Influence of cattle grazing and glade areas on invertebrate assemblages in a savanna ecosystem, northern Kenya
Abstract
Northern Kenya is a vast semi-arid area where large tracts of land are dedicated to ranching,
conservancies and pastoralism. Various studies have been conducted in this region to
understand how grazing influences the vegetation patterns, its composition and re-growth
characteristics. However, few studies have been conducted on the effect that grazing has on
invertebrate assemblages. This study therefore investigated the effect of cattle grazing on
invertebrate assemblages within Lewa Wildlife Conservancy, and across glade patches
established from areas that held kraals in past years.
Sampling of invertebrates was carried out across seasons over a ten month period using pitfall
trap and sweep net methods, at a six-week interval, and the results obtained subjected to
analysis of variance. Vegetation data for plant community characteristics and biomass was
collected using line intercept method. Soil was collected once during the study period in all
treatment sites and tested for soil pH, nitrogen, bulk density and organic carbon. Soil pH was
measured using a soil solution ratio of 1:5 (1 part soil to 5 parts 0.01M Calcium chloride),
Total Nitrogen was measured using the Kjeldahl method, a soil auger and metal core rings
were used to collect disturbed and undisturbed soil samples respectively and Organic Carbon
content was measured using the Walkley and Black method.
A total of 339 invertebrate species from 296 families were collected; pitfall trap method
accounted for 200 species [59%] and sweep net method had 139 species [41%]. Of this total
invertebrate collection, 160 families were from the order Coleoptera [54%] while Orthoptera,
Hemiptera, Hymenoptera, Diptera, had 51 [17%], 33, [11%], 32 [11%], 20 [7%] families
respectively.
Invertebrate diversity and abundance collected by both sweep net and pitfall trap methods
were not significantly different across the three treatments; glades, grazing and control.
Invertebrate species diversity and invertebrate species abundance were not significant at α ≤
0.05 for invertebrates collected using sweep net method. Similarly, for invertebrate species
collected using pitfall method, invertebrate diversity and abundance were not significant.
However, when the collected invertebrates were grouped into their different orders,
invertebrate species revealed significant differences in abundance (α ≤ 0.05). Invertebrate
species mean abundance was highest in the order Coleoptera in glades and grazed treatments.
Percentage total invertebrate abundance from glades of the year 2010 was the highest (24%)
compared to that of glades established in the other years. Glades from the year 2009 and 2012
had the lowest total invertebrate abundance at 18% while those of the year 2008 and 2011
recorded total invertebrate abundance of 20%. However, these differences in total
invertebrate abundance, across the years, were insignificant at α ≤ 0.05.
Invertebrate species diversity was not significantly different across glades of different years,
either by using sweep net or pitfall trap methods. However, an analysis of variance on
invertebrates collected across glades of different years, using pitfall method, revealed a
significant difference on invertebrate species abundance (α ≤ 0.05) that was attributed to
differences in glades of 2009 and 2010.
Invertebrate species diversity and abundance were significantly different across seasons α ≤
0.05 and invertebrate species abundance was higher in the wet seasons than in the dry one.
Vegetation biomass was not significantly different across the three treatments but was
significantly different across seasons within these treatments (α ≤ 0.05) with the highest
amounts of vegetation biomass occurring in the first wet season and the lowest amounts in
the dry season. A negative and significant correlation was established between Diptera
species and forbs biomass (r = - 0.29, α ≤ 0.05) and between Hemiptera species and
perennials biomass (r = - 0.26, α ≤ 0.05). Other invertebrate species namely Hymenoptera,
Coleoptera and Orthoptera did not significantly correlate to vegetation parameters, showing
that vegetation has an indirect effect on these species.
There was a negative and significant relationship between invertebrate species from the order
Coleoptera and soil bulk density (r = - 0.52, α ≤ 0.1). Soil bulk density was within the ideal
range of 1.10 and 1.47 g/cm3 for plant growth that is linked to increased invertebrate diversity
and abundance. Invertebrate species from the other orders, this are, Diptera, Hymenoptera,
Hemiptera and Orthoptera, did not give significant correlations with all measured soil
attributes, indicating lack of a direct relationship.
From this study, invertebrate species patterns reveal important ecological trends in grazed
conservancies and can therefore be used for ecological monitoring. Controlled grazing in
such conservancies should be encouraged as it does not affect the ecological integrity for
biodiversity conservation. Moreover, in practicing this, these conservancies will meet their
corporate responsibility of supporting neighboring grazing communities
Citation
Master of science in land and water managementPublisher
University of Nairobi