key: cord-0752942-lpwb38hh authors: Warburton, Elizabeth M. title: Untapped potential: The utility of drylands for testing eco-evolutionary relationships between hosts and parasites date: 2020-05-16 journal: Int J Parasitol Parasites Wildl DOI: 10.1016/j.ijppaw.2020.04.003 sha: 5f367017900844d4ad17f644e51252c861e521f7 doc_id: 752942 cord_uid: lpwb38hh Drylands comprise over 41% of all terrestrial surface area and are home to approximately 35.5% of the world's population; however, both free-living and parasitic fauna of these regions remain relatively understudied. Yet, the very conditions that make these regions challenging to study – extreme environmental conditions and low population density for various organisms – also make them potentially untapped natural laboratories for examining eco-evolutionary relationships between hosts and parasites. Adaptations and ecological patterns illustrated by desert parasite communities can serve as exemplars within the extremes regarding the evolution of virulence, breadth of host spectra, and lifecycle strategies. This review provides relevant examples for each of these three topics using parasites from dryland regions in order to encourage future empirical tests of hypotheses regarding parasite ecology and evolution within dryland ecosystems and stimulate wider investigation into the parasitofauna of arid regions in general. As global climate changes and anthropogenic disturbance increases, desertification is a growing problem which has been labeled as a threat to global health. Thus, deserts not only provide useful natural laboratories in which to study parasite transmission but understanding parasite transmission within these habitats becomes increasingly important as larger, likely highly resource insecure, populations are projected to live on the margins of desert regions in the future. Drylands, or areas with an annual precipitation to evapotranspiration ratio (i.e. aridity 48 index) of 0.65 or less (Safriel et al. 2007) , comprise approximately 41.3% of all terrestrial 49 surface area (Patz et al. 2012) . Additionally, as a whole, drylands are home to approximately 50 35.5% of the world's population and hold a share of 65% of the planet's rangelands as well as 51 25% of its cultivated lands (Safriel et al. 2007 ). Thus, from both a habitat and a land use 52 perspective, drylands are critical areas for assessing the interface between humans and nature. 53 Drylands themselves consist of three major types of habitat ( Fig. 1) : forests within dry subhumid 54 areas (aridity index = 0.50-0.65), grasslands within semiarid areas (aridity index = 0.20-0.50), 55 and deserts within arid (aridity index = 0.05 -0.20) as well as hyper-arid (aridity index < 0.05) 56 areas, with deserts accounting for slightly over 17% of the Earth's surface (Safriel et al. 2007 ). 57 Despite the significant extent of drylands on earth, these areas are understudied (Trimble 58 and van Aarde 2012). This could be due to preconceived perceptions of arid environments and 59 that working in them can be logistically challenging, as precarious environmental conditions, 60 limited resources, and remote locations can make research in deserts difficult. Additionally, 61 deserts occupy well over 5 times the land area of developing nations as compared to 62 industrialized ones, possibly making them an afterthought to researchers in typical Western 63 industrialized countries (Safriel et al. 2007 ). Unfortunately, deserts might also have been 64 ecological afterthoughts in the past, with less motivation for studying desert communities than, 65 for example, temperate forest communities. Desert ecosystems have historically been considered 66 to be of little complexity given that their aboveground net primary productivity (ANPP) is low, 67 typically three times lower than temperate forest ecosystem ANPP (Webb et al. 1978, Hadley 68 and Szarek 1981, Garcia-Pichel and Belnap 1996); therefore, these communities might not be 69 given the full consideration by ecologists that they deserve. This is unfortunate because not only Two barriers to parasite transmission in deserts are immediately apparent: 1) low host 79 density (e.g. Munger et al. 1983 , Polis 1991 , Krecek et al. 1995 , and 2) harsh abiotic conditions 80 (e.g. Dobson 1989). Thus, the contact rate between infective stages and potential hosts becomes 81 increasingly small as: 1) low host density means that the likelihood of host to host or host to 82 vector decreases, and 2) survival of infective stages in the environment likewise decreases as 83 abiotic conditions become increasingly inhospitable. However, given these two sets of 84 conditions, deserts can act as a type of natural laboratory in which to gather test predictions 85 related to key aspects of parasite transmission. As reviewed by Dobson (1989) , transmission can 86 be considered a "birth" in the ecological sense as it represents parasite recruitment while 87 virulence can similarly be considered a source of mortality. The basic reproductive rate (R 0 ) of a 88 parasite, or any pathogen, represents its reproductive success and is directly influenced by the 89 rate of new infections, the mortality rate of infected hosts, and the mortality rate of infective 90 stages (Box 1). Thus, the means by which a parasite can increase its reproductive success are to: 91 1) increase transmission rate, 2) decrease host mortality rate, or 3) decrease the mortality rate of 92 infective stages. Given that host density is low in deserts and their harsh abiotic conditions make However, the availability of susceptible hosts can greatly modify optimal virulence levels in eco-107 evolutionary models. This is because as the number of susceptible hosts in the population 108 increases, parasite fitness increases due to increased infectiousness; however, the fitness cost of One approach to improve empirical support for virulence predictions is to take advantage breadth hypothesis postulates that this pattern is due, at least in part, to interspecific variation in 186 niche breadth; thus, a species that can exploit a variety of habitats and resources can attain a 187 broad distribution and high local density (Brown 1984 (Brown , 1995 . short, this monogenean uses seasonal host mating aggregations to increase its likelihood of 232 success (Tinsley 1990 ). The life cycle of P. americanus is unique among helminths in that the 233 larvae, which infect the host via its nares, migrate to the bladder where adults mature and start 234 producing offspring that are retained in utero until the next rainy season (Tinsley and Earle 235 1983). Indeed, P. americanus can release its entire annual reproductive output within a seven-236 hour period that corresponds to host spawning (Tinsley 1990 ). Male toads, which typically enter 237 multiple spawning assemblies, exhibit nearly 100% prevalence and a mean intensity of over 100 However, since small lizards that do not exclusively feed on termites can also host these 277 encysted juveniles (Jones 1995), upward incorporation seems a logical scenario in this case. As there is a relative paucity of information concerning parasite virulence, specificity, 354 and life cycle evolution in arid environments, we also have comparatively little information with 355 which to identify potential emerging infectious diseases in these areas. These emerging 356 infections are not only of concern to dryland human population but to other regions as well. 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