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The spatial relationship between herbivorous arthropods, predators and vegetation in barley fields depends on management

Gröndahl, Sophie (2012) BIOM24 20121
Degree Projects in Biology
Abstract
Abstract

Organically managed cereal fields have a higher plant diversity compared to conventionally managed fields. The presence and spatial distribution of plant resources may affect the spatial relationships between prey organisms and their predators and could therefore alter the feeding relationships between predators and pest prey. As invertebrate predators play an important role in biological control, a change in trophic links between predators and pest prey may affect the provision of pest suppression services in cereal fields. This study focused on how different management systems affect the spatial relationships between invertebrate predators, invertebrate prey and vegetation characteristics. Three spring barley fields that... (More)
Abstract

Organically managed cereal fields have a higher plant diversity compared to conventionally managed fields. The presence and spatial distribution of plant resources may affect the spatial relationships between prey organisms and their predators and could therefore alter the feeding relationships between predators and pest prey. As invertebrate predators play an important role in biological control, a change in trophic links between predators and pest prey may affect the provision of pest suppression services in cereal fields. This study focused on how different management systems affect the spatial relationships between invertebrate predators, invertebrate prey and vegetation characteristics. Three spring barley fields that differed in management (conventional, young organic and old organic) were sampled in a spatially-explicit design with an insect-suction sampler for invertebrates and with visual estimates for vegetation. The vegetation characteristics varied as the conventionally managed field had almost no weeds whereas the young organic managed field had maximum weed coverage. Results show that vegetation complexity and weed coverage can lead to association between predators and prey and that predator-prey associations were spatially decoupled by a higher availability of alternative prey under organic management systems. Prey groups occurred aggregated which was most likely caused by the distribution of host plants and refuges. Predators showed a tendency towards segregation from each other, probably as a consequence of intraguild predation or cannibalism. The two older management systems – conventional and old organic – were more similar to each other than the young organic field. In conclusion, although organically managed fields may have higher abundances of predators, biological control may not necessarily be higher.

Popular science summary:

Spatial relationships between arthropods depend on management

Organically managed barley fields have a higher number of plant species compared to conventionally managed fields and weed distribution differs between farming systems. Under organic farming application of chemical fertilizers or pesticides is prohibited and the abundance of natural enemies is reduced. These changes may affect the important role of arthropod predators in pest management. Pests feed on cereals and cause significant economic damage to farmers, but natural enemies can function as biological control agents to reduce pest damage.

This study focus on how different management systems affect the spatial relationships between arthropod (an invertebrate animal having an external skeleton, a segmented body and jointed outgrowths) predators, arthropod prey and vegetation characteristics. The results will give us an insight in the biological control potential of natural enemies in barley fields with different management history. Three fields were sampled with an insect suction sampler: 1.) a conventionally managed field, 2.) an organically managed field that has recently been transformed and 3.) an organically managed field that is more than ten years under such management. Samples were taken in a 8 x 8 regular grid of sample points (1600m²) to test if management and vegetation characteristics affect the spatial relationships between predators and prey. Coverage with grassy and broad leaved weeds, tiller height and tiller density were recorded at all points.

The main difference was found between the conventionally managed field – which had almost no weeds – and the young organic managed field – which had extensive weed coverage. Results suggest that vegetation complexity (weed coverage, tiller density and tiller height) caused association between predators and prey. Plants provide refuges for prey to hide from their predators, therefore prey do not need to avoid areas with high spider abundances to reduce risk of predation. However, predator-pest associations were decoupled by a higher availability of alternative prey under organic management. Prey groups generally occurred aggregated independent of management. The distribution of plants which provide food resources and substrate might be the reason for this. Predators showed a tendency towards avoiding each other, this could be caused by active avoidance due to cannibalism. The spatial patterns in the two older management systems – conventional and old organic – were more similar to each other than to the ones observed in young organic fields. A surprising result, since it was assumed that the type of management should be the most important driver of spatial interactions. In conclusion, results show that although organically managed fields may have higher abundances of predators, biological control may not necessarily be higher, as predators may switch from focusing on crop pests to feeding on prey that utilizes weeds.

Advisor: Henrik Smith, Klaus Birkhofer
Master´s Degree Project in Nature Conservation, 30 credits, 2012
Department of Biology, Lund University (Less)
Please use this url to cite or link to this publication:
author
Gröndahl, Sophie
supervisor
organization
course
BIOM24 20121
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
3633592
date added to LUP
2013-04-16 14:39:19
date last changed
2013-04-16 14:39:19
@misc{3633592,
  abstract     = {Abstract

Organically managed cereal fields have a higher plant diversity compared to conventionally managed fields. The presence and spatial distribution of plant resources may affect the spatial relationships between prey organisms and their predators and could therefore alter the feeding relationships between predators and pest prey. As invertebrate predators play an important role in biological control, a change in trophic links between predators and pest prey may affect the provision of pest suppression services in cereal fields. This study focused on how different management systems affect the spatial relationships between invertebrate predators, invertebrate prey and vegetation characteristics. Three spring barley fields that differed in management (conventional, young organic and old organic) were sampled in a spatially-explicit design with an insect-suction sampler for invertebrates and with visual estimates for vegetation. The vegetation characteristics varied as the conventionally managed field had almost no weeds whereas the young organic managed field had maximum weed coverage. Results show that vegetation complexity and weed coverage can lead to association between predators and prey and that predator-prey associations were spatially decoupled by a higher availability of alternative prey under organic management systems. Prey groups occurred aggregated which was most likely caused by the distribution of host plants and refuges. Predators showed a tendency towards segregation from each other, probably as a consequence of intraguild predation or cannibalism. The two older management systems – conventional and old organic – were more similar to each other than the young organic field. In conclusion, although organically managed fields may have higher abundances of predators, biological control may not necessarily be higher.

Popular science summary:

Spatial relationships between arthropods depend on management

Organically managed barley fields have a higher number of plant species compared to conventionally managed fields and weed distribution differs between farming systems. Under organic farming application of chemical fertilizers or pesticides is prohibited and the abundance of natural enemies is reduced. These changes may affect the important role of arthropod predators in pest management. Pests feed on cereals and cause significant economic damage to farmers, but natural enemies can function as biological control agents to reduce pest damage.

This study focus on how different management systems affect the spatial relationships between arthropod (an invertebrate animal having an external skeleton, a segmented body and jointed outgrowths) predators, arthropod prey and vegetation characteristics. The results will give us an insight in the biological control potential of natural enemies in barley fields with different management history. Three fields were sampled with an insect suction sampler: 1.) a conventionally managed field, 2.) an organically managed field that has recently been transformed and 3.) an organically managed field that is more than ten years under such management. Samples were taken in a 8 x 8 regular grid of sample points (1600m²) to test if management and vegetation characteristics affect the spatial relationships between predators and prey. Coverage with grassy and broad leaved weeds, tiller height and tiller density were recorded at all points.

The main difference was found between the conventionally managed field – which had almost no weeds – and the young organic managed field – which had extensive weed coverage. Results suggest that vegetation complexity (weed coverage, tiller density and tiller height) caused association between predators and prey. Plants provide refuges for prey to hide from their predators, therefore prey do not need to avoid areas with high spider abundances to reduce risk of predation. However, predator-pest associations were decoupled by a higher availability of alternative prey under organic management. Prey groups generally occurred aggregated independent of management. The distribution of plants which provide food resources and substrate might be the reason for this. Predators showed a tendency towards avoiding each other, this could be caused by active avoidance due to cannibalism. The spatial patterns in the two older management systems – conventional and old organic – were more similar to each other than to the ones observed in young organic fields. A surprising result, since it was assumed that the type of management should be the most important driver of spatial interactions. In conclusion, results show that although organically managed fields may have higher abundances of predators, biological control may not necessarily be higher, as predators may switch from focusing on crop pests to feeding on prey that utilizes weeds.

Advisor: Henrik Smith, Klaus Birkhofer
Master´s Degree Project in Nature Conservation, 30 credits, 2012
Department of Biology, Lund University},
  author       = {Gröndahl, Sophie},
  language     = {eng},
  note         = {Student Paper},
  title        = {The spatial relationship between herbivorous arthropods, predators and vegetation in barley fields depends on management},
  year         = {2012},
}