Animals are shrinking, researchers say — and that could affect the boreal forest

A new study published in the journal Science suggests that body size in many species across the animal kingdom is declining. 

Researchers reviewed 60 years of data from all over the world and discovered that all kinds of species, from fish to plants and animals, are shrinking over time. Not only are some species smaller than they used to be, but those smaller species seem to be increasingly outcompeting bigger ones.

The researchers looked at 5,000 ecosystems over those 60 years, and found significant changes in 4,300 unique species.

As ecosystems are often structured in terms of species' size — big animals eat smaller animals which in turn prey on smaller animals — this may present a serious disruption, according to the authors of the study.

"As organisms become smaller, or we remove the top predators, we can expect ecosystems to fundamentally change as well," said lead researcher Inês Martins, from Scotland's University of St. Andrews. "Which would have considerable measurable consequences to the species that live in them, the services they provide and how humans benefit from them."

The changes in body size can be attributed to a number of different factors, the researchers say, from warmer temperatures to decreased food availability. Previous research has also drawn a link between the selective exploitation of bigger fish by commercial fishers. 

The new study found that the changes in body size were particularly prevalent in fish. 

"Fishing competitions have seen smaller and smaller trophy fish," said Martins. "We also know that most of the threatened species we have nowadays are large-body species. Humans have long preferred to hunt and eat the big things."

Some people in the Yukon say they've also been noticing changes similar to what the researchers observed.

"Across the board with chinook [salmon], we're seeing them coming in smaller," said Tom Buzzell, manager of transboundary rivers operations with Fisheries and Oceans Canada in Whitehorse.

"These are changes that have happened with the run everywhere. The really large chinook are less and less."

Outside of the water, the trend isn't as straightforward. The data indicated that more complicated changes are taking place on land — the impacts of climate change have some species getting smaller over time while others get bigger. 

We often rely on population figures to tell us everything we need to know about the health of a species, but Martins says this approach leaves out key details; determining the viability of a species by recording the number of individuals doesn't tell the full story.  

"To fully understand the effects of environmental change and the impact of human activity on living things, we must not neglect changes within species themselves," she said.