RESULTS
With the exception of Gole Lough, E. canadensis was encountered
in all study sites (Fig. 1). The highest mean percentage occurrence ofE. canadensis per lake was in Group 1 lakes (48%),
followed by Group 2 lakes (32%), and Group 3 lakes
(28%). Current native macrophyte species richness and E.
canadensis sample occupancy were positively correlated among lakes (r=
0.44; P= 0.08) (Fig 2a). This positive relationship became significantly
stronger (r= 0.74; P< 0.01) after excluding Kilturk Lake,
which was identified as an outlier by having 19 native macrophytes
species but an E. canadensis occupancy of just 9% (Fig. 2a).
Positive correlations between current native macrophyte species richness
and E. canadensis were similarly observed within lakes for two
thirds of the sites (including all four basins in the ULE which were
treated as a single lake) (Fig. 2). Positive correlations between lake
taxon richness of plant macrofossils and E. canadensis abundances
were observed over time for Castle, Cornabrass, Killymackan, Head and
Gole Lakes (Fig. 5). No association between surveyed native macrophyte
species richness and E. canadensis abundance was observed for the
Lakes Killymackan, Derrysteaton, Derryhowlaght and Head.
pBRTs showed that the importance of the pure abiotic fraction in
explaining E. canadensis abundance variation declined from 30%
in Group 1 to 13% in Group 3 (Fig. 3). Within the pure
abiotic fraction, latitude explained almost half of the variation (48%)
in Group 1 , but just 9% in Group 3 . Water clarity inGroup 3 explained 68% of the abundance variation compared with
only 24% in Group 1 (24%). Longitude effects remained
relatively constant across the three lake groups, explaining 24% of the
abiotic fraction in Group 1 , 21% in Group 2, and 23% inGroup 3 .
The importance of the pure diversity fraction in explaining E.
canadensis abundance variation in the pBRTs almost doubled from 17% inGroup 1 to 31% in Groups 2 and 3 (Fig. 3). Native
beta diversity emerged as the most important predictor, accounting for
almost two thirds of the pure diversity fraction in Groups 1 and2 (61% and 62% respectively), and 40% of the variation inGroup 3 . The importance of native Shannon diversity showed an
increasing trend from relatively low levels of explained variation inGroup 1 (8%) to nearly four fold higher (31%) in Group
3. The explanatory importance of floating plant cover increased fromGroup 1 (10%) to Group 3 (16%), whilst overall plant
cover was most influential in Group 1 (16%) compared toGroup 2 (8%) and Group 3 (7%). The influence of
submerged plant cover was generally low among the three groups,
explaining just 5% - 7% of the pure diversity fraction. The
proportion of variance jointly attributable to the fraction of both
abiotic and diversity descriptors increased from Group 1 (37%)
to Group 3 (51%). The proportion of unexplained variation
declined from 16% in Group 1 to 5% in Group 3 .
The pBRT fitted function plots (Figure 4) show that E. canadensisabundance variation was spatially related to latitude and longitude
sampling points. There were also marked reductions in E.
canadensis abundances with declining water clarity (index values
<1.2) and with increases in native plant cover (>
60%), in particular for Groups 2 and 3 . A nonlinear
pattern with three distinct phases of E. canadensis abundances
and native macrophyte beta diversity also emerged, characterised by: i)
abundant E. canadensis co-occurring with diverse native
macrophyte communities and high submerged plant cover; ii) low
abundances or absences of E. canadensis coupled with high native
plant cover and low native macrophyte diversity; and iii) abundantE. canadensis co-occurring with diverse native macrophyte
communities and high floating plant cover.
RFA on the palaeo-data in Group 1 lakes identified beta diversity
as the most important predictor in explaining E. canadensisabundance variation through time (Fig. 6a). Shannon diversity and
floating plant cover were also influential. E. canadensisabundances were positively related to all three diversity predictors.
For Group 2, variation in submerged plant cover was the most
important driver of E. canadensis abundances followed by beta
diversity and Shannon diversity, respectively (Fig. 6b). Here, E.
canadensis abundances were positively related to beta diversity and
Shannon diversity values, whilst negatively related to submerged plant
cover. The analysis of Group 3 lakes identified plant cover as
the most important variable in explaining E. canadensis abundance
variation through time (Fig. 6c). Beta diversity, submerged plant cover,
and Shannon diversity also positively influenced E. canadensisabundances.