Lost Animal Species Can Have Massive Impact on Plant Survival

Plants depend on birds and mammals to disperse their seeds.

American robin with berry
marcophotos / Getty Images

It’s quite the domino effect. As bird and mammal species start to disappear, so will many plants that rely on those animals to disperse their seeds.

A study by U.S. and Danish researchers finds that the ability of those plants to keep pace with climate change has been reduced by 60% globally. With the loss of the animals that spread their seeds, the plants are less likely to be able to adapt to climate warming.

About half of all plant species depend on animals to disperse their seeds and dispersal is critical for plants in several ways, the study’s first author, Evan Fricke from Rice University, tells Treehugger.

First, when animals spread seeds, it helps plants reproduce in the habitats where they already exist.

“For example, seed dispersal allows seeds to reach areas suitable for growth. Seed dispersers can also increase the chance that seeds within fruits turn into seedlings by removing fruit pulp and scratching up the seed coat in ways that can improve germination,” Fricke says.

Seed dispersal also allows plant species to spread into new areas or into areas where they have disappeared.

“This includes moving back to areas affected by deforestation and other land use changes, as well as movement to areas newly suitable for growth, survival, and reproduction under climate change,” Fricke says. 

“The relationship between fleshy-fruited plant species and their dispersers is a mutually beneficial one. The animal gets a nutritious reward and the plant gets its seeds dispersed across the landscape.”

Mapping Seed Dispersal

For their study, researchers used data from thousands of scientific studies to map how birds and mammals dispersed seeds around the world. They looked at various components of the process, including which animals disperse seeds from which plants, how far the seeds are spread, and how likely a seed is to turn into a seedling once it has been dispersed.

With that data and information on the animal and plant species like seed size, plant height, and animal body mass, researchers used machine learning to estimate how each bird and mammal species disperse seeds.

This includes some species like elephants, bears, and hornbills that disperse many seeds at great distances, as well as some species such as eagles and penguins that don’t disperse any seeds at all.

“This allowed us to estimate how much seed dispersal is provided by the animal species that exist at any location around the world. Then, we could compare how much seed dispersal is performed currently versus how much seed dispersal would be performed if animal extinctions and range contractions hadn't occurred,” Fricke says.

“Overall, we estimate that seed disperser declines have reduced the dispersal of seeds far enough to track climate change by 60% on average around the world. We also estimate that, if endangered species were to go extinct in the future, there would be a further 15% global reduction in climate-tracking dispersal.”

The results were published in the journal Science.

Cutting Important Ties

The study shows that when bird and mammal species are lost, it can have an effect on the plants in the ecosystem that rely on them.

“These mutualistic ties between plant and seed disperser are cut. This means that the ecological process of seed dispersal is disrupted, likely causing negative impacts on regeneration and reducing the ability for plant species to respond to climate change by shifting their geographic ranges,” Fricke says.

There can be so many negative impacts when this happens.

“The consequences can include a decline in regeneration of impacted plant species, and potentially even the complete loss of plant species from ecosystems where dispersers have declined,” Fricke says.

“This sets up the potential for many negative knock-on consequences. Not only a loss of plant biodiversity in ecosystems experiencing seed disperser declines, but the loss of the ecological functions that plant biodiversity supports. This includes storing carbon, providing habitat for wildlife, and supporting the livelihoods of people that depend on forests and other vegetation.”

The findings are important because the analysis suggests that drops in biodiversity reduce the climate resilience of forest ecosystems and other vegetation.

“This shows how important conserving and restoring animal biodiversity is for plants' ability to adapt to climate change,” Fricke says.

“Not only does the work underscore how important it is to conserve endangered seed dispersers, but also emphasizes the need to support seed dispersal function as part of our land management, protected area planning, and ecosystem restoration.”

View Article Sources
  1. Fricke, Evan C. et al, "The effects of defaunation on plants' capacity to track climate change." Science, vol. 375, no. 6577, 13 Jan. 2022. doi:10.1126/science.abk3510

  2. the study’s first author, Evan Fricke from Rice University