Comparing species interaction networks along environmental gradients

Abstract

Knowledge of species composition and their interactions, in the form of interaction networks, is required to understand processes shaping their distribution over time and space. As such, comparing ecological networks along environmental gradients represents a promising new research avenue to understand the organization of life. Variation in the position and intensity of links within networks along environmental gradients may be driven by turnover in species composition, by variation in species abundances and by abiotic influences on species interactions. While investigating changes in species composition has a long tradition, so far only a limited number of studies have examined changes in species interactions between networks, often with differing approaches. Here, we review studies investigating variation in network structures along environmental gradients, highlighting how methodological decisions about standardization can influence their conclusions. Due to their complexity, variation among ecological networks is frequently studied using properties that summarize the distribution or topology of interactions such as number of links, connectance, or modularity. These properties can either be compared directly or using a procedure of standardization. While measures of network structure can be directly related to changes along environmental gradients, standardization is frequently used to facilitate interpretation of variation in network properties by controlling for some co-variables, or via null models. Null models allow comparing the deviation of empirical networks from random expectations and are expected to provide a more mechanistic understanding of the factors shaping ecological networks when they are coupled with functional traits. As an illustration, we compare approaches to quantify the role of trait matching in driving the structure of plant-hummingbird mutualistic networks, i.e. a direct comparison, standardized by null models and hypothesis-based metaweb. Overall, our analysis warns against a comparison of studies that rely on distinct forms of standardization, as they are likely to highlight different signals. Fostering a better understanding of the analytical tools available and the signal they detect will help produce deeper insights into how and why ecological networks vary along environmental gradients.

The geographical variation of network structure is scale dependent: understanding the biotic specialization of host–parasitoid networks

“…Similarly, our results show the importance of considering different spatial scales to get a broader understanding of the specialization pattern and their determinants. Additionally, if network structure varies across spatial scales (Pillai et al 2011, Roslin et al 2014, Wood et al 2015, Galiana et al 2018), then network studies estimating the causes of variation in network structure along any environmental gradient (Dormann et al 2017, Tylianakis and Morris 2017, Pellissier et al 2018) might benefit from understanding the spatial scaling of network structure along the gradient. However, caution must be exerted when interpreting the comparison between different spatial scales.…”

Finding flies in the mushroom soup: Host specificity of fungus‐associated communities revisited with a novel molecular method

“…() discovered a total of 82 coleopteran species from 427 fruiting bodies from 22 genera when sampling tropical bracket fungi, which often produce hard and long‐lived fruiting bodies. While these numbers could easily be translated to simple metrics of host specificity—such as number of associated species per host taxon—the different resolution and the different biases associated with different methods often render direct comparisons between metrics of ecological interaction structure problematic (Morris, Gripenberg, Lewis, & Roslin, ; Pellissier et al., ; Wirta et al., ). Simple metrics of host specialization will namely vary with the set of host fungi included for comparison and thus with the study design.…”

Unveiling the food webs of tetrapods across Europe through the prism of the Eltonian niche

“…The history of land use in particular has been shown to be a major driver of diversity patterns (Niedrist, Tasser, Lüth, Dalla Via, & Tappeiner, 2009;Pimm & Raven, 2000), especially in Europe where a complex history of local extinctions, species reintroductions and translocations has unfolded over the past centuries, particularly for large tetrapods such as the wolf or the bear (Chapron et al, 2014). Our study does not disentangle the factors upon which the environment selects species, and in fact, it would be vain to attempt to tease apart these drivers.…”