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Functional divergence between Vachellia and Senegalia could underpin differences in invasiveness and Eltonian niche partitioning in African savannas
For several decades Vachellia and Senegalia have been assumed to be ecological equivalents in African savannas. Their supposedly close evolutionary relationship has, however, been totally revised in light of recent molecular phylogenetic data. These data highlight the deep divergence (± 30 Ma) betwe...
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| Format: | Thesis |
| Language: | English |
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Department of Biological Sciences
2020
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| _version_ | 1867613263212052480 |
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| access_status_str | Open Access |
| author | Lewis, Joel Robin |
| author2 | February, Edmund C |
| author_browse | February, Edmund C Lewis, Joel Robin |
| author_facet | February, Edmund C Lewis, Joel Robin |
| author_sort | Lewis, Joel Robin |
| collection | Thesis |
| description | For several decades Vachellia and Senegalia have been assumed to be ecological equivalents in African savannas. Their supposedly close evolutionary relationship has, however, been totally revised in light of recent molecular phylogenetic data. These data highlight the deep divergence (± 30 Ma) between these genera and also their independent transitions into African savannas. Distant divergence and independent transitions into savannas between Vachellia and Senegalia suggest their possible ecological niche divergence, which could explain the observed differences in invasiveness between them. This thesis provides the first ecophylogenetic study to assess the level of ecological niche divergence between these economically, socioeconomically and ecologically important tree genera in Africa. I address this question by first determining the scale(s) of coexistence between Vachellia and Senegalia and determine their phylogenetic community structure at both the landscape-regional scale and the plot-scale. I then conduct a large glasshouse experiment using seven species from each genus to determine conserved functional differences between them during seedling establishment in the presence, and also the absence, of grass competition. While I find evidence of some Grinnellian niche differentiation between Vachellia and Senegalia at the landscape-regional scale, their greater-than-expected co-occurrence in plots suggests they generally partition an Eltonian niche in savannas. Glasshouse trait data also reveal that Vachellia has a conserved strategy for rapid vertical growth when free from grass competition and Senegalia has a conserved root tissue density response to grass competition. I argue that the rapid growth (bolting) strategy of Vachellia in the absence of grass competition provides a functional explanation for why this genus is observed to be relatively more invasive in grassy ecosystems. I also argue that the root tissue density response of Senegalia enables stable coexistence at the plot scale with Vachellia as it increases competitive ability among grasses leaving establishment less tied to times of low grass biomass. I conclude that this study provides overwhelming support for ecological niche divergence between Vachellia and Senegalia and that this divergence may underpin their observed differences in invasiveness. That the coexistence of these genera is largely attributable to Eltonian niche partitioning also provides fresh support for the contention that tree-tree competition structures tree communities in African savannas. Finally, niche divergence suggests that, when promoting diversity and managing invasion, Vachellia and Senegalia species will require different management practices. |
| format | Thesis |
| id | oai:open.uct.ac.za:11427/31698 |
| institution | University of Cape Town (South Africa) |
| language | eng |
| last_indexed | 2026-06-10T12:33:21.255Z |
| license_str | Not specified — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from UCTD — University of Cape Town Open Access Repository |
| publishDate | 2020 |
| publishDateRange | 2020 |
| publishDateSort | 2020 |
| publisher | Department of Biological Sciences |
| publisherStr | Department of Biological Sciences |
| record_format | dspace |
| source_str | UCTD — University of Cape Town Open Access Repository |
| spelling | oai:open.uct.ac.za:11427/31698 Functional divergence between Vachellia and Senegalia could underpin differences in invasiveness and Eltonian niche partitioning in African savannas Lewis, Joel Robin February, Edmund C Verboom,Tony G Biological Sciences For several decades Vachellia and Senegalia have been assumed to be ecological equivalents in African savannas. Their supposedly close evolutionary relationship has, however, been totally revised in light of recent molecular phylogenetic data. These data highlight the deep divergence (± 30 Ma) between these genera and also their independent transitions into African savannas. Distant divergence and independent transitions into savannas between Vachellia and Senegalia suggest their possible ecological niche divergence, which could explain the observed differences in invasiveness between them. This thesis provides the first ecophylogenetic study to assess the level of ecological niche divergence between these economically, socioeconomically and ecologically important tree genera in Africa. I address this question by first determining the scale(s) of coexistence between Vachellia and Senegalia and determine their phylogenetic community structure at both the landscape-regional scale and the plot-scale. I then conduct a large glasshouse experiment using seven species from each genus to determine conserved functional differences between them during seedling establishment in the presence, and also the absence, of grass competition. While I find evidence of some Grinnellian niche differentiation between Vachellia and Senegalia at the landscape-regional scale, their greater-than-expected co-occurrence in plots suggests they generally partition an Eltonian niche in savannas. Glasshouse trait data also reveal that Vachellia has a conserved strategy for rapid vertical growth when free from grass competition and Senegalia has a conserved root tissue density response to grass competition. I argue that the rapid growth (bolting) strategy of Vachellia in the absence of grass competition provides a functional explanation for why this genus is observed to be relatively more invasive in grassy ecosystems. I also argue that the root tissue density response of Senegalia enables stable coexistence at the plot scale with Vachellia as it increases competitive ability among grasses leaving establishment less tied to times of low grass biomass. I conclude that this study provides overwhelming support for ecological niche divergence between Vachellia and Senegalia and that this divergence may underpin their observed differences in invasiveness. That the coexistence of these genera is largely attributable to Eltonian niche partitioning also provides fresh support for the contention that tree-tree competition structures tree communities in African savannas. Finally, niche divergence suggests that, when promoting diversity and managing invasion, Vachellia and Senegalia species will require different management practices. 2020-04-28T13:15:23Z 2020-04-28T13:15:23Z 2019 2020-04-28T10:28:45Z Master Thesis Masters MSc https://hdl.handle.net/11427/31698 eng application/pdf Department of Biological Sciences Faculty of Science |
| spellingShingle | Biological Sciences Lewis, Joel Robin Functional divergence between Vachellia and Senegalia could underpin differences in invasiveness and Eltonian niche partitioning in African savannas |
| thesis_degree_str | Master's |
| title | Functional divergence between Vachellia and Senegalia could underpin differences in invasiveness and Eltonian niche partitioning in African savannas |
| title_full | Functional divergence between Vachellia and Senegalia could underpin differences in invasiveness and Eltonian niche partitioning in African savannas |
| title_fullStr | Functional divergence between Vachellia and Senegalia could underpin differences in invasiveness and Eltonian niche partitioning in African savannas |
| title_full_unstemmed | Functional divergence between Vachellia and Senegalia could underpin differences in invasiveness and Eltonian niche partitioning in African savannas |
| title_short | Functional divergence between Vachellia and Senegalia could underpin differences in invasiveness and Eltonian niche partitioning in African savannas |
| title_sort | functional divergence between vachellia and senegalia could underpin differences in invasiveness and eltonian niche partitioning in african savannas |
| topic | Biological Sciences |
| url | https://hdl.handle.net/11427/31698 |
| work_keys_str_mv | AT lewisjoelrobin functionaldivergencebetweenvachelliaandsenegaliacouldunderpindifferencesininvasivenessandeltoniannichepartitioninginafricansavannas |