The widespread distribution and niche breadth of Rhadinopsylla fleas: via dispersal routes of small rodents
Introduction:
In biogeography, mechanisms of physical and environmental barriers shape the distribution of species. According to Darwin, these barriers also play an important role in the geographical ranges of organisms (Darwin 1873). The geographic range of a species reflects the species ecological niche (Brown and Lomolino 1998). An ecological niche is defined as the resources required for a species to persist in an environment. Two factors influence the breadth of an ecological niche: abiotic and biotic conditions. Abiotic factors include the physical space a species occupies, the temperature, and seasonality, whereas biotic factors include food requirements and the interactions between species (Hutchinson 1959). A species' niche breadth relies heavily on its specialization because its ecological niche is shaped by interactions with other species.
The specialization of a species is its adaptation to a specific resource, function, or environment (Dictionary.com 2021). Species specialization influences the distribution of a species as it shapes the niche breadth of these organisms. A negative correlation has been discovered between the degree of specialization and the geographic range of a species (Krasnov et al. 2005). This correlation aligns with the niche breadth hypothesis which states that species able to tolerate a broad range of environmental conditions tend to have more suitable habitats to occupy, and therefore, have wider geographic ranges than species that tolerate only a narrow range of conditions (Brown 1984).
This specialized relationship is frequently demonstrated through parasite-host interactions. This relationship determines species distributions as the parasite and host often coevolve (Parasitic Relationships 2021). The interaction of parasites living on hosts and adapting to their environment is due to the parasite's dependency on the host for food. As a result, the distribution of hosts determines the geographic range of many species of parasites (Shenbrot et al. 2007). Consequently, the niche breadth of the parasites is also determined by the geographic ranges of hosts. As discussed above, specialization shapes niche breadth. In parasite-host relationships, host-specificity is a determinant in the livable distribution of the parasite. Host-specificity falls into three categories: host-specific, host-opportunistic, and opportunistic . Host-specific parasites are highly specific and thus they will often have identical tolerances and geographical ranges as a single host species as their livability depends on having access to that specific host. Host-opportunistic parasites can have several different host species and thus are often distributed across the geographic ranges of several hosts. Finally, opportunistic parasites can exploit many hosts and therefore achieve a geographically scattered abundance (Shenbrot et al. 2007). The parasite-host relationship of fleas and small rodents offers the ideal model of this relationship, although these interactions occur in many other organisms as well.
Due to the importance of host-specificity in the geographic ranges of organisms, we will be studying the parasite-host relationship of Rhadinopsylla subspecies and their small rodent hosts. Specifically, the Rhadinopsylla subspecies of R. rectofrontia, R. heiseri, and R. syriaca.
These subspecies of Rhadinopsylla are categorized as “nest fleas”; nest fleas typically remain under host shelter for their entire lifetime (Medvedev et al. 2020). Rhadinopsylla subspecies reside in the Holarctic region, which includes North America and Europe. This genus of fleas offers a model for testing the distribution of fleas as they disperse via parasitized rodents like ground squirrels, gerbils, voles, and mice across the Holarctic region (Medvedev et al. 2020). Although host-specificity varies among the Rhadinopsylla subspecies, most can feed on many small mammal hosts (Medvedev et al. 2020). The level of host-specificity within R. rectofrontia, R. heiseri, and R. syriaca will vary amongst each subspecies, but it also reveals the niche breadth and therefore, the geographic range of these fleas.
The parasite-host relationship between Rhadinopsylla subspecies and small rodents provides us with a better understanding of their distribution and geographic range. These interactions allow us to form two hypotheses for the pattern and mode of distribution for fleas.
First, we hypothesize that Rhadinopsylla rectofrontia, R. heiseri, and R. syriaca have similar fundamental niches and geographic ranges despite their global distribution. Second, we hypothesize that these subspecies formed similar fundamental niches and distribution via dispersal of their small rodent hosts.