. We are still far from understanding urban seed dispersal networks and the connectivity between urban green areas and the surrounding natural forests. Studying seed dispersal networks in urban habitats can provide information on the resilience status of the local community. 

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Eastern Grey Squirrels are dependent on tree cover and canopy connectivity when it comes to seed dispersal in urban spaces. Photograph: Susana Candusso

Understanding urban seed dispersal networks is not only interesting in terms of the particularities of urban habitats and the implications it may have at a landscape management level, but it can also act as a model for seed dispersal dynamics beyond the urban landscape. Common characteristics of urban environments such as fragmentation, edge effects (changes that occur at the boundary of two habitats) and the prevalence of non-native species mirror the widespread threats to ecosystems across landscapes, thus serving as an exceptional and easily accessible experiment.