Invaders or survivors

This talk deals with plant species whose populations show rapid and unwanted growth in some parts of the country or some parts of a landscape. These unpopular plants have drawn the attention of landusers, economists, scientists, agro-chemical companies and politicians over the past five decades because they have massive negative impacts on the productivity and value of grazing land and watersheds, pose intellectual challengers for those in the fields of biological and chemical control and population modelling, and opportunities for perpetual generation of employment opportunities in the public and private sectors. These unwanted plants often form dense thickets that shade out grazing and exclude livestock. Many of them are thorny, poisonous or simply unpalatable. But are the drivers of population increase the same for all such undesirables? Here I propose that unwanted plants fall into two distinct categories, namely invaders and survivors (Table 1) Table 1 Term Invader Survivor Synonym Aggressor, encroacher, interloper, trespasser, intruder Outlaster, endurer, remnant Definition One who enters by force in order to conquer One who lives through affliction, remaining alive after an event in which others have died Implication The sense of coming from the outside and taking over The sense of being a remnant of a local damaged community Origin Alien Indigenous Savanna examples Chromolaena odorata, Lantana camara, Melia azedarach, Psidium guajava, Solanum mauritianum Dichrostachys cinerea, Vachellia hebeclada, V. karroo,  V. nilotica, V. tortilis, Senegalia  mellifera, Grassland examples Acacia dealbata, A. mearnsii, Populus x canescens, Eucalyptus spp. Vachellia karroo, Stoebe plumosa, Dicerothamnus rhinocerotis Rhigozum trichotomum Karoo examples Cactaceae, Prosopis hybrids, Pennisetum setaceum Chrysocoma ciliata, Euphorbia mauritanica, Galenia africana, Homeria miniata, Pteronia pallens , This simple framework could influence the way in which managers think about the drivers of invasion, bush-encroachment and unwanted vegetation changes in grazing lands, and the approaches to solving the problems. It might help predict patterns of plant population increase and spread, in response to changes in management regimes and climate change. Both categories of unwanted plants occur, and are increasing, in the main livestock-producing biomes, the Karoo, Grassland and Savanna. Here I will review, for each biome, the history of increase of the most prevalent invader and survivor species in context of the natural conditions, resources and disturbances regimes characteristic of the biome, and management practices applied in the biome. I will compare the life-histories of the invader and survivor species and discuss outcomes of various approaches to their control and management.