All for One and One for All

By Shaun Coutts

Human behaviour plays an enormous role in the spread or control of invasive weeds.

Weeds can cross property boundaries but weed management often cannot. Controlling invasive species across the landscape is therefore not only about the effectiveness of control strategies; it’s also how, when and where those strategies are employed. And these factors are all the result of human behaviour.

We recently modelled the impact of different types of human behaviour together with our ecological understanding of two significant weeds. We wanted to know how the collective decisions of many property managers affected the spread of two damaging weeds: serrated tussock and African lovegrass. We wanted to look at the spread of these two weeds at a large scale and over a longer time period (50 years), and the only practical way to do this was to build a model.

In our model there were 4096 decision-makers who all decided whether or not to control the weed based on a simple rule: if the benefit of controlling the weed was higher than the cost then that decision-maker was more likely to undertake control. If the reverse was true then they were less likely to control.

Using this model we could change aspects of the behaviour of the decision-makers. Our model wasn’t perfect, but it was detailed enough to give us an idea of how the behaviour of multiple land managers might affect the spread of weeds.

Differences in the impact and ease of control between serrated tussock and African lovegrass highlighted an important point about which types of weeds are likely to become widespread. Serrated tussock is very damaging to graziers but there are some practical steps land managers can take to reduce it density and impact. African lovegrass, on the other hand, is slightly less damaging but is very hard to control. This meant the decision to control serrated tussock was very obvious, while the decision to control African lovegrass was more ambiguous.

As a result, most of our modelled decision-makers controlled serrated tussock straight away and its spread was greatly reduced. African lovegrass, by comparison, very often took over the entire modelled landscape because there was always a pool of infested areas available to spread it. This suggests that weeds that become widespread might be species that have a medium impact.

The modelled outcome for serrated tussock, however, does not tally with the reality we see around us. Serrated tussock is widespread in Australia. However, a lot of the spread of serrated tussock happened before systematic efforts were targeted at controlling it.

This highlights another important finding of our model. For concern about a weed to have a large effect on its spread, that concern must be present while the weed is still rare. If concern only grows as the species becomes widespread, by the time there is enough concern that everybody acts, the weed is already so well established that landscape-wide control is very difficult.

A further complication when thinking about multiple actors is that not all of those deciding whether to control or not have the same goals or resources. Some might be operating large commercial properties, others small hobby farms. Some may not even want to control invasive species at all. We included this in the model by making some decision-makers needing extra benefit to control weeds. We called these decision-makers “unmotivated”, and in our model they very rarely controlled the weed.

Because unmotivated decision-makers act as a constant source of invading weed, they have an especially large effect on the rest of the landscape when there is lots of long-distance dispersal. We found that a few unmotivated land managers (1–2%) didn’t make much difference, but if they made up about 10% of all the land managers then they could cause the weed to spread across the whole landscape.

Unmotivated land managers are only a problem if the land they are responsible for gets infested in the first place. When there are more unmotivated mangers in the landscape acting as a constant source of infestation, it is more likely that other unmotivated managers will get infested and become sources themselves.

All of this demonstrates that to really appreciate how weed control works over multiple properties, factoring in the behaviour of multiple land managers is essential.

Shaun Coutts is a researcher with the Environmental Decisions Group at the University of Queensland.

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