Little patches of diversity: the value of small fragments of pristine grasslands
Original article published in ENVIRA (Spring Edition 2021), quarterly Newsletter for the Unit for Environmental Sciences and Management at North-West University (UESM).
By Marlize Muller (PhD student) Forb Ecology Research Group
Supervisors: Frances Siebert (NWU), Anja Linstädter (University of Potsdam), Dave Thompson (SAEON), Stefan Siebert (NWU)
Grasslands have long been neglected as an important ecosystem, since it is erroneously believed that forests represent the dominant climax state. This notion has been challenged by the fact that more than 58% of the planet’s land area is covered by non-forested, ancient open-ecosystems (including deserts, grasslands and savannas). Despite this, the threats faced by forests are well understood, documented and resourced (think Amazon rain forests), while open-ecosystems such as grasslands are poorly managed and conserved (Bond, 2019).
The Orange River lily (Crinum bulbispermum), grows in moist grasslands.
Grassland systems are host to an exceptionally high diversity of animal and plant species, habitats and communities. For instance, there are 161 orchid taxa in the Grassland Biome of South Africa, of which 67% are endemic. Grasslands are, however, threatened by human activities, as species richness decreases by nearly 50% after land transformation (Muller et al., 2021). Many grassland species are not tolerant to anthropogenic soil disturbance and disappear completely from the system. This is especially true for plant species with underground organs adapted to survive harsh winter conditions, drought and fire. The loss of species when grasslands are transformed could have a significant impact on the ability of these systems to provide vital Ecosystem Services (ESs). Untransformed grasslands provide a variety of ESs that are necessary for our survival (Figure 1), including the provisioning of food, fresh water, medicinal resources and the regulation of systems, such as erosion prevention and carbon sequestration (TEEB, 2011).
Figure 1. Some of the ecosystem services provided by grasslands.
The Grassland Biome is one of the most threatened biomes in South Africa, as 40-60% has already been irreversibly modified, 60% of the remaining grassland is threatened, and only about 15% remains natural grassland with less than 3% of grasslands formally protected. One grassland ecosystem that is particularly threatened is the Woodbush Granite Grassland (WGG) that is found close to the small town of Haenertsburg in Limpopo. Only about 6% of the WGG is still in a natural state, with the largest fragment being 192 ha in size (Dzerefos et al., 2017).
This small fragment of WGG contains ~660 plant species, making the conservation value of the area especially high. Most of these species are forb species, since grasses represent only a sixth of the total richness. Herbaceous bulbs and forbs are the life forms that contribute to the high diversity even though they occur in low densities.
Figure 2. Grasslands consist of a high diversity of forb- and grass species that are maintained by inter-annual rainfall variability, fire and herbivory. Indigenous trees are scarce, although large areas of natural grasslands are covered by exotic timber plantations.
Pristine patches of grasslands, such as the WGG furthermore provide a natural buffer to extreme events, such as droughts. Grasslands have evolved in the presence of endogenous disturbances, such as lightning-ignited fires, rainfall variability and large mammalian herbivory and are therefore tolerant and to some extent dependent upon these natural disturbances (Figure 2). The Forb Ecology Research Group (FERG), in close collaboration with the South African Environment Observation Network (SAEON) and Potsdam University assessed the effect of the severe drought of 2014-2016 on the floristic diversity of fragmented grasslands, with a specific focus on the WGG. Results revealed that the floristic diversity of the WGG remained largely intact when pre-drought, in-drought and post-drought data were compared. Our further investigations into the functional ecology of these grassland patches are expected to improve the understanding of ecosystem resilience in grasslands and why they need to be protected, restored and managed sustainably.
(Left & middle) An orchid species, Eulophia ovalis var. ovalis (left) occurs in open grassland and a bulbous geophyte, the thick-leaved gladiolus (Gladiolus crassifolius) (right) is found in rocky grasslands.
An orchid species, Eulophia welwitschii, is often found in marshy areas and seasonally flooded grasslands.