Abstract
With climate change leading to poleward range expansion of species, populations are exposed to new daylength regimes along latitudinal gradients. Daylength is a major factor affecting insect life cycles and activity patterns, so a range shift leading to new daylength regimes is likely to affect population dynamics and species interactions; however, the impact of daylength in isolation on ecological communities has not been studied so far. Here, we tested for the direct and indirect effects of two different daylengths on the dynamics of experimental multitrophic insect communities. We compared the community dynamics under “southern” summer conditions of 14.5‐hr daylight to “northern” summer conditions of 22‐hr daylight. We show that food web dynamics indeed respond to daylength with one aphid species (Acyrthosiphon pisum) reaching much lower population sizes at the northern daylength regime compared to under southern conditions. In contrast, in the same communities, another aphid species (Megoura viciae) reached higher population densities under northern conditions. This effect at the aphid level was driven by an indirect effect of daylength causing a change in competitive interaction strengths, with the different aphid species being more competitive at different daylength regimes. Additionally, increasing daylength also increased growth rates in M. viciae making it more competitive under summer long days. As such, the shift in daylength affected aphid population sizes by both direct and indirect effects, propagating through species interactions. However, contrary to expectations, parasitoids were not affected by daylength. Our results demonstrate that range expansion of whole communities due to climate change can indeed change interaction strengths between species within ecological communities with consequences for community dynamics. This study provides the first evidence of daylength affecting community dynamics, which could not be predicted from studying single species separately.
Highlights
Climate change has led to an increase in global temperatures (Hansen, Sato, Ruedy, Schmidt, & Lo, 2016), which is predicted to continue, with a projected increase in the mean global surface air temperature of 3.0°C by the end of the 21st Century (2071–2100), relative to the period between 1961 and 1990 (Flato et al, 2013; Houghton et al, 2001)
We expected that longer daylength would increase parasitoid attack rate, which would in turn (a) negatively affect the host aphid population and, through reduced interspecific competition, (b) positively affect the other aphid species
M. viciae did not respond to daylength when it was the only aphid species present in the food web experiment
Summary
Climate change has led to an increase in global temperatures (Hansen, Sato, Ruedy, Schmidt, & Lo, 2016), which is predicted to continue, with a projected increase in the mean global surface air temperature of 3.0°C by the end of the 21st Century (2071–2100), relative to the period between 1961 and 1990 (Flato et al, 2013; Houghton et al, 2001). The harvesting of a single parasitoid species led to a community-wide extinction cascade in a recent experiment, an effect that was transmitted indirectly via competition at the herbivore level (Sanders, Kehoe, & van Veen, 2015). Many are major pest species, especially when acting as vectors for plant viruses, causing critical damage to agricultural crops (Dedryver, Le Ralec, & Fabre, 2010) Their population and community dynamics have been studied extensively, including in the context of indirect species interactions (Hassell, 2000; Kaiser-Bunbury & Müller, 2009; Müller & Godfray, 1999; Sanders, Sutter, & Veen, 2013; Sanders, Kehoe, & van Veen, 2015; Snyder & Ives, 2001) as well as climate change (Forrest, 2016). We show that while the host-parasitoid interaction was not affected by daylength, we discovered that the competitive strength of the two aphid species changed with daylength resulting in higher Megoura viciae abundance under long days
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