Perennial Weeds in Agricultural Landscapes: Managing Threats to Smallholder Livelihoods.

A number of highly invasive perennial weeds, both woody and herbaceous, are capable of becoming dominant vegetation in  small-holder agroecosystems, resulting in major, costly and intractable changes in plant community composition and agroecosystem function in these systems. Such plant invasions are particularly common in the global South, and can seriously threaten small-holder livelihoods in these agroecosystems.  Therefore, these may be among the most damaging of plant invasions, in terms of effects on human well-being. Invaded agroecosystems include grasslands, savannas, and fallows-based systems; invading species include Pteridium aquilinum, Acacia nilotica, Imperata cylindrica, Chromoleana odorata.  These plant invasions can be understood as emergent properties of ‘eco-social’ systems, in which biophysical and social factors interact to enable and enforce invasions.  These invasions may create ‘persistant degraded states’ in agroecosystems that are difficult to reverse.  Approaches to management and restoration of invaded agroecosystems are being explored in many settings; progress and prospects for these efforts will be reviewed.