That humans are affecting ecosystems across local to planetary scales is news to relatively few people today. Anthropogenic climate change, alteration of nutrient cycles, and land conversion are widely cited drivers of contemporary ecosystem novelty. These factors have absolutely resulted in new configurations of biotic and abiotic interactions which have no historical analog. However, these human impacts are relatively modern. Human ecosystem impacts can certainly be traced back millennia using historical records of fire, but at such small human population sizes these impacts were rarely likely to result in widespread creation of novel ecosystems (Koch & Barnosky, 2006).
But before the invention of the internal combustion engine, the Haber-Bosch process, or even the plow, humans may have been unknowingly creating novel ecosystems as hunters on the landscape. While there is ongoing debate regarding the degree to which the Quaternary megafaunal extinctions of North and South America were the result of human harvest or rapid climate change, it appears that humans arrived on the scene shortly before two-thirds of the world’s largest animals (>44 kg) went extinct (Barnosky & Lindsey, 2010). The loss of megaherbivores has been suggested to have increased fire frequency and created novel species assemblages in North America (Gill et al. 2009), suggesting that harvest may have been the first human-induced driver of novelty.
It may not be hard to imagine that the loss of two-thirds of the world’s megafauna as resulting in novel species assemblages and ecosystem functions. But perhaps more interesting to consider is the way in which harvest may affect various dimensions of ecosystem novelty. In one dimension, when a species is harvested to extinction the result is a novel community assemblage: previous species richness minus one. But, what of harvest to historically low levels? One might argue that as long as a species persists in its historical habitat, the ecosystem is not necessarily novel. Alternatively, one could argue that it depends on the relative difference in current abundance relative to a baseline of previous abundance. Further yet, one could argue that unless there are measurable and/or qualitative changes in ecosystem function as a result of high levels of harvest-induced mortality, harvest has not resulted in novelty.
Let’s consider two examples of more modern harvest effects in order to discuss novelty. First, the harvest of passenger pigeons (Ectopistes migratorius), ultimately to extinction. Passenger pigeons have been estimated to have once numbered between 3 and 5 billion, winging their way across northeastern North America in column-shaped flocks measuring 1 km wide by up to 450 km long. Ellsworth and McComb (2003) estimated that 3 billion passenger pigeons would have consumed between 300,000 and 1,500,000 hectares red oak acorn production—per day. Further, they suggest that the sheer roosting of these birds on trees would have led to damaged and overturned trees, and where the trees did remain erect, the ground would have been covered with up to 50 cm of excrement at regular roosting sites. The authors argue that passenger pigeons may have been responsible to maintaining white oak forest dominance in North America, and that their extinction may have facilitated the spread of red oak forest northward. The regular damage and seed consumption of oaks caused by the pigeons, the authors argue, may have created a more fire-prone landscape that favored white oaks due to their increased fire-tolerance relative to red oaks. The harvest to extinction of passenger pigeons has resulted in reduced species richness across their range, an event that Aldo Leopold eulogized at the dedication of the monument to the passenger pigeon at Wyalusing State Park, Wisconsin in 1947: “We meet here to commemorate the death of a species. This monument symbolizes our sorrow. We grieve because no living man will see again the onrushing phalanx of victorious birds, sweeping a path for spring across the March skies, chasing the defeated winter from all the woods and prairies of Wisconsin. Men still live who, in their youth, remember pigeons; trees still live that, in their youth, were shaken by a living wind. But a few decades hence only the oldest oaks will remember, and at long last only the hills will know.”
Their loss means a novel species assemblage in northeastern North America. But, did the loss of such great numbers of pigeons result in novelty in other dimensions? It’s highly likely, but the data to show if the disturbance regime has been substantially altered or if there are other organisms that now predate upon acorns or cycle nutrients with the same or similar efficacy are yet lacking.
A second modern case study of harvest leading to novelty comes from the commercial harvesting of whales. Commercial whaling has been going on for over 1000 years, and it’s estimated that total biomass of whales has been reduced by up to 85% from the pre-whaling era (Roman et al., 2014). While no whale species has been harvested to extinction, their cumulative scarcity in the oceans has resulted in the identification of 4 mechanisms of ecosystem alteration that have been consequently reduced. First, at their pre-whaling abundances in the North Pacific, whales would have required an estimated 65% of all primary production to maintain such large populations. While the same amount of primary production required to support one blue whale could support 1,500 penguins, the combined biomass of the penguins would only by approximately 8% of the blue whale owing to vast differences in metabolic efficiency. Therefore, as a result of reduced whale populations, less carbon is stored in these ecosystems or transported to the ocean floor upon death (Roman et al. 2014). The loss of whales is also have thought to result in altered phenotypes among krill, a favorite food of baleen whales.
The second mechanism by which reduced whale populations have altered oceanic ecosystems is the loss of important food sources for killer whales. It is thought that relative depletion of whales as prey for killer whales led to greater predation on smaller mammals, like sea otters, creating cascading effects that ultimately reduced the abundance of kelp forests off the coasts of Alaska and British Columbia. Thirdly, whales often feed at great depths but require breathing at the ocean surface. This vertical movement facilitates the movement of critical nutrients like nitrogen and iron by two mechanisms: whales, having filled up on food at depths defecate nutrient-rich plumes in surface waters causing increased surface productivity; and also by the mere movement of water along the drag of their bodies across the nutrient-rich depths and nutrient-poor surface waters. Finally, in death whales transport immense amounts of nutrients to the ocean floor. In fact, a single 40-ton gray whale can deliver the equivalent of >2000 years of carbon flux to the ocean floor to the area on which the carcass lands. In the North Pacific, 60 species of animals are known only from whale carcasses. Together, the restoration of whales to historical numbers would result in the transport of equivalent amounts of carbon to the ocean floor as proposed iron enrichment climate mitigation strategies have proposed (Roman et al. 2014).
Although whales haven’t become extinct, their size and critical historical ecosystem roles indicates that even the reduction in total numbers has created massive alterations and created novel pathways by which ecosystems function today.
~by Aaron Koning~
Barnosky AD, Lindsey EL. 2010. Timing of Quaternary megafaunal extinction in South America in relation to human arrival and climate change. Quaternary International, 217:10-29.
Ellsworth JW, McComb, BC. 2003. Potential Effects of Passenger Pigeon Flocks on the Structure and Composition of Presettlement Forests of Eastern North America.Conservation Biology, 17(6): 1548-1558.
Gill JL, Williams JW, Jackson ST, Lininger KB, Robinson GS. 2009. Pleistocene megafaunal collapse, novel plant communities, and enhanced fire regimes in North America. Science, 326:1100-1103.
Koch PL, Barnosky, AD. 2006. Late Quaternary Extinctions: State of the Debate. Annu. Rev. Ecol. Evol. Syst. 2006. 37:215–50.
Roman J, Estes JA, Morissette, L, Smith, C, Costa D, McCarthy, J, Nation JB, Nicol S, Andrew Pershing A, Smetacek V. 2014. Whales as marine ecosystem engineers. Frontiers in Ecology and the Environment, 12(7): 377–385.