Bramon Mora, B. & Cirtwill, A.R., & Stouffer, D.B. (2018) pymfinder: a tool for the motif analysis of binary and quantitative complex networks. bioRxiv: doi: https://doi.org/10.1101/364703     pdf
This preprint introduces pymfinder, a python package for calculating the frequencies of motifs (unique patterns of interactions among small groups of species) in complex networks. Includes functions to describe network structure overall as well as functions to define the roles of species or links in terms of their participation in different motifs.
Albouy, C., Archambault, P., Appeltans, W., Araújo, M.B., Beauchesne, D., Cazelles, K., Cirtwill, A.R., Fortin, M.J., Galiana, N., Leroux, S., Pellissier, L., Poisot, T., Stouffer, D.B., Wood, S.A., & Gravel, D. (2019) The marine fish food web is globally connected. Nature Ecology and Evolution: in press. doi: 10.1038/s41559-019-0950-y
This paper examines how spatial turnover in marine fish community composition affects the structure of the fish food web. We found that the network was generally highly connected and did not show strong spatial modules, but the expected increase in species richness towards the tropics did result in changes to network structure. The high spatial connectivity may help marine food webs to resist perturbations, but could also allow the effects of disturbances to spread rapidly.
Cirtwill, A.R., Eklöf, A., Roslin, T., Wootton, K., & Gravel, D. (2019)
A quantitative framework for investigating the reliability of empirical network construction.
Methods in Ecology and Evolution: 10, 902-911.
doi: 10.1111/2041-210X.13180     preprint doi: https://doi.org/10.1101/332536     submitted pdf
This paper begins by describing the nested layers of uncertainty which affect the assembly of empirical ecological networks (e.g., due to true variation in interactions between sites, variation in interactions due to changes in abundances, or variation in the detectability of different interactions). To address the uncertainty inherent in empirical networks, we propose a simple and adaptable Bayesian framework. Using this framework makes the assumptions used to build the network explicit and can account for different levels of information about different interactions.
Baiser, B., Gravel, D., Cirtwill, A.R., Dunne, J.A., Fahimipour, A.K., Gilarranz, L.J., Grochow, J.A., Li, D., Martinez, N.D., McGrew, A., Poisot, T., Romnuk, T.N., Stouffer, D.B., Trotta, L.B., Valdovinos, F.S., Williams, R.J., Wood, S.A., & Yeakel, J.D. (2019) Ecogeographical rules and the macroecology of food webs. Global Ecology and Biogeography: 28, 1204-1218. doi: https://doi.org/10.1111/geb.12925
This paper summarizes several well-known ecological rules (Rapoport's rule, Bergmann's Rule, etc.) and their potential relationships with food-web structure. These relationships have different levels of support, and we aim to promote further study about the trends which are currently under-studied.
Simmons, B.I., Cirtwill, A.R., Baker, N., Dicks, L.V., Stouffer, D.B., & Sutherland, W.J.
(2019) Motifs in bipartite ecological networks: uncovering indirect interactions.
Oikos: 128, 154-170.
https://doi.org/10.1111/oik.05670     open-access early view edition
This paper demonstrates that motifs (unique patterns of interaction among small groups of species) can explain variation in the structure of bipartite networks such as plant-pollinator networks that is masked by other measures of structure such as connectance, nestedness, and modularity. Bipartite motifs can be calculated using the software introduced in Simmons et al., 2019.
Cirtwill, A.R., Dalla Riva, G.V., Gaiarsa, M.P., Bimler, M.D., Cagua E.F., Coux, C., & Dehling, D.M. (2018)
A review of species role concepts in food webs. Food Webs: 16, e00093.
download pre-publication version
This paper reviews several major concepts of species roles in food webs, including centrality, trophic position, and motif roles. We use Eltonian niches as a framework to relate these role concepts to each other and to the broader ecological context.
Cirtwill, A.R. & Eklöf, A. (2018)
Feeding environment and other traits shape species' roles in marine food webs.
Ecology Letters: 21(6), 875-884. doi: 10.1111/ele.12955.
pre-publication main text     pre-publication SI
Dryad data repository
This paper identifies the motif positions which explain the most variation in species' roles in marine food webs. We then relate the frequencies of these positions to species traits such as body mass and feeding environment. We found that positions in motifs describing apparent and direct competition and the three-species food chain explained the most variation and were related to feeding environment, body mass, trophic level, and other traits.
Cirtwill, A.R., Roslin, T., Rasmussen, C., Olesen, J.M., & Stouffer, D.B. (2018)
Between-year changes in community composition shape species' roles in an Arctic plant-pollinator network.
Oikos: 127(8), 1163-1176. doi:10.1111/oik.05074.
pre-publication main text     pre-publication SI
Dryad data repository
This paper describes changes in the structure of an Arctic plant-pollinator network over two decades and relates changes to network structure and species' motif roles to changes in community composition. We found that networks with greater turnover in composition tended to include species with different roles.
Cirtwill, A.R., Lagrue, C., Poulin, R., & Stouffer, D.B. (2017)
Host taxonomy constrains the properties of trophic transmission routes for
parasites in lake food webs. Ecology: 98(9), 2401–2412.
pre-publication version     pre-publication SI
This paper explores how links' roles (e.g., centrality, proportion of predator's diet contributed, and prey abundance) and host taxonomies relate to their use as parasite transmission routes. Host taxonomy plays a major role, as most parasites are restricted to small numbers of hosts. After taking host taxonomy into account, we found that transmission routes generally involved intermediate prey with high local biomasses, while links between intermediate and definitive hosts tend to be dynamically weak.
Cirtwill, A.R., Stouffer, D.B., Poulin, R., & Lagrue, C. (2016) Are parasite diversity and abundance linked to individual diet range and feeding preferences in fish hosts? Parasitology: 143(1), 75-86. doi:10.1017/S003118201500150X
In this paper, we test whether the parasite loads of individual fish are related to their diets. We examined both diet range (number of prey) and preferences for particular prey (indicated by high proportions of the fish's diet), but did not find any relationship between individual diets and parasite loads. This contrasts with earlier work which has found strong effects of species-level diets on species-level parasite loads. There may be a mis-match between drivers of parasite load at the levels of species and individuals.
Poisot, T., Cirtwill, A., Gravel, D., Fortin, M.-J., & Stouffer, D.B. (2016)
The structure of probabilistic networks. Methods in Ecology and Evolution: 7(3), 303-312.
Most ecological networks treat interactions as fixed entities, which occur or do not occur. Interaction strengths or frequencies may be included, but these are also usually treated as fixed values. This ignores the variability of interactions between species over space and time as community composition, species traits, and local conditions change. This paper introduces probabilistic equivalents for many commonly-used network metrics and advocates for a more realistic consideration of the variability in interactions in the future.
Poisot, T., Gravel, D., Leroux, S., Wood, S.A., Fortin, M.-J., Baiser, B., Cirtwill, A.R., Araujo, M.B.,
& Stouffer, D.B. (2016) Synthetic datasets and community tools for the rapid testing of ecological hypotheses.
Ecography: 39(4), 402-408.
This paper introduces software and an analysis pipeline designed to collect and combine data from disparate sources into predicted food webs. These webs can then be used as first steps towards understanding areas where food webs have not yet been constructed, or to compare observed food webs with those expected based on species occurrances and ecological rules. The ever-increasing volume of open ecological data means that such approaches have great potential to augment the time- and resource-intensive construction of ecological networks for each site of interest.
Cirtwill, A.R. & Stouffer, D.B. (2016) Knowledge of predator-prey interactions improves predictions
of immigration and extinction in island biogeography. Global Ecology and Biogeography: 25(7), 900-911.
pre-publication version     pre-publication SI
In this paper, we combine a classic island-biogeography dataset with a more recently-constructed food web for the community in order to test whether information about species' trophic links can improve our predictions of community turnover. This work draws upon the Trophic Theory of Island Biogeography (TTIB), which aims to refine the classic Theory of Island Biogeography by incorporating the dependence of predators upon their prey. Consistent with the TTIB, we found that the presence of prey did affect species' probabilities of extinction on small islands. We did not detect and effect of predators, but note that predators were nearly omnipresent in this dataset.
Cirtwill, A. 2016. Species roles and link roles: a richer perspective on network ecology. University of Canterbury [PhD Thesis].
In this thesis, I explored several aspects of species' and links' roles in ecological networks. All chapters except one have since been published, and the remaining chapter is currently in preparation. Please check out the individual articles that interest you.
Cirtwill, A.R., Stouffer, D.B., & Romanuk, T.N. (2015)
Latitudinal gradients in biotic niche breadth vary across ecosystem types. Proceedings of the Royal Society B: 282(1819), 20151589.
The latitudinal gradient in species richness is well known but poorly understood. One possible explanation for this widespread trend is that species tend to be more specialised in the tropics, leading to smaller niches and allowing greater numbers of species to coexist. Specialisation, however, also depends directly upon species richness. In this paper, we test whether the scaling of specialisation with species richness varies over latitude in different ecosystem types (marine, freshwater, or terrestrial). We found that, in freshwater food webs, the scaling of specialisation with species richness did vary over latitude. In other ecosystem types, however, there was no trend. This suggests that the latitudinal diversity gradient may have different drivers in different types of ecosystems.
Cirtwill, A.R. & Stouffer, D.B. (2015) Concomitant predation on parasites is highly variable but
constrains the ways in which parasites contribute to food web structure. Journal of Animal Ecology: 84(3), 734-744.
Parasites are involved in several different types of interactions: parasitism, predation by free-living species on free-living parasite life stages, intraguild predation among parasites, and concomitant predation on parasites inside their hosts. We tested how adding these different types of interactions affects how we see the roles of parasites and free-living species in food webs, and how the roles of different types of links compared. Concomitant predation is the only interaction type which clearly distinguishes the roles of parasites from those of free-living species. The roles of concomitant predation links are, however, much more variable than those of other interaction types.
In this paper, we used paired pollination networks including and excluding exotic plants to determine how these species may affect network structure. We found that vulnerable pollinators appear to depend strongly upon exotic plants, suggesting that removing exotic species may place some pollinators at greater risk of extinction. This is not to say that exotic plants are, in general, beneficial to native communities, but that they may be useful when attempting to conserve particular pollinator species.
Cirtwill, A. 2012. Latitude, temperature, and productivity affect ecological network structure. Dalhousie University [Honours Thesis].
This thesis explored environmental drivers of food-web structure. A significantly revised version was later published as an article in Proc R Soc B. Please see that manuscript for more details.