Mutual Facilitation Among Invading Nuttall's Waterweed and Quagga Mussels.

Benjamin Wegner,Mikael Gillefalk,Jan Köhler,Anna Lena Kronsbein,Klaus Van De Weyer,Michael T Monaghan,Sabine Hilt,Elisabeth Funke

doi:10.3389/fpls.2019.00789

Abstract

Nuttall’s waterweed (Elodea nuttallii) is the most abundant invasive aquatic plant species in several European countries. Elodea populations often follow a boom-bust cycle, but the causes and consequences of this dynamics are yet unknown. We hypothesize that both boom and bust periods can be affected by dreissenid mussel invasions. While mutual facilitations between these invaders could explain their rapid parallel expansion, subsequent competition for space might occur. To test this hypothesis, we use data on temporal changes in the water quality and the abundance of E. nuttallii and the quagga mussel Dreissena rostriformis bugensis in a temperate shallow lake. Lake Müggelsee (Germany) was turbid and devoid of submerged macrophytes for 20 years (1970–1989), but re-colonization with macrophytes started in 1990 upon reductions in nutrient loading. We mapped macrophyte abundance from 1999 and mussel abundance from 2011 onwards. E. nuttallii was first detected in 2011, spread rapidly, and was the most abundant macrophyte species by 2017. Native macrophyte species were not replaced, but spread more slowly, resulting in an overall increase in macrophyte coverage to 25% of the lake surface. The increased abundance of E. nuttallii was paralleled by increasing water clarity and decreasing total phosphorus concentrations in the water. These changes were attributed to a rapid invasion by quagga mussels in 2012. In 2017, they covered about one-third of the lake area, with mean abundances of 3,600 mussels m−2, filtering up to twice the lake’s volume every day. The increasing light availability in deeper littoral areas supported the rapid spread of waterweed, while in turn waterweed provided surface for mussel colonization. Quantities of dreissenid mussels and E. nuttallii measured at 24 locations were significantly correlated in 2016, and yearly means of E. nuttallii quantities increased with increasing mean dreissenid mussel quantities between 2011 and 2018. In 2018, both E. nuttallii and dreissenid abundances declined. These data imply that invasive waterweed and quagga mussels initially facilitated their establishment, supporting the invasional meltdown hypothesis, while subsequently competition for space may have occurred. Such temporal changes in invasive species interaction might contribute to the boom-bust dynamics that have been observed in Elodea populations.

Highlights

Positive interactions can play a decisive role in shaping communities and regulating ecosystem structure and function (Halpern et al, 2007; Brooker et al, 2008; Soliveres et al, 2015)
Total N (TN) loads decreased from 140 ± 38 g TN m−2 a−1 in the 1980s to 30 ± 11 g TN m−2 a−1 in the decade 2007–2016
Total coverage of submerged macrophytes in Lake Müggelsee increased from 5% in 1999 to 25% in 2017 (Figures 1A,C)

Summary

Introduction

Positive interactions can play a decisive role in shaping communities and regulating ecosystem structure and function (Halpern et al, 2007; Brooker et al, 2008; Soliveres et al, 2015). Based on the widespread occurrence of positive interactions between non-native species, Simberloff and Von Holle (1999) developed the “invasional meltdown hypothesis (IMH)” suggesting that non-indigenous species can facilitate one another’s invasion in various ways, increasing the likelihood of survival and/or of ecological impact, and possibly the magnitude of impact. While the majority of studies supported the IMH, studies at the community or ecosystem level were rare, especially for cases where two non-native species interact and both species are affected. There are only few studies on indirect positive interactions, as these are often difficult to detect and measure, and often neglected (White et al, 2006)

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Conclusion