Quiz 18 :
Primary Production and Energy Flow
Forests are habitats in which trees are the most dominant form of vegetation. Forest structure refers to the distribution of vegetation both horizontally and vertically across a given area. Forest structure is determined by several physical attributes of forest vegetation. Quantifying forest structure is important to relate forest structure to specific forest functions. Forest processes refer to the various ecological processes including the following: • Movement and persistence of particular species • The susceptibility and prevalence of disturbances such as forest fires, pest outbreaks, • Redistribution of matter and nutrients Ecologists study both structure and processes in different ecosystems. A population refers to a set of individuals that belong to the same species and that live in the same geographical area. Population ecologists are concerned with the study of factors that affect populations and how and why populations change over time. They study forests from the point of view of structure and dynamics of forest populations. Some of the properties of populations studied by population ecologists include population size, population density, patterns of dispersion, population growth, and so on. A community is a large collection of two or more populations of different species living in the same geographical area. Community ecologists study interactions between species in different communities on various spatial and temporal scales. These include abundance of organisms, their distribution, structure of community, and relationships between co-existing populations. The major focus of community ecologists is on the genotypic and phenotypic characteristics of the populations involved in a community. They also study patterns in species richness, productivity, and food web structure. Ecosystem ecologists study the interactions between organisms and their environment in a given ecosystem. The major focus of ecosystem ecologists is on functional processes or ecological mechanisms which help to maintain the structure and function of an ecosystem as a whole. These processes include primary production, decomposition, energy flow, and tropic interactions.
The t -test is a statistical test that assesses; whether, the means of two groups are statistically the same or different from each other. In the given example, two populations of the diatom feeding caddisfly, Neothremma alicia have been compared with respect to the biomass per unit area in flooded and unflooded streams. Consider the hypothesis to be "means of the two populations are the same". Use the t -test to test this hypothesis. If the calculated value of t is less than the critical value of t , accept the hypothesis. If the calculated value is greater than the critical value, reject the hypothesis. While comparing the calculated value of t with the critical value, consider the level of significance, P. In the example, P 0.05 the level of significance, and the calculated value of t (3.480) is greater than the critical value of t (2.12). Hence, reject the hypothesis. The meaning of P 0.05 is that 'the probability of the two means are the same, is less than 0.05'. Consider P 0.01, the critical t value would have changed to 2.92. Thus, for P 0.01, critical value of t is less than calculated value of t. Hence, we would reject the hypothesis. P 0.01 mean that the probability of the two means are same, which is less than 0.01. Thus, this does not change the earlier statistical comparison.
a. The monthly net primary production (NPP) of the given forests is calculated by dividing the annual net primary production value by the length of the growing season. Thus, the missing data in the table can filled as follows: The monthly NPP represented here is the monthly productivity obtained during the growing season of the forest. b. The annual NPP in the high latitude forests corresponding to the boreal forests is the lowest among the forests at the other two latitudes. However, the short-term or the monthly NPP for boreal forests is the highest among the three forests. This is largely because the growing season in these forests lasts for only for a period of 3 months. It is during this period that most of the annual net primary production is achieved. The total annual NPP in the middle latitude forests (temperate forests) is more than that of the boreal forests and less than the tropical forests. However, the monthly NPP in the growing season is greater than that of the tropical forests because the length of growing season in these forests lasts for a period of six months. In case of the tropical forests, the annual NPP is the highest among the three forests, yet the short-term (monthly) NPP is the lowest. This is because, as the growing season in these forests lasts throughout the year, the total productivity is evenly distributed over 12 months. c. Looking at the higher values of short-term productivity of the high and middle latitude forests as compared to the lower values of short term productivity of the tropical forests, one may tend to think that the high latitude forests are more productive. However, this is not true because the short term productivity values have been calculated on the basis of the length of growing season in these forests. Since the tropical forests have the longest growing season, their net annual productivity gets divided by 12 to yield the short term productivity. As a result, it has a lower value. Hence, while comparing productivity of forests, it would be more appropriate to consider the annual NPP values rather than the short-term NPP values.