Warming and nitrogen affect size structuring and density dependence in a host–parasitoid food web

Abstract

Body size is a major factor constraining the trophic structure and functioning of ecological communities. Food webs are known to respond to changes in basal resource abundance, and climate change can initiate compounding bottom-up effects on food-web structure through altered resource availability and quality. However, the effects of climate and co-occurring global changes, such as nitrogen deposition, on the density and size relationships between resources and consumers are unknown, particularly in host-parasitoid food webs, where size structuring is less apparent. We use a Bayesian modelling approach to explore the role of consumer and resource density and body size on host-parasitoid food webs assembled from a field experiment with factorial warming and nitrogen treatments. We show that the treatments increased resource (host) availability and quality (size), leading to measureable changes in parasitoid feeding behaviour. Parasitoids interacted less evenly within their host range and increasingly focused on abundant and high-quality (i.e. larger) hosts. In summary, we present evidence that climate-mediated bottom-up effects can significantly alter food-web structure through both density-and trait-mediated effects.

Climate change in size-structured ecosystems

“…In a field manipulation of the temperature and nitrogen load experienced by a host-parasitoid food web, however, increasing temperature or nitrogen yielded higher host densities and consequently more generalized feeding by parasitoids. Interestingly, increasing temperature and nitrogen simultaneously did not yield an additive response in host density or parasitoid generality, suggesting that they do not interact linearly [20]. Differences in whether warming counteracts enrichment [17] or if both effects are non-additive [20] may be explained by the presence of a strongly size-structured predator -prey community in the aquatic experiment of the former and the lack thereof in the host-parasitoid community of the latter study as suggested by model analyses [11].…”

“…Interestingly, increasing temperature and nitrogen simultaneously did not yield an additive response in host density or parasitoid generality, suggesting that they do not interact linearly [20]. Differences in whether warming counteracts enrichment [17] or if both effects are non-additive [20] may be explained by the presence of a strongly size-structured predator -prey community in the aquatic experiment of the former and the lack thereof in the host-parasitoid community of the latter study as suggested by model analyses [11]. Interestingly, the size structure of host-parasitoid communities may be strengthened by warming [18,20], which may modify interactions with enrichment.…”

“…This theme issue integrates novel approaches for addressing the consequences of climate change in complex, sizestructured ecosystems. This includes the development of novel concepts [7][8][9][10], model analyses [11][12][13] and empirical studies in marine [14,15], freshwater [16,17] and terrestrial ecosystems [18][19][20][21][22]. In the following, we will introduce the framework of this theme issue by describing the complexity and the size structure of natural communities and how they might interact with climate change in determining top-down or bottom-up control, community stability and spatial processes.…”

Tree phylogenetic diversity promotes host–parasitoid interactions

“…To our knowledge, only two recent studies from subtropical Australia have addressed this topic and found a decrease of parasitism with elevation [18,46], which we support. Higher parasitism at lower and warmer lower elevation might be caused by higher attack rates of parasitoids, as demonstrated in controlled warming experiments [60]. Thus, increasing temperatures with climatic change will probably increase parasitism rates but influence host-parasitoid interactions in unpredictable ways, as it is unclear if hosts and parasitoids will react similarly [5].…”

Potential for climate effects on the size-structure of host–parasitoid indirect interaction networks

“…Interestingly, studies in this issue on both theoretical predator-prey systems [38] and empirical litter feeding arthropod communities [39] predict changes in diet breadth as a result of changes in foraging behaviour in response to climate change, indicating the potential generality of these effects. While our predictions are based on weather variability, de Sassi et al [40] demonstrate that changes in mean temperature and nitrogen deposition can also lead to changes in electivity. Both these factors affect host density, which should lead to a higher host encounter rate for parasitoids, making it more likely that they will be egg-limited and therefore more elective.…”