What causes it? A combination of species replacement and changes within species. So says new research led by the University of St Andrews. Living things are getting smaller and smaller, and the trend is becoming more pronounced.
The research was published in Science by an international team of scientists from 17 universities. They analyzed data from around the world over the past 60 years and from many types of animals and plants. Previous research showed that the size of trophy fish in fishing competitions declines. Many of the most endangered species are large. The new study connects the dots.
The lead author is Dr. Inês Martins, of the University of St Andrews. She said in a statement, “In some locations, for example, smaller and smaller individuals of thornback ray are observed. Smaller-bodied species, such as mackerel, are increasing in abundance. The big fish just can’t seem to catch a break.”
Contraction was most common among fish. But it also occurred among other groups of organisms (such as plants and invertebrates). In these groups of species, the study reveals that some complex changes are taking place. Some organisms become larger while others shrink. “When large organisms disappear, others try to take their place and consume the resources that remain available.”
The study also observed the replacement of a few large organisms by many small ones, keeping the total amount of life, known as biomass, constant. Ecosystems tend to compensate for the change. They keep the overall biomass of the species studied in a particular habitat stable. To what is this stability attributed? It is a trade-off between reductions in body size and simultaneous increases in abundance among organisms.
How does the fact that living things are getting smaller and smaller affect us? The researchers say, “It’s clear that there is a widespread replacement of species that we’re seeing around the world. And it is having measurable consequences. The fact that organisms are getting smaller has important effects. The size of animals mediates their contribution to the functioning of ecosystems and how humans benefit from them. For example, larger fish feed more people than smaller fish.”