Learning to live with novelty: Implications of exotic earthworms and their interactions with seeds, mulch, and wood ash for ecological restoration

Abstract

A major challenge in restoration ecology is the biological invasion of ‘exotic’ species, some of which may spread widely and have undesirable impacts as ‘invasive’ species. Ongoing debates and changing perspectives suggest we may be overlooking opportunities to consider exotic species more broadly, not only as adversaries but also as potential null players or even allies in restoration. This may be exemplified by the invasion of exotic earthworms in North America, a long-term and widespread invasion of ecologically-influential organisms without practical ways to control it. The purpose of this dissertation is to consider the integration of exotic earthworms into restoration by exploring how they interact with three restoration interventions: seeds, mulch, and wood ash. I used laboratory microcosms and field-based experiments with a focus on the ecosystem engineering nightcrawler earthworm (Lumbricus terrestris L.). Overall, earthworms had effects that might be contextually beneficial or detrimental to ecological restoration: earthworms selectively consumed and buried seeds which could reduce recruitment from seed mixes or contribute to seed bank formation (Chapter 2); earthworms collected and buried mulch which exposed the soil underneath but could help mix organic matter into degraded soils (Chapter 3); and earthworms responded behaviourally and in population density to different wood ashes and helped mix surface-applied wood ash into the soil (Chapter 4). I propose that by recognizing exotic earthworms as a novel and increasingly common ecosystem feature in North America and by learning how to mitigate their undesirable impacts and take advantage of their benefits, we could more efficiently and effectively restore these changing ecosystems. This dissertation contributes to our expanding knowledge of earthworm ecology, facilitates increased integration of earthworm interactions into restoration, and offers insights into the broader implications of biological invasion for conservation. Studying the case of exotic earthworms in North America raises important questions about why we restore and conserve, the value of case-by-case management of invasions based on impact, the importance of considering the longer-term outcomes of invasion and naturalization, and – in some cases – the merit in learning to live with novelty.