In my last post, I mentioned that habitat degradation and human-caused mortality were some of the biggest issues facing river otters. Recent research has tended to support those views, while providing more detail about where, how, and when those issues loom largest. We’ve also seen that otter populations can recover with protection, but there are no over-night successes.
Asian short-clawed otter- photo by Dulup. License
Degraded waterways are a big deal for otters– and they tend to like forested areas along water bodies, as I mentioned before. Some research from Argentina suggests that neotropical river otters are willing to use areas set aside for forestry (Gomez et al. 2014), so there appear to be ways that we can share these riparian forests with otters. But it takes more than a few good trees to make good habitat for river otters- vegetation within the river itself is important. Rivers that are canalized and/or dredged have little to no vegetation along their bottoms either because they are sealed by concrete or regularly scoured to ensure vessels can pass- this results in a different fish community than rivers with natural beds. In the case of European otters in Poland, not only did the fish community in canal systems contain poorer food resources for otters, but the otters started eating more amphibians which aren’t as energetically rewarding as fish (Kloskowski et al. 2013). And naturally-occurring water bodies seem to be preferred by river otters in North America- reservoirs created by beavers were utilized more often than man-made reservoirs in Massachusetts (Newman and Griffin 1994).
We do know that otter populations can increase with protection, which is great, but it’s clear that it is not a fast process and there are limits to what can be accomplished. Researchers surveying for giant river otters in Brazil found that, although protected by national and international legislation, populations were experiencing slow population growth at best (dos Santos Lima et al. 2013). When a population increases in size, it can only take the species so far- at a certain point there isn’t enough space or food or some other resource. Once a studied population in Finland got to a certain density, litter size decreased, so that, even with more otters reproducing, the population was not growing as quickly (Sulkava et al. 2007). Those animals which moved into unoccupied areas to get away from the crowding faced challenges of their own since not all habitat is equal- during harder winters in Finland, very few otters survived when they weren’t in prime habitat.
So it’s a matter of preserving habitat, and enough of it, plus making sure that adequate fish resources are available. Plus we have to deal with both vehicle-caused mortality- in the Czech Republic this was responsible for more than 75% of recorded otter deaths in between 1990 and 2011 (Polednik et al. 2011) – and intentional killing, for example as a result of competition for fish between otters and fishermen (Kafle 2009). Why should we make the effort? Well, in addition to the role of otters as sentinel species whose presence or absence can let us know when our waterways are in trouble, there is some suggestion that otters may help limit smaller carnivores, such as mink, which can be a good thing in places where mink are threatening the survival of smaller animals, like water voles (McDonald et al. 2007).
How can we help river otters continue to do their thing? In my next post, I hope to have some good answers for that question!
Works cited:
Gomez, J. J., J. I. Túnez, N. Fracassi, and M. H. Cassini. 2014. Habitat suitability and anthropogenic correlates of Neotropical river otter (Lontra longicaudis) distribution. Journal of Mammalogy 95:824–833.
Kafle, G. 2009. A Review on Research and Conservation of Otters in Nepal. IUCN Otter Specialist Group Bulletin 26:32–43.
Kloskowski, J., J. Rechulicz, and B. Jarzynowa. 2013. Resource availability and use by Eurasian otters Lutra lutra in a heavily modified river-canal system. Wildlife Biology 19:439–451.
McDonald, R. A., K. O’Hara, and D. J. Morrish. 2007. Decline of invasive alien mink (Mustela vison) is concurrent with recovery of native otters (Lutra lutra). Diversity and Distributions 13:92–98.
Newman, D. G., and C. R. Griffin. 1994. Wetland Use by River Otters in Massachusetts. The Journal of Wildlife Management 58:18–23.
Polednik, L., K. Polednikova, J. Vetrovcova, V. Hlavac, and V. Beran. 2011. Causes of deaths of Lutra lutra in the Czech Republic (Carnivora: Mustelidae). Lynx 42:145–157.
dos Santos Lima, D., M. Marmontel, and E. Bernard. 2013. Reoccupation of historical areas by the endangered giant river otter Pteronura brasiliensis (Carnivora: Mustelidae) in Central Amazonia, Brazil. Mammalia 78:177–184.
Sulkava, R. T., P. O. Sulkava, and P. E. Sulkava. 2007. Source and sink dynamics of density-dependent otter (Lutra lutra) populations in rivers of central Finland. Oecologia 153:579–588.