What is it that makes a plant rare or abundant? Heo and colleagues set their sights on an intriguing subject – the Hart’s Tongue Fern (HTF) complex. This research, published in the Annals of Botany, sheds new light on how global climatic and land-use changes shape the abundance and distribution of this fern complex, which comprises five geographically segregated species. Their findings serve as a model for understanding similar dynamics in other plant species and ecosystems.
The Hart’s-tongue fern, scientifically known as Asplenium scolopendrium, is a distinctive fern species due to its unique leaf structure. Unlike many ferns with complex, divided leaves, the Hart’s-tongue fern has simple, undivided leaves that are long, flat and said to look like the tongue of a deer or hart.
Hart’s-tongue fern is mainly found in regions with a temperate climate, such as Europe, but also in certain areas in North America, East Asia, and New Zealand. It prefers damp, shady habitats and often grows in limestone-rich soils, like those found in gorges, ravines, or rocky outcrops.
From a scientific standpoint, Hart’s-tongue fern is interesting as it’s a fern complex, meaning it includes various closely related species which can provide insights into evolutionary biology, species adaptation, and the impacts of climate change on biodiversity.
Also, the Hart’s-tongue fern has a relatively narrow ecological niche, which makes it more sensitive to environmental changes. This specificity means it can serve as an “indicator species”, meaning the health and abundance of this fern can give scientists valuable clues about the overall health of the ecosystems they inhabit. In their article, Heo and colleagues write:
Ferns, often considered bio-indicators, are particularly sensitive to environmental conditions. Clear correlations between local abundance and climatic conditions have been reported for several fern species (Kessler et al., 2011; Pouteau et al., 2016; de Gasper et al., 2021; Bonari et al., 2022), and their vulnerability to anthropogenic threats has also been widely recognized (Giudice et al., 2011; Marini et al., 2011; Brummitt et al., 2016). Nonetheless, the spatial heterogeneity of local abundance in the HTF complex suggested a potential role for regional or local factors in their abundance patterns. Hierarchical systems of environmental controls probably determine abundance patterns, such that climatic and land-use dynamics act globally but are modulated by regional- to local-scale determinants such as topography, geology and microclimate, which operate at increasingly finer scales (Howard, 2015).Heo et al. 2023
It’s this interplay between global and local factors that Heo and colleagues wanted to understand. To get to grips with it, they used an approach like looking at a map of the world divided into a grid, where each cell is 100 km by 100 km, to understand the distribution and abundance of this plant species in each region.
The researchers collected data on the number of fern populations in each grid cell. They then used information about the environment, such as climate, soil conditions, landscape, and human impacts, to explore what could affect where and how densely these ferns are found.
A part of the study was also about understanding how environmental changes (like climate change) could affect where these plants could grow. They used sophisticated computer modelling to simulate past and future climatic conditions and understand how the plants’ suitable habitats could have shifted. They combined this information with an examination of conservation efforts. They calculated how much of each region was within protected areas and used it as a proxy to estimate the impact of human-induced changes on the plant distribution.
Heo and colleagues made a case study of the fern populations in the USA. Here, the researchers analyzed long-term data, collected over a century, to see how the number of ferns has changed over time and whether this could be linked to climate change or human activities.
To understand how these different factors might influence the abundance of ferns in a region, the researchers used a statistical model called a boosted regression tree. It can handle complex relationships between factors and is good at managing noisy data.
It wasn’t just a loss of habitat that Heo and colleagues searched for. They also looked for “hot spots” – areas where these plants were particularly abundant, as they could be important for the species’ survival.
The research findings illustrate that the Hart’s-tongue fern complex prefers specific bioclimatic conditions, being most abundant in temperate broadleaf and mixed forests scattered across regions of the Northern Hemisphere. The uneven distribution of these habitats explains the scattered global abundance pattern of Hart’s-tongue ferns.
The research also revealed that climate change is causing a shift in Hart’s-tongue fern abundance patterns, with tropical populations diminishing and higher latitude populations increasing since the Last Glacial Maximum around 21,000 years ago. The distance decay effect indicates that populations in lower latitudes face stronger extinction pressure, explaining the rarity of two southern taxa.
Another significant global driver is land-use changes due to anthropogenic threats. However, these effects vary regionally, likely due to the differing levels of habitat protection. East Asian Hart’s-tongue ferns are primarily found in highly protected regions, suggesting that less protected areas have suffered local extinction due to intensive land use.
Regional factors also play a vital role in Hart’s-tongue fern abundance patterns. For instance, in Europe, precipitation seasonality is the primary determinant of abundance patterns, whereas, in East Asia, spatial heterogeneity of precipitation, temperature, and terrain ruggedness are significant influencers.
The study also noted trends over the last 100 years demonstrating a northward shift of Hart’s-tongue fern populations in the USA, with increases in the north and decreases in the south. Interestingly, populations under the protection of conservation authorities showed increasing trends compared to unprotected populations at the same latitude.
The paper highlights how climate change is shifting the ideal habitats for these ferns away from the tropics towards higher latitudes – areas further away from the equator. This means that, as the Earth’s climate changes, places where these ferns could typically grow and flourish are changing as well, which could potentially impact the species’ future survival.
Human activities also play a significant role, particularly regarding protected areas, where human activity is limited to protect wildlife. North America had the highest proportion of fern populations exposed to low levels of protection, while East Asia had the highest proportion in highly protected areas. All fern populations in New Zealand are under intermediate protection.
The research emphasizes the complexity of factors impacting the distribution and survival of species like Hart’s-tongue ferns. It shows that both climate change and human activity can significantly impact these plants, indicating the importance of protective measures and careful land management in preserving biodiversity.
The findings are important not just for the fern species studied but potentially for other plant species, as many are similarly affected by climate conditions and human activity. However, as the study highlights, the exact impacts can vary greatly depending on the specific environmental factors of each location. This reinforces the idea that there’s no ‘one size fits all’ approach to conservation – strategies must be designed with a keen understanding of each region and the unique characteristics and needs of the species.
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Heo, N., Leopold, D.J., Lomolino, M.V., Yun, S. and Fernando, D.D. (2023) “Global and regional drivers of abundance patterns in the hart’s tongue fern complex (Aspleniaceae),” Annals of Botany, 131(5), pp. 737–750. Available at: https://doi.org/10.1093/aob/mcac129.