chapter 54

Introduction

In this chapter we will examine ecological interactions between different species that live together in a community. Then we will look at factors that affect community structure, the number of species found in a community, the particular species that are present, and the relative abundance of these species.

community structure,

the number of species found in a community, the particular species that are present, and the relative abundance of these species.

Interactions between Species.

Interactions between different species, interspecific interactions, include competition, predation, herbivory, parasitism, mutualism, and commensalism. When defining interspecific interactions these relationships can have a positive (+) effect, a negative (-) effect or no effect (0) on the organisms in the relationships

interspecific interactions

Interactions between different species

interspecific interactions ideas

1. Community

2. Predation

3. Herbivory

4. Parasitism

5. Mutualism

6. Commensalism

Competition

Competition is a -/- interaction that occurs when individuals of different species each use a resource that limits the survival and reproduction of both individuals. When one species has a competitive advantage over another in the same environment, the weaker competitor will be eliminated, an outcome called competitive exclusion. Competition for limited resources can cause evolutionary change in populations. One way to examine how this occurs is to focus on an organism’s ecological niche, the specific set of biotic and abiotic resources that an organism uses in its environment. Two species cannot coexist permanently in a community if their niches are identical. However, two species with similar niches can coexist if natural selection enables one species to use a different set of resources or similar resources at different times of the day or year. This differentiation is referred to as resource partitioning.

competitive exclusion

When one species has a competitive advantage over another in the same environment, the weaker competitor will be eliminated, an outcome

ecological niche

the specific set of biotic and abiotic resources that an organism uses in its environment.

resource partitioning.

However, two species with similar niches can coexist if natural selection enables one species to use a different set of resources or similar resources at different times of the day or year. This differentiation is referred to as resource partitioning.

Predation

Predation is a +/- interaction in which one species, the predator, kills and eats the other species, the prey. Both predators and prey are adapted through natural selection in order to eat or to avoid being eaten. Animals display a variety of morphological and physiological defensive adaptations. Mechanical or chemical defenses protect species such as porcupines and skunks. Animals with effective chemical defenses often exhibit bright warning coloration such as poison dart frogs. Cryptic coloration, or camouflage, makes prey difficult to see. In Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful species.

warning coloration

Animals with effective chemical defenses often exhibit bright warning coloration such as poison dart frogs.

Cryptic coloration

or camouflage

Batesian mimicry

A palatable or harmless species mimics an unpalatable or harmful species.

Herbivory

Herbivory is a +/- interaction in which an organism, the herbivore, eats part of a plant or algae, thereby harming it but usually not killing it. Herbivores have specialized adaptations to detecting and processing their food. Many insects have chemical sensors on their feet that enable them to distinguish between plants based on their toxicity or nutritional value. Many herbivores also have specialized teeth and digestive tracts adapted for processing vegetation. Plants have also evolved chemical toxins and specialized structures such as thorns and spines to reduce the impact of herbivores.

Parasitism

Parasitism is a +/- interaction in which one organism, the parasite, derives its nourishment from another organism, the host, which is harmed in the process. Parasites that live within the body of their host, such as tapeworms, are endoparasites; parasites that feed on the external surface of a host, such as ticks, are ectoparasites.

endoparasites

Parasites that live within the body of their host, such as tapeworms

ectoparasites.

parasites that feed on the external surface of a host, such as ticks,

Mutualism

Mutualism is a +/+ interaction that benefits both species involved in the relationship. Mutualism is common in nature such as microoganisms in the digestive tract of termites, animals that pollinate flowers or disperse seeds, fungi on plant roots and photosynthetic algae in corals.

Commensalism

Commensalism is a +/0 interaction in which one species benefits from the relationship and the other is neither harmed nor benefits from the relationship. One example is the growth of flowers that require low light levels growing only on forest floor environments. The trees provide the shade necessary for the flower growth but, are neither benefitted nor harmed by the flower’s presence.

Diversity and Trophic Structure

Ecological communities can be classified by certain general attributes, including how diverse they are and the feeding relationships of their species.

Species Diversity.

1. The species diversity of a community is the variety of different kinds of organisms that makeup the community.

2. Species diversity has two components, species richness, the number of different species in the community, and relative abundance, the proportion each species represents of all individuals in the community.

3. If two communities each containing 100 individuals of four different species (A, B, C, and D) but with different percentages of the same species, the species diversity of the two communities would be different. For example, in Community 1: A = 25, B =25, C =25, and D = 25; and in Community 2: A = 80, B = 5, C = 5, and D = 10.

4. Community 1 is more diverse than community 2 because the relative abundance is greater. Higher-diversity communities generally are more productive and are better able to withstand and recover from environmental stresses. More diverse communities are also more stable year to year and in their productivity.

species diversity

of a community is the variety of different kinds of organisms that makeup the community.

species richness,

the number of different species in the community,

relative abundance,

the proportion each species represents of all individuals in the community.

Trophic Structure.

1. The trophic structure is the feeding relationships among the members of a community.

2. The trophic structure represents the transfer of chemical energy from plants and other autotrophs (primary producers) to herbivores (primary consumers), carnivores (secondary, tertiary, and quaternary consumers) and eventually decomposers.

3. This transfer of energy is referred to as a food chain and the position that each organism occupies in the food chain is called its trophic level.

4. Food chains are not an isolated unit, separate from other feeding relationships in a community. Instead, a group of food chains are linked together to form a food web.

5. Ecologists diagram the trophic relationships of a community using arrows that link species according to who eats whom.

6. A given species may weave into a food web at more than one trophic level depending on what it is eating at a particular time and what is eating it.

7. Each food chain within a food web is only a few links long because of the inefficiency of energy transfer along the chain, called the energetic hypothesis.

8. On average only about 10% of the energy stored in the organic matter of each level of the food chain is converted to organic matter at the next trophic level. Thus, a producer level consisting of 100 kg of plant material can support about 10 kg of herbivore biomass and 1 kg of carnivore biomass.

trophic structure

the feeding relationships among the members of a community.

food chain

transfer of energy

trophic level

the position that each organism occupies in the food chain

food web

Food chains are not an isolated unit, separate from other feeding relationships in a community. Instead, a group of food chains are linked together in the form of a web

energetic hypothesis

Each food chain within a food web is only a few links long because of the inefficiency of energy transfer along the chain