Diversifying crop rotations with temporary grasslands - Université de ...

Table 2
Overview showing (i) mean seed predation rates for three weed species; (ii) coefficients of determination of ANOVA models explaining ‘seed predation rates’ by ‘treatment’(R2 ); and (iii) coefficients of correlations between seed
predation rates and canopy light interception (r). N = 20 for each weed species at each trial.
Predator guild Trial 1 (April) Trial 2 (May) Trial 3 (July)
Weed species Mean predation (%) ANOVA treatment (R 2 ) Correlation light (r) Mean predation (%) ANOVA treatment (R 2 ) Correlation light (r) Mean predation (%) ANOVA treatment (R 2 ) Correlation light (r)
(A) Total
A. myosuroides 21 ns ns 15 ns ns 51 0.42 † +0.46 *
S. arvensis 12 0.49 * +0.47 * 2 ns +0.56 ** 47 0.48 * +0.59 **
V. arvensis 58 0.85 *** +0.46 * 10 ns +0.44 † 58 0.46 * +0.52 *
All species 30 0.21 * ns 9 ns +0.31 * 52 0.41 *** +0.52 ***
(B) Vertebrates
A. myosuroides 18 ns ns 13 ns ns 49 0.46 * +0.52 *
S. arvensis 12 0.48 * +0.47 * 2 0.41 † +0.53 * 42 0.39 * +0.48 *
V. arvensis 50 0.78 *** +0.52 * 8 ns +0.51 * 39 ns ns
All species 27 0.25 ** +0.24 8 ns +0.34 ** 44 0.26 ** +0.32 *
H. Meiss et al. / Agriculture, Ecosystems and Environment 138 (2010) 10–16 13
(C) Invertebrates
A. myosuroides 4 ns ns 4 ns ns 2 ns ns
S. arvensis 1 ns ns 1 ns ns 4 0.54 * +0.37 †
V. arvensis 9 ns −0.50 * 3 ns ns 24 ns +0.51 *
All species 5 ns −0.23 † 3 ns ns 10 0.18 * +0.32 *
ns, not significant.
† p < 0.1.
* p < 0.05.
** p < 0.01.
*** p < 0.001.
was obtained by dividing the total amount of additive variance
explained by ‘light interception’ by the total amount of additive
variance explained by ‘treatment’.
3. Results
Seed predation rates varied considerably between the three
trials (F 2,338 = 34.5, p < 0.0001, see Table 2 for mean values).
Exclusion treatments (12 mm cages) suggested that vertebrates
contributed much more to total seed losses than invertebrates
(F 1,338 = 74.2, p < 0.0001). Predation rates also differed between the
three weed species (F 2,338 = 13.1, p < 0.0001). Losses of V. arvensis
seeds were highest during all periods and exclusion treatments
except in May, where they were similar to A. myosuroides. Losses
of S. arvensis were mostly lower than the two other species
(Table 2).
3.1. Impact of vegetation cover
Weed seed predation was positively related to vegetation cover
in most of the cases. In ANCOVA models, the additive effect of
‘light interception’ was highly significant for Total (p < 0.0001)
and Vertebrate predation (p = 0.0004), but not for Invertebrates.
Moreover, several two or three-fold interactions between light
interception, weed species and trial period were significant but
explained less variance than the main effects. When analyzing
each trial separately, Total seed predation was always positively
related to light interception (Table 2). Correlations were strongest
in the July trial (r = 0.52, p < 0.0001), intermediate in May (r = 0.31,
p < 0.0149), but not significant in April (r = 0.19, p = 0.1491). For
Vertebrates, correlations were always positive but again weakest
in April. For Invertebrates, correlations were only significant
in July, probably because Invertebrate predation rates were very
low in April and May (Table 2). When looking at each weed
species-trial combination separately, predation rates increased
with vegetation cover in 15 out of 27 individual cases, correlations
were not significant in 11 cases, mainly for treatments
with low overall predation levels, and only one negative correlation
was detected for Invertebrates feeding on V. arvensis in April
(Table 2).
In three-way ANOVAs, the additive effect of ‘treatment’ was
significant for Total (p < 0.0001), Vertebrate (p < 0.0001), and Invertebrate
predation (p = 0.0175). Significant interactions with ‘trial
period’ showed that the differences between the treatments differed
with time. When calculating separate one-way models for
each trial (but pooling all weed species), the differences between
the treatments were significant for Total and Vertebrate predation
but not for Invertebrates in April; differences were always
significant in July but never in May, where predation rates were
low in every crop (Table 2). A similar pattern appeared when
analyzing each weed species-trial combination separately, but differences
were more often significant for S. arvensis and V. arvensis
than for A. myosuroides. In summary, 9 out of 27 individual models
were significant at p < 0.05 and two at p < 0.1 (Table 2). In
April, Total predation was high in all Medicago plots (averaging
at 49%) and low in both Dactylis and bare soil plots (15–20%,
Fig. 2). In May, predation was always low except in uncut Medicago,
where it was significantly higher (20%). In July, the predation
levels were highest in uncut Medicago and uncut Dactylis (both
about 75%), lowest in bare soil plots (about 18%) and intermediate
in cut Medicago (36%) and cut Dactylis (58%, see Fig. 2 for
details). Cutting always had a negative impact, especially in Medicago,
where it reduced the Total predation rates by about 53% in
July and 77% in May, compared to 21% and 27% in Dactylis, respectively.