Posted by: oikosasa | October 30, 2012

Battlefield study: Grasshoppers vs. wolf spiders

That predator-prey interactions can be temperature-dependent is something that Angela Laws and Anthony Joern shows in the new Early View paper in Oikos “Predator–prey interactions in a grassland food chain vary with temperature and food quality”

Read their background story here:

“Grasshoppers are important components of most grassland ecosystems.  These abundant herbivores can influence many ecosystem processes such as nutrient cycling and primary productivity.  But the effects of grasshoppers on ecosystem processes often depend on the outcome of their interactions with other species, including predators.  For example, spiders are common predators of grasshoppers that alter grasshopper behavior and can limit grasshopper population size. But the outcome of species interactions can be sensitive to changes in many biotic and abiotic environmental factors.

We were interested in learning how temperature can influence predator-prey interactions between grasshoppers and wolf spiders.  Ectothermic organisms like grasshoppers and spiders are likely to be especially impacted by shifting temperatures, because temperature affects many physiological processes including feeding, activity, and digestion.  But temperature may also alter species interactions with effects on food web functioning.  In our system, grasshoppers prefer warm temperatures and are active during the day while wolf spiders prefer cooler temperatures and are crepuscular.  Therefore, we predicted that shifting temperatures can alter predator effects on grasshoppers by expanding (through cooling) or contracting (through warming) the total amount of time each day that both grasshoppers and wolf spiders are active.

We conducted a three-year field experiment to test these predictions using common species of grasshopper (Orphulella speciosa) and wolf spider (Rabidosa rabida). We set up field cages and stocked them with grasshoppers only or grasshoppers and spiders.  To alter temperature, we surrounded some of the cages with temperature chambers constructed of steel frames covered with shade cloth (decreased temperatures) or with plastic sheeting (increased temperatures).  Other cages were left uncovered as a control.  The roofs of these temperature chambers were mounted on garage door tracks and could be opened and closed.  The roofs only covered the cages during the morning and were left open for most of the day.

We found that spiders had strongest effects on grasshopper survival in the cooled treatments, and weakest effects on grasshopper survival in the warmed treatments, as predicted.  In some years, this led to the appearance of a trophic cascade (an indirect effect of predators, where predator presence leads to an increase in plant biomass) in cooled treatments, but not warmed treatments.   Our results show that the outcome of predator-prey interactions between grasshoppers and wolf spiders, and their effects on plant production, can shift with temperature.  Our data also suggest that wolf spiders may be less effective at limiting the size of some grasshopper populations under warmer conditions.”

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