Diversity of Toxoplasma gondii strains shaped by commensal communities of small mammals

Commensal rodent species are key reservoirs for Toxoplasma gondii in the domestic environment. In rodents, different T. gondii strains show variable patterns of virulence according to host species. Toxoplasma gondii strains causing non-lethal chronic infections in local hosts will be more likely to persist in a given environment, but few studies have addressed the possible role of these interactions in shaping the T. gondii population structure. In addition, the absence of validated techniques for upstream detection of T. gondii chronic infection in wild rodents hinders exploration of this issue under natural conditions. In this study, we took advantage of an extensive survey of commensal small mammals in three coastal localities of Senegal, with a species assemblage constituted of both native African species and invasive species. We tested 828 individuals for T. gondii chronic infection using the modified agglutination test for antibody detection in serum samples and a quantitative PCR assay for detection of T. gondii DNA in brain samples. The infecting T. gondii strains were genotyped whenever possible by the analysis of 15 microsatellite markers. We found (i) a very poor concordance between molecular detection and serology in the invasive house mouse, (ii) significantly different levels of prevalence by species and (iii) the autochthonous T. gondii Africa 1 lineage strains, which are lethal for laboratory mice, only in the native African species of commensal small mammals. Overall, this study highlights the need to reconsider the use of MAT serology in natural populations of house mice and provides the first known data about T. gondii genetic diversity in invasive and native species of small mammals from Africa. In light of these results, we discuss the role of invasive and native species, with their variable adaptations to different T. gondii strains, in shaping the spatial structure of T. gondii genetic diversity in Africa.     

Shifts from native to invasive small mammals across gradients from tropical forest to urban habitat in Borneo

Urbanization has paved the way for the spread of commensal rodents at global scale. However, it is largely unknown how these species use tropical anthropogenic landscapes originally covered with forests and inhabited by diverse small mammal assemblages. We surveyed non-flying small mammals in various urban and suburban habitat types and adjacent forest in the tropical town of Kota Kinabalu in Borneo. We used occupancy and polynomial regression models to determine variation in species occurrences along gradients of land-use intensity. Müller’s sundamys (Sundamys muelleri) was the only native small mammal species found in urban and suburban landscapes with a continuous decrease in occurrence probability from forests to urban habitats. The invasive Asian black rat (Rattus rattus species complex) and the invasive Asian house shrew (Suncus murinus) had the highest occurrence probabilities in habitats of intermediate land-use intensity, but Asian black rats are also likely to occasionally invade forested habitats and occupied urban habitats in sympatry with the Norway rat (Rattus norvegicus). In urban and suburban habitats, fallow land possibly favoured the occurrence of S. muelleri and S. murinus. Other native small mammal species (Muridae, Sciuridae, Tupaiidae) were found only in forested areas. Our study shows that native small mammals found in forest are largely replaced by invasive species in urban and suburban habitats. Due to their occurrence in habitats of various land use intensities, S. muelleri and R. rattus comprise central links between forest wildlife and urban species, an association that is important to consider in studies of parasite and disease transmission dynamics.     

Toxoplasmosis seroprevalence in urban rodents: a survey in Niamey,Niger

A serological survey of Toxoplasma gondii was conducted on 766 domestic and peridomestic rodents from 46 trapping sites throughout the city of Niamey, Niger. A low seroprevalence was found over the whole town with only 1.96% of the rodents found seropositive. However, differences between species were important, ranging from less than 2% in truly commensal Mastomys natalensis, Rattus rattus and Mus musculus, while garden-associated Arvicanthis niloticus displayed 9.1% of seropositive individuals. This is in line with previous studies on tropical rodents - that we reviewed here - which altogether show that Toxoplasma seroprevalence in rodent is highly variable, depending on many factors such as locality and/or species. Moreover, although we were not able to decipher statistically between habitat or species effect, such a contrast between Nile grass rats and the other rodent species points towards a potentially important role of environmental toxoplasmic infection. This would deserve to be further scrutinised since intra-city irrigated cultures are extending in Niamey, thus potentially increasing Toxoplasma circulation in this yet semi-arid region. As far as we are aware of, our study is one of the rare surveys of its kind performed in Sub-Saharan Africa and the first one ever conducted in the Sahel.     

Dietary niche differentiation among three species of invasive rodents (Rattus rattus, R. exulans, Mus musculus)

The diets of sympatric rodents partially define their realized niches. Identifying items in stomachs of introduced rodents helps determine rodents’ trophic positions and species most at risk of consumption. In the Hawaiian Islands, which lacked rodents prior to human arrival, three rodents (Rattus rattus or black rat, R. exulans or Pacific rat, Mus musculus or house mouse) commonly coexist in native habitats where they consume a wide range of plants and animals. These three rodent species were trapped in montane forest for 2.5 years; their stomach contents were analyzed to determine short-term diets (n = 12–95 indiv. per species), and isotopic fractions of δ¹⁵N and δ¹³C in their bone collagen were analyzed to further estimate their trophic positions (n = 11–20 indiv. per species). For all three species, >75 % of individuals had plants and >90 % had arthropods in their stomachs, and significant differences in mean relative abundances were found for food items in stomachs among all three rodents. Rodents may be dispersing some native and non-native seeds, including the highly invasive Clidemia hirta. Most identifiable arthropods in rodent stomachs were non-native, and no stomachs contained birds, snails, or lizards. The δ¹⁵N and δ¹³C signatures were consistent with trophic feeding differences revealed from stomach contents. Dietary niche differentiation by coexisting rodent species is evident in this forest, with Pacific rats being intermediate between the mostly carnivorous house mouse and the mostly herbivorous black rat; such findings can help forecast rodent impacts and direct management efforts in ecosystems where these invasive animals coexist.     

Shifts in an invasive rodent community favoring Black rats (Rattus rattus) following restoration of native forest

One potential, unintended ecological consequence accompanying forest restoration is a shift in invasive animal populations, potentially impacting conservation targets. Eighteen years after initial restoration (ungulate exclusion, invasive plant control, and out planting native species) at a 4 ha site on Maui, Hawai'i, we compared invasive rodent communities in a restored native dry forest and adjacent non‐native grassland. Quarterly for 1 year, we trapped rodents on three replicate transects (107 rodent traps) in each habitat type for three consecutive nights. While repeated trapping may have reduced the rat (Black rat, Rattus rattus) population in the forest, it did not appear to reduce the mouse (House mouse, Mus musculus) population in the grassland. In unrestored grassland, mouse captures outnumbered rat captures 220:1, with mice averaging 54.9 indiv./night versus rats averaging 0.25 indiv./night. In contrast, in restored native forest, rat captures outnumbered mouse captures by nearly 5:1, averaging 9.0 indiv./night versus 1.9 indiv./night for mice. Therefore, relatively recent native forest restoration increased Black rat abundance and also increased their total biomass in the restored ecosystem 36‐fold while reducing House mouse biomass 35‐fold. Such a community shift is worrisome because Black rats pose a much greater threat than do mice to native birds and plants, perhaps especially to large‐seeded tree species. Land managers should be aware that forest restoration (i.e. converting grassland to native forest) can invoke shifts in invasive rodent populations, potentially favoring Black rats. Without intervention, this shift may pose risks for intended conservation targets and modify future forest restoration trajectories.     

Observations on commensal rats and their status to plague in Bombay

The distribution pattern of various types of commensal rodents in Bombay city reveals that Bandicota bengalensis constitutes the predominant commensal rodent species followed by R. rattus and Rattus norvegicus. Apart from these three types, Bandicota indica, M. musculus and an insectivore (Suncus murinus) are the three species of commensal small mammals that are frequently encountered in or near human habitations. These small mammals are prevalent throughout the year and their percentage distribution varies very little during different months of the year. None of the rodent species examined during the years 1976-85 revealed presence of Y. pestis infection by bacteriological or serological methods. From these findings, it could be concluded that in the city of Bombay a focus of zoonotic plague infection does not exist.     

Urban Market Gardening and Rodent-Borne Pathogenic Leptospira in Arid Zones: A Case Study in Niamey,Niger

Leptospirosis essentially affects human following contact with rodent urine-contaminated water. As such, it was mainly found associated with rice culture, recreational activities and flooding. This is also the reason why it has mainly been investigated in temperate as well as warm and humid regions, while arid zones have been only very occasionally monitored for this disease. In particular, data for West African countries are extremely scarce. Here, we took advantage of an extensive survey of urban rodents in Niamey, Niger, in order to look for rodent-borne pathogenic Leptospira species presence and distribution across the city. To do so, we used high throughput bacterial 16S-based metabarcoding, lipL32 gene-targeting RT-PCR, rrs gene sequencing and VNTR typing as well as GIS-based multivariate spatial analysis. Our results show that leptospires seem absent from the core city where usual Leptospira reservoir rodent species (namely R. rattus and M. natalensis) are yet abundant. On the contrary, L. kirschneri was detected in Arvicanthis niloticus and Cricetomys gambianus, two rodent species that are restricted to irrigated cultures within the city. Moreover, the VNTR profiles showed that rodent-borne leptospires in Niamey belong to previously undescribed serovars. Altogether, our study points towards the importance of market gardening in maintain and circulation of leptospirosis within Sahelian cities. In Africa, irrigated urban agriculture constitutes a pivotal source of food supply, especially in the context of the ongoing extensive urbanization of the continent. With this in mind, we speculate that leptospirosis may represent a zoonotic disease of concern also in arid regions that would deserve to be more rigorously surveyed, especially in urban agricultural settings.     

Do seeds from invasive bromes experience less granivory than seeds from native congeners in the Great Basin Desert?

In part, the enemy release hypothesis of plant invasion posits that generalist herbivores in the non-native ranges of invasive plants will prefer native plants to exotic invaders. However, the extent to which this occurs in natural communities is unclear. Here, I examined the foraging preferences of an important guild of generalist herbivores—granivorous rodents—with respect to seeds from a suite of native and invasive Bromus (“brome”) species at five study sites distributed across?≈?80,000 km² of the Great Basin Desert, USA. By examining only congeners, I accounted for a potentially large source of interspecific variation (phylogenetic relatedness). In general, granivorous rodents removed seeds from native bromes at a 23% higher rate than seeds from invasive bromes, suggesting a preference for native species. This preference was not entirely explained by seed size, and patterns of seed removal were consistent across study sites. These findings suggest that invasive bromes in the Great Basin might experience less rodent granivory than native congeners, which is consistent with a key prediction derived from the enemy release hypothesis.     

Evolutionary history of rat‐borne Bartonella: the importance of commensal rats in the dissemination of bacterial infections globally

Emerging pathogens that originate from invasive species have caused numerous significant epidemics. Some bacteria of genus Bartonella are rodent‐borne pathogens that can cause disease in humans and animals alike. We analyzed gltA sequences of 191 strains of rat‐associated bartonellae from 29 rodent species from 17 countries to test the hypotheses that this bacterial complex evolved and diversified in Southeast Asia before being disseminated by commensal rats Rattus rattus (black rat) and Rattus norvegicus (Norway rat) to other parts of the globe. The analysis suggests that there have been numerous dispersal events within Asia and introductions from Asia to other regions, with six major clades containing Southeast Asian isolates that appear to have been dispersed globally. Phylogeographic analyses support the hypotheses that these bacteria originated in Southeast Asia and commensal rodents (R. rattus and R. norvegicus) play key roles in the evolution and dissemination of this Bartonella complex throughout the world.     

Helminth Diversity in Synanthropic Rodents from an Urban Ecosystem

Richness and diversity of parasites depend on a set of interrelated factors related to the characteristics of the host, the environment and the parasites itself. In the City of Buenos Aires, rodent communities vary according to landscape structure. The goal of this paper was to study the variations of helminth richness and diversity among invasive rodent species in different landscape units of the City of Buenos Aires. 73% of the rodents were parasitized with at least one of the 10 identified helminth species. Each rodent species presented its own characteristics in terms of richness, diversity and helminth composition, keeping these characteristics still occupying more than one landscape unit. The infracommunities with greater diversity corresponded to R. norvegicus due to its high values of parasitic richness, proportion of infected hosts and parasite prevalence. Instead, R. rattus and M. musculus infracommunities had lower diversity since a high percentage of them presented a unique helminth species. Within the city, the inhabitants of shantytowns would be the most exposed to zoonotic diseases transmitted by rodents due to high abundance of rodents harboring a high parasite load, including species like Hymenolepis nana and H. diminuta, recognized worldwide from a zoonotic aspect.     

Unexpectedly high diversity of trypanosomes in small sub-Saharan mammals

The extremely species-rich genus Trypanosoma has recently been divided into 16 subgenera, most of which show fairly high host specificity, including the subgenus Herpetosoma parasitizing mainly rodents. Although most Herpetosoma spp. are highly host-specific, the best-known representative, Trypanosoma lewisi, has a cosmopolitan distribution and low host specificity. The present study investigates the general diversity of small mammal trypanosomes in East and Central Africa and the penetration of invasive T. lewisi into communities of native rodents. An extensive study of blood and tissue samples from Afrotropical micromammals (1528 rodents, 135 shrews, and five sengis belonging to 37 genera and 133 species) captured in the Central African Republic, Ethiopia, Kenya, Malawi, Mozambique, Tanzania, and Zambia revealed 187 (11.2%) trypanosome-positive individuals. The prevalence of trypanosomes in host genera ranged from 2.1% in Aethomys to 37.1% in Lemniscomys. The only previously known trypanosome detected in our dataset was T. lewisi, newly found in Ethiopia, Kenya, and Tanzania in a wide range of native rodent hosts. Besides T. lewisi, 18S rRNA sequencing revealed 48 additional unique Herpetosoma genotypes representing at least 15 putative new species, which doubles the known sequence-based diversity of this subgenus, and approaches the true species richness in the study area. The other two genotypes represent two new species belonging to the subgenera Ornithotrypanum and Squamatrypanum. The trypanosomes of white-toothed shrews (Crocidura spp.) form a new phylogroup of Herpetosoma, unrelated to flagellates previously detected in insectivores. With 13 documented species, Ethiopia was the richest region for trypanosome diversity, which corresponds to the very diverse environments and generally high biodiversity of this country. We conclude that besides T. lewisi, the subgenus Herpetosoma is highly host-specific (e.g., species parasitizing the rodent genera Acomys and Gerbilliscus). Furthermore, several newly detected trypanosome species are specific to their endemic hosts, such as brush-furred mice (Lophuromys), dormice (Graphiurus), and white-toothed shrews (Crocidura).     

Effect of an Invasive Plant and Moonlight on Rodent Foraging Behavior in a Coastal Dune Ecosystem

Understanding how invasive plants may alter predator avoidance behaviors is important for granivorous rodents because their foraging can trigger ripple effects in trophic webs. Previous research has shown that European beach grass Ammophila arenaria, an invasive species in coastal California, affects the predation of other seeds by the rodents Microtus californicus, Peromyscus maniculatus, and Reithrodontomys megalotis. This may be due to lower perceived predation risk by rodents foraging in close proximity to the cover provided by Ammophila, but this mechanism has not yet been tested. We examined the perceived predation risk of rodents by measuring the ‘giving up density’ of food left behind in experimental patches of food in areas with and without abundant cover from Ammophila and under varying amount of moonlight. We found strong evidence that giving up density was lower in the thick uniform vegetation on Ammophila-dominated habitat than it was in the more sparsely and diversely vegetated restored habitat. There was also evidence that moonlight affected giving up density and that it mediated the effects of habitat, although with our design we were unable to distinguish the effects of lunar illumination and moon phase. Our findings illustrate that foraging rodents, well known to be risk-averse during moonlit nights, are also affected by the presence of an invasive plant. This result has implications for granivory and perhaps plant demography in invaded and restored coastal habitats. Future research in this system should work to unravel the complex trophic links formed by a non-native invasive plant (i.e., Ammophila) providing cover favored by native rodents, which likely forage on and potentially limit the recruitment of native and non-native plants, some of which have ecosystem consequences of their own.     

Patterns of Privet: Urbanizing Watersheds,Invasive Ligustrum sinense,and Performance of Native Plant Species in Piedmont Floodplain Forests

Floodplain forests in rapidly changing landscapes with increased urbanization may reshape habitat conditions to the detriment of native biota and favor invasive species. We assessed whether introduced Ligustrum sinense distribution and abundance are causally linked to urbanization and whether mechanisms that promote L. sinense cause the demise of native plant species. We surveyed vegetation in 12 independent floodplain forests along an urban to rural gradient in South Carolina, USA. We then used a seedling transplant experiment in nine watersheds to assess how increased urban development affects survival and growth of L. sinense and three native species over two growing seasons. Urban development ranged from 1 to 45% and L. sinense cover was positively associated with development. However, in our transplant experiment, growth and survival of L. sinense did not differ significantly among watersheds. Native species were able to survive at all sites, but performance varied greatly among sites and species but not as a function of urban development. Our results suggest that although L. sinense invasion and urbanization are related (likely due to proximity of propagules from urban sources), the demise of native species cannot be explained by increased urbanization or changes in edaphic conditions and points to L. sinense as an agent of change in our floodplain forests. These results indicate that all floodplain forests are at risk of invasion if propagules arrive and that land managers need to be vigilant against new species introductions regardless of the proximity to urban areas.     

Existing and emerging high impact invasive species are characterized by higher functional responses than natives

Predicting ecological impacts of invasive species and identifying potentially damaging future invaders are research priorities. Since damage by invaders is characterized by their depletion of resources, comparisons of the ‘functional response’ (FR; resource uptake rate as a function of resource density) of invaders and natives might predict invader impact. We tested this by comparing FRs of the ecologically damaging ‘world''s worst’ invasive fish, the largemouth bass (Micropterus salmoides), with a native equivalent, the Cape kurper (Sandelia capensis), and an emerging invader, the sharptooth catfish (Clarias gariepinus), with the native river goby (Glossogobius callidus), in South Africa, a global invasion hotspot. Using tadpoles (Hyperolius marmoratus) as prey, we found that the invaders consumed significantly more than natives. Attack rates at low prey densities within invader/native comparisons reflected similarities in predatory strategies; however, both invasive species displayed significantly higher Type II FRs than the native comparators. This was driven by significantly lower prey handling times by invaders, resulting in significantly higher maximum feeding rates. The higher FRs of these invaders are thus congruent with, and can predict, their impacts on native communities. Comparative FRs may be a rapid and reliable method for predicting ecological impacts of emerging and future invasive species.     

Dynamic species distribution models reveal spatiotemporal habitat shifts in native range‐expanding versus non‐native invasive birds in an urban area

Urbanisation as a major driver of changes leads to the extinction of some species while others increase in abundance, especially non‐native species. Spatiotemporal distribution patterns of these successful species are likely to be shaped by their response and tolerance to urban features. This study assesses the anthropo‐ecological requirements of two co‐occurring bird species, the native range‐shifting jackdaw Corvus monedula and the non‐native invasive ring‐necked parakeet Psittacula krameri. We built yearly models over an eight‐year period using an ensemble modelling approach assessing response differences through time and between species. Predictors describing human‐made structures, socio‐ecological proxies and resources availability were extracted from temporally coincident databases. Dispersal and habitat constraints were implemented in final models to provide more realistic forecasts of species future distributions. Ensemble models evaluated with a random partition of the training dataset showed a higher accuracy than those evaluated with an independent dataset from another time period. Our results highlight temporal variations in the relative importance of predictors for both studied species. Single‐season occurrence data may thus be insufficient to characterize species ecological requirements. The ring‐necked parakeet and the jackdaw showed different responses to urban features. Jackdaws preferred the more urbanized part of the city while the distribution of parakeets was strongly positively associated with the density of exotic ornamental trees. We concluded that ring‐necked parakeet range expansion is likely to be driven by its effective ability to exploit urban resources which native species do not or under exploit, suggesting an open window of foraging opportunities. However, the jackdaw may be misled by a high cavity availability and a large amount of low‐quality anthropogenic food in the urban core. We suggest that dynamic SDMs are a critical tool not only to forecast the future expansion of invasive species but also for a better understanding of processes driving urban biodiversity persistence.     

Rodent Diversity in a Highly Degraded Tropical Landscape: Hong Kong, South China

The diverse ecological roles played by different rodent species mean that the loss of some species and superabundance of others could potentially influence a wide range of ecological processes. Hong Kong (22° N, 114° E), with seven million people in a land area of 1100 km², could be considered a `worst case scenario' for the survival of mammalian diversity. Existing information on rodents in Hong Kong was compiled from previous published and unpublished studies, and additional trapping was conducted at 17 non-urban sites. The rodent fauna of modern Hong Kong consists of eight species of rats and mice (Bandicota indica, Mus caroli, M. musculus, Niviventer fulvescens, Rattus norvegicus, R. rattus, R. tanezumi, R. sikkimensis: Muridae), one porcupine (Hystrix brachyura: Hystricidae), and one recently introduced tree squirrel (Callosciurus erythraeus: Sciuridae). Six of the murids are urban or agricultural commensals, so only the porcupine and two murids, N. fulvescens and R. sikkimensis, are likely survivors of Hong Kong's pre-deforestation native rodent fauna. The two murids co-dominate in forest and shrubland, but can also move through grassland, which has probably enabled their survival through repeated cycles of fragmentation and regrowth. Additional forest rodents that may have inhabited Hong Kong in the past are tentatively identified from information on their recent distributions in the region. One possible ecological consequence of Hong Kong's depleted rodent fauna is a shift in the balance between seed predation and seed dispersal, in favor of the former.     

Movement Patterns of Small Rodents in Lassa Fever-Endemic Villages in Guinea

The Natal multimammate mouse (Mastomys natalensis) is the reservoir host of Lassa arenavirus, the etiological agent of Lassa fever in humans. Because there exists no vaccine for human use, rodent control and adjusting human behavior are currently considered to be the only options for Lassa fever control. In order to develop efficient rodent control programs, more information about the host’s ecology is needed. In this study, we investigated the spatial behavior of M. natalensis and other small rodents in two capture-mark-recapture and four dyed bait (Rhodamine B) experiments in Lassa fever-endemic villages in Upper Guinea. During the capture-mark-recapture studies, 23% of the recaptured M. natalensis moved between the houses and proximate fields. While M. natalensis was found over the entire study grid (2 ha), other rodent species (Praomys daltoni, Praomys rostratus, Lemniscomys striatus, Mus spp.) were mostly trapped in the surrounding fields. Distances between recapture occasions never exceeded 100 m for all rodent species. During the dyed bait experiments, 11% of M. natalensis and 41% of P. daltoni moved from the fields to houses. We conclude that commensal M. natalensis easily moves between houses and proximate fields in Guinea. We therefore consider occasional domestic rodent elimination to be an unsustainable approach to reduce Lassa virus transmission risk to humans, as M. natalensis is likely to reinvade houses quickly from fields in which rodents are not controlled. A combination of permanent rodent elimination with other control strategies (e.g., make houses rodent proof or attract predators) could be more effective for Lassa fever control, but must be further investigated.     

Native Rodent Species Are Unlikely Sources of Infection for Leishmania (Viannia) braziliensis along the Transoceanic Highway in Madre de Dios,Peru

An estimated 2.3 million disability-adjusted life years are lost globally from leishmaniasis. In Peru''s Amazon region, the department of Madre de Dios (MDD) rises above the rest of the country in terms of the annual incidence rates of human leishmaniasis. Leishmania (Viannia) braziliensis is the species most frequently responsible for the form of disease that results in tissue destruction of the nose and mouth. However, essentially nothing is known regarding the reservoirs of this vector-borne, zoonotic parasite in MDD. Wild rodents have been suspected, or proven, to be reservoirs of several Leishmania spp. in various ecosystems and countries. Additionally, people who live or work in forested terrain, especially those who are not regionally local and whose immune systems are thus naïve to the parasite, are at most risk for contracting L. (V.) braziliensis. Hence, the objective of this study was to collect tissues from wild rodents captured at several study sites along the Amazonian segment of the newly constructed Transoceanic Highway and to use molecular laboratory techniques to analyze samples for the presence of Leishmania parasites. Liver tissues were tested via polymerase chain reaction from a total of 217 rodents; bone marrow and skin biopsies (ear and tail) were also tested from a subset of these same animals. The most numerous rodent species captured and tested were Oligoryzomys microtis (40.7%), Hylaeamys perenensis (15.7%), and Proechimys spp. (12%). All samples were negative for Leishmania, implying that although incidental infections may occur, these abundant rodent species are unlikely to serve as primary reservoirs of L. (V.) braziliensis along the Transoceanic Highway in MDD. Therefore, although these rodent species may persist and even thrive in moderately altered landscapes, we did not find any evidence to suggest they pose a risk for L. (V.) braziliensis transmission to human inhabitants in this highly prevalent region.     

Potential Role of Masting by Introduced Bamboos in Deer Mice (Peromyscus maniculatus) Population Irruptions Holds Public Health Consequences

We hypothesized that the ongoing naturalization of frost/shade tolerant Asian bamboos in North America could cause environmental consequences involving introduced bamboos, native rodents and ultimately humans. More specifically, we asked whether the eventual masting by an abundant leptomorphic (“running”) bamboo within Pacific Northwest coniferous forests could produce a temporary surfeit of food capable of driving a population irruption of a common native seed predator, the deer mouse (Peromyscus maniculatus), a hantavirus carrier. Single-choice and cafeteria-style feeding trials were conducted for deer mice with seeds of two bamboo species (Bambusa distegia and Yushania brevipaniculata), wheat, Pinus ponderosa, and native mixed diets compared to rodent laboratory feed. Adult deer mice consumed bamboo seeds as readily as they consumed native seeds. In the cafeteria-style feeding trials, Y. brevipaniculata seeds were consumed at the same rate as native seeds but more frequently than wheat seeds or rodent laboratory feed. Females produced a median litter of 4 pups on a bamboo diet. Given the ability of deer mice to reproduce frequently whenever food is abundant, we employed our feeding trial results in a modified Rosenzweig-MacArthur consumer-resource model to project the population-level response of deer mice to a suddenly available/rapidly depleted supply of bamboo seeds. The simulations predict rodent population irruptions and declines similar to reported cycles involving Asian and South American rodents but unprecedented in deer mice. Following depletion of a mast seed supply, the incidence of Sin Nombre Virus (SNV) transmission to humans could subsequently rise with dispersal of the peridomestic deer mice into nearby human settlements seeking food.     

Invasion by Rattus rattus into native coastal forests of south‐eastern Australia: are native small mammals at risk?

The black rat, Rattus rattus, is an alien rodent in Australian ecosystems where niche overlap with native small mammals may lead to competition for resources and displacement of native species. In coastal habitats surrounding Jervis Bay in south‐eastern Australia, R. rattus co‐occurs with the native bush rat, Rattus fuscipes, and brown antechinus, Antechinus stuartii. Relative distributions and abundances, and fine‐scale space use suggest invasive and native rodents compete for use of space and habitat. Such competitive interactions were not evident between R. rattus and native A. stuartii, which was negatively influenced more by disturbance to habitat. Differences in rodent communities between spatially separate forests forming the northern and southern peninsulas of Jervis Bay potentially reflect symmetrical competition and differences in competitive outcomes. In southern forests, R. rattus was largely restricted to patches of disturbed forest associated with campgrounds. Competitive interference by native rodent populations inhabiting surrounding intact forests may have so far limited R. rattus colonization of these areas. In northern forests, R. rattus was the predominant rodent irrespective of disturbance, while populations of R. fuscipes were unusually low seemingly due to poor juvenile recruitment. Native individuals avoided areas frequented by adult R. rattus and given that species did not partition use of microhabitats, R. rattus most likely precluded R. fuscipes from suitable habitat and in doing so limited native populations. We discuss how natural disturbance of habitat and human activity have potentially facilitated successful invasion by R. rattus of the northern forests. Studies that manipulate rodent populations are required to support these interpretations of observed patterns.