Abstract
Non-native earthworms have invaded ecosystems around the world but have recently received increased attention as they invaded previously earthworm-free habitats in northern North America. Earthworms can affect plants by ingesting seeds and burying them in the soil. These effects can be negative or positive but are expected to become increasingly negative with decreasing seed size. Orchids have some of the smallest seeds of any plants, so we hypothesized that earthworm consumption of seeds would decrease seed viability and lead to burial of ingested seeds. We used a combination of mesocosms and field measurements to determine whether native and non-native earthworms would affect Goodyera pubescens seed germination by decreasing seed viability through digestion or burial. To determine soil depths at which seed burial would decrease chances of germination, we used field measurements of the abundance of mycorrhizal fungi needed for G. pubescens ger- mination at different soil depths. We found that the combined effects of earthworm ingestion and burial would be expected to result in a loss of 49 % of orchid seeds in mature forests and 68 % of those in successional forests over an average year. Differences in seed ingestion and burial among soils from mature and successional forests were probably driven by differences in their ability to support earthworm biomass and not by differences in earthworm behaviour as a function of soil type. The combined effects of earthworm ingestion and burial have the potential to result in substantial loss of orchid seeds, particularly in successional forests. This effect may slow the ability of orchids to recolonize forests as they proceed through succession. Determining whether this strong effect of earthworms on G. pubescens viability and germination also applies to other orchid species awaits further testing.
Highlights
Worldwide, the role of invasive species in contributing to the decline of rare native species is receiving increased attention (e.g. Augustine and Frelich 1998; Mack et al 2000; Gundale 2002; Pawson et al 2010)
In preliminary tests neither orchid seeds nor fluorescent beads were actively ingested by earthworms, but rather both were ingested in the process of feeding at the soil surface, so we considered the beads to be an accurate representation of movement of orchid seeds
We used post hoc comparisons with Scheffecritical values (Ruxton and Beauchamp 2008) to test for differences in how earthworm species ingested beads, and how deeply they buried beads in mature compared with successional forest soils, to better understand what was driving the significant interaction between earthworm species and site age in bead burial depth and differences between earthworms in bead ingestion
Summary
The role of invasive species in contributing to the decline of rare native species is receiving increased attention (e.g. Augustine and Frelich 1998; Mack et al 2000; Gundale 2002; Pawson et al 2010). Augustine and Frelich 1998; Mack et al 2000; Gundale 2002; Pawson et al 2010) Many invasions, especially those in the soil, are cryptic and can proceed unnoticed for many years. Nonnative earthworms have invaded ecosystems around the world but have recently received increased attention as they invaded previously earthworm-free habitats in northern North America (Hendrix et al 2008). There they have been shown to dramatically reduce leaf litter cover on the forest floor and endanger many
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