Ecology is the study of relationships among organisms and between organisms and environment. Consideration for ecology in a project allows a far more in-depth and realistic portraits of biological diversity, populations, the way organisms interact with each other and how this shapes their adaptations, on every scale of life - from the raw amount of energy available on an entire world, down to the morphology and behaviour of the single organisms.
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The first step to determine how energy is transferred through living systems is to find ut how much energy is available on the planet. Not every form of energy can be realistically used by organisms - the kinetic energy of the planet itself or its gravitational potential energy, for example. See here for a summary of the different energy sources that could conceivably be exploited by living organisms.
On Earth, the most used source of energy is sunlight. Earth receives 1.74·1017 W on the whole from the Sun; remember that this value is directly proportional to the surface of the planet, to the fourth power of the star temperature and inversely proportional to the square of the distance between star and planet. Since the Earth's surface area is 510 millions of square km, this means an insulation of 340 W per square meter on the top of the atmosphere. Since the 34% of this sunlight is reflected into space from Earth's albedo, the value drops to 220 W/m2, or 4550 kcal/d/m2 (kilocalories per day per square meter).
The other energy source used on Earth is reducing (electron-rich) chemicals. It's harder to estimate a worldwide availability of chemical energy; one of the most common and profitable, the oxidation of sulfur and ferric iron into sulfuric acid and ferrous iron (S + 6Fe(III) + 4H2O → HSO4- + 6Fe(II) + 7H+), gives off 60 kcal per occurrence. Sulfur makes up the 0.042% of Earth's crust, but a great part of it is already oxidised and it isn't useful for chemiosynthesis.
Thermotrophs feed on heat leaking from the core; on Earth (where no examples are known) they could rely on 3.0-4.4·1013 W (much more on large planets with a hotter core and stronger tectonic activity). As for the magnetic field, an even more exotic source of energy, it contains a fixed (non-renewed) amount of energy of roughly 6.37·108 J. In contrast, note that sunlight and heat leakage enrich Earth's surface with respectively 2.0·1012 J and 3.5-5.1·108 J each day.
Also see: BiomeOf course, only a part of the available energy will be useful to living systems. While the average square meter of Earth's surface receives 220 W/m2 (4550 kcal/d/m2), at the Equator the insulation is greater: 750 W/m2. Considering night, the insolation over a day falls to 300 W/m2 (6230/kcal/d/m2). This is the amount of energy that reaches a meter of Earth's surface at the Equator. The value for different latitudes (ignoring the axial tilt) can be obtained by multiplying it by the cosine of the latitude in degrees:
Value on Earth (per m2)
|0° (Equator)||1.000||6230 kcal/d||300 W|
|10°||0.985||6140 kcal/d||296 W|
|20°||0.940||5860 kcal/d||282 W|
|30°||0.866||5400 kcal/d||260 W|
|40°||0.766||4770 kcal/d||230 W|
|50°||0.643||4010 kcal/d||193 W|
|60°||0.500||3120 kcal/d||150 W|
|70°||0.342||2130 kcal/d||103 W|
|80°||0.174||1080 kcal/d||52 W|
|90° (Pole)||0.000||0 kcal/d||0 W|
Given the nature of photosynthesis (see below), Earth plants can use only roughly 4/9 of the incoming light: therefore, at the Equator, 2770 kcal/d/m2 are available for photosynthesis. About 1% of the energy used by plants can be converted into biomass; thus, in ideal conditions, a square meter at the Equator produces 28 kcal/d/m2 available for consumption, which corresponds to about 6.5 grams of plant matter (see below).
|Surface||Biome||Daily productivity||Yearly productivity||Yearly biomass|
|29%||11%||Rainforest||22 kcal/m2||8200 kcal/m2||2000 g/m2|
|5.0%||Monsoon forest||17 kcal/m2||6300 kcal/m2||1500 g/m2|
|1.3%||Marsh, swamp||13 kcal/m2||4800 kcal/m2||2000 g/m2|
|8.4%||Deciduous forest||13 kcal/m2||4700 kcal/m2||1000 g/m2|
|1.0%||Scrubland||11 kcal/m2||3900 kcal/m2||800 g/m2|
|4.7%||Open woodland||7.7 kcal/m2||2800 kcal/m2||600 g/m2|
|10%||Savanna||7.7 kcal/m2||2800 kcal/m2||700 g/m2|
|9.4%||Farmland||7.4-16 kcal/m2||2700-6000 kcal/m2||650-1400 g/m2|
|8.1%||Boreal forest||6.6 kcal/m2||2400 kcal/m2||500 g/m2|
|1.3%||Lakes, rivers||6.3 kcal/m2||2300 kcal/m2||500 g/m2|
|6.0%||Prairie||5.5 kcal/m2||2000 kcal/m2||500 g/m2|
|5.4%||Tundra||< 2 kcal/m2||600 kcal/m2||140 g/m2|
|12%||Arid scrub||< 1 kcal/m2||300 kcal/m2||70 g/m2|
|16%||Desert, glaciers||~ 0 kcal/m2||~ 0 kcal/m2||< 3 g/m2|
|71%||0.55%||Coral reef||25 kcal/m2||9000 kcal/m2||2000 g/m2|
|7.4%||Coastal water||4.4 kcal/m2||1600 kcal/m2||350 g/m2|
|92%||Open ocean||< 2 kcal/m2||600 kcal/m2||130 g/m2|
Thus we obtain the primary productivity, that is, the amount of energy found in producers (vegetation and analogue organisms) and available for consumption to other organisms. Humans are currently estimated to consume a fifth of the worldwide net productivity.
Knowing the energetic needs of an organism, and by extension a population, we can easily derive its range (inhabited geographical area) in a particular biome. Assuming a 50 kg marsupial mammal, living in a deciduous forest, its consumption can be estimated (see here) in 49·50¾ = 49·18.8 = 921 kcal/d. Since each day a square meter of deciduous forest produces 13 kcal, the mammal will need 921/13 = 71 m2. If it lived in a tundra, it'd need 921/2 = 461 m2; if it lived in a lush rainforest, 921/22 = 42 m2 would be enough.What about higher-order consumers? A predator feeds on the energy contained in the tissues of its preys, which in turn got them from producers. These levels of organisms (producer → first-order consumer → second-order consumer → ... → decomposer) are called trophic levels. However, when the first-order consumer feeds, it uses most of the energy for itself, and only a small part remains available for the second-order consumer, in form of chemical energy in the cells: more precisely, about two thirds of the original energy is dispersed by respiration, while two thirds of what remains is expelled in waste or lost in the decay of tissues: generally, only 10% of the energy passes from a level to another.
This means that, if a predator is to gain 1 kcal from its prey, this needs to gain 10 kcal from its food, and each more level multiplies by 10 the amount of energy required at the base. Let's add to the forest 40 kg placentate predators: they'd need , each day, 72·40¾ = 1145 kcal, which in turn means that their preys need at least 11,450 kcal worth of plant matter, and thus 11450/13 = 881 m2 of forest (or 5725 m2 of tundra, or 521 m2 of rainforest). Beside the raw energy value, we can also know that each predators will need to eat, on average, 1145/921 = 1.2 preys per day.
This calculation assumes a perfect environment with constant productivity, ignoring bare rocks, water pools, inedible plants, bad weather, etc.; let's double the needed range to account for that and we'll get 142 m2 for the herbivores and 1762 m2 for the predators. Assuming that they're the only consumers in the environment, a square km of deciduous forest will be able to feed roughly 7000 herbivores and 570 predators: this is their saturation density.
Energy pyramids and biomass
The energy flux in an environment can be represented in graphical form by a pyramid with the producers at its base and the consumers on top of them; the organisms at each (trophic) level can be measured in number of individuals, biomass or directly in energy. Biomass is the total mass (usually discounting water) of all the organisms in a certain category of habitat: globally about 4.38·1013 W are converted into biomass, and since plant matter contains on average 4.25 kcal per gram, this makes for 1.95·1011 kg of new biomass each day, all over the Earth. The availability of solar energy is directly proportional both to the planet's surface and the star's luminosity (see here).
|Trophic level||Global available energy||Global biomass|
|Autotrophs||4.38·1013 W||8.3·1014 kcal/d||3·1013 kg|
|Herbivores||4.38·1012 W||8.3·1013 kcal/d||3·1012 kg|
|Carnivores I||4.38·1011 W||8.3·1012 kcal/d||3·1011 kg|
|Carnivores II||4.38·1010 W||8.3·1011 kcal/d||3·1010 kg|
|Carnivores III||4.38·109 W||8.3·1010 kcal/d||3·109 kg|
There are three ways to build an energy pyramid:
- A pyramid of numbers counts the number of individuals at each trophic level. Since the size, and thus the energetic needs, of different species can be wildly different, such a model isn't very reliable: a single host can feed a large number of smaller parasites.
- A pyramid of biomass considers the total biomass of each trophic level, usually discounting water and inorganic material (such as calcareous shells). While it's more reliable than a pyramid of numbers, it can create a paradox as in the case of arctic seas, where the biomass of autotrophic plankton is smaller than the total biomass of the consumers that feed on it: this can happen because the plankton's life cycle is extremely short, so that in a given moment the living biomass is disproportionately small.
- A pyramid of energy directly measures the flow of energy through each trophic level: at the base there's the environment's primary productivity, and each level is roughly ten times smaller than the one below. A gross productivity and a net productivity must be distinguished: the first one (green in the image above) is the amount of energy absorbed by organisms (primary productivity for producers). Two thirds of this energy are dispersed through respiration; what remains is the net productivity (orange in the image above). Two thirds of this is dispersed through excretion and decay, leaving the gross productivity of the next level.
The term "ecological niche" refers to the position or role that a species occupies in a certain environment: the physical space it lives in, the time range in which it's active, the type of resources it consumes, how it responds to fruitful or harmful variations in the environment (e.g. natural disasters, predators, parasites, diseases, food scarcity) and its relationships with the other organisms (see below).
There is a fundamental niche, the range of conditions in which a species is able to survive, and a realized niche, the more specific conditions it actually lives in when imperfections of the environment and other organisms in competition are accounted for. When two species occupy the same niche in the same habitat, they enter in direct competition with each other; according to Georgy Gause's competitive exclusion principle, the less specialized of the two species inevitably goes extinct, unless it moves into a different niche. A possible strategy to survive is niche differentiation, that is, an adaptation to consume the same resource in different times of the day or thew year (temporal partition), in different regions of the environment (spatial partition), in different conditions of a variable environment (conditional partition), or by developing different body structures to consume the resource in different ways (morphological partition).
The concept of guild is similar to the niche: it refers to a group of species that exploit the same resources in similar ways, often with similar body structures. Terms such as "tree", "shrub", "vine", "grass", "plankton" are all simple examples of guilds; more complex definitions can include "underbrush detritivore" (e.g. woodlice and millipedes), "savanna grazer" (e.g. antelopes and kangaroos), "forest canopy folivore" (e.g. sloths and howler monkeys), "colonial filter-feeder" (e.g. corals and bryozoans), etc.
(Examples for each existing niche are provided in form of links)
r-strategy and K-strategyIn the block of adaptations of each particular species, a tendency can be found towards two basic survival strategies, concerning individual development, reproduction, populations and ecological role. They can be understood through the logistic function devised by Pierre-François Verhulst and Alfred Lotka in 1920, that describes the growth of living populations through time:
dP/dt = rP (1 - P/K), or, dividing by K and establishing x = P/K, dx/dt = rx (1-x)
Here, P represents the size of the population in a given moment and K the carrying capacity (the maximum population that can exist in a certain environment), while dP/dt is the derivative of P in function of t (time). The natural growth rate (r) can be calculated as r = ln(P2/P1)/t, where P1 and P2 are the size of the population in two different moments and t is the time span (usually a year) in which the population grows from P1 to P2. K is the limit the growth of P tends to, given infinite time.
The inversion of the function can be used to found the population P2,in function of the former P1 and of the time t: P2 = K/(1+qe-rt), where q = (K/P1)-1. An example: say there is a population of 10 individuals in a habitat where the maximum carrying capacity is K=1000, and the growth rate is r=0.095 (which means +10% per year). In this case, q = 1000/10-1 = 99. The population will grow in this way:
- after one year will be P2 = 1000/(1+99e-0.095) = 1000/(1+90) = 11 individuals;
- after two years, P2 = 1000/(1+99e-0.191) = 1000/(1+82) = 12 individuals;
- after ten years, P2 = 1000/(1+99e-0.953) = 1000/(1+38) = 26 individuals;
- after fifty years, P2 = 1000/(1+99e-4.77) = 1000/(1+0.84) = 543 individuals;
- after a hundred years, P2 = 1000/(1+99e-9.53) = 1000/(1+0.007) = 992 individuals.
As the equation shows (and see the graph above), initially the population grows very quickly, according to the rate r, but at a certain point the term rP2/K (the braking effect of the already existing population) takes over, and the growth slows down, until the population reaches K and stops growing at all.
A species can maximize its own its own success either by investing on r (by having a high reproductive rate) or on P/K (by having always a population very close to the carrying capacity). These two strategies are thus known as r-strategy and K-strategy. Species following the r-strategy, more useful in unstable environments, favour the capacity of producing quickly a vast offspring, by dispersing a large number of gametes/seeds/eggs in the environment, so that the sheer number will ensure the survival of at least someone of them; the r-strategy traits are found in all bacteria and protists, grasses, insects, fish and amphibians and, among mammals, in rodents.
K-strategy works best in stable environments with a high carrying capacity. Such species tend to be highly specialized to extract as much energy as possible from their resources, and thus keeping their population large at all times (not as large as a r-population in favourable times, but much larger than the same population in unfortunate times). Each individual has a small number of children, but takes a great care of them; the K-strategy traits are found and animals such as elephants, cetaceans and primates.
There are, of course, species that combine both strategies: trees and turtles have the K-trait of a large size and long lifespan, but also the r-trait of producing a large offspring and not taking care of it. Among mammals, males are often slightly more oriented towards r-strategy than females.
|Populations||More individuals||Fewer individuals|
|Offspring per parent||More children||Fewer children|
|Reproductive strategy||Dispersal||Parental cares|
|Food specialization||Less specialized||More specialized|
|Long-time survival||More likely||Less likely|
|Intelligence||Less likely||More likely|
No organism exists in perfect isolation: every biological activity requires interacting both with the environment and with other organisms. A species can act on others in an indirect way, for example by consuming a common resource or modifying the number of shared predators. Interactions can be classified accoreding to their effect on the affected species:
|Effect on B||Effect on A|
In neutralism there's a null (or negligible) effect on both species involved. Given the extension and complexity of biological relationships on Earth, it's possible that true neutralism doesn't actually exist.
Amensalism is harmful for a species and neutral for another. Since no species gains an advantage from it, amensalism is usually an accidental byproduct of another action: for example, a large-sized animal can trample and crush smaller organisms.
Competition is an interaction that damages both species. As there is no one to benefit from it (except possibly for a third species that takes advantages from the other two being hurt), natural selection tends to remove it, turning it into antagonism, commensalism or mutualism, or simply with niche differentiation. There are three forms of competition:
- Interference competition: the individuals of a species directly interfere with the activities of the other, for example stealing their food (like lions do to hyenas and wild dogs) or damaging their environment.
- Exploitation competition: the two species need the same resources (food, light, space, etc.), so each of them reduces their availability for the other. Usually, the clash is quickly won by the more specialized species.
- Apparent competition: the two species share a predator or parasite, so that one of them, expanding its population, also makes the predator/parasite more common, thus damaging all its other preys/hosts.
Competition can also be intraspecific (within a species), for example when males compete for the right to reproduce in several animal species. While competition is common between species, populations or individuals, it's very rare between larger clades. When competition is lethal for both involved parties (a rare and shortlived occurrence), it's called synnecrosis.
Antagonism is beneficial for a species and harmful for another. It includes every form of predation (which includes herbivores eating plants) and parassitism. It can occur between species with a close ecological role (intraguild predation), as in the case of lions and lynxes, coyotes and foxes, sharks, spiders, etc.
Predation is a form of antagonism in which an organism, the predator, consumes the tissues of another organism, the prey. See Speculative bioenergetics#Carnivores for examples of predatory specialization.
Parasitism is an antagonistic symbiosis in which a parasite lives in close - and often permanent - contact with a host. Parasites are called ectoparasites when they live on the outer surface of the host (e.g. fleas, ticks, lice) and endoparasites when they live inside the host's body (e.g. tapeworms, hookworms, whipworms); parasitoides eventually kill their host (e.g. ichneumon wasps). Specializations for parassitism are usually very advanced, and they include:
- very small body size;
- simplification of the body, especially the digestive system and sense organs;
- loss of limbs, but development of adhesive organs (hooks, suckers, glue glands, etc.);
- increased absorption of chemicals through the skin (in endoparasites);
- buccal organs to perforate the host's skin (in ectoparasites);
- high reproductive rate, common asexual reproduction;
- larval stages very different from the adult to migrate between host bodies.
A commensalistic relationship is beneficial for a species but has a negligible effect on the other one. In the strictest sense, it involves taking food from other organisms' waste, such as scavengers or the many species (rats, gulls, raccoons) that feed on trash in human cities. In a wider sense, herons and other birds follow the herds of large ungulates (or the swarms of army ants) to eat the burrowing insects and worms disturbed from their pass.
Other forms of commensalism do not involve feeding. In phoresy, a very small organism is carried by a larger one: mites, millipedes and pseudoscorpions on terrestrial animals, while remoras cling on to sharks; epizoochory is phoresy applied to plant seeds. In inquilinism, the smallest organism lives inside the space created by the larger one: for example, vines and ivy on trees, lizards inhabiting anthills or the several invertebrates that live in the cavities of pitcher plants. Finally, metabiosis is a more indirect relationship where an organism makes the environment more suitable for another, for example necrophages clearing it from carrions, or mollusks producing the shells used by hermit crabs.
A mutualistic relationship is beneficial for both species involved. It can be classified according to the kind of exchanged benefits:
- Resource-resource relationships are the most common. The most important examples include lichens (associations of algae and fungi, where the alga produces sugars through photosynthesis for the fungus, and the fungus provides support, water and minerals to the alga), mycorrhizae (fungi associated to plant roots, to which they provide phosphates and nitrates in exchange for sugars) and rhizobia (bacterias living in the roots of legume plants, extracting nitrogen for the atmosphere).
- Service-resource relationships are also relatively common. The most obvious example is zoophilous pollination, where an animal feeds on the nectar of a flower while carrying its pollen; in endozoochory, an animal eats fruit to disperse its seed; aphids secrete a sugar-rich fluid (honeydew) for the ants that protect them from predators. Phagophiles, such as oxpeckers and cleaner fish, eat leftover food, dead tissue, excess secretions and parasites from another organism. Domestication can be considered such a relationship, as the domesticated species provides food or other resources while being fed and protected from predators.
- Service-service relationships are the most rare kind. Sea anemones and clownfish protect each other from predators, but the clownfish also provides a resource (ammonia for the symbiotic algae living within the anemone). Pseudomyrmex and Crematogaster ants live within the bulging, hollow thorns of some acacia species, and protect them by attacking the herbivores, or destroying competitor plants.
Ecosystems and biomes
Also see: Biomes
- ↑ Symbiosis is commonly used as a synonim, but it applies only to mutualistic relationship that are necessary to the survival of the organisms. For some, commensalistic or parasitic relationships that are equally necessary to the survival of the profiting organism should be also considered symbiosis.