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Animal ethology and ecology. Sociobiology --- Birds

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Evolution. Phylogeny

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The human population growth in many parts of the world causes a rising demand for resources. This often induces deforestation, resulting in small forest relicts embedded in a human-modified landscape consisting of agricultural fields, exotic tree plantations, and settlements. The Taita Hills (Kenya), which was the study area for this doctoral research, is a prime example of these processes. The tree species composition and structure was inventoried in the twelve remaining forest relicts, the exotic plantations, and the matrix. With these data we quantified the remaining biodiversity, the related ecosystem services, and the potential for restoration actions.First, the indigenous forest relicts were studied. Four forest communities were defined, including two (i.e. Millettia-Craibia and Chrysophyllum-Strombosia) with typically late successional tree species and two (i.e. Phoenix-Maesa and Tabernaemontana), which consisted mainly of early-successional species. We suggest that the species composition of the larger forest relicts, where all four communities were found, resembles the historical or natural state. The occurrence of the Phoenix-Maesa or Tabernamontana community was restricted to forest edges and the smaller forest relicts. These relicts can be defined as hybrid ecosystems due to the absence of late successional species. Such hybrid ecosystems differ from the historical species composition, but still hold the potential to be restored with adapted management measures. We found the larger forest relicts, on higher altitudes, further away from the forest edges and on sites with low human disturbance to be significantly more biodiverse.In these forest relicts, we also analysed the reproductive success of the tree species. The forest regeneration determines future forest structure and composition, but is often severely hampered in degraded forests. The extinction debt, recruitment, and colonization credit was quantified for each forest relict and on landscape level based on the co-occurrence of tree species in the seedling, sapling or mature tree layer. On landscape level, the potential extinction debt was 9% of the regional species pool. This corresponds with seven species likely to become extinct in the Taita Hills. On the scale of a forest relict, several species were absent in the upper canopy layer while they did occur in the regeneration layer. This recruitment credit was three and six species for the large and small forest relicts, respectively, while the potential extinction debt was twelve and four species respectively. Circa 20% of all tree species in a forest relict were threatened by extinction. The conservation value of the smaller forest relicts was higher than expected because of the relatively high recruitment credit, which even included late-successional species.In the second part, the role of exotic tree plantations for biodiversity conservation was studied. Therefore, the regeneration of indigenous and exotic species was measured in plantations dominated by Pinus patula, Eucalyptus saligna, Cupressus lusitanica, Grevillea robusta or Acacia mearnsii. Regeneration of indigenous species, even late-successional species, was relatively higher in Grevillea plantations. The other plantation types had a significantly lower performance, partly due to the invasive character of the dominant species.In the third part, solitary trees were studied in the human-modified landscape. In total, 65 tree species were observed (63% of these were indigenous). A non-random distribution of tree traits was observed in the landscape. For instance, more exotic tree species occurred on croplands where they may deliver marketable products or services, play a role in nitrogen fixation or can be used in agroforestry systems. The indigenous species also delivered multiple products or ecosystem services, but no other services than exotic species. The matrix clearly functions as a refuge for a subset of the indigenous tree species and becomes a place for domestication and preservation of useful species. Future changes in key social and ecological drivers are expected and ecosystem service trade-offs may arise. For instance, the loss of local knowledge on indigenous plant species can result in a replacement of indigenous species by exotics.Faced with the challenges of demography and poverty, the area of cultivated land is increasing worldwide and the production of goods and services is intensified and enhanced. Within intensified systems, indigenous tree species are at risk due to several, non-exclusive reasons. Especially the conservation of late-successional tree species in these landscapes is precarious, because they are restricted to the forest interior of large forest relicts. In contrast, early-successional species seem to persist throughout the landscape. These pioneer trees can catalyse all production processes, both cultivated and natural, while they also diminish the pressure on the remaining forest relicts. The protection of the remaining indigenous forest relicts, combined with the enlargement of secondary forest, for instance by conversion of exotic plantations, and the protection of the solitary trees in the matrix is necessary for the protection of the regional biodiversity and the ecosystem services they deliver. Inheemse bosrelicten, boomplantages en de landbouwmatrix. Zij verzekeren de permeabiliteit van de matrix of kunnen de omvorming van exotische boomplantages versnellen, terwijl ze verschillende ecosysteemdiensten kunnen leveren aan de lokale bevolking. Het voorbestaan van climaxboomsoorten is echter precair en enkel gewaarborgd binnen grote bosrelicten waar menselijke verstoring minimaal is. Het behoud van de inheemse bosrelicten of het uitbreiden ervan, bijvoorbeeld door de omvorming van exotische plantages, met het behoud van de vele solitaire bomen in de matrix, is noodzakelijk om de biodiversiteit en haar geleverde ecosysteemdiensten te behouden.