Editor’s Choice @Biotropica 47(4): Do bats fertilize their roost trees?
I am pleased to announce the July 2015 Editor’s Choice Article: Voigt, C. C., Borissov, I. and Kelm, D. H. (2015). Bats Fertilize Roost Trees. Biotropica, 47: 403–406.
This paper was a theoretically simple but technically challenging attempt to answer a long-standing questions: do animals that roost in trees fertilize them? Using nitrogen isotope ratio as proxy for nutrient transfer, Voigt et al. tracked nutrients from bat excreta to seed exocarp in Dipteryx panamensis that host bat roosts, suggesting that trees may benefit from nutrients of bat guano deposited into hollow tree trunks. It’s a perfect example of an Insights paper: a combination of an important question, a straightforward design, and an advance in our understanding of a geographically widespread system in the tropics. Congratulations to Christian and colleagues, who explain the motivation and insights in their essay below.
PS It’s not that often research published in Biotropica is featured in The Economist, but this paper got great write up there entitled “Blood Earth”. Perhaps it’s not surprising that an article about creatures of the night and fecal matter would appeal to a readership that includes hedge fund managers! Jokes aside, kudos again to the research team.
Bats fertilize roost trees
When looking down from a tower at the lush rainforest canopy, it is an impressive thought that many tropical forests grow on nutrient poor soils. To overcome the shortage in nutrients, tropical plants have evolved various mechanisms to extract efficiently nutrients from soil. Some plants have even entered tight relationships with animals, such as ants, to profit from additional nutrient input from excreta or food items. Already in the eighties, the ecologists Kubíkova and Janzen suggested that this co-evolutionary process related to nutrient supply, might even work on a larger scale where trees with rotten cores may benefit from excreta deposited by vertebrates using these hollow cores as shelters. This may seem impossible because hollows inside trees lack roots for nutrient uptake. Yet, this is not the case for those trees that Kubíkova and Janzen had in mind: They argued that nutrients from faeces falling on the soil inside of hollows at the base of tree trunks may be absorbed by a mesh of fine roots. And here, bats enter the stage: bats are greatly limited in the availability of daytime roosts. Exchanging nutrient-rich excreta for a daytime shelter would in fact make sense and could be considered of adaptive value for a bat. Dan Janzen even proposed that rotten cores in tree trunks – despite their appearance as a sign of decay – might be of adaptive value for trees when the extra nutrient uptake enhances the Darwinian fitness of trees. Even though this idea was formulated more than 30 years ago it has remained untested since then.
In the forest reserve of La Selva Biological Station, a well-known Costa Rican field station run by the Organization of Tropical Studies, hollow trunks can be encountered in many large trees, particularly in trees of the species Dipteryx panamensis. Individuals of this species often exceed the top canopy layer of Neotropical forests. Tree hollows vary in size, ranging from small crevices to hollows that have the size of a small garage. Looking for daytime roosts of fruit-eating bats, these hollows caught the attention of my collegues Detlev Kelm, Ivailo Borrisov and me in 2006. Some of the hollow trees had huge openings because large parts of the decaying trunk had already vanished. Entries to other hollows were relatively small, consisting merely of a hole that was big enough for us to crawl inside. I remember one of these roosts with a particularly small entry and a particularly large interior. I had to lie down in the litter of the forest floor and glide like a snake inside the roost. Indeed, it was also worthwhile to always look out for venomous snakes, such as Bothrops asper, because they would often sit close to the entry to ambush on bats flying in and out during dawn and dusk. As bat biologists, we were interested in the diversity of bat species that occupied these tree roosts. We identified a wide variety of species, such as fruit-eating, nectar-feeding, insect-feeding, frog-eating and even blood-feeding species. The grounds of the roosts were usually covered with bat faeces. In case fruit-eating bats occupied the roost, small seedling would sometimes grow from defecated seeds. When vampire bats occupied the hollow trees, the floor was covered by a thick, blackish liquid, which consists of digested blood. Knowing Janzen’s opinion paper, Detlev, Ivo and I asked ourselves how to measure a possible nutrient input from bats via their faeces to the tree. Stable isotopes had already been used before in the study of trophic interactions, and thus we decided to look into nitrogen isotope ratios of soils and plants.
During two field seasons, we collected soil from the inside and outside of hollow Dipteryx panamensis. For a comparison, we also looked at exterior soil next to Dipteryx panamensis without hollows. Indeed, soils from the exterior of hollow trees had similar nitrogen isotope ratios compared to conspecifics without hollows, thus providing evidence that both types of trees were growing on similar soils. In soil collected from the interior of hollow trees, we found higher nitrogen isotope ratios values in some but not all trees. We wondered if roots were taking up nitrogen-15 inside tree hollows, so that we were not able to detect the isotopic signal in the interior soil. In order to test this idea, we used a setup that one of us established over the past years in and around the La Selva biological reserve: Detlev Kelm had set up several dozens of artificial bat roosts to support local colonies of fruit-eating bats and to facilitate seed dispersal into deforested areas. For our study, we looked at nitrogen isotope ratios in soil of the interior and exterior of these artificial daytime roosts. In contrast to hollow trees, we found an enrichment of nitrogen-15 in soils collected from the inside of artificial roosts compared with soils from the exterior. Thus, when roots are lacking, we found a strong effect of nitrogen input from bat faeces. The yet missing piece of this puzzle was the question whether nitrogen-15 from bat faeces could be tracked in the seeds of Dipteryx panamensis. And indeed, we found a higher enrichment of nitrogen-15 in seed exocarp of Dipteryx panamensis with hollows than in seed exocarp of D. panamensis without hollows. We considered this as convincing evidence that nitrogen isotopes from bat faeces are transported from the soil in the interior of bat roosts, some 30 or 40 m into the canopy where it is assimilated into the tissue of seed exocarp. Consistent with this finding, we found that trees with large colonies in their hollows and those with vampire bats tended to have higher nitrogen isotope ratios in their seed exocarps than those with smaller colonies or those hosting other species than vampire bats.
In summary, our isotopic data is in line with the hypothesis formulated by Kubíkova and Janzen some 40 years ago. This tree-bat interaction could be considered a win-win situation for all partners: Bats benefit from having a daytime shelter and plants profit from the extra nutrients deposited by defecating bats. When I now look around from the canopy tower of the La Selva Biological reserve, I am amazed that some of the large Dipteryx panamensis reaching higher than most other trees are in fact fertilized by bats at the base of their trunk.