Warming the tundra: how do invertebrates and plants respond?

Submitted by editor on 5 May 2015.Get the paper!

Caterpillar of the Arctic moth (Gynaephora groenlandica) feeding on Salix arctica. Accelerated rates of warming in tundra ecosystems can simultaneously affect plants, herbivores and their interactions. Invertebrate herbivores are likely to respond strongly to warming because their life cycles depend on temperature; yet, their effects on tundra plants have received little attention.

Most studies investigating the effects of warming and herbivory on tundra ecosystems manipulate temperature and measure the responses of the plants (Elmendorf et al. 2012).  But warming can also directly affect herbivores and herbivory rates, and this will in turn influence their interactions with forage species and the entire plant community. Indeed, in tundra plant communities, herbivory might counteract the effects of warming on plants.  For example, grazing by reindeer can prevent warming-related shrub encroachment (Olofsson et al. 2009), and consequently grazing management might buffer some of the adverse effects of warming on tundra rangelands (Speed et al. 2013).

Understandably, it is difficult to experimentally ‘warm’ large herbivores, which have been the focus of most tundra research. However, invertebrate herbivores can respond more strongly to warming, because their behaviour and life cycles strongly depend on temperature.  Relative to vertebrates, non-outbreak invertebrate herbivores consume less biomass and tend to have more localized effects on plants.  Few studies have investigated whether invertebrate herbivory can have similar effects as those reported for vertebrate herbivory.

In our recent paper “Warming the tundra: reciprocal responses of invertebrate herbivores and plants” we report results of a field experiment using passive warming chambers and enclosures at two elevations, to simultaneously warm plants and their herbivores in a full-factorial design. As a study system we used a non-outbreak insect herbivore, the Arctic moth (Gynaephora groenlandica) and two widespread tundra plants, the Arctic willow (Salix arctica) and the mountain avens (Dryas octopetala). We measured changes in herbivore performance, herbivory rates and plant responses during one growing season.

We found that temperature (treatments were achieved through elevation or experimental warming) affected the performance of our cold-adapted caterpillars, with higher respiration rates and lower growth rates in warmer plots. Interestingly, there was a shift in the relative intensity of herbivory on the two plant species, so that in warmer plots caterpillars fed more on the more nutritious Salix arctica. These changes in herbivory rates and selective foraging have potential consequences for the longer-term responses of tundra plants to warming. Individual responses of plants were weak, as expected in a short term manipulation, but the presence of invertebrate herbivores still affected the responses of plants to elevation or experimental warming.

The effects of invertebrate herbivory in tundra can have consequences to other herbivores too. Future studies of tundra herbivory need to incorporate the combined effects of co-occurring vertebrate and invertebrate herbivores.

Our results indicate that invertebrate herbivory, even at non-outbreak levels, could play an ecologically relevant role in tundra ecosystems. The impacts of invertebrate herbivores can also have consequences for other co-occurring herbivores(Barrio et al. 2013), affecting vegetation dynamics at a larger scale. The predicted changes in the abundance and distribution of invertebrate herbivores in warming tundra ecosystems requires more research.  Collecting baseline information on invertebrate distribution and abundance in tundra ecosystems is required to further evaluate their ecological effects at a larger scale under current and future warming conditions.

Isabel C. Barrio, C. Guillermo Bueno and David S. Hik

REFERENCES

Barrio, I. C. et al. 2013. After the frass: foraging pikas select patches previously grazed by caterpillars. - Biol. Lett. 9: 20130090.

Elmendorf, S. C. et al. 2012. Global assessment of experimental climate warming on tundra vegetation: heterogeneity over space and time. - Ecol. Lett. 15: 164–175.

Olofsson, J. et al. 2009. Herbivores inhibit climate-driven shrub expansion on the tundra. - Glob. Chang. Biol. 15: 2681–2693.

Speed, J. D. M. et al. 2013. The response of alpine Salix shrubs to long-term browsing varies with elevation and herbivore density. - Arctic, Antarct. Alp. Res. 45: 584–593.

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