Community Ecology

Chapter 54: Community Ecology

Biological Community

A biological community consists of groups of populations of different species living close enough to interact. Factors affecting community structure include the number of factors in the community, the species present, and the relative abundance of different species.

Factors Influencing Community Structure

Several factors influence the structure of a community:

  • Foundation species: Large or abundant species that significantly affect community structure by providing habitat and food for other organisms.

  • Interspecific interactions: Predation and other interactions affecting the number and specific types of species present.

  • Disturbances: Events such as marine heat waves, storms, and human activities that remove organisms or alter resource availability.

Interspecific Interactions

Interspecific interactions occur between individuals of different species. These interactions can have positive (+), negative (-), or no effect (0) on the individuals involved.

Types of interactions:

  • Competition (-/-)

  • Predation (+/-)

  • Herbivory (+/-)

  • Parasitism (+/-)

  • Mutualism (+/+)

  • Commensalism (+/0)

Ecological interactions are broadly categorized into competition, exploitation, and positive interactions.

Competition (-/-)

Competition occurs when individuals of different species use a resource that limits the survival and reproduction of both individuals.

  • Competitive exclusion: Local elimination of the inferior competitor when two species use the same limited resources. Two species competing for the same limiting resources cannot coexist permanently in the same place.

  • G.F. Gause's Study: Study by G.F. Gause using three Paramecium species, where P. caudatum went extinct when grown with P. aurelia due to competition for the same resources. When P. caudatum and P. bursaria were grown together the coexist.

Ecological Niches and Natural Selection

  • Ecological niche: The specific set of biotic and abiotic environmental resources an organism uses, defining its ecological role.

  • Competitive exclusion principle: restated based on the niche. Two species cannot coexist permanently in a community if their niches are identical. Ecologically similar species can coexist if there are significant differences in their niches.

Resource Partitioning

Resource partitioning is the differentiation of niches that enables similar species to coexist in a community.

Fundamental vs. Realized Niche

  • Fundamental niche: The niche potentially occupied by a species, representing the optimum range of conditions.

  • Realized niche: The actual portion of the fundamental niche occupied by a species. The fundamental niche may differ from the realized niche due to competition or other factors.

Ecological Niches in Time and Space

Species can partition their niches in time as well as space. For example, the common spiny mouse and the golden spiny mouse are normally nocturnal, but where they coexist, the golden spiny mouse becomes diurnal.

Exploitation (+/-)

Exploitation refers to any interaction in which individuals of one species benefit by feeding on individuals of another species, which are harmed. This includes:

  • Predation

  • Herbivory

  • Parasitism

Predation (+/-)

Predation is an interaction in which an individual of one species (the predator) kills and eats an individual of another species (the prey).

  • Predator adaptations: Claws, fangs, poison, speed, agility, camouflage, and sensing organs.

  • Prey behavioral defenses: Hiding, fleeing, forming herds or schools.

  • Morphological and physiological defense adaptations: Mechanical defenses, chemical defenses, coloration, mimicry.

Chemical Defense

  • Synthesizing toxins.

  • Accumulating toxins from the plants they eat, as seen in monarch butterflies and bombardier beetles.

Coloration

  • Aposematic coloration: Bright warning coloration indicating toxicity; predators avoid brightly colored prey.

  • Cryptic coloration: Camouflage that makes prey difficult to see in their environment.

Mimicry

Mimicry involves resemblance to other species.

  • Mullerian mimicry: Two or more unpalatable species resemble each other.

  • Batesian mimicry: A palatable or harmless species mimics an unpalatable or harmful model. Example: scarlet king snake mimicking the poisonous coral snake.

  • Mimicry can also evolve in predators to help them approach prey. Example: Mimic octopus.

Antipredator Defenses

  • Displays of Intimidation: Porcupine fish inflates itself.

  • Fighting: Horns and antlers.

  • Agility: Grasshoppers’ powerful jumping ability.

  • Masting: Synchronous production of progeny satiate predators and allow some young to survive; high seed production in trees; periodical cicadas.

Herbivory (+/-)

Herbivory occurs when an herbivore eats parts of a plant or alga, harming but usually not killing it.

  • Herbivore adaptations: Insects with chemical sensors; mammals with specialized teeth or digestive systems and a strong sense of smell.

  • Plant defenses: Mechanical defenses (spines, cuticles, thorns) and chemical defenses (toxins, non-toxic chemicals that cause abnormal herbivore development or are distasteful).

Parasitism (+/-)

Parasitism occurs when organisms (parasites) live in or on another living organism (the host) and obtain their nutrition at the host’s expense. Parasitism is the predominant lifestyle on earth.

  • Endoparasites: Live within the body of the host.

  • Ectoparasites: Live on the external surface of the host.

  • Parasitism can affect the survival, reproduction, and density of host populations.

Many different organisms are parasites, including flatworms, segmented worms, roundworms, protists, bacteria, arachnids, and arthropods.

Parasitic Plants

  • Mistletoe: A hemiparasite that photosynthesizes but steals water and nutrients from the host plant using a haustorium.

  • Rafflesia arnoldii: A plant with no chlorophyll, completely dependent on a host plant for water and nutrition; lives inside the host vine and only shows its massive flower.

Parasite Life Cycles

Parasites often have complex life cycles that involve multiple hosts. Parasites can alter the host's behavior to increase the likelihood of the parasite being eaten by a new host, thus completing the parasite’s life cycle. Example: Toxoplasma gondii.

Positive Interactions

At least one species benefits, and neither is harmed. This includes:

  • Mutualism (+/+)

  • Commensalism (+/0)

Mutualism (+/+)

Mutualism is a common interspecific interaction that benefits individuals of both species. In some cases, each species depends on the other for survival and reproduction; in others, both species can survive alone.

  • Coral symbiosis with algae

  • Ant-Acacia mutualism

Commensalism (+/0)

Commensalism is an interaction in which individuals of one species benefit while members of the other species are neither harmed nor helped.

  • Shade-tolerant wildflowers

  • Herbivores and egrets

Diversity and Trophic Structure

Ecological communities can be characterized by general attributes, such as diversity and the feeding relationships of their species. A few species in a community exert strong control on that community’s structure.

Species Diversity

Species diversity has two components:

  • Species richness: The number of different species in the community.

  • Relative abundance: The proportion each species represents of all individuals in the community.

  • Two communities can have the same species richness but different relative abundance. Higher-diversity communities are often more resistant to introduced species.

Trophic Structure

The feeding relationships between organisms in a community are a key factor affecting community structure and dynamics.

  • Food Chain: The transfer of energy between different trophic levels.

    • Autotrophs: Primary producers at the base of the food chain, harvest light or chemical energy.

    • Heterotrophs: Consume other organisms.

      • Herbivores: Primary consumers that eat plants.

      • Carnivores: Secondary, tertiary, and quaternary consumers that eat consumers in the level below.

    • Decomposers: The final link in the chain, break down dead organisms.

  • The position an organism occupies in a food chain is called its trophic level.

Food Webs

Food webs are groups of food chains linked together, forming complex trophic interactions. Arrows link species in the food web according to who eats whom.

  • Complicated food webs can be simplified by grouping species with similar trophic relationships into broad functional groups or by isolating a portion of a community that interacts very little with the rest of the community.

Limits on Food Chain Length

Each food chain in a food web is usually only a few links long.

  • Energetic hypothesis: Food chain length is limited by inefficient energy transfer; only approximately 10% of the energy absorbed at one level is transferred to the next level due to heat loss.