by Elizabeth Medford
The value of biodiversity for the resilience of ecological systems has become common knowledge in ecology spheres. Moreover, the affects of humans on the diversity of many ecosystems around the world have been proven previously. This study connects these two pieces of knowledge in stating that the combination of diversity loss caused by human activities and environmental change increase the risk for sudden ecosystem collapse. MacDougall et al. (2013) demonstrated this connection in a degraded but species-rich grassland that was subject to fire suppression techniques as well as invasion by non-native species. The authors conclude that human disturbance created a negative relationship between diversity and function but that the elimination of the buffering effects of high species diversity has led to a vulnerability to sudden environmental change. The findings of this study can be applied more generally to many different ecosystems because of the prevalent combination of long-term land management and species loss. This study relates to climate change because as global temperatures cause fires to increase in occurrence and severity, trophic collapse because of human-caused diversity loss may also increase.
Since the mid-nineteenth century many forest communities have been managed similar to agricultural crops. Fire-suppression has been an integral part of such forest management. While these fire-suppression practices have long been associated with loss of plant species, communities also have a relatively high annual productivity level and invasion resistance compared to other managed low-diversity agricultural systems. To test the vulnerability of managed grassland areas, the authors established 80 meter sized treatment plots on Vancouver Island, British Columbia, Canada. This savanna has 115 plant species, 75% of which are native and include grasses, forbs, and woody plants. Biomass was measured in half of the plots annually and nutrient levels were measured in 2000 and two years after burning. The authors burned twenty of the plots once a year for ten years while twenty other plots were burned similarly but only for five years.
Redundancy can be crucial for stability in complex systems because one species can substitute the function and niche of another. In the case of the human influenced grassland however, no species was distributed widely enough beforehand to compensate solely for the effects of fire. MacDougall et al. observed that biodiversity levels not only influenced invasion after burning but also the dynamics of the fire regime itself. With the re-introduction of fire the grasslands only survived in areas with remnant native species where rare plants prevent extensive invasion and conversion toward woodland. Therefore, the system collapsed within one growing season with immediate dominance by invasive species. This study illustrates the ways that human activity can reduce biodiversity levels and therefore homogenize the structure and function of ecological systems. We can see from this study that this homogenization simultaneously weakens compensatory mechanisms for responding to sudden changes. Considering that human activity has been persistent in many different ecosystems around the world, there may be similar vulnerability to sudden environmental change. Unfortunately, this vulnerability will only be evident after the collapse occurs making preventative action difficult. Considering the environmental disturbances predicted to occur from changing temperature and precipitation levels, these collapses may occur more frequently than they are currently, making ecosystem vulnerability even more of a pressing topic. The authors urge for further studies on the effects of human disturbance in other ecosystem types to better understand possible future system collapse.
MacDougall, A., McCann, K., Gellner, G., 2013. Diversity Loss With Persistent Human Disturbance Increases Vulnerability to Ecosystem Collapse. Nature 494, 86–89. http://www.nature.com/nature/journal/v494/n7435/abs/nature11869.html