Positive Species Interactions Notes

Species Interactions

• Interactions between species:

  • Competition: (-, -) Both species are harmed.

  • Predation/Herbivory: (-, +) One species benefits and the other is harmed.

  • Parasitism: (-, +) One species benefits and the other is harmed.

• Positive interactions: (+, +) Both species benefit.

Positive Interactions

• Mutualism and Commensalism are positive interactions.

Introduction to Positive Interactions

• Positive interactions: One or both species benefit, and neither is harmed.

• Example: Most plants form associations with fungi.

• Positive interactions have influenced key events in the history of life and continue to shape communities and influence ecosystem functions.

Influence of Positive Interactions on Biodiversity

• Positive interactions influence biodiversity by creating alliances between species that allow them to coexist.

• Benefits include:

  • Provision of food

  • Habitat

  • Specialized services such as pollination, dispersal, predator defense

  • Reduction of physical stress

Examples of Positive Interactions

• Egyptian Plover Bird.

• Clownfish and sea anemones:

  • Clownfish benefit by receiving a safe place to live and prey to eat.

  • Clownfish provide food to the anemone, rid it of harmful parasites, and chase away fish that feed on anemones.

Positive Interactions (Facilitation)

• Mutualism: Mutually beneficial interaction between individuals of two species (+/+ relationship).

• Commensalism: Individuals of one species benefit; individuals of the other species do not benefit but are not harmed (+/0 relationship).

Benefits and Costs of Positive Interactions

• Benefits of positive interactions: food, shelter, transport, etc.

• In a mutualism (+/+), there is a cost to one or both partners, but the net effect is positive.

• For each species, the benefits are greater than the costs.

Mutualistic Associations

• Mutualistic associations are widespread.

• Most plants form mycorrhizae (fungus root): symbiotic associations between roots and various fungi.

  • Fungi increase surface area for uptake of water and nutrients.

  • Plants supply fungi with carbohydrates.

  • Fungi improve plant growth and survival in a wide range of habitats.

Challenges Faced by Early Plants

• Algal ancestors of plants obtained water, minerals, $CO_2$ from water (immersed).

• They faced challenges when they moved to land 500 million years ago.

Mycorrhizae Example

• Plants grown with mycorrhizae have better success compared to those without mycorrhizae.

Coral and Algae Mutualism

• Corals form mutualisms with symbiotic algae.

  • Coral provides the alga with a home, nutrients (nitrogen and phosphorus), and access to sunlight.

  • Alga provides the coral with carbohydrates produced by photosynthesis.

• The relationship began more than 210 million years ago; corals inhabited nutrient-poor marine environments.

• Effects of climate change: warming sea, thermal stress, algae expelled, coral bleaching.

Herbivores and Gut Microbes

• Herbivores, such as cattle and sheep, depend on bacteria and protists that live in their guts to help metabolize cellulose.

• Cellulase enzyme is absent in herbivores.

• Ruminant animals.

Protist Gut Mutualist

• Wood-eating insects also have gut protists that can digest cellulose.

• Insects would starve if gut mutualists such as protists did not help them digest wood.

• The protist can break down cellulose, a major structural component of wood that the cockroach cannot digest on its own.

Commensalism and Foundation Species

• Millions of species form +/0 relationships with foundation species that provide habitat:

  • Lichens on trees

  • Bacteria on human skin

• In kelp forests (ocean), many species depend on the kelp for habitat/shelter.

• Insects and understory plants in tropical rainforests depend on the trees for habitat.

Species-Specific and Obligate Interactions

• Some positive interactions are highly species-specific and obligate (not optional for either species).

• The leaf cutter ants and fungus cannot survive without each other, and both have evolved unique features that benefit the other species.

• Fungus-growing ants started cultivating fungi for food at least 50 million years before the first human farmers.

Leaf-Cutter Ants and Fungi

• The relationship benefits both species:

  • Ants cannot survive without their fungi.

  • Many of the fungi cannot survive without the ants.

• The fungi are cultivated in underground gardens.

• Leaf-cutter ants cut bits of leaves from plants and feed them to the fungi; fungi produce structures called gongylidia, on which the ants feed.

Fungal Garden of a Leaf-Cutter Ant

• The garden chamber contains a specialized structure called a gongylidia, which is produced by the cultivated fungus and eaten by the ants.

• Gongylidia is rich in lipids and carbohydrates.

Leaf-Cutter Ant Colony Structure

• Fungal garden.

• Fungus chambers.

• Foraging tunnels.

Ant Contributions to the Fungus

• Ants scrape a waxy covering from the leaves so that fungi can penetrate.

• The fungus digests and detoxifies the chemicals that plants use to deter insect herbivores.

• Weeding by ants increases, increase antimicrobial toxins produced in specialized glands.

• Nonresident fungi, pathogens, and parasites can sometimes invade the colonies.

Facultative Positive Interactions

• Many mutualisms and commensalisms are facultative (not obligatory) and show few signs of coevolution.

• In deserts, the shade of adult plants creates cooler, moister conditions.

• Seeds of many plants can only germinate in this shade.

• The adult is called a nurse plant.

• A single species of nurse plant can protect the seedlings of many different species.

Seed Dispersal by Herbivores

• Large herbivores, such as deer or moose, consume seeds of herbaceous plants.

• Many seeds pass through unharmed and are deposited with feces.

• Feces becomes a dispersal mechanism.

• The plants benefit by having their seeds dispersed; the herbivores benefit from the food source.

Categorizing Mutualisms

• Mutualisms are categorized by the type of benefits that result.

• Trophic mutualisms: Mutualist receives energy or nutrients from its partner.

  • Examples: Leaf-cutter ants and fungus, mycorrhizae (associations between roots and fungi), coral-alga symbiosis.

Habitat Mutualisms

• One partner provides the other with shelter, living space, or favorable habitat.

• Example: Pistol shrimp dig burrows that they share with goby fish.

• The goby gets a refuge and, in turn, serves as a “seeing eye fish” for the nearly blind shrimp.

Costs in Mutualism

• Although both partners in a mutualism benefit, there are also costs.

• In the coral–alga mutualism, cost to the coral includes supplying nutrients and space; cost to the alga is giving up some carbohydrates it could use for itself.

• In a mutualism, net benefits must exceed net costs for both partners.

Environmental Conditions and Mutualism

• If environmental conditions change, and benefit is reduced or cost increased for either partner, the outcome may change.

• Treehoppers and ants: Some ants protect treehoppers from predators, and the treehoppers secrete “honeydew” (sugar solution), which the ants feed on.

Shifts in Interaction Type

• Treehoppers always secrete honeydew, so ants always have this resource.

• But if predators are few, the treehoppers may get no benefit.

• The interaction shifts from +/+ (mutualism) to +/0 (commensalism) or +/– (parasitism), if consumption of honeydew by ants reduces treehopper growth or reproduction.

Withdrawal of Rewards

• Mutualists may withdraw rewards.

• In high-nutrient environments, plants can easily get nutrients and may reduce the carbohydrate reward to mycorrhizal fungi.

• The costs of supporting the fungus become greater than the benefits the fungus provides.

• If environmental conditions change…

Plant Discrimination Among Fungi

• The plant barrelclover, Medicago truncatula, can discriminate among mycorrhizal fungi, allocating more carbohydrates to the fungal hyphae that are supplying the most phosphorus.

• Fungal hyphae (long, branching filament).

Rewarding Beneficial Partners

• Plant roots were supplied with sucrose labeled with $^{14}C$.

• Plant transferred more carbohydrates to fungal hyphae that had access to phosphorus.

• These fungal hyphae were supplied with either 35 or 700 $μM$ of phosphorus.

  • $0 μM$ phosphate = ~60% carbohydrate transferred to hyphae

  • $35 μM$ phosphate supply to fungal hyphae = ~80% carbohydrate transferred to hyphae

  • $700 μM$ phosphate supply to fungal hyphae = ~90% carbohydrate transferred to hyphae

Cheating in Mutualistic Relationships

Cheaters

• Cheaters: Individuals that increase offspring production by overexploiting their mutualistic partner.

• If this happens, the interaction probably will not persist.

• Several factors contribute to the persistence of mutualism.

Penalties on Cheaters

• “Penalties” may be imposed on cheaters.

• In an obligate mutualism between a yucca (shrub) and a yucca moth, the female moth collects pollen in one yucca and lays eggs in another, depositing the pollen in this flower.

• Larvae complete development by eating some of the seeds in the flower.

Yucca and Moth Cheating

• Cheating can occur if moths lay too many eggs and the larvae eat too many seeds.

• Yuccas can selectively abort flowers with too many eggs, before the moth larvae hatch.

• The question is posed: How should the plant punish or impose a penalty?

Partners are Not Altruistic

• Partners in a mutualism are not altruistic.

• Both partners take actions that promote their own best interests.

• In general, a mutualism evolves and is maintained because the net effect is advantageous to both partners.

Ants and Acacia Trees

• The ants live in large thorns on the tree and feed on nectar and high-protein Beltian bodies produced by the tree.

• In exchange, ant workers patrol the tree 24 hours a day, aggressively attack insect and mammal herbivores, and even destroy plant competitors.

Ant-Plant Mutualism Benefits

• Ants have removed the plants that grew near this acacia, creating a competitor-free zone for the plant

• The ants are tending to larvae and pupae inside an acacia thorn.

Benefits for Acacias

• To determine benefits for the acacias, Janzen (1966) removed ants from some trees and compared them to trees with ants.

• Acacias with ant colonies weighed over 14 times as much as trees without, had higher survival rates, and were attacked less frequently by insect herbivores.

Obligate and Coevolved Mutualism

• Acacias without ant colonies are often killed by herbivores in 6–12 months, and the ants can’t survive without the trees.

• Both species have evolved unusual characteristics that benefit the other species.

• The ant–acacia partnership is an example of an obligate and coevolved mutualism.