Ecological Archives E096-029-A1

Sonia Kéfi, Eric L. Berlow, Evie A. Wieters, Lucas N. Joppa, Spencer A. Wood, Ulrich Brose, and Sergio A. Navarrete. 2015. Network structure beyond food webs: mapping non-trophic and trophic interactions on Chilean rocky shores. Ecology 96:291–303. http://dx.doi.org/10.1890/13-1424.1

Appendix A. Elements about the data set.

General Composition of the Web

Note that plankton includes phytoplankton and zooplankton and this group, along with "benthic bio-film", is composed of many species but represented as one node in the network (see bias 4 in next paragraph).

Common types of non-trophic interactions present in the data set

a These categories correspond to distinct functional types of non-trophic interactions distinguished by how they are modeled as proposed in Kéfi et al. 2012.

Competition for space is considered direct link between species because space is not included as a node in the network. This interaction is distinguished from competition for prey which is implicit in the food web as exploitation competition.

Potential biases of our web: A work in progress

1. Missing species and their potential interactions with those inhabiting rocky platforms

While missing species from wave exposed platforms are comparatively few and do not present apparent systematic bias with respect to their trophic position or link type, omissions could alter statistics of the web.

Subtidal-intertidal connections may have been under-estimated due to lack of knowledge about subtidal species (e.g., fish) that utilize the intertidal zone during high tides and spread the effect onto wave exposed platforms. Fish effects around channels and boulder fields in the lower shore have been demonstrated (Muñoz and Ojeda 1998, Ojeda and Muñoz 1999), but the extent of incursions into wave exposed platforms is unclear and we opted to leave them out. Similar decision were taken regarding small native and introduced rodents and marsupials, which also feed on intertidal organisms (Navarrete and Castilla 1993), but it is not entirely clear which habitat they exploit.

Species found almost exclusively in specialized habitats, such as vertical walls (e.g., Lottia orbingy), tidepools (e.g., Loxechinus albus), or overhangs (Trimusculus peruvianus) were not included either. Further research might suggest the need to include them in the tightly connected ecological web and that might alter web statistics, but at this point it is unclear whether that could bias results in any direction.

2. Over-estimation of interference competition among basal (algal) species

Measuring the relative importance of interference competition among rare species under natural conditions is particularly challenging and requires methodological advances to conduct more realistic experiments. For instance, many common algal species occur in low abundance (rare), but in close proximity to other more abundant species, making it difficult to perform sensible experimental manipulations under field conditions. Future experiments could therefore determine that some of our cases of interference competition are really cases of no interaction (e.g. rare species limited by environmental conditions) or even facilitation. When local experimental information was lacking for a pair of sessile species, we probably had a greater tendency in assigning (i.e., benefit of doubt) the interaction to competition for space than when dealing with pairs of mobile species at higher trophic levels. This would create a bias in favor of negative NTI's at lower trophic levels. However, the sheer number of species at bottom versus high trophic levels would make it difficult to alter the general pattern.

Along this same line, several of our -/- links might actually be 0/- (amensalisms). When we knew a species pair engaged in competition, but were uncertain as to competitive abilities, we assumed a symmetrical interaction. However, competition for space can be highly asymmetrical and some of negative links would be reduced. Indeed, part of the reason why negative non-trophic interactions are so dominant in our web is because the web is binary (i.e., an interaction is either present or absent). If the network (at some further stage) becomes quantitative, we will be able to identify competitively dominant species and thereby take highly asymmetrical competition into account.

3. Under-estimation of non-lethal, long-term effects of predators

Beyond behavioral (escape, habitat shifts) or morphological changes (shell thickness, byssus threads), little is still known of the long-term effects of predator-induced stress in invertebrates. Recent research in terrestrial mammal predator-prey species has evidenced long-lasting consequences of predators on prey reproductive cycles, energy allocation and overall fitness (Hawleena and Schmitz 2010, Boonstra 2013). Further developments in stress physiology of marine invertebrates might evidence unsuspected predator effects on prey species that do not respond behaviorally or morphologically, which would then increase the number of negative non-trophic interactions initiated at top trophic levels. But since the total number of top trophic level species is low in comparison to basal species, it is unlikely that the broad patterns presented here are significantly affected.

4. Lumping

In Chile, as in most rocky shores of the world, biofilms represent a complex multi-taxa assemblage, within which there might be negative and positive interactions (Hillebrand et al. 2000, Aguilera et al. 2013). However, due to lack of information we, as most ecologists, have treated it as a black box that is included as a single node in our ecological network. Disentangling interactions within the biofilm assemblage and between these taxa and the different grazers that feed on them can bring up a whole new set of interactions into light, although we do not anticipate new types of interactions or any bias into the broad patterns reported here.

Plankton (zooplankton and phytoplankton) are also treated as a single node upon which benthic filter-feeders prey. However, unlike biofilms, we do not see any way in which differential feeding, even if it could alter plankton composition/abundance, could feedback into the intertidal web. Therefore, lumping plankton species seems less consequential for the wave exposed web than lumping biofilm taxa.

We know that a large fraction of invertebrate species at all trophic levels of the web have pelagic larval stages that can feed on phyto- and/or zoo-plankton. It is well-known that food availability can influence larval performance, and therefore these feeding links are important and are part of the basic life history of these species. However, at this point we have no way to incorporate them into any marine ecological web and we cannot begin to anticipate the consequences on the structural patterns of the rocky shore interaction web.

5. Incidental trophic links

As with all trophic webs, a relatively small fraction of feeding links may in reality be quite trivial as their representation in diet of a consumer could be incidental and relatively rare. The fact that our rocky shore web was constructed considering the local co-occurrence and the vertical (tidal) distribution of organisms makes it less prone to incorporate the "freak" observations of nature (e.g., crabs feeding on seals washed-up onshore, seastars eating ghost crabs used as bait by fishers).

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