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14 H. Meiss et al. / Agriculture, Ecosystems and Environment 138 (2010) 10–16
Fig. 2. Mean seed predation rates in five treatments as a function of vegetation cover (light interception). Each dot represents the mean value of three weed species and four
replicates per treatment (N = 12), error bars show ±1SD. Bold lines indicate that the correlations between predation and light interception rates were significant (Table 2).
3.2. Crop species vs. cutting
For models of Total predation, the interaction between crop
species and cutting was nearly significant (p = 0.073) while all other
interactions were not significant. Interestingly, the additive effect
of ‘cutting’ was highly significant (p = 0.0004) while ‘crop species’
had no additive effect (p = 0.72). A very similar pattern appeared
for Vertebrate and Invertebrate predation, the only difference being
that the additive effect of ‘crop species’ was significant for Invertebrates
(p = 0.047). The impact of cutting was thus stronger for both
predator guilds at both trial dates than the differences between the
crop species.
The comparison of ANOVA and ANCOVA models described in
Section 3.1 showed that ANOVA models including ‘treatment’ (five
levels) explained more variance than the alternative ANCOVA models
using ‘light interception’ (one regressor); the coefficients of
determination (R 2 ) of the ANOVA models were 11–16 percent
points higher than for the ANCOVA models. However, the adjusted
R 2 differed only by 0–9 percent points, reflecting the lower number
of parameters in the ANCOVA models.
The variance partitioning analysis indicated that 16–64% of the
variance in predation rate explained by ‘treatment’ may equally
be explained by ‘light interception’ (Table 3). This percentage
tended to be higher for trials and exclusion treatments with higher
predation rates (cf. Table 2). Fig. 2 supports the results of the
statistical analysis and shows that differences in mean predation
rates between the crop species and cuttings were mostly positively
related to mean light interception rates, except for Invertebrates in
April and May, where predation rates were low.
Variance partitioning also indicated that the sum of the variance
explained by ‘treatment’ and ‘light interception’ (additive effects
and interactions) varied between 18% and 55%, which was higher
than the variance explained by weed species (1–41%, Table 3).
Together, the two “environmental” variables determining the habitat
quality of seed predators were thus more important than the
differences between the weed species.
4. Discussion
Strong differences of seed predation rates between weed species
have been frequently reported (e.g., Kollmann and Bassin, 2001;
Westerman et al., 2003b; Mauchline et al., 2005). Interestingly,
weed species preferences observed here were similar to findings
of a previous study conducted in organic wheat fields in the same
geographical area (Alignier et al., 2008). However, results suggested
that predator habitat quality may be even more important than
differences between seeds (Table 3).
Westerman et al. (2003a) observed high contributions of vertebrates
to total weed seed losses like in the present analysis. In
contrast, many other studies suggested that invertebrates cause
Table 3
Variance decomposition of seed predation rates. The additive variances of ‘treatment’ and ‘light interception’ are divided into exclusive and common parts. The variable
‘weed species’ has no common variance and no significant interactions with the two other variables.
Effect Type of variance Trial 1 (%) Trial 2 (%) Trial 3 (%)
Total Vertebrate Invertebrate Total Vertebrate Invertebrate Total Vertebrate Invertebrate
‘Treatment’ Additive, exclusive 18 19 5 9 11 11 14 15 9
‘Light’ and ‘treatment’ Additive, common 3 6 0 1 4 2 26 10 9
‘Light’ Additive, exclusive 0 0 5 10 11 0 1 0 1
‘Light * treatment’ Interaction 1 1 8 7 8 5 13 11 1
‘Weed species’ Additive, total 41 30 41 16 11 5 1 1 17
Whole model 63 56 60 43 46 23 56 38 37
Total ‘habitat effect’ a 22 26 19 27 35 18 55 37 20
% of ‘treatment’ explained by ‘light’ b 16 24 (5) (13) (26) (18) 64 39 49
a The total variance explained by additive effects of, and interactions between ‘treatment’ and ‘light interception’.
b The percentage of variance explained by ‘treatments’ that may equally be explained by ‘light interception’. Values are in brackets for the cases where the differences
between the ‘treatments’ were already not significant (Table 2).