organisms in time and space ecology

Organisms In Time and Space: Ecology

Ecology

BIO101 - Bora Zivkovic - Lecture 3 - Part 2

Ecology is the study of relationships of organisms with one another

and their environment. Organisms are organized in populations,

communities, ecosystems, biomes and the biosphere.

A population of organisms is a sum of all individuals of a single

species living in one area at one time.

Individuals in a population can occupy space in three basic patterns:

clumped spacing, random spacing and uniform spacing.

Metapopulations are collections of populations of the same species

spread over a greater geographic area. There is some migration (ths

gene-flow) between populations. Larger populations are sources and

smaller populations are sinks of individuals within a metapopulation.

Population size is determined by four general factors: natality,

mortality, immigration and emigration.

Natality depends on a number of factors: the proportion of the

population that are at a reproductive age (as opposed to

pre-reproductive and post-reproductive), proportion of the

reproductively mature individuals that get to reproduce, sex-ratio of

the reproductives, the mating system, the fertility of individuals

(sometimes affected by parasites), the fecundity (number of offspring

per female), the maturation rate (the amount of time needed for an

individual to attaint sexual maturity), and longevity (amount of time

an individual can live after reproducing).

Mortality is affected by bad weather, predation, parasitism and

infectious diseases. It depends on the mortality of pre-reproductive

stages (from eggs and embryos, through larva and juveniles), mortality

of reproductive stages, and mortality of post-reproductive stages

(often from disease or aging).

A population can, theoretically, grow exponentially indefinitely.

However, in the real world, the growth is limited by the amount of

space, food (energy) and predators. Thus, the population size often

plateaus at an optimal number - the carrying capacity of that

population.

Some organisms produce a large number of progeny, most of which do not

make it to maturity. This is r-strategy. The population size of such

species often fluctuates in boom-and-bust patterns.

Other organisms produce a small number of progeny and make a heavy

investment into parenting and protecting each offspring, This is

K-strategy. The population size of such species grows more slowly and

tends to stabilize around the carrying capacity.

All populations show small year-to-year fluctuations of population

sizes around the optimum number. Some species, however, exhibit

regular oscillations in population sizes. Such oscillations often

involve populations of two different species, usually a predator and

its prey, the most famous example being that of the snowshoe hare and

the lynx.

Correct prediction of future changes in a population size is essential

for the assessment of the populations viability and for its

protection.

A biological community is a collection of all individuals of all

species in a particular area. Those species interact with each other

in various ways, and have evolved adaptations to life in each others'

presence.

Niche is a term that describes a life-role, or job-description, or one

species' position in the community. An example may be a large

herbivore, a nocturnal burrowing seed-eater, a seasonal fruit-eater,

etc.

Within one community only one species can occupy any particular niche.

If two species share some of their niche, they are in competition with

each other. If two species occupy an identical niche, they cannot

coexist - one of the species will be forced to move out or go extinct.

If two species compete for the same resource (food, territory, etc.),

one will utilize the resource better than the other. Competitive

exclusion is a process in which one species drives another species out

of the community.

Complete exclusion is not inevitable. The competition between two

species can be reduced by natural selection, i.e., one of the species

will be forced to assume a slightly different niche. For instant, two

species can geographically partition the territory, e.g., one living

at higher altitude than the other on the same mountain-side. Two

species can also temporally partition the niches, for instance one

remaining active at night and the other becoming active during the

day.

Predation is one of the most important interaction between species in

a community. Predation often causes evolutionary arms-races between

predators and prey. For instance, by killing the slowest zebras, lions

select for greater speed in zebras. Greater speed in zebras selects

for greater speed in lions.

The most interesting examples of evolutionary arms-races between pairs

of enemies are those in which the prey is dangerous to the predator,

often by being toxic or venomous. For example, garter snakes and tiger

salamanders on the West coast are involved in one such arms-race. Prey

- the salamander - secrete tetrodotoxin from its skin. This toxin

paralyzes the snake. Locally, some snakes have evolved an ability to

tolerate the toxin, but the side-effect of such evolution is that

these snakes are slow and sluggish - themselves more vulnerable to

predation by birds.

Ground squirrels (prey) in the Western deserts have evolved immunity

to rattlesnake venom, so the rattlesnakes (predators) are becoming

more venomous. Similarly, and in the same area, desert mice have

evolved immunity to the toxin of their prey - the scorpions, resulting

in increasing toxicity of the scorpion venom in that region (but not

in areas where these two species do not overlap). A Death's-head

sphynx moth steals honey from beehives and has evolved partial

immunity to honey-bee venom.

Many plants have evolved thorns or toxic chemicals to ward off their

enemies - the herbivores. Monarch butterflies are capable of feeding

on milkweed despite this plant's toxic content. Moreover, the Monarchs

store the noxious chemical they extracted from milkweed and that

chemical makes the butterflies distasteful to their own predators.

The shape and color of the prey often evolves to protect from

predation. Warning coloration, usually in very bright colors, informs

the predators that the prey is dangerous. Aposomatic coloration is one

commonly found kind of warning coloration - the black and yellow

stripes on the bodies of many bees and wasps are almost a universal

code for dangerous venomous stings.

Cryptic coloration, or camouflage, on the other hand, allows an animal

to blend in with its surroundings. Many insect look like twigs, leaves

or flowers, effectively hiding them from the eyes of predators. Some

animals have evolved behavioral color-change, e.g., chameleons, some

species of cuttlefish and the flounder.

Batesian mimicry is a phenomenon in which non-toxic species evolve to

resemble a toxic species. Thus, some butterflies look very similar to

Monarch butterflies and some defenseless flies and ants have

aposomatic coloration.

Mullerian mimicry is a phenomenon in which two or more dangerous

species evolve to look alike. This is "safety in numbers" strategy as

a predator who tastes and spits out one of them, will learn to avoid

all of them in the future.

Co-evolution does not occur only between enemies. It can also occur

between species that positively affect each other. The best example is

co-evolution of flowers and insect pollinators.

Symbiosis is a relationship between organisms that are not direct

enemies (e.g,. predator and prey) to each other. Commensalism,

mutualism and parasitism are forms of symbiosis.

In commensalism, one partner benefits, while the other one is not

affected at all. For instance, birds building nests in a tree do not

in any way affect the fitness of the tree.

Mutualism benefits both partners. The best known examples are lichens,

mycorrhizae, and legumes. Birds that clean the skin or teeth of

crocodiles, hippos or rhinos are protected by their hosts.

Parasitism is detrimental to one of the partners. Parasites that are

too dangerous, i.e., those that kill their host, are not successful

since they also die without leaving offspring. Thus, parasites evolve

to be minimally harmful to their hosts. The same logic goes for

infectious agents - the disease should help propagate the

microorganism (e.g, by causing sneezing, diarrhea, etc.) without

killing the host.

The organisms that make up ecosystems change over time as the physical

and biological structure of the ecosystem changes. Right now, one of

the effects of global warming is that some species migrate and others

do not. Thus, old ecosystems break down and new ones are formed. The

ecosystems are in a process of remodelling. During that process, many

species are expected to go extinct.

When an ecosystem is disturbed to some extent, but not completely

eradicated, the remodelling process that follows is called primary

succession.

When an ecosystem is completely wiped out (e.g,. a volcanic eruption

on an island), secondary succession occurs, with a predictable order

in which species can recolonize the space. One species prepares the

ground (quite literally) for the next one. The process may start with

bacteria, lichens and molds, continuing with mosses, fungi, ferns and

some insects, etc, finally ending with trees, birds and large mammals.

The final structure of the ecosystem is quite stable over time - this

is a mature ecosystem.

Read:

Peter H. Raven, George B. Johnson, Jonathan B. Losos, and Susan R.

Singer, Biology (7th edition), McGraw-Hill Co. NY, Chapters 53-57.

Previously in this series:

Biology and the Scientific Method

Cell Structure

Protein Synthesis: Transcription and Translation

Cell-Cell Interactions

Cell Division and DNA Replication

Cell Differentiation and Embryonic Development

Genotype and Phenotype

Evolution

Behavior

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posted by coturnix at 11:22 PM

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