Prediction of the effects of climate change on parasite distribution
A common wisdom holds that parasites will spread extensively, and will expand their geographic range northward as climate will go warmer. Moreover, the association between climate and parasites spread has recently been questioned. It has been argued (i) that parasites would undergo range shifts (and even range restriction) rather than range expansion and (ii) that many non-climatic factors may affect, and even overshadow, the effects of the climate.
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Such controversy has important fundamental and societal implications. Accordingly, the next generation of model-based predictions concerning climate change and parasites obviously need to consider all the processes that link parasites to their environment. Including non-climatic factors in predictive models is an efficient way to obtain a balanced and objective view of the risk posed by climate change on parasites spread. My research contribute substantially to the ongoing debate about climate change and parasites, notably by providing an objective view of the relative effects of climatic vs. non climatic factors on parasites spread.
I use a specific host-parasite interaction (Tracheliastes polycolpus / Leuciscus leuciscus) to identify climatic and non-climatic factors that may affect parasite colonization.
Combining empirical observations, experimental works and molecular tools, I focus on potential barriers to T. polycolpus colonization, and I evaluate the strength of each of the following barriers to colonization:
(i) The dispersal ability of T. polycolpus
(ii) The abiotic requirement of T. polycolpus
(iii) The ability of T. polycolpus to shift on new hosts species
(iv) The ability of T. polycolpus to adapt to foreign L. leuciscus genotypes
I develop a predictive statistical framework that would consider conjointly the effects of climatic and non-climatic factors on parasite spread under several climatic scenarios. This integrative framework will aim at improving the forecasting of the effect of climate change on the future geographic range of T. polycolpus.
Finally, I generalize the findings by using meta-analyses of the existing literature to quantify numerically changes in parasite geographic range in the last decades (over taxon and ecosystem types). This meta-analysis as well as simulations will be used to assess the relative effects of various biological and environmental variables (climatic and human-related) on those changes in parasite geographic range.
Combining empirical observations, experimental works and molecular tools, I focus on potential barriers to T. polycolpus colonization, and I evaluate the strength of each of the following barriers to colonization:
(i) The dispersal ability of T. polycolpus
(ii) The abiotic requirement of T. polycolpus
(iii) The ability of T. polycolpus to shift on new hosts species
(iv) The ability of T. polycolpus to adapt to foreign L. leuciscus genotypes
I develop a predictive statistical framework that would consider conjointly the effects of climatic and non-climatic factors on parasite spread under several climatic scenarios. This integrative framework will aim at improving the forecasting of the effect of climate change on the future geographic range of T. polycolpus.
Finally, I generalize the findings by using meta-analyses of the existing literature to quantify numerically changes in parasite geographic range in the last decades (over taxon and ecosystem types). This meta-analysis as well as simulations will be used to assess the relative effects of various biological and environmental variables (climatic and human-related) on those changes in parasite geographic range.
Parasites facing climate change: adapt now or perish?
The conventional wisdom holds that parasites facing with climate change will track their favourable habitats and spread extensively northward, hence colonizing new environments. However, given their small size and their limited intrinsic mobility, many parasites will not be able to track alone their favourable environment and will depend about the dispersal ability of their hosts to do that. However, climate change is occurring at a time when natural environments are becoming increasingly fragmented through habitat destruction also imposing strong selective pressures on host dispersal rates. Consequently, the last issue for numerous parasites will be to adapt in situ. In other words, evolutionary adaptation to environmental changes might be the only way that species can mitigate the stressfull conditions of changing environment if they are unable to disperse.
However, with few exceptions, the importance of adaptation tends to be ignored both in broader discussions about the effects of environmental changes on parasites and in models for predicting parasite responses to these changes. Thus, the new generation of approaches must consider all the evolutionary processes that link parasites to their environment and my researches propose to replace the evolutionary potential of parasites at the core of this debate.
A key evolutionary change may be the pre-adaptation scenario. In populations living in relative heterogeneous environments, pre-adapted populations would better survive future global change, and could also expand their ranges by replacing non-evolvable (or not pre-adapted) local populations. For instance, populations currently living in environments with high temperature fluctuations may evolve thermal generalism, whereas populations living in stable environments should evolve thermal specialization toward the local optima; we can expect generalist populations to better cope with future climate change through pre-adaptation. Thus, rather than considering species as a uniform entity, the pre-adaptation hypothesis considers species as a heterogeneous entity that may include population subsets pre-adapted to future climate change. Pre-adapted populations have not yet received sufficient attention in view of the increasing role they will play in prediction of parasites responses to environmental changes. Explicitly considering the adaptive potential of parasite permit to provide an integrative overview of the impact of climate change on host-parasite interaction.
However, with few exceptions, the importance of adaptation tends to be ignored both in broader discussions about the effects of environmental changes on parasites and in models for predicting parasite responses to these changes. Thus, the new generation of approaches must consider all the evolutionary processes that link parasites to their environment and my researches propose to replace the evolutionary potential of parasites at the core of this debate.
A key evolutionary change may be the pre-adaptation scenario. In populations living in relative heterogeneous environments, pre-adapted populations would better survive future global change, and could also expand their ranges by replacing non-evolvable (or not pre-adapted) local populations. For instance, populations currently living in environments with high temperature fluctuations may evolve thermal generalism, whereas populations living in stable environments should evolve thermal specialization toward the local optima; we can expect generalist populations to better cope with future climate change through pre-adaptation. Thus, rather than considering species as a uniform entity, the pre-adaptation hypothesis considers species as a heterogeneous entity that may include population subsets pre-adapted to future climate change. Pre-adapted populations have not yet received sufficient attention in view of the increasing role they will play in prediction of parasites responses to environmental changes. Explicitly considering the adaptive potential of parasite permit to provide an integrative overview of the impact of climate change on host-parasite interaction.
The study model
I focus on a wildlife host-parasite interaction involving a crustacean copepod ectoparasite, Tracheliastes polycoplus and a freshwater fish host, Leuciscus leuciscus. T. polycolpus has a direct life cycle; only sexually mature females are parasitic and they attach to fish fins whereas male are free-living (A, B). Females feed on the epithelial cells and mucus of the fish’s fins. The grazing activity of T. polycolpus results in local lesions and secondary bacterial inflammations leading to the partial or total destruction of fins. These direct pathogenic effects reduce the fitness of the hosts by reducing their swimming performance, feeding success and growth rate and increasing their mortality. After reproduction, females develop two egg sacs (C) releasing free-living copepodid larvae into the water column (D), which constitute the infective stages of the parasite.
T. polycolpus is probably highly sensitive to variation in temperature throughout its lifetime. Indeed, small increases in aquatic temperatures may be directly perceptible by the free-living larvae. Particularly, temperature may affect the physiological condition of the copepodid larvae (growth, survival time, mobility) and its ability to infect the hosts. Temperature may also affect the ectoparasitic adult performances (growth, fecundity, virulence). In the meantime, dace may be subjected to a reduced defense against infection (tolerance or resistance) due to thermal stresses.