Lack of mutualists reduce species richness on low latitude islands: study

One of the most basic patterns of biodiversity on Earth shows that species diversity is highest near the equator, declining toward the poles. New global biodiversity research published in Nature reveals a break from this pattern on oceanic islands. The diversity on islands in the tropics is not much higher than on those in higher latitudes. The loss of mutualistic interactions on low-latitude islands may be the explanation for this situation.

Wind, water, or birds assist many plants in reaching oceanic islands. However, mutualistic plants require their partners to arrive as well because they rely on relationships with other organisms to establish. A new study determines that the significant decline of mutualistic plants on islands contradicts a common global pattern: plant diversity always increases near the equator.

Mutualism shapes island plant colonization

The study suggests that the absence of mutualistic interactions, such as relationships with pollinators and fungi, on low latitude islands may be responsible for this unexpected finding.

Unlike the common understanding of biodiversity distribution along latitude, known as the latitudinal diversity gradient (LDG), which typically shows higher species richness near the equator, this study found that this pattern doesn't hold for oceanic islands.

While previous research on island biogeography often focused on physical factors like size and isolation, this study highlights the crucial role of mutualistic interactions in determining species colonization on islands.

The study, spearheaded by Dr. Camille Delavaux, suggests that mutualistic interactions, or the symbiotic relationships between species, are a critical factor in the successful colonization of islands by plants. Unlike mainland regions where mutualistic plants thrive, oceanic islands often lack the necessary partners for these plants to establish themselves. As a result, fewer mutualistic plants are found on islands, particularly in tropical regions.

“Not only are mutualisms important for plant colonisation to oceanic islands, but they are more important than all other traditionally studied variables combined,” says Dr. Camille Delavaux, the lead author of the study.

Dr. Delavaux stresses mutualism's importance

In an interview with Ground Report, Dr. Delavaux highlights that the study, along with others, indicates the native community's lesser reliance on mutualisms compared to mainland floras. This suggests that non-native species may introduce novel mutualisms, posing a higher invasion risk on islands.

She highlighted the importance of monitoring the transport of such species and their associated organisms to mitigate potential threats.

Dr. Delavaux further explained that the loss of mutualist-associated plant species from mainlands to islands, particularly in tropical regions, contributes significantly to species loss. This underscores the importance of mutualism in maintaining high tropical diversity on mainlands at low latitudes. She stressed that conservation efforts in these regions should not only target plants but also their various mutualisms.

Regarding island conservation, Dr. Delavaux advocated for preserving native ecosystems and avoiding artificial management for higher diversity. She emphasized the importance of conserving native plants and their mutualistic interactions to safeguard the unique biodiversity of islands. Despite their lower species richness compared to mainland areas, tropical islands host numerous endemic species found nowhere else, making their conservation imperative.

Island plant diversity linked to mutualism

Researchers analyzed data from 354 islands and 608 mainland regions and discovered that the absence of mutualistic partners such as pollinators and mycorrhizal fungi contributes significantly to the lower plant diversity on tropical islands. In some cases, mainland areas in the tropics may have up to 95% more plants dependent on pollinators than an island at the same latitude.

“It is astonishing to think that tropical islands are not considerably more diverse than those in the higher latitudes. This important role of mutualisms completely transforms our fundamental understanding of how biodiversity is shaped,” said Prof. Dr. Thomas Crowther, professor of ecology at ETH Zurich.

Prof. Dr. Thomas Crowther, an ecology professor at ETH Zurich, emphasized the profound implications of this study for our understanding of biodiversity. The findings highlight the importance of mutualistic relationships in shaping global biodiversity patterns and suggest that conservation efforts should focus on protecting these interactions.

However, the findings do not provide a reason to believe that urgently introducing more pollinators or soil fungi to oceanic islands is necessary. The islands just possess less diversity than mainland areas, and we must protect their native flora and fauna.

Instead, the study raises awareness about the importance of mutualisms for global biodiversity. They play a critical role in both understanding the lower plant diversity on islands and, conversely, in maintaining the high plant diversity on the mainland. This insight not only has value for future scientific research but also holds importance for conservationists and ground restoration efforts.

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