In the past, when climates changed, forests became distributed more to the north or to the south of their previous locations. This is not to say that the trees, themselves, moved. Rather, the ranges of the tree species changed. For example, fossil plant and pollen records show that the ranges of tree species shifted northward at a rate of 50km per century as the climate warmed after the retreat of the North American ice cap.
Tree range migration is facilitated by the movement of seeds. When the climate changes, trees begin to form more seeds; these get spread around by dispersal agents and may germinate in areas that are more favorable for growth. Although wind and water may carry some seeds, the vast majority of trees depend on animals to disperse them to new areas. But now, many of these seed-dispersing animals are themselves threatened, as climate changes and habitats are lost.
In a study featured on the cover of this week’s issue of Science, U.S. and Danish researchers consider how the loss of species of birds and animals will impact the ability of plants to adapt to human-induced climate change in future. While animals can often fly, walk, run or swim to more suitable areas, trees cannot escape the heat. Tree species that need to move their offspring away from warmer temperatures will only be able to do so via the activities of seed-dispersal agents, such as mammals and birds.
“Some plants live hundreds of years, and their only chance to move is during the short period when they’re a seed moving across the landscape,” said Rice University ecologist Evan Fricke, the study’s first author. “If there are no animals available to eat their fruits or carry away their nuts, animal-dispersed plants aren’t moving very far.”
In collaboration with researchers from the University of Maryland, Iowa State University and Aarhus University, the Rice team used machine learning and data from thousands of field studies to map the contributions of seed-dispersing birds and mammals worldwide. They compared maps of seed dispersal today with maps showing what dispersal would look like if there were no human-caused extinctions or species range restrictions.
The experts also used data synthesized from field studies around the world to train a machine-learning model for seed dispersal, and then used the trained model to estimate the loss of climate-tracking dispersal caused by animal declines.
Fricke explained that developing estimates of seed-dispersal losses due to animal and bird extinctions or changes in their ranges required two significant technical advances: “First, we needed a way to predict seed-dispersal interactions occurring between plants and animals at any location around the world.”
For this, they used data on species interactions from over 400 field studies that detailed how many seeds are dispersed by particular species of birds and mammals, how far they are dispersed and how well those seeds germinate. Based on this data, the researchers could predict interactions between plants and seed dispersers.
“Second, we needed to model how each plant-animal interaction actually affected seed dispersal,” said Fricke. “For example, when an animal eats a fruit, it might destroy the seeds or it might disperse them a few meters away or several kilometers away.”
The results of the study showed that many seed-dispersing mammals and birds have already been lost, particularly in temperate regions across North America, Europe, South America and Australia. In addition, if currently endangered species go extinct, tropical regions in South America, Africa and Southeast Asia would be severely affected.
“We found regions where climate-tracking seed dispersal declined by 95%, even though they’d lost only a few percent of their mammal and bird species,” said Fricke.
The findings showed that the ability of animal-dispersed plants to keep pace with climate change has been reduced by 60% due to the loss of mammals and birds that help such plants adapt to environmental change.
“In addition to the wake-up call that declines in animal species have vastly limited the ability of plants to adapt to climate change, this study beautifully demonstrates the power of complex analyses applied to huge, publicly available data,” said Doug Levey, program director of the National Science Foundation’s (NSF) Directorate for Biological Sciences, which partially funded the work.
Fricke explained that the study is the first to quantify the scale of the seed-dispersal problem globally and to identify the regions most affected. He emphasized that declines in the populations of seed dispersers highlight an important intersection between the climate and biodiversity crises impacting natural environments today.
“Biodiversity of seed-dispersing animals is key for the climate resilience of plants, which includes their ability to continue storing carbon and feeding people,” he said, adding that restoration of ecosystem connections in natural habitats could counteract some declines in seed dispersal, but not all of them. People also rely, both economically and ecologically, on seed-dispersing birds and mammals to ensure the supply of important resources.
“Large mammals and birds are particularly important as long-distance seed dispersers and have been widely lost from natural ecosystems,” said Svenning, the study’s senior author. “The research highlights the need to restore faunas to ensure effective dispersal in the face of rapid climate change.”
“When we lose mammals and birds from ecosystems, we don’t just lose species. Extinction and habitat loss damage complex ecological networks,” said Fricke. “This study shows animal declines can disrupt ecological networks in ways that threaten the climate resilience of entire ecosystems that people rely upon.”