Thesis Examination - MSc Student Jesse L. Rodgers

Wed. Aug. 15 10:00 AM - Wed. Aug. 15 12:00 PM
Location: 1RC028 (Graduate Studies Boardroom, 1st Floor of Richardson College)

The Effects of Livestock Grazing on Vegetation and Lepidopterans in Endangered Alvar Sites in Manitoba’s Interlake


Alvar is a globally rare type of ecosystem characterized by open, flat, windy terrain which typically experience dry summers and frozen winters, and by incomplete vegetation cover dominated by sedges, grasses, and herbs with patches of exposed calcareous pavement. Alvars often contain unique limestone land formations, and are hot spots for biodiversity, containing species rich plant and animal communities. This ecosystem supports a mixture of tall grass prairie and boreal forest species that are not found together in any other type of eco-system and therefore alvars make a unique contribution to the biodiversity of Manitoba. Alvars have a restricted geographical distribution and are only found in the temperate boreal regions of northern Europe and North America.

Several sites in the southern Interlake area of Manitoba contain areas identified by the Manitoba Alvar Initiative (MAI) as true alvars. The Interlake region of Manitoba where alvars occur is sparsely populated and the dominant economic land uses are mining and agriculture. Livestock production is prevalent especially in areas with exposed rock surfaces and thin soils that are not suitable for crop agriculture, and as such many alvars in Manitoba are grazed. Alvar makes a unique contribution to the biodiversity of the Interlake area and little is known about the extent and quality of the various alvar sites in the Interlake, or the impacts of anthropogenic activities such as livestock grazing on alvar diversity.

Disturbance including domestic grazing may be necessary to maintain diverse alvar plant communities. In European alvars livestock have been used to remove encroaching shrubs and young trees, which pose a threat to the biotic diversity of alvars. Where alvar vegetation communities are subjected to minimal grazing pressure or no grazing and the absence of fire, the alvar may experience a progression in successional stages eventually resulting in a forested ecosystem. However, intense grazing can cause damage to desired alvar plant species, increased frequency of invasive species, drought susceptibility, nutrient loading, and soil compaction. Despite the importance of grazing to the maintenance of alvars, the effects of grazing in Manitoba’s alvar sites have not been thoroughly studied in the province, and there is a need to determine the effects of grazing on these fragile ecosystems. Changes to alvar soils and plant responses to grazing may lead to impacts on the butterfly and moth communities (Lepidoptera), if larval food or adult nectar resource plants are affected.

Indicator species of plants and animals are useful as predictors when comparing the effects of disturbance on ecosystems. Bioindicator studies using indicator species to reflect the impacts of disturbance can be used to determine if natural areas are being damaged, areas that most need protection and rehabilitation, or indicate if a particular land management strategy is effective. In this study, plants and Lepidopterans were used as indicators of alvar health between grazed and ungrazed areas. Lepidoptera and plants have a long evolutionary relationshipsand that represents several trophic levels in alvar ecosystems. Changes in Lepidopteran diversity may signal future ecological changes and indicate areas sensitive to anthropogenic habitat disturbance.

The process to recover degraded alvars is often long and it is important to focus conservation efforts on preserving the remaining areas and enlarging existing habitat fragments. Maintenance of biodiversity is important for alvars and indentification and monitoring bioindicators in alvars may be useful as a predictive method to reduce future loss of diversity. I hypothesized that there would be differences in environmental conditions, and in plant and Lepidopteran diversity between between grazed and ungrazed treatment sites, and that the effects of grazing would result in different species of plants, butterflies, and moths associated with these two treatments. The intention of this research is to help policy makers and land managers determine which alvars should be given priority for protection and better understand the effects of livestock grazing on plants and Lepidopterans Manitoba’s alvars.

Assessment of soil and structural variables of the alvar sites showed that soils in grazed sites were significantly more compacted than soils in ungrazed sites, with significantly higher levels of soil nitrate, which may be linked to lower plant species richness in grazed alvar sites. The sodium concentration was also significantly higher in grazed sites compared to ungrazed sites with higher levels of sodium, which may lead to soil salinization and effects on the growth of certain plant species.
Plant species richness was significantly higher in the ungrazed sites, likely as a result of encroaching forest species. Ungrazed sites had significantly higher numbers of individual trees and trees with larger diameter at breast height. Ungrazed sites supported a variety of shade-tolerant plant species that were less prevalent in the grazed sites. Invasive grasses, and shade intolerant/grazing tolerant plant species were also found in grazed sites. There was considerable variation between grazed alvar sites in plant community composition on a landscape level. Ungrazed sites were more uniform in plant species and several indicator species for each treatment types were identified.

There was less of a trend in associations of specific butterfly or moth species with either grazed or ungrazed sites although some species could potentially be used as indicator species in future studies. Rarefied moth species richness however was significantly higher in the ungrazed sites. The butterflies appeared to be more affected than moths when linked to the presence of their larval host plants when comparing grazed and ungrazed alvars. Feeding guild analysis of moths indicated that there are a higher proportion of Tree and Shrub feeders, and Generalist species in ungrazed sites, while grazed sites had higher proportions of Shrub/Ground specialist species.

If alvars are not managed appropriately they will be subject to encroachment by shrubs and trees, and if they are managed too intensely their fragile soils may be damaged and the alvar community may not recover. The overall grazing pressure on the alvars in this study was considered to be on the low end of the scale of grazing intensity. While effects of grazing were found for some plants and Lepidopterans, the best management strategy for the short-term is to ensure future grazing is maintained at a low intensity, perhaps later in the season to maximize Lepidopteran access to nectar scources and to ensure rotations are kept relatively short. This strategy is useful for the maintenance and enlargement of existing alvar fragments, allowing livestock to more uniformly remove encroaching plants from alvars, encouraging the establishment of native prairie plants, and allowing the plant community time to regenerate while not being actively grazed. All grazed alvars should be subject to regular soil tests to monitor the levels of compaction, nitrate and sodium, as these variables may be detrimental to the plant community when in excess, and may have indirect effects on animals as well.