Do urban street trees improve landscape connectivity for bats?

Having recently shared a study that looked at the urban forest and bird species diversity, I thought we could use the momentum from that article and build upon it by looking at bats and the urban forest. The bird species diversity article highlighted the criticality of good landscape connectivity within urban environments (the opposite of habitat fragmentation), and the authors of this study suggest much the same from the very beginning – alongside fragmentation (usually coupled with urban sprawl / urbanisation) comes a decline in species diversity within urban areas, and it can be anticipated that bats are no exception to this.

The authors undertook this study in Vitoria, a city in the southeastern part of Brazil, with an intent of assessing whether wooded streets were used by bats (this would allow the authors to conclude whether urban street trees are beneficial in terms of connectivity for bats). Vitoria, home to 1.8 million people, expanded significantly during the 1940s and onwards, following industrial advancements. Such expansion lead to the destruction of the surrounding environment (including the Atlantic Forest) and its ecosystems, and the increased pollution levels have further taxed remaining vestiges. However, the city is still marked as a site important for biodiversity conservation, likely because of its eight municipal parks acting as habitat for a great variety of wildlife to the now (locally) fragmented Atlantic Forest.

This study chose to look at three of those eight parks for the presence of bats: (1) Pedra da Cebola Municipal Park, with 100,005 square metres of land that once acted as a transition between coastal shrubland at the Atlantic Forest; (2) Horto de Maruipe Municipal Park, with 60,000 square metres of land and covered by native Atlantic Forest vegetation (though with many introduced species of wildlife), and; (3) Fazendinha Municipal Park, with 22,653 square metres that – like Pedra da Cebola Municipal Park – was a transition zone between the coast and the Atlantic Forest. Three wooded streets, and three non-wooded streets, were also surveyed for bat presence. By surveying the parks and the streets, comparisons could be drawn between the two, and there would be scope to determine whether bats remain isolated to the parks. Sampling for the bats (with the use of mist nets) took place over the course of a year, with visits taking place for three days each month.

In total, 172 individuals were captured a total of 174 times. Most were found within the municipal parks, followed by the wooded streets, and then the non-wooded streets. However, the vast majority were only found within the urban parks (both in terms of species diversity and species abundance), with wooded and non-wooded streets not displaying a marked difference between one another. The authors do note that limited samplings perhaps lead to this, and suggest that more sampling visits to all areas would potentially show more individuals of more species using all three environments (only 10 species were recorded, whilst the region has 36 bat species – the authors do suggest that Vitoria may not harbour all species, however). The table below breaks down species presence by environment.

The number of individuals of each bat species recorded during the study period of one year.

In light of their research, the authors in fact suggest that wooded streets are not particularly important for bat species, though they do recognise that increasing sampling visits and having sampling methods utilise ultrasonic devices may have improved the results obtained. For example, Noctilo sp. bats were observed, though not recorded with the netting method.

The fact that Artibeus lituratus was the most frequently observed one across all three environments, the authors allege, is due to its opportunistic behaviour. It has previously been recorded using urban environments, and may have a tolerance to urbanisation as a result (unlike other species). However, other research papers have suggested more species of bat (some found in this study) do use urban areas for feeding, particularly where lights attract insects (insectivorous bat species will benefit most for this).

Artibeus lituratus (giant fruit-eating bat). Source: Mammalia Web.

So what does this study mean? As sampling was admittedly quite limited, perhaps it’s not a foregone conclusion that bats don’t use street trees as vehicles to move between larger park areas (or for other reasons), though it does suggest that maybe the urban forest is not structurally supportive of many bat species. Could more be done in the planning stages of development, or even following development, to attract more bats to the urban streets of Vitoria? The authors suggest that using native trees in place of exotic ones may be a good starting point.

Source: Oprea, M., Mendes, P., Vieira, T., & Ditchfield, A. (2009) Do wooded streets provide connectivity for bats in an urban landscape?. Biodiversity and Conservation. 18 (9). p2361-2371.

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Do urban street trees improve landscape connectivity for bats?

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