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Under the Shade of Eucalypt Trees

Credit: mraoraor/Adobe

Credit: mraoraor/Adobe

By Maider Iglesias-Carrasco

Eucalypt trees are iconic in Australia, with around 900 species spread around the continent. However, a new study has found that toxic compounds in their leaves are having undesirable ecological impacts when eucalypt plantations have been established in other parts of the world.

Australia is famous worldwide for its curious animals, but most of the people around the globe also know Australia for its plants. Eucalypts are a diverse group of flowering trees and shrubs that dominate much of the Australian landscape, with around 900 species covering more than 90 million hectares of the country.

Some Australian animals, such as possums and koalas, have adapted to exploit eucalypt trees and forests after evolving together over millions of years. But what happens to the animals in other parts of the planet when humans move eucalypt trees from Australia to their environments?

All over the world, vast areas of native forest have been converted into exotic tree plantations. In total, 7% of the global forested area is occupied by plantations, which are mainly used to produce paper, timber or charcoal. Eucalypts are a main character of this transformation of the habitat across the continents.

One of the advantages of eucalypt trees is their quick growth. In Europe, for example, eucalypts reach maturity for harvesting within 10 years of being planted, while a native oak tree needs more than 100 years to reach the same size.

Although eucalypt trees are a significant economic resource, they can have lasting detrimental consequences for the animals living among them.

The Effects of Exotic Plantations on Native Animal Diversity

Researchers have carefully studied how the transformation of the habitat affects biodiversity by comparing the number of species present in natural forests with the number of species that are able to remain in an exotic tree plantation. Some studies have demonstrated that in extreme cases, where natural forests become scarce and the landscape has suffered drastic transformations, plantations can actually contribute to the conservation of diversity.

However, the vast majority of studies show that exotic tree plantations reduce the number of species of animal groups that are important for the original ecosystems, such as arthropods, reptiles and birds. Natural forests are usually composed of many species of trees and shrubs of different sizes and ages, and provide an abundance of structures for animals to hide, feed or nest in. In contrast, exotic plantations are often composed of trees of a single species and age, which reduces the structural complexity and hence the possibility for many animals to survive in these novel human-created habitats.

European Amphibians Living in Exotic Eucalypt Plantations

Although some animal species are able to survive in exotic plantations, an important question is whether they are doing well in these novel environments or are struggling to survive. Does living in exotic plantations affect animal behaviour and morphological traits in a way that can compromise the survival of populations in the long-term?

To explore these questions, a group of Australian and European collaborators and I performed a series of experiments over a period of 4 years in eucalypt plantations in northern Spain, with a focus on amphibian species. Why amphibians? Globally, amphibians are among the most threatened animal groups, and understanding the causes of this diversity loss is a major challenge for the scientific community.

Habitat loss, for example, is devastating for amphibians due to their low mobility, which impedes their migration when the environment changes. Therefore, amphibians are usually key indicators of the quality of a given habitat.

Amphibians are also very sensitive to the presence of toxic substances in the environment because they have a highly permeable skin. As a consequence, small changes in the chemical characteristics of their terrestrial or aquatic habitats (depending on their life-history phase) can directly affect their well-being.

Critically, eucalypt trees are quite effective at changing the chemical environment wherever they grow. The leaves from eucalypt trees have several chemical compounds called tannins and phenols that, in their native Australian habitat, inhibit the growth of other plants, thus reducing competition and repelling herbivorous predators.

These compounds can also be toxic for the native fauna of other distant environments when eucalypts are planted in large amounts. For instance, these compounds are released by the leaves falling on the ponds where amphibians live and breed. Therefore, the potential for these chemicals to negatively affect amphibians is high, and this is what we wanted to explore.

To explore the effects of eucalypt chemical compounds on European amphibians, we first looked for a species that inhabited both natural oak forest as well as the novel eucalypt plantations. Since oak forests are the natural environment of amphibians in the study area in Spain, we can measure different traits and behaviours and compare them to those of individuals exposed to eucalypts to see if there are any differences.

After rigorous exploration, we found the perfect amphibian species to meet our goals: the palmate newt (Lissotriton helveticus). This colourful little newt was present in several ponds located under both eucalypt plantations and oak forests, enabling us to compare these habitats.

During the breeding season, males develop several morphological structures to attract females, including duck-like membranes between the toes called palmates (Fig. 1a). The size of these structures, and hence the attractiveness of these males, depends on their health, so this newt was the ideal candidate to test their susceptibility to the toxicity of eucalypt compounds.

This species also relies on chemical communication, in addition to visual signals, in order to detect potential mates and avoid predators. These behaviours were also likely to be affected by the presence of eucalypt chemical compounds in the water.

We first explored whether exposure to eucalypts affects the ability of males to develop large morphological traits and an efficient immune response. One easy technique to compare the immune response is an assay called phytohemagglutinin (PHA). The injection of PHA induces a slight local inflammation at the point of injection – in the case of newts, at the base of the tail. By measuring the width of the tail in the same point before and 24 hours after the injection of PHA, we can estimate the immune response of individuals. The bigger the inflammation, the stronger the immune response!

Due to the high toxicity of the pond water in exotic eucalypt plantations, we found that males not only have less energy to invest in the morphological structures that are so important for attracting females, but also fail to produce a strong immune response.

However, these males are somewhat lucky. Although females from the natural oak habitat would have found them unattractive, their own exposure to eucalypt chemical compounds left them unable to differentiate between high quality males with big morphological structures and a strong immune response, and low quality males with small morphological structures and a weak immune response (Fig. 2b). This advantages low quality males because they get to reproduce, but disadvantages females that, by not choosing the best males, will have offspring of lower viability.

Eucalypt chemicals can affect not only communication between individuals of the same species, but also interactions with other species present in their habitats. For example, newts and their larvae can fall prey to several species, including snakes and dragonfly larvae, so it is very important for them to rapidly detect these predators and the cues that signal their risk of predation. For instance, organisms often react to the odour of predators, but also to the odour of injured individuals of their own species.

We gave newts the opportunity to choose between a small pond that contained the odour of an injured newt plus oak-infused or eucalypt-infused water, and a small pond with only oak-infused or eucalypt-infused water without cues of predation risk. Newts tested in the water infused with natural oak leaves usually avoid submerging in ponds in which they can sense predation risk (Fig. 1c). However, after their exposure to eucalypt chemicals, newts became confused and were not able to differentiate between ponds free of predators and ponds with predators. This kind of behavioural changes can risk the life of both adult and juvenile newts, increasing the mortality of individuals in exotic plantations.

The Consequences

Exotic plantations not only reduce the diversity of native species, but in some cases they also affect negatively the individuals that are able to survive. Weaker immune responses in exotic plantations can alter the dynamics of pathogens, making these species more sensitive to diseases. At the same time, the reduced ability of females to choose the best males, as well as the risk-prone behaviour induced by the presence of toxic substances in the water, could lead to significant reductions in population numbers.

These cryptic effects cannot be detected by only exploring for the presence or absence of a species in a specific habitat, but require a closer look at the behaviour and condition of local creatures. Future generations of scientists have a fundamental role determining the consequences of establishing exotic species like eucalypts for European timber plantations.

Maider Iglesias-Carrasco is a postdoctoral researcher in the University of Auckland’s Department of Ecology, Evolution & Genetics.