Dispersal, dormancy, life history and breeding systems of southern African Asteraceae : risk-reducing strategies in unpredictable environments

[摘要] ENGLISH ABSTRACT: How organisms respond to unpredictable environments is a fundamental question in evolutionaryecology. For example, plants may reduce the risk of reproductive failure by spreading theirreproductive effort in space (dispersal) or in time (dormancy, iteroparity). Similarly, different plantbreeding systems, (for example the ability to autonomously self-fertilise) may reduce the risk ofreproductive failure in environments where pollination in particular is unreliable. Each of thesestrategies may be affected by selective pressures exerted by heterogeneous abiotic and bioticenvironments (e.g. unreliable rainfall patterns or range edge habitats). However, there is littletheoretical or empirical consensus on how these strategies are related.In Chapter 2, I explore the association between dispersal and breeding system traits andrange edge proximity. I show that annual daisies from Namaqualand, South Africa, arecharacterised by two discreet syndromes: high selfing ability associated with good dispersal andobligate outcrossing associated with lower dispersal, regardless of range position. This chapterillustrates that selection on both breeding system and dispersal traits may act consistently acrossdistribution ranges.Because co-flowering plants often share pollinators, their fecundity is likely affected bychanges 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 spatialdistribution pattern on pollination and fecundity in annual Namaqualand daisies. I show thatincreasing conspecific density and aggregation enhanced fecundity through increased mateavailability and reduced heterospecific interference, independent of pollinator visitation rates.Moreover, I demonstrate the benefits of autonomous selfing when mates are limited and thepotential for interspecific pollen transfer is high.In Chapter 4, I examine relative investment in dispersal vs. dormancy in seed heteromorphicDimorphotheca (Asteraceae) species in relation to life history, rainfall unpredictability and rangeedge proximity. I show annuals and perennials differ significantly in the relative investment indifferent dispersal strategies. However, my findings provide little support for theoretical predictionsof bet-hedging strategies in unpredictable or range edge habitats. This chapter emphasises the roleof local environmental factors on fruit set that may obscure expected patterns across broad climaticgradients.Because of different costs and benefits of dispersal in space and time, we may expectnegative patterns of covariation among dispersal and dormancy as alternative risk-reducingstrategies. 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, butwas inconsistent at different levels of biological organisation. I also show that the effects of lifehistory on spatial and temporal dispersal were inconsistent.Taken together, my research illustrates the importance of simultaneously investigatingdifferent risk-reducing strategies, because associations among them are clearly complex and oftencontradict theoretical expectations. Moreover I show that the effects of life history and phylogeneticrelatedness cannot be disregarded. My findings underscore the importance of dispersal in space andtime as well as autonomous selfing as risk-reducing responses to unreliable environments.

[发布机构] Stellenbosch University