It's the shift in diet itself that has ramifications throughout the entire ecosystem in question. Many goldenrod species are highly competitive when left to their own devices. If left untouched, abandoned fields can quickly become a monoculture of goldenrod. That is where the spiders come in. By causing a behavioral shift in their grasshopper prey, the spiders are having indirect effects on plant diversity in these habitats. Because grasshoppers spend more time feeding on goldenrods in the presence of spiders, they knock back some of the competitive advantages of these plants.

The researchers found that when spiders were present, overall plant diversity increased. This is not because the spiders ate more grasshoppers. Instead, it's because the grasshoppers shifted to a diet of goldenrod, which knocked the goldenrod back just enough to allow other plants to establish. It's not just plant diversity that changed either. Spiders also caused an increase in both solar radiation and nitrogen reaching the soils!

In knocking back the goldenrod, the habitat became slightly more open and patchy as various plant species of different shapes and sizes gradually established. This allowed more light to reach the soil, thus changing the environment for new seeds to germinate. Also, because goldenrod leaves tend to break down more slowly, they can have significant influences on nutrient cycles within the soil. As a more diverse set of plants establish in these field habitats, the type of leaf litter that falls to the ground changes as well. This resulted in an overall increase in the nitrogen supply to the soil, which also influences plant diversity.

In total, the mere presence of spiders was enough to set in motion these top-down ecosystem effects. It's not that spiders eat more grasshoppers, it's that they are changing the behavior of grasshoppers in a way that results in a more diverse plant community overall. This is a radically different narrative than what has been observed with examples such as the reintroduction of wolves to the greater Yellowstone ecosystem yet the conclusions are very much the same. Predators have innumerable ecosystem benefits that we simply can't afford to ignore.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2]