Trophic Cascades – do we know all we need to know?Posted: May 4, 2014
By Dominic Maher
The four horsemen of the ecological apocalypse; habitat loss, pollution, invasive competitors and hunting, have gained a fifth and very powerful ally of late; climate change. Some alarming reports of drastic change in the next few centuries predict an anthropogenic cause for the next mass extinction. With this in mind, and the constant reminder from the global press, conservationists are under pressure to a new gear to ‘save the world’. As always, resources and knowledge are limited, and thus conservationists are looking to employ methods with far reaching effects, for the smallest economic input.
Owing to the simplicity and effectiveness of the trophic cascade paradigm, it has been utilised umpteen times in conservation to preserve whole ecosystems, many with great success. The temptation is to attempt to replicate that success as quickly and efficiently as possible. Fredreich Hayek (1960) observed that imitation of successful institutions and habits drives social evolution. I propose the same is true of the evolution of conservation practices. The danger however, is that we don’t know when a trophic cascade will occur, and when it will not. The lack of clearly defined characteristics for an ecosystem in which a cascade will invariably occur has led to much debate. This gap in our knowledge has potential for catastrophic effects both economically and ecologically if we get it wrong. That is, if we justify ecological manipulations based off false assumptions that ecosystems are uniform and trophic cascades are assured. This has never been more apparent than now, when the pressure to get it right is the highest it has ever been, and the access to information is so readily available.
What is a trophic cascade?
A trophic cascade is a conceptual theory used to explain or predict the major ecological structure and dynamics of the trophic levels within an ecosystem (Carter et al. 2007). More specifically trophic cascades can affect the diversity and abundance of the organisms therein (Power, 1992) via trophic regulation by predators (top-down) and food limitation (Bottom-up) (Bowlby & Roff, 1986). Hairston et al. (1960) are credited with pioneering the concept, though zoologist Robert Paine actually coined the term after performing experimental manipulation of top predators (Paine, 1980). In the context of the study performed by Paine, the term refers only to the effects of top-down pressure. However the term is now widely documented including bottom-up pressures, principally in aquatic, and specifically in marine systems. Trophic cascades are most readily illustrated in a three trophic level ecosystem, where interactions between predator, prey and primary producers are clearly defined and visible.
When have they occurred?
The documented cases of trophic cascades are many. They have been shown to control species composition and biomass as well as the production of herbivores and plants. Arguably the most documented would be the reintroduction of the Grey Wolf in the Yellowstone National Park. In this case, a subspecies of Grey Wolf (Canis lupus occidentalis) was introduced into the park, despite their native range being from the Upper Mackenzie River Valley (Canadian Northwest Territory) down to central Alberta, north of Yellowstone. The classification of the wolves as “experimental and non-essential” by the Endangered Species Act in the United States, poses serious questions as to the level of acceptable risk the government was willing to take. In this case, the (re)introduction was a raging success, but could it have gone wrong?
When have they not?
Food webs and their intricate interactions differ in their complexity. This has brought into question the ‘paradigm’ of trophic cascades and has fuelled the ongoing debate among ecologists. Specifically, the debate is concerned with the generality of the model, the absence of factors such as behavior, the varied measures employed to illustrate the cascade, and the ubiquity of the model. Can we apply as broad a theory to the wide array of complex interactions between different trophic levels? Poorly documented, but paramount to this issue, are the instances where trophic cascades do not occur. Epitomised by the idiom ‘absence of evidence is not evidence of absence’, there are ecosystems in which cascades do not occur. One study by Carter et al. (2007) tested the generality of the trophic cascade paradigm by recording changes in density of macroalgae in San Juan Channel, Washington, as a result of three levels of experimental urchin harvest:
(1)simulated sea otter predation (monthly complete harvest of sea urchins);
(2)simulated commercial urchin harvest (annual size-selective harvest of sea urchins); and
(3)no harvest (control)
The two experimental urchin removal treatments did not significantly increase the density of perennial or annual species of macroalgae after 2 years, despite significant and persistent decreases in urchin densities.
This is particularly interesting given Estes & Palmisano’s (1975) work with sea otters in the Aleutian Islands, and the manipulation of sea urchin populations by Kitching and Ebling (1961), and Ogden et al. (1973) all found opposing results. One could argue this calls into repute the work done by Carter et al. I believe that given the same species in different spatial zones react differently; these studies serve to exemplify the need for caution when manipulating ecosystems.
What limits the occurrence of trophic cascades?
Fretwell (1977) observed that in reality, ecosystems often have more or less than three trophic levels. He proposed that in ecosystems with an odd number of levels predators would limit grazers. Whereas in systems with an even number, plants would be grazer limited. Although Fretwell (1977) was proposing different ecosystems would cascade differently, no doubt he would be the first to admit the more trophic levels, the more intricate the interactions between them. This leads us onto the factors that can limit or prevent cascades from occurring. It appears that acting in opposition to cascades are compensatory mechanisms, which can dampen or even prevent a cascade from occurring (Pace et al. 1999). These can include omnivory (feeding on more than one trophic level (Pimm, 1982) by top predators and/or mid level consumers), species diversity and species replacement within trophic levels. Stein et al. (2011) assessed the generality of the trophic cascade paradigm by assessing the conceptual strength in reservoir food webs. They focused on the species Dorosoma cepedianium (Gizzard shad), an open-water omnivore, and found that they regulated community composition. As opposed to being regulated by top-down or bottom-up forces. A similar study performed by Pringle and Hamazaki (1998) found that no trophic cascade resulted from the experimental exclusion of omnivorous fish and shrimp in the Sabalo River, Costa Rica. Pringle and Hamazaki (1998) predict that tropical streams that are characterised by large omnivores that affect more than one trophic level, would rarely see a trophic cascade occur. Furthermore, they highlight the importance of distinguishing per capita impacts versus the impacts at natural population densities, when predicting a trophic cascade. The presence of omnivores introduces a third form of trophic pressure that is, linear, which only further complicates the paradigm.
What happens if we get it wrong?
The implications of getting a conservation program wrong, specifically one that involves the manipulation of the food webs, could be ecologically and economically catastrophic. Especially if the manipulation is justified based on the assumption of total uniformity across ecosystems, and assumption that a cascade will occur. One such example that illustrates the danger is that of introduction of arctic foxes (Alopex lagopus) to the Aleutian archipelago as a result of the collapse of the fur trade in the late 19th, early 20th centuries. A study performed by Croll et al. (2005) found that not only did a trophic cascade not occur, but that the introduction transformed the islands from grasslands to maritime tundra. The islands were essentially nutrient impoverished, and thus no longer supported productive grasses and sedges. A major ecosystem change resulted, one that was certainly not predicted. Croll et al. (2005) conclude; “the mechanisms by which predators exert ecosystem-level effects extend beyond both the original conceptual model provided by Hairston et al. and its more recent elaborations”. This work showed that predators can have substantial indirect effects on entire ecosystems in ways that are both different from trophic cascades, and poorly understood.
Factors limiting the occurrence of a trophic cascade are many. Presence of omnivores, species diversity, and species replacement are only a few. However, the fact that so many questions arise when applying trophic cascade paradigm to widely varying ecosystems may hinder the application of conservation practices. I recommend the advice of Hunter and Price (1992) be heeded. Specifically conservation practitioners first must consider which factors modulate resource limitation and predation in a given system. Also relevant is under what conditions will predators or resources predominate the regulation of populations. It may not be possible to create a one-size-fits-all model for whether a trophic cascade will or will not occur in a given system. However the proposed framework provides a starting point for conservation managers, before actually implementing in situ interventions.
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