Emergence of “Boundaries” by the Spread of Invasive Plant Species

Boundaries are not only spatial separations between territorial units, they are also zones of interaction between neighboring entities. Leaving the mere spatial interpretation, this dual nature of boundaries also exists in social-ecological systems. In the context of the project ”Future Invasions” we address both the materialities (invasive spread ecology and dynamics, infrastructure development, household assets) and the imaginations of boundaries (actors’ aspirations and views on possibilities and probabilities). The transformation of interactions between people and the environment is studied in areas that are increasingly affected by two invasive plant species in the Marigat plains around Lake Baringo in Kenya. Mesquite (Prosopis juliflora) is a woody shrub or tree originally introduced in the area in the context of an afforestation project, which currently occupies former grazing land and restricts the access of livestock to watering places. The Parthenium weed (Parthenium hysterophorus) is an emergent problem in the area benefiting from intensification of land use, improvement in irrigation infrastructures and the enhanced connectivity within the area. Both species are considered as highly noxious plants, also beyond Eastern Africa.

Dense stand of Mesquite (Prosopis juliflora) in former pasture land. Photo courtesy of Alvares. M

Physical boundaries correspond to environmental attributes favoring the establishment and aggressive spread of Mesquite and the Parthenium weed. Contrasting geological formations and respective soil attributes, natural and artificial dynamics of hydrology and their interaction with land uses, hypothetically define the physical boundaries for plant invasions. Both species invade primarily pasture lands. Mesquite thrives on pastures around the shoreline of freshwater water bodies, in river banks or in areas where roots can access the groundwater table. Temporary high soil moisture content along canals and roads and no disturbance by mechanical soil tillage of crop land define the boundaries of current invasion sites of Parthenium weed. With sedentary crop production and associated frequent soil disturbances by mechanical tillage operations, the establishment and invasive spread of both species may be hampered.

Social-ecological boundaries are affected by changes in the human realm. This relates for instance to land management changes and emerging anthropogenic disturbances. Such changes in land uses comprise the construction of canals or road infrastructures, soil tillage for crop farming or fallowing, the transformation of pastures to crop land, and pasture management (here especially the shift from cattle to goats acting as vectors for seed spread by zoochory). Such changes entail disturbances that contribute to shifting the boundaries by stimulating spread dynamics and expanding the invasion to previously non-affected areas.

Socio-economic boundaries may affect the ability to adapt to changes such as the invasion of species threatening the basis of people livelihoods. The boundaries differ between households depending on the access to physical, human as well as social capital. For instance, boundaries between households with and without access to land allow the former to shift from pastoralism to sedentary field crop production when confronted with massive invasion dynamics. Limits of rural labor and product markets are other examples of boundaries to adaptation. The ability to cross these boundaries is affected by infrastructural development.

Aspiration boundaries refer to land users’ desires for the future with respect to dimensions like income, household assets or education for their children. Their capacity to aspire gradually differs with available resources such as current household assets and social capital such as access to agricultural extension services. Additionally, institutional mechanisms such as cooperative societies contribute strongly and positively to the formation of income and asset aspirations. Stricter boundaries in terms of effects on aspirations could be implied between those experiencing an ecological shock such as massive invasion dynamics and those that do not. However, foregoing aspirations of remunerating land use for some may foster higher aspirations of others when considering competition for land between pastoralists and sedentary farmers.

As mentioned boundaries are not static; they entail associated dynamics such as the rapid sedentarization of former pastoral, transhumant people and their market-oriented production of food crops on former pasture land. Thus, semi-nomadic Il Chamus and Tugen, as well as Pokot pastoralists along the southern and northern fringes of Lake Baringo, respectively, are increasingly claiming parts of former communal pasture land for producing maize and forage seeds in rural, and perishable vegetables in peri-urban settings. Such trends are on the one hand driven by uncertainties in pastoral land ownership, but increasingly also by declining pasture productivity associated with the spread of invasive plant species. On the other hand, the observed system changes are accelerated and occur in response to new marketing opportunities and reduced transaction costs associated with infrastructural developments, i.e. those related to the exploitation of geothermal energy. In addition, these transformations have feed-backs on the invasive spread dynamics, affecting the different alien invaders. While infrastructure development with canals, roads and reservoirs will promote invasive spread, the shift towards crop cultivation can contribute to controlling the economic damage associated with invasion. Thus, invasive species act as a push factor (pushing traditional pastoralism to the margin), while market access acts as a pull factor (pulling field crop production into former pastoral areas), and the associated system shifts are affecting future invasive spread with multiple shifts of social-ecological boundaries.

While primarily contributing to the operationalization of the bridging concept of “boundaries”, the expected transformations and their determining push-pull dynamics occurring at larger spatial scales will induce feedback processes on the ecological and the social-economic sub-system. Thus, the project concomitantly addresses also the bridging concepts of coupling and cross-scalar linkages.

By: Sub-project B01, Future Invasions.