Impact of human land use ‘comparable to climate effects’ in determining survival of plants

Human activity can be as influential as climate impacts in determining which types of plants live where, according to a global study led by Irish scientists.

It concludes species are not affected equally as slow-growing plants like trees are less able to cope with more intensive human land use than disturbance-tolerant species like grasses and weeds.

Researchers at the School of Natural Sciences in Trinity College Dublin demonstrated that human activity acts as a filter, shaping the occurrence of plant species alongside climate factors including global warming.

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The study carried out at an unprecedented global scale analysed data from more than 4,800 species across six continents and 41 countries. It found plant occurrence can change as much between different land uses as between different climates.

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As human influence is increasing over the dry surface of the Earth, and disproportionately affects many areas where the climate is most suitable for slow-growing species, “human land use acts on top of climate as a filter, shrinking the area where certain species are most competitive”, they conclude.

“Temperature variation in a given area, a key dimension of climate change, has the biggest impact on which species occur where, while human land use is as important as temperature, rainfall and rainfall variation in a given area in driving plant distributions,” said Caroline McKeon who led the research.

Climate change and its impact on species receive huge research interest and investment, and there is a wealth of evidence for the impacts climate change has on biodiversity, said Dr McKeon, “however, the leading cause of biodiversity loss and ecosystem change is currently human land use”.

The research confirms 75 per cent of the Earth’s surface is subject to anthropogenic land use, while the rate of land use change is accelerating most rapidly in areas with the highest biodiversity.

The Government has committed to developing a national land use plan to balance addressing the climate and biodiversity emergencies. This will address the multiple requirements of capturing carbon by afforestation and rewetting bogs, while maintaining sustainable food production, providing for development of bioenergy and reducing water pollution.

The TCD study, which emphasises the need to consider land use as a key driver of biodiversity, is published in Global Ecology and Biogeography.

“Humans are in direct competition with plant species for highly productive environments, and we find that species that have historically dominated these climates are the least able to cope with human disturbance. Human land use is acting as a filter, contracting the area that is most suitable to slow-growing species while allowing disturbance-tolerant plants to expand their range,” said Dr McKeon.

“Species have evolved different strategies for surviving, growing and reproducing in different climates,” she added. “What human activity is doing is making it much more difficult for certain, slow-growing, species to survive.”

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In Ireland, climate conditions are favourable for over 80 per cent of the surface to be covered in forest. “However, what we actually have is less than 10 per cent native forest and more than 50 per cent grassland. So we have climate determining which plant species can occur, but human activities determine which species ultimately do occur,” she said.

“Because human influence is so pervasive, this really changes how suitable species’ strategies are for the world they are trying to live in. A key thing to consider is that human land use is incredibly prevalent on the surface of the Earth. And with the exception of the colder, dryer places like boreal taiga and tundra, we like to live and farm in the same places that are suitable for a lot of plants. So certain types of species are really under pressure.”