| Summary: | Understanding relationships between large herbivores and plant species diversity in dynamic riparian
zones, and more specifically sodic zones, is critical to biodiversity conservation. Sodic patches form
an integral part of savanna ecosystems because of the ecosystem services and functions they provide,
i.e. accumulation of nutrients, provision of open spaces for predator vigilance and formation of wet
season wallowing points. Furthermore, these key resource areas sustain body condition for dry season
survival and support reproduction through nutritional benefits, making them „nutrient hotspots.‟
The Nkuhlu research exclosures in the Kruger National Park (KNP) provide a unique opportunity to
investigate spatial and temporal heterogeneity patterns within riparian zones, and how these patterns
are affected by fire and herbivory. A monitoring project was initiated to answer questions pertaining
to the dynamics of the herbaceous layer and was aimed at determining (a) whether there exists
meaningful variance in herbaceous plant species richness and diversity across different treatments of
fire and herbivory in the ecologically sensitive sodic zone, (b) if temporal shifts in plant species
composition and diversity occurs, (c) whether an increase in herbaceous biomass, an artifact of
herbivory and fire exclusion, suppresses herbaceous plant species diversity and richness, and (d)
whether there exists a significant relationship between herbaceous biomass and species
richness/diversity. The Nkuhlu exclosures consist of three herbivory treatments, each divided into a
fire and no-fire treatment, hence six treatment combinations overall. Herbivory treatments consisted
of, (1) a partially fenced area designed to specifically exclude elephants (giraffes are also excluded
due to body size), (2) an open, unfenced area and (3) a fully fenced area, designed to exclude all
herbivores larger than a hare. Herbaceous vegetation was sampled in two 1 m2 circular sub-plots in
the eastern and western corners of each of the 82 fixed plots. Biomass of each plot was estimated with
a Disc Pasture Meter (DPM) by sampling ten points diagonally within each plot. DPM-readings were
converted to kg/ha according to latest conversions for the Lowveld Savanna.
Species richness and biomass showed significant variance across treatments for the 2010 dataset,
whereas no significant variation in herbaceous species diversity was perceived. Combined treatment
of fire absence and herbivore presence contributed to higher forb species richness in the sodic zone.
Biomass was significantly higher in fully fenced areas where herbivores were excluded, opposed to
the open and partially fenced areas. Although no significant variation was recorded for diversity
across treatments, lowest diversity was recorded in the absence of all herbivores, especially in
combination with fire treatment. After nine years of herbivory exclusion, diversity of herbaceous
species varied significantly. Herbaceous species composition changed over time in areas exposed to herbivory, while composition of fully fenced treatments did not reveal change. A hump-shaped
relationship exists between herbaceous species richness/diversity and field biomass, at least for areas
with biomass levels not exceeding 2500 kg/ha. Herbivores are therefore considered essential in
sustaining herbaceous plant species richness and system heterogeneity in the sodic zone, since
herbaceous species richness/diversity was higher in herbivore presence and herbaceous species
composition changed over time in areas exposed to herbivory. Although statistically non-significant,
fire seems to suppress species richness.
Conservation implications: This study could be used as framework to advance and develop sciencebased
management strategies for, at least, the sodic zones of the KNP. Research in these exclosures
contributes to our understanding of these landscapes and benefit ecosystem conservation planning. It
also provides valuable long-term data for key ecological processes. === MSc (Environmental Sciences), North-West University, Potchefstroom Campus, 2014
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