More grazing geese does not always mean less harvest
More grazing geese does not always mean less harvest
It would seem logical: more geese eat more grass, leaving less for the farmer to harvest. But a recent study suggests it may not be so straightforward. An international team led by the Netherlands Institute of Ecology (NIOO-KNAW) looked at the impact of different goose species and the number of geese on agricultural damage. Their first conclusion: “The number of geese does not translate directly to a smaller grass yield.”
Large numbers of geese graze on agricultural grassland, where they eat grass meant for cattle. Geese are migratory birds, so they’re not stopped by borders. Around 2.4 million geese now winter in the Netherlands each year. One third stays in a single part of the country: the province of Friesland (Fryslân). This includes more than half a million barnacle geese alone.
Not everyone sees the geese as welcome guests, on account of the feared agricultural damage. Goose populations have skyrocketed over the past few decades, and conflict between geese and agriculture has intensified accordingly. Different methods can be used to control the geese, including shooting at them. But to justify this, it’s important to know how goose abundance and interactions between species affect the amount of damage.
“Fewer geese doesn't automatically mean less damage,” says animal ecologist Nelleke Buitendijk from NIOO-KNAW. It’s one of the most striking conclusions of a scientific study by Buitendijk and her colleagues – the first in a series of three – that was recently published in the Journal of Applied Ecology.
It may be hard to imagine now, but only a few decades ago many goose species were declining. The barnacle goose was even threatened with extinction, and became a protected species. This means hunting barnacle geese is not allowed. However, it is possible to get a special permit to shoot at the geese with the goal of chasing them away, as a means of protecting crops.
A management approach used in Friesland and elsewhere is to divide agricultural land into two zones: an area where geese are actively chased away – by walking onto the field, shining lasers across the grass or shooting (with a permit) – and a refuge or accommodation area. In the accommodation area, the geese can eat in peace without intentional disturbance.
“The goal is to teach the geese that some areas are dangerous, and they should avoid these. If this works well, geese should cluster in accommodation areas. This will lead to higher damages in those areas, but the overall damage - across the entire province or region - might turn out to be lower,” Buitendijk thinks.
Not a straight line
Buitendijk and her colleagues used data from detailed monthly goose counts in the Frisian accommodation areas. They combined this with movement data from geese carrying special GPS-transmitters. This allowed them to estimate grazing pressure per field, and compare thisit to the estimated yield reductions cited in official damage reports.
“Only for barnacle geese did we find a clear relationship between geese and yield reductions,” explains Buitendijk. “But that relationship is a curve rather than a straight line: the damage per barnacle goose decreases when fields are grazed more intensively.” In addition to the number of geese, it is also important to consider when the geese are present. In the Netherlands, barnacle geese may stay well into spring, past the first harvest. The larger greylag and white-fronted geese leave much earlier, before the grass starts growing. These larger species also prefer taller grass as they have a larger bill. Because of such differences, not all species are equally responsible for agricultural damage, and the larger species are less commonly found in areas with the highest spring damage.
Consequences for management
Measures such as population control should only be used with the utmost caution, the researchers argue. This is because management approaches don’t always have the expected or desired effect.
“We also see that the different species do not all have the same impact on agriculture. So it is important to make sure that management practices target the right species,” warns Buitendijk.
More pieces of the puzzle
“These research results are part of a larger puzzle. We need to understand how geese decide when and where to graze, how they affect their food source, and how this can be influenced by people,” explains Buitendijk. “By exploring this further, we can improve goose management, making sure that goose populations continue to do well, while at the same time decreasing conflict with humans.”
As a follow-up to their initial research, Buitendijk and her colleagues are also looking at the effects of grazing on grass development in a field study. A third study will combine all knowledge in a model, for a better understanding of the effectiveness of goose management methods.
Buitendijk also points out a positive side to the story: “The recovery of goose populations in recent years is actually a great success for conservation, in a world where more and more species are lost. Now we just need to find a way to live together with these geese.” The results of the other two studies, expected in early 2023, will hopefully help with this.