Data from: Parasites affect food web structure primarily through increased diversity and complexity

Comparative research on food web structure has revealed generalities in
trophic organization, produced simple models, and allowed assessment of
robustness to species loss. These studies have mostly focused on
free-living species. Recent research has suggested that inclusion of
parasites alters structure. We assess whether such changes in network
structure result from unique roles and traits of parasites or from changes
to diversity and complexity. We analyzed seven highly resolved food webs
that include metazoan parasite data. Our analyses show that adding
parasites usually increases link density and connectance (simple measures
of complexity), particularly when including concomitant links (links from
predators to parasites of their prey). However, we clarify prior claims
that parasites “dominate” food web links. Although parasites can be
involved in a majority of links, in most cases classic predation links
outnumber classic parasitism links. Regarding network structure, observed
changes in degree distributions, 14 commonly studied metrics, and link
probabilities are consistent with scale-dependent changes in structure
associated with changes in diversity and complexity. Parasite and
free-living species thus have similar effects on these aspects of
structure. However, two changes point to unique roles of parasites. First,
adding parasites and concomitant links strongly alters the frequency of
most motifs of interactions among three taxa, reflecting parasites'
roles as resources for predators of their hosts, driven by trophic
intimacy with their hosts. Second, compared to free-living consumers, many
parasites' feeding niches appear broader and less contiguous, which
may reflect complex life cycles and small body sizes. This study provides
new insights about generic versus unique impacts of parasites on food web
structure, extends the generality of food web theory, gives a more
rigorous framework for assessing the impact of any species on trophic
organization, identifies limitations of current food web models, and
provides direction for future structural and dynamical models.