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, directly connected to physical phenomena.

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)



Structure and community

Structural components of a habitat will impact community composition and potentially trajectory of succession.

For example – trees, by providing perch sites for birds, can play a role as nuclei for seed dispersal, thereby impacting species composition, including at early successional stages:

Influence of vegetation structure on spatial patterns of seed deposition by birds.  R. N. FERGUSON and  D. R. DRAKE.  New Zealand Journal of Botany, 1999, Vol. 37: 671-677


This can affect both native and non-native, and invasive and non-invasive (and, more generally, desired and not) plants:

Contagious seed dispersal and the spread of avian-dispersed exotic plants.  N. Omar Bonilla and Elizabeth G. Pringle.  Biological Invasions.  December 2015, Volume 17, Issue 12, pp 3409–3418


Put glibly – the presence of trees can increase rates of colonization and succession, by providing perching sites for seed dispersers, but what gets dispersed in is not highly predictable.

Sources and sinks

Just because an animal survives somewhere, doesn’t mean it thrives there.

Fitness, in the evolutionary sense, refers not only to survival, but also to reproduction.  That is, when we talk about fitness, in the context of selection, an animal or plant or any other kind of organism needs to reproduce in order to be fit.  This, of course, requires survival as well (dead organisms don’t reproduce) – but while survival per se is necessary, it is not sufficient, for fitness.

And so, population density, alone, of a species is not a perfect indicator of habitat quality for that species – there could be a lot of that species there, but that doesn’t necessarily mean that it’s reproducing there. This would be what we call a sink habitat (i.e, a place where a species is present – and perhaps even abundantly – but not enough to replace individuals lost through death and out-migration).

If such a habitat were in isolation, then it would of course eventually run out of individuals of that species – however, if there’s a source habitat (i.e, a place where reproduction exceeds loss due to death and migration) nearby, providing new individuals to the sink habitat that replace those lost through death or out-migration, then that sink habitat can have a consistent (and perhaps even consistently high) population density, even though its population isn’t reproducing enough to replace itself.

This means that density alone is not a clear indicator of the quality of the habitat… or, put another way:

Density as a Misleading Indicator of Habitat Quality.  B. Van Horne.  The Journal of Wildlife Management Vol. 47, No. 4 (Oct., 1983), pp. 893-901

One implication of this is that the habitat that is being conserved for a species might not be optimal for the reproduction of that species, and this could guide errant conservation strategies – for example, if most of the habitat were sink habitat, and it were supposed that that were the optimal habitat for the species, when in reality nearby (but seemingly less important because it includes less total area of the species of interest) is the source replenishing that population, then effort might be spent on that sink habitat at the expense of effort that would have been spent more fruitfully on the nearby source habitat.

Sinks can benefit a metapopulation, as they may decrease competition in source habitats.

Another implication of this is that the realized niche of a species might actually be larger (with respect to area) than its fundamental niche:

Sources, Sinks, and Population Regulation.  H. Ronald Pulliam.  The American Naturalist Vol. 132, No. 5 (Nov., 1988), pp. 652-661

A species’ niche is, broadly speaking, the setting (or, more precisely – the environmental factors) required for a species to survive and reproduce.  The fundamental niche is the set of environmental factors that one would idealize that the species requires to survive and reproduce – the realized niche, though, is what actually happens, in the context of real biological communities.

It is often considered that a species fundamental niche (that is, and broadly speaking, the places where it lives) will occupy less space than its realized niche, due to competition – that is, there are places a species can live, but doesn’t, because it gets outcompeted in its niche.

A population is a set of individuals of a given species, in a given geographic area.  They are presumed to be interbreeding.  A metapopulation is a set of populations among or between which there is gene flow.

Density dependent effects

Increase in population density can increase or decrease per capita mortality and fitness.

The decrease in fitness can come from competition, especially at higher population densities:
Interspecific Competition and Niche Differentiation Between Plantago lanceolata and Anthoxanthum odoratum in a Natural Hayfield.  F. Berendse.  Journal of Ecology Vol. 71, No. 2 (Jul., 1983), pp. 379-390


All else being equal, at increased population densities, per capita resource availability will go down – that is, there will be less food, water, territory, etc for each individual, as the population level increases.  If one (or more) of those resources is what’s limiting survival and/or reproduction… then increase in population density will be expected to limit the population’s rate of increase.

Increased population density can also increase the probability of contact between individuals – if there is a communicable disease that limits population abundances (by increasing mortality or decreasing birth rates) in a given area, then increased population densities could increase the probability of disease transmission, and then that could translate into reduction of population growth rates through a reduction of individual fitness.

That being said, an increase in population density can increase fitness –

Increase in fitness can come from facilitation:


“Facilitation Shifts Paradigms and Can Amplify Coastal Restoration Efforts,” Brian R. Silliman, Elizabeth C. Schrack, Qiang He, Rebecca Cope, Amanda Santoni, T. van der Heide, Ralph Jacobi, Mike Jacobi, Johan van de Koppel; Nov. 2, 2015, Proceedings of the National Academy of Sciences


At low population densities, “Allee effects” can decrease fecundity:


This can be caused, for example, by mate limitation.




Very broadly speaking all organisms have habitat requirements, and these can be distilled into four broad categories: food (nutrients/energy), water, cover (protection), and space (territory).


Anon.  2002.  The Four Essential Elements of Habitat.  Partner in Flight International Migratory Bird Day document.

This means that if you want to have a plant or animal, you have to supply all of its habitat needs (and ones required for all of its life stages)  – and if you want to get rid of an animal, you need to remove at least one of those needs.

Some habitat resources are consumed (and therefore need to be replenished), such as food and water, while others are not consumed, even though they may need to be repaired or replaced at some point; these include such structural elements as sites for sleeping/resting/bedding or for hibernation, and territory.

Other needs might be for mutualists, such as mycorrhizae or pollinators, or for prey or plants to eat (i.e, there may be biological requirements for the habitat, as well as physical).

There might also be a need for a lack of barriers, so that animals can travel for food, or for water, or to find mates, or to expand into other territories; roads can be barriers, and this can impede habitat usage by certain animals:

Richard T. T. Forman and Lauren E. Alexander.  1998.  ROADS AND THEIR MAJOR ECOLOGICAL EFFECTS.  Annual Review of Ecology and Systematics.  Vol. 29:207-231


For a case study in the complexity of understanding habitat, see here:

Roger L.H. Dennis.  2004.  Just how important are structural elements as habitat components? Indications from a declining lycaenid butterfly with priority conservation status.  Journal of Insect Conservation.  March 2004, Volume 8, Issue 1, pp 37–45


Habitat requirements can be quite specific (that is, far more narrowly defined than the four broad categories listed above):

Muturi EJ, Allan BF, Ricci J.  2012 Influence of leaf detritus type on production and longevity of container-breeding mosquitoes.  Environ Entomol. 2012 Oct;41(5):1062-8. doi: 10.1603/EN11301.


While habitat, broadly, refers to the physical and biological needs, in a particular geographic location, of an organism, more specific definitions can and do vary:

The habitat concept in ornithology.  WM Block, LA Brennan – Current ornithology, 1993



So then what is the difference between “habitat” and “niche“?

There is an aphorism that habitat is the address and niche is the occupation – that is, and put glibly, habitat has an emphasis on geography and place (“where is it?”), while the emphasis of the niche is on what the organism does and, more importantly, needs (“what does it do?”).

Put broadly, and in practice – if you want to find an organism (or make a place suitable for an organism), you focus on habitat, if you want to understand an organism’s impact on the environment, you consider the niche.