Institutionen för fysik, kemi och biologi Examensarbete 16 hp
Farming system and landscape complexity affects
pollinators and predatory insect communities differently
Michaela Håkansson
LiTH-IFM- Ex--14/2872--SE
Handledare: Dennis Jonason, Linköpings universitet Examinator: Anders Hargeby, Linköpings universitet
Institutionen för fysik, kemi och biologi Linköpings universitet
Rapporttyp Report category Examensarbete C-uppsats Språk/Language Engelska/English Titel/Title:
Farming system and landscape complexity affects pollinators and predatory insect communities differently
Författare/Author:
Michaela Håkansson
Sammanfattning/Abstract:
It has been argued that organic farming sustains a higher biodiversity than conventional farming. This might promote the ecosystem services that exist in agricultural landscapes such as pollination and pest control. Here, I examined the effect of farming system (organic vs. conventional) with respect to the time since farming system transition,
landscape heterogeneity and plant richness on pollinating and predatory insects. In total, data from 30 farms were used, of which 20 were organic and 10 were conventional. The data were analyzed using general linear models and model averaging. The results show that insect groups responded differently to various factors. Pollinators were more sensitive to landscape complexity, showing an increase of abundance and species richness with an increased
heterogeneity. Predators on the other hand reacted to farming system, where there was an increase in abundance and species richness on organic farms.
ISBN
LITH-IFM-G-EX— 14/2872—SE
__________________________________________________ ISRN
__________________________________________________
Serietitel och serienummer ISSN
Title of series, numbering
Handledare/Supervisor Dennis Jonason
Ort/Location: Linköping
Nyckelord/Keyword:
Organic farming, agricultural intensification, landscape heterogeneity, time since transition, bumblebees, solitary bees, hoverflies, natural enemies.
Datum/Date
2014-09-01
URL för elektronisk version
Institutionen för fysik, kemi och biologi
Department of Physics, Chemistry and Biology
Avdelningen för biologi
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Index
Contents
1 Introduction ... 2
2 Materials and methods ... 4
2.1 Data ... 4 2.2 Analysis ... 4 3 Results ... 5 4 Discussion ... 10 4.1 Farming system ... 10 4.2 Landscape complexity ... 10 4.3 Interactions ... 11
4.4 Social and ethical issues ... 12
4.5 Conclusions ... 12
5 Acknowledgements ... 12
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1 Introduction
There are many benefits humans get from the ecosystems, so called ecosystem services, such as waste decomposition and water purification. They are services that occur naturally in the environment and are essential for human life. One important ecosystem service is pollination, which is managed by pollinators, the majority being invertebrates such as hoverflies, bumblebees and bees. In tropical areas vertebrates like bats and birds also pollinate (Buchmann et al. 1996).
About 75% of all food crops grown globally (for example tomatoes, cucumber, fruit trees, berries and almond) are to at least some degree dependent on
pollinators and stand for about 35% of the global production volume (Klein et al., 2007). Bartomeus et al. (2014) showed that pollination by insects enhanced the average crop yield by 18% to 71% depending on the crop (crops used in the experiment was spring oilseed rape, buckwheat, field bean, and strawberry). The same study showed that crop yield was enhanced by higher visitation rate from pollinators, but visitation rate was not always higher with higher species
richness, which means that higher species richness does not always result in higher yields.
Pollination is not just important for food production but consequently also for the economy. In 2009 the value of crop pollination in Sweden was estimated to around €21-36 million (189-325 million SEK)(Jordbruksverket, 2013) and €153 billion globally during 2005, which is about 9,5% of the total value of food production (Gallai, et al. 2009). Unfortunately we are seeing an alarmingly large decrease of pollinators today (Biesmeijer, et al. 2006). A continued decline of pollinators may result in economic losses when productivity falls (Gallai, et al. 2009) and declining yield stability (Garibaldi, et al. 2011)
The way the modern agricultural landscape is managed is thought to be the major reason for the decline of pollinators (Stoate, et al. 2001). We produce more food on less area to meet the demands of a growing population. This has resulted in an increased use of fertilizers, pesticides and heavy machinery compared to a few decades ago (Tilman, et al. 2001). Important habitats for the pollinators and other plants and animals have become fragmented and polluted, resulting in a loss of individuals and species (Steffan-Dewenter, et al. 1999; Björklund, et al. 1999). Kremen et al. (2002) showed that intensive agricultural management leads to a reduced diversity of wild bees in such a way that the pollination service they provide becomes hampered.
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The former mosaic shaped farming landscapes with wetlands, small fields and meadows that are important habitats for many species has become more
homogenous and has put a limit to the biodiversity in these areas (Rundlöf, 2009; Stoate, et al. 2001). We now have fewer but larger farms with larger fields that fragment the landscapes and create barriers for species (Stoate, et al. 2001; Ricketts, et al. 2008). There is a decrease of pollinators with increasing isolation (Ricketts, et al. 2008; Steffan-Dewenter, et al. 1999).
Organic farms are managed without the use of inorganic pesticides and
fertilizers. Studies have been made to see what impact conventional and organic farming has on pollinators and plants. The results have been varied, but
generally it is concluded that organic farming has a positive effect on the
biodiversity compared with conventional farming (Bengtsson, et al. 2005).
Many studies support that (Holzschuh, et al. 2008; Winqvist, et al. 2012; Tuck,
et al. 2014), but there are others who say that different factors such as
surrounding natural vegetation and landscape diversity plays a larger role in the effects on biodiversity. Rundlöf et al. (2007) and Power et al. (2012) concluded that organic farming does support a higher biodiversity, but that landscape heterogeneity can be used to increase the abundance and species richness. In Rundlöf’s et al. (2007) study, landscape heterogeneity showed to have a larger positive effect on conventional farms than on organic farms and organic farms had a larger effect in homogeneous compared to in heterogeneous landscapes.
The objective of this study was to analyze the effect of farming system (organic and conventional) and landscape complexity on the abundance and species richness of pollinators and predatory insects. This study will also examine the effect of the time since transition (when the farms changed from conventional to organic farming) and the specie richness of plants.
Based on previous studies I expected to find 1) a higher species richness and abundance in farms managed organically compared to conventionally, 2) a higher biodiversity of pollinators when the surrounding area is heterogeneous and that 3) long time since transition from conventional to organic farming system will affect the pollinators positively.
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2 Materials and methods
2.1 Data
The data used in this study were collected as a part of a PhD project at the Swedish University of Agricultural Sciences in Uppsala, but has yet not been analyzed.
Data was collected in 2009 on 30 farms (one cereal field per farm), of which 20 were organic and 10 were conventional. One of the organic farms was later removed from the dataset due to that the field was set aside by the farmer. The organic farms differed in the time since transition between 1-23 years. Bumble bees, hoverflies and solitary bees were collected using the pan trap method, which uses cups painted in UV-reflecting paint to attract pollinators. For this collection dispensable coffee cups were used and painted in three colors (blue, white and yellow) and placed in height with the surrounding vegetation. The cups were filled with 50% propylene glycol to preserve the trapped insects and to reduce the surface tension. The drowned insects were later collected, stored in 70% ethanol and sent to an expert that determined them to species. There were five groups of cups used for each farm, each group containing one cup per color (n=15 per farm), and they were evenly distributed over 250 m along the field margin and emptied five times during the season.
A landscape complexity index was calculated using ArcGIS to measure the amount of arable land within a radius of 1000 m from the sampling site (low proportion arable land=low index=heterogeneous and vice versa). The plant diversity was measured twice in 2009 by counting the number of species in totally 20 inventory squares (0.3*0.3 m) per farm. 10 of the squares were distributed along the field margin and the other 10 were split into two groups and placed along two within-field transects at a 1, 5, 10, 20 and 40 m distance from the field border.
2.2 Analysis
The data were analyzed using the statistical program R v 3.0.3 (R Development Core Team, 2014). The given data contained information not only of pollinators but also the bycatch that were predators, mostly wasps. Hoverflies have a
predatory larval stage and for that reason they represented both pollinators and predators. All the predators were treated together in the analyses and the
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bees), bumblebees (Apidae) and hoverflies (Syrphidae). Honey bees (Apis
mellifera) were excluded from the data set due to that their presence would be an effect of where their hives have been placed rather than by farming system or landscape complexity.
I wanted to know the effect of farming system, plant diversity, landscape complexity and time since transition, and their two-way interactions, on the species richness and abundance of pollinators. To test this I created two global models, one where I examined the effects on pollinator and predator species richness and abundance, using the independent factors farming system, plant diversity and landscape complexity (with interactions). The other global model only included the organic farms and thus excluded the factor farming system as a factor and replaced it with time since transition.
I used a general linear model (GLM) to test the effect of the factors. For species richness a Poisson regression model was used and for the abundance a Gaussian regression model (the abundance was log-transformed (log + 1) to fit the
model). The global models were standardized using the arm package (Gelman, et al. 2014), to estimate the relation between parameters by placing them on a comparable scale. I used the dredge function in the MuMIn package (Barton, 2014) to find all possible model combinations. Finally, model averaging was used. Model average weights the different model after how well they interpret the data in order to get average model parameter estimates. Parameter estimates with confidence intervals not overlapping zero were treated as statistically significant (in a frequentist statistics point of view)
3 Results
Farming system had an effect on solitary bees and predators. The abundance of bees increased on organic farms and so did the species richness and abundance of predators (Figure 1). Hoverflies and bumblebees were not affected by farming system but by landscape complexity, where hoverfly species richness and
bumblebee abundance increased with increasing landscape heterogeneity (Table 1 andTable 2). Landscape complexity had no effect on predators or bees and
plant species richness had no effect on either of the test group.
Solitary bees were affected by the interaction between landscape complexity and plant diversity. Both the abundance and species richness increased with higher plant species richness, but only in the heterogeneous landscapes whereas in the
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homogeneous landscapes the effect of plants richness was neutral or slightly negative (Figure 3 and Figure 4).
The interaction between the time since transition and plant diversity affected the abundance of bees. Bee abundance increased with increasing plant richness, but only on farms with short time since transition (below the median) whereas on farms with long time (over the median) it decreased (Figure 5).
Figure 1. Average species richness (±95%CI) of bumblebees, solitary bees, hoverflies and predatory insects on conventional and organic farms.
Figure 2. Average abundance (±95%CI) of bumblebees, solitary bees, hoverflies and predatory insects on conventional and organic farms.
0 2 4 6 8 10 12 14 16 18
Bumblebees Solitary bees Hoverflies Predators Species Sp e c ie s ri c h n e s s Conventional Organic 0 10 20 30 40 50 60
Bumblebees Solitary bees Hoverflies Predators Abundance Av e ra g e a b u n d a n c e Conventional Organic
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Table 1. Results from the global model analysis showing estimates (Est), standard errors (SE) and 95% confidence intervals for abundance of the four test groups (Bumblebees, solitary bees, hoverflies and predatory insects). Farm.sys= farming system; Index=landscape complexity; Plants=plant species richness; TST=time from transition.
ABUNDANCE Bumblebees Bees Hoverflies Predators
Est SE CI low CI high Est SE CI low CI high Est SE CI low CI high Est SE CI low CI high -1,592 0,822 -3,282 0,099 -1,592 0,822 -3,282 0,099 -1,592 0,822 -3,282 0,099 -1,592 0,822 -3,282 0,099 Farm.sys -0,142 0,169 -0,489 0,206 0,748 0,361 0,006 1,489 -0,199 0,249 -0,711 0,313 0,623 0,274 0,060 1,185 Index -0,382 0,157 -0,704 -0,059 -0,214 0,315 -0,863 0,435 -0,201 0,236 -0,686 0,284 -0,366 0,261 -0,904 0,171 Plants -0,091 0,178 -0,456 0,275 -0,356 0,402 -1,177 0,465 0,072 0,259 -0,459 0,602 -0,268 0,326 -0,935 0,399 TST -0,027 0,197 -0,443 0,389 0,458 0,384 -0,357 1,273 0,289 0,302 -0,349 0,928 -0,126 0,304 -0,769 0,517 Farm.sys*Index 0,180 0,327 -0,493 0,854 0,307 0,650 -1,037 1,650 -0,411 0,492 -1,424 0,603 0,180 0,534 -0,921 1,280 Farmsys*plants -0,138 0,391 -0,945 0,670 -0,145 0,762 -1,718 1,428 -0,336 0,577 -1,524 0,852 -0,613 0,615 -0,921 1,280 Index*plants -0,659 0,394 -0,456 0,275 -2,643 0,820 -4,332 -0,954 0,916 0,621 -0,361 2,193 -0,514 0,704 -1,965 0,936 Index*TST 0,027 0,461 -0,956 1,011 0,592 0,949 -1,442 2,625 0,927 0,694 -0,555 2,409 -0,564 0,749 -2,160 1,033 TST*plants 0,286 0,391 -0,550 1,121 -1,900 0,712 -3,417 -0,382 -0,612 0,624 -1,940 0,716 -0,604 0,589 -1,860 0,652
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Table 2. Results from the global model analysis showing estimates (Est), standard errors (SE) and 95% confidence intervals for species of the four test groups (Bumblebees, solitary bees, hoverflies and predatory insects). Farm.sys= farming system; Index=landscape complexity; Plants=plant species richness; TST=time from transition.
SPECIES Bumblebees Bees Hoverflies Predators
Est SE CI low CI high Est SE CI low CI high Est SE CI low CI high Est SE CI low CI high -1,592 0,822 -3,282 0,099 -1,592 0,822 -3,282 0,099 -1,592 0,822 -3,282 0,099 -1,592 0,822 -3,282 0,099 Farm.sys -0,150 0,120 -0,397 0,097 0,441 0,313 -0,196 1,079 0,104 0,127 -0,157 0,365 0,698 0,297 0,087 1,309 Index -0,180 0,118 -0,423 0,064 0,019 0,250 -0,494 0,532 -0,274 0,120 -5,421 -0,098 -0,335 0,251 -0,851 0,182 Plants -0,057 0,124 -0,312 0,199 0,071 0,306 -0,553 0,695 0,038 0,137 -0,521 -0,027 -0,223 0,316 -0,871 0,425 TST -0,105 0,153 -0,429 0,218 0,009 0,281 -0,586 0,604 0,064 0,150 -0,255 0,383 0,020 0,286 -0,585 0,624 Farm.sys*Index 0,087 0,238 -0,404 0,577 0,028 0,493 -0,990 1,047 -0,236 0,248 -0,747 0,275 0,040 0,581 -1,157 1,238 Farmsys*plants -0,002 0,279 -0,577 0,573 0,362 0,589 -0,851 1,575 0,360 0,278 -0,215 0,935 -1,914 0,474 -1,914 0,474 Index*plants 0,016 0,297 -0,597 0,628 -1,677 0,691 -3,092 -0,262 0,102 0,312 -0,539 0,744 -2,042 0,903 -2,042 0,903 Index*TST -0,186 0,369 -0,973 0,602 0,557 0,669 -0,873 1,987 0,205 0,349 -0,541 0,952 -0,572 0,710 -2,083 0,939 TST*plants 0,325 0,313 -0,342 0,993 -0,736 0,512 -1,830 0,357 -0,454 0,300 -1,095 0,186 -0,829 0,576 -2,052 0,394
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Figure 3. The interaction between landscape complexity (low index= more heterogeneity, high index more homogeneous) and plant diversity on the species richness of solitary bees. High and low index, respectively, is based on the median index of all farms.
Figure 4. The interaction between landscape complexity (low index= more heterogeneity, high index more homogeneous) and plant diversity on the abundance of solitary bees. High and low index, respectively, is based on the median index of all farms.
0 2 4 6 8 10 20 30 40 50 S pe c ies r ic hn es s Plant diversity High index Low index 0 5 10 15 20 25 10 20 30 40 50 A bu nd an c e Plant diversity High index Low index
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Figure 5. The interaction between the time since transition and plant diversity on the abundance of solitary bees. High and low index, respectively, is based on the median index of all farms.
4 Discussion
4.1 Farming system
Organic farms are thought to sustain higher biodiversity (Bengtsson, et al. 2005;
Holzschuh, et al. 2008; Winqvist, et al. 2012). Therefore, it was not expected
that the effect of organic farming on pollinators would be so weak. Results from previous studies suggest that both abundance and species richness of all test groups should have improved on organic farms (Holzschuh, et al. 2008; Tuck, et al. 2014), but in this study only bees and predators responded positively to
organic management. It was only for the predators that both the abundance and species richness increased on organic farms, whereas for bees only the
abundance did.
My results support the results of Bengtsson et al. (2005) who found that predatory insects are more sensitive to farming system than other insects. Interesting to notice in his study was that while there was an increase of predators on organic farms, there were no noticeable increase of other insects and pests in comparison to conventional farms. My results thus provides indirect support for the notion that predators may manage pest control in organic farms just as well as pesticides in conventional farms (Bengtsson, et al. 2005).
4.2 Landscape complexity
A heterogeneous landscape contains a larger variation of habitats such as ponds, ditches, meadows and woodland edges, than homogeneous landscapes. This is
0 5 10 15 20 25 20 25 30 35 40 45 50 A bu nd an c e Plant diversity Long time Short time
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thought to be the reason for the increased biodiversity that exist in these areas. The heterogeneity provides habitat for a larger variety of species.
I found that the species richness of hoverflies and the abundance of bumblebees increased with higher landscape heterogeneity. This supports my hypothesis that greater landscape heterogeneity supports a higher biodiversity. However, the fact that landscape complexity did not affect all groups still stands. One possible reason could be the size of the farm. Small farms have been shown to sustain a higher biodiversity then larger farms. These farms often have smaller field size and this has shown to have a positive effect on biodiversity (Belfrage, et al. 2005). This could have affected my result and evened out the differences both between farming system and landscape complexity. Because I had no
information regarding the size of the farms used in this test I could neither confirm nor reject this hypothesis.
Another possible explanation is that in a landscape with lots of plants the insects would favor the flowers instead of the pan trap. This would give a concentration effect were more insects fall into the traps in areas with low compared to high plant abundances.
4.3 Interactions
Both the abundance and species richness of bees were affected by the interaction between landscape complexity and plant species richness. These interactions are hard to explain and I can only speculate the reason behind them. The
interactions showed that the effect of plant species richness on bees was only evident in landscape with low proportion arable land whereas in landscapes with high proportion arable land the effect of plants richness decreased slightly or was neutral.
Higher plant species richness in an area dominated by arable land was thought to increase diversity for bees and not decrease, because plants provide food for the pollinator.
The negative effect of plant richness on the bees in homogeneous landscapes was not as clear as the positive trend in the heterogeneous landscapes with high plant species richness. This could be interpreted as when the surrounding area reaches a certain level of homogeneity the amount of plant species does nothing to improve the living conditions for the bees. Hence, the negative effects of homogeneous landscapes cannot be overcome by high (local) plant diversity (Jonason, et al. 2012).
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The interaction between the time since transition and plant diversity might be explained by a concentration effect. A farm that has been managed organically for long times might support a higher diversity of plats due to the absence of herbicides and a long time of possible immigration of plants from the
surroundings. This might result in the farms having such a high amount of plants that the abundance of bees gets diluted over the real plants and the pan traps. This leaves less pollinators to fall in the pan traps, or the pollinators simply prefer the real plants.
4.4 Social and ethical issues
Ethical issues are of less importance in my part of the project which is
theoretical and only handles already collected data. However, the data I used is based on the collection of a large amount of insects that have been killed, and there is chance of red-listed species have been represented. The amount of collected insects, however, is fairly insignificant in relation to the abundance of the species in nature.
The ecosystem services that pollinators and predators provide are essential for many plants and for human welfare. We would not only lose biodiversity but also crop yield as pollinators improve production of certain crops. This would mean an economic and productivity loss that could have severe impact on society.
4.5 Conclusions
The results from my study suggest that pollinators are more affected by
landscape composition than farming system. This means that organic farming are not as effective in enhancing biodiversity for pollinators as the promotion of a varied landscape. Predatory insects on the other hand reacted stronger to farming system as both abundance and species richness increased on organic farms.
My study also suggests that high plant diversity cannot overcome negative effects of homogeneous landscapes. This reinforces the importance of varied landscape for biodiversity.
5 Acknowledgements
I want to thank Dennis Jonason for guiding me through this project and sharing his data with me.
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