WHY this variation in vulnerability?

Submitted by editor on 13 February 2019.Get the paper!

As I child, I could only dream about the Amazon Rainforest. Few places had such a stronghold on my imagination – a mystical and remote place teeming with unparalleled diversity and exotic creatures. But when, decades later, I made my inaugural trip to Manaus, Brazil, I was no longer so innocent. Although my first naive impressions still rang true, the Amazon Rainforest was also being fragmented, clearcut, and carved up by roads. The forest and its denizens are now a diminished version of their former selves.

 

Today we know that certain groups of birds are disproportionately vulnerable to these degraded habitats, which bare the footprint of humanity. Still, a nagging, yet simple, question persists – why? What is it that causes some birds to be more vulnerable to Amazonian deforestation and fragmentation than others? One evocative idea posits that perhaps birds which are accustomed to dwelling in the dark understory of continuous forest are visually sensitive to the bright light found in gaps, degraded forest, or at the edges of fragments.

This Black-headed Antbird was captured in a mist net and now showcases a metal ring with a unique combination on its right leg.

To answer this ‘why’ question, we first began by trying to rigorously determine the vulnerability of each of the most common understory birds. To do this, we used more than 4000 mist-net captures gathered across the span of a decade. Some of these were from primary forest, untouched by humans, whereas others were in secondary forest and small fragments. Using the species-specific differences in capture frequency between these two forest types, we could quantify which species were positively and negatively affected by disturbance.

In degraded habitats, we found that more than half of the birds we sampled declined significantly in relative abundance compared to primary forest, whereas only a few species appeared to increase. This is noteworthy as it allows us and other researchers to then explore what might explain the interspecific variation in vulnerability.

A portrait photo of a Black-headed Antbird with a scale overlay, which was used to subsequently measure the exposed diameter of the bird’s eye using image-processing software.

But we had set out to specifically tackle the visual constraints hypothesis. Unfortunately, however, visual sensitivity in a bird is difficult to measure directly. So instead, we relied on two indirect metrics to infer visual sensitivity: the size of a bird’s eye and when that bird begins singing at dawn. Both of these characteristics presumably reflect a species’ visual capacity under low light intensities. We then looked for correlations between these two visual metrics and our species’ vulnerability scores, but did not find support for visual sensitivity to light.

In attempting to do so, however, we introduce a novel method to investigate the ‘why’ question, which we hope may one day shed light on these diminished forests and their impoverished fauna.

 

This work would not be possible without support from the Biological Dynamics of Forest Fragments Project and funding by the US National Science Foundation (LTREB 0545491 and 1257340).

Cameron L. Rutt on behalf of all co-authors

 

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