What can bat poop tell us about past tropical landscapes?

Rachel Reid is a postdoctoral researcher at Washington University in St. Louis. She uses isotope chemistry to answer questions about ecology, geology, and conservation – including questions that can help build reference models for ecological restoration. Note: This blog is republished with permission from Amigos (No. 91 May 2019), the newsletter of Las Cruces Biological Station.

 Many people head to Costa Rica for spring break to see monkeys and sloths at Manuel Antonio National Park or to try their hand at surfing in the Pacific. While we did stop to gawk at the crocodiles that hang out under the bridge over the Tárcoles River with a busload of tourists, the goal of our trip diverged significantly from the spring break crowd – we were heading off the beaten path to southern Costa Rica to collect samples of modern and ancient bat guano (aka poop).

Bats sometimes visit the same caves over thousands of years, and the accumulated piles of guano offer a unique opportunity to study past environments. Just like a core of sediment from the bottom of a lake or the ocean, a core of bat guano collected from a cave contains useful information about the past, both recent and distant. The material at the bottom of the core is the oldest and that at the top is the youngest, so by sampling the length of a core, we can essentially take a short, stinky walk back in time.

We are interested in detecting changes in bat guano chemistry (particularly the carbon isotope values) through time as a way of evaluating what type of vegetation would have been on the landscape in the past. This works because information about the plants at the base of the food chain gets propagated up to the plant-eating insects and then to the insect-eating bats whose guano we’re sampling.

Bat Food Chain

Like other animals, bats and insects both gain carbon and nitrogen through the food they eat. Bats eat insects, which are in turn eating the local vegetation. Different types of plants have different carbon isotope values, such that most trees and shrubs (C3 plants) have much lower carbon isotope values than most grasses (C4 plants). Shifts in tropical bat guano carbon isotope values, therefore, are indicative of landscape-level changes in vegetation between more open, grassland plants and tropical forest.

How does bat poop inform conservation?

In the late 1940s, southern Costa Rica was nearly 100% forested. We know this from aerial photos – the earliest ones are from 1948. In later years, aerial photos show that most of that forest was cleared for coffee plantations; two thirds of it was cleared by 1980, for example.

This recent deforestation has motivated forest restoration efforts such as the creation of biological corridors and international scientific studies. Nonetheless, several studies (such as this and this) suggest that extinction rates in this region may be lower than would be predicted from recent habitat loss. One explanation for this could be that the regional flora and fauna evolved for several thousand years in a mixed forest and non-forest landscape managed by humans. By piecing together records of past vegetation from bat guano cores, we’ll be able to gain a better picture of what the landscape would have looked like in the past and potentially refine landscape-scale conservation and restoration targets.

For this first trip, our goals were to visit several caves to collect samples and to scout out future sampling opportunities. Southwestern Costa Rica has the highest concentration of karst caves in the country, so we were in the right place. In four days of fieldwork we visited three different caves (two of them twice!), collected 77 cm of core material, and took dozens of samples of modern bat poop.

At Bajo los Indios Cave, also known as Corredores, along the Rio Corredor, we ventured into a restricted, elevated chamber in hopes of finding deeper, more protected accumulations of guano. We were disappointed to find that even in this higher chamber, the cave was very wet and muddy and any significant guano accumulations appeared to have washed away. We collected a guano/mud core anyway and we’ll see what we can learn from it.

Bat Guano Team by JF

The bat guano team. From left to right: Leighton Reid & Christy Edwards (Missouri Botanical Garden), Rachel Reid & Alice Xu (Washington University in St. Louis), and Jeisson Figueroa (Organization for Tropical Studies). Photo by Jeisson Figueroa.

Taking a guano core by JF

Leighton Reid uses a peat corer to extract a sample of bat guano from a karst cave. Photo by Jeisson Figueroa.

One additional important piece to our project is to try to get a better idea of what modern insectivorous bats, such as the mesoamerican mustached bat (Pteronotus parnellii mesoamericanas), are eating. We’ll then use that information to better interpret our results back in time. We’re excited to start analyzing samples!

This pilot study was generously funded by grants from the Living Earth Collaborative and from the International Center for Energy, Environment and Sustainability.

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