Rewilding tropical forests: dung is the key

Rewilding means different things to different people. Basically, it involves restoring a species, or several species to an area from which they have been extirpated by humans. Conservation biologists might study the population size and distribution of the returned species, ecologists might focus on interactions between the returned species and other species, while anthropologists might investigate how humans in the area are adjusting to having a new species in their lives.  One of the most famous examples of rewilding is the return of gray wolves to the Greater Yellowstone Ecosystem in western U.S.A., which can be looked at from the perspective of how the wolf populations are doing numerically, how they affect their prey (elk) or their prey’s prey (willow and aspen in the case of elk), and how they affect ranchers in the surrounding areas.

Conservation ecologists have begun a major rewilding program in Tijuca National Park in Brazil, introducing agoutis in 2010 and brown howler monkeys (Alouatta guariba clamitans) in 2015. Howler monkeys were extirpated from this park over a century ago, so ecologists worried that the monkeys might interact with the remaining species in unexpected ways.  For example, this forest hosts several species of invasive fruit trees, such as the jackfruit (Artocarpus heterophyllus). Luisa Genes and her colleagues were concerned that howler monkeys might eat fruits from these trees, and poop out the seeds in new forest locations, causing the invasive species to spread more rapidly.

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Introduced howler monkey holding the second baby born to her in the forest. Credit: L. Genes.

Even a disturbed rainforest such as Tijuca National Park hosts a large number of plant species, so the interactions can be complex and difficult to study.  As is so often the case in ecology, one very important complex of interactions involves poop.  Specifically, howler monkeys eat fruit off of trees, and poop the seeds out, usually at a new location, effectively dispersing the seeds.  But there is a second link in this seed dispersal interaction.  Twenty-one species of dung beetles use howler monkey poop for food for themselves and their offspring, breaking off small sections into balls and rolling the balls to a new location.  This process of secondary dispersal is nice for the beetles, but also for the seeds within the balls, which can now germinate in a new location without competing with the large number of seeds in the original howler monkey pile.

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Two dung beetles battle over a dung ball. Credit: Rafael Brix.

Genes and her colleagues were interested in two basic questions.  First, were the howler monkeys eating fruit from a few select tree species, or were they eating from many different types of trees, thereby dispersing seeds from many species?  Before releasing the monkeys (two females and two males), they attached radio transmitters to the monkeys so they could easily track them, and note what they ate.  Based on 337 hours of observation, the howler monkeys ate fruit from 60 different tree species out of 330 possible species in the forest (18.2%).  This is an underestimation of actual howler monkey contribution to seed dispersal, because the researchers observed the monkeys for a relatively brief time, and fruit consumption by the monkeys should increase over time as the population of monkeys (and possibly tree diversity), continues to increase.

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Male howler monkey released in 2016.  Note the radio transmitter on its right rear leg. Credit: L. Genes.

The second question is whether secondary dispersal by dung beetles was reestablished following reintroduction of howler monkeys.  To answer this question quantitatively, Genes and her colleagues set up an experiment that used plastic beads of various sizes instead of seeds. The researchers set up circular plots of 1m diameter with 70 grams of howler monkey poop in the middle.  Each pile was mixed with seeds (actually beads) of four different sizes (3, 6, 10 and 14 mm diameters) to mimic the range of seed sizes. The researchers measured secondary seed dispersal by returning 24 hours later and counting the remaining beads, reasoning that the rest had been moved by dung beetles (along with the poop) to a new location.

Genes and her colleagues discovered that the median rate of seed dispersal (bead removal) was 69% with larger seeds being moved at a significantly lower rate than smaller seeds.  Thus secondary seed dispersal by dung beetles was still operating in this ecosystem even after howler monkeys had been absent for over 100 years.

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Removal rate of beads (seed mimics) from dung piles by dung beetles in relation to bead size.  Different letters above treatments indicate statistically significant differences between treatments. 

Overall, ecological interactions among howler monkeys, plants, and dung beetles were rapidly reestablished once howler monkeys were reintroduced to the community.  There are plans to introduce five more howler monkeys this year, which should further increase beneficial seed dispersal, and hopefully allow plant diversity to increase as well.  One problematic observation was that howler monkeys also ate invasive jackfruit, which could promote its dispersal within the community.

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Luisa Genes monitors howler monkeys in the forest. Despite its apparent lushness, the forest still lacks many species and interactions that you would expect to find in an intact forest. Credit: L. Candisani.

The researchers discovered only 21 species of dung beetles, which was somewhat lower than other studies have found.  It is probable that conversion of this land into farmland in the 19thcentury led to the decline and/or demise of some dung beetle species.  With reintroduction of howler monkeys, and the passage of time, Genes and her colleagues expect that this rewilding effort should lead to a more robust ecosystem, with increased howler monkey populations supporting high dung beetle abundance and diversity, and more effective dispersal of many plant species. To understand the overall impact on forests, the researchers recommend that future studies should compare seedling survival and forest regeneration in areas where howler monkeys were reintroduced to areas where howler monkeys are still missing.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Genes, L. , Fernandez, F. A., Vaz‐de‐Mello, F. Z., da Rosa, P. , Fernandez, E. and Pires, A. S. (2019), Effects of howler monkey reintroduction on ecological interactions and processes. Conservation Biology, 33: 88-98. doi:10.1111/cobi.13188. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2019 by the Society for Conservation Biology. All rights reserved.