White tailed deer is a species native to Kentucky, where I live, and to much of the eastern hardwood forest region of the USA. However, white tailed deer have attained such high abundance that they cause major ecological problems normally associated with invasive species. For example, white tailed deer browse tree seedlings, so in areas with high deer populations, growth of new trees may not occur. Absence of tree regeneration affects other native species such as birds that prefer to nest in younger trees. Deer may also heavily browse some herb species, like ginseng, leading to their local extinction. Species like this can be a real threat to the native ecosystem. A recent article by Michael P. Carey et al. in Frontiers in Ecology and the Environment discusses the case of such “native invaders.” The authors explain how white tailed deer and other super abundant, high impact native species present challenges for policy, management, and society.
First, Carey et al. outline how native invaders occur. Non-native species like wild boar and cane toad are introduced by humans from long distances (or escape local captivity), and then have detrimental impacts on local ecological systems. Native invaders differ in that they are not transported, but their invasive/damaging stage is often sparked in some way by some human activity. People may cause high reproduction of a native species, such as providing supplemental food or removing a predator (white tailed deer often have no predators) or competitor. Second, human-modified habitat may provide a niche that a particular native species is good at exploiting (white tailed deer thrive in fragmented forest). Lastly, some native species are “stocked” (sport fish and game birds), in which large numbers of animals are bred and intentionally released into the wild. Therefore, because of humans, populations of native species may achieve very large numbers, especially in particular habitats, without actually leaving their native geographic range.
Carey et al. discuss several native invaders in detail, such as rainbow trout, a popular sport fish. These fish have been stocked in historically fish-less lakes within their native range, and can wreck the native ecosystem and its co-evolved trophic relationships. Another problem with stocked populations is with stocking individuals of large size (or some other characteristic), making the population genetically and perhaps ecologically different from previous wild populations. Using examples, Carey et al. explain how native invaders pose similar environmental problems as non-native invasives, but special problems for scientific research, management, public education, and policy. One research challenge is to determine how abundant the native species once was (a challenge in much of conservation biology), how much population sizes fluctuated, what historical impacts occurred, and whether current impacts are “off the chart.” The geographic range and population sizes before Europeans entered North America may be difficult to ascertain, as is determining what is “natural.” However, this information is necessary for managers to know if/when a native should be considered problematic.
One management challenge is that a native invader may be simultaneously an invader in one part of its range, a threatened species in another part of its range, and perhaps even a non-native invader outside its range (e.g. rainbow trout). Another problem is to control a native invader. In the case of one native invasive fish, northern pikeminow, fishermen are rewarded for catching this fish (a reward for being an invasivore!), which can help reduce density. While this does provide recreation and education about the problem, the underlying cause of the invasive habits of this fish (stream and river management practices) are not addressed. Furthermore, for some native invasives, there is no evidence regarding whether a control program has measurable effects. A final issue is that some agencies have to balance conservation and natural resource goals.
A final and large challenge is to convince the general public about the potential harm that native species can cause when they become invasive and to demonstrate the need for occasionally lethal control measures. Public opinion will often be divided, and media attention can simplify the ecological picture. Still, a broader discussion of what we mean by “invasive,” and a thoughtful reflection on the social and ecological reasons behind the control measures we use (including eating!), can help advance our understanding and management of native invaders.