Self-incompatible annual weed species, with similar flowers, flowering time and ecology were used to investigate the relationship between relative abundance, pollinator service and seed-set in two-species mixtures. In mixed stands of the mayweeds <i>Anthemis cotula</i> L. and <i>Tripleurospermum inodorum</i> (L.) Schultz Bip. the common capitulum-visiting insects (hoverflies and other diptera) seemed to find the two species equally attractive, frequently flying from one to the other. In a survey of wild populations of these species the number of insect visitors per capitulum was independent of the number of capitula per m<SUP>2</SUP>. These observations suggest that where <i>A. cotula</i> and <i>T. inodorum</i> co-occur the number of insect visits to each is not affected by the presence of the other. However, when mixtures of <i>A. cotula</i> and <i>T. inodorum</i> were grown in garden plots the number of insect visitors per capitulum was lowest in the plots where the number of capitula per m<SUP>2</SUP> was highest. Thus it would seem that sometimes when <i>A. cotula</i> and <i>T. inodorum</i> grow together each species reduces the number of insect visits to the other. In 9:1 and 1:9 mixtures of <i>A. cotula</i> and <i>T. inodorum</i> the percentage of seeds set by each species when the minority type was sometimes significantly lower than when forming the majority in a nearby plot. It is likely that this occurred because most pollinators visited both species (ie. they were not 'constant'), and thus there were few direct flights between minority individuals, so that pollination of these plants was very ineffective. Container-grown <i>Sinapis arvensis</i> L. and <i>S. alba</i> L. were placed in an experimental garden in single-species groups (6 plants) and 1:9 and 9:1 mixtures (60 plants, a 'majority' of 54 and a 'minority' of 6). Individuals were moved between sites and their seed-set assessed when in single-species, majority and minority situations. Majority and single-species seed-sets were similar, but minority seed-sets were significantly lower (in both species). The low minority seed-sets were due to ineffective pollination which occurred because most insects visited both species whilst foraging in the mixed groups, and thus few visits to minority flowers were immediately preceded by visits to flowers on another minority plant. The incompatibility system of <i>Sinapis arvensis</i> was studied in plants collected from farmland in South Wales. The results obtained are consistent with there being a single locus, multiallelic, sporophytic incompatibility system in this species. In the population investigated there were at least 14 S-alleles, and probably more than 24. By labelling individuals it was shown that females of the solitary bee species <i>Colletes daviesanus</i> Smith returned repeatedly to forage at the same sites.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:636991 |
| Date | January 1986 |
| Creators | Ford, M. A. |
| Publisher | Swansea University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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