Rice University biologist Amy Dunham has spent decades studying the mountainous rainforests of Madagascar's Ranomafana National Park, a UNESCO World Heritage site that was designated a national park in 1991. In a project co-led by Dunham and Rice's Matt McCary together with a team of U.S. and Malagasy researchers, the group recently published a study showing that strawberry guava, an invasive plant, can prevent natural forest generation in areas of Ranomafana with a history of past disturbance, even decades after deforestation has ended.
Native to Brazil, strawberry guava was introduced to Madagascar during the colonial era in the 1800s. Its tall woody stems make excellent construction materials, while its fruits are enjoyed by humans and animals alike. In fact, lemurs, the beloved, endangered ambassadors of Madagascar's incredibly diverse ecosystem, are often seen snacking on the red fruits. A disturbance-favoring plant, strawberry guava takes hold easily in areas where the rainforest canopy is damaged, often through deforestation. And as Dunham observed, once strawberry guava becomes established, the rainforest's recovery often stalls, even after many decades.
"Following forest disturbance, we would normally expect to see a gradual process of natural regeneration," said Dunham, the corresponding author on the paper and associate professor of biosciences. "Our study shows that when strawberry guava becomes established, that natural regeneration process can stall, arresting native species recovery at an early stage and disrupting the soil, insect and plant communities that support the rest of the forest."
Dunham and McCary's team worked in collaboration with Malagasy researchers to compare guava-invaded rainforest plots with nearby uninvaded plots, examining effects on vegetation, insects and invertebrates, soil and forest regeneration.
At nearly every level, the researchers found that guava was changing the forest. Guava-invaded soils were less nutrient dense, lacking in carbon, nitrogen, ammonia and organic matter. The dense guava thickets increased the density of understory vegetation more than threefold, and the forest floor underneath the guava supported fewer kinds of insects, while favoring a small number of decomposers.
Most striking, however, was the difference the researchers saw in the forest floor plants — plants that were under 1 meter tall, including tree seedlings. Diversity of this forest floor vegetation was lower in the guava-invaded plot, but diversity in tree seedlings alone was not. In fact, tree seedlings grew at the same diversity and density in the guava-invaded plots as they did in the native ones. Yet tree seedlings were not able to overtake the guava-invaded plots, even after decades of natural recovery. They could sprout and grow into seedlings but no further.
"The presence of non-guava tree seedlings at the same density and diversity in both sets of plots shows that native trees are able to sprout," said Julieanne Montaquila, a graduate student and co-author on the paper. "But the invaded plots remain arrested at the seedling stage, unable to progress into the next step of the regeneration cycle."
The exact reason for this arrest is unclear. It could be the increased density of the guava plants outcompeting the seedlings for space. Or the decreased soil quality. Or the allelochemicals the guava extruded that inhibited growth of native species. Perhaps the changed insect populations played a role. Whatever the reason, it was clear that strawberry guava is unlikely to release its stranglehold on the forest without intervention.
At the same time, the strawberry guava patches attract higher numbers of fruit-eating animals such as the endangered lemurs. Typically, conservation efforts to save a beloved animal, like lemurs, result in benefits for their entire ecosystem. "Here, we have an invasive species that is feeding lemurs while also destroying their habitats," said Eric Wuesthoff, a graduate student and co-author on the paper. The realization that strawberry guava is a food source for lemurs has, for some conservationists, lessened their perception of the invasive nature of the plant.
"What this paper highlights is the complexity of conservation efforts," Dunham said. "Strawberry guava is extremely difficult to eradicate, useful to the Malagasy people and positively associated with lemurs. But its presence disrupts the forest at many levels and may prevent the rainforest from fully recovering after deforestation."
This project was funded by Rice University's Sustainable Futures Fund, the Explorer's Club Youth Activity Fund, the Garden Club of America's Clara Carter Higgins Summer Environmental Studies Scholarship, and the Rice University Wagoner Fellowship. Two authors worked on this project while supported by NSF GRFP fellowships.
