The summary of the paper is given in a short, numbered bit at the end of the paper.
Their summary:
1. Populations are limited by their resources. 2. Interspecific competition must exist. 3. Herbivores are seldom food limited, and are instead, predator-limited.
This last one is the big-cheese:
Terse though it is, this is the type of summary I would like to see in modern papers.
The idea that herbivores are limited by predators, that is, predators keep the world green, is also referred to as a top-down effect. Again, that is, predators (from the top) control the biodiversity within the system.
What are the impacts of this hypothesis?
Where does it stand now?
Robert Paine did some science on the effects of predators. Wait, let me back track. The whole idea, that is the impact of predators on herbivores that allow the earth to be green, some will argue, was first proposed by Aldo Leopold in 1949, in a much more casual essay. He was much less distinct with his details.
Back to this Paine guy:
He set up some experiments, published in 1966, where predators were excluded from rocky intertidal zones along the Pacific shoreline. The major result of these experiments, biodiversity collapse. Some invertebrates and algae went locally extinct. The main, and largest overarching predator, was the intertidal sea star Pisaster ochraceus.
Predator exclusion creates biodiversity havoc.
Smaller predators did not appear to have an impact, or as much of an impact. In this experiment, species of carnivorous snails were present. The author suggest that even a handful of snails do not have the consummation power of a single sea star.
This idea, this pinnacle species, that had such influence on the system, is referred to as the Keystone species. Massive effects like these, that is a change in the system, by removing the keystone predator for example, that lead to a major shift in the system, are referred to as Trophic cascades.
There are other terms that refer to a system that has changed, regime shift is one of them. As influences on systems are many, environmental changes being the most forefront these days, systems themselves are often the focus. If the systems is strong enough, ‘strong’ in some sense, then it will resist shifting, or resist a regime change. The word stable is sometimes used, but there is danger in this terminology since stable is used in so many other ways. Resilience is used as well, but resilience is also used in other ways. Two main definitions of resilience are used, one is resistance to change, the other, perhaps seeing systems as ever changing, is the speed at which systems return to the steady state (or the past state) after some large altering influence.
Some work on marine systems shows that nature of the biodiversity matters; that is, if the ecosystem is biodiverse enough, then the number of predators does not influence the system greatly. So systems, diverse with organisms, are more resilient. Perhaps this is a reminder that ecosystems are complex and one pattern, that of a keystone predator greatly influencing the system, is not universal.
The tale of Yellowstone wolves has highlighted these concepts in the terrestrial world. In Yellowstone of course, wolves were reintroduced to areas where they had been decimated and locally extinct. The introduced wolves had massive influence on the Yellowstone ecosystem. The National Park Service monitors the wolves, of course. I cited just one annual report below, much more information out there for those interested.
Take note, predators alter the ecosystem (did I make this point already?). In the case of Yellowstone, they alter it in fascinating and beneficial ways. We humans, continue to relearn the lesson, yet again, ecology is complex, and we need to mess with it as little as possible.
References:
Leopold A. 1949. Thinking like a mountain, in Sand county almanac.
Hairston NG, Smith FE, Slobodkin LB. 1960. Community structure, population control and competition. American Naturalist 94 (879): 421–425. doi:10.1086/282146.
O’Connor MI and Bruno JF. 2009. Predator richness has no effect in a diverse marine food web. Journal of Animal Ecology 78: 732–740.
Paine RT. 1966. Food web complexity and species diversity. The American Naturalist 100: 65–75.
National Park Service: Yellowstone Wolf Project. 2017. https://www.nps.gov/yell/learn/nature/upload/WOLF-REPORT-2017-YELL-FINAL.pdf
FYI, I'm trying to get my drawing mojo back. Then I can post mediocre drawings on here again.