Habitat availability alters the relative risk of a bovine tuberculosis breakdown in the aftermath of a commercial forest clearfell disturbance

In their new study, Murphy et al. discuss the relationship between human modification of landscapes and zoonotic disease emergence and spread, through their case study of bovine tuberculosis in Ireland.

The interconnectedness of ecosystems is one of the most endearing facets of landscape ecology. Yet, it presents the biggest challenge for applied ecologists seeking to understand the cause and effect of ecosystem modification. Changes to the landscape, the environment or biological communities can create cascading effects that reverberate across ecosystems subtly over time. The reintroduction of wolves to Yellowstone National Park is a popular example that sparked a public awareness of this phenomenon, with a short YouTube video garnering 43 million views linking  wolves to changes in the river ecosystems in the park. Despite recent public interest, it is an understudied field in applied ecology.

Unfortunately, the science underlaying these processes is more complex than a simple cause and effect story. While wolves may indeed influence riparian systems in Yellowstone by modifying the “landscape of fear” and altering large herbivore grazing behaviour, they are not the sole actors in this system and many unseen processes are likely at play. For researchers to disentangle how changing one characteristic of an ecosystem may impact another requires careful study, a plethora of data, and sound interpretation of the findings. As humans in ‘modern society’ and their associated infrastructure continue to proliferate into wild spaces, a strong research effort should be directed toward understanding these effects in time and space to design science-based policy and management of landscapes, as well as industrial development into the future.

Our study aimed to examine how land-use change in the form of industrial clearfell forestry rotations effected the relative risk of bovine tuberculosis breakdowns on farms in the vicinity of the forest property. This is a common forest management practice in Ireland, where our study occurred, however it is also present in many other countries across the world. We suspected that the removal of large tracts of forest habitat would change the macroecology of the locality and may influence the behaviour and movement of bovine tuberculosis wildlife reservoirs, the European badger (Meles meles, pictured below) and possibly Ireland’s three prevalent deer species, Red deer (Cervus elaphus), Sika deer (Cervus nippon, pictured below) and Fallow deer (Dama dama), which are yet to be confirmed as wildlife reservoirs in our study system.

Using a case-control study design incorporating 38,255 farms across all of Ireland, we found that the relationship between a forest clearfell disturbance and relative change in bovine tuberculosis risk for farms nearby was not straightforward. Rather, the change in risk was dynamic in time and space depending on the size of the clearfell, the time that elapsed after a clearfell, the distance to the farm and the area of natural habitats that occur around the farms.  The presence of mixed-forestry (deciduous and coniferous tree mixtures) was shown to have a positive effect on bovine tuberculosis risk for farms, with the presence of this habitat resulting in a reduction in bovine tuberculosis risk after a clearfell.

P02[103:104] TT[767] E[107:0000]G[188:0x00] BV[30:-3] IR[F:40] 2294
Photo: Kilian Murphy
P02[396:396] TT[767] E[107:0000]G[064:0x00] BV[62:-4] IR[F:40] 2325
Photo: Adam F. Smith 

In comparison, when natural grassland habitats are present in the landscape mosaic, we found that for farms within 2km of the forest property where the cut occurred, the risk of a bovine tuberculosis breakdown dropped significantly in the first two years. In contrast, farms that are greater than 5km from the forest property, risk increased significantly in the initial two years after a clearfell when natural grassland was present between the forest and the farm. Paradoxically, this effect reverses in the third year post-clearfell, where farms within 2km were observed to have an increased risk of bovine tuberculosis breakdown and farms at 5km experienced a reduction in bovine tuberculosis risk when natural grassland was present. The dynamic changes in risk we detected is representative of the relative changes to the landscape that has occurred in the three-year period after a clearfell disturbance.

Trees do not transmit bovine tuberculosis and yet we found a dynamic relationship between the removal of forest habitat and bovine tuberculosis breakdown risk for farmers in the region despite lacking wildlife data for our study. This suggests that other ecological agents, such as wildlife reservoirs, may be impacted by the removal of forest habitats. It is our interpretation that the initial removal of the forest drives these species away from the property due to the presence of mechanised forestry equipment, vehicles and humans coupled with the disturbances to wildlife associated with these instruments. It is plausible that this disturbance pushes wildlife away from the forest and from the farms located nearby (within 2km), resulting in the observed decrease in risk, whereas this increases risk for farms further away from the forest (c. 5km).

Clearfell_Year1
Photo: Kilian Murphy
Clearfell_Year3
Photo: Kilian Muphy

As the clearfell site begins to regenerate and is recolonised by early succession plants and shrub, it becomes increasingly attractive habitat for species such as badgers and deer, which may then be drawn back to the site to establish new territories. This might offer an explanation for the changes in risk we detect in year three after a clearfell, where farms within 2km experience an increase in risk and farms further than 5km observe a significant decrease in risk. If clearfell forestry operations create a source-sink habitat mosaic for wildlife reservoirs in agroforestry ecosystems, it may result in increased changes to wildlife movement, territoriality and behaviour which could in turn influence their encounters with domestic animals and thus, disease transmission. 

Ecologists are only scratching the surface on the complexity of ecosystem connectivity and understanding the role of land-use change on wider systems. Unravelling the mechanism of the trends we detect will require much further study and iterative collective understanding. Approaching these challenges in ecology requires many different approaches and perspectives to garner a holistic understanding and accurate interpretation to inform science-based management and policy.

Read the full paper Habitat availability alters the relative risk of a bovine tuberculosis breakdown in the aftermath of a commercial forest clearfell disturbance in Journal of Applied Ecology

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