- Changes in the abundance of one prey species may indirectly affect other prey species by affecting the numbers or behaviour of generalist predators.
- A 27-year study investigated the relationships between field voles, red grouse, and four predator species that eat both voles and grouse (red fox, weasel, hen harrier, buzzard) on a moor in southwest Scotland.
- When field vole numbers were high, red grouse chick survival was lower and abundance of buzzards and foxes (when foxes were not controlled) was higher.
The field vole is a common rodent in Britain whose numbers naturally fluctuate in cycles. As voles are an important prey for many predators, these fluctuations can have a large impact both on predators and on other prey species. For example, some predators may switch to hunting alternative prey when vole abundance is low, or predators may be attracted when vole abundance is high, which could increase incidental predation of other prey in the same habitat.
This 27-year study examined these possible indirect relationships between field voles and red grouse, a gamebird inhabiting heather moorland. There is no direct competition between voles and grouse for resources. Although both are herbivores, grouse feed on heather and prefer heather-dominated areas, whereas voles feed on grasses and herbs and are more abundant in grass-dominated areas. However, they occur in close proximity to each other in the heather-grass mosaic found on many moors in the British uplands, and a range of predators predate both voles and grouse.
The authors investigated the relationships between voles, grouse and four shared predators (red fox, weasel, hen harrier and buzzard) on a moor in southwest Scotland, which was subject to alternating periods with and without grouse moor management. When the moor was managed for grouse, foxes and weasels were legally controlled, while hen harrier and buzzard were legally protected.
What they did
The study took place between 1992 and 2018 on Langholm Moor in southwestern Scotland, which was dominated by a mosaic of heather and grass moorland. During the study period, Langholm Moor was periodically managed for red grouse (1992-1999 and 2008-2016), which included the legal control of foxes and weasels, while hen harrier and buzzards were protected throughout.
The authors estimated the abundance of field voles, red grouse and the four predator species (although data were not available for all species in all years). Vole abundance in spring was estimated each year using snap traps in late March to early April. Red grouse were counted in March and July using pointing dogs to estimate spring and summer densities. Grouse counted in July were aged as young or adult to calculate adult survival from spring to summer as well as two measures of breeding success (proportion of females with broods, number of young per adult). The authors also estimated nesting success of radio-tagged females by calculating the proportion of clutches where one or more chicks hatched.
From 2003 onwards, red fox activity in spring was estimated by counting the number of scats along surveyed transects. Between 2002 and 2015, weasel activity was estimated in spring using footprint tracking tunnels. Each year, the authors recorded the number of breeding female hen harriers and, in 1993 and then from 2003 onwards, the number of breeding buzzards. In addition, sightings of buzzards and hen harriers were also recorded during the annual breeding bird surveys.
The authors then tested whether vole abundance was related (1) to red grouse densities, survival, or breeding success, and (2) to indicators of predator abundance (fox, weasel, hen harrier, buzzard). They also tested whether these relationships differed in relation to grouse moor management, which included the removal of foxes and weasels.
What they found
The authors found no relationship between vole abundance and grouse densities, adult survival, or nesting success. However, the number of young per adult grouse in July and the number of females with broods were lower in years with higher vole abundance, irrespective of whether the moor was managed for grouse or not.
The relationship between foxes and voles varied in relation to grouse management. When the moor was not managed for grouse, fox activity was higher in years when there were more voles. However, when the moor was managed and foxes were controlled, the overall fox activity was lower and not associated with vole abundance. The activity of weasels was not related to vole abundance or grouse management.
The numbers of breeding hen harriers and buzzards were neither associated with vole abundance nor with grouse management, but there were more buzzards seen during breeding bird surveys in years with higher vole abundance.
What does this mean?
The authors found that high field vole abundance in spring was associated with lower red grouse breeding success and higher abundance of some generalist predators (red fox and buzzard). As vole abundance did not affect grouse nesting success, this suggests increased predation of grouse chicks in years with high vole abundance.
Fox numbers only showed a positive association with vole abundance when foxes were not controlled, whilst weasel indices showed no relationship with voles. Vole abundance did also not affect the number of breeding hen harrier or buzzards. However, the number of buzzards seen during breeding bird surveys was higher in vole peak years. When voles are common on the moor, buzzards spent more time hunting in moorland habitat, therefore incidentally coming across and eating more grouse.
Although the underlying mechanisms behind the negative relationship between vole numbers and grouse chick survival remain unclear, it may be at least partly explained through increased predation by rodent-hunting raptors such as buzzards and, in periods without grouse management, foxes. However, incidental predation of grouse chicks by other rodent specialists such as stoats or short-eared owls, which were not included in this study, could also have contributed.
Further research would help scientists and land managers to better understand the mechanisms driving the indirect relationship between voles and red grouse in a multi-predator community.
About the LMDP
This study was part of the Langholm Moor Demonstration Project, a long-running collaboration between the GWCT, SNH, Buccleuch Estates, the RSPB and Natural England, which aimed to resolve the conflict between driven grouse shooting and raptor conservation. Further papers from the Langholm Moor Demonstration Project can be found on the LMDP website.
Read the original paper
Ludwig, S.C., Roos, S., Baines, D. (2020) Fluctuations in field vole abundance indirectly influence red grouse productivity via a shared predator guild. Wildlife Biology 2020: wlb.00642.