Groom et al, 2006 identified 7 major threats to biodiversity that often interact speeding up loss of biodiversity. Although landscape fragmentation and habitat loss have long been recognised as some of the major contributors that threaten global biodiversity there are other threatening processes such as the introduction of invasive species and disease that are also of concern. This essay will focus on the introduction of a disease hosting species that due to habitat fragmentation now threatens specialist forest species and is also responsible for a disease that can be fatal to humans. The review study is one conducted by Suzan (2008) and his researchers focusing on the loss of species diversity in small mammals and the higher densities of rodents hosting hantavirus in varying fragmented habitats throughout western Panama.
It is suggested that habitat fragmentation and destruction is responsible for the creation of simplified communities that are dominated by generalist species. Generalist species have a high adaptability to a range of habitats, whereas the specialist species are adapted to a narrowly defined habitat and are prone to become endangered or extinct due to destruction and fragmentation of their habitats (Suzan et al, 2008). Prior to undertaking the study Suzan and his team hypothesized that habitat fragmentation and loss of diversity due to human activities, such as deforestation and agriculture favours the spread of the hantavirus reservoirs and increases the risks of hantavirus outbreaks.
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Generalist species are often associated with the transmission of pathogens to humans, such as the parvovirus, rabies and this case hantavirus. Hantaviruses are transmitted to humans by inhaling of the virus from rodent urine, faeces, saliva or by direct contact such at bites (Tsai, 1987). Hantaviruses are known to cause hemorrhagic fever with renal syndrome in Asia and Europe and hantavirus pulmonary syndrome (HPS) in the America's (Suzan et al, 2008) whilst the number of cases reported of HPS is relatively low, mortality is considered quite high.
The study compared communities of small mammals in differing habitats with varying degrees of fragmentation. It included forests, forest edges and disturbed pasture areas across 6 Panamanian national parks and surrounding areas. At each park 3 types of habitats were chosen, pristine forest, edge areas where there was distinct transition between forest and disturbed habitat and anthropogenically disturbed areas. For each of the habitat types they compared small mammal communities, species composition, richness, evenness and relative abundance. In each sampling area a 10m x 10m grid was sampled with 100 live traps (8 x 8 x 2cm), with each trap baited the same. Sampling was undertaken over 3 consecutive nights during 2 sampling periods, June 2001 and November 2002.
The data was analysed in Annova to test for difference in species diversity and richness. Species richness, composition and diversity of the small mammals were also determined. The diversity was determined using the N2 of Hill's index (Hill, 1973) with evenness (Krebs index of evenness, 1999) calculated as the relative distribution of individuals among the species in the community. Cluster analyses for species distribution was performed based on the Sorensen index with the Jaccard similarity index (Jaccard, 1912) used to assess the similarity of species composition with habitat types.
All the trapped small mammals were identified by species, sex and weight, with a total of 237 individual's sampled belonging to 22 species, consisting of 3 marsupial species and 19 rodent species. The study found that 13 species (59%) occurred in the forest and forest edges with a further 3 species (13%) occurring in the edge and disturbed pasture habitats. This included the Oligoryzomys flavescens and Zygodontomys brevicauda, hosts for the Choclo and Calabazo viruses, respectively. An introduced species Mus musculus was exclusive to the disturbed habitats. The forest habitats included the more specialist species with the more generalist species found in the forest edges and disturbed habitats.
The species compositions from each of the habitat types clearly showed differences in mammal composition and abundances in each of the habitat areas. The forest habitats hosted higher species diversity with several specialist forest species present. Generalist species were common throughout the edges and disturbed areas, although the overall abundance of species was greatest in the forest areas, with the edge areas having the greatest species diversity. The results showed that the edge and disturbed areas had significantly higher abundances of the hantavirus hosts, O. flavescens (Choclo virus) and Z. brevicauda (Calabazo virus). Species richness and diversity was lowest in the disturbed areas, than in the forest and edge areas.
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Whilst it is unclear what theory the study was based on, given that the study focuses mainly on the increasing distribution of the hantavirus host species it is thought that the theory on metapopulation dynamics could be applied to the study. This would benefit the study if it was applied to the hantavirus hosts to model the potential expansion of the species. This would help in determining the overall effect on the species that inhabit the edge and forest areas. One such model was devised by Hess (1996) with his initial model built around Levin's (1969) metapopulation model. Hess combined his mathematical epidemiology model with a metapopulation model and showed that highly contagious diseases of moderate severity can spread widely and in turn increasing the probability of metapopulation extinction. Another similar study using a metapopulation model was undertaken in Kanagawa Prefecture, Japan to predict the range and expansions of the feral raccoon (Procyon lotor) (Koike et al, 2006). Whilst modelling results may not always be accurate they can highlight data weaknesses, assist in general thinking and help provide internally consistent arguments, leading to better developed models and improved predictions (Burgman et al, 1993, Lindenmayer and Burgman, 2005).
The study demonstrates that fragmented disturbed habitats are associated with a lower diversity of small forest mammals and have higher densities of rodent populations that are host to potentially fatal hantaviruses. It also highlighted that specialist forest species that tend to only survive in a narrowly defined habitat are prone to endangerment or extinction due to continual habitat destruction from deforestation and urban spread. The study also showed that generalist species are able to survive in small forest areas and in human dominated environments. It is possible that with the increased abundances of generalist species there can be other ecological consequences such as the endangerment of forest fauna and the alteration of food chains.
This review clearly highlights that more studies are needed to help better understand the effects of fragmentation and the spread of infectious diseases in varying geographical regions and habitats, especially since the conversion of forested areas for agriculture and human activities remains high. Given that this is occurring it is expected that there will be further increases into hantavirus outbreaks and other potentially infectious diseases. Often modelling centred on the movement among species populations and reducing the risk of metapopulation extinctions (Hanski, 1989), however this study highlights the need for more models to focus on epidemiology and the spread of disease which also causes a loss of biodiversity as well as having severe impacts to human health and wellbeing. This seems particularly significant given the recent outbreaks of bird and swine flu's throughout the world.