Life

Animal grazing increases plant species diversity and prevents fires

Credit: Adobe Stock
Credit: Adobe Stock

Animal grazing contributes to increased biodiversity and ecosystem recovery after moderate fires, and reduces the risk of future fires. At the same time, large herbivorous mammals are more likely to feed in areas that have been burned, scientists from Poland and Sweden have shown.

'We show that fire can attract cattle grazing in a temperate wood pasture and that increased grazing on burned areas can reduce the fuel (tall, dry grass - ed. PAP) and thereby the extent of future fires. Furthermore, we show that fire and herbivory may alter plant species composition and the relative cover of different plant life forms,’ the biologists write in the Journal of Applied Ecology (https://doi.org/10.1111/1365- 2664.14618).

Pyric herbivory is a term coined in 2009 to describe how fire and mammals that feed exclusively on plant food interact. It has been noticed that burned areas are attractive to large herbivorous animals. In turn, fires are less common in grazing areas because animals eat excess vegetation that could fuel the fire. This phenomenon has been quite well studied in North American, Australian and African ecosystems, where huge fires are common, but it has remained poorly researched in Europe.

Dr. Marcin Churski and Dr. Dries Kuijper from the Mammal Research Institute of the Polish Academy of Sciences - conducted an experiment that helped fill this gap. They investigated the interactions between grazing herbivores (cows of three different breeds) and wildfires in the European forest-pasture landscape of southern Sweden. They showed that burned areas clearly attracted such animals, which affected the future composition of local vegetation. On this basis, they concluded that pyric herbivory could serve as an effective tool to counteract biodiversity loss also in European ecosystems.

HERBIVORS AND FIRE SHAPE ECOSYSTEMS

'Fires and large herbivorous mammals significantly affect the structure and functioning of ecosystems around the world,’ says Dr. Churski. This impact is visible even in places where either only fires occur or vegetation is only browsed by animals. In areas where both of these factors occur, their combined impact is even more significant.

The researcher and his colleagues emphasize that pyric herbivory, i.e. the interaction between fire and grass grazing by cattle, is 'crucial for maintaining the heterogeneity of vegetation.’

For the study, scientists set up experimental plots in southern Sweden. Each plot was divided into four subplots: grazed, burned, grazed and burned, and a control plot where there was no interference. They used camera traps to observe the behaviour of cattle in burned and unburned areas. They measured the height of grass and the proportion of burned area, the two variables that could influence animal preferences, and then assessed how grazing changed the fire behaviour, i.e. the way the fuel ignites, the flame develops and the fire spreads. They also examined the impact of the presence of herbivores in burned areas six years after a fire.

It turned out that cows spent more time grazing in burned fields than in unburned fields, and the larger part of the field that had been burned, the more willing cows were to stay there. Why? 'Herbivores are more likely to choose previously burned areas for feeding and spend more time on these surfaces compared to areas that have not been burned. However, the consequences of this process should be expected after a longer period of time, not immediately after the fire, but from the moment when plants (mainly grass) start to grow back,’ says Dr. Churski.

'In our study,' he adds, 'the areas were burned in April, and the herbivores were released into the pasture in May, about a month after the fire. At this point, the surface is already covered with regrowing young plants, and all last year's biomass (less attractive and of poorer nutritional quality) has been removed by fire. These more visible, regrowing plants that are attractive to herbivores.’

GRAZING AND THE RISK OF FIRE

Dr. Churski and his colleagues also calculated that the percentage of the field that was burned was positively related to the grass height before the fire, and over the six-year observation period, the height of vegetation on the burned plots decreased significantly. In other words: the taller the grasses in an area, the greater the risk of a serious fire occurring there. In turn, mammals grazing on already burned areas in subsequent years limited the growth of grass beyond a certain height, which reduced the risk of another fire.

'In our experiment, both the height of vegetation and the proportion of burned area decreased over the 6-year study period. And lower vegetation reduces the risk of subsequent fires,’ says Churski.

All these elements lead to the conclusion that herbivore grazing in burned areas seems to be a good method to not only reduce future fire risk, but also prevent biodiversity loss.

'We found the largest number of plant species on burned and grazed areas, and the lowest on ungrazed areas. This is linked to reducing competition between plants through burning and browsing. The effect of moderate burning and grazing is associated, among other things, with improving light conditions, increasing space for more plants and increasing potential niches for seed germination. This generally translates into a larger number of species that can grow in such places,’ says Churski. 

Pyric herbivory is already used in other places around the world, for example in North America, to protect nature and biodiversity, and indirectly - to reduce the risk of fire that threatens nature and people. In these places, controlled fires are carried out by specialized personnel, within the framework of applicable law. These are large, well-coordinated actions, often in the presence of the fire brigade. 'I think it could work similarly in Europe. However, it requires further research,’ Dr. Churski adds.

The researchers emphasise that the need to restore and maintain open habitat biodiversity is greater than ever and, at the same time, we need to identify cost-efficient and sustainable management methods.

'Pyric herbivory-based management has been shown to maintain cattle stocking rates and provide high-quality forage throughout the whole season and still increase biodiversity and could therefore be considered as an alternative to traditional conservation management methods also in Europe,’ they conclude.

Katarzyna Czechowicz

kap/ bar/ kap/

tr. RL

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