Pests and pest controlling organisms across tropical agroecological land-

scapes in relation to forest and tree-cover

Debissa Lemessa

© Debissa Lemessa, Stockholm University 2014 Cover photo and trophic cascade illustration: Debissa Lemessa ISBN 978-91-7447-881-5 Printed in Sweden by US-AB, Stockholm 2014 Distributor: Department of Ecology, Environment and Plant Sciences

To my children: Marartuu, Ilillii and AagaYah

Doctoral dissertation Debissa Lemessa Department of Ecology, Environment and Plant Sciences, Stockholm University SE – 106 91 Stockholm, Sweden

Pests and pest controlling organisms across tropical

agroecological landscapes in relation to forest and tree-

cover Abstract – A major challenge in agroecosystems is how to manage the systems so that it reduces crop pests and enhances natural pest control. This thesis investigates patterns of crop pests and top-down effects of birds and arthropod predators in relation to land-use composition across spatial scales. In paper (I) I examined the crop distribution and land-use types in relation to the crop raiding patterns in 15 transects in sites close to and far from forests along with a questionnaire survey at household level. I found severe crop raiding close to forests, but it had no impact on crop composition growing between the two sites. In paper (II) I examined the effect of forest and tree-cover, at local and landscape scales, on the abundance of arthropod predators by collecting specimens from 40 homegardens. My result showed higher abundance of arthropod predators when either the homegarden or the surroundings had a high tree-cover, compared to when tree cover at both scales was similarly either high or low. In paper (III) I investigated the effect of excluding birds and arthropod predators on leaf damage on rapeseed in 26 homegardens. I found stronger top-down impacts from arthropod predators on crop pests in tree-poor gardens than in tree-rich gardens. There was no effect of birds. In paper (IV) I explored the effect of landscape complexity on bird and arthropod predation using plasticine caterpillars in 36 homegardens across landscapes. The rate of arthropod predation on caterpil-lars was higher in simple than in complex landscapes. The rate of bird preda-tion did not vary between complex and simple landscapes. In simple land-scapes, arthropod predation was higher than that of birds. The overall results suggest that simplified gardens/landscapes still have enough habitat hetero-geneity to support arthropod predators for the significant top-down control-ling effect on crop pests. However, I did not find clear effect of complexity on the top-down effect of birds.

Key-words: Activity abundance, Crop raiding, Exclosure experiment, Homegarden, Leaf damage, Predation, Spatial scales, Structural complexity

List of papers

The thesis consists of the following four papers which are referred to by roman numerals:

I. Lemessa, D., K. Hylander and P.A. Hambäck. 2013. Com-position of crops and land-use types in relation to crop raiding pattern at different distances from forests. Agriculture, Ecosystems and Environment 167: 71-78.

II. Lemessa, D., P.A. Hambäck and K. Hylander. The effect of local and landscape land-use composition on arthropod predators in a tropical agricultural landscape (submitted)

III. Lemessa, D., Ulrika Samnegård, P.A. Hambäck and K.

Hylander. Tree cover mediates the effect of excluding ar-thropod predators on crop leaf damage, but in an unex-pected way (submitted)

IV. Lemessa, D., P.A. Hambäck and K. Hylander. Birds and arthropod predation on plasticine caterpillars across tropi-cal agricultural landscapes (manuscript)

Paper I is reprinted with permission from Agriculture, Ecosystem and Environment.

Contents

Introduction ........................................................................................ 11 Background .............................................................................................. 11

Crop pests and natural enemies ................................................................ 12

Objectives of the thesis ............................................................................ 15

Materials and Methods ....................................................................... 16 Study areas and study systems ................................................................. 16

Location, topography and climate ............................................................ 16

The vegetation type of the study areas ..................................................... 17

Agricultural landscapes of the study areas ............................................... 17

Crops and crop raiding patterns from forest edges (Paper I) .................... 20

Abundance of arthropod predators in homegardens (Paper II) ................ 20

Field cage experiment on crop leaf damage (Paper III) ........................... 21

Birds and arthropod predation across landscapes (Paper IV) ................... 22

Results and Discussion ....................................................................... 23

Concluding remarks ............................................................................ 27

Acknowledgements ............................................................................ 28

References .......................................................................................... 29

Svensk sammanfattning ...................................................................... 34

Tack/Thanks ....................................................................................... 38

Abbreviations

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Introduction

Background Human land-use for food production through intensification of agri-culture is the primary cause of the many modified or changed land-scapes that we observe today. Fragmentation and loss of habitats in landscapes are some of the many outcomes of the human land man-agement mainly for agriculture throughout the world (Whitmore 1997). Disruption of biological interactions, for example, natural pest control functions in agroecosystems, uneven distribution of organisms or the isolation of species in to small subpopulations and changes in the community structure across agroecosystems have been taking place as a result of the fragmentation of natural habitats (Gaston 2000; With et al. 2002; Cayuela et al. 2006). These ecological dynamics and processes are the challenges for how the biodiversity could be man-aged in agricultural landscapes for the sustainable utilization of the resources and services (Millennium Ecosystem Assessment 2005; An-go et al. 2014). These are challenges because the systems in agricul-tural landscapes are either positively or negatively affected by the management practices for agricultural production. For example, clear-ing away of the forests at the forest−farmland interface or cutting trees from crop lands to mitigate the problem of crop pests could negatively affect different kinds of beneficial organisms such as natural enemies and pollinators. Hence, the management of ecological disservices (e.g., mammal and insect pests of crops) and services (e.g., natural pest control) needs a critical understanding of the effects of the habi-tats formed in agricultural landscapes. Such understanding is even more crucial in tropical regions where the landscapes are typically heterogeneous and consist of small different land-use types that are rapidly changing as a result of extensive and subsistence farming sys-tem, rapid deforestation and land degradation and where little is known in terms of how these land-use dynamics affect biodiversity and related ecosystem services.

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Crop pests and natural enemies Humans get both positive and negative provisions from nature. The provisions from nature also harm the well-being of the humans by reducing productivity or by increasing production costs and also by threatening the health of humans. This concept is known as ecosystem disservices (Zhang et al. 2007). For example, crop pests reduce agri-cultural production and hence could be categorized as one of the eco-system disservices (Zhang et al. 2007; Dunn 2010). On the other hand, the beneficial provisions from nature, for example, top-down control of pests by natural enemies is an ecosystem services. Ecosystem ser-vices are defined by Daily (1997) as ‘the conditions and processes through which natural ecosystems, and the species that make them up, sustain and fulfill human life’. The use of the concept of ecosystem disservices is controversial and debated. To exemplify, in many aspects, forest patches and trees pro-vide important economic, social and ecological services including food, wood values, fibers, shade and improving of soil fertility and microclimate in agroecosystems. However, at the same time these forest patches and trees in agricultural landscapes also harbor pests or are hosts for agricultural pests and diseases and these harm the liveli-hoods of the farmers. The farmers, mainly those who live close to the forests, face severe impacts from crop raiders and hence may develop negative attitude towards forests (Paper I) and their management prac-tices most likely also gear towards how to mitigate these problems, for instance, by clearing off forests and trees (Ango et al. 2014). Howev-er, what farmers consider only as disservice (e.g. crop raiders) could provide important ecological functions or services such as seed dis-persal by baboons, monkeys and birds. For this reason in this thesis I considered both crop pests and natural pest control as ecological func-tions or processes taking place in agricultural landscapes. Basically, crop pests include herbivores, frugivores, seed-eaters and pathogens (fungal, bacterial and viral diseases) that decrease the productivity and sometimes cause a complete crop loss. Revenue loss from insect pests and pathogens can be disproportionately high for some crops that are sold in fresh condition and for which its price is dependent on such quality (Babcock et al. 1992). Major crop pests in

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the tropics that affect agricultural production are: armyworms, locusts, stem borers, aphids, sorghum shoot flies, termites and coffee berry disease (Bijlmakers 1989). In tropical homegardens the common in-sect pests that mainly cause damage to cruciferous vegetable and oil crops are aphids, diamondback moth and flea beetles (Gera district agricultural offices unpublished document). Crop raiding by mammals such as olive baboons, monkeys, bush pigs, porcupines etc. also caus-es considerable crop damage in the tropics and this impact is most often severe close to forest edges (Naughton-Treves 1997). The top-down trophic interactions between natural enemies (both ver-tebrate and invertebrate) and crop pests, also called ‘natural pest con-trol’, is one example among the many regulatory services that humans get from nature in agricultural landscapes. Natural pest control is the ecological process by which naturally occurring predators and parasi-toids suppress the population of pests within the habitat they share. For example, avian insectivores, spiders and predatory beetles are among the many natural enemies which are important for pest control in agricultural landscapes (Schmidt et al. 2008; Maisonhaute et al. 2010; Johnson et al. 2010). Several studies have shown that insectivo-rous birds reduce insect pests and their damage to crops (Van Bael et al. 2008; Sinu 2011). Spiders are generalist predators but mostly prey on aphids from crops, while predatory beetles are also non-selective but vigorously attack larvae of lepidopteran crop pests (Nyffeler and Sunderland 2003; Shough 1940). However, the services provided by these predator organisms are influenced in different ways by the envi-ronmental variables, such as forest cover, tree cover and open non-crop cover (e.g. grazing/grass lands) across local and regional scales (Rusch et al. 2010; Maisonhaute et al. 2010; Oõrourke and Blitzer 2011; Şekercioğlu 2012). Throughout history human beings have been altering the natural habi-tats of the organisms mainly for food production. Loss of habitat and degradation, crop fertilization and insecticide application are some of the outcomes of human land management that in turn has resulted in the disruption of the trophic interactions in ecosystems (Borer and Gruner 2009). Several conceptual models were developed to illustrate how the human land management causes loss of biodiversity and af-fects the trophic interactions in agroecosystems (cf. Swinton et al. 2007; Zhang et al. 2007; Rusch et al. 2010).

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Land degradation causes soil erosion and impoverishment of soil fer-tility which in turn reduces agricultural production. Loss of habitat due to land degradation also increases pest pressure on crops for the reason that crop pests move to crop fields for more food resources and for refuges. In fact, natural habitat conversion to agricultural land af-fects both natural enemies and pests. Plowing the land and application of pesticides directly kills both insect pests and pest controllers such as ground predatory beetles and hunting spiders (Rusch et al. 2010; Mazzi and Dorn 2012). In crop fields the natural enemies may fail to perform well in suppressing crop pests (top-down effects) since re-sources are insufficient to sustain their population (Gurr et al. 2003). Moreover, seasonal farming practices such as soil tillage, annual crop growing and harvesting may increase the temporal variation in the availability of resources (Rand et al. 2006; Oõrourke and Blitzer 2011). The crop fields are also not stable habitats for crop pests, for example, when crops are harvested they survive in the surrounding semi-natural habitats available for overwintering or for food resources (Bianchi et al. 2006; Tscharntke et al. 2007). The modification of the landscape does not only affect the top-down trophic interaction between natural enemies and the herbivores but also the interaction between herbivores and their host plants (Ben`itez-Malvido and Lemus-Albor 2005). The clearing of the forests to reduce the impact of mammal crop pests such as baboons, monkeys and bush pigs could negatively influence the beneficial organisms such as pred-ators, parasitoids, seed dispersers, pollinators, and decomposers. Moreover, the modern agricultural system, for example, monoculture and intensive use of agrochemicals (artificial fertilizers and various pesticides) to increase food production could negatively affect these beneficial organisms. Therefore, to conserve or manage and enhance the service they provide in agroecosystems it is necessary to under-stand how they are affected by the different land-use systems across spatial scales.

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Objectives of the thesis

The general objective of my thesis is to explore how the composition of land-use types including forest and tree-cover affects the ecological patterns and processes related to crop pests and pest controlling organ-isms across spatial scales and across landscapes in agroecosystems of southwest Ethiopia. Specific objectives of the thesis were:

• To examine how the variation in distribution of land-use types and grown crops is related to crop raiding patterns of larger mammals (olive baboons and bush pigs) from forest edges across a landscape of southwestern Ethiopia.

• To investigate how the land-use composition (at local and landscape level) influences the abundance and species compo-sition of arthropod predators in homegardens of southwest Ethiopia.

• To explore the effect of birds and ground arthropod predators on crop leaf damage and how this effect is mediated by a tree cover gradient across a landscape.

• To examine the effect of structural complexity on the rates of predation on plasticine caterpillars by birds and by arthropods across agricultural landscapes of southwest Ethiopia.

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Materials and Methods

Study areas and study systems

Location, topography and climate

For this thesis, all my study sites were situated in agricultural land-scapes of Jimma zone, southwest Ethiopia (7o 24'─8o 4' N and 35o58'─37o 14' E, Fig. 1). The study landscapes lie within the altitudi-nal range of between 1600−2500 m a.s.l. and their topographic fea-tures comprise of gentle, undulating, hilly and rugged slopes. The ma-jor soil type is Nitosol (Dubale 2001; Nigussie and Kissi 2012). Nito-sols are characterized by reddish color with either silt or clay texture, has low levels of nitrogen and phosphorus but high levels of iron (Fe). All over the landscapes the main rainfall season is from May to Sep-tember and the amount varies between 1150 and 2080 mm per annum. The monthly mean temperature varies between 10 to 26 °C (Nigussie and Kissi 2012, District agricultural development offices unpublished document).

Figure 1. Agricultural landscapes of southwest Ethiopia: the map of study landscape produced from satellite image in Google Earth for paper I, II and III is shown by (A) and study landscapes for paper IV are shown by (B).

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The vegetation type of the study areas The vegetation type of Ethiopia in general and of southwest Ethiopia in particular have been described several times in the past mostly based on climate and physiognomy (cf. Friis et al 1992; Senbeta 2006). According to Friis et al (2010) the main vegetation type of the southwest Ethiopia is described as moist evergreen afromontane for-est. The dominant tree species in this forest include: Pouteria adolfi-friederici, Croton macrostachyus, Schefflera abyssinica, Apodytes

dimidiata, Ficus spp, Syzygium guineense, Allophyllus abyssinicus, Cordia africana, Millettia ferruginea, Sapium ellipticum, Albizia spp, Acacia abyssinica and Olea welwitschii. In most parts of southwest of Ethiopia, Coffee arabica grows as a forest understory shrub (Senbeta 2006; Hundera et al. 2013), but also in different systems as semi-forest coffee, garden coffee and plantation coffee (Teketay 1999).

Agricultural landscapes of the study areas The once pristine forest landscapes in southwest Ethiopia have been transformed to human dominated heterogeneous agricultural land-scapes (Hylander et al. 2013). As a result of this human land transfor-mation the landscapes in southwest Ethiopia now comprise of differ-ent settings such as homegardens surrounded by live fences, croplands, grazing lands, grasslands, wetlands, large and small forest patches, scattered trees, semi-forest coffee patches, garden coffee, ensete (a perennial tuber and root crop) and khat farming (a perennial stimulant crop) (Paper I).

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The variation in forest and tree-cover and tree diversity across the landscape may reflect a management intensity gradient from forest edges (Fernandest and Nair 1986; Peyre et al. 2006; Tolera et al. 2008). Homegardens are interesting from several dimensions since they often have a high diversity of crops and are interspersed through-out the landscape (Abate et al. 2000; Paper I). The homegardens I studied were situated both in the vicinity of the forest edges as well as several kilometers away from forests and the landscape around them varies in forest/ tree cover as well as in several other respects (e.g. in distribution of grasslands, wetlands etc.). Subsistence farming of an-nual and perennial food crops is the main land-use system in the open agricultural landscape apart from coffee production in aforementioned systems. The views of representative landscape and homegardens of southwest Ethiopia are shown in Fig. 2. The annual crops grown are teff (Eragrostis teff), maize, barley, sor-ghum, wheat, pulse and oil crops while the perennial crops are coffee (Coffea arabica), khat (Catha edulis), ensete (Ensete ventricosum) and avocado. Here, coffee and khat are stimulant plants and are commonly grown by most farmers. Ensete (Ensete ventricosum, Musaceae) is a perennial herbaceous plant and is called ‘false banana’ as it is mor-phological similar to banana. In the area, ensete abundantly grows in homegardens mainly at upper altitudes and people extracts food from its pseudostem and corm mainly for household consumption.

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Figure 2. Examples of the agricultural landscapes of southwest Ethio-pia with different level of complexity (A vs. B) in terms of forest and tree cover and land-use compositions (Photo: Debissa Lemessa).

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Crops and crop raiding patterns from forest edges (Paper I) To assess the impact of the mammal crop raiding close to and far away from the forest edges I made field survey on field crops and homegarden crops. In the surveys I walked on 1 km transects, 15 tran-sects were laid out close to and 15 transects far away from forest edg-es in a pair-wise design. I also interviewed farmers (30 close to and 30 far from forests) at the household level regarding the impact and fre-quency of crop raiding by olive baboons and bush pigs both in the gardens and in the fields (Paper I, Fig. 1 A). Abundance of arthropod predators in homegardens (Paper II) For this study I selected 40 homegardens situated in different locations from the forest edges and differ in structural complexity from Google Earth picture. I installed six plastic pitfall traps in each homegardens to study how the community dynamics of arthropod predators was affected by the composition of local land-use types and landscape for-est and tree cover (Paper II, Fig. 1 A). At the local scale (within 100 x 100 m surrounding the central houses), I inventoried the land-use types that include tree cover, number of trees and tree species, annual crop species, ensete cover, fallow lands, grazing lands and grassland cover. From these land-use types, I pooled the proportion of the land-use types for grazing lands, grasslands and fallow lands into a catego-ry named “open non-crop cover”. Moreover, at landscape scales of 200 and 500 m radii buffers I quantified forest and tree cover from a pan-sharpened high resolution (0.5 m) big world view2 satellite image (October-November 2011, projected in UTM WGS 84) of the land-scape using ArcGIS 10 (ESRI 2011) and program ChorosLandCover (Izolde and Choros Cognition Company 2012).

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Field cage experiment on crop leaf damage (Paper III) For Paper (III) I examined how excluding birds and ground-dwelling arthropod predators affected the insect herbivory damage to leaves of rapeseed plants (Brassica napus). For this, I established field exclo-sure experiment in 26 homegardens (Fig. 1 A). In the same study ap-proach as in paper (II), I selected 26 homegardens from the landscape with different level of complexity and which were also located at dif-ferent distances from forest edges. I sowed rapeseed in each of 26 homegardens on a plot size of 5 x 2.5 m during the crop growing sea-son of the area. When the plants developed ≥ 3 leaves I built a bird exclosure with woody frames and bird net cage (20 x 20 mm cell size) to exclude birds on the plot (2.5 x 1.5 m) leaving the rest for control. The bird exclosure cage excluded birds only but insect herbivores and arthropod predators could enter and go out through the nets. Along with this, after I randomly selected six groups of plants (five individu-als each) inside the bird net cage and six groups from control plots, I excluded the ground arthropod predators from three groups of plants inside the bird exclosure and from three groups in open plots using undercut plastic buckets while three groups in respective plots were used as control. Data on leaf damage per plant due to insect herbivores was estimated for four rounds with approximately one week time in-terval in between. To study the effect of environmental variables: (1), at the plot scale (20 x 20 m) surrounding the rapeseed plots, I estimated tree cover. Moreover, (2) at the local scale (within 100 x 100 m surrounding the central houses), I inventoried the land-use types that include tree cov-er, number of trees and tree species, annual crop cover, fallow lands, grazing lands and grassland cover. From these land-use types, I pooled the proportion of the land-use types for grazing lands, grasslands and fallow lands into a category named “open non-crop cover” and (3) at landscape scales of 200 and 500 m radii buffers, I also quantified for-est and tree cover for each homegardens from a pan-sharpened high resolution (0.5 m) big world view2 satellite image (October-November 2011, projected in UTM WGS 84) of the landscape using ArcGIS 10 (ESRI 2011) and program ChorosLandCover (Izolde and Choros Cognition Company 2012).

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Birds and arthropod predation across landscapes (Paper IV) The effect of the structural complexity on the patterns of predation rates of birds and arthropod predators was investigated using plasti-cine caterpillars across six landscapes (Paper IV, Fig. 1 B). For this purpose, I identified three complex landscapes that had higher forest and tree-cover (> 30 %). and three simple landscapes with lower forest and tree-cover (< 20 %) from the pictures in Google Earth. From each landscape, I selected three simple and three complex homegardens for a total of 36 homegardens in a full factorial design. At the beginning of the study I recorded the number of trees and the number of stems in each homegarden within 100 x 100 m surrounding the central houses. During this inventory work I identified two perennial crops: coffee and avocado, which are commonly grown by the farmers in their gar-dens. The aim of the identification of these two shrub species was to use them for attaching plasticine caterpillars and so that I could con-trol or avoid the bias that might come from the tree species preference of birds and arthropod predators while preying on plasticine caterpil-lars. After I randomly selected two avocado and two coffee shrubs which were located in different places within the garden, I attached 100 plasticine caterpillars (diameter = 2.5 mm and length = 10 mm) per homegarden on their leaves approximately at the height of be-tween 1-1.5 m from the ground using liquisole glue (Casco). I collect-ed data on the number of plasticine caterpillars attacked and re-moved/missed in each garden either Monday-Wednesday-Friday or Tuesday-Thursday-Saturday, since I did not check for predation on Sundays. This was done for seven to nine consecutive weeks (August 26 to October 25, 2013).

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Results and Discussion

The results of this thesis suggest that the human land management induced habitat heterogeneity such as forest/tree cover and open non-crop cover across spatial scales are shaping the pattern of distribution of crop pests and natural enemies or natural pest control services in agricultural landscapes of southwest Ethiopia. In my study for paper (I), I hypothesized that the spatial distribution of crop species compo-sition would differ between sites close to and far from the forests across a landscape. The result of the questionnaire survey on the pat-tern of crop raiding showed that the farming communities dwelling close to the forest edges are more prone to the crop raiding by olive baboons and bush pigs than those farmers situated at far away from forest edges. Close to the forest edges farmers are always in conflict with the crop raiders and this impact may shape the management prac-tices of the farmers’ related forests and trees in the landscape, for ex-ample, farmers may clear off and cut trees to avoid the problem of crop raiding. The different crop raiding avoidance measures taken by the farmers mainly related to the different tree clearing strategies such as cutting, burning and ringing or debarking of trees was also shown by another study performed in the same landscape (Ango et al. 2014). However, amazingly, my result from this paper (I) was in contrary to my expectation and it showed no significant difference in species composition of crops between sites close to and far away from forest edges despite that farmers close to the forest edges are victims of se-vere crop raiding by mammal pests. My explanation for the lack of such differences in crop distribution between these sites is that there could be lack of alternative crop species that are not attacked and which could also grow in that agro-climatic condition. Moreover, the farming communities may not want to cease growing of the crop spe-cies despite the pest damage as these crops are an integral part of their cultural feeding system that they have adapted for several decades (Medley et al. 1995). The results from papers (II and III) are somewhat related to each oth-er. In paper (II) I found that the abundance of pooled arthropod preda-tors (spiders, predatory beetles and predatory true bugs) was much higher in heterogeneous landscapes where the tree cover was high at either local or landscape scales than in homogeneous landscapes where the tree cover was either high or low at both local and land-

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scape scales. Similarly, the abundance of spiders was highest at an intermediate land use cover or in heterogeneous habitats that com-prised of both open non-crop cover (e.g. grazing lands) and some tree cover compared to the landscapes that consisted of either higher for-est/shrubby habitats or simplified open fields. The possible interpreta-tion for this pattern could be that these organisms may benefit more from the mixed habitats than from a single habitat type. For example, such heterogeneous habitats may provide niche complementarity for different species in terms of different resources for reproduction and stability during local disturbances (Maudsley et al. 2002; Rusch et al. 2010). Moreover, during local disturbances, due to heavy grazing or crop cultivation, some tree cover in the landscape may provide better refuge (physically and in terms of a sheltered micro-climate) (Schmidt and Tscharntke 2005; Chaplin-Kramer and Bremen 2012; Thomson and Hoffmann 2013). Hence, my result from paper (II) suggest that maintaining both open non-crop habitats and some tree cover can en-hance the abundance of arthropod predators and perhaps also the cor-responding natural pest control services cover in tropical heterogene-ous landscapes. In paper (III) my hypothesis was that excluding birds and ground ar-thropod predators would release higher number of crop pests which in turn would cause higher leaf damage to crops, and these effects were expected to be stronger in tree-rich gardens or in the surrounding landscape with higher tree compared to in tree-poor gardens or in the landscapes. Surprisingly, my results were in contradictory with my hypotheses and instead showed no effect of bird exclosure on leaf damage. One possible explanation for the lack of significant impact of birds on the leaf damage could be that pest suppression activity of birds may depend on the density of insect herbivores (Singer et al. 2012; Schönrogge et al. 2013). In this case, I observed only low level of mean leaf damage per plant from the whole study period or from all four round visits (i.e., 3.2 % and 3.0 % inside/outside the bird net cage, respectively) most likely showing low abundance of herbivores that were acting on rapeseed plants. Another interpretation is that this experiment was performed during the active crop growing season so that the top-down effects of birds may be diluted among the different kinds of vegetables and crops found surrounding my plots and this might have lowered the effect of bird predation.

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In contrast, ground arthropod exclosure has caused higher leaf damage compared to the control (not excluded) rapeseed plants which is more in line with my hypothesis. These results show that ground arthropod predators were more important for top-down control of insect herbi-vores than birds in homegardens of southwest Ethiopia. However, unexpectedly, the top-down effect of ground arthropods was stronger in tree-poor gardens or in landscapes with low tree cover than in tree-rich gardens or in the surrounding with higher tree cover. One explanation for this pattern is that in tree-poor gardens there are higher density of herbivores and also more arthropod predators are present there to control them. This result shows that had I had studied the leaf damage on the rapeseed plants without excluding arthropod predator I would have made a wrong conclusion that there is no effect of local and landscape variables (e.g. tree cover, open non-crop cover) on the leaf damage. The results from paper (I) and (II) are not fully similar. In paper (II) I found low abundance of ground arthropod predators in gardens when tree cover is lower similarly both at local and landscape scales, while in paper (III) I instead found the strongest top-down effects in tree-poor gardens/landscapes. In most cases, the land-use type that is dominant in tree-poor gardens are either the annual crop fields or open non-crop cover (e.g. graz-ing/grasslands) or both. Moreover, this study was performed during the active crop growing season and hence there is high food resources in tree-poor gardens which possibly attracts more herbivores. The presence of high number of herbivores also in turn may attract their natural enemies (Rand et al. 2006; Bianchi et al. 2013). It is known that higher number of herbivores utilizes annual crops as the potential resource for food in simple landscapes while their abundance is most likely lower in complex landscapes (Rusch et al. 2010; Caballero-López et al. 2012). Moreover, according to the prediction of the re-source concentration hypothesis (Root 1973) the population of herbi-vores is lower in tree-rich habitats and if so, excluding arthropod predators in tree-rich gardens may not show high leaf damage. In paper (IV), I had two hypotheses; firstly, I hypothesized that the rates of predation of both birds and arthropods on plasticine caterpil-lars would be higher in complex gardens/landscapes than in simple gardens/landscapes. The assumption here is that there would be more refuges and alternative resources for predation in complex gar-

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dens/landscapes than in simple gardens/landscapes. Secondly, I hy-pothesized that there should have been larger variation in rates of pre-dation between complex and simple gardens in simple landscapes than in complex landscapes. I expected this pattern assuming that the sim-ple homegardens in simple landscapes are more simplified, poorer in tree cover and less connected to buffering habitats than the simple homegardens in complex landscapes. Surprisingly, my results were contradictory to my expectation. Using plasticine caterpillars, I found a mean daily predation rate of 1.45 % in complex landscapes and 1.46 % in simple landscapes for birds. For arthropods the mean daily predation rates were 1.34 % in complex landscapes and 1.85 % per day in simple landscapes. As these preda-tion rates indicated, the rate of predation by arthropod predators was higher in simple landscapes than in complex landscapes but also that there was no difference in the rate of bird predation between these landscape types. In a similar way, there were no differences in the rates of predation between complex and simple gardens either in com-plex or in simple landscapes for both birds and arthropods. In simple landscapes, but not in complex landscapes, the rate of arthropod pre-dation was higher than that of birds. The lack of effect of complexity on the pattern of bird predation may be due to that birds are highly mobile (higher dispersal ability) and broadly forage from different habitats to cope with the changing habi-tats in human dominated landscapes (Şekercioğlu 2012). In this regard, a meta-analysis performed by Van Bael et al. (2008) also showed a lack of difference in level of bird predation between habitats that vary in complexity in the tropics, for example, between forests (complex habitat) and agroforestry (simplified habitat). Several previous studies have similarly showed that the difference in habitat or landscape com-plexity in agroecosystems has small effect impact on the pattern of bird predation (Gunnarsson 1996; Greenberg et al. 2000; Posa et al. 2007). Nevertheless, the overall rates of predation by birds and arthro-pods varied among the study landscapes, even though this was not captured by the measured variables. This pattern may indicate that apart from the vegetation structure, my study landscapes may also vary by other landscape specific processes.

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Concluding remarks Over all, the results of this thesis suggest that both pests and pest con-trollers are affected by the human land-use practices across scales in the landscape. Farmers situated close to forests face severe crop raid-ing problem almost throughout the year (Paper I). The influence of this frequent negative interaction between mammal crop raiders and the farmers at the crop fields–forest interface may impact the farmers to develop a negative attitude towards forest conservation. Arthropod predators are important top-down controllers of crop pests and this effect is stronger in tree-poor gardens or in simple landscapes compared to in tree- rich gardens or complex landscapes. My result from paper (IV) also suggest that birds are important predators for reduction of crop pests in agricultural landscapes. However, I did not find clear evidence for the effect of the complexity on the top-down effect of birds. Taken together, the simplified gardens/landscapes of southwest Ethiopia still have sufficient habitat heterogeneity that can support arthropod predators to a certain level so that they can have significant top-down controlling effect on crop pests (Paper II, III and IV). However, had these gardens/landscapes been extremely simpli-fied or cleared they might not have enough such habitat heterogeneity and resources to support these predator organisms (cf. Tscharntke et al. 2012).

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Acknowledgements

I would like to express my deepest gratitude to Kristoffer Hylander and Peter Hambäck for their valuable comments on this summary. My thanks also goes to Niklas Lönnell and Maria Johansson for their help to translate my summary to Swedish.

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Svensk sammanfattning Landskapets utseende påverkas starkt av människans markutnyttjande, inte minst för matproduktion. Intensifieringen av jordbruket är en av huvudorsakerna till de förändringar i landskapet vi ser idag. Fragmentering och habitatförlust är några effekter av människans brukande av marken, vilket kan gå ut över människors välbefinnande, leda till förlust av biologisk mångfald och påverkan på biologiska interaktioner. Samtidigt kan det omgivande landskapet påverka människans odlingar både positivt och negativt. Dessa effekter påverkas i sin tur av hur människan brukar landskapet. Min avhandling undersöker mönster och processer kopplade till ekosystemtjänster och otjänster i det tropiska jordbrukslandskapet. Mer konkret så behandlar avhandlingen hur sammansättningen av landskapet (mätt t.ex. som mängden skog och träd) påverkar skadedjur och naturliga skadedjursfiender på olika skalor i jordbruksekosystem i sydvästra Etiopien. De studerade landskapen är relativt kuperade med böljande kullar samt ligger på en höjd mellan 1600 och 2500 m.ö.h. De rödaktiga, leriga-siltiga, svagt sura jordarna (nitosoler) i området har låga halter av kväve och fosfor och höga halter av järn (därav färgen). Regntiden infaller mellan maj och september och nederbörden varierar mellan 1150 och 2080 mm per år. Månadsmedeltemperaturen är mellan 10 och 26 °C. Den ursprungliga vegetationen utgörs av fuktig bergsregnskog där ett stort antal träd förekommer. Kaffe (Coffee arabica) växer vilt i buskskiktet i sydvästra Etiopiens skogar men odlas nu i hemträdgårdar, i halvöppna skogar där övrigt buskskikt har tagits bort för att gynna kaffet och till viss del även i planteringar. Jordbrukslandskapet i sydvästra Etiopien är ofta mosaikartat och består ofta av många olika element såsom hemträdgårdar, åkrar, betesmarker, andra gräsmarker, stora och små skogsdungar och solitära träd under vilka ofta kaffe odlas. Framför allt används marken för odling av livsmedelsgrödor till husbehov. Till detta kommer en odling av kaffe i omgivande skogar. Ettåriga grödor som odlas är t.ex. teff (Eragrostis teff), majs, korn, durra (Sorghum bicolor), vete, balj- och oljeväxter. Fleråriga grödor för avsalu som odlas är kaffe, kat (Catha edulis), etiopisk banan (Ensete ventricosum) och avokado. Njutningsmedlena kaffe och kat odlas av de flesta jordbrukare. Etiopisk banan, en flerårig ört som liknar banan mycket i växtsättet,

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odlas ofta i hemträdgårdar till husbehov på högre altituder i området. Det är jordstammen och delar av den överjordiska skenstammen som äts. Hemträdgårdar är de odlingar som omger ett eller flera bostadshus och är i allmänhet inte större än 100×100 m och kan omges av ett staket av trädstammar som sedan slår rot (levande staket). Hemträdgårdar har en hög mångfald av grödor och förekommer överallt i jordbrukslandskapet och varierar i många hänseenden och är därför intressanta att studera. De förekommer både nära skogskanten som i ett mer öppet landskap och sammansättningen av markanvändningstyper runt dem varierar i fråga om t.ex. mängden trädbeklädd mark, gräsmarker och våtmarker samt vilka grödor som odlas. I min första studie undersökte jag påverkan av däggdjurs skadegörelse på grödor på sammansättningen av grödor som en lantbrukare odlar i hemträdgårdar och på fälten, nära och längre ifrån skogskanter. Detta gjorde jag genom att notera vilka grödor som odlades i 15 transekter nära skogskanten och 15 längre ifrån skogskanten. Dessutom intervjuade jag också 60 jordbrukare (30 nära skogskanten och 30 längre ifrån) om effekten och frekvensen av skadegörelse av anubisbabian (Papio anubis) och busksvin (Potamochoerus larvatus) i hemträdgårdarna och på fälten. I min andra studie studerade jag hur dynamiken av spindlar, rovlevande skalbaggar och skinnbaggar påverkas av fördelningen av olika markanvändningsslag på lokal nivå och landskapsnivå. I fyrtio hemträdgårdar med olika sammansättning satte jag ut sex fallfällor i plast (diameter = 8 cm, djup = 6 cm) som tömdes med 4–7 dagars mellanrum. På den lokala skalan så inventerade jag de olika markanvändningsslagen i fält (i en ruta 100 m×100 m runt huset i hemträdgården). På landskapsnivå så mätte jag skogs- och trädtäckning från satellitbilder i cirklar med 200 och 500 m radier runt huset.

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I min tredje studie undersökte jag hur insektskadorna på blad av raps (Brassica napus) påverkades av att jag utestängde fåglar och rovlevande insekter och spindlar. Tjugoåtta hemträdgårdar valdes ut i landskap med olika komplexitet. I fält så mätte jag de olika markanvändningstyperna i en ruta 100×100 m runt huset i hemträdgårdarna (här benämnt lokal skala). Från satellitbilder så uppskattade träd och skogstäckning i cirklar med 200 m och 500 m radie runt huset (här benämnt landskapsskala). Jag sådde rapsfrön inom ett område 5×2.5 m under odlingssäsongen. När plantorna hade åtminstone tre blad satte jag upp burar (maskstorlek: 2×2 cm) som stänger ute fåglar men inte rovlevande leddjur från halva området. Jag valde sedan ut sex grupper om fem plantor inne i buren och sex grupper utanför. Runt hälften av dessa (3 grupper i buren och 3 utanför) satte jag en hink utan botten över för att stänga ute de rovlevande insekterna och spindlarna. Jag mätte sedan ätskadorna på rapsbladen en gång i veckan under fem veckor.

Till sist så studerade jag angrepp av rovlevande arter på larver gjorda av modellera i sex olika landskap. Detta gjorde jag för att undersöka hur predationen av leddjur och fåglar påverkas av landskapets strukturella komplexitet. Jag valde tre enkla landskap med låg skog- och trädtäckning (< 20 %) och tre mer komplexa landskap (> 30 %). I varje landskap så valde jag även ut tre komplexa hemträdgårdar och tre strukturellt enkla hemträdgårdar. I fält räknade jag antalet trädarter och trädstammar i varje av de 36 hemträdgårdarna (inom en ruta 100×100 m). Jag fäste dock bara mina modellerelarver bara på de allmänt förekommande grödorna kaffe och avokado för att undvika att grödan som larven satt på skulle påverka resultatet. I samlade in data på antalet modellarver som var attackerade och borttagna/missade i varje trädgård-varannan dag i början av veckorna och var 3:e dag i slutet av veckorna, eftersom vi inte besökte trädgårdarna på söndagar. Data samlades in under sju till nio veckor i följd (26 augusti−25 oktober, 2013). Huvudresultatet i denna avhandling antyder att den heterogenitet i livsmiljöer människans markanvändning leder till, påverkar ekosystemtjänster och otjänster i jordbrukslandskapet både på lokal nivå och på landskapsnivå. Till exempel så har jordbrukarsamhällen nära skogskanten större risk att råka ut för däggdjursskador på grödorna jämfört med dem som ligger längre ifrån skogen. Detta har

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dock inte lett till någon signifikant skillnad i sammansättningen av grödor mellan dessa två grupper (nära och långt ifrån skogskanten). Möjliga orsaker till detta kan vara att det inte finns så många alternativa lämpliga grödor som inte utsätts för skador eller en ovilja att överge en viss gröda t.ex. av hävd eller kulturella skäl. Jag fann också en högre abundans av leddjurpredatorer (mest spindlar) i heterogena miljöer mellan olika skalor (d.v.s. när trädtäckningen är bara är hög på den lokala eller landskapsskalan) än när trädtäckningen är liknande både på den lokala som på landskapsnivån. Detta mönster kan bero på nischkomplementaritet mellan olika arter och olika resurser för reproduktion och stabilitet under lokala störningar. Jag fann också att bladskadorna var högre på plantor från vilka marklevande rovleddjur hade stängts ute. Att utesluta fåglar hade dock ingen effekt på bladskadorna. Detta visar att marklevande spindlar och insekter är viktigare än fåglar för att reglera mängden växtätande insekter i hemträdgårdar i sydvästra Etiopien. Effekten av att stänga ute leddjuren var starkare i trädfattiga trädgårdar än i de där det fanns många träd. Jag föreslår att detta beror på det i trädfattiga hemträdgårdar odlas fler ettåriga grödor där tätheten av både växtätande insekter och rovlevande leddjur kan vara högre under odlingssäsongen. Resultaten av avhandlingens sista studie visar att både fåglar och rovlevande leddjur är viktiga för att reglera skadeinsekter i jordbrukslandskapet. Den dagliga insektspredationen var högre i enkla än i komplexa landskap. Det var däremot ingen skillnad i insektspredation mellan komplexa och enkla hemträdgårdar, varken i enkla eller i komplexa landskap. Jag hittade ingen skillnad i fågelpredation, varken mellan komplexa och enkla hemträdgårdar, eller mellan komplexa och enkla landskap. Ej heller hittade jag någon skillnad mellan komplexa och enkla landskap.

Sammanfattningsvis indikerar mina resultat att i enkla trädgårdar/landskap är det fortfarande tillräckligt mycket habitatsvariation för att upprätthålla insektspredatorer vid en viss nivå där de kan ha en kontrollerande effekt på skadeinsekter. Men jag hittade ingen tydlig effekt av variationen i komplexitet på fågelpredationen.

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Tack/Thanks I would like to express my deepest gratitude to Kristoffer Hylander and Peter Hambäck for their thorough supervision and encouragement throughout my study period. Moreover, my whole hearted thanks goes to the following persons. 1. Kristoffer Hylander och hans fru Eva samt barnen May, Ruben och Lydia, Kristoffers föräldrar Nils Olof och Gunnel, samt Kristoffers syster Karolina och bror Samuel med familjer. Jag är tacksam för deras support och ansträngningar att få mig att känna mig som hemma under mina studier i Sverige. 2. Tack tidigare och nuvarande doktorander, Joakim Hansen, Gundula Kolb, Petter Andersson,Tove von Euler, Karin Lönngren, Helena Forslund, Niklas Lönnell (hans så betydelsefulla uppmuntran och hjälp att lösa de utmaningar som jag stött på under mina studier), Ulrika Samnegård, Lisa Fors, Tenna Toftegaard, Veronika Johansson, Ellen Schagerström, Alma Strandmark, Johan Dahlberg, Malin Köning, Bryndis Marteinsdotter, Tola Gemmechu och Yihun Dille. 3. Min uppskattning går också till Victor Johansson (duktig på R), Maria Johansson, Anita D'agostino, Leila Ahonen, Jessica Oremus och Johan Klint. Mina erfarenheter har berikats mycket genom er. 4. Så vill jag uttrycka mitt hjärtligaste tack till Elisabeth Lundqvist som inhyst mig och gjort min vistelse så trivsam här i Sverige. Slutligen är jag tacksam mot Julia Hedjärn Swaling för hennes uppmuntran och hjälp under min studietid. 5. I am grateful to Sileshi Nemomissa and Feyera Senbata for their unreserved help to pursue my study. I am also grateful to Fekadu Bezayene, Shimelsis Gebre and Mesele (Abu) for their help and encouragment from different perspectives. 6. I would like to thank my father, Lammeessaa Baay’isaa and my mother Dashii Mararaa for their devotion to help me from the very beginning of my schooling to this level. 7. In the end, I am deeply grateful to my family: Simanyi Admaasuu (my wife) and our children: Marartuu, Ilillii and AagaYah, for letting me continue what my parents started; without their patience and coop-eration it would not have been possible for me to complete my study.