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- Lemessa, Debissa, 1973-
(author)
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Pests and pest controlling organisms across tropical agroecological landscapes in relation to forest and tree-cover
- 2014
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Doctoral thesis (other academic/artistic)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 transectsin 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 home gardens. My result showed higher abundance of arthropod predators when either the home garden 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 rape seed in 26 home gardens. 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 home gardens across landscapes. The rate of arthropod predation on caterpillars was higher in simple than in complex landscapes. The rate of bird predation did not vary between complex and simple landscapes. In simple landscapes, arthropod predation was higher than that of birds. The overall results suggest that simplified gardens/landscapes still have enough habitat heterogeneity to support arthropod predators for the significant top-down controlling effect on crop pests. However, I did not find clear effect of complexityon the top-down effect of birds.
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| 2. |
- Lönnell, Niklas, 1969-
(author)
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Dispersal of bryophytes across landscapes
- 2014
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Doctoral thesis (other academic/artistic)abstract
- Dispersal, especially long-distance dispersal, is an important component in many disciplines within biology. Many species are passively dispersed by wind, not least spore-dispersed organisms.In this thesis I investigated the dispersal capacity of bryophytes by studying the colonization patterns from local scales (100 m) to landscape scales (20 km). The dispersal distances were measured from a known source (up to 600 m away) or inferred from a connectivity measure (1–20 km). I introduced acidic clay to measure the colonization rates over one season of a pioneer moss, Discelium nudum (I–III). I also investigated which vascular plants and bryophytes that had colonized limed mires approximately 20–30 years after the first disturbance (IV).Discelium effectively colonized new disturbed substrates over one season. Most spores were deposited up to 50 meters from a source but the relationship between local colonization rates and connectivity increased with distance up to 20 km (I–III). Also calcicolous wetland bryophyte species were good colonizers over similar distances, while vascular plants in the same environment colonized less frequently. Common bryophytes that produce spores frequently were more effective colonizers, while no effect of spore size was detected (IV). A mechanistic model that take into account meteorological parameters to simulate the trajectories for spores of Discelium nudum fitted rather well to the observed colonization pattern, especially if spore release thresholds in wind variation and humidity were accounted for (III).This thesis conclude that bryophytes in open habitats can disperse effectively across landscapes given that the regional spore source is large enough (i.e. are common in the region and produce spores abundantly). For spore-dispersed organisms in open landscapes I suggest that it is often the colonization phase and not the transport that is the main bottle-neck for maintaining populations across landscapes.
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| 3. |
- Schmalholz, Martin, 1981-
(author)
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Resistance and recolonization of bryophyte assemblages following disturbances : - detecting patterns and exploring mechanisms
- 2010
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Doctoral thesis (other academic/artistic)abstract
- Disturbances are ubiquitous features of most northern forest ecosystems. The subsequent response of plant assemblages on both short (resistance or not) and long term (recolonization or not) will depend on a number of factors operating at several spatial scales. In boreal forest ecosystems, bryophyte assemblages are a conspicuous and species rich group of plants for which these processes are poorly understood. Using a combination of experimental and observational approaches this thesis explores these questions for closed-canopy bryophyte assemblages in relation to a) microtopography (both for the initial and long-term response), b) environmental constrains during post-logging succession and c) disturbance type. My results clearly show that the shade and shelter provided by microtopographic surface structures can increase survival rates of bryophytes following clear-cut logging by decreasing mortality from microclimatic stress and mechanical disturbance. Following clear-cutting, the recovery of forest floor and dead wood living bryophytes seems to be a relatively steady and progressive process without any major bottleneck episodes in the young or semi-mature forest stages with much of the pre-disturbance composition recovered after 50 years. Although boulders were found to increase the initial survival on clear-cuts and hence increase disturbance resistance, we found no evidence that boulders influenced the subsequent recolonization process. Lastly, strong compositional dissimilarities were found in young forests (40 years) following clear-cut logging, wildfire and insect outbreak, indicating divergent trajectories to occur following different disturbances. Hence, early seral stages of forest ecosystems regenerating after natural disturbances seem to compliment young managed forests in maintaining landscape level diversity.
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