Communities in Motion

A biological community is defined as an assemblage of populations of various species living close enough for potential interaction.
- Example: The “carrier crab” carries a sea urchin on its back for protection against predators.

Interactions within a community may help, harm, or have no effect on the species involved.
- Ecologists refer to relationships between species in a community as interspecific interactions.
- Examples of interspecific interactions include:
- Competition
- Predation
- Herbivory
- Symbiosis (which includes parasitism, mutualism, and commensalism)
- Facilitation
Interspecific interactions can significantly affect the survival and reproduction of each species with effects categorized as follows:
- Positive (+)
- Negative (−)
- No effect (0)

Interspecific competition (−/− interaction) occurs when species compete for a resource that limits their growth or survival.
Strong competition can lead to competitive exclusion, defined as the local elimination of a competing species.
- Competitive Exclusion Principle: States that two species competing for the same limited resources cannot coexist in the same place.

Evolution is evident through the concept of the ecological niche, which is a specific set of biotic and abiotic resources utilized by an organism.
- An ecological niche can also be understood as an organism's ecological role within an ecosystem.
Ecologically similar species can coexist in a community if significant differences exist in their niches.
Resource partitioning: The differentiation of ecological niches that allows similar species to coexist within a community.

Fundamental niche: The niche potentially occupied by a species.
Realized niche: The niche actually occupied by the species due to competition.
- Competition may result in a difference between fundamental and realized niches.
- Example: The presence of one barnacle species limits the realized niche of another barnacle species.

Results demonstrated the impact of competition on the realized and fundamental niches of Chthamalus and Balanus barnacles.

Character displacement refers to the observed phenomenon where characteristics are more divergent in sympatric populations of two species than in allopatric populations.
- Example: Variation in beak size between populations of two species of Galápagos finches illustrates this concept.

Predation (+/− interaction) describes the interaction where one species, the predator, kills and consumes the other species, the prey.
Feeding adaptations of predators may include:
- Claws
- Teeth
- Stingers
- Poison
Defensive adaptations of prey include:
- Behavioral defenses: hiding, fleeing, forming herds/schools, active self-defense.
- Morphological and physiological defenses.

Cryptic coloration: Camouflage that makes prey difficult to spot (e.g., Canyon tree frog).
Aposematic coloration: Bright warning coloration in animals with effective chemical defenses (e.g., Poison dart frog).
Batesian mimicry: Harmless species mimic a harmful one (e.g., Nonvenomous hawkmoth larva mimicking a venomous green parrot snake).
Müllerian mimicry: Two unpalatable species mimic each other (e.g., Cuckoo bee and Yellow jacket).

Herbivory (+/− interaction) occurs when an herbivore consumes parts of a plant or alga.
Herbivores may have adaptations such as:
- Behavioral adaptations for feeding.
- Specialized teeth or digestive systems.
Plant defenses include:
- Chemical toxins.
- Physical protective structures.

Symbiosis refers to a close relationship where two or more species live in direct and intimate contact with one another.

Parasitism: One organism (the parasite) benefits at the expense of the host.
- Many parasites have complex life cycles involving multiple hosts.
- Parasites may alter the behavior of hosts to enhance their own fitness.
Mutualism: (+/+ interaction) is a beneficial interaction where both species benefit.
- In some cases, one species cannot survive without the other, while in others both can live independently.
- Mutualisms often involve coevolution of adaptations in both species.
Commensalism: (+/0 interaction) where one species benefits, and the other is unaffected.
- Hard to document due to the likelihood of some impact on both species.

Species diversity is a key feature of community structure, comprising:
- Species richness: The number of different species in a community.
- Relative abundance: The proportion of each species amidst all individuals in the community.
Determining both components can be challenging, particularly for small organisms.
- Molecular tools can assist in determining microbial diversity.

Experimental manipulation of diversity shows that communities with higher species diversity tend to:
- Be more productive and stable in productivity.
- Consistently produce biomass than single-species plots.
- Better withstand and recover from environmental stresses.
- Be more resistant to invasive species.

The trophic structure represents the feeding relationships among organisms within a community.
- Plays a critical role in community dynamics.
Food chains link trophic levels from producers to top carnivores.
Food webs illustrate the complex interactions and relationships in feeding across multiple species.

Certain species exert a substantial impact on community structure due to their abundance or role.
- Dominant species: Most abundant or highest biomass, competitive in resource exploitation or successful at evading predators.
- Keystone species: Significant ecological roles without a necessarily high abundance; their removal significantly impacts the community.
- Ecosystem engineers (foundation species): Cause physical changes in environments that affect community structure (e.g., beaver dams).

Disturbance: An event that changes a community by removing organisms and altering resource availability.
- Examples include fire, flood, and human activities.
The nonequilibrium model suggests communities constantly change due to disturbances rather than existing in equilibrium.
- High disturbance levels are linked to both high intensity and frequency.

Ecological succession refers to the process of community and ecosystem changes following disturbances.
- Primary succession: Occurs where no soil exists.
- Secondary succession: Begins in areas where soil remains after disturbance.
Early and late-arriving species interact in succession, influencing each other based on three mechanisms:
- Facilitation, inhibition, and tolerance.

Human disturbances tend to reduce species diversity worldwide (e.g., trawling in marine ecosystems).
Latitudinal gradients in species richness: Species richness is higher in the tropics and declines toward the poles, influenced by evolutionary history and climate.

Pathogens significantly alter local and global community structures, often when introduced to new habitats.
Zoonotic pathogens can be transferred from animals to humans directly or through vectors, necessitating an understanding of host-vector communities to prevent disease transmission.