Plant communities of Kilombero wetlands in Tanzania: structure, invasion and cattle grazing
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Date
2014
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Sokoine University of Agriculture
Abstract
Wetlands arc dynamic ecosystems appreciated worldwide for supporting biodiversity.
sustaining livelihoods and climate change mitigation and adaptation. However, the increased
degradation of wetlands and changes in their plant species composition and diversity due to
anthropogenic disturbances, alterations of natural environmental conditions and plant
invasions is an increasing global concern. Thus, to restore the species composition and
diversity and protect existing plant communities in wetlands, timely understanding of
important factors that regulate and maintain plant community structure as well as how
communities become vulnerable to alien plant invasions is essential. Therefore, using
herbaceous vegetation, studies were conducted to investigate key drivers of spatial
heterogeneity in plant species composition, richness and diversity, to examine invasibility of
plant communities and to assess how large domestic herbivores (cattle) select foraging sites
within Kilombero Valley Floodplain wetland. Tanzania.
The results show that patterns of spatial variation in plant species composition.
richness and diversity were associated with natural environmental conditions, human
disturbance and spatial position variables. In particular, variation in species composition of
all plant species and functional groups (graminoids. forbs and shrubs) were best explained by
natural environmental conditions (total organic carbon, elevation and distance from
Kilombero River) and human disturbance (cow dung density and distance to cattle holding
pen area (kraal)). Total species richness and the richness of functional groups decreased with
the increase in distance from the river, suggesting that, annual flooding is important in the
study site. Grazing reduced total species diversity and the diversity of graminoids possibly
through changing local nutrient balance. Most shrub species were found on relatively high
ixelevations, and shrub richness related negatively to total organic carbon suggesting that
shnibs are less tolerant to inundation and high levels of soil fertility.
Plant community properties such as species evenness and bioinass have rarely been
used in biological invasion studies. Interestingly, these properties promoted community
resistance against invasion of alien plant species Echinochloa colona through reducing its
abundance. Moreover, it appears that grazing intensity creates gaps which are occupied by
fast growing opportunistic species such as £. culona which may then alter the structure of
native plant communities. Thus, disturbed areas with rich environmental resources (e.g. light.
water, nutrients) and low plant biomass and evenness of native species are hot spots for plant
invasions.
Cattle selected foraging sites using a nesting feeding choice model. On a larger scale.
cattle selected patches with low diversity, at high elevations, and at longer distances from
agricultural fields. Moreover, cattle selected patches with a higher abundance of palatable
plant species with high biomass at large scale. It appears that once the large scale patch is
selected, cattle tend to graze on small patches with high plant species richness and moderate
biomass to achieve nutritional balance and maximize energy gains.
In conclusion, maintenance of plant species richness and biomass together with
optimal stocking rates could improve cattle production and rangeland quality. Since natural
environmental conditions and human disturbance variables shapes plant community structure.
intensification of human activities (e.g. overgrazing by livestock) and alterations of natural
environmental conditions (e.g. through water diversion and drainage) should be discouraged.
Finally, improved management strategies including increasing biomass and evenness of
native species together with reduction in grazing pressure may reduce community
vulnerability to alien plant invasions and improve biodiversity conservation.
Description
PhD Thesis
Keywords
Plant communities, Kilombero wetlands, Cattle grazing, Structure, Invasion