5.1, 5.2 - Ecosystems and distribution factors

Population, community, habitat and ecosystem

Ecologists choose the group of organisms and area they are studying before explaining a pattern.

For example, the individuals of one pond-snail species in a particular pond form a population. The snails in a different, isolated pond can be studied as a different population. The boundary and time matter: “all snails everywhere” does not identify one local population, and several snail species together are not one population.

A pond community includes its snail, plant, algal, fish, fungal and bacterial populations, including organisms too small to see unaided. It is more than the conspicuous animals. Water temperature and dissolved oxygen are conditions affecting the community; they are not populations within it.

A habitat is the place where an organism lives. A pond is a habitat, while the underside of a submerged stone can be a smaller habitat within it. Different species can share a habitat. As you saw when studying niches, sharing a place does not mean using the same resources or tolerating exactly the same conditions.

The pond ecosystem includes the community, water, mineral ions, light and other physical or chemical conditions, together with their interactions. For example, algae use light and dissolved substances to grow; snails consume algae; the organisms also alter their surroundings. An ecosystem can be studied at a small scale, such as one pond, or a much larger scale. It is not defined by a fixed size.

These terms answer different questions: population asks which one species, community asks which interacting populations, habitat asks where an organism lives, and ecosystem includes the community and its interactions with the non-living environment.

How many and where

The number of organisms and their locations describe different features of a habitat.

A species can be abundant in one small patch but absent from most of a habitat. Another can occur throughout the habitat, with only a few individuals in each place. Saying “there are many” does not describe where they are; saying “they occur near the stream” does not state how many there are. Comparisons of numbers need a stated area and time. A larger area may contain more individuals simply because more ground was examined.

Consider Willow Glade, an original fictional woodland-edge habitat used throughout this lesson. An ecologist examines one complete 10-square-metre area in each of three zones on the same June day. Every visible rooted individual of each herb species in those areas is counted. The two herb species, P and Q, are illustrative species, not names of real plants. The following values are constructed teaching data, not published field measurements.

P is most abundant in the examined canopy area and was not recorded in the examined open area. Q shows a different distribution, occurring in the edge and open areas but not the canopy area. These are statements about the recorded places and date. They do not establish that P is absent from every open part of the glade, or describe the pattern in winter. Nor does the table yet tell us which factor produced either pattern.

The equal examined areas make the local counts directly comparable. Estimating the whole population from samples, choosing quadrats or transects, and measuring habitat factors are developed in the next lesson; a count from one area is not automatically the population size of the whole habitat.

Abiotic conditions and tolerance limits

Physical and chemical conditions affect the processes that allow an organism to survive, grow and reproduce.

Abiotic factors are non-living physical or chemical features of the environment. Their effects need a mechanism, rather than a list of names. For a green plant, reduced light can reduce photosynthesis, leaving less organic material for growth and reproduction. Low soil moisture can reduce water uptake; loss of turgor and stomatal closure can reduce growth and carbon dioxide entry. Fewer seedlings may establish, or existing plants may die, reducing local abundance and restricting the places where the species persists.

Other abiotic factors act through different processes:

An organism has a tolerance range for an environmental factor: values within which it can function sufficiently to survive. Growth and successful reproduction may require a narrower range. Near a limit, individuals may remain alive but produce few surviving offspring. A long-lived adult's presence therefore does not by itself show that a self-sustaining population can establish there. Limits can also differ between life stages, such as seedlings and mature plants.

One favourable condition does not remove every other limit. Extra light may increase photosynthesis when water and other requirements are sufficient. In very dry soil, however, stomatal closure and water shortage may prevent the expected growth increase. In Willow Glade, P's absence from the open area could be consistent with moisture intolerance, but P might instead be affected by competitors or another unmeasured factor. Its actual tolerances must be investigated rather than inferred from its name or one observation.

A bright but dry site therefore need not support faster growth than a shaded, moist one.