Invasions of non-native earthworms related to population declines of ground-nesting songbirds across a regional extent in northern hardwood forests of North America

Abstract

Non-native invasive earthworms (Lumbricus spp.) substantially change previously earthwormfree hardwood forests of North America by consuming the leaf litter layer, reducing cover and richness of herbaceous plants, and increasing dominance of sedges and grasses. These changes have been associated with reduced density of Ovenbirds (Seiurus aurocapilla) and Hermit Thrushes (Catharus guttatus) in 10-20 ha forest stands, and with reduced Ovenbird nesting success. Whether earthworms reduce songbird populations across a regional extent is unclear. We investigated relationships among Lumbricus, vegetation structure, landscape patterns of forest cover, and density of four ground-nesting songbird species at points scattered across the Chequamegon-Nicolet (Wisconsin) and Chippewa (Minnesota) National Forests, USA. In both national forests, Ovenbird density was significantly lower at invaded points than Lumbricus-free points, but only in sugar maple (Acer saccharum) and sugar maple/basswood (Tilia americana) (hereafter, maple-basswood) woodlands. Density of the Hermit Thrush, Black-and-white Warbler (Mniotilta varia), and Veery (Catharus fuscescens) did not differ in relation to Lumbricus. In maple-basswood forests, Lumbricus biomass was the best predictor of Ovenbird density, with greater biomass associated with reduced density. Vegetation structure and landscape pattern variables received weak support as density predictors. Across all forest types, Ovenbird density was most strongly related to forest cover within 500 and 1,000 m radii. Our results suggest that earthworm invasions may pose a regional threat to Ovenbirds within maple-basswood forests of the U.S. northern Midwest.

Under the radar: mitigating enigmatic ecological impacts

“…With devastating ecological impacts globally, it is listed as one of the key threatening processes in Australia [73]. Other examples of cryptic invasions include the destructive invasion by nonnative earthworms in northern America [41], and devastation caused by invasive rodents on islands, tramp ants, and numerous weeds [20,74,75]. Indeed, secondary impacts are frequently associated with increased access to relatively undisturbed areas through such road networks.…”

“…Such accessibility can attract poachers, loggers, miners, graziers, arsonists, land speculators, recreationalists, and even researchers. These uses almost inevitably result in further impacts that can extend far beyond the initial impacts of a development both in space and time, such as introductions of invasive organisms with major ramifications for ecosystems (e.g., [41]). Thus, secondary impacts are also called 'human invasions', or the 'Pandora's box effect' [20,26].…”

Spatial patterns and spread of exotic earthworms at local scales

“…These invasions are leading to cascading changes in soil structure (Mclean and Parkinson 1997), nutrient cycling (Alban and Berry 1994), plant-community composition (Scheu and Parkinson 1994;Hale et al 2006), and songbird populations (Cameron and Bayne 2012;Loss et al 2012). Although European earthworms are thought to have been introduced to North America during European settlement with soil used for ship ballast or soil associated with plant material (Tiunov et al 2006), information on the timing of introduction in western Canadian forests is limited.…”

Earthworm Invasions in Northern Hardwood Forests: a Rapid Assessment Method

“…Changes to the soil eliminate sensitive plant species (Gundale 2002), reduce cover and diversity of herbaceous plants and tree seedlings, and increase cover of sedges and grasses Holdsworth et al 2007a). These changes can reduce abundance of salamanders (Maerz et al 2009) and ground-nesting songbirds (Loss and Blair 2011;Loss et al 2012).…”