Concept of r-selected and K-selected Organisms
Organisms adapted to survive in unstable environments are referred to as r-selected. r-selected
organisms’ live in settings where population levels are well below the maximum number that the
environment can support i.e. the carrying capacity, there numbers are growing exponentially at the
maximum rate at which the population can increase if resources are unlimited. Organisms that are
r-selected tend to be small, short lived, and opportunistic, and to grow through irregular boomand-burst population cycles. Examples include insects, annual plants, bacteria, frogs and rats etc.
Species considered pests typically are r-selected organisms that are capable of rapid growth when
environmental conditions are favorable.
Organisms adapted to survive in stable environments are referred to as K-selected. This is because
they live in environments in which the number of individuals is at or near the environment's
carrying capacity (often abbreviated as K). K-selected species are typically larger, grow more
slowly, have fewer offspring and spend more time parenting them. Examples include large
mammals, birds, and long-lived plants such as redwood trees. K-selected species are more prone
to extinction than r-selected species because they mature later in life and have fewer offspring with
longer gestation times.
Fig 5: Typical characteristics of r-selected and k-selected organisms
Table 1: Reproduction in r-selected and K-selected species
Feature
Norway rat (r-selected) African elephant (K-selected)
Reaches sexual or reproductive maturity 3-4 months
10-12 years
Average gestation period
22-24 days
22 months
Time to weaning
3-4 weeks
48-108 months
Breeding interval (female)
Up to 7 times per year
Every 4 to 9 years
Offspring per litter
2-14 (average 8)
1 average, 2 high
Source: http://animaldiversity.ummz.umich.edu/site/index.html.
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Many organisms fall between these two extremes and have some characteristics of both types.
Ecosystems tend to be dominated by r-selected species in their early stages with the balance
gradually shifting toward K-selected species. In a growing population, survival and reproduction
rates will not stay constant over time. Eventually resource limitations will reduce one or both of
these variables. Populations grow fastest when they are near zero and the species is uncrowded. A
simple mathematical model of population growth implies that the maximum population growth
rate occurs when the population size (N) is at one-half of the environment's carrying capacity, K
(i.e., at N = K/2).
In theory, if a population is harvested at exactly its natural rate of growth, the population will not
change in size, and the harvest (yield) can be sustained at that level. In practice, however, it can
be very hard to estimate population sizes and growth rates in the wild accurately enough to achieve
this maximum sustainable yield.
Availability of resources, such as light, water, and nutrients, is a key control on growth and
reproduction. Some nutrients are used in specific ratios. For example, the ratio of nitrogen to
phosphorus in the organic tissues of algae is about 16 to 1, so if the available nitrogen concentration
is greater than 16 times the phosphorus concentration, then phosphorus will be the factor that limits
growth; if it is less, then nitrogen will be limiting. To understand how a specific ecosystem
functions, it thus is important to identify what factors limit ecosystem activity.
Resources influence ecosystem activity differently depending on whether they are essential,
substitutable, or complementary. Essential resources limit growth independently of other levels: if
the minimum quantity needed for growth is not available, then growth does not occur. In contrast,
if two resources are substitutable, then population growth is limited by an appropriately weighted
sum of the two resources in the environment. For example, glucose and fructose are substitutable
food sources for many types of bacteria. Resources may also be complementary, which means that
a small amount of one resource can substitute for a relatively large amount of another, or can be
complementary over a specific range of conditions.
Resource availability serves as a so-called "bottom-up" control on an ecosystem: the supply of
energy and nutrients influences ecosystem activities at higher trophic levels by affecting the
amount of energy that moves up the food chain. In some cases, ecosystems may be more strongly
influenced by so-called "top-down" controls namely, the abundance of organisms at high trophic
levels in the ecosystem. Both types of effects can be at work in an ecosystem at the same time, but
how far bottom-up effects extend in the food web, and the extent to which the effects of trophic
interactions at the top of the food web are felt through lower levels, vary over space and time and
with the structure of the ecosystem.
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Food Chains and Food Webs
The simplest way to describe the flux of energy through ecosystems is as a food chain in which
energy passes from one trophic level to the next, without factoring in more complex relationships
between individual species. Some very simple ecosystems may consist of a food chain with only
a few trophic levels.
In food chains, each organism is shown as feeding on only one other type of organism or otherwise
as a linear feeding relationship. Most organism feed on more than one other organism. Some feed
in both grazing and detrital food chains. Carnivores at the higher trophic level have highly varied
diets and operate as secondary, tertiary, quaternary and higher consumers. Some animals,
including humans, feed on organisms at all trophic levels; plants, animals and fungi (omnivores).
Grazing and detrital food chains interlink in a complex manner and the mesh of interlinking food
chains is called a food web.
Fig. 6: Food chains and Food webs
Drastic changes at the top of the food web can trigger trophic cascades or domino effects that are
felt through many lower trophic levels. The likelihood of a trophic cascade depends on the number
of trophic levels in the ecosystem and the extent to which predators reduce the abundance of a
trophic level to below their resource-limited carrying capacity. Some species are so important to
an entire ecosystem that they are referred to as keystone species, connoting that they occupy an
ecological niche that influences many other species. Removing or seriously impacting a keystone
species produces major impacts throughout the ecosystem.
Within ecosystems, different species interact in different ways. These interactions can have
positive, negative, or neutral impacts on the species involved (Table 2).
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Table 2: Relationships between individuals of different species.
Type of
interaction
Effect of interaction
Examples
Both species are harmed
Oak trees and maple trees competing for light in a
(population growth rates are
forest, wading birds foraging for food in a marsh
reduced).
Predation
One species benefits, one is Predation:
wolf
and
rabbit
Parasitism
harmed.
Parasitism: flea and wolf
Both
species
benefit.
Humans and house pets, insect pollination of
Mutualism
Relationship may not be
flowers
essential for either.
One species benefits, one is
Commensalism
Maggots decomposing a rotting carcass
not affected.
Allelopathy (plants that produce substances
One species harms another
harmful to other plants): rye and wheat suppress
(typically by releasing a
Amensalism
weeds when used as cover crops, broccoli residue
toxic substance), but is not
suppresses growth of other vegetables in the same
affected itself.
plant family
Competition
Ecological Pyramids
Elton (1920) prepared the first pyramids which were based on mere field observations of numbers
of animals in different sizes and classes. He observed that predators wee larger than their prey and
that their relationships were quite specialized. Lindeman (1940) adapted pyramids to trophic levels
irrespective of their sizes but discovered that it is more difficult to identify their trophic level.
Feeding relationships and the efficiency of energy transfer through the biotic component of
ecosystems have traditionally been summarized with pyramid diagrams. These give an apparently
simple and fundamental basis for comparing:


Different ecosystem
Seasonal variation within particular ecosystem
Change in an ecosystem
In summary, pyramids have been used according to the following: Pyramid of numbers: Based on counting of organisms at each trophic levels
 Pyramid of Biomass: Based on weight (Dry weight) of each organisms at each level
 Pyramid of energy: Based on energy contents of organisms at each level (The most important).
Pyramid of numbers
The organisms of a given area or habitat are first counted and then grouped into their trophic
levels. A progressive decrease in the number of organisms at each successive level is determined.
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A diagrammatic rectangular box is used to denote the proportion of each trophic organism. Three
important difficulties may arise:
 Producers vary greatly in size, but a single grass plant/algae is given the same status as a
single tree, therefore, a true pyramid shape is difficult to obtain, e.g. parasitic food chains
may also give inverted pyramids.
 Number range is so great that it becomes difficult to draw the pyramids to scale except
with logarithmic scales which often becomes difficult to interpret.
 Exact trophic level of an organism may be difficult to ascertain.
Pyramid of biomass
Pitfalls in the use of pyramid of numbers can be overcome by using pyramid of biomass in which
total mass of the organisms (biomass) is estimated for each trophic level. Each estimate involves
weighing representative individuals as well as recording numbers. Ideally dry masses are
compared. The rectangles used in constructing the pyramid represents the masses of each organism
at each trophic level per unit area/volume.
Standing biomass
The biomass at the time of sampling, at a given time is referred to as standing biomass or standing
crop biomass. This figure gives no indication of rate of production (productivity) or consumption
of biomass. This can be misleading in two ways: If the rate of consumption (loss through being used as food) more or less equals the ate of
production, the standing crop does not give any indication of productivity e.g. a fertile
intensely grazed pasture may heave a smaller standing crop of grass, but a higher
productivity, than a less fertile and ungrazed pasture.
 If the predators are small e.g. algae, they have a high turn-over rate (a high growth rate and
reproduction balanced by a high rate of consumption/death). Although size may be small
compared with trees, productivity may the same.
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Fig.
7:
Comparing the three Pyramids
Ecosystem as a Cybernetic System
Cybernetics concerns itself with control and communication in systems formed by living
organisms and their artifacts. Every system is a set of different elements or compartments or units,
any one of which exists in many different states, such that the selection of a state is influenced by
the state of other components of the system. Elements linked by reciprocal influences constitute a
feedback loop. The loop may be negative, or stabilizing, like the one formed by a heating unit and
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a thermostat or the mechanisms regulating sugar level in the blood. Or the loop may be positive
or disruptive, like the spread of an annihilating epidemic. Cybernetics can be found at different
levels of life, e.g. cellular level, and organismal level and at levels where the interacting elements
are individuals. Ecosystem is the study of systems at a level where interacting organisms may be
considered elements of interaction, either among themselves, or with a loosely organized
environmental matrix. A characteristic of negative feedback is that not only the entire system but
also some selected states of the system show a considerable persistence through time, e.g. A
predator – prey interaction.
 Human ecosystems are complex cybernetic systems that are increasingly being used by
ecological anthropologists and other scholars to examine the ecological aspects of human
communities in a way that integrates multiple factors as economics, socio-political
organization, psychological factors, and physical factors related to the environment.
•
Predator-prey interactions: each keeping the other in a sustainable state
•
A small hymenopteran parasite can keep the number of grasshoppers in check, thereby
maintaining the grass populations in a grassland ecosystem
•
The regulator has a minute proportion of the total biomass and represents < 1 % of the
energy flow through the system
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Density / Production
Diagrammatic Representation of 'r' and 'k' Strategies
70
60
50
40
30
20
10
0
+ve feedback
+ve and -ve
feedbacks
+ve feedback
1
2
3
4
5
6
7
8
9
10 11 12
Time
Reduction in
Biomass
Occurs For the following Reasons:
1. Not everything in the lower levels gets eaten.
2. Not everything that is eaten is digested.
3. Energy is always being lost as heat.
It is important to remember that the decrease in number is best detected in terms or biomass.
Numbers of organisms are unreliable in this case because of the great variation in the biomass of
individual organisms.
A generalization exists among ecologists that on average, about 10% of the energy available in
one trophic level will be passed on to the next; this is primarily due to the 3 reasons given above.
Therefore, it is also reasonable to assume that in terms of biomass, each trophic level will weigh
only about 10% of the level below it, and 10x as much as the level above it.
Ecosystem Homeostasis
Self-maintenance and self-regulation are the hallmark of all natural ecosystems. It has a tendency
to resist change and to remain in a system of equilibrium. Cybernetics helps the ecosystem in
achieving homeostasis by feeding back output information from the system to control future input.
A feedback in this control must be a negative feedback.
Homeostasis is the dynamic equilibrium among the living members of an ecosystem, and with
their ever-changing environmental conditions, such as wind, rainfall, nutrient availability, air
quality, and climate.
Homeostasis mechanisms chiefly concern:
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1. Matters cycling and energy flow,
2. Protection of production level,
3. System structure.
The homeostatic mechanisms operate within the biotic structures of an ecosystem which, in turn,
are fitted to the range of variability in the environmental factors of a given biotope.
A homeostatic condition within an ecosystem implies that all aspect of ecosystem function are in
balance. A balance must exist between production, consumption and decomposition, as well as
between species within the system. No species can continue to increase beyond a reasonable limit,
resources will place a check on growth and overcrowding. Environment also has a way to limit
recycling wastes. Many a population regulates its own densities (self-regulation). For instance,
phytoplanktons exerts positive feedback on zooplanktons and in turn, zooplanktons places a
negative feedback on phytoplanktons. Phytoplanktons are autotrophs with a high reproductive
potential; therefore zooplanktons maintain the outrageous growth pattern of zooplanktons.
An increase in water temperature in the springtime, increases metabolic rate and respiration of
aquatic plants and animals, it results in an increase in C02 and a decrease in O2. The higher level
of free CO2 and increasing H2O temperature stimulate more rapid photosynthesis and plant growth
which utilize CO2 and produce O2. Both O2 and CO2 tend to return to normal limits.
Failure to regulate these ecological indices may result in a breakdown otherwise referred to as
ecological catastrophe (ecocatastrophe). When ecocatastrophe occurs, a new structural and
functional balance develops by a complete replacement of one type of component organism.
Studies of biological systems proved that populations were controlled by homeostatic mechanisms
that maintain their genetic structure.
Two factors are responsible for the introduction of the concept of homeostasis in ecology:
1.
It is necessary for ecological models,
2.
A homeostasis system is based on the assumption of interrelated components of the
internal structure of the biological system.
Examples of Ecosystem Homeostasis
1. Attack Avoidance: In mammals and birds especially, members of the same species
spend a lot of time and energy harassing each other, chasing each other, fighting (but
not hurting each other much), and avoiding each other. This is called Attack-Avoidance
behavior.
2. Overcrowding: An entirely different homeostatic process arises when a population of
mammals explodes its numbers for some reasons.
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 In small consumers such as mice, rabbits and lemmings, over-crowding creates
an increasing stress that damages the thyroid gland, which manufactures
essential hormones.
 When population densities reach a critical point, the stress becomes extreme
and much of the population simply drops dead from endocrine (hormone)
system damage.
3. Prey Abundance: When the prey of predators such as owls is super-abundant, as at the
peak of lemming and hare cycles, the predator will have more young. The snowy owl,
for example, will lay up to eleven eggs at peak prey abundance where it normally lays
three. So the predators become as super-abundant as prey. If a niche opens in an
ecosystem, life fills it as quickly as possible. The prey population crashes. The
predators have little choice. They can travel, or starve.
4. Multiple Births: In some species, such as deer, crowding results in single births. When
the same species is not crowded, twins are the rule. Populations adjust themselves to
"under crowding" as well as to overcrowding. If there is a niche (a way to make a
living) available in an ecosystem, life will fill it.
5. Predation: This can also be seen as a homeostatic process of communities.
Predator/prey relationships have an odd element of cooperation in them. Predators tend
to kill the weakest members of the prey population, which include animals who are ill,
old, or very young. Predators are unlikely to kill the strongest and fastest prey, who
survive to pass along those traits to their offspring. Through natural selection, predation
shapes the prey species as much as it shapes the predator. In other words, deer run fast
because the slow ones were eaten. The conclusion is that predation helps regulate the
health and stability of the community. In natural communities, what is good for the
community is not always what is good for single lives.
6. Sustainability: Natural ecosystems are sustainable. That means that they can maintain
themselves with some integrity. It does not mean that they remain in a fixed state;
nothing can. Sustainable systems can and do change.
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