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.


Chemical inhibition (“allelopathy”) has been hypothesized to play a role in ecological succession:

Allelopathy as Expressed by Helianthus annuus and Its Role in Old-Field Succession.  Roger E. Wilson and Elroy L. Rice.  Bulletin of the Torrey Botanical Club ; Vol. 95, No. 5 (Sep. – Oct., 1968), pp. 432-448


For more details on allelopathy:


Plant abundance and animal abundance

A corollary to the “trophic pyramid” is that there will be more vegetation in a terrestrial ecosystem than animals (animals eat plants, either directly or indirectly by eating other animals that have eaten plants).

This is also true globally:


The biomass distribution on Earth.  Yinon M. Bar-On, Rob Phillips, and Ron Milo.  PNAS May 21, 2018.


Temporal dynamics of colonization

When a species arrives in a novel ecosystem, there can be a delay in establishment – that is, it can take a while for a new species to naturalize in a given ecosystem.  There can also be temporal lags in expansion of naturalized populations (i.e., a species might be present in an area for some time prior to its populations experiencing rapid growth in local abundance).

This means that arrival of a species and the recognition of its having arrived can be separated by a significant time interval (and this may impact management of undesirable and/or pathogenic species).

This can be due to the biology of the organism and how it interacts with its environment:

Lag times and exotic species: The ecology and management of biological invasions in slow-motion.  Jeffrey A. Crooks.  Ecoscience 12(3):316-329. 2005; https://doi.org/10.2980/i1195-6860-12-3-316.1


Cholera epidemic in Yemen, 2016–18: an analysis of surveillance data. Camacho et al.  The Lancet Global Health  Volume 6, No. 6, e680–e690, June 2018 https://www.thelancet.com/journals/langlo/article/PIIS2214-109X(18)30230-4/fulltext

It could also be due to observer effects (e.g., no one’s looking for it, and so it isn’t reported).

Keystone species

A keystone species is one that has an ecological impact out of proportion to its abundance (i.e., it has more of an effect on an ecosystem than you might expect, based on how many of them they are).

One way in which this effect can be exerted is by predators reducing abundance of more competitive species, thereby allowing other, less competitive species to survive and perhaps to thrive (and thereby increase species richness):

Robert T. Paine.  1966. Food Web Complexity and Species Diversity.  The American Naturalist. Vol. 100, No. 910 (Jan. – Feb., 1966), pp. 65-75


This effect (removal of predators increasing species richness of prey) has been documented in terrestrial ecosystems:

Henke and Bryant. 1999. Effects of coyote removal on the faunal community in western Texas.  The Journal of Wildlife Management, Vol. 63, No. 4 (Oct., 1999), pp. 1066-1081


It is not just removal, but also behavioral responses, that can cause ecological effects, including trophic cascades.

Justin P. Suraci, Michael Clinchy, Lawrence M. Dill, Devin Roberts & Liana Y. Zanette.  2016.  Fear of large carnivores causes a trophic cascade. Nature Communications; doi:10.1038/ncomms10698


Smith JA, Suraci JP, Clinchy M, Crawford A, Roberts D, Zanette LY, Wilmers CC.  Fear of the human ‘super predator’ reduces feeding time in large carnivores.  Proc Biol Sci. 2017 Jun 28;284(1857). pii: 20170433. doi: 10.1098/rspb.2017.0433.


These effects can potentially impact physical properties of an ecosystem:


Competition structures communities

The principle of “competitive exclusion” basically says that two organisms that occupy the same niche (put glibly – that they do the same thing) will not occupy the same place – one will outcompete the other.

This is related to “niche pre-emption” – which essentially says that an organism already present in an ecosystem will exclude newcomers who occupy that same niche (and “want to” occupy that same habitat).

And so, competition guides presence or absence of species in a biological community (i.e, competition structures the community composition).

This has been looked at, for example, for cats and coyotes in urban areas:

Gehrt SD, Wilson EC, Brown JL, Anchor C (2013) Population Ecology of Free-Roaming Cats and Interference Competition by Coyotes in Urban Parks. PLoS ONE 8(9): e75718. https://doi.org/10.1371/journal.pone.0075718


And for “colonization resistance”:

Lawley TD, Walker AW.  Intestinal colonization resistance.  Immunology. 2013 Jan;138(1):1-11. doi: 10.1111/j.1365-2567.2012.03616.x


Competitive exclusion can vary based on environmental conditions, and co-existence can be possible in some environments, for species that would exclude one another in other environments:

Jeffrey Edmunds, J. M. Cushing, R. F. Costantino, Shandelle M. Henson, Brian Dennis
R. A. Desharnais.  Park’s Tribolium competition experiments: a non‐equilibrium species coexistence hypothesis.  Journal of Animal Ecology:  Volume72, Issue5;  September 2003; Pages 703-712



Dilution effects

It has been proposed that an increase in biodiversity can lead to a decrease in disease transmission, due to a “dilution effect” – that is, basically, that hosts (of disease vectors) that are not competent for transmission of a disease will block (or dilute) environmental cycles of transmission of that disease, and therefore the presence of those hosts in an ecosystem (which presumably is also an increase in biodiversity) will reduce environmental load and subsequent transmission of that disease.

(note: “host”, here, refers to non-human hosts of parasitic disease vectors that are also parasitic on and disease vectors for humans)


However –


The above theory is not reliant on an increase in biodiversity per se, but about an increase in biodiversity increasing the probability of the ecosystem including non-competent hosts.


For a critical review, see here:

S. E. RANDOLPH and A. D. M. DOBSON.  Pangloss revisited: a critique of the dilution effect and the biodiversity-buffers-disease paradigm.  Parasitology; Volume 139, Issue 7
June 2012 , pp. 847-863


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.


Introduction to a new environment can cause such rapid evolution:



This rapidity contributes to the long-term unpredictability of evolutionary trajectories:

Peter R. Grant, B. Rosemary Grant.  Unpredictable Evolution in a 30-Year Study of Darwin’s Finches.  Science 26 Apr 2002: Vol. 296, Issue 5568, pp. 707-711
DOI: 10.1126/science.1070315