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

higher weed seed losses (e.g., Menalled et al., 2000; Gallandt et
al., 2005; Holmes and Froud-Williams, 2005; Mauchline et al.,
2005). Yet, some authors have reported methodological biases in
the assessment of the relative impact of seed predator guilds. Some
predators may avoid the exclusion cages even though their body
size would permit them to pass through the mesh openings. Smaller
invertebrates might also be unable to remove the seeds glued to the
sandpaper cards (Shuler et al., 2008). However, possible underestimations
of both the total predation rates and the contribution
of invertebrates in this study would be systematic and would not
challenge the comparisons between the treatments.
4.1. Vegetation cover
The impact of vegetation cover was nearly always positive
among the trials, weed species, and exclusion treatments (Table 2).
Positive impacts of vegetation cover were in line with most of the
previous studies conducted in intensively managed and more natural
ecosystems in various locations (Table 1). In our study, about
12% of the variation in seed predation rates could be explained by
vegetation cover and this value was above 30% when global predation
rates were high. Similar rates were observed by Heggenstaller
et al. (2006). Vegetation cover was thus probably a major factor
affecting weed seed predation rates.
Vegetation cover may change the habitat quality for seed predators
by modifying (a) the microclimate (light, temperature) and soil
characteristics (humidity, plant litter), (b) the presence of alternative
food items such as leaves or insect larvae, (c) the presence
of living or dead plant material that may be used as substrates
for reproduction, and (d) the risk of being predated by carnivores
(Manson and Stiles, 1998; Landis et al., 2005). Given this variety of
possible mechanisms, it may be expected that different predator
guilds react differently to the quantity (and quality) of vegetation
cover. Several studies indicated that most granivorous beetles
and rodents prefer denser vegetation (Hulme, 1997; Manson and
Stiles, 1998; Honek and Jarosik, 2000; Shearin et al., 2008), while
granivorous birds and ants may prefer open patches (Hulme, 1997;
Moorcroft et al., 2002; Butler et al., 2005). While most of the previous
studies focused either on vertebrates or on invertebrates
(Table 1), our exclusion treatments indicated that vegetation cover
increased weed seed predation by both guilds, except for periods
with very low predation rates. Field observations and pitfall
trapping suggested that both mice and granivorous beetles were
abundant in the experimental field, while ants were rarely captured
(data not shown). There is also no reason to assume that predation
rates would be always linearly related to vegetation cover.
To our knowledge, the study by Navntoft et al. (2009) is the first
one to report non-linear impacts of vegetation cover on weed seed
predation (Table 1). In our study, some relationships were rather
exponential, e.g., for predation by Invertebrates in July (Fig. 2).
4.2. Crop species vs. cutting
Heggenstaller et al. (2006) found that seed predation rates
follow the seasonal crop biomass development and would be
temporarily reduced after mowing in perennial forage crops. Our
results based on simultaneous comparisons of cut and uncut plots
(reducing potential confounding temporal effects) support this
hypothesis. In uncut crops, predation rates were higher in Medicago
compared to Dactylis crops. Several authors reported tendencies
towards higher seed predation in legume crops compared to nonlegume
crops (Andersson, 1998; Gallandt et al., 2005; Heggenstaller
et al., 2006), but the reason why predators might prefer legume
crops over grasses is still unclear.
In our experiment, the greater explanatory power of cutting
compared to crop species indicated that vegetation quantity
H. Meiss et al. / Agriculture, Ecosystems and Environment 138 (2010) 10–16 15
(biomass) was more important than vegetation quality, as already
observed by Gallandt et al. (2005) for predation by invertebrates.
The low predation rates observed on plots without any vegetation
agree with this hypothesis. Differences between the five treatments
were mainly linked to the differences in cutting and to the complete
absence of plants in bare soil plots. This was probably the reason
why quite large parts of the variation between the treatments could
also be explained by canopy light interception (Table 3). The use
of continuous environmental variables instead of categorical factors
has proved to be more successful in predicting other biological
phenomena including species richness and spatial distributions of
organisms (Lindegarth and Gamfeldt, 2005). In our case, the use
of a continuous measurable variable allowed (i) reducing the number
of parameters in the models (parsimony/Occam’s razor) and (ii)
testing a more general hypothesis (“vegetation cover affects weed
seed predation rates”) which may be helpful to develop predictive
models and facilitate the meta-analytical comparison of different
studies (Lindegarth and Gamfeldt, 2005).
Results suggested that weed seed predation may be enhanced
by maintaining a high and temporally extended vegetation cover.
Farmers may thus potentially favour the ecological service of weed
seed predation by implementing crop management practices that
maximize vegetation cover on arable fields. This might be achieved
by using cover crops, undersowing techniques, crop mixtures, or
by including perennial crops in the rotations.
Acknowledgements
We thank Florence Strbik, Cyril Naulin, François Duguet, Pascal
Farcy, Philippe Chamoy and Denis Lapostolle for assistance in
the field experiment; Fabrice Dessaint for statistical advice; Pavel
Saska, Audrey Alignier, Aline Boursault, Richard Gunton and two
anonymous reviewers for helpful comments on earlier versions
of this manuscript. This work received funding from ECOGER and
ADVHERB (ANR-08-STRA-02) projects, and AgroSupDijon. H. Meiss
was granted a scholarship by the French Research Ministry.
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