Grasshopper ecology and conservation in the Nama-Karoo.
Loading...
Date
2001
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
This study was undertaken in the Karoo, a semi-arid grazing land in South Africa, to
elucidate the interaction between grasshopper assemblages and various aspects of the Karoo
landscape. It falls into four sections, the first of which was a three-year study which was
undertaken on and around a prominent South African mesa to determine its role as an
elevational conservation refugium for grasshoppers in a sea of grazed flatlands (Chapter 2).
The number of grasshopper species and individuals on the summit, slopes and flatlands
varied significantly, in relation to measured environmental variables. The summit, through
inaccessibility to livestock grazing, was effectively a conservation refugium for one
grasshopper species, Orthochtha dasycnemis. There was no significant difference in species
richness between years of sampling, although there were significant variations in
grasshopper abundance between years. The difference in rainfall between years was
significant and appeared to be the key factor influencing grasshopper population dynamics.
This clearly shows that a mesa can act as a conservation island and refugium supporting an
insect assemblage that would be otherwise altered by heavy livestock grazing on the
surrounding flatlands. This summit assemblage is strongly linked with that on the slopes and
below, and is determined not so much by an island effect per se, but by low grazing intensity
and associated soil and vegetation structure.
The second part of the study focussed on the interaction between grasshopper assemblage
response and three hill sizes at a regional scale (Chapter 3). Small hills contained a
significantly higher grasshopper species richness and abundance than medium and large
hills. There were significantly higher number of small-sized grasshopper species and
individuals than medium and large-sized ones. Flatlands surrounding small hills had
significantly higher grasshopper species richness and abundance than those surrounding
medium and large hills. The slopes of the three hill sizes did not show significant difference
in species richness and abundance. There was no significant variation between the summits
of the three hill sizes in species richness but they varied in grasshopper abundance. The
summits of small hills had significantly higher grasshopper abundance than the summits of
medium and large hills.
Detrended Correspondence Analysis showed two clear grouping of sampling site and
grasshopper species. While the flatlands of small hills formed a separate assemblage of
several grasshopper species, slopes and summits of all hills formed another clump of few
grasshopper species. Canonical Correspondence Analysis revealed that flaltands
surrounding small hills occurred along increasing gradients of shrub cover whereas those
surrounding medium and large hills occurred along increasing gradients of grass cover,
vegetation density and greenness of grasses. Slopes and summits of all hill sizes occurred
along increasing gradients of rock cover, cragginess, grass height and soil temperature.
Patterns of grasshopper dominance were markedly variable among sites. There were low
dominance patterns on flatlands of small hills where most species were rare. The
distributional patterns varied of higher taxonomic groups varied among the three hill sizes.
Small hills contained species from four families and nine subfamilies, but medium and large
hills had only members of Acrididae in five subfamilies. About 50% of the total grasshopper
abundance were associated with small hills. The study revealed the patterns of grasshopper
assemblages at regional scale, and showed that variability in hill sizes across the Karoo has marked role in grasshopper conservation, and that grasshoppers interact differentially with
variable hill sizes across the Karoo.
The third part of the study was undertaken at twelve grassland sites in the Mountain Zebra
National Park (MZNP) and the surrounding farms to assess changes in grasshopper
assemblages to grazing by indigenous mammals inside the park in comparison with grazing
by domestic cattle outside (Chapter 4). The MZNP has been restored from cattle-grazed
farmland to indigenous mammal parkland for 62 years. The number of grasshopper species
and families inside the park was not significantly different from outside the park, but the
number of individuals inside the park was significantly higher.
Multivariate statistics did not reveal any strong site groupings based on simple inside/outside
comparisons, but there were clear groupings of sites based on vegetation characteristics
and other environmental variables. The park boundary, therefore, does not significantly
determine grasshopper assemblages, although intensity of grazing does. The indigenous
mammals inside the park had the same effect as the domestic cattle outside, and it was the
level of defoliation and trampling that was important rather than type of mammal. Very
intensive livestock grazing and trampling leads to bush encroachment and reduction in cover
and/or disappearance of several grass species. In response to this pressure, grasshopper
populations dropped, with localized extirpation of some species. Vegetation composition
and structure showed a significant influence on grasshopper assemblages, particularly grass
height and percentage cover. The MZNP is thus a localized area of elevated grasshopper
abundance in comparison with the surrounding farm landscape, and presumably represents
a situation prior to the current, intensive farming activities. Such elevated grasshopper The significantly lower population of grasshoppers on the surrounding farms, with local
extirpation of some species also suggests that the MZNP could be viewed as a local centre
to which species with higher capacity for mobility may seek refugia from anthropogenic
pressures. Hence the MZNP serves as a reference showing the difference between restored through-
natural-succession and anthropogenically-disturbed habitats, and compares
desirable with undesirable ecosystem changes for herbivorous invertebrates such as
grasshoppers.
The fourth part of the study was on grasshopper assemblage response to seasonal grazing
(including summer, winter, spring and autumn grazing), rotational grazing, continuous
resting and continuous grazing at a long-term experimental site (Chapter 5). Rotationally grazed
sites supported the highest number of grasshopper species and abundance, while
continuously-grazed sites had the lowest. Cluster analysis revealed that spring-grazed and
winter-grazed sites were the most similar, with continuously-rested sites being the next most
similar to these. Rotationally-grazed sites showed the lowest similarity to the rest of the
sites. DCA showed clear groupings of sites and grasshopper species, with most species
associated with rotationally grazed sites. Continuously-grazed sites had a different
grasshopper assemblage. CCA showed that the assemblages followed definite gradients of
measured environmental variables. Rotationally-grazed sites occurred along gradients of
increasing bare ground, while continuously-grazed and summer-grazed sites occurred along
increasing gradients of shrub cover and soil temperature. Spring-grazed, autumn-grazed, winter-grazed and rotational1y-grazed sites were characterized by high vegetation density.
Grasshopper dominance patterns were very different at different sites. Summer-grazed sites
had the highest percent dominance (40%) by Picnodictyaflavipes, while winter-grazed sites
showed higher percent dominance (32%) by Pseudogmothela sp. The significance of
variable grazing management systems for maintaining floral and grasshopper diversity is
discussed. It is recommended that rotational grazing in this arid system is most suited to
maintaining plant and insect diversity.
These four parts in this study all clearly showed that grasshoppers interact with the
landscape in a way that their assemblage patterns are dictated by patch as well as by
regional dynamics. Topography in particular contributes significantly to biodiversity patterns
at the spatial scale of landscape. But these patterns are also strongly determined by
differential grazing pressures from domestic livestock which in turn interact with the various
topographical features. These findings enable recommendations to be made on optimal
grazing regimes relative to the hilly features of the landscape. The results also show that
restoration which incorporates low-pressure grazing regimes and which takes cognizance
of topographical features can maintain grasshopper abundance and diversity in the long
term.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2001.
Keywords
Grasshoppers--Karoo., Grasshoppers--Ecology., Grasshoppers--Habitat., Karoo--Ecology., Theses--Zoology.