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Are Generalist Parasites Being Lost from Their Hosts?

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Are Generalist Parasites Being Lost from Their Hosts? Journal of Animal Ecology 2016 doi: 10.1111/1365-2656.12470 FORUM Response to Strona & Fattorini: are generalist parasites being lost from their hosts? Maxwell J. Farrell1*, Patrick R. Stephens2 and T. Jonathan Davies1,3 1Biology Department, McGill University, 1205 Docteur Penfield, Montreal, QC H3A 1B1, Canada; 2Odum School of Ecology, University of Georgia, Athens, GA 30602, USA; and 3African Centre for DNA Barcoding, University of Johannesburg, PO Box 524, Auckland Park, 2006 Johannesburg, South Africa Key-words: asymmetry of interactions, coextinction, conservation, host range, host speci- ficity, infectious disease, macroecology, parasite richness 3 The inclusion of both host life-history traits and mea- Introduction sures of current extinction risk as predictors in our Strona & Fattorini (2015) raise concerns about the con- analyses was not properly justified and could lead to clusions of a recent analysis by Farrell et al. (2015) on contrasting results if we were attempting to explain patterns of host–parasite associations among free-living the evolution of parasite specialization. carnivores and terrestrial ungulates. Contrary to expecta- We believe these criticisms arise from a few simple tions from coextinction theory, our study found ungulate misunderstandings; we address each point in detail below. species threatened with extinction are associated with a higher proportion of single-host parasites compared to non-threatened species. The increased proportion of sin- gle-host parasites in threatened ungulates appears to Non-random distributions of specialist and result from decreased richness of multi-host parasites as generalist parasites we do not observe significant differences in the richness of In various species interaction networks, non-random dis- single-host parasites between threatened and non-threa- tributions of specialist and generalist affiliates have been tened ungulates. We suggest one explanation may be that observed in which specialists interact more often with multi-host parasites are being lost from declining ungulate highly connected species in the network (Bascompte & populations due to decreased cross-species contact as spe- Jordano 2003; Ings et al. 2009). This pattern, reflecting an cies’ ranges contract. We offer the hypothesis that strong ‘asymmetry of interactions’, has been shown for macro- Allee effects and selective pressures to maintain minimum parasites of Canadian freshwater fish and of small mam- group sizes in these species facilitate the transmission of mals in Siberia (Vazquez et al. 2005). single-host parasites within host groups, even while the Strona & Fattorini suggest that hosts harbouring many species as a whole is in decline. parasites should also be associated with a greater propor- We appreciate the thoughtful commentary by Strona & tion of specialist parasites. Under their assumptions, Fattorini and are pleased that our recent work is sparking threatened hosts are associated with fewer parasite species debate regarding the ecological mechanisms behind our and, due to a process of passive sampling, will be associ- unexpected results. However, Strona & Fattorini raise ated with fewer specialist parasites. Our results do not three main concerns about our analysis: match these predictions. We find threatened ungulates are 1 Our results may be explained by non-random distribu- associated with a higher proportion of specialist parasites. tions of specialist and generalist parasites (Vazquez The bias described by Strona & Fattorini is thus in the et al. 2005) leading to hosts that harbour greater num- opposite direction of our findings, and their proposed bers of parasite species being more likely to harbour mechanism is unable to explain our results. This may sug- specialist parasites. gest that our finding is even more striking than we ini- tially presented, or that their assumption of an asymmetry 2 Analysing parasite host specificity as a binary variable of interactions is not supported by the data. (single-host vs. multi-host) may bias our interpreta- We agree that threatened hosts should be associated tions as this classification may not reflect the evolu- with fewer parasite species, and this has been found in tionary and ecological implications of host specificity. previous comparative studies (Altizer, Nunn & Lindenfors 2007) and again in our study. However, Nunn & Altizer *Correspondence author. E-mail: [email protected] (2005) did not show any pattern of asymmetry of interac- © 2016 The Authors. Journal of Animal Ecology © 2016 British Ecological Society 2 M. J. Farrell et al. tions in their original description of the data, and we host range of all parasites associated with a particular additionally find no evidence of this pattern or the host, we find no trend for hosts with high parasite rich- suggested mechanism of assembly. ness to harbour a greater proportion of specialist parasites Strona & Fattorini purport to show a pattern of (Fig. 1c). This indicates that drawing conclusions on the asymmetric interactions in our data by depicting the distribution of host ranges in a parasite assemblage using relationship between minimum host range of parasites only the minimum observed value, as Strona & Fattorini that infect a given host and parasite richness on that have done, can be misleading. Unfortunately, this biased host. Host range refers to the number of host species a description of host–parasite networks is further propa- parasite infects, and the minimum host range is defined gated in Strona (2015). as the smallest observed value of host range for all par- While we agree that an asymmetry of interactions in asites associated with a particular host species. Mini- host–parasite networks could be an exciting pattern to mum host range is a property of the suite of parasites explore, and may hint at consistent properties similar to that are associated with a particular host species and as those of mutualistic networks, it is important to re-iter- such, each data point in Strona & Fattorini’s original ate that networks with asymmetric interactions are not figures represents the parasite assemblage of a single- found across all ecosystems (Poulin 2007; Campiao~ et al. host species. We take issue with their analysis for two 2015). The application of network analysis to host–para- reasons. site associations is in its early stages (Poulin 2010), and First, Strona & Fattorini disguise important variation additional work is needed to determine the extent to in the data by rounding the logged values of minimum which properties of host–parasite networks generalize, host range and then using boxplots to visualize the and the causal mechanisms responsible (Vazquez et al. relationship with parasite richness. We re-plotted this rela- 2005). tionship using a scatterplot (Fig. 1a). We find consider- able variability in the data and show that single-host Defining parasite host specificity parasites are not only relegated to parasite-rich hosts, and parasite-poor hosts do not interact mainly with generalist We appreciate that there are many ways of quantifying parasites. host specificity depending on the question of interest and Secondly, we are concerned that Strona & Fattorini are information at hand (Poulin, Krasnov & Mouillot 2011), drawing conclusions about the central tendency of a dis- and we previously explained in Farrell et al. (2015) our tribution by investigating the behaviour of its minimum rationale for using the single-host/multi-host distinction. value (i.e. using minimum host range). If we visualize the Pragmatically, it is much easier to make inference about same relationship between host range and parasite rich- extinction dynamics using this framework compared to ness, but use maximum rather than minimum host range, continuous measures of host specificity. For example, the we can see that hosts with greater numbers of parasite extinction of a host will result in the extinction of all species also harbour additional generalist parasites host-specific parasites, but when parasites are found on (Fig. 1b). However, assessing whether networks are char- two or more hosts (which may differ in threat status), it is acterized by an asymmetry of interactions typically much harder to make predictions on the extinction risk of involves examining average host range across parasite the parasite. In this context, we suggest that using the sin- assemblages (e.g. Vazquez et al. 2005; Poulin 2007), which gle-host/multi-host distinction allows us to make more takes into account the host range of all parasites in the explicit predictions, and we can see no reason why this assemblage. When visualizing the relationship using mean should bias our results. (a) (b) (c) 100 50 20 50 100 10 10 20 25 25 Parasite richness per host Parasite richness per host Parasite Parasite richness per host Parasite 1 1 1 2 5 10 20 50 100 12 5102050 12 5102050 12 5102050 Minimum host range Maximum host range Mean host range Fig. 1. Relationships between the minimum (a), maximum (b) and mean (c) host range of parasites associated with a particular host spe- cies, and the parasite richness of that host species (data from Farrell et al. 2015). Transparency of points is set to a = 0Á3 to allow visual- ization of overlapping data. © 2016 The Authors. Journal of Animal Ecology © 2016 British Ecological Society, Journal of Animal Ecology Extinction of multi-host parasites 3 ficity and risk of coextinction for the rhinoceros stomach Life-history traits vs. extinction risk botfly, and it is the potential for changes in the specificity In our study, we investigated how extinction processes and structure of host–parasite systems resulting from host resulting from human influence change host–parasite asso- decline that we hope to highlight with our work. Perhaps ciations. It is possible that Strona & Fattorini misunder- the perceived shift in host specificity of parasites as their stood the goals of our analysis as it was not our intention alternative hosts decline to extinction may also be a possi- to explain the evolutionary drivers of parasite specificity. ble explanation for our results, and potentially a promising We recognize that there are many host life-history traits area for future research.
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