The impacts of Crassula helmsii on Northwest European freshwater macroinvertebrates

Abstract

Crassula helmsii (New Zealand pygmyweed) is an Australasian aquatic plant which has invaded European freshwaters since the 1950s, provoking widespread concern amongst conservationists. The ecological impacts of C. helmsii invasion remain unclear, however, particularly with respect to macroinvertebrates. The main aim of this thesis is therefore to determine how Crassula helmsii impacts recipient macroinvertebrate assemblages in small lentic waterbodies across its invasive range. To this end, I surveyed small lentic waterbodies across the UK, Belgium and the Netherlands, including invaded and ecologically similar uninvaded sites in each region, assessing the impacts of C. helmsii invasion on macroinvertebrates using a suite of taxonomic and functional diversity metrics. I found that Crassula helmsii invasion was associated with slightly elevated macroinvertebrate taxon richness, but that alien taxa were more abundant in C. helmsii sites. Additionally, assemblage composition apparently shifted following invasion, particularly with respect to taxa and traits associated with detritivory.

An additional aim of the thesis is to determine the mechanisms underpinning C. helmsii’s impacts on macroinvertebrate assemblages. To achieve this, I conducted mesocosm experiments investigating the preference of four herbivore and detritivore species for C. helmsii versus native macrophytes, as well as a field experiment to investigate the colonisation and breakdown of C. helmsii litter in nature. Macroinvertebrate consumers exhibited divergent preferences for C. helmsii vs. native macrophyte tissues, possibly associated with interspecific differences in feeding mode and tolerance to phenolic defences. In the field, Crassula helmsii litter was colonised by a comparable macroinvertebrate assemblage to the native macrophyte Callitriche stagnalis, but decomposed at a slower rate, indicating resistance to mechanical and/or microbial breakdown. By the end of the trial, C. helmsii detritus hosted more abundant macroinvertebrates. The recalcitrance of C. helmsii detritus, along with its year-round abundance, is likely to drive impacts on macroinvertebrate detritivores that were observed in field surveys.

This thesis also aimed to contextualise C. helmsii’s impacts by summarising the global ecological consequences of alien aquatic macrophyte invasions. To achieve this, I conducted a meta-analysis, finding overall negative effects of alien macrophytes on the diversity of native biota, but a significant positive effect of submerged alien macrophytes on macroinvertebrates, congruent with the elevated macroinvertebrate taxon richness observed amongst C. helmsii in my field surveys.

In summary, Crassula helmsii appears to have limited, and not wholly negative, impacts on recipient NW European macroinvertebrate assemblages associated with dense vegetation in shallow waters. The palatability of C. helmsii varies between consumers but is not clearly different to that of native macrophytes. Impacts of C. helmsii on macroinvertebrates appear likely to be driven (in part) by the palatability, abundance and recalcitrance of C. helmsii detritus.