Ecology PDF

# Characters Used in Community Structure It is pointed out above that each community is characterised by its species diversity, growth forms and structure, dominance, successional trend etc. To study the details of these aspects of any community there are taken into consideration a number of charac...

# Characters Used in Community Structure It is pointed out above that each community is characterised by its species diversity, growth forms and structure, dominance, successional trend etc. To study the details of these aspects of any community there are taken into consideration a number of characters (parameters). These are then used to express the characteristics of a community. For instance, if we say that this particular species is dominant in the community, what may be the possible characters on the basis of which such a conclusion can be drawn. These various characters used for such purposes are broadly classified into two major categories: 1. **Analytical characters**. These are generally expressed in terms of 5-point scale. 2. **Synthetic characters**. These are actually computed from analytical characters. ## Analytical characters These are of two types. (1) quantitative, which are expressed in quantitative terms, and (2) qualitative, which are expressed only in qualitative way although finally grouped under 5-point scale. ### Quantitative characters These include such characters as frequency, density, abundance, cover and basal area etc. #### Frequency Various species of the community are recorded by different phytosociological methods, by taking any sampling unit like quadrat, the transect and point. Details of these methods will be described later. Frequency is the number of sampling units (as %) in which a particular species occurs. Thus frequency of each species is calculated as follows: $Frequency (%) = \frac{No. of sampling units in which \newline the species occurred}{Total no. of sampling units studied} \times 100$ After determining the percentage frequency of each species, various species are distributed among Raunkiaer's (1934) five frequency classes depending upon their frequency values as follows: | Frequency % | Frequency class | |---|---| | 0-20 | A | | 21-40 | B | | 41-60 | C | | 61-80 | D | | 81-100 | E | #### Density Frequency does not give correct idea of the distribution of any species, unless it is correlated with other characters, such as density etc. Density represents the numerical strength of a species in the community. The number of individuals of the species in any unit area is its density. Density gives an idea of degree of competition. It is calculated as follows: $Density = \frac{Total no. of individuals of the species \newline in all the sampling units}{Total no. of sampling units studied}$ The value thus obtained is then expressed as number of individuals per unit area. #### Abundance This is the number of individuals of any species per sampling unit of occurrence. It is calculated as follows: $Abundance = \frac{Total no. of individuals of the species\newline in all the sampling units}{No. of sampling units in which \newline the species occurred}$ But, abundance thus obtained in quantitative terms gives little idea of the distribution of the species. #### Cover and basal area Quantitative values of frequency, density etc. do not give correct information on the distribution of a species. Since, different species differ in their growth forms, such values are to be supplemented with other characters like cover and basal area. Cover, although sometimes used in general sense for the area occupied by a plant, (which may be the herbage cover or the cover or basal area), it is generally used for aboveground parts. Thus cover or herbage cover signifies primarily the area of ground occupied by the aboveground parts of plants, such as leaves, stems and inflorescence as viewed from above. It is a good measure of the herbage availability. It is estimated by chart-quadrat, line intercept or point frame. Basal area refers to the ground actually penetrated by the stems and is readily seen when the leaves and stems are clipped at the ground surface. It is one of the chief characteristics to determine dominance. It is measured either 2.5 cm above ground or actually on the ground level. It is measured by callipers, line-interception or point-centered quadrat method. Since plants differ in their growth form the relationship between herbage cover and basal area differs in different types of plants as shown in Figure 5. Based on their cover values, species are grouped into six groups as follows: Group 1 (less than 5%), 2 (5-25), 3 (25-50%), 4 (50-75%), 5(75-95%) and 6 (95-100%). Braun-Blanquet (1932) included only five groups. #### Dominance Infact, dominance is a synthetic character. However, Daubenmire (1959) considered it as a analytical character and expressed its six types. Infact the number of organisms sometimes may not give correct idea of the species. If we base our conclusion on number, a single or few trees in a grassland, or few grasses in a forest should be of little value. But if we consider the species on the basis of area occupied or weight (biomass), the situation may be different. ## Qualitative characters These include physiognomy, phenology, stratification, abundance, sociability or gregariousness, vitality and vigour, life form (growth form) etc., most of which are described, and may be grouped in point scales. ### Physiognomy This is the general appearance of vegetation as determined by the growth form of dominant species. Such a characteristic appearance can be expressed by single term. For example, a simple look to such a community where larger trees are dominant with some shrubs, would tell that is a forest. Similarly on the basis of appearance it may be a grassland, desert etc. ### Phenology It also includes periodicity, aspection etc. Phenology is the scientific study of seasonal change i.e. the periodic phenomena of organisms in relation to their climate. Different species have different periods of seed germination, vegetative growth, flowering and fruiting, leaf fall, seed and fruit dispersal etc. Such data for individual species are recorded. A study of the date and time of these events is phenology. In other words, phenology is the calendar of events in the life history of the plant. These events are shown by phenograms. Phenograms of some Indian grasses and sedges are shown in Figure 4. Thus phenology of Thus in dominance, cover is included as important character. The single tree in grassland may occupy fairly a large area and may have much mass. ### Sociability It denotes the proximity of plants to one another. Plants generally grow as isolated individuals, in patches, colonies or groups. Plants of some species grow better when nearer to each other and produce thick population. Others become weak or die in such an association. Fruit and seed dispersal etc. and nature of reproduction are affected by the way in which they are aggregated. Species with same density, may differ in sociability values. Thus sociability expresses the degree of association between species. Some divide the sociability into as many as ten classes, but Braun-Blanquet (1932) used only five sociability groups as follows: - $S_{1}$ - Plants (stems) found quite separately from each other, thus growing singly. - $S_{2}$ - A group of 4-6 plants at one place - $S_{3}$ - Many smaller scattered groups at one place - $S_{4}$ - Several bigger groups of many plants at one place - $S_{5}$ - A large group occupying larger area. ### Vitality This is the capacity of normal growth and reproduction, which are important for successful survival of a species. The vitality depends upon weight of species. In plants, stem height, root length, leaf area, leaf number, number and weight of flowers, fruits, seeds etc. determine the vitality. On this basis plants are grouped into different vitality groups. Misra and Puri (1954) gave three groups as (i) well developed plants completing their life cycles regularly, producing viable seeds, (ii) plants reproducing vegetatively and (iii) ephemerals, short lived. Braun-Blanquet (1932) gave four groups. Daubenmire ((1968) gave vitality groups as follows : - $V_{1}$ - Plants whose seedlings die - $V_{2}$ - Seedlings grow, but unable to reproduce - $V_{3}$ - Reproduce only vegetatively - $V_{4}$ - Reproduce sexually, but uncommon - $V_{5}$ - Reproduce sexually and grow regularly. ### Life-form (growth form) In past, ecologists used the general appearance of plant communities (as determined by the stature of the plant species, their spread and characters of life-form), as the basis of the study of communities. These methods are known as physiognomic methods. Of these, Raunkiaer's (1934) life-form method has been commonly employed. A life-form is "the sum of the adaptation of the plant to climate". He considered that the way in which different species overcome the adverse environmental conditions determines their limits of distribution. Thus the plants' climate can be expressed by the statistical distribution of life-forms in the flora of a particular region. On the basis of the position of perennating buds on plants and the degree of their protection during adverse conditions, Raunkiaer classified plants into five broad life-form categories which are shown in Figure 6. These are as follows: - **(i) Phanerophytes**. Their buds are naked or covered with scale, and are situated high up on the plant. These life-forms include trees, shrubs and climbers generally common in tropical climates. Depending upon the size, they may be further subdivided into megaphanerophytes (over 30 meters high), mesophanerophytes (8-30 meters high), micro-phanerophytes (2-8 meters high), and nano-phanerophytes (under 2 meters). Epiphytes are either included in phanerophytes, or sometimes included under a separate life-form. - **(ii) Chamaephytes**. Their buds are situated close to the ground surface. They are common at high altitudes. Example-Trifolium repens. - **(iii) Hemicryptophytes**. These are mostly found in cold temperate zone. Their buds are hidden under soil surface, protected by the soil itself. Their shoots generally die each year. Examples-most of the biennial and perennial herbs. - **(iv) Cryptophytes or Geophytes**. Their buds are completely hidden in the soil, as bulbs and rhizomes. Most of them are found in arid zones. Hydrophytes are the cryptophytes whose buds are found below the water surface. - **(v) Therophytes**. These are seasonal plants, completing their life cycle in a single favourable season, and remain dormant throughout the rest unfavourable period of year in the form of seeds. They are common in deserts. Braun-Blanquet (1951), on the basis of Raunkiaer's life-forms, divided plants into as many as ten categories. ## Synthetic characters These are determined after computing the data on the quantitative and qualitative characters of the community. For comparing the vegetation of different areas, community comparison needs the calculation of their synthetic characters. These are determined in terms of: 1. **Presence and constance**. It expresses the extent of occurrence of the individuals of a particular species in the community i.e. how uniformly a species occurs in a number of stands of the same type of community. The species on the basis of its percentage frequency may belong to any of the following five presence classes: - **(a) Rare**-present in 1 to 20% of the sampling units - **(b) Seldom present**-present in 21-40% of the sampling units - **(c) Often present**-present in 41-60% of the sampling units - **(d) Mostly present**-present in 61-80% of the sampling units - **(e) Constantly present**-present in 81-100% of the sampling units. 2. **Fidelity**. This is the degree with which a species is restricted in distribution to one kind of community. Such species are sometimes known as indicators. The species have been grouped into five fidelity classes as follows: - **Fidelity 1** - Plants appearing accidentally. - **Fidelity 2** - Indifferent plants, may occur in any community.