Ecological Archives E090-077-A3

Lauren A. Harrington, Andrew L. Harrington, Nobuyuki Yamaguchi, Michael D. Thom, Pablo Ferreras, Thomas R. Windham, and David W. Macdonald. 2009. The impact of native competitors on an alien invasive: temporal niche shifts to avoid interspecific aggression? Ecology 90:1207–1216.

Appendix C. Spatial distribution of mink and otter signs along rivers in the upper Thames Valley.

The possibility of interspecific spatial avoidance between mink and otters was further examined by investigating current spatial patterns of the field signs of these two species at the R. Thames study sites and at an additional three study sites within the same river catchment: the R. Windrush, R. Evenlode, and the R. Cherwell. All sites are approximately 20 km in length (<2–20 m wide, <0.2–3 m deep).

Field sign survey methods

Between September 2003 and March 2006, field sign (‘scat’) surveys (details in Harrington et al. 2008) were carried out four times per year (March, July, September, and November) at at least two of the four survey sites per survey session (n = 32 surveys). For each survey at each site, we searched the entire riverbank for mink and otter faeces (referred to hereafter as ‘mink scats’ and ‘otter spraints’) and recorded a GPS location for all scats and spraints detected. DNA analysis of mink scats confirmed that our identification of mink scats in the field was highly reliable (Harrington et al. 2008).

For monitoring purposes as part of a separate study, we also carried out an alternative survey method whereby ‘tracking rafts’, placed throughout the site at 1-km intervals, were used to detect mink tracks (Reynolds et al. 2004, Harrington et al. 2008). Tracking raft surveys were carried out at all sites, on all survey sessions (as above). For this study, we used data from three surveys, during which the presence of otter spraint on the rafts was recorded at the beginning of each survey period (all rafts had been left in situ between 2003 and 2006 and were often ‘marked’ by otters; personal observation) to assess the effect of otter marking on the subsequent use of rafts by mink; rafts were checked for the presence of mink tracks two weeks later.

Analysis

To examine the current spatial deposition of mink scats and otter spraints and to test for spatial avoidance of otters by mink on the river, we divided all four river sites into contiguous sections within ArcMap, plotted scat and spraint locations and classified each section, for each separate scat survey, as containing both mink and otter, mink only, otter only, or neither. We tested for an association between mink scats and otter spraints at two scales (based on 50-m and 500-m river sections). We restricted this analysis to surveys with ≥ 10 faeces of each species over the entire 20-km site. To test for spatial avoidance in a situation where otter spraint was known to be present prior to a visit by mink, we used similar tests of association to examine the use, by mink, of tracking rafts with and without otter spraints.

Results

In contrast to what might be expected if there was interspecific spatial avoidance on the river, we found a significant positive association between the distribution of mink scats and otter spraints at the 50-m scale in 11 of 14 surveys included in the analysis (individual χ² tests P < 0.003, Bonferroni adjustment). No such association (either positive or negative) was detected at the 500-m scale (13 of 14 tests, NS). Similarly, there was no evidence of avoidance of tracking rafts with otter spraint (based on the presence of mink tracks) (χ², P > 0.2 in all cases, n = 3 tests).

Discussion

We found no evidence of spatial avoidance by mink of areas on the river used by otters. Spatial analyses of the locations of scats and spraints suggested that both species were at least sometimes attracted to the same areas at the 50-m scale (but not at the 500-m scale) perhaps reflecting similar requirements for feeding and/or resting areas. Similar patterns have been observed between southern river otters, Lontra provocax, and American mink in Argentina, where mink are less likely to occur at lakes where otters are present but within a lake where both species occur, a positive association is found in their distributions (Aued et al. 2003). In apparent contrast, Erlinge (1972) found a reciprocal distribution between mink and otters in Sweden; mink appearing to be largely restricted to habitats that were suboptimal for otters. It is difficult to interpret faecal locations in terms of habitat preferences (Kruuk 2006) (and we make no attempt to do so here) but at the very least the coincidence of spraints and scats indicates that mink and otters, whatever their motivational status, were at that location. In the absence of detailed temporal data it is not possible to test hypotheses such as that mink scats occurred first, were counter-marked by an otter and thereafter avoided by mink. This possibility is not supported, however, by our observation that visits to tracking rafts by mink appeared to be unaffected by the presence of otter spraint (prior to visits) and by experimental tests of the response of mink to otter scent (anal gland secretion) in which we found no consistent evidence of avoidance (Harrington 2007).

LITERATURE CITED

Aued, M. B., C. Chehebar, G. Porro, D. W. Macdonald, and M. H. Cassini. 2003. Environmental correlates of the distribution of southern river otters Lontra provocax at different ecological scales. Oryx 37:413–421.

Erlinge, S. 1972. Interspecific relations between otter (Lutra lutra) and mink (Mustela vison) in Sweden. Oikos 23:327–334.

Harrington, L. A. 2007. The American mink, Mustela vison: its management and interactions with two native mustelids, the European polecat, Mustela putorius, and the Eurasian otter, Lutra lutra. DPhil thesis, University of Oxford, Oxford, UK.

Harrington, L. A., A. L. Harrington, and D. W. Macdonald. 2008. Estimating the relative abundance of American mink Mustela vison on lowland rivers: evaluation and comparison of two techniques. European Journal of Wildlife Research 54:79–87.

Kruuk, H. 2006. Otters: ecology, behaviour and conservation. Oxford University Press, Oxford, UK.

Reynolds, J. C., M. J. Short, and R. J. Leigh. 2004. Development of population control strategies for mink Mustela vison, using floating rafts as monitors and trap sites. Biological Conservation 120:533–543.


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