Journal of Applied Ecology 2013, 50, 1387–1396
doi: 10.1111/1365-2664.12148
New methods of crop production and farmland birds:
effects of plastic mulches on species richness and
abundance
rka1*, Magdalena Lenda2, Dawid Moron
3 and Piotr Tryjanowski1
Piotr Sko
1
Institute of Zoology, Poznan University of Life Sciences, Wojska Polskiego 71 C, 60-625 Poznan, Poland; 2Institute
of Nature Conservation, Polish Academy of Sciences, Mickiewicza 33, 31-120 Krakow, Poland; and 3Polish Academy
of Sciences, Institute of Systematics and Evolution of Animals, Sławkowska 17, 31-016 Krakow, Poland
Summary
1. Modern methods of crop production are regarded as one of the major factors moderating
ecosystem processes in agricultural landscapes and may negatively affect farmland
biodiversity. One method which is on the increase is mulching: the use of plastic foil to cover
crops at sowing in order to promote early growth by reducing the negative effects of variable
temperatures and to limit pesticide use. However, almost nothing is known of the role of
mulching on farmland biodiversity.
2. In this study, carried out in southern Poland in 2011, we investigated the impact of
mulching with plastic foil on the number of species and abundance of farmland birds at two
spatial scales.
3. At the local scale, bird species number and abundance were lower in areas where foil was
used than those in the control areas, both during the period when the foil was used and after
it was removed from the crops. At the landscape scale, we found a significant negative
relationship between the proportion of crops covered by foil and bird species richness and
abundance. Farmland specialists, nonfarmland birds, ground nesters and above-ground
nesting species were all negatively affected by foil mulching. Foil had a negative effect on
potential resources for birds including adult butterflies and their larvae and weed species.
4. Synthesis and applications. Our results provide the first evidence that the use of foil for
mulching has negative effects on farmland bird populations, probably through the trophic
cascade and habitat disturbance. Therefore, foil mulches must be considered as another factor
contributing to the decline of farmland biodiversity. We suggest limiting the use of this
method of vegetable production at the farm level. Decreasing the field size and converting
some arable fields into grassland patches are proposed as mitigation measures in landscapes
with high foil cover.
Key-words: agricultural intensification, breeding habitat, dynamic landscapes, foraging
habitat, habitat loss, management, vegetables
Introduction
Modern agriculture in Europe is based on large-scale food
production. To achieve this, farmers introduce new methods of crop management in order to increase yield, limit
the effects of pests and unfavourable weather conditions
and, ultimately, to maximize their own economic benefits.
These methods include genetically modified crops that are
herbicide-tolerant and insect-resistant or UV-blocking
foils that disrupt insect pest infestation (Diaz & Fereres
*Correspondence author. E-mail: [email protected]
2007). Plastic mulches are a relatively new method used in
agriculture and date back to the 1950s (Lamont 1991).
The use of plastic mulch has become a common practice
for all vegetable farmers; benefits include reduced
evaporation and soil compaction, weed control, reduction
in herbicide and pesticide application and elevated soil
temperatures and thus frost protection, which promotes
earlier plant maturity compared with conventional vegetable farming (Lamont 1991; Gustavsson 1999; Tarara
2000; Waterer 2000; Franczuk, Kosterna & ZaniewiczBajkowska 2010). Though very effective and affordable,
plastic mulch has become an environmental management
concern owing to disposal issues (Durham 2003). At the
© 2013 The Authors. Journal of Applied Ecology © 2013 British Ecological Society
1388 P. Sk
orka et al.
beginning of the 21st century, almost 13 million hectares
world-wide were covered with plastic mulch (Timsina &
Connor 2001). In Europe, foil mulches covered about 430
thousand hectares (Scarascia-Mugnozza, Sica & Russo
2011).
New methods of crop production in agriculture often
have detrimental effects on wildlife (Schifferli 2001;
Tryjanowski et al. 2011). Many of the organisms inhabiting farmland are amongst the currently most endangered
species, with rapid population declines (Thomas et al.
2004; Wretenberg et al. 2007). For example, common
farmland birds, which are often regarded as indicators of
ecosystem health (Gregory et al. 2005), have declined
dramatically in Europe compared with birds in other
habitats (Donald, Green & Heath 2001). Almost nothing
is known of the effect of plastic mulching on farmland
birds.
Biodiversity conservation on farmland encompasses a
range of different measures that take into account withinfield processes and spatial heterogeneity at both local and
larger spatial scales (Tscharntke et al. 2005; Bat
ary et al.
2011; Kleijn et al. 2011). At the local (field) scale,
biodiversity is mostly shaped by management intensity
and disturbance rates (Kruess & Tscharntke 2002; Kleijn
& Sutherland 2003). At larger spatial scales, habitat
heterogeneity and species dispersal abilities are important
for sustaining biodiversity (Tscharntke et al. 2005; Kleijn
et al. 2011). Thus, local (field scale) alpha species diversity
was shown to be poorly, if at all, related to the
landscape-wide beta species diversity (Tscharntke et al.
2012).
Foil mulches may be regarded as a high-level disturbance
at the local scale as far as birds are concerned. Plastic foils
are used mostly in early spring (April–May) when many
farmland bird species establish breeding territories and start
nesting. Fields covered with plastic foil prevent access to
basic requirements such as nesting sites and food resources.
However, foils are removed in the middle of May, so the
effect of foil mulching is temporary and vacant sites could
be quickly filled by birds from neighbouring areas (spillover
effect; Tscharntke et al. 2012). However, several studies
have shown that foil mulches permanently reduce species
richness of weeds and density of several invertebrates,
which may be food for farmland birds (Ricotta & Masiunas
1991; Radics & Szne Bognar 2004; Frank & Liburd 2005;
Coolong 2012). Thus, according to food web theory
(Cohen, Briand & Newman 1990; Williams & Martinez
2000), the impact of foil mulching on taxa from lower
trophic levels should also be manifested in a negative
impact on populations of taxa from higher trophic levels,
for example, birds, even after removal of the foil from
crops. Hence, one may hypothesize that at the local scale,
foil mulches have a strong negative effect on farmland birds
both during mulching and after its removal.
The conservation of biodiversity and ecosystem services
in agricultural systems requires a landscape perspective
(Tscharntke et al. 2005, 2012). Therefore, the effect of foil
mulching on birds should also be considered at a larger,
landscape scale. Farmland birds with high dispersal
abilities may be less susceptible to small-scale changes in
resource availability. Thus, at a landscape scale, the
negative effect of foil mulches on farmland birds may be
mitigated by spatial heterogeneity, for example, the
presence of marginal habitats such as grassland patches
providing nest sites and food resources, or by crop
diversity. Therefore, when studying the effects of foil
mulches on farmland birds, the impact of other,
potentially important, factors should be taken into the
account. Obviously, if a level of habitat disturbance is
spatially extensive equating to a high cover of foil mulch
in a landscape, then this should act in a negative manner
on farmland birds. Thus, a negative relationship between
foil cover in a landscape and bird species richness and
abundance is expected.
When considering farmland as a habitat of birds, it is
important to realize that species inhabiting agricultural
landscapes are not a homogenous group. Some management schemes may affect particular species or functional
groups of species in different ways than others, leading to
a species group filtering effect (landscape-moderated
functional trait selection hypothesis; Tscharntke et al.
2012). For example, some birds are farmland specialists
but many other species are nonfarmland birds that occur
in several other habitats, for example, forests and human
settlements (Tryjanowski et al. 2009, 2011). Bird species
also differ in their breeding biology, especially nesting site
choice. Many farmland birds are ground nesters, while
others build nests in shrubs, trees or buildings. It is clear
that foil mulches should negatively affect farmland
specialists and species breeding on the ground, but
nonfarmland birds and above-ground breeding species
may remain unaffected by the presence of foil mulches.
Use of plastic foils in agriculture increases every year
(Scarascia-Mugnozza, Sica & Russo 2011); thus, it is
important to understand how foil mulches affect the
farmland bird community. We hypothesized that both the
number of species and abundance of birds would be lower
in sites covered by plastic foil than those in control sites
without foil at the local scale and that at the landscape
scale, higher cover of foils in a landscape will decrease the
number of species and their abundance (Kruess &
Tscharntke 2002; Kleijn & Sutherland 2003). However, the
decrease may be counteracted by the cover of semi-natural
habitats and crop diversity (Kleijn et al. 2011; Tscharntke
et al. 2005). We expected that foil mulches cause profound
changes to the food resources of farmland birds (weed and
invertebrate populations) that extend beyond the period of
foil cover, and therefore, the negative effect of foil mulches
on farmland birds would also be sustained during the period after foil is removed from crops (Cohen, Briand &
Newman 1990; Williams & Martinez 2000). We also predict
that foils affect mostly farmland specialists and ground
nesting birds but not the above-ground nesters and nonfarmland species (Tscharntke et al. 2005, 2012).
© 2013 The Authors. Journal of Applied Ecology © 2013 British Ecological Society, Journal of Applied Ecology, 50, 1387–1396
Effects of plastic mulches on farmland birds
1389
Materials and methods
SELECTION OF PLOTS AND BIRD CENSUSES
STUDY AREA
The effect of mulching on birds was studied in two ways, which
corresponded with two spatial scales, namely small, circular local
plots (314 ha) and large landscape plots of 1 9 1 km (100 ha).
The study area was located in southern Poland, in agricultural
landscapes situated to the north of the city of Krak
ow (Fig. 1).
The elevation of the study area varies between 180 and 250 m
a.s.l. The average annual temperature is 7 °C, and the annual
precipitation is 600 mm (Kondracki 2002). The agriculture in this
area is focused on the production of vegetables. Vegetable crops
constitute over 50% of all crops, and cabbage fields predominate
especially in the parishes of Igoomia, Nowe Brzesko and Charsznica. The other most common crops are cereals (25%). Many of
the cabbage fields are mulched with plastic foil typically for a
period of around 6 weeks from the beginning of April to the middle of May (Appendix S1, Supporting information). The mulched
fields are entirely covered with white foil during this period.
LOCAL SCALE SURVEY
We selected areas with a high cover (over 80%) of foil mulch
and compared these with areas without mulching. We selected
50 points in areas with a high cover of foil mulch and an
equal number of points in the foil-free control areas. At each
point, birds were counted within a 100 m radius; thus, the
sample unit, hereafter referred to as ‘the point’, was 314 ha.
Both foil and control points were chosen in accordance with
several criteria in order to keep landscape structure and other
potentially confounding variables as constant as possible
(Appendix S2, Supporting information).
Between the beginning of April and the end of June 2011, we
made four bird counts. We divided the breeding season into two
periods: during mulching, from the beginning of April to the
middle of May and once the foil was removed, from the middle
of May to the end of June. Two counts were carried out in each
period. Counting dates were homogeneous between the points
with and without foil mulches. At each survey point, birds were
counted for a period of 10 min. The surveys were conducted
between sunrise and 10 A.M. under favourable weather conditions,
with no rain and wind rating below 3 on the Beaufort scale
(Bibby, Burgess & Hill 1992).
LANDSCAPE SCALE SURVEY
We selected 25 100 ha landscape plots (191 km) differing in the
percentage cover of foil mulch (Table 1). We chose the plots in
such a way as to ensure that all possibly confounding variables
were not correlated either with each other or with the foil mulch
cover, and we included them as covariates in the subsequent
analysis (Table 1).
Between the beginning of April and the end of June 2011,
we carried out four surveys. Two surveys were included during
Table 1. Characteristics of the 25 landscape plots (100 ha each)
Variable
code
Foil cover
Field size
Settlement
Forest
Cabbage
Grassland
Crop diver
Fig. 1. Map of Poland showing the study area and the 25
191 km landscape plots (grey squares).
Description
Mean
SE
Min
Max
Foil mulch
cover (%)
Mean field
size (ha)
Human
settlement
cover (%)
Forest cover (%)
Cabbage crop
cover (%)
Grassland
cover (%)
Diversity of crops
(Simpson
reciprocal
diversity index)
17
27
0
43
13
01
03
25
4
05
0
8
4
59
05
18
0
40
9
70
8
13
0
18
20
013
112
© 2013 The Authors. Journal of Applied Ecology © 2013 British Ecological Society, Journal of Applied Ecology, 50, 1387–1396
353
1390 P. Sk
orka et al.
the mulching period and two when the foil was removed, as
detailed in the section on local scale survey methods above.
The observers walked through the plot (both along field borders and inside crop fields) for three hours, taking an arbitrary
path (about 5 km long); however, every plot was covered, visually and aurally, in its entirety. The exact location and number
of all the birds seen or heard within the sampling area were
recorded on a map, with the exception of those which simply
passed over in flight. The surveys were conducted between one
hour after dawn and 11 A.M. and under favourable weather
conditions (see above).
LEPIDOPTERA AND WEED COUNTS
We estimated two food resources of farmland birds: species richness and density of butterflies and species richness of weeds.
We counted all adult butterflies, and we also estimated abundance of larvae of two of the most common species: large white
Pieris brassicae and small white P. rapae that are regarded as
pests of cabbage crops.
To count adult butterflies, we established forty 100 m transects
(20 in mulched fields and 20 in foil-free cabbage fields). We
counted butterflies in a 5-m-wide belt (25 m on both sides of the
transect path). We made six counts, three during mulching and
three after foil was removed (at the beginning of June and July
and in late July).
Butterfly larvae were counted in late July in the same 40 fields
(20 covered by foil and 20 without foil) where adult butterflies
were counted. In each field, we inspected 30 cabbage heads to
search for larvae of the two Pieris butterflies. We noted presence
and number of larvae of each butterfly species. In total, 1200 cabbage heads were inspected.
Weed species were counted in the same fields as butterfly larvae. In each field, two square 100 m2 plots were set. Counts were
performed twice, at the end of May and in late July. Forty fields
were investigated (20 covered by foil and 20 without foil) with 80
plots in total.
DATA HANDLING AND STATISTICAL ANALYSIS
The larger of the two bird counts for both the points and the
landscape plots during each period was used in the subsequent
summaries and analyses; the analysis of mean counts gave very
similar results. Each species was classified as: a farmland
specialist or nonfarmland species, and ground nesting or aboveground nesting species, based on Tryjanowski et al. (2009)
(Appendix S2, Supporting information). Statistical analyses were
performed on total species number and abundance as well as on
data split into the aforementioned groups.
To compare the bird species richness and abundance between
the foil points and the control points during the mulching period
and after the removal of the foil, we used a General Linear
Mixed Model (GLMM). Habitat type (two levels: mulched sites
and foil-free controls sites) and period (two levels: before 15 May
and after 15 May) were introduced as fixed effects, and point ID
nested in the habitat type was a random factor.
Most of species recorded at a point were single individuals;
therefore, we compared species occurrences between the two
habitat types, rather than comparing species abundance. To
compare the occupancy rate of a given species between mulched
and control points, we used a GLMM with logit link function
and binomial error variance. The fixed and random effects were
the same as in models described above.
A GLMM with identity link function was also used to compare the butterfly species richness and abundance between the
transects on mulched and control fields during the mulching period and after removal of the foil. The structure of the model was
the same as for the bird models. To compare the proportion of
cabbage heads infested by Pieris brassicae and P. rapae, a
GLMM with logit link function and binomial error variance was
used. Habitat type was introduced as a fixed effect, and field ID
nested in habitat type was a random factor. To compare the
number of larvae of both pest species on cabbage heads, we used
a GLMM with identity link function. Only cabbage heads with
at least one larvae were considered in this analysis.
To compare weed species richness between mulched and control fields in the second half of May and at the end of July, we
used a GLMM with the identity link function. Habitat type and
period were introduced as fixed effects, and plot ID nested in
field ID and the latter nested in habitat type were random
factors.
In all GLMMs, the interaction between habitat type and period was introduced in initial parameterization. The interaction
was removed from models if it was nonsignificant.
To identify the factors affecting bird species richness and abundance in the 1 9 1 km landscape plots, we used model selection
procedures based on information theory (Burnham & Anderson
2002). The Akaike Information Criterion, corrected for small sample size (AICc), was used to identify the most parsimonious models
from each candidate set, with 127 models tested for each dependent variable (Appendix S2, Supporting information). Finally, we
ranked all the models built in accordance with their ΔAICc values
and used those with the lowest AICc, together with the associated
weight values, the probability that a given model is the best, as
those best describing the data. We considered models with ΔAICc
lower than two as equally good (Burnham & Anderson 2002). We
used model averaging for estimates of the function slopes of the
parameters of interest (Burnham & Anderson 2002). Finally, the
model weights were used to define the relative importance of each
explanatory variable across the full set of models evaluated by
summing up the weight values of all models that contained the
explanatory variable of interest (Burnham & Anderson 2002).
All GLMMs were performed in SPSS version 19 for Windows
(SPSS Inc. 2010, Armonk, NY, USA). Model selection and averaging were run in the SAM 4.0 statistical software (Rangel,
Diniz-Filho & Bini 2010). We did not find evidence for statistically significant spatial autocorrelation in our data by examining
Moran’s statistics. When necessary, we used logarithmic transformation to reduce the effects of outlier observations (Quinn &
Keough 2002). We considered that the function slopes, the betas,
were significant if their 95% confidence intervals (95% CI) did
not overlap with zero. All the statistical parameters, betas and
means, are quoted standard error (SE).
Results
THE EFFECT OF FOIL MULCHES ON BIRD
COMMUNITIES AT THE LOCAL SCALE
We noted 31 bird species during the point counts (Appendix S2, Supporting information). Both the number of species (GLMM F1, 98 = 205034, P < 0001) and the
© 2013 The Authors. Journal of Applied Ecology © 2013 British Ecological Society, Journal of Applied Ecology, 50, 1387–1396
Effects of plastic mulches on farmland birds
abundance of birds (GLMM F1, 98 = 130174, P < 0001)
were significantly lower at the mulched points than at the
control ones (Fig. 2a). The number of species (GLMM
F1,
P < 0001)
and
abundance
98 = 49707,
1391
(GLMM F1, 98 = 33324, P < 0001) were lower during
mulching than after foil removal (Fig. 2a). These analyses
gave similar results when performed separately for
farmland specialists, nonfarmland birds, ground nesters
(a)
(b)
(c)
(d)
(e)
Fig. 2. Species richness (white bars) and number of individuals (grey bars) per point of (a) all species, (b) farmland specialists, (c) nonfarmland birds, (d) ground nesting species and (e) above-ground nesting species during the mulching period and the period after the foil
was removed at the points (314 ha each) covered with foil and the foil-free control sites. Whiskers are 95% confidence intervals.
© 2013 The Authors. Journal of Applied Ecology © 2013 British Ecological Society, Journal of Applied Ecology, 50, 1387–1396
1392 P. Sk
orka et al.
and above-ground nesters (Fig. 2b–e; Appendix S2, Supporting information). Analysis of the occurrence of
species at the points revealed that 13 species had a
significantly lower and none had significantly higher
occupancy in the mulched points compared with the
control points in either of the periods (Appendix S2,
Supporting information).
THE EFFECT OF MULCHING ON BIRD COMMUNITIES AT
THE LANDSCAPE SCALE
In total, we noted 64 species in 25 landscape plots
(Appendix S2, Supporting information).
There were three equally good models explaining 67%
of the variation in the number of species during the
mulching period (Table 2). Species richness was negatively
affected by foil cover in the landscape (Table 3, Fig. 3a),
crop diversity and by average field size, but positively by
forest cover in a landscape (Table 3).
There were five best models explaining the number of
individuals during the mulching period (Table 2). These
models explained 86% of the variation in the number of
individuals. The abundance was negatively affected by foil
cover in the landscape (Fig. 3b) and average field size
(Table 3) but positively by grassland cover and crop
diversity (Table 3).
There were three equally good models explaining 60%
of the variation in species richness in the period after the
Table 2. Best models describing the species richness and
abundance of birds in the farmland landscape plots during the
mulching period and the period after removal of the foil. The
number of parameters in a model (k), the variance explained by
the model (r2), the Akaike Information Criterion score (AICc),
the difference between the given model and the most
parsimonious model (D) and the Akaike weight (w) are listed.
Field size – mean field size, foil cover – foil mulch cover in a
landscape, crop diver – diversity of crops (Simpson diversity
index), forest – forest cover, grassland – grassland cover, cabbage
– cabbage crop cover
Model
k
r2
AICc
Species richness during mulching
Field size + foil cover
3 072
138164
Field size + foil cover
4 074
139823
+ crop diver
Field size + forest
4 073
140108
+ foil cover
Abundance during mulching
Foil cover
2 085
229089
Field size + foil cover
3 087
229669
Cabbage + foil cover
3 086
230846
Foil cover + crop diver 3 086
230924
Grassland + foil cover
3 086
231085
Species richness after the removal of the foil
Foil cover + crop diver 3 0622 142029
Foil cover
2 0554 143327
Grassland + foil cover
4 0638 144009
+ crop diver
Abundance after the removal of the foil
Foil cover
2 057
266261
D AICc
w
0
1659
0215
0094
1944
0071
0
0580
1756
1834
1996
0128
0096
0053
0051
0047
0
1298
1980
0176
0092
0063
0
0244
removal of the foil (Table 2). Species richness in this
period was negatively affected by previous foil cover in
the landscape (Fig. 3c) and crop diversity but positively
by grassland cover (Table 3).
There was only one best model explaining 57% of the
variation in the number of individuals in the period after
the removal of foil (Table 2). The only variable in this
model was the previous cover of foil in the landscape,
negatively affecting bird abundance (Fig. 3d, Table 3).
When the above analyses were performed separately on
farmland specialist, nonfarmland birds, ground nesting
species and above-ground nesting species, the effect of foil
mulches was still negative and statistically significant in
two periods after controlling for other variables
(Appendix S2, Supporting information). The effect of
other factors was highly variable depending on the species
group. Only grassland cover always had a positive impact
on all species groups when it was in the best models
(Appendix S2, Supporting information).
THE EFFECT OF MULCHING ON BUTTERFLIES AND
WEEDS
We noted 207 individuals of 18 butterfly species (Appendix S3, Supporting information). Both the number of
species (GLMM F1, 38 = 13530, P = 0007) and the abundance of butterflies (GLMM F1, 38 = 14211, P = 0004)
were lower in mulched transects (Fig. 4a, Appendix S3,
Supporting information). The number of butterfly species
(GLMM F1, 39 = 41294, P < 0001) and their abundance
(GLMM F1, 39 = 41294, P < 0001) were lower during
mulching than after foil removal (Fig. 4a).
The proportion of cabbage heads infested by larvae was
smaller in mulched fields than that in foil-free fields for
large white (GLMM F1, 30 = 10639, P < 0001) but not
for small white (GLMM F1, 39 = 1529, P = 0224;
Fig. 4b). The mean number of larvae per cabbage head
was smaller in mulched fields than that in foil-free fields
for large white (GLMM F1, 36 = 17875, P < 0001
Fig. 4b) but not for small white (GLMM F1, 27 = 0411,
P = 0527; Fig. 4b).
In total, we noted 32 weed species (Appendix S4,
Supporting information). We found a statistically
significant interaction between habitat type and period for
weed data (GLMM F1, 78 = 4120, P = 0046). Tukey’s
post hoc tests showed that mean number of weed species
per 100 m2 plot was lower in mulched fields than that in
control fields during the mulching period but not after
removal of the foil in late July (Fig. 4c; Appendix S4,
Supporting information).
Discussion
Our study has revealed for the first time that the use of
plastic foil for mulching had a negative effect on the
species richness and abundance of farmland birds at both
the small scale of individual fields and the landscape scale.
© 2013 The Authors. Journal of Applied Ecology © 2013 British Ecological Society, Journal of Applied Ecology, 50, 1387–1396
Effects of plastic mulches on farmland birds
1393
Table 3. Estimates of the function slopes of variables present in the most parsimonious models describing the species richness and abundance of birds in the farmland landscape plots during the mulching period and the period after the removal of the foil. Importance of
each explanatory variable was calculated by summing up the weight values of all models that contained the variable. Standard errors
(SE) and 95% confidence limits (CL) are also presented. A variable which confidence limits of the estimate of function slope overlapped
with zero is in italic. Foil cover – foil mulch cover in a landscape, field size – mean field size, crop diver – diversity of crops (Simpson
diversity index), forest – forest cover, cabbage – cabbage crop cover, grassland – grassland cover
Variable
Importance
Estimate
Species richness of birds during mulching
Foil cover
1000
Field size
0789
Crop diver
0280
Forest
0248
Abundance of birds during mulching
Foil cover
1000
Field size
0352
Grassland
0321
Crop diver
0310
Cabbage
0280
Species richness of birds after the removal of the foil
Foil cover
0999
Crop diver
0544
Grassland
0259
Abundance of birds after the removal of the foil
Foil cover
1000
Lower 95%
CL
SE
Upper 95%
CL
0306
3134
1981
0282
0062
1061
0587
0071
0427
5214
3131
0143
0185
1054
0831
0421
3641
11031
1347
22492
0798
0352
2918
0409
8126
0556
4332
16750
0545
6565
0291
2951
5311
2149
38419
1887
0285
2805
0115
0060
0978
0037
0403
4723
0041
0167
0887
0188
3768
0719
5177
2360
Interestingly, the negative effect of foil mulch was also
still present after foil removal in the middle of May.
Contrary to expectations, foil did not act as a group-specific environmental filter and all groups of birds (farmland
specialists, nonfarmland birds, ground nesting species and
above-ground nesting ones) were negatively affected by
mulching. Potential food resources for birds, adult and
larval butterflies and weed species, also had lower density
and species richness in mulched fields. This suggests that
in accordance with our hypotheses, widespread foil use
for crop mulching acted as a habitat disturbance
preventing birds from access to food resources and
breeding sites. Mulches may also have altered resources
that, probably via the trophic cascade, negatively affected
birds even after the foils were removed. These may add to
the general negative trends in farmland biodiversity and
the decline in the suitability of agricultural landscapes for
birds.
The use of plastic foil for mulching is now widespread
(Scarascia-Mugnozza, Sica & Russo 2011). Compared
with other mulch types, such as living mulches or straw
mulches, plastic foil is very much cheaper, enables large
areas to be covered and is easier to handle and to remove
(Durham 2003). Covering a field with foil means that all
(a)
(b)
(c)
(d)
Fig. 3. The effect of a foil mulch cover in
the landscape on species richness and
abundance of all birds during the
mulching period (a, b) and the period after
the foil was removed (c, d).
© 2013 The Authors. Journal of Applied Ecology © 2013 British Ecological Society, Journal of Applied Ecology, 50, 1387–1396
1394 P. Sk
orka et al.
(a)
(b)
(c)
Fig. 4. The effects of mulching on (a) species richness (white
bars) and mean number of individuals per count (grey bars) of
butterflies, (b) proportion of infested cabbage heads (white bars)
and number of larvae on the cabbage head (grey bars) of Pieris
brassicae and P. rapae and (c) number of weed species. Whiskers
are 95% confidence intervals.
the habitat resources for birds are unavailable for
breeding and foraging. We also cannot exclude the
possibility that the reflection from the foil repels some
species, preventing them from settling in these fields.
However, this does not explain why the negative effect of
foil use extends into the period after it is removed. After
removal of foils, the number of birds did not increase to
the level noted in control mulch-free sites and the
landscapes without mulched fields.
Overall, species richness and abundance of species
increased during the period after the foil was removed.
This is probably linked in part with phenology and the
arrival of late, long-distance migrants such as whinchat
Saxicola rubetra, red-backed shrike Lanius collurio or
marsh warbler Acrocephalus palustris. However, even
they did not entirely fill the gaps brought about by the
plastic foil. As we demonstrated, mulches altered habitat
and food resources (weed species and adult butterflies
and their larvae) even after foil removal, probably
making those parts of the farmland less suitable for all
group of birds. This result is in agreement with other
studies on the effects of foils on invertebrates (Ricotta
& Masiunas 1991; Hooks & Johnson 2003; Mahajan
et al. 2007). It is also possible that some nonterritorial
individuals known as floaters (Newton 1998) and
breeding dispersers may have settled in fields after the
removal of the foil, since occupancy at the mulched
points increased slightly after foil removal for some
species such as grey partridge Perdix perdix, Eurasian
skylark Alauda arvensis and corn bunting Miliaria
calandra, while occupancy remained stable in the control
areas. Both the mulched and control sites were treated
with pesticides and herbicides; however, the mulched
sites were usually sprayed once, whereas pesticides were
applied two or three times at the control sites. However,
it is unlikely that this could explain why the number
and abundance of bird species were higher at control
points than mulched points. Numerous works have
shown that pesticide and herbicide use has both direct
and indirect detrimental effects on farmland bird species
richness, abundance, reproduction and food resources of
birds, such as weeds and insects (Moreby & Southway
1999; Holland et al. 2006; Geiger et al. 2010). Thus, it
is possible that if we were able to control for the rate
of pesticide use, the negative effect of foil mulches
would be much more pronounced than those recorded
in our study. The lower pest density and weed species
number, together with lower pesticide and herbicide use
in mulched sites, indicate that this method of crop
production is somehow more environmentally friendly
than conventional crop production. As we demonstrated, however, this decreases the number of farmland
birds and ecosystem services potentially provided by
these organisms (e.g. natural pest control). This raises a
serious conservation dilemma of how to minimize the
negative effects of pests and weeds on crops while sustaining high species richness and abundance of birds.
Our surveys on the large landscape plots showed that
other variables also had significant effects on bird species
richness and abundance. Thus, according to theory
© 2013 The Authors. Journal of Applied Ecology © 2013 British Ecological Society, Journal of Applied Ecology, 50, 1387–1396
Effects of plastic mulches on farmland birds
(Kleijn et al. 2011; Tscharntke et al. 2012), spatial
heterogeneity may, at least to some degree, mitigate the
bird habitat disturbance caused by the use of foils on
crops. However, most of the analysed factors (crop
diversity, forest and human settlement covers, cabbage
crop cover) had different (positive or negative) effects
depending on the group of species (Appendix S2,
Supporting information); thus, they are of little practical
application when conservation and landscape planning for
birds are concerned. Only grassland cover in the
landscape always had a positive effect on birds when it
was in best models. Grassland patches spread through a
landscape dominated by intensively managed crops play a
key role in sustaining farmland biodiversity (S€
oderstr€
om
& P€art 2000; Batary, Baldi & Erd}
os 2007). For birds, they
provide suitable breeding and foraging habitats
(Concepci
on et al. 2012). In our study area, the grasslands
were usually small patches that were either intensively
mown or grazed by cows, which is particularly good for
birds foraging on the ground because prey are easy to
catch in those conditions (Morris & Thompson 1998).
Thus, the possibility of mitigating the negative effect of
foil mulches on birds in a landscape is the conversion of
some fields into grassland patches.
Amongst other factors that potentially could be of use
when mitigating the effects of foils, field size is
particularly interesting. More bird species were present in
landscapes with smaller fields. This was also true for
farmland specialists. We counted birds in different parts
of large fields; thus, this effect could not be attributed to
greater species detectability in smaller fields. Agricultural
landscapes with smaller fields are inhabited by more
species (e.g. S€
oderstr€
om & P€art 2000; Herzon & O’Hara
2007). Fields in Poland are separated from each other by
narrow grass strips (Appendix S1, Supporting
information); thus, the smaller fields increase the density
of these potentially important microhabitats for birds
(Concepci
on et al. 2012). Hence, another initiative that
could help to maintain species richness of farmland birds
in landscapes with mulched fields is to divide large fields
into smaller ones.
SYNTHESIS AND APPLICATION
Plastic foil mulches are an efficient measure to control
invertebrate pests and weeds; however, they are
detrimental to farmland birds. This raises a serious
conservation dilemma. If conservation of farmland birds
is a priority in a given landscape, the use of foil mulches
should be carefully considered. This would require a
monitoring of species richness and abundance in a target
area before the use of foils to estimate their potential
impact. We suggest maintaining a low cover of plastic foil
mulches in the landscape; on the basis of our regression
equations, it seems probable that a cover of less than
20% is sufficient to support c. 80% of all the species that
occur in the areas without the foil mulches. Small field
1395
sizes and a high proportion of grassland cover in the
landscape may also alleviate the negative effect of foil
mulches on birds. This would require actions involving
many farmers because average farm size in our study area
is small (<10 ha). The problem of foil mulches should be
regulated in the framework of the common agricultural
policy of the European Union. For example, farmers
should be encouraged to limit the area covered with
plastic mulches and select late-growing vegetable varieties.
However, to state specific recommendations, more
detailed studies are required and they should focus on the
estimation of potential profits resulting from lower pest
densities and weed species in mulched areas which may
counterbalance potential costs linked with the loss of the
ecosystem services provided by birds. A comparison of
different types of mulch with mechanical and chemical
methods of weed and pest control (including at a
landscape scale) is also required in future studies.
Acknowledgements
We thank David Kleijn and three anonymous referees for helpful
comments on the manuscript. We are grateful to Tim H. Sparks for
linguistic improvements.
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Handling Editor: David Kleijn
Supporting Information
Additional Supporting Information may be found in the online version
of this article.
Appendix S1. Photos of the study area.
Appendix S2. Additional bird data. A comparison of the landscape composition around mulched and control sites. List of all
birds noted during point counts and in the landscape plots. Generalized liner mixed models for species occupancy in points. Full
set of model tested from landscape plots. Details on specific
groups of species: farmlands specialists, non-farmland birds,
ground nesting and above-ground nesting species. Summary of
all results.
Appendix S3. List of butterfly species and their abundance.
Appendix S4. List of all weed species recorded during the study.
© 2013 The Authors. Journal of Applied Ecology © 2013 British Ecological Society, Journal of Applied Ecology, 50, 1387–1396