Chapter 07 COMMUNITY STRUCTURE.docx

1、Chapter 07 COMMUNITY STRUCTUREChapter 07 COMMUNITY STRUCTURE （群落结构）THE COMMUNITY DEFINED （定义）An ecological community is a system made of species populations interacting; they are bound by interactions.Plant and animal populations living together and interacting directly or indirectly form a communit

2、y. Different species populations. In the same area. Interacting in spatial and trophic relationships. Characterized by dominant species.Botanists use the term associations for plant communities possessing a definite species composition.Communities may be recognized as autotrophic or heterotrophic. A

3、utotrophic communities use energy from the sun or the center of the earth to synthesize carbohydrates. Heterotrophic communities consume more food than they make, and are dependent on imports of food from the surrounding area.PHYSICAL STRUCTURE （物理结构）A community can be described by analyzing differe

4、nt aspects of its structure or organization:Species composition: the species that form part of the community and represent a subset of the species of the region. There are patterns of relative abundance among the species.Physiognomy: stratification and spatial pattern of the species.Temporal: the da

5、ily and seasonal cycles of activity.Trophic: the patterns of energy transfer involving food chains and trophic levels.Guilds and niches: different species perform different functions in the same habitat; some species utilize the habitat and resources in a similar manner.Why is the community made of

6、these species and not others?Community structure is related to coactions, the life history of its constituent species and the physical environment.Raunkier recognized five life forms based on the position of the overwintering buds. This system provides a standard mean of describing the structure of

7、a community VERTICAL STRATIFICATION （垂直结构）Communities usually have a noticeable vertical structure.On land, plants determine the vertical structure of the community. In this structure, other forms of life are distributed and adapted to live.This vertical distribution has an effect on the amount of l

8、ight that penetrates to lower layers.The gradient of light affects the vertical distribution of plants and indirectly of animals.Layers or strata, e.g. trees form the canopy, smaller trees form the understory, shrubs layer, herb layer, forest floor, and subterranean layer.Stratification is applicabl

9、e to underground ecosystems as well as aquatic ecosystems.The canopy is principal layer for photosynthesis. The structure of the canopy (dense or open) determines how much light penetrates to the lower layers. The canopy has a major influence on the rest of the community. Understory trees most be ab

10、le to tolerate shade including the young of canopy trees. Species that are unable to tolerate shade will die; others will eventually grow and reach the canopy after some of the older trees die.The nature of the herb layer will depend on the density of the canopy and understory, topography, soil cond

11、itions, etc. The process of decomposition takes place in the forest floor. Nutrients are released here and reused by plants. Aquatic ecosystems have a layering determined by light penetration, and profiles of oxygen and temperature. There are usually three layers recognized in lakes of certain depth

12、 depending on light. The upper layer, the photic zone, is where most photosynthesis occurs and is dominated by phytoplankton; this is called the epilimnion. The metalimnion is characterized by the thermocline, a sharp drop in temperature The lower layer, the benthic zone or hypolimnion, is where mos

13、t decomposition takes place; it is a cold zone about 4C, usually poor in oxygen. Communities, whether terrestrial or aquatic, have similar biological structure. They possess an autotrophic layer, which fixes the energy of the sun. The community also posses a heterotrophic layer that utilizes the foo

14、d stored by autotrophs, transfer the energy and circulates the nutrients.Communities also have a characteristic horizontal pattern of dispersion.Each vertical layer in the community is inhabited by characteristic organisms. There is considerable interchange between the layers. In general, the greate

15、r the vertical stratification of a community, the more diverse its animal life.In general, the more complex the vertical stratification of the community, the more diverse the animal life.HORIZONTAL STRUCTURE （水平结构）Terrestrial and aquatic communities may also exhibit a horizontal distribution of spec

16、ies produced by variation in environmental conditions, resulting in a patchy distribution of species.Walking across the land, patches are noticed: grassland, forests, old abandoned fields, etc. A patchy environment in turn influences the distributional pattern of animal life across the landscape. BI

17、OLOGICAL STRUCTURE （生物结构）To determine dominance plant ecologists use relative abundance, relative dominance and relative frequency. Relative abundance or density of species compares the numerical abundance on one species with the total abundance of all species. Relative dominance uses the ratio of t

18、he basal area or biomass of one species to total basal area or biomass for all species. Relative frequency is based on the number sample points or plots in which a species is found to occur relative the total number of samples take. These three measurements are combined into one, the importance valu

19、e for each species. Importance value = relative density + relative dominance + relative frequency Multiplying the importance value by 100 gives the % importance.Species that reach a high level of importance are called index species.Species richness refers to the number of species in a community.SPEC

20、IES DIVERSITY （物种多样性）Shannon-Wiener index (H) measures the likelihood that the next individual will be the same species as the last. H = -(pi) (log2 pi) pi = number of species i / total number of individuals of all species; the proportion of individuals of species i in a community. Log2 C = ln C ln

21、2this is an alternate formula to find the log2 or of any other base. The higher the value of H, the greater is the uncertainty, or the probability that the next individual chosen at random from a collection of species containing N individuals will NOT belong to the same species as the previous one.

22、The lower the value of H, the greater the probability that the next individual encountered will be the same species as the previous one. Low index means little probability of selecting the same species in the next run, which means high diversity. In a stream, H 3 usually indicates clean water. There

23、 is great diversity. Lowest value = 0; highest value = 6.64, using log2.The maximum possible species diversity, Hmax, for a community of s species would be the condition where the individuals composing the community were evenly distributed among all s species. This is the condition of maximum evenne

24、ss. Species evenness can be calculated by dividing the species diversity of the community, H, by the maximum possible diversity for the community, Hmax. J = H/HmaxWhen H = Hmax the community has reached its maximum diversity.The value of J will approach 0 as the community becomes dominated by a sing

25、le species. This means that diversity is decreasing.J = H/Hmax = -(pi) (ln pi) / ln s s = number of species in the community. Alpha diversity: when species diversity is compared in different localities of a community, e.g. different localities in a 30 ha area. Beta diversity: when species diversity

26、between two separate communities or habitats is compared. Gamma diversity: when the comparison is made between two geographically different regions, e.g. west Tennessee bottomland forest and high altitude forest in the Smokey Mountains.SPECIES ABUNDANCE （种的多度）Two communities with the same indexes do

27、 not necessarily have the same species richness and evenness.The rank-abundance diagram gives the picture of species abundance in a community. By plotting the relative abundance of a species in the y axis against the species sequence in the x axis, starting with the most abundant species. Abundant s

28、pecies preempt a larger portion of space and resources than do less abundant species. There are several proposed mathematical models that attempt to summarize the abundance relationships within the community. None is perfect.INFLUENCE OF POPULATION INTERACTIONS ON COMMUNITY STRUCTUREThe biological s

29、tructure of the community is a result of a rich array of factors relating to both the physical and biological environment. COMPETITIONSince Darwin, ecologists have considered interspecific competition, especially competitive exclusion, as the cornerstone of community structure.To prove this hypothes

30、is with studies in the wild has proven to be very difficult.The difficulty arises because patterns that appear to be the consequence of competition may have alternative explanations, like variations in environmental factors that have a direct impact on population dynamics, such as climate, or other

31、types of species interactions. Competition may important only at certain times, e.g. fewer seed in time of drought increases competition between seed eating birds. Parasitism or predation may give a competitive advantage to other competitors, e.g. oaks defoliated by gypsy moth lose to yellow poplar

32、and sugar maple competitors. Field studies have shown that competition is often important - over 90% of the cases studied.There seems to be an absence of strong competition in many ecosystems.One species arises in the habitat or immigrates from elsewhere and it overlaps with another species in their habitat distribution.1. One of the two is very ef