Oribatid mite communities after ecosystem disturbance in Alberta

Many northern Alberta soils have a thick forest floor that houses the majority of soil biogeochemical processes and biological interactions. Microarthropods dominate the faunal communities in these soils, and oribatid mites are the key detritivores that initiate litter decomposition and maintain forest floor structure. Soil disturbance is becoming more prevalent across Alberta and may threaten the long-term sustainability of the forest floor as both a nutrient reservoir and a habitat for oribatid mite communities. The objective of this research was to characterize oribatid mite abundance, richness, diversity, and species composition after two growing sources of soil disturbance in Alberta: oil sands mining and non-native earthworm invasion. Bitumen extraction in the Athabasca oil sands region has disrupted over 800 km2 of boreal forest habitat to date, forcing soils to be reconstructed from the ground up. The specific objectives following soil reconstruction were to identify: i) which overstory vegetation may favor mite recovery, and ii) at what point in time would mite communities begin to resemble mite communities in natural stands. A chronosequence of 15 reclaimed soils was sampled to assess the influence of canopy (aspen or white spruce) and time-since-reclamation (8-31 yrs.) on oribatid mite communities, and was compared to five undisturbed soils. Species-level identification revealed that the presence of a novel forest floor at sites undergoing reclamation had the biggest impact on oribatid mite reestablishment. Reclaimed stands with a novel forest floor thickness 2 cm had similar oribatid mite species richness and diversity to that of undisturbed stands and at times had higher abundances than undisturbed stands regardless of time since reclamation or stand type. Compared to soil reconstruction, non-native earthworm invasion is a less drastic disturbance. However, the presence of another keystone detritivore may threaten the stability of endemic oribatid mite communities in northern boreal forest floors. The objectives following non-native earthworm invasion were to: i) characterize earthworm and oribatid mite assemblage present in a boreal aspen stand and ii) assess how oribatid mite assemblages respond to changes in soil biogeochemical properties as a result of earthworm disturbance. An invaded aspen stand near Wolf Lake, Alberta was surveyed for earthworm invasion and divided into two areas representing different invasion stages: (1) a low density, single species invasion area (Dendrobaena octaedra only), and (2) a high density, multiple species invasion area (Dendrobaena octaedra and Aporrectodea spp.). At each area, oribatid mite communities were sampled. The high density invasion area had a higher forest floor bulk density and a thinner leaf litter. As a result, a drastic loss of the mite species Gymnodamaeus ornatus was observed and overall species richness decreased in the high density invasion area. Ultimately, it was the physical loss of and/or alteration of the forest floor habitat that was the main driver of oribatid mite community response to soil disturbance in both studies.