Amazonian dirt roads are choking Brazil’s tropical streams

(The Conversation is an independent and nonprofit source of news, analysis and commentary from academic experts.)

<a target=”&mdash;blank” href=”″>Cecilia Gontijo Leal</a>, <a target=”&mdash;blank” href=”″>Museu Paraense Emílio Goeldi, Belém</a>

(THE CONVERSATION) The first time I traveled to the Amazon, in 2010, I had no idea what to expect. A doctoral student from the far-off Brazilian city of Belo Horizonte, I imagined that my field work – studying fish habitats in the largest tropical forest on Earth – would be all boat rides on immense rivers and long jungle hikes.

In fact, all my <a target=”&mdash;blank” href=””>research team</a> needed was a car. That’s because the Brazilian Amazon isn’t just a rainforest: This 1.6 million square mile area is also home to villages, <a target=”&mdash;blank” href=”″>farming</a>, logging and even <a target=”&mdash;blank” href=”″>mining</a>. That means there are <a target=”&mdash;blank” href=””>roads</a>, most of them dirt.

Scientists have long known that roads contribute to deforestation, both <a target=”&mdash;blank” href=””>in the Amazon</a> and <a target=”&mdash;blank” href=”″>in other rainforests</a>. But <a target=”&mdash;blank” href=”;hl=en”>my research</a> reveals that these transportation networks also harm Amazonian waterways, affecting the fish that thrive in this delicate habitat and endangering local communities.

Water, water everywhere but not a bridge in sight

Our original 2010 study examined water conditions and fish diversity in 99 small streams – called “<a target=”&mdash;blank” href=””>igarapés</a>” – in the Amazonian state of Pará. To reach them, we drove long distances past pastures, large plantations and small villages, through dense virgin forests and young regenerating jungle.

We also crossed a lot of rivers and streams – the waterways that nurture this rich tropical ecosystem. Rarely did we use a bridge, though.

Instead, we discovered, most Amazonian igarapé crossings are an informal affair. Streams are narrowed with packed dirt, and water is channeled underneath via a metal pipe.

Most of the makeshift culverts we saw were either too narrow for the water flow or were installed too high – “perched” such that they created a sort of mini waterfall on the downstream side of the crossing, visibly disrupting the water flow.

A fish would really struggle to get through a perched culvert, we thought. Clearly, these informal stream crossings must be affecting local fish populations in other ways, too. I’ve spent the past eight years documenting how.

Roads threaten Amazonian fish

The Amazon’s rivers and streams contain a brilliantly <a target=”&mdash;blank” href=””>diverse array of fish</a> – so many <a target=”&mdash;blank” href=””>thousands of species</a> that scientists are <a target=”&mdash;blank” href=”″>still finding new fish families</a>. We counted 44 species of fish in just one small Amazonian stream. That’s <a target=”&mdash;blank” href=””>more than live in all of Denmark</a>.

After many hours measuring various attributes of small streams – including water depth, channel width, water flow, temperature and types of substrate – and sampling the fish that lived there, the data from <a target=”&mdash;blank” href=””>our 2010 study confirmed that dirt roads are ravaging Amazonian streams</a>.

We found that makeshift road crossings cause both shore erosion and silt buildup in streams. This <a target=”&mdash;blank” href=””>worsens water quality</a>, hurting the <a target=”&mdash;blank” href=””>fish that thrive in this delicately balanced habitat</a>.

The ill-designed road crossings also act as barriers to movement, preventing fish from finding places to feed, breed and take shelter. This can lead to what’s called “<a target=”&mdash;blank” href=””>faunal homogenization</a>” because the most vulnerable aquatic species die out, leaving a less diverse fish community.

Disrupted water flows particularly affect carnivorous species, which tend to hunt across long distances. The dogtooth fish, or Acesthrorhynchus falcatus, for example, typically <a target=”&mdash;blank” href=”;lr=&amp;id=iz3NBQAAQBAJ&amp;oi=fnd&amp;pg=PA1&amp;dq=Acestrorhynchus+falcatus+hunting+amazon&amp;ots=DT75TW%E2%80%94MaB&amp;sig=dWmvE6e-iewmwvIAwYxQbi1DTdU#v=onepage&amp;q&amp;f=false”>hunts in many different streams</a>, swimming from one to the other in search of prey.

We don’t know yet what happens to carnivorous Amazonian fish whose range is shrunk by road crossings. But malnutrition, inbreeding and even species endangerment seem like likely outcomes.

Fracturing the Amazon’s watery network

Amazonian streams are <a target=”&mdash;blank” href=””>interconnected</a>, a natural network perfectly calibrated to transport nutrients and organic matter downstream, control water flow and regulate water quality. In short, igarapés are the essential interface between forests and the Amazon’s big rivers.

Informal stream crossings create blockages in this water network. I estimate that there are some 3,000 informal stream crossings in Paragominas municipality alone – and that’s just one small part of one state of the nine states that comprise the Brazilian Amazon.

Taken together, these makeshift “bridges” – some <a target=”&mdash;blank” href=””>belonging to roads built illegally</a> – have profoundly fragmented the Amazon’s freshwater ecosystem.

This article was originally published on The Conversation. Read the original article here: <a target=”&mdash;blank” href=”″></a>.

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