The Ecology of Playing Dead – when not needed to…something that Xinqiang Xi, John N. Griffin and Shucun Sun have diged deeper into in their new Early View paper “Grasshoppers amensalistically suppress caterpillar performance and enhance plant biomass in an alpine meadow”.
Read Shucun Sun’s story about Playing dead behaviour in grasshoppers:
As a child, I would often wake in the middle of the night thinking I could hear a burglar in the kitchen downstairs (in reality, my family cat coming through the cat flap). I would lay there, alert in bed, not daring to move in case I would be heard. By mistaking sounds of the cat for those of a burglar, I had inappropriately employed a danger avoidance strategy, costing myself much-needed sleep. We know that, in nature, prey are often highly-tuned to the signals of their predators and take action to avoid predation, like growing defensive body armor, shifting habitats, or even playing dead. Prey take these energetically demanding measures because being eaten tends to be rather more costly to one’s fitness! However, just like my childhood anecdote, prey species can get it wrong and misidentify a friend for a foe, reacting to cues from animals within their trophic level (competitors) that pose no predation threat. This sort of interaction could be common in nature and may not only incur a cost for the ‘victim’ but also have knock-on effects to other species that interact with them.
In our paper, we describe and explore the direct and indirect consequences of interactions between two common grazing insects in alpine meadows of the Tibetan Plateau in northwestern China. We ran a season-long field experiment in which we manipulated the presence and absence of the caterpillar, Gynaephora alpherakii, and grasshopper, Chorthippus fallax, in enclosures, and measured responses of both grazer species and their plant resources. After two months we discovered a strong negative one-way interaction between these species – the seldom considered form of interaction known as amensalism. While caterpillars showed reduced growth, survival, egg production, and delayed metamorphosis in the presence of grasshoppers, there was no reciprocal negative affect of caterpillars on grasshoppers. Because caterpillars are voracious grazers, changes in their activity and survival caused by the presence and absence of grasshoppers propagated to influence the composition and biomass of plants.
We put the amensalistic interaction down to a case of mistaken identity. We observed that by whacking into – and landing heavily upon – grass stems as they move about the meadows, grasshoppers trigger a death-feigning response in caterpillars whereby caterpillars, upon perceiving risk, drop to the ground from the grass stems and leaves where they forage, cease movement, and curl up for about 20 minutes before resuming foraging. Repeated disturbances from the seasonally abundant grasshoppers could have significant effects on feeding time and energy uptake. Indeed, caterpillars in the same enclosures as grasshoppers were observed actively foraging significantly less frequently than when they were alone – evidence of the cost of mistakenly playing dead and helping to explain grasshopper effects on caterpillar growth, timing of metamorphosis, and grazing impact on plants.
Our study provides a rare example of amensalism in a natural ecosystem and shows that it can result from a previously unappreciated mechanism – mistaken identity. Furthermore, this work highlights that such interactions can have significant consequences for the functioning of ecosystems, as revealed by marked shifts in the relative abundances of plant functional groups and overall biomass. Strong amensalistic interactions, if common, could have consequences for our understanding of key issues, such as the evolution of risk-reducing behaviors and traits and the link between consumer biodiversity and ecosystem functioning.