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