What are the effects of livestock grazing on ecosystem services?

Livestock husbandry represents the most important way that humans utilize arid ecosystems.  However, because of the low productivity of these systems, herbivory by domestic ungulates can have profound impacts on the function and diversity of such regions.  Mediterranean ecosystems have been grazed by livestock for thousands of years and have literally been shaped by herbivory. As such, one would expect them to be highly resilient to grazing. Arguing against this, are the increasing numbers of cases where extreme overgrazing has led to desertification.  

Because rangeland systems are so complex, and because different ecosystem components respond in different ways, it can be difficult to obtain coherent and generalizable answers. Research conducted in the lab aims to elucidate how different livestock grazing regimes affect Mediterranean habitats and the ecosystem services they provide to human communities. 

Ongoing lab work investigates how different stocking rates affect species diversity and biomass for plants, invertebrate and vertebrate organisms. For example, regarding vegetation structure, we have shown that strong herbivory has a negative effect on most aspects of plant cover including biomass and species diversity – however, it is not clear whether lower levels of grazing may actually be even beneficial for local heath and maquis habitats. 

Ecosystem services provided by native plant communities include pest control and erosion control, as well as pollination of crops (e.g. various vegetable species and fruit tree species including orange and apple trees). They are critically important for local communities who have dependent on them for thousands of years. We investigate these questions using a combination of observational and experimental approaches. 

Does livestock grazing affect honey production? 

The reason was that because goats don't like to consume thyme, this species benefits by light- to intermediate grazing regimes, which remove most of thyme's plant competitors. As a result, moderately grazed areas tend to harbor large thyme populations, and beehives at such sites produce higher honey yields. However, outside the main thyme producing season (June-July) such areas produce very few resources (nectar, pollen) to support bee populations throughout the rest of the year. Beehives appear to be more stressed in grazed areas, requiring extensive food supplementation and drug treatments to combat invading pathogenic Varroa mites. Because of these additional costs, shortened colony life expectancy, and increased labor, and despite elevated honey yields, beekeeper profits are ultimately lower in grazed rather than in ungrazed areas. 

In a project led by S. Brenton, lab members evaluated how increasingly intense levels of livestock grazing affect honey production in Mediterranean plant communities. On the study islands, honey is mostly produced by the unifloral thyme (Coridothymus capitatus) the main honey-producing plant in the Aegean region. Although grazing leads to an overall decline of plants and amount of flowers, honey production was actually higher in areas that were grazed relative to ungrazed areas. 

One particularly valuable natural product generated by Aegean plant communities is honey, which commands high prices on the market and is an important export product. Because livestock grazing leads to broad decreases in the amount of flowers in an area, it has been postulated that it may also impact honey production.