Mammals, freshwater reference states, and the mitigation of climate change

Abstract

Freshwater habitats have been severely altered by human activities, and legislation has variously been passed requiring assessment of the damage in preparation for its repair. The current condition is generally measured against a reference state, which may be 'natural', meaning pre-Columbian, in the United States, pre-European in Australia or, in Europe, a habitat with 'no, or negligible human influence'. Such standards are mostly unachievable at present because of a current economic philosophy that promotes exploitation of nature, but they are also nebulous. Even pristine habitats naturally change, so that there can be no single immutable reference state. Nonetheless, in Europe at least, the concept of the reference state has been particularly compromised, compared with the aspirations of the legislation. Apart from a very liberal approach to setting reference standards, there has been a major omission in contemplating the general nature of the reference state. The potential former roles of large herbivorous land mammals, such as horses, bovines, deer and, early in the Holocene, elephants and rhinoceri, and their predators, in nutrient transfers, have been ignored. The effects of such large mammals, particularly in influencing lakes, are assessed in the light of major losses of much of the Pleistocene fauna during the hunter-gatherer phase of human colonisation, and then subsequent near-complete attrition when agriculture, husbandry and forestry converted the majority of biomes to 'anthromes'. Animals, and particularly large herbivorous mammals, transfer nutrients across catchments. Large mammals are likely to have maintained a state of turbid water and algal blooms in pristine, poorly flushed, shallow floodplain lakes. In contrast, we currently perceive these to be in a reference state when dominated by submerged plant communities in clear water. Although of conceptual interest, this may seem to have little practical relevance because the reference state has become a pragmatic and political construct rather than an ecological one. I counter such a view by arguing that mitigation of climate change must require replacement of substantial carbon sinks that have been lost, that ecological sinks within restored biomes are likely to be the most effective and that restoration of such systems must involve the important roles of large mammals, with the implications these have for our concepts of lake restoration.