Individuality – not as obvious as you think
Population – the unit of ecology and evolution
Communities and ecosystems as assemblages of populations
Larger-scale ecological patterns
Like any other field of science, ecology has its own language and its own way of using everyday English words in specific senses. It is essential that you develop a fluency in ecological language, and also to appreciate how the terminology of ecology relates to that used in evolutionary biology and other fields of study.
We find the concept
of an individual to be very obvious among humans. Even when confronted with a
set of identical twins, we automatically presume that each is a separate and
complete individual (though urban myths abound concerning the supposed special
psychic connections twins possess!). A set of conjoined twins may give us
pause, but even then we usually think they would be better off separated to
“live their own lives”. Individuality is the basis of legal responsibility, of
property rights, indeed of personhood itself. Yet this kind of individuality is
far from universal.
Many organisms exist
in a more colonial or interconnected state, as conjoined clone-copies. Consider
mosses, or corals, or mushrooms – large amounts of genetically identical tissue
may be connected “behind the scenes” by filaments or other living links, though
the tissues we usually observe may appear to be formed into distinct bodies. In
such organisms, a significant mode of reproduction may be simple fragmentation,
the physical breaking-apart of the larger body into separate pieces, each of
which may go on to become a linked-up superorganism in its own right. An individual thus must be
thought of as a genetic unit, physically distinct (separate from) other such
units. Being aware of the surprising complexity of individuality is a
useful starting point when considering larger-scale organization of organisms
into sets.
Individuals, whether
they are discrete like a human being or linked as colonies like a string of
strawberry-plants, may be grouped meaningfully into the larger collections
called populations. A population
is a set of organisms of the same kind, that is genetically similar
enough to be capable of interbreeding with each other, found occupying a
single locale. It is relatively easy to make a definition of this style,
but often difficult to be sure how to apply it in practice. Where can a line be
drawn around a locale? Are individuals at opposite ends of this area really
capable of interbreeding, compared to nearer-neighbour individuals? And how can
organisms in one population be reliably distinguished from those in adjacent
populations?
These questions about
the nature and boundaries of populations lead directly to a consideration of
why the term is useful. A population of a given sort of organism is the
self-sustaining unit that enters into interactions with other populations in a
community (see below), so ecologically
speaking a population is meaningful. If the interactions in which it is
involved are not stable, either the population will disappear, or it will cause
other populations to do so, over ecological timescales. Evolutionarily, a
population composed of genetically variant individuals is the crucible of
change. Under prevailing environmental conditions, some variants will do better
than others, and the changing composition of a population as differential
success accumulates is actually evolution.
The locale occupied
by a population (and, by extension, an entire set or community of populations,
see below), a site enjoying certain consistent abiotic features like
temperature, moisture, seasonality, soil type, and so forth, is usually
referred to as a habitat.
Habitats can exist independently of the organisms that occupy them (a pond
poisoned to remove all organisms in it is still a pond habitat), but the
organisms will occupy a site only if appropriate habitat conditions are met.
The habitat with its organisms is an ecosystem (see below). A habitat type (for
example, a pond) can be occupied by different communities in different sites
(pond species in North America differ from those in Africa and in Asia by
having at least partially different sets of species), though the appearance of
the complete ecosystem will be similar.
Unfortunately, it is
common parlance to use the word “species” in two different ways: to describe a
distinct type of organism (more or less equivalent to using the old-fashioned
“kind”), and to encompass the full range of population-variations which may
still permit interbreeding. This latter usage is technically the proper one: a species is the combined
membership of all populations of a given sort of organism, irrespective of
their geographic distribution, as long as all are potentially interfertile.
Stated this way, the species is much more an evolutionary than an ecological
concept, and as far as ecology is concerned we need not be too concerned about
species – in a community, there will be only one population of a species
present, and even if the species exists as a single population rather than
many, it will interact within the community just as any other population would.
Further discussion of species concepts can be found here.
No population can
live without the presence of other populations. Consumer organisms cannot exist
without the plant or animal populations they regard as prey, plants can exist
only if microbes have produced nutrients they can use, eaters of dead things
live only in places where members of living populations die. Usually there are
recognizably distinct sets of organisms which tend to co-occur, and which
regularly make their living by interacting with each other. Such a set of
populations, living together, is referred to as a community, and the community along
with its abiotic environment is referred to as an ecosystem. (Since the organisms
won’t be there without the habitat, the terms can be considered functionally
equivalent.)
A community is
characterized by fitness-influencing interaction among populations present:
organisms in a community feel competitive impacts from other organisms within
the community, and not from organisms outside the community, and similarly
predators feed mainly on prey organisms they find within the community and less
so on any outside it. Membership in a community implies that one’s fitness is
determined mainly by other community-members. There is considerable dispute about
how obligatory community membership is – organisms may be interacting
not so much because they need one another, but rather because they simply share
similar habitat requirements. Thus not all interactions in a community will be
strong, and communities may not be highly coherent, or contain species not also
found elsewhere. This makes the boundaries of communities more diffuse than we
might wish!
Going “up” one level from
communities/ecosystems, a term you often see is biome; for example, there are several
different types of forest ecosystem on the west coast of North America,
dominated in different sites by hemlock, by cedar, by redwood, and so forth,
but all are sufficiently similar in other ways for them to be considered a
single biome, or set of ecosystems.
Another term, used
mainly in biogeography, describes an area which may contain a large number of
distinct ecosystems and/or biomes, but distinct from other large areas – this
term is the region,
sometimes also referred to as the realm. For example, North America contains
forest biomes, grasslands, deserts, and tundra, and so does Asia, but the set
of organisms present in a North American desert differs from what you would
find in an Asian desert, mainly because of biogeographic isolation of the
continents for long periods of their history.
“Above” the biome and
region levels, all ecological systems are united in the biosphere, the totality
of living things and their supportive environments across the globe.