Rules of Ecology

This is the post excerpt.


Ecology, due to its stunning complexity, is a science without much in the way of laws, but there are some rules…. rules that determine what can happen, what can’t happen, and, sometimes, what will or will not happen (i.e., actual laws).

Generally speaking, though, these are rules that allow or guide but don’t necessarily require or prevent.


Also – just to note that “science”, here, is meant as a system where generalities (like rules or laws – or theories) are supported (or not) by reproducible observations (from the field or the lab; via experiments or other kinds of studies), or other kinds of direct evidence. Additionally, there is generally a proposed mechanism (that is, a cause for the effect – a “because clause”, if you will) whose validity can be tested by comparing systems with and without the proposed cause, and determining if, respectively, the effect is present in the former and absent from the latter.

It is also a system where terms are clearly defined (or, at least, definable) and, generally and preferably, directly connected to physical phenomena.

And, natural history – or qualitative, or narrative, or anecdotal – studies are an excellent fit for ecology, as they highlight that each data point is truly unique.  This makes for an excellent complement to “big data” research, as that statistical style of research looks for (or assumes, even) similarity (or fungibility, even) of data points.

Big data needs little data, and little data needs big data.

The rate of evolution

Evolution can happen very quickly (in just a few generations).  This can be the case for rapid shifts in characteristics critical to ecological impacts, such as fecundity (reproduction) and growth.

Pimentel, D., W. P. Nagel and J. L. Madden, 1963: Space-time structure of the environment and the survival of parasite-host systems. Amer. Nat. , 97, 141-167


“Within eight generations in a 16-cell system, reproductive capacity of the parasite declined 40 per cent. In 20 generations in a 30-cell system, reproductive capacity of the parasite declined 68 per cent.”

And, for another example:

Conover, D.O. and S.B. Munch. 2002. Sustaining fisheries yields over evolutionary time scales. Science 297 (5578): 94-96.

Over the span of four generations, after strong selective pressure (either the largest 90%, smallest 90%, or random 90% (for control) were removed) there was substantial (~2-fold) difference between the two directed selective regimes, in mean weight of individuals in those populations.


Well-mixed vs. geographically structured habitats

If populations of a species are well-mixed (geographically), then they may be more vulnerable to extirpation (extirpation is basically locally extinction) than if there are secluded or sequestered populations.

This can be because – to put it glibly – if there’s somewhere to hide from predators or avoid parasites, then populations can persist in those sites, while being extirpated elsewhere, instead of being extirpated everywhere in the ecosystem.

This is an important corollary to discussions of corridors and connectivity.  That is, a potential downside of connectivity is vulnerability to predators or parasites.

Pimentel, D., W. P. Nagel and J. L. Madden, 1963: Space-time structure of the environment and the survival of parasite-host systems. Amer. Nat. , 97, 141-167


Huffaker, C. B., and C. E. Kennett. 1959. A ten-year study of vegetational changes associated with biological control of Klamath weed. J. Range Management 12: 69-82.



Huffaker, C. B. “Experimental Studies on Predation: Dispersion Factors and Predator–Prey Oscillations.” Hilgardia: A Journal of Agricultural Science 27 (1958): 795–834


This can also affect competition among genetically distinct populations of a species, and subsequent evolution of those populations – for example if one population has a competitive advantage over another population, and if those populations are in contact with each other (versus being segregated), then that can enrich for the population with the advantage over the other.

Chao L, Levin BR.  1981.  Structured habitats and the evolution of anticompetitor toxins in bacteria.  Proc Natl Acad Sci U S A. 1981 Oct;78(10):6324-8.




Apparent competition

For competition, “typically, the term is used for direct inhibitory interactions or for more indirect inhibitory effects arising from the sharing of resources in short supply” (Holt 1977) –

That is, two organisms are competing for a resource (it could be food, it could be territory, it could be for a mate – the list goes on), and for both of them, growth and/or reproduction are reduced (that is, there’s a reduction of fitness, in the evolutionary sense).

However – organisms can affect each others’ fitness without interacting directly.  For example, if a predator eats two different kinds of prey, then there can be “prey switching”, that is, the predator can eat the more abundant prey species, thereby reducing predation pressure on the less abundant one.  This shared predation would affect both species in the ecosystem.

This would also mean that if one prey species is at a low abundance, the predator can just switch to the other instead of starving (and subsequent reduction of fitness for the predator), which is what would happen if that predator could only eat one type of prey (the example of the lynx and hare in the Arctic is commonly and controversially called on for this); this prey-switching would then allow that predator to continue eating its prey at the same or a similar rate.  Therefore, the presence of the one prey species would effectively (though indirectly) decrease the abundance of the other, and it might appear as if these prey species were competing directly with each other.

Robert D. Holt.  1977.  Predation, Apparent Competition, and the Structure of Prey Communities.  Theoretical Population Biology 12(2): 197-229


Bergerud, A. T. 1967. The distribution and abundance of arctic hares in Newfoundland. Canadian Field Naturalist 81(4): 242-248.


This means that introducing one species into an ecosystem could reduce populations of a second species that’s already there, even if those two species don’t directly compete with each other.

This is, by the way, similar to the idea of food subsidies, that is, providing an alternate food source maintains predator populations in the face of scarcity of prey, thereby further forcing down the abundances of those prey populations (perhaps even to extirpation).

Hawkins, C.C., W.E. Grant, and M.T. Longnecker, Effect of Subsidized House Cats on California Birds and Rodents.  Transactions of the Western. Section of The Wildlife Society 35: 29-33


Top down – Bottom up

The trophic pyramid has a base and a top – and if lower levels (in this arrangement) are manipulated, upper levels can be affected, and vice versa.

“Bottom up” effects come from changes in what is eaten, while “top down” effects come from changes in what is doing the eating.

Both can impact population abundances – for example, fertilization of plants can increase herbivore abundances, and excluding predators can increase abundances of prey.

Krebs CJ, Boutin S, Boonstra R, Sinclair AR, Smith JN, Dale MR, Martin K, Turkington R. Impact of food and predation on the snowshoe hare cycle.Science. 1995 Aug 25;269(5227):1112-5.


Shifts in abundance of predators of herbivores can impact vegetation.

Terborgh J, Lopez L, Nuñez P, Rao M, Shahabuddin G, Orihuela G, Riveros M, Ascanio R, Adler GH, Lambert TD, Balbas L.  Science. 2001 Nov 30;294(5548):1923-6.  Ecological meltdown in predator-free forest fragments.


The effects can be highly variable (and therefore not readily predictable) –

C. John Burk.  1973.  The Kaibab Deer Incident: A Long-persisting Myth. BioScience 23(2): 113-114


Graeme Caughley.  1970.  Eruption of Ungulate Populations, with Emphasis on Himalayan Thar in New Zealand.  Volume 51, Issue 1  January 1970 Pages 53–72

This raises the question – if a golf course is fertilized, how much of that is feeding geese?

Seed banks – Seed germination – Seed dispersal

The present composition of a plant community is strongly influenced by persistence of seeds underground (the seed bank), patterns of seed germination (including influence of light and moisture), and what seeds have gotten there (dispersal).  These all differ across species in a community.

This means that it can be difficult, if not impossible, to accurately predict the future composition of a plant community based on what is presently growing (above-ground) there – and also that what is planted, for example, in a restoration planting, will not be the only predictor of what will end up growing there.

References –

Different species in a community will have different germination requirements (light, moisture, chilling, dry storage) –  Grime, J.P., Mason, G., Curtis, A.V., Rodman, J., Band, S.R., Mowforth, M.A.G., Neal, A.M., & Shaw S. (1981) A comparative study of germination characteristics in a local flora. Journal of Ecology, 69, 1017–1059

Different species persist for different lengths of time underground: Laura A. Hyatt and Brenda Casper. Seed bank formation during early secondary succession in a temperate deciduous forest Volume 88, Issue 3; June 2000; Pages 516–527

Seed bank and dispersal have varying impacts on plant community development: Mary Allessio Leck.  Seed-bank and vegetation development in a created tidal freshwater wetland on the Delaware River, Trenton, New Jersey, USA.  Wetlands;  June 2003, Volume 23, Issue 2, pp 310–343 … “Donor or stockpiled soils or transplantations (e.g., van der Valk and Pederson 1989, van der Valk et al. 1992) were not necessary because of the tremendous influx of seeds into the created wetland. In addition to a number of exotics and invasives, both the seed bank and vegetation contained a number of rare species.”


Native plants

At least two of the rationales put forth for the use of native plants are based in ecological and evolutionary principles: local adaptation and co-evolution.  The former is the basis of the idea that plants will do better in areas where they have lived for a long time (because they’ve adapted to local conditions), and the latter is the basis of the idea that plants will serve as better wildlife (including pollinator) hosts and partners in areas where they have lived with those animals for a long time (because they have evolved along with those animals).

This of course means that we need to have a clear temporal and spatial/geographic component to our definition of “native”, and/or, more broadly, that we need to be able to assess how long a given plant has been continuously (i.e., without significant temporal disjunction) growing in a given area, to know if that is a long enough time to expect those plants to have adapted to local conditions, and for those plants and local animals to have co-evolved.  That in turn, requires that we have an understanding of how rapidly evolution occurs.  It also means we need to have an understanding of how specialized a given plant is as a host, and to the environment in which it grows.  Additionally, while plants certainly do adapt to climate (i.e, temperature and precipitation), there are other variables, such as soils (including drainage and chemistry), hydrology, aspect (which direction a slope faces), for example, to which plants adapt, and therefore a local plant may not be adapted to a specific site in a region, even if it has grown nearby for centuries or millenia.


“Nativeness” was reviewed in a chapter of “Fifty Years of Invasion Ecology: The Legacy of Charles Elton”; edited by David M. Richardson (2011)