Community PP

Environmental Science (BIOL2) Study Notes

Definition of a Community

  • A community is defined as a set of populations of more than one species in a given place, at a given time.

    • Example Species: Elephant seals, Seagulls.

Example Community in a Specific Location

  • A community observed in a disused garden area behind Trader Joe's consists of:

    • Various species of small plants, e.g.,

    • Lil’ grass

    • Some lil’ herb species

    • Lil’ Crassula species

    • At least 3 wild plant species have been identified in this area.

Key Properties of Communities

Species Richness

  • Definition: The number of species present in the community.

Species Evenness

  • Definition: How evenly the representation or abundance of each species is within the community.

    • Example Comparison: Two communities with equal richness (3 species each).

    • The community on the right demonstrates greater evenness in species representation.

Categorization of Communities

  • Communities are often categorized and named based on dominant plant species.

    • Example: “Bigberry manzanita chaparral” located in Marshall Canyon, San Gabriel Mountains foothills.

Understanding Chaparral Communities

  • Definition of Chaparral: A shrubland community found across California and similar climates.

    • Characteristics:

    • Dense, impenetrable thicket of shrubs or dwarf trees.

    • Dominated by woody evergreen shrubs (do not lose leaves seasonally).

    • Leaves often contain woody material that makes them stiff.

    • Chaparral plants are adapted to withstand regularly recurring wildfires.

    • Note: Significant presence of chaparral vegetation in the San Gabriel Mountains.

Impenetrability of Chaparral

  • Definition and Characteristics:

    • Described as “Too high to go over, too low to go under, and too thick to go through.”

    • Characteristics contributing to impenetrability:

    • High plant density

    • Hard, woody branches

    • Presence of thorns and other defensive structures.

Historical Perspective on Chaparral

  • John Muir's (1918) description of the San Gabriel Mountains:

    • Muir depicted the chaparral as a “thorny” defense, describing how it dominates the landscape and makes human activities insignificant.

    • His quote highlights the lushness and adaptability of chaparral creatures across varied topographies, emphasizing the exuberance of natural growth over human endeavors.

Understanding Species Within Communities

  • The focus shifts to understanding the roles of species within communities.

Definition of Species Niche

  • Niche: The range of resources a species can utilize and the conditions it can tolerate.

    • Note on Pronunciation: The word “niche” can vary in pronunciation; it originates from French.

Types of Niches

  • Fundamental Niche: Refers to all resources and conditions a species could potentially utilize and occupy.

  • Realized Niche: Represents the actual resources and conditions a species uses and occupies.

Factors Influencing Niche Realization

  • Species do not occupy their fundamental niche entirely due to competition with other species.

Competition in Ecological Communities

  • Definition of Competition: The interaction that occurs either within species (intraspecific competition) or between species (interspecific competition).

    • Impact: Competition limits the success (fitness) of the involved species.

    • Example: Lions and hyenas compete for food resources on the African savannah, creating challenges for both species.

Competitive Exclusion Principle

  • Definition: States that two species cannot stably coexist in the same niche.

    • Outcome of Overlapping Niches:

    • The species that is the superior competitor will prevail, while the other will become extinct.

Realistic Niche Overlap

  • While niches may not be identical, they can be too similar to allow for coexistence.

    • Example: Homo sapiens and Neanderthals had overlapping niches that were unsustainable in the long term.

Long-term Coexistence Strategies

  • Coexistence among species often requires niche partitioning, which can occur through:

    • Spatial Variation: Utilizing different areas or habitats.

    • Temporal Variation: Utilizing resources at different times, allowing for resource specialization among slightly different resources.

    • Example: Hawks and owls share the same prey types and habitats but hunt during different times of the day (hawks in daylight, owls at night).

Categories of Species Interactions

  • Species interactions can be categorized based on their effects on the fitness of the interacting species:

    • Competition: (-/-)

    • Consumption: (+/-)

    • Includes predation/herbivory and parasitism.

    • Commensalism: (+/0)

    • Mutualism: (+/+)

    • Note: Close species interactions are often referred to as “symbioses.”

    • The term “symbiosis” comes from the Greek roots “sym” (together) and “bio” (life).