By looking at prehistoric developments, researchers have made the far-reaching importance of herbivores for fire activity clear: The extinction of these animals in the period 50,000 to 7,000 years ago led to increased fires in the grasslands of the world, according to their analyzes. The more intense the loss in a region, the more the flames blazed there. The results suggest that the ecological effect of grazers should be better considered when assessing global fire activity in the past, present and future, the scientists say.
Partly huge regions of the earth are characterized by vegetation that dries up again and again. A small spark is enough there to start fires that ultimately burn entire swathes of land to ashes. This is an at least partially natural process of great importance: It shapes the diversity of species in ecosystems as well as the release of climate-relevant gases and thus also has effects across geological time scales. In order to understand the importance of fires, it is important to know their causes and the factors of frequency. It seems clear that the climate plays a central role – droughts, for example, can significantly increase the likelihood of fires. But herbivore activity can also play an important role: They limit fire activity by eating plant material that would otherwise form fuel.
On the trail of the fire protection effect of grazers
This relationship has already been documented in some ecosystems. At the local level, it is becoming apparent that the disappearance of grass-eaters will lead to more frequent and stronger fires in the corresponding ecosystems. However, the researchers led by Carla Staver from Yale University in New Haven now wanted to check whether this role is also reflected on a large-scale and temporally comprehensive scale. To do this, they took an analytical look at developments around 50,000 to 7,000 years ago.
In this late Quaternary era, it is known that some of the world’s grassland ecosystems experienced severe extinction among the large herbivores. The researchers addressed the question of the extent to which these losses were reflected in the development of fire activity in these regions. To do this, they evaluated information from databases and studies that provided information about the extent of extinction in certain regions of the world. They were then able to compare these results with research results on paleo-fire activities based on investigations into traces of ash in sediment layers.
As they report, the data analyzes initially showed: In South America there were the greatest losses among grass-eaters during the study period: 83 percent of all species died out there, followed by North America with 68 percent and Australia with 44 percent. As the researchers point out, there is also evidence that the extinction events not only meant a loss of species, but also a general decline in grass-eater biomass. The research also confirmed the well-known fact that Africa’s fauna was relatively spared from extinction: only 22 percent of the grass-eater species there disappeared during the study period.
Links are emerging
The comparison of the data with the analysis results of the paleo-fire activities in the respective regions then showed: The fire frequency had increased the most where the extinction among the representatives of the grass-eaters was most pronounced. South America was at the top, followed by North America. In Australia, however, the fire frequency increased only slightly and in Africa the scientists even found evidence of a slight decrease over the course of the study period. The bottom line from the results is that the loss of grazers is reflected in changes in fire activity at the continental level, the scientists sum up.
“This work makes it clear how important grass-eating animals can be in the development of fires. We need to pay close attention to these interactions if we are to predict future fire patterns, ”says Staver. This is especially important against the background of the changes in herbivore communities in many grassland ecosystems: Due to the influence of humans, the populations of wild grass-eating animals are dwindling in many places. So far, however, this has generally not been taken into account in large-scale fire models. “Our results underscore the need to include herbivore control in our understanding of fire activity and explicitly in fire models. This can help to compensate for inconsistencies between observations and modeled scenarios for fire activity in the grasslands, ”the researchers write.
Source: Yale University, Article: Science, doi: 10.1126 / science.abj1580