Chapter 4- SPECIES INTERACTIONS AND COMMUNITY DYNAMICS

Competition for scarce resources and Predation are major factors in evolution and adaptation.

Competition

Law of competitive exclusion: no two species will occupy the same niche and compete for exactly the same resources in the same habitat for very long.

One species will have a competitive edge, and will gain a larger share of resources. Other species will migrate to a new area, become extinct, or change its behavior in a way to minimize competition. Process of niche evolution is called resource partitioning. Niche specialization can create behavior separation that allows subpopulations of a single species to diverge into separate species.

For what do organisms compete?

• Energy and matter in usable forms.
• Space
• Specific sites for life activities

Intraspecific competition: competition among members of the same species

Interspecific competition: competition among members of different species

• Competition among animals may not be in the form of fighting. In fact, many animals tend to avoid fighting if possible as it is not worth getting injured.
• Intraspecific competition can be especially intense because members of the same species have the same space and nutritional requirements.
• Plants have developed mechanisms to cope with intraspecific competition.
  • Seedlings unable to germinate in the shady conditions created by parent plants.
  • Plants disperse seeds to other sites by water, air, or animals.
  • Plants secrete substances that inhibit the growth of seedlings near them.
• Animals have developed mechanisms to cope with intraspecific competition.
  • Varied life cycles ( e.g. different habitats and feeding in juvenile and adult invertebrates)
  • Occupy different ecological niches.
  • Territoriality: intense form of intraspecific competition in which organisms define an area surrounding their home site or nesting site and defend it.
• These mechanisms (plant and animal):
  • Help allocate resources of an area by spacing out the members of a population
  • Promote dispersal into adjacent areas.

Predation

Predator: an organisms that feeds directly upon another living organism, whether or not it kills the prey to do so.

All forms of organisms which feed on living things can be considered predators:

• Carnivores
• Herbivores
• Omnivores
• Parasites
• Pathogens

Exceptions include scavengers, detritivores, and decomposers (which feed on dead things)

Predation is a potent and complex influence on the population balance of communities involving:

• All stages of the life cycles of predator and prey species
• Many specialized food-obtaining mechanisms
• Specific pre-predator adaptations that either resist or encourage predation

Predators play a role in evolution by

• Preying most successfully on the slowest, weakest, least fit members of their target population
• Reducing competition
• Preventing excess population growth
• Allowing successful traits to become dominant in the prey population

Coevolution: process in which species exert selective pressure on each other

Prey species evolve many protective or defensive adaptations to avoid predation.

Predators evolve mechanisms to overcome the defenses of their prey.

Keystone Species

Keystone species: species or set of species whose impact on its community or ecosystem is much larger and more influential than would be expected from mere abundance.

Both top predators (e.g. wolves) as well as less conspicuous species (e.g. tropical figs and some microorganisms) play essential community roles.

Often a number of species are intricately interconnected in biological communities so that it is difficult to tell which is the essential key.

The filter-feeding Krill is a keystone species in the complex food web of the Antarctic.

Symbiosis

Symbiosis: the intimate living together of members of two or more species.

In contrast to predation and competition, symbiotic interactions between organisms can be non antagonistic. Symbiotic relationships often entail some degree of coadaptation or coevolution of the partners, shaping, or at least in part, their structural or behavior characteristics (mutualistic coadaptation). Commensalism: type of symbiosis in which one member clearly benefits and the other is neither benefited nor harmed. Cattle and cattle egrets Many mosses, bromeliads, and other plants growing on trees. Mutualism: association in which both members of the partnership benefit. Lichens (combination of fungi and a photosynthetic partner) Parasitism: type of symbiosis in which one member benefits and the other is harmed.

Defensive Mechanisms

Many plants and animals have toxic chemicals, body armor, and other defensive adaptations to protect themselves from competitors or predators.

• Arthropods, amphibians, snakes, and some mammals produce noxious odors or poisonous secretions
• Plants also produce a variety of chemical compounds that make them unpalatable or dangerous to disturb
  • Poison ivy
  • Stinging nettles

Batesian mimicry: harmless species will evolve colors, patterns, or body shapes that mimic species that are unpalatable or poisonous.

• Wasps and longhorn beetle

Muellerian mimicry: two species, both of which are unpalatable or dangerous have evolved to look alike so that when predators learn to avoid either species, both benefit.

Species also evolve amazing abilities to avoid being discovered.

• Insects that look exactly like dead leaves or twigs

Predators use camouflage to hide as they lay in wait for their prey.

• Scorpion fish

POPULATION GROWTH DYNAMICS

Population growth (r) is controlled by variety of factors depending on scale examined:

Global Scale -- no. of births v. no. of deaths.

r = b - d

"r" is expressed as a proportion

(e.g., 0.1 , -0.05 ; equiv. to 10% increase and 5% decrease).

Example --

Given: Population of 10,000

400 births/yr (40 per 1,000 people)
200 deaths/yr (20 per 1,000 people)

Calcs: r = b - d

b = 400/10000 = 0.04
d = 200/10000 = 0.02

Therefore: r = 0.04 - 0.02 = 0.02

(The population is growing at an annual percentage rate of 2%.)

Local Scale -- (b-d) + migrations in and out

Migrate in = immigration (i)
Migrate out = emigration (E)

So, on local scale, population growth rate (r) is calculated taking into account births, deaths, immigrants and emigrants:

r = (b - d) + (i - E)

Example--

Given: Population of 10,000

400 births/yr (40 per 1,000 people)
200 deaths/yr (20 per 1,000 people)
20 immigrants/yr
50 emigrants/yr

Calcs: r = (b - d) + (i - E)

b = 400/10000 = 0.04
d = 20010000 = 0.02
i = 20/10000 = 0.002
E = 50/10000 = 0.005

Therefore: r = (0.04 - 0.02) + (.002 - .005) = 0.017

(The population is growing at an annual percentage rate of 1.7%.)

Strategies of Population Growth

• r-adapted species (adapted for high rates of growth)
  • Insects, rodents, marine invertebrates, parasites, and annual plants
• K-adapted species (adapted for living at or near carrying capacity)
  • Wolves, elephants, whales, and primates

Characteristics of contrasting reproductive strategies
r-adapted species	K-adapted species
Short life Rapid growth Early maturity Many small offspring Little parental care or protection Little investment in individual offspring Adapted to unstable environment Pioneers, colonizers Niche generalists Prey Regulated mainly by extrinsic factors Low trophic level	Long life Slower growth Late maturity Fewer large offspring High parental care and protection High investment in individual offspring Adapted to stable environment Later stages of succession Niche specialists Predators Regulated mainly by intrinsic factors High trophic level

POPULATION DYNAMICS

A. What is Diversity and/or Biodiversity
Species diversity - is the extent to which an ecosystem possesses differences in species in terms of genetic variation and distribution.

B. What is population?
Population is a group of interbreeding organism belonging to the same species. It is the interaction between organism that causes a population change (i.e., the national census is taken every ten years in a place at one time).

C. Characteristics of Population
1. Size - pertains to the number of individuals in a population (i.e., the recorded population of people in the Philippines as of August 2008 is 88.9 M)

Factors that contribute to the size of a population
a. Natality - the number of species that are born
b. Mortality - the number of species that die
c. Immigration - the number of species that entered the land
d. Emigration - the number of species that leave the land

TOP 20 LARGEST COUNTRIES BY POPULATION (LIVE)

1  China1,405,671,965

2  India1,289,649,648

3  United States326,255,319

4  Indonesia257,105,474

5  Brazil204,459,292

6  Pakistan189,514,149

7  Nigeria185,800,304

8  Bangladesh161,308,058

9  Russia141,943,539

10  Japan126,793,591

11  Mexico125,936,882

12  Philippines102,594,161

13  Ethiopia100,068,784

14  Vietnam93,809,429

15  Egypt85,335,412

16  Germany82,545,553

17  Iran79,963,363

18  Turkey77,155,144

19  Congo72,100,934

20  Thailand        67,529,331

 


                            Density - it is the number of individuals of a species living in a particular area of that population (i.e., 100 cows/hectare, 200 trees/hectare)

It is dependent upon such factors as availability of space, food, predators, water, light, and heat.

3. Distribution - it tells us how these individuals are located in that area. It is the arrangement of the individuals of a population within a particular space.
Structure: patterns of spatial distribution of individuals and population within the community and the relation of a particular community to its surroundings.

• Individuals within a population can be distributed randomly, clumped together, or in highly regular patterns.
• Larger communities often contain a mosaic of smaller units or subsets of the whole assemblage.
• Subunits develop because each species has a preference for specific, localized conditions.
• Patchiness: patterns of smaller units or subsets of the whole assemblage.
• Distribution in a community can be vertical as well as horizontal.

Law of the Limiting Factors, Law of the Minimum, Law of Tolerance

Critical Factors and Limiting Limits

Every living organism has limits to the environmental conditions it can endure

Environmental factors must be within appropriate levels for life to persist


These factors are primarily responsible for determining the growth and/or reproduction of an organism or population. It may be a physical factor such as temperature or light, a chemical factor such as particular nutrient, or a biological factor such as a competing species. The limiting factor may differ at different times and places.

The Law of Limiting factors states that too much or too little of any abiotic factor can limit or prevent growth of a population of a species in an ecosystem

Examples of limiting factors of a population growth
A. Terrestrial Ecosystem
1. Temperature
2. Water
3. Moisture
4. Soil nutrients

B. Marine Ecosystem
1. Salinity
2. Temperature
3. Sunlight
4. Dissolved Oxygen

Law of the Minimum

Proposed by Justus von Liebig in 1840.
It says that the success of organism determined by crucial ingredient that is in short supply.
As abundance of one resource increases another resource may become limiting.
Also known as Liebig's Law of Minimum - a system maybe limited by the absence or minimum amount (in terms of that needed) of any required factor.

What this law states is that the rarest requirement of an organism will be the limiting factor to its performance.

As an example a crop's yield is restricted by the lack of a single element, in this case lets suppose the soil is low in Nitrogen, adding more phosphorus will not improve the crops yield. Once the soil has nitrogen added crop yield will increase until another element becomes the limiting factor. And no further improvement in yield is possible until more of that element is made available.

Law of Tolerance

Proposed by Victor Shelford in 1913.
This is an extension of the Law of the Minimum.
It refers to the upper and lower bounds to physical environment an organism can tolerate.
These boundaries affect the ability to function, grow, and reproduce. These changes can be broad and narrow.
There are seasonal shifts in tolerance ranges, but within physiological limits.
Implication - no organism can live everywhere.

The law of Tolerance states that the existence, abundance, and distribution of a species in an ecosystem are determined by whether the levels of one or more physical or chemical factors fall above or below the levels tolerated by the species.

Carrying Capacity

The maximum population of a given species that an ecosystem can support without being degraded or destroyed in the long run. The carrying capacity maybe exceeded, but not without lessening the system's ability to support life in the long run.

Population Growth and Carrying Capacity
a. An ecosystem can support only a given number of individuals at a given time. When the capacity level exceeded, an imbalance in the ecosytem occurs.
b. Carrying capacity - is the size of the population of users a resource is able to keep in good condition. Or a number of factors in the environment, such as food, oxygen, diseases, predators and space, determine the number of organisms that can survive in a given area.

Four Factors interact to set the Carrying Capacity
1. the availability of space
2. the availability of energy
3. the accumulation of waste products and their means of disposal
4. the interaction and amount of organisms

Community Properties

This section focuses on how fundamental properties of biological communities and ecosystems are affected by factors such as tolerance limits, species interactions, resource partitioning, evolution, and adaptation.

Productivity

Primary productivity: rate of biomass production is an indication of the rate of solar energy conversion to chemical energy.

• The energy left after respiration is the net primary production.

  

• Photosynthetic rates are regulated by many factors.
  • Light levels
  • Temperature
  • Moisture
  • Nutrient availability

Tropical forests, coral reefs, and estuaries have high levels of productivity because they have abundant supplies of all of the above resources.

Other systems do not have sufficient levels of the necessary resources.

• Lack of water in deserts limits photosynthesis.
• Cold temperatures in Arctic tundra or high mountains inhibit plant growth.
• Lack of nutrients in the open ocean reduces the ability of algae to make use of plentiful sunshine and water.

Even in the most photosynthetically active ecosystems, only a small percentage of the available sunlight is captured and used to make energy-rich compounds.

• Much of the light reaching plants is reflected by leaf surfaces
• Most of the light that is absorbed by leaves is converted to heat is either radiated away or dissipated by evaporation and water.

Abundance and Diversity

Abundance: expression of the total number of organisms in a biological community

Diversity: measure of the number of different species, ecological niches, or genetic variation present.

• Abundance of a particular species often is inversely related to total diversity of the community.
• Communities with a very large number of species often have only a few members of any given species in a given area.
• Climate and history are important factors that dictate the abundance and diversity in a biological community.
• Productivity is related to abundance and diversity, both of which are dependent a several factors.
  • Total resource availability in an ecosystem
  • Reliability of resources
  • The adaptations of the member species
  • Interactions between species.

Complexity

Complexity: number of species at each trophic level and the number of trophic levels in a community.

• Diverse community may not be very complex if all species are clustered in only a few trophic levels.
• Diverse community may be complex if it has many interconnected trophic levels that can be compartmentalized into subdivisions.

Resilience and Stability

Three types of stability or resiliency in ecosystems

• Constancy: lack of fluctuations in composition or functions
• Inertia: resistance to perturbations
• Renewal: ability to repair damage after disturbance

The more complex and interconnected a community is, the more stable and resilient it will be in the face of disturbance.

In highly specialized ecosystems, removal of a few keystone species can eliminate many other associated species.

Chapter 3 - Ecosystems, Population and Species Interaction

Introduction - What is an Ecosystem?

An ecosystem consists of the biological community that occurs in some locale, and the physical and chemical factors that make up its non-living or abiotic environment. There are many examples of ecosystems -- a pond, a forest, an estuary, a grassland. The boundaries are not fixed in any objective way, although sometimes they seem obvious, as with the shoreline of a small pond. Usually the boundaries of an ecosystem are chosen for practical reasons having to do with the goals of the particular study.

Each organism and population has a HABITAT - the place or type of place where an organism or population naturally lives within a community.

Ecological niche - role or work performed by an organism.

The study of ecosystems mainly consists of the study of certain processes that link the living, or biotic, components to the non-living, or abiotic, components. Energy transformations and biogeochemical cycling are the main processes that comprise the field of ecosystem ecology. As we learned earlier, ecology generally is defined as the interactions of organisms with one another and with the environment in which they occur. We can study ecology at the level of the individual, the population, the community, and the ecosystem.

Studies of individuals are concerned mostly about physiology, reproduction, development of behavior, and studies of populations usually focus on the habitat and resources needs of individual species, their group behaviors, population growth, and what limits their abundnance or causes extinction. Studies of communities examine how populations of many species interact with one another, such as predators and their prey, or competitors that share common needs or resources.

In ecosystem ecology we put all of this together and, insofar as we can, we try to understand how the system operates as a whole. This means that, rather than worrying mainly about particular species, we try to focus on major functional aspects of the system. These functional aspects include such things as the amount of energy that is produced by photosynthesis, how energy or materials flow along the many steps in a food chain, or what controls the rate of decomposition of materials or the rate at which nutrients are recycled in the system.

Components of an Ecosystem

You are already familiar with the parts of an ecosystem. You have learned about climate and soils from past lectures. From this course and from general knowledge, you have a basic understanding of the diversity of plants and animals, and how plants and animals and microbes obtain water, nutrients, and food. We can clarify the parts of an ecosystem by listing them under the headings "abiotic" and "biotic".

ABIOTIC COMPONENT	BIOTIC COMPONENTS
Sunlight	Primary producers
Temperature	Herbivores
Precipitation	Carnivores
Water or moisture	Omnivores
Soil or water chemistry (e.g., P, NH4+)	Detritivores
etc.	etc.
All of these vary over space/time

By and large, this set of environmental factors is important almost everywhere, in all ecosystems.

Usually, biological communities include the "functional groupings" shown above. A functional group is a biological category composed of organisms that perform mostly the same kind of function in the system; for example, all the photosynthetic plants or primary producers form a functional group. Membership in the functional group does not depend very much on who the actual players (species) happen to be, only on what function they perform in the ecosystem.