Seasonal small mammal diversity, endocrine correlates, and diet behaviour in rodents across transformed and pristine ecosystems in the Magaliesberg

Abstract

Landscape transformations due to a growing human population lead to increased resource use, declines in natural habitat quality and increased human-wildlife conflicts across ecosystems. As a result, animal population dynamics, stress-related behaviour, and diet may be affected in unknown ways. Rodents can be ideal model species for monitoring ecosystem health. Their relatively small size allows them to use various niches that are inaccessible to larger animals. By investigating the effects of anthropogenic activities and changes in global climate on rodent species, we can assess the impacts these variables will have on wildlife over time. We can also determine the effects of these variables on species abundance, diversity, stress-related endocrine correlates, and diet preference at inter- and intra-specific levels. This study investigated the effects of anthropogenic activity and human-mediated land transformation on capture/recapture frequencies, species richness, and diversity of native small mammal community assemblages in the Magaliesberg Biosphere, North West province, South Africa. Additionally, the study aimed to establish minimally invasive methods for quantifying faecal glucocorticoid metabolites (fGCMs) in six southern African rodent species (Aethomys ineptus, Lemniscomys rosalia, Mastomys coucha, Micaelamys namaquensis, Otomys irroratus, and Steatomys pratensis) by evaluating five enzyme immunoassays (EIAs). Following this, the adrenocortical responses of dominant rodent indicator species to human-mediated practices and seasonal variation were monitored to determine their efficacy in informing on ecosystem health. Methods of keratinaceous (hair and nails) tissue sample collection and preparation methods for rodent diet stable isotope analysis (SIA) were also investigated. The least minimally invasive method was used to assess the diet preferences of two dominant rodent species across differently disturbed landscapes.

Five anthropogenically transformed land-use types were investigated: animal rehabilitation and ecotourism centre, agricultural farmstead, residential farmstead, mine-adjacent agricultural farmstead, and nature conservancy. Over three consecutive years, live traps were used to sample rodents during the dry and wet seasons. For validation of fGCM quantification methods, faecal samples were collected daily from enclosures of captive females and males and frozen immediately at -20°C. Biological and physiological stimuli were used, and fGCM concentrations were determined from samples collected before, during, and after each event. Following this, a suitable assay was found to assess the adrenocortical hormone responses of two dominant southern African murids, Lemniscomys rosalia and Mastomys coucha. Faecal samples were collected from traps of captured individuals and measured for fGCM concentrations using the previously validated assay. For diet analysis, hair and nail samples were prepared for SIA mass spectrometry, and carbon and nitrogen isotope ratios were compared.

My study found that capture/recapture frequencies differed significantly between sites and seasons, with the highest capture frequencies recorded at the agricultural and residential farmsteads. Species richness and diversity were highest at the residential and industrial-agricultural farmsteads, both of which experienced intermediate levels of anthropogenic disturbance throughout the sampling period. For quantifying fGCMs in rodents, my study identified the 5α-pregnane-3β,11β,21-triol-20-one EIA (detecting fGCMs with a 5α-3β, 11β-diol structure) as most suited in all six species assessed. Using this EIA, it was found that fGCM concentrations of M. coucha were significantly higher in the wet season compared to the dry. Land-use variation had no significant impact on either species, which supports reported synanthropic behaviour (to various extents) in both species. The reduced adrenocortical responses observed in these dominant species in human-transformed areas suggest that when monitoring fGCM concentrations, dominant species such as M. coucha and L. rosalia are not always an ideal indicator of anthropogenic disturbance.

Diet-related sampling methods were compared, and hair sample collection was found to be faster and more animal-friendly, resulting in a calmer demeanour and reduced anxious behaviour in animals compared to the effects of nail sample collection. Correlation analyses of hair and nails showed high similarity in δ13C and δ15N isotope ratios within four of the six species analysed. Hair clipping may be classified as the less invasive sampling method and can be used exclusively on rodents, as isotopic variation between hair and toenail samples is negligible.

Overall, the findings of the study lend support to the use of minimally invasive methods of data collection and a combination of both dominant and low-population rodent species as biological indicators of ecosystem health to ensure repeatable, long-term data collection and capturing of ecological responses from both generalist and specialist species to elucidate the impacts of anthropogenic activities on wildlife populations and their natural environments.