Problem: frugivores are too small these days

Plants can't poop. They need animals for that. For this vital service, they pay a price in the form of fleshy pulp. The animals eat the fruit, and as a thank you, they leave behind a little stool somewhere: seed 'bombs', surrounded by a nutritious coating. The animal gets a meal, and the plant gets dispersed. It's a win-win situation. This mutual dependence is enormous: fruit-eating animals need food, and conversely, in some forests, over 90 percent of trees rely on frugivores for their dispersal.

The tally

When something is this crucial, ecologists quickly appear to tally who eats what. Ecologist Daniel Guerra, from Naturalis Biodiversity Center, cataloged those types of records from the past sixty years. "102 locations, 1,900 plant species, and 1,100 animal species", he lists. "12,000 observations of animals eating a specific fruit. It was quite a task."

Avocado

From this study on consumption patterns, a correlation emerges. A division of labor, if you will. Small animals eat small fruits with small seeds, and large animals eat the large ones. Like most things in biology, this relationship is not set in stone. Large grizzly bears eat small berries. Humans will eat avocados, but they fail to excrete entire avocado stones – downright cowardly, from an avocado tree's perspective. But the correlation exists, and the more fruit in an animal’s diet, the stronger that relationship is.

In a new article in the scientific journal Ecology Letters, Guerra, principal investigator Renske Onstein and their colleagues expose the drawback of the win-win relationship. If one party loses, the other loses too. In areas heavily disturbed by humans, this is reflected in the composition of the frugivore community. There are fewer of them, and crucially, fewer large species like great apes or rhinos. This loss of diversity leads to a 'decoupling' of the size-based relationship, which undermines the resilience of the entire tropical forest. ”It is as if the community is sick”, Guerra says. 

Not hit yet

The publication focuses primarily on disturbances from the last few decades, and because trees can often live for decades or even centuries, the full consequences of the disappearing frugivores are not yet apparent. “This ecological train has not hit us yet”, says Guerra. “Or, more optimistically: we can still fix this by reintroducing the large-bodied frugivore species that have vanished from an area.”

More information:

• Read the article Human-Induced Downsizing of Animal Communities Weakens Trait Matching Between Tropical Plants and Frugivores in het vakblad Ecology Letters.
• Fruit-eating fish are essentieel for forests as well. Read about it in a former Nature Today article, 25 January 2025.

Text: Naturalis Biodiversity Center
Images: Tom Wout (lead photo: a collared araçari (Pteroglossus torquatus) from the toucan family feeds on ripe palm fruits in Panama); Getty Images; Gijsbert Praamstra