Heating up arthropod interactions in situ

Submitted by editor on 29 April 2016.Get the paper!

With advancing climate change, species ranges are likely to shift. However, climate change may also affect interactions among the co-occurring species remaining in the landscape. Most interactions on the globe involve insects, whether in the tropics, the temperate zone or the High Arctic, and whether being antagonistic trophic interactions or mutualistic plant-pollinator interactions. Given that insects are ectotherms and have (relatively) short generation times, we can expect insect individuals and populations to react fairly sensitively to ambient temperature. A key question is then how climate will affect interactions among insect species and the ecological functions stemming from these interactions.

 

Very much of what we know about the effects of climate on species interactions stems from plants warmed up in situ. This is (relatively) easy to do, as plants can be surrounded by passive heating devices, called Open Top Chambers (OTCs). You warm up the plant and you record how it responds – and how its interactions with other sedentary organisms, like bacteria or mycorrhiza, change. But where plants stay in place and cooperate, insects typically don’t; they move. This complicates matters, as few treatments can then be easily applied to the scale of an insect population or community.

Recently, multiple experiments have applied OTCs, or local heating slings, to infer what will happen to insect-sustained functions like herbivory or pollination with climate change. But if the insects can come and go into such treatments, then we have a problem.  If insects simply zoom in on plants thriving in the warm, or get stuck in an OTC when trying to feed on the plant inside, then what we may infer in terms of functioning is likely to be biased. We need to match the scale of the experiment with the scale of the phenomenon.

Yet, the world does not end with a plant in a plastic cone. In our recent paper, we apply OTCs in a new context: to heat up the sedentary phase in the life of highly mobile insects. Under field conditions, we subjected dung beetle communities to an experimental warming treatment, and assessed the net effect on two functions that they provide: dung decomposition and plant productivity. We found species interactions and functions changed with an increase in average temperature of less than a degree. To safeguard functioning under environmental change, we may thus need diverse sets of species to back up continued functioning.

Of course, an OTC around a cow pat is no magic template applicable to dissecting ecosystem functioning around the world. But it is a start, and it shows how we need to get creative. If we deal with mobile species, then maybe we can still pin them down for given life stages. For species tied to given resources, we can allow them to colonize, then heat up the resultant community. To explore climatic effects, we can do anything from subjecting lab cultures to controlled climate chambers to building full-tree enclosures. Full-forest or full-pasture warming is perhaps the next step, but has so far been difficult. Yet, a few years ago no one had thought of excluding ants from a full forest to expose their interactions with other species – then Vojtech Novotny just went out and did it. We guess no one had told him it was impossible…

Eleanor M. Slade and Tomas Roslin

Twitter handles: @eleslade @foodwebs

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