Browsing by Author "Ojija, Fredrick"

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The distribution, control techniques and impact of Parthenium hysterophorus on flower visitors and soil chemical properties in Meru district, Tanzania
(2020-08) Ojija, Fredrick
In Tanzania, the invasive plant Parthenium hysterophorus threatens natural and semi–natural environments. Little is known about how this species affects plant–pollinator interactions and soil–chemical properties. Roadside survey was conducted to assess P. hysterophorus invasion status within and outside Arusha National Park (ANP), Tanzania. Soil samples were also collected in invaded and uninvaded plots to investigate the impact of P. hysterophorus on soil–chemical properties. To test alternative natural–based management strategies, the study examined the bio–herbicide potential of Desmodium uncinatum leaf (DuL) crude extract and the competitive ability of fodder legume plant species (Lablab purpureus, Desmodium intortum and Medicago sativa) to suppress P. hysterophorus in pot and plot experiments. Results showed that ANP is currently uninvaded with P. hysterophorus, but some adjacent villages i.e. King’ori, Maleu, Napoco, Ngongongare, Ngurdoto, Oligilai and Sakila have already invaded by this invasive. Parthenium hysterophorus invaded site was more acidic with lower electrical conductivity, less calcium and phosphorus, and high cation exchange capacity. The DuL extract suppressed P. hysterophorus growth vigour, particularly at higher concentrations (>70%). At these concentrations, the invasive seedling stem height was >30% shorter, and seed germination was inhibited by >55% compared to seedlings sprayed with lower concentrations. When P. hysterophorus was grown in combination with all three test plants, its seedling stem heights and total fresh biomass were reduced by >60% and >59% in pots, and >40% and >45% in field plots respectively, compared to when grown alone or in mixture with just D. intortum or M. sativa. Moreover, surveys conducted on invaded and uninvaded sites to investigate the impact of P. hysterophorus on plant–pollinator interactions when two common target plants (Ocimum gratissimum and Ageratum conyzoides) were present showed that flower visitation rate to target plants was significantly lower in invaded plots than in uninvaded plots. This implies that P. hysterophorus may be disrupting pollen flow. The study recommends that the use of bio–herbicide and suppressive plant species to control the invasive should be promoted. However, it emphasizes the use of native suppressive plant species because non–natives may turn into invasives in the future.
Impacts of alien invasive Parthenium hysterophorus on flower visitation by insects to co-flowering plants
(Springer Nature, 2019-05-11) Ojija, Fredrick; Arnold, Sarah; Treydte, Anna
The exotic invasive plant Parthenium hysterophorus is invading many tropical habitats. While much work has been done on its interactions with other native plants, little is known about its interaction with insect floral visitors and how it impacts pre-existing pollination networks when it invades a site. We carried out surveys on sites with and without P. hysterophorus (invaded and uninvaded, respectively) to investigate its impact on plant–pollinator interactions with two common indicator or target plants (Ocimum gratissimum and Ageratum conyzoides) in Tanzania. During multiple 15 min observation periods in quadrats, the number of arriving flower visitors, duration of visits and visitation rate were measured and compared between sites. Visitation networks of flower visitors were developed by observing flower visitor taxonomic groups and plants visited across both invasion categories. Parthenium hysterophorus was heavily visited by a diversity of flies as taxonomic groups. Indicator plants received fewer flower visitors overall in the invaded site, implying P. hysterophorus may be disrupting pollen flow. Foraging behaviour and flower visitation by Apis mellifera and flies on target plants were particularly negatively affected in the invaded quadrats. Flower visitation rate to target plants was significantly lower in invaded quadrats than in uninvaded quadrats. This study supports work in other parts of the world demonstrating that invasive species can strongly disrupt pollination networks. By attracting flower visitors that could otherwise serve as pollinators of native plant species, P. hysterophorus which is rapidly spreading in eastern African ecosystems could have complex deleterious effects on the wider ecosystem.