The Petalidium Pages

A few photos (above) and Trip(p) reports (below) from our Petalidium fieldwork in Namibia… see Angola on another page.






Per unit geographic area, wet tropical regions such as the Amazon Basin, and Mediterranean ecosystems such as the Cape and California Floristic Provinces, host the highest levels of plant species diversity worldwide (Ackerly, 2009; Bass et al., 2010; Klak, Reeves, & Hedderson, 2004; Martínez‐Cabrera, Schlichting, Silander, & Jones, 2012). Correlates of high diversity in these regions include several abiotic factors such as time, geographic space, ample precipitation, and high temperature, as well as biotic factors such as competitive interactions and density and diversity of symbioses (Dobzhansky, 1950; Ehrlich & Raven, 1964; Mittelbach et al., 2007; Rull, 2011; Stebbins, 1970; Tripp & McDade, 2013; Tripp & Tsai, 2017). In contrast, deserts often have strikingly low levels of plant diversity, particularly infrageneric diversity, even those found in tropical latitudes (Heibl & Renner, 2012; Rundel et al., 1991). One example of this is the flora of the Atacama that, while highly endemic, has very few genera that contain >10 species (Rundel et al., 1991). Empirical data from lineages that represent marked deviations from the above pattern are needed to explain discrepancies in standing diversities between wet tropical and Mediterranean versus desert ecosystems.

Except for one intriguing, divergent species in India, the remarkable genus Petalidium is confined entirely to Namibia, Angola, and minimally, adjacent countries. Species diversity is centered in the ultra-arid northwestern mountains of northwestern Namibia and southern Angola, or the ‘Kaokoveld’ (Gil-Romera et al. 2006). Annual precipitation, most of which occurs in summer months, is less than 100 mm. In comparison, the nearby Nama and Succulent Karoo of South Africa receive 100-500 mm annually, with most precipitation in winter months, thus creating a Mediterranean climate. Additionally, Kaokoveld rainfall is extremely variable interannually, unlike the reliable precipitation of the Karoo (Jürgens 1991; Dean & Milton 1999). These two factors may be primary contributors to striking differences in species richness between the Kaokoveld and Karoo, and in the former, dictate an environment extremely sensitive to climate change (Dupont et al. 2011; but see Salguero-Gomez et al. 2012).

Yet, despite life in an extreme and unpredictable environment, Petalidium is apparently flourishing and actively speciating. Fossil midden evidence indicates that Petalidium has increased in abundance over the last 1,000 years, coincident with the drying of the environment in this region, and decrease in abundance of Poaceae (Gil-Romera et al. 2006).

Our preliminary work on an intriguing genus, so emblematic of the Namibian landscape, has convinced us that an in-depth, comprehensive taxonomic revision is a required for all downstream evolutionary and ecological investigation. The species richness in the genus – not too big and not too small – makes it possible to develop and implement a “total evidence” approach in revisionary systematics. At present, we are seeking funding in order to:

  • Document and describe diversity in Petalidium via a next generation monograph that utilizes evidence from numerous sources: taxonomic, morphological, cytological, anatomical, biochemical, molecular genetic, ecological, geographical, and climatological
  • Use these data in combination with extensive niche modeling to evaluate drivers of speciation (allopatry, habitat niche divergence, pollination system divergence) and develop Petalidium as a model system for studying ecology and evolution in ultra-arid environments
  • Conduct rigorous IUCN species assessments; because of the ecological importance and exceedingly small ranges of many species of Petalidium in a region highly susceptible to climate change, we predict Petalidium could serve as an “indicator genus” for environmental change in ultra-arid landscapes.

We believe that the products of this effort will be well-received by a diverse community of users (scientists, conservationists, land managers, government officials, and laypeople) and that the IUCN assessments will represent a long-lasting contribution to science and conservation in southern Africa. In addition to contributing to completion of Target 1 of the Global Strategy for Plant Conservation, it will also contribute to Target 16, which aims to build, and strengthen existing, human networks for conservation activities. 



Enjoy a bit of diversity that colleague Dr. Kyle Dexter (University of Edinburgh) and I have encountered over the years, by perusing these species pages…

Petalidium angustitibum

Petalidium bracteatum

Petalidium canescens

Petalidium coccineum

Petalidium crispum

Petalidium cymbiforme

Petalidium englerianum

Petalidium giessii

Petalidium halimoides

Petalidium “koppie”

Petalidium lanatum

Petalidium linifolium

Petalidium lucens

Petalidium “lucinda”

Petalidium luteo-album

Petalidium “magenta”

Petalidium ohopohense

Petalidium physaloides

Petalidium pilosi-bracteolata

Petalidium ramulosum

Petalidium rautanenii

Petalidium rossmanianum

Petalidium setosum

Petalidium sp. nov. — halimoides x setosum

Petalidium subcrispum

Petalidium variabile

Petalidium welwitschii

Kyle Dexter & Erin Tripp, in extreme northwestern Namibia (the Kaokoveld); video footage documents tremendous abundance of Petalidium welwitschii in the landscape (and some good Indigo Girls tunes as an accompaniment), which is typical of most species in the genus Petalidium: rare globally (restricted endemic), but hyperabundant locally…

3 thoughts on “The Petalidium Pages

    • Hi there,
      In fact, yes – we just published our first paper on phylogenetic of Petalidium. Much more to come but for the time being, I hope you enjoy it! Here is the reference:

      Tripp et al., 2017
      Radseq dataset with 90% missing data fully resolves recent radiation of Petalidium (Acanthaceae) in the ultra-arid deserts of Namibia
      Ecology & Evolution

      Greetings from Boulder, Colorado!


  1. I have fallen in love with Petalidiums from becoming acquainted with 3 species in The Conservatory at the Kirstenbosch Botanical Gardens.
    I found your website through an internet search for information on the genus and am loving it. Your Angola trip report is most absorbing.
    Do you have any open access references with ore information on P. englerianum please?


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