Urban areas are known to be prone to habitat fragmentation. We’ve seen the intense urbanization in New York City lead to the rapid divergence of mouse populations and the subdivision of bobcats in Los Angeles. But the adverse effects of habitat fragmentation are difficult to quantify given that they differ based on the ability of an organism to disperse and the spatial structure of the landscape.

Historically, birds have been viewed as less impacted by habitat fragmentation due to their ability to fly over and around patchy habitat. However, birds that are understory specialists have been shown to be affected by breaks in forest patches and may go locally extinct after a forest is fragmented. So how does the level of forest dependency in an organism influence connectivity and gene flow? This is what Emilie Cros and colleagues set to find out.

The island nation of Singapore encompasses a little more than 700 square kilometers and is home to over 5.5 million people (~7,800 people per square km). With this intense urbanization, Singapore has lost about 95% of its forested land leading to the local extinction of over 100 birds (to learn more about the urban wildlife in Singapore be sure to check out the Planet Earth II episode on cities).

To investigate their question, Cros et al. (2020) focused on six bird species with varying dependence on forest cover:

Short-tailed Babbler, Pellorneum malaccense (insectivore, forest-dependent)

Chestnut-winged Babbler, Cyanoderma erythropterum (insectivore, forest-dependent)

Abbot’s Babbler, Turdinus abbotti (insectivore, edge-tolerant)

Striped Tit-babbler, Mixornis gularis (insectivore, edge-tolerant)

Cream-vented Bulbul, Pycnonotus simplex (frugivore, forest-dependent)

Olive-winged Bulbul, Pycnonotus plumosus (frugivore, edge-tolerant)

The researchers then collected blood samples 218 of these birds on the island of Singapore as well as the smaller nearby islands of Sentosa and Ubin, and the contiguous forests in Vietnam and Borneo. To assess population connectivity the authors performed double-digest restriction-associated DNA (ddRAD) sequencing to detect SNPs (single nucleotide polymorphisms) across the genome. ddRAD sequencing is a type of reduced representation genome sampling that has become popular among population biologists for allowing scientists to examine thousands of loci and infer relatedness among individuals.

Inbreeding & Genetic Diversity

Cros et al. (2020) found that closely related kin in each of the six species, but inbreeding was low for all species except for the Cream-vented Bulbul suggesting that inbreeding is not impacting most of the species examined.

However, the researchers found that genetic diversity was lower in the Singaporean populations of the Short-tailed Babbler and the Chestnut-winged Babbler than in the unfragmented Bornean populations. This suggests that fragmentation of the forest in Singapore is leading to reduced genetic diversity in these two species. Interestingly, these two species are the insectivore, forest-dependent species.

Population Structure

Cros et al. (2020) also found that the two insectivore, forest-dependent species each formed two populations, one from the North Central Catchment and one from the South Central Catchment. These two populations are separated by less than a mile and the authors detected that open reservoirs, scrubland, and anthropogenic structures may be barriers to gene flow. In contrast, the other four species did not have such clear divisions and showed minimal or no spatial clustering.

What does this tell us?

This study reiterates how important life-history strategies are when understanding how urbanization influences gene flow. The two habitat specialists (the Short-tailed Babbler and the Chestnut-winged Babbler) are restricted to the last forest reserve located in the center of the city. And even within this forest reserve, both species formed fragmented populations, separated by anthropogenic landscape features.

However, four other species found on the island including the two edge-tolerant insectivores (Abbot’s Babbler and the Striped Tit-babbler) as well as the frugivore forest-dependent species (Cream-vented Bulbul) and the frugivore edge-tolerant species (Olive-winged Bulbul) did not show population differentiation across Singapore. This supports the idea that some species (in this case insectivorous forest-dependent) are more susceptible to fragmentation than other species and reiterates the need to understand the life-history strategies of species when assessing how urbanization influences gene flow and genetic drift.

Full paper here:

Cros, E., Ng, E. Y. X., Oh, R. R. Y., Tang, Q., Benedick, S., Edwards, D. P., … Rheindt, F. E. (2020). Fine-scale barriers to connectivity across a fragmented Southeast Asian landscape in six songbird species. Evolutionary Applications. https://doi.org/10.1111/eva.12918

Featured image:

A. Short-tailed Babbler (Malacocincla malaccensis) by Lip Kee licensed under CC BY-SA 2.0. B. Chestnut-winged Babbler, Danum Valley, Borneo by Nigel Voaden licensed under CC BY-SA 2.0. C. Abbot’s Babbler – Thailand_S4E7468 by Francesco Veronesi licensed under CC BY-SA 2.0. D. Yellow-breasted Babbler or Pin-striped Tit-babbler Mixornis gularis or Macronus gularis by Dr. Raju Kasambe licensed under CC BY-SA 4.0. E. Pycnonotus simplex by Lip Kee licensed under CC BY-SA 2.0. F. owb Pycnonotus plumosus by Lip Kee licensed under CC BY-SA 2.0.

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