| Summary: | The ecology and diversity of flowering parasitic plants and their hosts are poorly investigated and usually ignored in Saudi Arabian plant communities. Therefore, this work aimed at assessing the ecology and diversity of parasitic plants and their hosts along an elevation gradient in the Al-Baha region (1300–2400 m.a.s.l.). Different quantitative vegetation parameters were applied to analyze the collected data. Eight parasitic plants from six genera and four families were identified along the gradient, with 67% of them being zoochorously dispersed species. They accounted for approximately 23.5% (8 out of 34) of those found throughout Saudi Arabia. Perennials, stem hemiparasites, and biregional taxa accounted for around 62.5% of the total parasites, whereas indigenous species accounted for 75%. The dominant family of parasitic species was Loranthaceae (50%), and <i>Phragmanthera austroarabica</i> A.G.Mill. & J.A.Nyberg was the most important species (IVI = 107.28). <i>Orobanche cernua</i> Loefl. and <i>Loranthella deflersii</i> (Tiegh.) S.Blanco & C.E.Wetzel were restricted to the dry zone (low elevation) only, while the other parasites were distributed across the surveyed region. Twenty-three host plants were identified throughout the study region. About 83% of them were phanerophytes and bioregional plants, with 91% being perennial species. The prevalent host plant family across all sites was Fabaceae, with <i>Nicotina glauca</i> Graham being the most important host species (IVI = 32.44%). <i>P. austroarabica</i> and <i>Plicosepalus curviflorus</i> Tiegh. preferred <i>Vachellias</i> as host plants, while <i>Vachellia flava</i> (Forssk.) Kyal. & Boatwr. was the heavily infected host by <i>P. austroarabica</i>. <i>P. austroarabica</i> had a broad spectrum of host range (13 host plants), while <i>O. cernua</i> had a very narrow host range (only <i>Rumex nervosus</i> Vahl). Individual parasite and host species were markedly more abundant in the wet zone than in the low-altitude dry zone. Further research is needed to fully understand such distinctive groups of plants and their negative and positive ecological consequences on plant biodiversity and natural ecosystems.
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