Meredith Root-Bernstein raises the question of how we define overgrazing and highlights the recent findings of Oliva et al. in their article, Remotely sensed primary productivity shows that domestic and native herbivores combined are overgrazing Patagonia.
Can large wild herbivores live together with domestic livestock? This question is important to answer if we are going to reconcile the conservation of herbivore populations across large areas with human livelihoods in rangelands and silvopastoral systems. It can also be important to address this is the context of rewilding proposals: can rewilding with wild herbivores be compatible with production? There are a couple of key issues. One is disease transmission. Although most wildlife disease studies suggest that livestock are reservoirs of diseases that are transmitted to wild species populations, farming interests often make the opposite argument, claiming that wild species need to be controlled, or not reintroduced, to prevent the spread of disease to domestic stock. Another tricky question is whether grazing herbivores compete with livestock for food and whether this is acceptable from a socioeconomic perspective? Overgrazing, if it results in land degradation, will negatively affect both wild species populations, and biodiversity generally, as well as human livelihoods.
Interestingly, there is no clear definition of overgrazing. For wild species, theory says that there is a carrying capacity, K, for any species population in its environment. If the population overshoots K it will self-regulate to reduce population size to approach K. Rodent populations, for example, may oscillate dramatically around K, influenced by combinations of, or alternately stronger, top-down and bottom-up forces. Emerging evidence suggests that large herbivore populations may also overshoot K in some years and regulate this with high mortality; although this suggestion remains somewhat controversial. Overgrazing, in any case, a concept from rangeland management that often remains vague. It may refer to many different kinds of change in rangeland vegetation or dynamics of recovery; it may refer to small and medium-scale disturbance events, or to landscape-scale alterations. Oliva et al. in their paper, Remotely-sensed primary productivity shows that domestic and native herbivores combined are overgrazing Patagonia, address this last point.
The authors examine the case of wild guanaco (Lama guanicoe) that coexist with sheep and cattle in the Patagonian steppe of Argentina. Livestock were introduced to ranches there around 1880. Guanaco populations were initially reduced dramatically but in recent years have grown up to 2.1 million in the region. Over the same period, livestock initially increased to 3-4 million between the 1950s and 1990s, and then fell to a current level of almost 2.5 million. Oliva et al. estimate a carrying capacity for the region using ground-truthed remote sensing data to assess total productivity. They calculated stocking density by calculating the amount of consumable forage produced, divided by the estimated annual consumption for each species. They could then calculate how many individuals of each species could be supported, assuming that each species was present alone, or equivalently, that the species can substituted based on how much each eats, and different combinations of species can be equated with a total sum of consumed forage.
Oliva et al. find that while initially being over carrying capacity for several decades, the total stock of herbivores has recently fallen to within one standard deviation of the estimated carrying capacity. The rangelands, however, have not yet fully recovered, which may also be due to shrub encroachment creating a different habitat type. To understand the future dynamics of guanaco populations and their effects on sheep and cattle, the authors recommend further studies that distinguish between herbivory on different plant functional groups and across palatable or favoured and unfavoured species, and take into account the ability of guanacos to adapt to eating low quality forage, such as shrubs. They suggest that because guanacos and livestock can eat different things and use different parts of the landscape, mixed systems including guanacos may in fact prove to be more efficient than single-species livestock production, particularly if guanaco are also exploited for meat and fibre. Indeed, mixed livestock systems are considered superior for both grazing efficiency and reduction of disease and parasite transmission, from an agroecology perspective.
Although it seems that there is a long way to go to find suitable measures for calculating the carrying capacity of complementary herbivores in mixed systems, the general outline of Oliva et al.’s results are encouraging. Large wild herbivores can maintain sustainable population sizes in commercially grazed rangelands. Although commercial ranching is becoming less lucrative than it was during the boom years and some land abandonment has occurred, moving towards a more diversified, less intensive form of production is in line with agroecological ideas and sustainability more generally. This suggests ecologists and conservationists may also want to reconsider ideas about rewilding only in areas where farming has been abandoned and exploiting them only for tourism. They may find social acceptance of large herbivores living together with livestock in extensive rangelands if they accept reviving millennial traditions of human exploitation of large herbivores, sustainably of course, and with ecological science to back it up.
Read the full Research Article, Remotely sensed primary productivity shows that domestic and native herbivores combined are overgrazing Patagonia in Journal of Applied Ecology.