Climbing to Survive: Ants in the Rainforest Canopy May Be Resistant to Effects of Climate Change

Featured image caption: Weaver ants are found high in the trees of Australian rainforests. Research explored how life in the canopy differs from life on the forest floor for ants. (Image source: “Weaver Ants” by B. Dupont, CC-BY-SA 2.0, via Wikimedia Commons).

Source article: Leahy, L., Scheffers, B.R., Andersen, A.N., Williams, & S.E. Rates. of species turnover across elevation vary with vertical stratum in rainforest ant assemblages. Ecography. 2024. https://doi.org/10.1111/ecog.06972.

In introductory biology courses, we learn about different biomes – deserts, tundra, grasslands, rainforests, and so on. These biomes vary in their climate, and species’ climatic preferences determine what biome they are found in. However, there is considerable variation in climate even within each of these biomes. For instance, there are major differences between the canopy of a forest to the ground or the surface of a pond to the bottom, despite nominally being part of the same biome. Have we been focusing too much on the “forest”, and missing the “trees”, in studies of biogeography?

We can explore this question by quantifying the effects of macroscopic climatic variation (i.e., climate across wide geographic or regional scales) and fine-scale climatic variation (i.e., climate within different parts of a single region or biome). This interplay between this macro- and micro- level variation in environment is one of the key questions facing the fields of ecology and evolution, especially now when climate change is having a major impact on ecosystems globally. In a recent article in Ecography, researchers quantified the effects of this microgeographic variation in climate on the distribution of ant species in Australian rainforests, and compared these effects to those driven by macro-scale factors like elevation or region.

A few feet of distance, but worlds apart

Ants are a powerful system for asking these questions because they are very diverse, abundant, and occupy both arboreal and ground habitats. In rainforests, tree-dwelling ant species face a wider range of temperatures than those on the forest floor, increasing the niche space that is available to species living in trees. Thus, the authors hypothesized that the ground nesting ant communities would have greater rates of species turnover and lower species diversity than canopy nesting communities, despite living in the same biome. Furthermore, they predicted that the climatic effects of elevation changes (e.g., cooler temperatures going up a mountain) would have a greater impact on the ground nesting ant communities because species in the canopy have more space and opportunity to move around to find their optimal environment.

To do this, they sampled ant species from the ground and canopy of rainforests across four mountain ranges in Northeastern Australia, at five different elevations per mountain range. Across all sampling locations, 101 species were identified from 11,770 samples of individual ants. Notably, they found that the species present at just 3 meters high were distinct from those found on the ground, suggesting even ants living on the same tree can face contrasting environments.

There was 36% lower species turnover in the arboreal ant communities, suggesting that there is greater coexistence among species in the canopy than on the forest floor. Additionally, elevation had a greater impact on the ground nesting ants. Ant species on the floor were less likely to be present at multiple elevations than those in the canopy. This is most likely because arboreal species can move further up or down the canopy to adjust to temperature changes across elevations, whereas species on the forest floor are restricted in their climatic niche. 

Attention to detail matters in conservation

These findings could have major implications for evolution and conservation. Climate change not only impacts different biomes in unique ways, but may also have fine-scale effects on species within different sub-regions, or even within individual trees, of the same biome. This complexity needs to be accounted for when we are designing plans for rainforest conservation. On one hand, the success of the arboreal ant species suggests that many organisms are finding ways to adapt to changing environments. Yet, the climatic restrictions faced by the ground-nesting ant communities leave them more vulnerable. Losses of species diversity on the forest floor could have cascading effects on the rest of the ecosystem, including among the arboreal species that are not as directly impacted by these environmental changes.

These findings highlight the importance of conserving the forest floor habitats that many species rely on because they may be the first to incur the costs of climate change. Preserving these environments by reducing deforestation and limiting the construction of roads and trails in rainforests would have major positive effects on rainforest ecosystems as a whole. Ultimately, by studying communities at both micro- and macrogeographic scales, we can better understand the processes that shape species distributions and their effects on the ecosystem at large.

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