Biotic Interactions Notes

Part 1: Positive biotic interactions (Facilitation & Comm ensalism)

  • Overview: Positive biotic interactions include facilitation and commensalism, where at least one participant benefits and neither is harmed.
  • Facilitation: Facilitative or positive biotic interactions are encounters between organisms that benefit at least one of the participants and cause harm to neither.
  • Commensalism: An association between two organisms in which one benefits and the other derives neither benefit nor harm.
  • Phoresy: one animal attaches to another exclusively for transport. Example: Phoretic mites on a fly (Pseudolynchia canariensis).
  • Inquilinism: an association between members of two different species in which one (the inquiline) lives commensally on or in the other (the host), or inside the host's home.
    • Examples: Epiphytic Bromeliad; Barnacles on mussels; Siphon weed on egg wrack.
  • Metabiosis: a form of commensalism in which one organism creates or prepares a suitable environment for another.
  • Hermit crabs (and barnacles) use empty gastropod shells for shelter and protection.

Part 2: Mutualism

  • Mutualism: a mutual relationship where organisms of different species interact to their mutual benefit; usually involves direct exchange of goods or services (e.g., food, defense, or transport).

  • Subtypes:

    • Defensive mutualism
    • Resource-based mutualism
    • Dispersive mutualisms (seed/pollen transport in exchange for resources)

  • Example: Foraging and parasitism in leaf cutter ants Atta cephalotes

    • Ants are subject to parasitism by fly larvae Neodohrniphora curvinervis.
    • Larger ants are more likely to be attacked (smaller ant heads are too small to allow larval development).
    • Foraging strategies: larger ants forage at night (dark for flies); smaller ants forage during the day.

  • Mutualism between leaf cutter ants and basidiomycete fungus

    • Ants excavate 2–3 liter cavities in soil and culture a basidiomycete fungus on leaves cut from neighboring vegetation.
    • The ant colony may depend absolutely on the fungus for nutrition of their larvae.
    • Workers lick the fungus colonies and remove specialized swollen hyphae, which are aggregated into bite-sized ‘staphylae’; these are fed to the larvae and pruning may stimulate further fungal growth.
    • The fungus gains from the association: it is both fed and dispersed by leaf-cutting ants and has never been found outside their nests.

  • Dispersive mutualisms: interactions in which a species receives food in return for transporting the pollen or seeds of its partner (pollinators and seed dispersers).

  • Coevolution of plants and pollinators: Pollination syndromes (Table 32.2) describe suites of flower traits that match specific pollinators.

    • Bees: color vision includes ultraviolet (UV) and not red; good sense of smell; require nectar and pollen; colors often blue, purple, yellow (not red); fragrant; nectar and abundant pollen.
    • Butterflies: good color vision; sense odors with feet; need a landing place; feed with long, tubular tongue; colors blue, purple, deep pink, orange, red; open at landing; fragrance.
    • Moths: active at night; good sense of smell; long, thin tongue; open at night; white or bright colors; heavy, musky odors; nectar in deep, narrow floral tubes.
    • Birds (hummingbirds): color vision includes red; often require perches; poor sense of smell; feed in daytime; high nectar requirement; hover capability.
    • Bats: color blind; good sense of smell; active at night; high food requirements; navigate by echolocation; often red; strong, damage-resistant floral structures; open at night; copious nectar and pollen; pendulous flowers; light, reflective colors; strong odors.
    • Floral syndromes reflect coevolution with pollinators and influence visitation patterns.

Part 3: Symbiosis

  • Symbiosis: an intimate association between two species, typically to the advantage of both; often a physically close and long-term biological interaction; sometimes obligate.
  • Lichens: a mutualistic association between fungi (the mycobiont) and a photobiont (usually an alga; in some cases a cyanobacterium). The photobiont provides carbon via photosynthesis; cyanobacteria may provide fixed nitrogen. Mutualism in lichens is common; in 90% of lichens the photobiont is an alga.
  • Lichen morphology: Cortex; Algal zone; Medulla.
  • Mutualistic mycorrhizal associations between fungi and plants
    • Fungi and plants develop mutually beneficial associations in the soil; mycorrhizae are often essential to the host plant survival (obligate vs facultative).
    • Plants provide carbohydrate energy to fungi (fungi lack chloroplasts) and receive nutrient foraging from fungi.
  • Two dominant kinds of mycorrhizae:
    • Endomycorrhizae (Vesicular Arbuscular Mycorrhizae, VAM): fungal hyphae penetrate host root tissue; vesicles (storage) and arbuscules (transfer of nutrients) are formed; arbuscules comprise a complex dual-membrane interface between hyphae and plant cells; VAM is present in 80% of angiosperm plants.
    • Ectomycorrhizae (EM): fungal hyphae do not enter host cells; they form a Hartig Net between root cells and are enclosed by a Mantle (Fungal Sheath) around the growing root tip.
  • Interface and exchange
    • The exchange occurs at the interface between the two organisms and is often extensive and complex.
    • Plants supply carbohydrates; fungi forage for trace elements and essential nutrients throughout the soil.
  • Summary of concepts
    • Antagonistic biotic interactions (e.g., competition, predation) are associations where one benefits at the expense of the other.
    • Positive biotic interactions (facilitation, commensalism, mutualism) involve resource exchange that yields a net benefit for at least one participant.
    • Symbiosis is an intimate, often long-term association; many plants and fungi form mutually beneficial mycorrhizal associations.

Key numerical/named facts

  • Leaf-cutter ants Atta cephalotes cultivate a basidiomycete fungus on leaves cut from neighboring vegetation; colonies excavate 2-3 L cavities in soil for fungal culture. The fungus is nourished and dispersed by the ants; it has never been found outside the nest. 2−3 L2-3\,\text{L}

  • Endomycorrhizae (VAM) associations occur in about 80%80\% of angiosperm plants.

  • Lichens commonly involve photobionts that are algae in about 90%90\% of cases.

  • The leaf-cutting ant lineage is described as having evolved fungus farming, with numerous described species (the transcript notes ~210210 described species showing a common ancestor).

  • Real-world relevance: Biotic interactions underpin ecosystem structure, nutrient cycling, and service provision; human activities can disrupt these interactions with cascading ecological consequences.