Community Characteristics-IV

5.     Ecotone

The two integrating communities meets at a transitional area called Ecotone. The transition area between ecosystems like grassland and forest forms regional ecotone and between forest and field forms a local ecotone which can be wide or narrow. This ecotone may appear as clear boundaries with homogenous surfaces or in gradual blending forms between two communities.

Features

1.     A sharp vegetation transition for example change in grass colours indicates an ecotone.

2.     Physiognomy: a key indicator of ecotone where the plant species differ in physical appearance.

3.     Change in species is indicators of ecotone where we observe some specific organisms at one side of the ecotone boundary while some other specific organism on the other side.

4.     Spatial mass effect: New plant establishment or migration obscures an ecotone as they cannot form self sustaining population in different ecotone. But if survives between two communities, then species richness is exhibited by the ecotone.

5.     An ecotone can reveal the space sharing efficiency of two communities and the types of biomes by observing the exotic species abundance in ecotone.

6.     Best model to study diverse ecosystem.

7.     Shift in dominance represented by ecotone.

8.     Ecotone act as an ecological niche for the species colonizing at the junction called edge effect.

Formation of ecotone

When physical environment changes, example from forest to clean land, a clear and sharp interface is created between two communities. Moreover, gradual blended interface forms when unique local species and species common to both interacting community found together for example in Mountain ranges. Most Wetlands are ecotones (eg. woodlands of Western Europe).

6.     Ecoclines

A physical transition zone between biological systems termed as ecoline relates to ecotone. It depicts the physiochemical environmental changes microclimatically or chemically signalling an ecotone via signals such as gradient of hydrothermal, salinity or pH respectively.

Type of ecotone

a.     Halocline (gradient salinity)

b.     Thermocline (gradient in temperature)

c.      Pycnocline (water density gradient)

d.     Chemocline (chemical gradient)

7.     Seasonal and Diurnal Fluctuations

With space and time the population fluctuates in the communities.

8.     Pattern Diversity

Community is organized on the basis of pattern diversity. The patterns can be horizontal segregation or vertical stratification etc.

9.      Edge Effects

In ecology, ecotone exhibit changes in population constituting community structure allowing for greater biodiversity at the boundaries of the merged habitats and this is called as edge effects. When two habitats are separated by wise edge effects called ecotone than they develop their own type of vegetation and environmental conditions.

Types

1.     Narrow Edge effect: Abrupt ending of one habitat from where another habitat begins is a narrow edge effect.

2.     Wide Edge effect: Significant distance between two habitats is exhibited as Wide edge effect or ecotone.

3.     Induced Edge effect: The structural changes are induced over time either by the human interference or natural disturbances (eg. fire) and leads to induced edge effect.

4.     Inherent Edge effect: The border between two habitats are separated and stabilized by natural features are called as Inherent edge effect.

5.     Perforated Edge effect: The distance between two habitats has gaps in them which help in assisting other habitats.

6.     Convoluted Edge effect: A nonlinear division of two habitats leads to convoluted edge effect.

Edge effects on Succession

When vegetation spreads the succession is affected by edge effects. Different species colonizes to central portions or to the edge leading to differential species distribution. With the change in orientation the edge also changes, thus, participating in different vegetation patterns.

Community Characteristics-III

4.     Dominance

In a community different species interacts among themselves and in some communities the interaction results into dominance by one species or by a prominent species in group. The organisms dominating others are referred to as dominants.

In ecology the proportion of biomass or abundance of one species or taxon than other interacting species or taxon in a community. It’s the dominant species that defines the ecological community. For example Alnus glutinosa (Alder) is the tree dominating in the Western Europe woodland areas. They are used to classify or identify the type of ecology.

In a community we can consider a community as dominant on the basis of following:

Either they are occupying maximum space of community habitat or have highest biomass or play critical role in nutrient cycling, contribute maximum to energy flow or regulate other community organisms.

Sometimes numerically abundant (means more in number) makes organisms superior and dominant but not necessarily always. Microclimate within the community also effect and complicate this system by contributing more dominant species per microclimate. Microclimates have local environment differences like nutrients levels, moisture, topographic location etc.

Its only how impactful and important functions a species play in shaping the structure and function of community which decides its dominance. Sometimes even the low density group of species or a single species can be dominant.

Keystone species: Dominant species (plants/animals) playing crucial and unique role and highly effect community structure and function in relative to its abundance. These keystone species have very intense inter species associations thus, controls the number and types of other species in community. Therefore, if we remove keystone species the community will shift to new form dramatically and vary from its original structure and function.

A classic example of keystone species is Pisaster ochraceus, a starfish. This starfish is a keystone predator and the only natural predator for mussels, sea urchins and many other shellfishes. So, if we remove starfish, the mussels or urchin population will proliferate in an uncontrolled manner shifting the community.

Another example includes a prey predator system where small predators like weevil E. lecontei which forage on herbaceous species called E. watermifoil. E. watermifoil can eliminate dominant plant species of the inhabited community but it’s the predator E. lecontei which control E. watermifoil from doing so by feeding on it. E. watermifoil (prey) number is less and thus requires low density of predator (E. watermifoil). But if predator is eliminated out of the community, the prey will outgrow in number dramatically thereby, vanishing the dominant species of community and thus, by eliminating the small number predator, the community character will now be altered without its actual dominant species.

This example indicates that dominant species directly control the community character but keystone species indirectly alter the community character.

Several approaches are used to determine the ecological dominance.

If a sample is collected from a large area than the individuals of a species found in large number represents the abundance of species and its distribution within ecosystem is called as relative species abundances.

a.     Relative abundance: When the total abundance of all organisms is compared to numerically abundant one species it is called as species relative abundance. If a sample is collected from a large area than the individuals of a species found in large number represents the abundance of species and its distribution within ecosystem is called as relative species abundances.

b.     Relative dominance: Dominance among same sized species can be measured by occupying by a species to the entire area of community.

c.      Relative frequency: Among different sized species, the dominance is measured by the relative frequency.

All these three measurements summed up to provide an important value to each species. These values of species ranked them in a list and index species are the species with high level of important value.

Sporadically/Locally abundant

The frequency of species occurrence in all samples is termed as incidence which relates to abundance. If the incidence or frequency is low but the abundance of species in sample is high it is called sporadically abundant.

How to measure and calculate Relative species abundance?

Sampling methods such as:

1.     Track count

2.     Spotlight count

3.     Monitoring point pressure

4.     Roadkill counts

5.     Plant cover for plant species etc.

Relative abundance of species = No. of species from one sampling/ Total no. of species of all sampling

Community Characteristics-II

2.     Species Diversity

The major biodiversity measurements are species richness, Shannon Weiner Index and Simpson’s Index. Species diversity comprised of two factors species evenness and species richness (number of species).

Species evenness indicates the relative abundance of each species.

 Community “1”

Community “2”

Both communities have three types of plant species depicting same species richness but relative abundance vary.

In community “1” species A, B and C are equal in proportion (three each) indicating higher species evenness thus revealed higher species diversity.

In community “2” species C is more in proportion indicating low species evenness and species diversity.

Shannon-Weiner Index

It’s a common Index of species diversity represented as 

Hᵒ is the diversity index, the proportion of i^th species is represented as p_i while “s” is species richness i.e. number of species in community.

Community Characteristics-I

2.     Species Richness

Species Richness is given as number of species in a community. In an ecological habitat, landscape or community the count of different species represents the species richness. It does not indicate about the species abundance or relative abundance of species. For example beetles counted from a pitfall trap etc. Sample heterogeneity and the number of species influence the species richness. If the samples are collected from different environment and habitats then the collected data is higher for species richness. Thus sampling should be performed on large areas as the more heterogeneous environment prevails and large size of population.

Applications

Species richness helps in assessing the conservation values of landscapes or habitat by relative comparisons. Although it does not consider the type of species but areas with rare species have higher conservation values than same number of species which are commonly found. 

Growth forms/Life forms and structure of community

1.     Growth forms/Life forms and structure of community

A community consist of different growth form determining the community structure such as herbs, shrubs, trees. A growth form also have variations such as a tree can be long leaved or broad leaved etc. various growth form have different mode of arrangement classifying community into (a) Vertical stratification and (b) Horizontal Zonation, i.e. Populations assembled to form communities and these populations are dispersed into definite vertical or horizontal strata.

a.     Zonation

The spatial arrangement of community species exhibit patterns dividing it into sub-communities which are ecologically related. If the distribution pattern is horizontal it’s called zonation layering in community. For example in lakes or deep ponds majorly three zones are recognised i.e. littoral, limnetic (Photic or open-water) and profondal zone (Aphotic or Deep-water). The organism varies in each zone of zonation pattern. Another example include mountain associated vegetation, altitudinal and latitudinal variations of vegetation in relation to climate of the existing region.

A mountain depicting a horizontal zonation

b.     Stratification

Vertical change in the pattern of community structure is called stratification. Stratification can be simple such as in the horizontal zonation community of pond each zone has different vertical storey, or complex stratification. For example in grassland communities distinct floor with different yet characteristics growth forms are exhibited. The lowest vertical sub-division is called (1) subterranean, which includes roots of plants, debris and living organisms like soil bacterium, protozoas or fungi etc.

(2) Herbaceous substratum: Above the soil with roots of growth forms, the herbaceous sub-stratum includes upper parts of growth forms. The forest community stratification is much more complex with five vertical layering including:

1. Subterranean-beneath the soil

2. Forest floor-with the upper parts of growth form along with the litters, fungi, bacteria etc.

3. Herbaceous vegetation

4. Shrubs

5. Forest Strata (canopy)

Forest animal’s lives in different substrata and many of them may shift between substratums. The properties (requirement and adjustments) of one stratum can be similar to the same stratum of different community somewhere else in the world. For example forest floor of one community in country 1 share common requirements and adjustments to the community in country 2 although these countries are geographically separated.

The animals living in such geographically separated but similar substratum are called Ecological Equivalent.

Community

Different types of population in habitat are studied under community ecology. Same kind of species constitute population but when different populations (different kind of species) interacts together with beneficial and mutual adjustments sharing same environment and habitat, the association is called community.

Examples of community include grassland, a pond, or a forest where different animal organism (population) interacts with plants (another population) in a natural area and sharing environment. Thus, community includes only interacting biotic components of ecology i.e. living organisms.

If we consider abiotic component (environment) associated with the biotic community component (different organisms) then it will be called ecosystem. Ecosystem and community vary on the basis of environment factors.

While walking in a forest we not only see different types of plants/trees like pine tree, or maple tree but also get glimpse of deer or squirrel or spider sometimes indicating a collection of different kind of species constituting populations (different) in that natural area sharing uniform environment. These different population are adapted to the same environment and influence each other through their associations either positive or negative and thus, shaping/forming a biotic community.

The association and assembly of different populations shape the structure, diversity, niches and dominance properties of biotic community.

These interactions can be direct or indirect or can be negative or positive (competition, predation, mutualism and so on).

Community has different meanings; some say it’s an assemblage of organisms of same kind habituating same area such as birds in grasslands or lizards in deserts etc.

Some ecologists describe it as group of specialized organisms occupying same habitat like in the forest copy insect feeding birds form a community.

Some ecologists referred to a term “Guild”.

In a specific habitat the species group sharing same property of feeding or foraging are termed as “guild”. In all of the above described definitions the community is restricted to habitat sharing assembly of interest species.

A community can be autotrophic (self sustaining) or heterotrophic.

An autotrophic community gains solar energy through plant photosynthesis. It forms a major community.

Heterotrophic heterotrophic community depends on organic materials which are fixed energy from outside.

Different minor communities such as heterotrophic microcommunities (for example, fallen logs consists of different populations such as termite species constituting one population, bacteria and fungi or other small populations of different species together forming a small micro communities) assembled to form major communities (autotrophic community).

Different species of population have overlapping terrestrial ranges where they coexist in a particular limited area representing a community. A community is composed of both plants and animals as their interaction is must for the survival of community.

Just like population has certain group characteristics not applicable on single organisms similarly community also have characteristic different than their constituent populations.

Structure and characteristics of community

Under the similar environmental conditions similar type of species found in communities which tend to overlap each other and because of several variations and other variations animals may shuffle between communities.

Characteristics

a.     Stratification

b.     Ecotone- Principle of edges- denser population in ecotone than both communities.

c.      Ecological dominance

d.     Diurnal and seasonal variations

e.      Pattern Diversity

f.       Periodicity

g.     Turnover

h.     Interdependence

Ecotone also called as tensional zone.

In comparison to the adjacent communities, the ecotone population has higher density with more species and this rule is called as Principle of edges.

Lotka Volterra equation for competition and Predation

The Lotka Volterra equation are used to interpret the population dynamics in which two organisms interact in one of the two ways, (a) either compete for common resources or (b) associated in a prey-predator system.

The equation for first type of interaction i.e. competing for common resources is termed as the Competitive Lotka-Volterra equations while another type of interaction is described by predator-prey equation.

1.     Competitive Lotka-Volterra equations

The Lotka-Volterra equation for competition is based on the logistic equation. This equation is similar to Predation prey equation of Lotka-Volterra where species interact with others by one term and to itself by another term but this equation follows exponential mode rather than logistic model.

Ecologists used the equation for logistic model is given as:

dN/dt= rN (1- N/K)

·        N= population size

·        r=growth rate of population

·        K=carrying capacity

For competition between two species

In the Lotka-Volterra equation two additional terms were added to depict the species interactions between two given population N₁ and N₂ related logistic dynamics.

The equation is given as:

dN₁/dt= r₁N₁ [1-(N₁+a₁₂N₁/K₁)] for species 1

dN₂/dt= r₂N₂ [1-(N₂+a₂₁N₂/K₂)] for species 2

We all know that each organism has its own carrying capacity (K₁ and K₂ are different) and growth rate (r₁ and r₂ are different). As this equation for population dynamics is associated with interaction (competition) which are harmful to interesting species, in equation all a values are positive. In this equation a₁₂ termed as competition coefficient depicts the competitive effect on population one (N₁) by another population (N₂) that’s why represent as (a₁₂). Similarly for the equation of population of N₂, a₂₁ depicts the competitive effect on population N₂ by the population N₁ represented as (a₂₁). If a₁₂<1 it means species 1 has more effect on its own rather than the  effect of species 2 on species 1 i.e. more intense intra-specific competition.

In short the effected population comes first to the population affecting it.

Outcomes Interpretation

1.     If a₁₂=0, this means species 1 follow logistic model of population dynamics.

2.     If a₂₁=0, this means species 2 follow logistic model of population dynamics.

3.     If a₁₂=1, species 1 and 2 strongly compete with equal magnitude for common resources means intraspecific competition equals interspecific competition.

4.     If a₁₂= “-” negative, species 2 facilitates resource availability to species 1.

5.     a₁₂/ a₂₁ both negative indicate symbiotic relationships.

6.     If one is zero (0) among two, either a₁₂ or a₂₁ and the other is negative it indicates commensalism.

7.     If one is positive among two, either a₁₂ or a₂₁ and the other has no affect it means parasitism.

8.     If both have negative values it indicates competition.

Lotka Volterra Predation equations

The Predator prey equations given by Lotka-Volterra describe the interaction between prey and predation as a dynamic biological system.

The population follows a non-linear, first order differential equation and represented in pair of equation as

dx/dt = ax – βxy

dy/dt = δxy- γy

where “x” and “y” is the number of prey and predator population and “dx/dt” or “dy/dt” are the growth rate of prey and predator population in time “t”. The symbols α, β, γ and δ denotes the real and positive parameters related to the interaction between prey and predator species.

Assumptions

Lotka-Volterra predation equation is based on the assumptions:

1.     Predator can eat limitless.

2.     Supply of food resource (i.e. prey) depends on the prey population size.

3.     The rate of change of population directly depends on its size.

4.     Environment is constant, inconsequential genetic adaptations for both species.

5.     All time unlimited food supply for prey.

The equation is continuous and deterministic indicated continuous overlapping of prey and predator population.

Prey

The rate of growth for prey population is given by equation:

dx/dt = ax – βxy

The change in prey population follows exponential growth model represent as “ax” in the equation unless subjected to predation. The rate of predation is directly dependent on the rate of prey-predator interaction represented as “βxy”. “x” and “y” are the population size of prey and predator, if x/y is zero it clearly indicates that there is no predation.

Predator

The rate of growth for predator population is given by equation:

dy/dt = δxy – γy

The change in predator population equals to food supply mediated growth minus death of predators.

The growth of predator population is represented as “δxy”. “δ” is different from “β” as the rate of predator population growth is very much different from the rate of predation on prey. “γy” is the decay rate of predator either due to emigration or mortality. In the prey absence the predator follow an exponential decay.

The solution to equation is periodic and yields a simple harmonic motion where prey population is traced by predation population by 90ᵒ in cycle.

Figure 27: Harmonic motion of prey-predator system

Functional and numerical responses

In 1959, Holling suggested that as the prey density increases it leads to increase in Predation rates due to two effects:

(a)  Functional Response where the consumption rate of predator increases in presence of high prey population density.

(b)  Numerical Response in which increase in prey density leads to increase in predator population density.

a.     Functional Response

There are three types of functional response curves which relate density of prey population to prey consumption by single predator per unit time.

Type I Response curve

It’s a rare form of response in nature, for example in Filter feeders.

It’s an initial exponential relationship between prey and predator and its consumption by predator till reaches saturation point where predator cannot eat maximum from that rate.

Type II Response curve

It’s a common type of response in nature, for example rodents and weasels. At low population density the rate of consumption increases at decelerating rate (increases at slow rate).the rate of consumption is dependent on two factors, (a) the searching or locating a prey and (b) handling time of prey (capture, kill and eat). At low population density the searching for prey is important and predator kill prey at constant effort fashion while at high density of prey searching for prey becomes easy but handling time is limiting factor and thus the rate of consumption increases slowly. Subsequently, searching Is not required and rate of consumption levels off at maximum rate.

Type III Response curve

Type III response is also common in nature depicting logistic increase in the rate of consumption on increase in prey density.

Figure 28: Three types of Functional response curves

Numerical response curves

Numerical response means increase in predator density on increase in prey population density due to direct responses i.e. (a) on increase in prey density the fecundity rate of predator increases and (b) Aggregation response

a.     As the prey population size increases, the consumption rate of predator increases leads to high fecundity rate and low mortality rate.

b.     Aggregation response: Predators aggregation in prey hot spots is called aggregation response. This type of response increase prey-predator system stability.