April 15, 2021

Topanga Naturally: Of Bats and Bridges


Bridges, it seems, are home to multitudes of bat species in need of protection. Ken Wheeland knew that when he heard a jackhammer near his home one day and walked out to see workers pounding away at a bridge.

“I yelled for the workers to stop and told them that a colony of bats lived under the bridge they were refitting,” he recalls. “They argued with me, so I called the appropriate people and got the wheel rolling. Next day I saw Rebecca Andrews and she told me while she was waiting to cross the bridge, the workers were using a Bobcat with a huge jackhammer and when the pounding began hundreds of bats flew out from the bridge.”

Prior to refurbishing the bridge, Rosi Dagit had designed a low-impact construction plan that would cause minimal disruption to the bats. Although accepted by the County, the workers on the job neglected to follow the agreement.

“It was June 2010 when the bats left the bridge,” Wheeland continues. “I go down several times a week to check if they’ve returned. For months there was nothing. Then in December I saw a few. There are maybe two dozens bats now. In past years we counted up to 2,000.”

It is because of the relationship that Wheeland and other concerned Topanga residents have with the bats, that the information exists at all. In the late 1990s LA County Public Works decided that many bridges in Topanga were an “Emergency Project” that needed refurbishing. During a public meeting with Caltrans, Topangan Grant Brosius brought to light the colonies of bats living under these same bridges. As the meeting continued, citizens found that Caltrans did not have the necessary permits from Fish and Game or from the Coastal Commission to undertake the work proposed and, therefore, a bat study was launched. The Resource Conservation District (RCD) received a contract with LA County Works to determine baseline behaviors regarding the colonies.

Under the tutelage of Rosi Dagit, Hayley Safonov and her mother, Jackie, they monitored Topanga’s bat colonies between the years of 1999 and 2008 once a month, although frequency tapered off in the last few years.

To better understand bat behavior, Hayley and Jackie, with flashlights in hand, counted bat noses while the nocturnal bats roosted in bridge crevices during the day. Over the years of data collection trends started to emerge. Due to Hayley and Jackie’s commitment, research found that a majority of bats leave their roost by April 1 and return around June 1, pregnant and ready to give birth. The colonies then seem to leave again in late August and return around December 21. Interestingly, this data corresponds to bat data gathered in Pt. Mugu. When Pt. Mugu’s bat colony size shrivels, Topanga bat numbers flourish and vice versa.

As patterns emerged, other biologists came to Topanga to conduct further research. Through their studies it was found that some bridges housed juveniles and bachelors while other bridges became the nurseries for pregnant females and their young. Another study helped elucidate the bat species present in Topanga. Through use of an anabat–a device used to record and differentiate bat calls among species–it was determined that at least nine species of bats live in Topanga Creek’s watershed.

The three most common species living under the bridges are the big brown bat (Eptesicus fuscus), the little brown bat (Myotis lucifugus) and the Mexican free-tailed bat previously named Tadarida mexicana and has now been changed to Tadarida brasiliensis. The Mexican free-tailed bat is by far the most common under the Topanga bridges, although it also lives under roof tiles, in eaves, and amongst cave crevices. Two environmentally sensitive bat species were also found to live in Topanga, the pallid bat (Antrozous pallidous) and the Western mastiff bat (Eumops perotis).

Unfortunately bats have gained an bad reputation. They have been associated with the devil and vampires, rabies-ridden nuisances and pests whose only wish is to tangle in your hair. The truth is that bats are an essential component of a healthy ecosystem. As the only mammal capable of sustained flight, they are important pollinators, seed dispersers and insectivores. With exception of the Arctic, the Antarctic, and a few isolated oceanic islands, bats are found in every habitat. Their success as a species emphasizes their ecological importance. More than 70 percent of all bats feed solely on insects, eating up to one-third of their body weight each night, up to 600 insects per bat per night.

Given the thousands of bats counted by Hayley and Jackie, this is a staggering number of insects consumed by Topanga bats. Imagine the Canyon without them. Or for that matter, imagine Topanga without the people who take the time to care about our friends under the bridges. How many species might have been lost without the diligence and concern of Ken Wheeland, Grant Brosius, Hayley and Jackie Safono, and Rosi Dagit?

For more information regarding bat conservation, and effective ways to remove or exclude bats from your home, visit Bat Conservation International at www.batcon.org.