Factors Affecting Fruit Set in Aizoaceae Species of the Succulent Karoo
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ARTICLE IN PRESS Basic and Applied Ecology 9 (2008) 401–409 www.elsevier.de/baae Factors affecting fruit set in Aizoaceae species of the Succulent Karoo Gesine Pufala,Ã, Carolin Mayerb, Stefan Porembskia, Norbert Ju¨ rgensb aUniversity of Rostock, Institute of Bioscience, Department of Botany, Wismarsche Straße 8, 18051 Rostock, Germany bUniversity of Hamburg, Biocentre Klein Flottbek and Botanical Garden, Hamburg, Germany Received 22 February 2006; accepted 1 June 2007 Abstract By reducing the number of flowers and fruits, intense grazing activities of domestic livestock are considered to have negative effects on the plant recruitment of perennial plants in the Succulent Karoo, South Africa. In the present study, the fruit set of six perennial species belonging to the Aizoaceae was investigated under two different grazing intensities. Two species (Cheiridopsis imitans and Leipoldtia schultzei) were examined more closely to determine whether factors other than grazing impact fruit production of these plants. Apart from reproductive output, the population structure of these two species was explored in heavily and slightly grazed areas. For all investigated species, heavy livestock grazing was identified as a major threat to fruit production. Deviations from this pattern were recorded in a drought year, when harsh weather conditions reduced herd sizes dramatically and the usually high grazing pressure was alleviated. However, detailed analyses revealed that also other biotic factors can cause severe fruit and flower losses even exceeding that caused by grazing. Caterpillars of the noctuid moth, Diaphone eumela, and two rodent species, Otomys unisulcatus and Rhabdomys pumilio, proved to be further important herbivores of flowers and fruits. Experiments with caged plants, excluding livestock from grazing, confirmed though that livestock grazing definitely accounts for a continuous reduction of fruit production. Insufficient pollination success was another parameter reducing fruit set; however, this was almost exclusively observed under low grazing pressure. r 2007 Gesellschaft fu¨ rO¨ kologie. Published by Elsevier GmbH. All rights reserved. Zusammenfassung Allgemein wird angenommen, dass intensive Weidewirtschaft in der Sukkulenten-Karoo Su¨ dafrikas negative Effekte auf die Reproduktion und Verju¨ ngung mehrja¨ hriger Pflanzen hat, indem domestizierte Herbivoren die Zahl von Blu¨ ten und Fru¨ chten schma¨ lern. In der vorliegenden Studie wurde der Fruchtansatz bei sechs perennierenden Arten der Aizoaceae unter zwei unterschiedlichen Beweidungsintensita¨ ten untersucht. Ferner sollte die eingehendere Betrachtung zweier Arten (Cheiridopsis imitans und Leipoldtia schultzei) Aufschluss daru¨ ber geben, ob neben der Beweidung auch andere Faktoren die Fruchtproduktion dieser Pflanzen beeinflussen. Zusa¨ tzlich wurde in stark und gering beweideten Gebieten die Populationsstruktur dieser beiden Arten analysiert. Fu¨ r die untersuchten Arten erwies sich die Beweidung durch Nutzvieh als einer der wesentlichen Gru¨ nde fu¨ r die Verringerung der Fruchtproduktion. Extreme Witterung in einem Du¨ rrejahr fu¨ hrte zu Abweichungen von diesem Muster, da durch erheblich verringerte Weidetierbesta¨ nde der sonst starke Beweidungsdruck nur geringfu¨ gig war. ÃCorresponding author. Victoria University of Wellington, School of Biological Science, New Kirk Building, Wellington, New Zealand, Tel.: +64 4 463 8012. E-mail address: [email protected] (G. Pufal). 1439-1791/$ - see front matter r 2007 Gesellschaft fu¨ rO¨ kologie. Published by Elsevier GmbH. All rights reserved. doi:10.1016/j.baae.2007.06.001 ARTICLE IN PRESS 402 G. Pufal et al. / Basic and Applied Ecology 9 (2008) 401–409 Die detaillierten Untersuchungen zeigten allerdings, dass neben Beweidung andere biotische Faktoren noch gro¨ ßere Blu¨ ten- und Fruchtverluste verursachen ko¨ nnen. Als Blu¨ ten- und Fruchtfresser erwiesen sich die Raupen des Nachtfalters Diaphone eumela (Noctuidae), sowie zwei Nagetierarten, Otomys unisulcatus und Rhabdomys pumilio. Experimente mit Pflanzen in Ka¨ figen, die nur Nutzvieh ausschlossen, besta¨ tigten jedoch, dass Beweidung durch Ziegen zweifellos fu¨ r den kontinuierlichen Verlust von Fru¨ chten verantwortlich ist. Unzureichende Besta¨ ubung verringerte ebenfalls den Fruchtansatz, spielte jedoch fast ausschließlich bei geringem Weidedruck eine Rolle. r 2007 Gesellschaft fu¨ rO¨ kologie. Published by Elsevier GmbH. All rights reserved. Keywords: Cheiridopsis imitans; Herbivory; Lack of pollination; Land use; Leipoldtia schultzei; Livestock exclusion; Population structure Introduction Cheiridopsis imitans and Leipoldtia schultzei display different flowering strategies (steady state and big bang) The Succulent Karoo Biome in South Africa is one of and were therefore chosen for a detailed examination the most striking ecosystems in the world. With concerning the various influences on the development of approximately 4850 plant species, of which 40% are fruits and their current population structure. The endemic, it is one of the two arid regions that qualify as question was whether grazing is the major threat to biodiversity hotspots (Hilton-Taylor, 1996). Aizoaceae, the fruit set of indigenous species and, thus, their with most species being succulent dwarf shrubs, abundance and diversity in the Succulent Karoo or to contribute about 20% to the flora of the Succulent what extent other factors also have an impact on the Karoo (Cowling & Hilton-Taylor, 1999). Although this fruit production of perennial plants. biome is so unique, less than 4% of the area are under protection (Lombard, 1995). Major threats to the biodiversity of the Succulent Karoo Biome are anthro- Materials and methods pogenic influences such as livestock grazing (Davis & Heywood, 1994). It is therefore necessary to investigate The present study forms part of the botanical subproject the impact of different land use practices on the flora (S06) of BIOTA Southern Africa (Biodiversity Monitoring and fauna as well as restoration possibilities to manage Transect Analysis in Africa, www.biota-africa.org). Field the conservation of biodiversity in the area. investigations were conducted from 2002 to 2004 during For hundreds of years livestock herding has had a the main flowering season (August–October). major impact on the landscape (Hoffman, Cousins, Meyer, Petersen, & Hendriks, 1999). Nowadays, the Succulent Karoo forms a mosaic of communally Study site and study species managed land and privately owned farms. On commu- nal rangeland the stocking rates often exceed recom- The study sites were established on two adjacent mendations by the Department of Agriculture, leading BIOTA observatories (standardised monitoring sites of to its degradation with altered vegetation and signs of 1km2) in the Leliefontein Communal Reserve, Northern erosion (Allsopp, 1999; Todd & Hoffman, 1999). Due to Cape. Leliefontein is situated on the Kamiesberg ongoing grazing pressure, the floristic diversity has escarpment, which lies between 900 and 1500 m a.s.l. severely been altered by a shift from perennial shrub The whole region, known as Namaqualand, is char- vegetation to higher diversity and abundance of annuals acterised by a winter rainfall regime, Leliefontein and dominance of unpalatable shrubs (e.g. Galenia receives about 150–250 mm precipitation annually africana, Aizoaceae) (Todd & Hoffman, 1999). Com- (CV ¼ 33%; Todd & Hoffman, 1999). High tempera- mercial farms, on the other hand, often have the tures, low relative humidity and mostly no or very possibility of applying rotational grazing strategies with little cloud cover result in large annual and daily grazing forming a landscape that is considered near- temperature fluctuations (Desmet & Cowling, 1999). natural. The soils in Namaqualand are generally poor of Livestock can affect plant recruitment directly by nutrients and are rather infertile, but their nutrient grazing or browsing flowers, fruits and seedlings (Milton content may vary depending on surrounding vegeta- & Dean, 1990) or indirectly with continuous grazing tion, rock cover and elevation (Allsopp, 1999; Carrick, reducing plant fitness or the trampling preventing 2003b; Petersen, Young, Hoffman, & Musil, 2004). germination (Venter, 1962 in Esler, 1999). The present Rainfall and temperature data were recorded by an study investigates the fruit set of six Aizoaceae species automatic weather station (BIOTA) situated on the under different grazing intensities. Leliefontein site and soil samples were taken on ARTICLE IN PRESS G. Pufal et al. / Basic and Applied Ecology 9 (2008) 401–409 403 different locations on both study sites (Pufal, unpub- rodents feeding on flowers. The rodents were identified lished data). by faeces and tracks around damaged plants as well as Namaqualand belongs to the Succulent Karoo Biome remains of ‘‘harvested’’ C. imitans flowers, matching the (Rutherford & Westfall, 1986) with succulents from flower stalk, in a rodent burrow. Damage not only to many families (Cowling & Hilton-Taylor, 1999). The flowers but also to other plant parts (leaves) was observatories are Leliefontein on communal rangeland identified as grazing by livestock. Goats or donkeys (NW corner: 30.386S, 18.276E) with heavy browsing show a similar feeding pattern, but since donkeys passed and grazing pressure by goats and wild living donkeys, only very occasionally by the study area, damage by and Remhoogte (NW corner: 30.935S, 18.276E), a livestock is attributed to goats. Fruits were investigated commercial farm with very