The interconnectedness of ecosystems and their components is, today, a familiar concept. Top predators eat herbivores, herbivores eat plants, and top predators keep so-called meso-predators in check too. But perhaps it isn’t appreciated enough just how interconnected things can be. Cristina Eisenberg’s excellent 2010 book The Wolf’s Tooth: Keystone Predators, Trophic Cascades and Biodiversity draws on decades of ecological research to paint a complex picture of ecosystem interactions and cascades, of the crucial role of top predators, and of human impact on communities in the natural world. Fully referenced, meticulously researched and beautifully written, The Wolf’s Tooth is an absorbing read for anyone interested in biodiversity, ecology, conservation or wildlife management.
While Eisenberg’s case studies, and expertise, mostly centre around the wooded habitats of the United States, she devotes chapters to ecosystem change and community ecology in terrestrial, freshwater and marine habitats around the world. Discussion of Pleistocene extinctions, of deep time, re-wilding and the future should make this book, and her approach, interesting to archaeologists and palaeontologists as well.
In this review I hope not only to bring attention to a book that I rate very highly, but also to help promote and emphasise the concept of trophic cascades and the consequences of the anthropogenic removal of top predators.
Trophic cascades, keystones, and the ‘ecology of fear’
The focus of the book is a discussion of trophic cascades research and on what it means for community structure, conservation and management. The term ‘trophic cascades’ – coined by Robert Paine in his 1980 work on marine ecosystems – refers to the dynamic flow of energy through an ecosystem whereby keystones (a term originally applied specifically to top predators) assert a ‘top down’ influence. If there are more wolves, there are more songbirds and more butterflies because wolves keep deer in check; if deer are kept in check, vegetation is not over-browsed and floral communities are rich and luxuriant. A healthy flora not only means successful recruitment of seedlings and saplings into a multi-tiered woodland community, it also has such knock-on effects as the stability of riverbanks, the quality of groundwater, and the health of the deer themselves. [Yellowstone wolf pack image from wikipedia].
Good observational evidence indicates that the presence of keystone predators (like the wolf) results in an ‘ecology of fear’ (Brown et al. 1999) among local herbivores. If deer have to remain alert and on the lookout for predators, they rarely get the opportunity to over-browse: eating is done sparingly while the animal is on the move, constantly on the lookout. Contrary to what you might have read – or what you might have encountered should your experiences be (like mine) based on areas where large herbivores roam unchecked by predators – features such as browse lines are not necessarily a natural and ubiquitous feature of wooded environments. Rather, they occur where herbivores do not operate under an ecology of fear but subject foliage to too much browsing [adjacent image of deer feeding at a browse line from Blog of an Ancient Gardener].
Top-down vs bottom-up and the prevalence of trophic cascades
The concept of trophic cascades is not new. As Eisenberg notes, Darwin documented several cases in his writings. One example: more domestic cats meant less mice and therefore more bees, as mice plundered the bee’s hives. Both Charles Elton and Aldo Leopold drew attention to cascading ecological phenomenon in their pioneering works of the 1920s and onwards.
Nevertheless the importance of predators in controlling ecosystem health from the top down was largely unappreciated for much of the 20th century. Management practises that involved the planned and very much deliberate removal of wolves and others predators were applied, creating a legacy of mis-management where herbivores (deer) irrupted (that is, boomed in numbers) to form dense populations far exceeding ideal carrying capacities. This was then manifested in the affected forests by the obvious absence of saplings – they’d literally been eaten to death. A lack of sapling recruitment to the tree population means that those forests will be empty when the big, old trees die. [Image below by Doug Smith, from wikipedia].
In 1960, Nelson G. Hairston, Frederick E. Smith and Lawrence B. Slobodkin published their important paper on the ‘green world hypothesis’: a discussion of the idea that predation on herbivores controls the loss of vegetation in a top-down cascade (Hairston et al. 1960). Eisenberg shows that this crucial piece of research marked a turning point in our attitude to and understanding of ecosystem structure. While debates over the importance of ‘top-down’ versus ‘bottom-up’ processes would (and do) continue, the importance of trophic cascades have been increasingly recognised, with management and conservation strategies changing accordingly.
Among the many case studies involving trophic cascades discussed by Eisenberg are those pioneering works carried out in the H. J. Andrews Experimental Forest in Oregon and Wind River Experimental Forest in Washington. Here and elsewhere, cascading relationships involving wolves, wapiti, hemlocks, dwarf mistletoe, butterflies and other species have been documented. Eisenberg also discusses examples from freshwater habitats, and from rockpools and various other marine environments. One of the best known ecosystems affected by trophic cascades is the marine community around the Alaskan Aleutian Islands.
Sequential megafaunal collapse and ecosystem degradation
In those Alaskan waters, the recovery of sea otters Enhydra lutris resulted in a decline in Strongylocentrotus sea urchins and a recovery of the kelps that the urchins browse upon (Estes et al. 1978). Incidentally, it’s been suggested that the decline of Steller’s sea cow Hydrodamalis gigas was linked to harvesting of sea otters and a resulting increase in urchins and loss of kelp (Anderson 1995). However, study has shown that massive over-exploitation of the sea cows themselves better explains their rapid extinction (Turvey & Risley 2006). Anyway, when killer whales Orcinus orca began feeding on sea otters, the system flipped from top-down control to bottom-up: urchins surged in numbers again, and kelp became over-grazed (Springer et al. 2003). [The adjacent photo - showing a killer whale skull to scale with that of a sea otter - is from Williams et al. (2004)].
It seems that orcas were ‘fishing down’ the food chain. Historically, they’d predated on great whales, and then on seals and sea lions when the whales were gone, but a crash in the numbers of Common/Harbour seals Phoca vitulina and Steller’s sea lions* Eumetopias jubatus now meant that the orcas had switched to sea otters (Estes et al. 1998, Springer et al. 2003, Williams et al. 2004). The larger phenomenon at work here – termed sequential megafaunal collapse – resulted in substantial debate in the literature, with some workers questioning whether the collapse really was sequential, and whether killer whale predation was responsible for the observed otter decline. If sequential megafaunal collapse really was affecting killer whale behaviour and hence sea otter numbers, management strategies related to sea otter conservation would have to incorporate several species (rather than just the otters alone) and “reach across multiple geographic areas and food webs” (Eisenberg 2010, p. 65).
* Everyone still uses the term ‘sea lion’ but it should really be ‘sealion’, since they’re not actually lions (similar changes have been made elsewhere in zoological etymology).
Nevertheless the sequential megafaunal collapse hypothesis has generally been supported and mirrors our own over-exploitation of marine ecosystems: as humans have removed large, apex predators like shark, tuna and cod, the affected ecosystems have simplified. Species at lower trophic levels have boomed in numbers through release from predation. While this looks great if you want to exploit those ‘newly released’, now super-abundant species, another tier in the system becomes depleted as they become exploited too. This continues until all you’re left with is vast quantities of algae and nematodes (we previously covered sequential ecosystem collapse in my review of the movie The End of the Line) [and see The End of the Line's site. Extrapolated collapse of global fisheries shown below from Andrew's ENVR 2000 Blog].
This is why the removal of top predators (and other large animals) is so insidious. Car-loving moron Jeremy Clarkson’s claim that the extinction of the tiger is a good thing for people who want to go on back-packing holidays to tropical Asia looks blissfully naïve in view of the fact that sequential deterioration of ecosystems results from the removal of apex predators. When we kill all the wolves, tigers, sharks or bears, we don’t just lose charismatic, viscerally thrilling species – we fundamentally alter the makeup and health of the community. The knock-on effects both weaken the system’s resilience to change, and eventually result in forests composed of ‘living dead’ tree species, “[destined] to die without replacement” (Eisenberg 2010, p. 114). Eisenberg points to Amazonian regions where the forests appear ok at superficial glance, but which turn out on examination to prove empty and degraded in ecological terms. Similarly, those 20th century management policies in North America and elsewhere that led to the removal of wolves proved disastrous in the long term, resulting in depleted landscapes.
What’s more, the complexity and interconnectedness of ecosystems mean that collapse in one part of a system can result in a surprising effect elsewhere. Sea otter decline in Alaskan waters resulted, predictably enough, in a surge in urchins and a deforestation of kelp. But it also led to Bald eagles switching to a diet dominated (c. 80%) by seabirds, since the fish the eagles had previously preyed on had declined or disappeared due to loss of sheltering kelp (Anthony et al. 2008) [diagram below, from Anthony et al. (2008), shows changing Alaskan Bald eagle diet at four Aleutian sites between 1993-1994 and 2000-2002. In the sample diets for 2000-2002, note the stronger association with bird prey such as ptarmigan, pintail, gull and puffin].
Despite the focus here on top-down control, Eisenberg explains that ‘bottom-up’ effects are important too, and in some ecosystem the two processes occur in synergy. The world might sometimes be green because some or much of that greenery is inedible, not because herbivores have been prevented from running riot by their controlling predators. It might be that the dominance of one system over the over is controlled by overall environmental productivity: in the exploitation ecosystems hypothesis developed by Oksanen et al. (1981), only habitats with a high plant biomass allow the presence of top predators that then exert a top-down influence. Predators fail to have a major influence on herbivores in such unproductive habitats as semideserts, alpine regions, steppes and tundra regions.
Deep time and re-wilding
As mentioned earlier, there’s also much here of interest to palaeontologists and archaeologists, given that the loss of the Pleistocene megafauna across much of the world has very likely resulted in substantially simplified ecosystems. Megafaunal loss seems to have resulted in more powerful keystone roles for the large predators that remained, and plant communities seem to be restructured versions of what existed before. As Eisenberg says, “Ecosystems have been truncated or decapitated by the loss of larger animals. Beyond evolutionary entanglements, when one views these extinctions through trophic cascades glasses, the profound ecological wreckage humans have inadvertently wrought on this planet begins to become apparent” (p. 48).
These days the concept of re-wilding – of somehow reconstructing communities by restoring or reintroducing lost components – is popular and increasingly discussed as a feasibility, but I think that proper reconstruction of the ecosystems concerned is far more complex than has sometimes been intimated. You can’t just release Asian elephants and camels onto the North American plains and wait to see what happens [the photo above is from a re-wilding project currently underway: Chinese tigers have been taught to live a wild life in Africa before being released in China. So, yes, you're seeing a tiger chasing Blesbuck Damaliscus pygargus phillipsi. See Save China's Tigers].
As if it isn’t already clear from what I’ve said, everyone with a serious interest in ecology, conservation, ecosystem management and/or biodiversity should read Eisenberg’s book. I loved it, and developed an enhanced understanding of trophic cascades research and ecosystem change. In a world where habitats and communities are changing fast due to human action, such concepts as sequential faunal collapse and ecosystem degradation are going to become all too familiar.
Eisenberg, C. 2010. The Wolf’s Tooth: Keystone Predators, Trophic Cascades and Biodiversity. Island Press, Washington, pp. 254. ISBN 13: 978-1-59726-397-9.
The Island Press page is here. Buy it at amazon: The Wolf’s Tooth: Keystone Predators, Trophic Cascades, and Biodiversity. Cristina herself blogs at Island Press’s Eco-Compass Blog.
For previous Tet Zoo articles relevant to ecosystem degradation, trophic cascades and the loss of biodiversity, see…
- Get ready for 2008: Year Of The Frog
- The EDGE amphibian project launches today
- California’s declining frogs
- Harbour seal kills and eats duck
- The Global Amphibian Crisis, 2009
- The End of the Line: a must see
- Predatory animals are bad and should be allowed to go extinct, or should be modified to become kind and herbivorous
- Close up to Andrias, despite the smell and the teeth
- A future for vesper bats? (vesper bats part XX – last in series)
Refs – -
Anderson, P. 1995. Competition, predation and the evolution and extinction of Steller’s sea cow, Hydrodamalis gigas. Marine Mammal Science 11, 391-394.
Anthony, R. G., Estes, J. A., Ricca, M. A., Miles A. K., Forsman, E. D. 2008. Bald eagles and sea otters in the Aleutian Archipelago: indirect effects of trophic cascades. Ecology 89, 2725-2735.
Brown, J., Laundré, J., Gurung, M., & Laundre, J. (1999). The Ecology of Fear: Optimal Foraging, Game Theory, and Trophic Interactions Journal of Mammalogy, 80 (2) DOI: 10.2307/1383287.
Estes, J. A., Smith, N. S. & Palmisano, J. F. 1978. Sea otter predation and community organization in the western Aleutian Islands, Alaska. Ecology 59, 822-833.
- ., Tinker, M. T., Williams, T. M. & Doak, D. F. 1998. Killer whale predation on sea otters linking oceanic and nearshore ecosystems. Science 282, 473-476.
Hairston, N. G., Smith, F. E. & Slobodkin, L. B. 1960. Community structure, population control, and competion. American Naturalist 94, 421-425.
Oksanen, L., Fretwell, S. D., Arruda, J. & Niemelä, P. 1981. Exploitation ecosystems in gradients of primary productivity. American Naturalist 118, 240-261.
Springer, A. M., Estes, J. A., van Vliet, G. B., Williams, T. M., Doak, D. F., Danner, E. M., Forney, K. A. & Pfister, B. 2003. Sequential megafaunal collapse in the North Pacific Ocean: an ongoing legacy of industrial whaling? Proceedings of the National Academy of Sciences 100, 12223-12228.
Turvey, S. & Risley, C. 2006. Modelling the extinction of Steller’s sea cow. Biology Letters 2, 94-97.
Williams, T. M., Estes, J. A., Doak, D. F. & Springer, A. M. 2004. Killer appetites: assessing the role of predators in ecological communities. Ecology 85, 3373-3384.