How global heating can disconnect tropical forests from freshwater ecosystems

This blog post is provided by Liam Nash and tells the #StoryBehindThePaper for his article “Warming of aquatic ecosystems disrupts aquatic-terrestrial linkages in the tropics“, with co-authors Pablo Antiqueira, Gustavo Romero, Paula de Omena, and Pavel Kratina, which was recently published in the Journal of Animal Ecology. Liam is currently doing a PhD at Queen Mary University of London on aquatic-terrestrial linkages around the world.
Tank-bromeliads contain an aquatic microecosystem, closely linked with its surrounding terrestrial ecosystem. (Photo credit: Gustavo Romero)

Throughout Brazil’s Atlantic Rainforest are plants known as tank-bromeliads. Related to the pineapple, they mostly grow high off the ground on the branches of other trees. Between tightly overlapping, watertight leaves, these plants hold a small pool of water known as the “tank”. Encompassed by these tanks are microecosystems of diverse freshwater organisms. Beetles and midge larvae eat sunken, dead leaves. Mosquito larvae dart through the water column. Damselfly nymphs lurk in the shadows, topping the tank food chain as apex-predators. What all of these tank-dwelling insects have in common is that they are aquatic while young. However, after metamorphosis they are all destined for the terrestrial ecosystem, the land and air, as winged flying adults.

The young (left) of the Leptagrion damselfly develop aquatically in tank-bromeliads but become terrestrial when adult (right). (Photo credit: Liam Nash (left) and Pavel Kratina (right))

These aquatic-come-terrestrial insects go on to feed predators such as birds, bats and spiders, transferring resources from water to land. On the flip side, the aquatic tank-bromeliad communities are themselves heavily reliant on terrestrial resources, transferred from land to water in the form of leaf litter and other falling detritus.

This is because, despite being traditionally considered separately, aquatic and terrestrial ecosystems are in fact tightly interconnected by these flows of resources. These links allow for diverse and sometimes diverging impacts of human activity, such as climate change, to propagate between different ecosystem types.

One of the principal ways ecologists might understand how rising temperatures will affect these links is through mesocosm experiments – heating up large, artificial ponds under predicted warming scenarios. However, mesocosm experiments are still rare in the tropics compared to Europe and North America. This is where tank-bromeliads come in. The aquatic microecosystems they contain can be easily manipulated and replicated, while still allowing for natural environmental variability. They act as a natural microcosm and have already been used to test a range of ecological hypotheses throughout the American tropics.

The bromeliad species used in our study: Neoregelia johannis. This one was so big it had fallen from its tree. (Photo credit: Liam Nash)

For our study, carried out in Brazil’s Atlantic Rainforest, we also used tank-bromeliads. We wanted to test exactly how these links connecting tropical aquatic and terrestrial ecosystems are impacted by warming. Using a custom-made, bromeliad-heating system we warmed up fifty wild bromeliad tanks. The plants were enclosed with net traps to catch emerging insects (the water-to-land link) and contained strips of cotton to measure decomposition (representing leaf litter; the land-to-water link).

Collecting wild bromeliads for our warming experiment. (Photo credit: Liam Nash)

We observed sharp declines in the emergence of insects of almost 25% for every 2°C of warming. Smaller, faster developing insects such as midges and mosquitoes fared even worse. This bodes badly for insects, already suffering from widespread declines, with temperatures predicted to rise by 2 – 4°C in Brazil over the next century. Fewer aquatic insects emerging means less prey for predators which rely on them, which can have knock-on effects across the entire rainforest ecosystem.

Because insects are ectothermic, or “cold-blooded”, their internal temperature is dependent on the environment’s temperature. The tropics may be warm, but they are consistently warm, without large seasonal changes in temperature like in temperate regions. This means that tropical ectothermic insects are not adapted to large changes in temperature and are at increased risk of the negative effects of warming. Our findings directly contrast with results from cooler regions, where warming boosted the emergence of aquatic insects in terrestrial environments.

The decomposition results were more complex. For most bromeliads, warming increased the rate at which plant compounds broke down in water. Higher energy demands associated with higher temperatures increased resource demand and activity levels. This was mostly driven by microbes (such as fungi and bacteria) which are typically more adaptable to changing temperatures. This may have worrying consequences, as microbial decomposition can increase carbon emissions into the atmosphere. However, in the largest bromeliads the impact of warming was dampened, suggesting that larger, more diverse habitats can buffer some of the impacts of climate warming.

South-east Brazil’s Atlantic Rainforest is a hotspot of tank-bromeliad diversity. Aquatic microecosystems are everywhere. (Photo credit: Liam Nash)

Taken together it seems the connections between land and water in the tropics may be more vulnerable to rising temperatures than in other parts of the world. By affecting the very processes which connect separate ecosystems, warming can have wide-reaching and unexpected effects beyond the boundaries of any individual ecosystem. As we try and understand how our world will look over the next century of human activity, it is increasingly important to consider ecosystems not as separate entities, but as components of a complex, interconnected landscape.

Read the paper

Read the full paper here: Nash, L.N., Antiqueira, P.A., Romero, G.Q., de Omena, P.M. and Kratina, P. (2021), Warming of aquatic ecosystems disrupts aquatic‐terrestrial linkages in the tropics. Journal of Animal Ecology. Accepted Author Manuscript.

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