Consequences of self-fertilisation for fecundity and progeny performance in invasive plants.
Rodger, James Gordon.
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Plants that can self-fertilise should, on average, be more invasive than plants that can not self-fertilise because they can reproduce regardless of the availability of mates and pollinators. Self-fertilisation should have a strong effect on invasiveness because, to become invasive, introduced plants have to pass through bottlenecks of low plant abundance when mates and pollinators are likely to be scarce. Under these conditions, reproduction of plants that can not self-fertilise is often limited by pollen receipt. Selfing may thus contribute to invasiveness by alleviating pollen limitation Allee effects (pollen limitation caused by low abundance) especially as theoretical work indicates that ability to invade and rate of invasion are highly sensitive to fecundity of small and isolated populations and single individuals. Recently, a correlation between ability to self-fertilise and invasiveness has been observed in several invasive floras, consistent with the hypothesis that species that can self-fertilise should be more invasive. However, it has not yet been demonstrated that this relationship arises from reproductive assurance. To establish the causal basis of a correlation between a plant trait and invasiveness, a mechanism linking that trait to invasiveness must be demonstrated. For this it is necessary to show firstly that the trait actually affects performance in the introduced range and secondly that plant performance affects invasiveness. Self-fertilisation is hypothesised to increase invasiveness by enhancing reproductive performance. The first step in testing this hypothesis is therefore to show that being able to self-fertilise increases fecundity, i.e. that it provides reproductive assurance. However, progeny from self-fertilisation often suffer from inbreeding depression – they perform worse than those from cross-fertilisation – so it is also necessary to show that this cost does not outweigh the reproductive assurance benefit of selfing. So far, reproductive assurance has been assessed in only a few invasive plant species. These studies did not assess inbreeding depression and only one investigated reproductive assurance in relation to abundance, finding no relationship. In this thesis I have sought to understand the importance of self-fertilisation for reproduction of invasive plants in the introduced range through case studies. In particular, I assessed whether reproductive assurance from self-fertilization alleviates Allee effects via pollen limitation. To do this I tested whether pollen limitation and reproductive assurance were greater at low plant abundance. Further, I conducted progeny trials to assess inbreeding depression, as this cost of selfing potentially negates reproductive assurance benefits. I also conducted observations and experiments to identify the principle pollinators of my study species as reproductive assurance and its relationship to plant abundance depend on pollinator visitation, The Australian trees Acacia mearnsii and A. dealbata are highly invasive in the study region of KwaZulu-Natal, South Africa. Through controlled pollination experiments I established that A. dealbata was self-compatible and autonomously self-fertilising, while previous studies reported A. mearnsii as self-incompatible. I identified the native honeybee Apis mellifera scutellata as the principal pollinator of A. mearnsii, A. dealbata and a co-occuring related invasive species, Acacia decurrens, in the study region. I conducted pollen supplementation experiments in two of these species, aiming to indirectly assess reproductive assurance from selfing in the self-compatible A. dealbata by comparing pollen limitation between this species and the self-incompatible A. mearnsii. In both species, I conducted pollen supplementation in single isolated trees and trees in continuous populations, to test whether pollen limitation was more severe in isolation. These pollen supplementation experiments were inconclusive with respect to pollen limitation but indicated that if there was pollen limitation in A. mearnsii, it was not related to isolation. Progeny trials in A. dealbata revealed relatively strong inbreeding depression in progeny growth and survival. This suggests that selfed progeny may not reach reproduction, so even if self-fertilisation provides reproductive assurance, it may not contribute to invasion in this species. As floral morphology of Acacia species prohibits the use of emasculation experiments to directly measure reproductive assurance, I conducted further investigations on Lilium formosanum, a large-flowered, autonomously self-pollinating invasive geophyte native to Taiwan. I identified the long tongued hawkmoth Agrius convolvuli as its primary pollinator in its introduced range in KwaZulu-Natal, South Africa. Trials of progeny from self- and crosspollination in the field (to 31 months) and in a controlled shade-house environment (to 26 months) showed no evidence of inbreeding depression in germination, growth or survival. Flowering was assessed in the shade-house as most plants did not flower in the field. Only one of five populations showed inbreeding depression in probability of flowering in the second year of growth but none showed inbreeding depression in the third year. Inbreeding depression was thus generally undetectable in L. formosanum. I tested for reproductive assurance and pollen limitation in L. formosanum by conducting floral emasculations and pollen supplementations in multiple populations across a range of population size and isolation in three different years. These experiments demonstrated that reproductive assurance was substantial and that pollen limitation was low or absent. Contrary to expectations, reproductive assurance was not greater in smaller populations and was greater for more isolated populations in only one of three years. However, that study did not include many very small populations. To assess reproductive assurance at very low abundance, I created arrays of emasculated and intact plants within and around naturally occurring populations at two sites. Isolated plants had higher reproductive assurance than did plants placed inside the continuous population at one site, supporting the hypothesis that selfing provides reproductive assurance against pollen-limitation Allee effects. However, in these studies, generally inadequate pollinator visitation was the main reason that L. formosanum exhibited reproductive assurance through selfing. The substantial reproductive assurance and minimal inbreeding depression displayed by L. formosanum makes a compelling case for the hypothesis that self-pollination promotes invasion. Nevertheless, demographic modelling will be necessary to assess whether increased fecundity through reproductive assurance results in increased rates of population growth and spread, and hence invasion, in this species. To assess whether reproductive assurance accounts for the relationship between ability to self-fertilise and invasiveness in plants generally, the contribution of self-fertilisation to invasiveness will have to be evaluated for a larger sample of invasive and non-invasive introduced species, using the approaches taken in this thesis, followed up by demographic modelling.