Bio 1300, Ch 57 Species Interactions

Overview of Species Interactions

Species Interactions Types:

• Competition (+/-): This interaction occurs when two or more species compete for the same resource, leading to a negative impact on at least one participant.

• Amensalism (0/-): In this interaction, one species is harmed while the other is unaffected. An example is the effect of large trees shading smaller plants.

• Predation, Herbivory, Parasitism (+/-): These interactions involve one species benefiting at the expense of another, which can lead to population control dynamics. Predation involves killing prey, while herbivory involves grazing on plants, and parasitism involves living off another organism.

• Mutualism (+/+): A symbiotic relationship where both species benefit, such as bees pollinating flowers while obtaining nectar for food.

• Commensalism (+/0): In this interaction, one species benefits while the other remains unaffected, such as barnacles attaching to whales.

Competition

Types of competition:

• Intraspecific: This occurs between individuals of the same species, such as trees competing for sunlight in a dense forest.

• Interspecific: This occurs between individuals of different species, as seen in different bird species competing for nesting sites.

Forms of competition:

• Exploitation: This is indirect competition where individuals compete for limited resources, such as water or nutrients, with one species consuming more than the other.

• Interference: This is direct competition involving physical interactions, such as aggressive behaviors displayed by animals fighting for territory or mates.

• Allelopathy: A form of extremely asymmetric competition, often witnessed in plants, where one plant species releases chemicals that inhibit the growth of other species nearby.

Types of Competition

Intraspecific Competition:

• Exploitation: Each individual consumes resources such as leaves or fruits, affecting the growth and reproductive success of others.

• Interference: In this form, individuals physically intimidate or displace each other, leading to dominance hierarchies.

Interspecific Competition:

• Exploitation: Different species may compete for shared resources like food or shelter. For example, both aphids and caterpillars feed on plants, competing for the same leaves.

• Interference: This involves direct intimidation, where different species may engage in aggressive behaviors to assert dominance, such as when beetles compete for mates with caterpillars.

Ecological Studies on Competition

Commonness of Competition:

• A comprehensive review conducted in 1983 revealed that competition was documented in 55% of species and was prevalent in 90% of species when examined in pairs.

• It has been noted that as overall species richness increases, competition typically decreases due to niche differentiation, allowing more species to coexist.

Case Study: Paramecium Competition

• Gause’s Experiment (1934): This pivotal study examined the interactions between three species of Paramecium. When grown together in identical conditions, one species outcompeted the others, leading to its extinction if their environmental resource requirements were similar.

• Competitive Exclusion Principle: This principle asserts that two species competing for the same resources cannot coexist indefinitely; one will outcompete the other, leading to its decline.

Resource Partitioning

• Robert MacArthur (1958): In this influential study, five species of warblers were observed exploiting different parts of the same tree for foraging. This behavior demonstrated niche differentiation, allowing each species to coexist by utilizing different resources within the tree.

Niches Defined

• Fundamental Niche: This refers to the optimal range of conditions under which an organism can survive and reproduce without limitations, often determined through ideal environmental conditions.

• Realized Niche: This is the actual range that a species occupies in the presence of competition and other biotic factors, typically narrower than the fundamental niche.

• Connell’s Barnacle Experiment (1954): This classic experiment illustrated the concepts of fundamental and realized niches. The study of Chthamalus barnacles showed that their range was limited by competition with other barnacle species (realized niche) versus their potential distribution dictated by environmental conditions (fundamental niche).

Character Displacement

• Definition: Character displacement refers to the tendency for species to become morphologically or behaviorally different when they are in competition for similar resources, which reduces competition through differentiation.

• Example: Size differences observed in Galapagos finches, where size variations depend on whether species coexist (sympatric) or are geographically separated (allopatric), showcase adaptation due to resource competition and environmental pressures.

Predation, Herbivory, and Parasitism

Predation Categories:

• Predation can be categorized based on lethality and duration of the interaction, affecting predator-prey dynamics.

Antipredator Strategies:

• Chemical defenses: These are constituents such as toxins released by species like the Bombardier beetle to deter predators.

• Aposematic coloration: This warning coloration is exhibited in toxic species, signaling danger to potential predators.

• Camouflage: The ability of organisms to blend into their environments effectively (cryptic coloration) or utilize catalepsis, freezing in place when threatened.

• Mimicry:

  • Mullerian Mimicry: Involves noxious (toxic) species resembling each other, reinforcing predator learning.

  • Batesian Mimicry: Involves palatable species adopting the appearance of unpalatable ones to avoid predation.

Effects of Predators:

• Predators play a crucial role in regulating prey populations, contributing to maintaining ecological balance. Moreover, invasive species can significantly impact native ecosystems when they lack natural predators, such as the Guam brown tree snake.

Plant and Herbivore Dynamics

Plant Defenses:

• Plants have developed various defense mechanisms, including physical barriers (e.g., thorns) and chemical defenses via secondary metabolites that deter herbivory.

• Host Plant Resistance: This describes the evolution of plants to develop traits that inhibit herbivory, such as genetically modified organisms (GMOs) like Bt corn that produce insecticidal proteins.

Herbivore Adaptations:

• Various herbivores have evolved adaptations to overcome plant defenses, utilizing detoxification pathways to process harmful chemical compounds found in their food.

Parasitism

• Definition: Parasitism is an interaction where one organism, the parasite, benefits at the expense of the other organism, the host, often leading to reduced host fitness.

• Variations exist in parasites:

  • Ectoparasites: These parasites live on the surface of the host (e.g., ticks and fleas), often causing irritation and disease transmission.

  • Endoparasites: These organisms live inside the host's body (e.g., tapeworms and flukes), often causing significant morbidity and sometimes mortality.

Mutualism and Commensalism

• Mutualism: In this interaction, both species involved benefit, such as bees and flowering plants where bees obtain nectar and the plants achieve pollination, thereby enhancing reproductive success.

• Commensalism: In this type of relationship, one species benefits while the other is neither helped nor harmed, with examples like cattle egrets feeding on insects stirred up by grazing livestock, benefiting the egrets with minimal impact on the cattle.

Control Mechanisms in Ecosystems

• Bottom-Up Control: This ecological concept implies that resource availability, such as food and nutrients, affects population dynamics of higher trophic levels.

• Top-Down Control: Here, populations of higher trophic levels (predators) regulate the abundance of lower trophic levels (herbivores and plants), influencing community structure.

• Trophic Cascade: Changes at the top of the food web, such as the decline or increase of top predator populations, can cause cascading effects throughout the ecosystem, impacting plant and herbivore populations beneath them.

Lecture Questions

57.1

•What are the 5 types of interactions species can have with each other? What are the effects of each interaction on the species involved?

•What are the 4 different types of competition? Examples?

• Intraspecific competition – Between individuals of the same species

  • Example: Two deer competing for mates

• Interspecific competition – Between different species

  • Example: Trees and shrubs competing for sunlight

• Exploitative competition – Competing indirectly by using up a resource

  • Example: Two species of birds eating the same seeds

• Interference competition – One species actively interferes with the other

  • Example: A lion chasing away hyenas from a kill

•Can coexistence happen? Summarize the Gause (1934) experiment What is the competitive exclusion principle?

Gause’s Experiment (1934):

• Studied two species of Paramecium (single-celled organisms)

• When grown separately, both thrived

• When grown together, one species always outcompeted the other and caused extinction

📌 Conclusion:

• Led to the Competitive Exclusion Principle:

  Two species competing for the exact same resources cannot coexist indefinitely. One will always outcompete the other.

•What is resource partitioning? Explain the difference between a fundamental and a realized niche.

Resource partitioning is when species divide a niche to reduce competition and allow coexistence.

Example:

• Different species of warblers feed on different parts of the same tree to avoid direct competition.

• Fundamental niche = the full range of environmental conditions a species could live in (if there were no competitors)

• Realized niche = the actual conditions and space the species occupies due to competition or limitations

Example:

• A barnacle species could live from high to low tide (fundamental), but only lives in the upper zone due to competition (realized).

•What is character displacement? Examples?

🧬Character displacement is when species evolve physical or behavioral traits to reduce competition when they live in the same area (sympatry).

📚 Examples:

• Darwin’s finches on the Galápagos:

  • Beak sizes differ more when two species live together, so they feed on different seed types.

• Salamanders or frogs may differ in mating calls in overlapping areas to avoid hybridization or competition.

57.2

•What are the 4 categories of predation? What 2 factors are the categories of predation based on?

Predation categories are based on two factors:

1. Lethality (whether the prey dies)

2. Duration of association (how long the predator and prey interact)

The 4 Categories:

1. True Predators

   • Kill prey immediately

   • Short interaction

   • Examples: Lions, hawks, wolves

2. Grazers

   • Eat part of prey without killing it

   • Long interaction

   • Examples: Cows, deer, caterpillars

3. Parasites

   • Do not usually kill the host

   • Long-term relationship

   • Examples: Tapeworms, lice

4. Parasitoids

   • Eventually kill their host

   • Prolonged interaction

   • Examples: Wasps laying eggs in caterpillars

•What are the main strategies that prey can employ to avoid predation? Examples of each strategy?

1. Camouflage (Cryptic coloration)

   • Blending into the environment

   • Example: Leaf insects, snowshoe hares

2. Aposematic coloration

   • Bright colors warning of toxins

   • Example: Poison dart frogs, monarch butterflies

3. Mimicry

   • Batesian mimicry: Harmless species mimics a harmful one

     • Example: Viceroy butterfly mimics monarch

   • Müllerian mimicry: Two harmful species resemble each other

     • Example: Bees and wasps

4. Mechanical defenses

   • Physical structures

   • Example: Porcupine quills, turtle shells

5. Chemical defenses

   • Producing toxins or bad-tasting compounds

   • Example: Skunks, toxic plants

6. Behavioral defenses

   • Acting in ways to avoid detection or escape

   • Example: Schooling fish, alarm calls in meerkats

•What do we mean by donor vs predator- controlled systems? Examples?

• Donor-controlled system:

  • Prey (or resource) abundance is limited by its own reproduction, not predators.

  • Removing predators doesn’t affect prey population much.

  • Example: Grass growing back after herbivory.

• Predator-controlled system:

  • Predators limit prey populations.

  • Removing predators causes prey to increase.

  • Example: Wolves controlling deer populations

•What do we mean by a plant vs herbivore arms race? What are the 2 main types of plant defenses What are some examples of plant defenses?

It’s the ongoing co-evolutionary struggle between plants evolving defenses and herbivores evolving ways to overcome them.

🛡 2 Main Plant Defenses:

1. Mechanical defenses

   • Thorns, spines, thick leaves

   • Example: Cactus spines, tough grass blades

2. Chemical defenses

   • Toxins, deterrents, digestion inhibitors

   • Examples:

     • Alkaloids (nicotine, caffeine)

     • Tannins (reduce digestibility)

     • Latex/sap (clogs insect mouths)

•What is host plant resistance? How do humans utilize it?

Host plant resistance is a plant’s natural ability to resist or tolerate herbivory or disease.

🧑‍🌾 How humans use it:

• In agriculture, we breed or genetically modify crops to be more resistant to pests or pathogens

• Examples:

  • Corn resistant to rootworms

  • Wheat with fungal resistance

•What strategies to herbivores have to counteract plant defenses?

• Detoxification enzymes – Break down plant toxins

  • Example: Caterpillars digesting alkaloids

• Behavioral adaptation – Eat only certain parts or avoid defended areas

  • Example: Monkeys peeling bark to avoid toxins

• Storage and tolerance – Store toxins to use against predators

  • Example: Monarch caterpillars storing milkweed toxins

• Microbial symbionts – Gut microbes help digest tough or toxic materials

  • Example: Cows digesting cellulose

•What is parasitism? Examples?

Parasitism is a relationship where one species (the parasite) benefits at the expense of the host.

Examples:

• Tapeworms in intestines

• Fleas or ticks on dogs

• Mistletoe on trees

•What are the 2 main types of parasitic flowering plants? Examples?

• Holoparasites

  • Completely dependent on host for nutrients

  • No photosynthesis

  • Example: Rafflesia, dodder

• Hemiparasites

  • Can photosynthesize, but still tap into host for water/nutrients

  • Example: Mistletoe

•What are the various categories of parasites?

• Endoparasites – Live inside the host

  • Examples: Tapeworms, protozoa

• Ectoparasites – Live on the surface of the host

  • Examples: Lice, fleas, ticks

• Microparasites – Tiny (often reproduce inside host cells)

  • Examples: Viruses, bacteria

• Macroparasites – Larger (may live inside or outside)

  • Examples: Worms, arthropods

• Obligate parasites – Must have a host to complete life cycle

  • Example: Plasmodium (malaria)

• Facultative parasites – Can live independently but may parasitize

  • Example: Some fungi or bacteria

57.3

•What is a mutualism? What are the 3 types of mutualisms, and how do they differ? Examples of each?

Mutualism is a type of symbiotic interaction where both species benefit from the relationship.

Mutualisms differ in how essential or specific the relationship is for survival.

1. Obligate mutualism

• Both species require the relationship to survive or reproduce

• Example:

  • Lichens: A mutualism between fungi and algae—neither can live alone in harsh environments

  • Termites + gut microbes: Termites can’t digest wood without symbiotic protozoa

2. Facultative mutualism

• The relationship is beneficial but not essential—both species can survive alone

• Example:

  • Ants and aphids: Ants protect aphids; aphids provide sugary honeydew. But both can live independently

  • Birds eating parasites off large mammals

3. Trophic vs Defensive vs Dispersive mutualism (based on the type of benefit):

Some sources classify mutualisms based on what’s being exchanged, rather than how necessary it is:

• Trophic mutualism = exchange of nutrients or energy

  • Example: Mycorrhizal fungi and plant roots

• Defensive mutualism = one species provides protection, the other provides food or shelter

  • Example: Ants defend acacia trees from herbivores, acacia gives ants food

• Dispersive mutualism = one species helps with pollination or seed dispersal

  • Example: Bees pollinating flowers; birds spreading seeds after eating fruit

•What is commensalism? Examples?

Commensalism is a relationship where one species benefits, and the other is unaffected (neither helped nor harmed).

📌 Examples:

• Cattle egrets following cows: They eat insects stirred up by the cows’ movement

• Barnacles on whales: Barnacles get a free ride and better access to plankton, the whale doesn’t care

• Epiphytic plants (like orchids) growing on trees: They gain height and light, the tree is unaffected

57.4

•What do we mean by bottom-up, and top-down, control? What are the factors that drive each system?

Bottom-Up Control: This ecological concept implies that resource availability, such as food and nutrients, affects population dynamics of higher trophic levels. In bottom-up systems, the abundance of herbivores or predators is determined by the availability of plants or prey.

Top-Down Control: In contrast, this concept indicates that populations of higher trophic levels (predators) regulate the abundance of lower trophic levels (herbivores and plants), influencing community structure. In top-down systems, a change in the population of predators can lead to significant changes in the populations of herbivores and primary producers below them.