Abstract
Research on the structure of ecological networks suggests that a number of universal patterns exist. Historically, biotic specialization has been thought to increase towards the Equator. Yet, recent studies have challenged this view showing non-conclusive results. Most studies analysing the geographical variation in biotic specialization focus, however, only on the local scale. Little is known about how the geographical variation of network structure depends on the spatial scale of observation (i.e., from local to regional spatial scales). This should be remedied, as network structure changes as the spatial scale of observation changes, and the magnitude and shape of these changes can elucidate the mechanisms behind the geographical variation in biotic specialization. Here we analyse four facets of biotic specialization in host-parasitoid networks along gradients of climatic constancy, classifying the networks according to their spatial extension (local or regional). Namely, we analyse network connectance, consumer diet overlap, consumer diet breadth, and resource vulnerability at both local and regional scales along the gradients of both current climatic constancy and historical climatic change. While at the regional scale none of the climatic variables are associated to biotic specialization, at the local scale, network connectance, consumer diet overlap, and resource vulnerability decrease with current climatic constancy, whereas consumer generalism increases (i.e., broader diet breadths in tropical areas). Similar patterns are observed along the gradient of historical climatic change. We provide an explanation based on different beta-diversity for consumers and resources across the geographical gradients. Our results show that the geographical gradient of biotic specialization is not universal. It depends on both the facet of biotic specialization and the spatial scale of observation.
Highlights
IntroductionBiotic specialization has been thought to be higher in more constant environments
Biotic specialization has long fascinated biogeographers and ecologists
It is the first to explore the spatial scale-dependency of network biotic specialization across large biogeographical gradients
Summary
Biotic specialization has been thought to be higher in more constant environments. More historically constant environments reflect the available time for potential species coevolution due to temporal stability of local communities, which increases local adaptation and favours biotic specialization (Jansson and Dynesius 2002, Dalsgaard et al 2011, 2013, Schleuning et al 2012). Recent network studies have addressed the geographical variation in biotic specialization, showing non-conclusive and highly idiosyncratic results (Moles and Ollerton 2016). While some authors showed an increase in network specialization and higher predation risk towards the tropics (Olesen and Jordano 2002, Dalsgaard et al 2011, Trøjelsgaard and Olesen 2013, Roslin et al 2017), others found the opposite (Schleuning et al 2012). Some others found distinct trends for each hemisphere (Pauw and Stanway 2015) and for each measure of biotic specialization considered (Dalsgaard et al 2017), or no latitudinal trend at all (Ollerton and Cranmer 2002, Morris et al 2014)
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