Thesis (PhD)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: How organisms respond to unpredictable environments is a fundamental question in evolutionary
ecology. For example, plants may reduce the risk of reproductive failure by spreading their
reproductive effort in space (dispersal) or in time (dormancy, iteroparity). Similarly, different plant
breeding systems, (for example the ability to autonomously self-fertilise) may reduce the risk of
reproductive failure in environments where pollination in particular is unreliable. Each of these
strategies may be affected by selective pressures exerted by heterogeneous abiotic and biotic
environments (e.g. unreliable rainfall patterns or range edge habitats). However, there is little
theoretical or empirical consensus on how these strategies are related.
In Chapter 2, I explore the association between dispersal and breeding system traits and
range edge proximity. I show that annual daisies from Namaqualand, South Africa, are
characterised by two discreet syndromes: high selfing ability associated with good dispersal and
obligate outcrossing associated with lower dispersal, regardless of range position. This chapter
illustrates that selection on both breeding system and dispersal traits may act consistently across
distribution ranges.
Because co-flowering plants often share pollinators, their fecundity is likely affected by
changes in pollinator visitation rates or the transfer of conspecific relative to heterospecific pollen.
In Chapter 3 I experimentally investigate the effects of con- and heterospecific density and spatial
distribution pattern on pollination and fecundity in annual Namaqualand daisies. I show that
increasing conspecific density and aggregation enhanced fecundity through increased mate
availability and reduced heterospecific interference, independent of pollinator visitation rates.
Moreover, I demonstrate the benefits of autonomous selfing when mates are limited and the
potential for interspecific pollen transfer is high.
In Chapter 4, I examine relative investment in dispersal vs. dormancy in seed heteromorphic
Dimorphotheca (Asteraceae) species in relation to life history, rainfall unpredictability and range
edge proximity. I show annuals and perennials differ significantly in the relative investment in
different dispersal strategies. However, my findings provide little support for theoretical predictions
of bet-hedging strategies in unpredictable or range edge habitats. This chapter emphasises the role
of local environmental factors on fruit set that may obscure expected patterns across broad climatic
gradients.
Because of different costs and benefits of dispersal in space and time, we may expect
negative patterns of covariation among dispersal and dormancy as alternative risk-reducing
strategies. In Chapter 5, I provide evidence for a trade-off between these traits across 27 wind-
dispersed daisy species from South Africa. This trade-off did not depend on life history effects, but
was inconsistent at different levels of biological organisation. I also show that the effects of life
history on spatial and temporal dispersal were inconsistent.
Taken together, my research illustrates the importance of simultaneously investigating
different risk-reducing strategies, because associations among them are clearly complex and often
contradict theoretical expectations. Moreover I show that the effects of life history and phylogenetic
relatedness cannot be disregarded. My findings underscore the importance of dispersal in space and
time as well as autonomous selfing as risk-reducing responses to unreliable environments.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96736 |
| Date | 04 1900 |
| Creators | De Waal, Caroli |
| Contributors | Anderson, Bruce, Ellis, Allan George, Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xii, 166 pages : illustrations, maps |
| Rights | Stellenbosch University |
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