April 22

Enslaver Parasites and Host Behavior Modification

Definition and General Function: * Enslaver parasites are organisms that manipulate the behavior of their hosts to suit the parasite's life cycle, often facilitating dispersal, protection, or transmission. * Fungal Enslavers: Certain fungi infect flies and alter their "perching" behavior. This modification ensures that when the fly dies, the fungal spores are positioned for easier dispersal.

Case Study: Hymenopeimecis argyraphaga (Tropical Parasitoid Wasp)

Host Interaction: The larvae of this wasp species attach to a host spider and survive by sucking the spider's body fluids.
Behavioral Manipulation: * When the larva has grown to a specific stage, it induces the spider to build a specialized "cocoon web." * The normal web-building sequence of the spider is interrupted and replaced with a specialized structure designed to support the wasp's needs.
The Cocoon Web: * This specific web is incredibly durable and protects the wasp larva from torrential rains, which would normally destroy a standard spider web. * The larva then kills the spider by injecting a poison and consumes the body. * After eating the host, the larva spins its own cocoon and attaches it to the pre-constructed cocoon web.
Experimental Evidence (Eberhard 2001): * In experiments where wasp larvae were removed from host spiders just before the cocoon web was supposed to be built, spiders produced webs that were distinctly different from both normal webs and cocoon webs. * Some spiders were able to recover their original, normal web-making abilities after several days.
Mechanism of Action: * The manipulation is believed to be caused by a fast-acting chemical injected by the larva into the spider. * This chemical appears to act in a dose-dependent manner to disrupt the spider's innate sequence of web-building behaviors.

Parasitic Flatworms and the Green-Banded Broodsac

Host: Snails.
Mechanism: The flatworm hijacks the snail's nervous system.
Behavioral Change: The snail is forced to move into open, exposed areas where it is highly visible to predators.
Visual Mimicry: The parasite takes over the snail's eyestalks, transforming them into pulsating, caterpillar-like appendages.
Purpose: This attracts birds (the next host in the life cycle). The bird eats the snail, allowing the parasite to complete its life cycle.

Toxoplasma gondii: The Protozoan Enslaver

Biological Characteristics: * A protozoan parasite measuring roughly $5-50\,\mu\text{m}$ . * Immature/larval stages live in various hosts, but the definitive host (where the adult stage resides) is the gut of felines (cats).
Manipulation of Rodents: * Infected rats do not avoid cats; in some cases, they are actively attracted to them. * T. gondii makes the smell of cat urine appealing to the rat. * Infected male rats become more attractive to female rats because the parasite makes the male's scent more appealing. * Goal: Increases the probability that the rodent (intermediate host) will be eaten by a cat, allowing the parasite to return to its definitive host.
Human Impact and Latent Toxoplasmosis: * Approximately one in three people worldwide (and up to $80\%$ in some specific countries) have antibodies to T. gondii. * The parasite forms cysts inside muscle cells and neurons to avoid the immune system. * Research indicates that people with latent toxoplasmosis are more likely to be involved in traffic accidents. * Infected women show higher rates of suicide attempts. * Infected men find the smell of cat urine more attractive. * There are potential causal links being researched between T. gondii and Schizophrenia, Brain cancer, and Bipolar disorder.

The Jewel Wasp and the Zombie Cockroach

Species: The female parasitoid Jewel Wasp.
The Attack Sequence: * First Sting: Delivered to the thorax, which temporarily paralyzes the cockroach's front legs. * Second Sting: Delivered directly into the head, injecting venom into a specific area of the cockroach's brain.
Outcome: The venom impairs the cockroach’s ability to initiate movement. It becomes a "zombie" that the wasp can lead by its antenna back to the wasp's nest.
The Nursery: The wasp lays an egg on the cockroach and seals it in the nest. The larva hatches, feeds on the living cockroach for several days, and then pupates inside the cockroach's abdomen. An adult wasp emerges approximately one month later.

Evolutionary and Ecological Effects of Parasitism

Selection Pressure: Enslaver parasites exert strong selection pressure on host populations. While resistance is expected to evolve, there is currently more evidence of previous evolutionary changes (adaptations) than ongoing coevolution.
Evolutionary Links: Ecological interactions (like parasitism) affect evolution, and evolution affects ecology.
Ecological Structure: Parasites play a prominent role in food webs, competition, and biodiversity patterns. Without them, nature could be overrun by specific organisms; they structure ecological communities.

Biological Competition: Fundamentals

Definition: Competition is a $-/-$ (negative-negative) interaction where organisms compete for limited resources.
Resource Requirements: Resources are environmental components required for growth, survival, or reproduction that can be consumed or depleted. Examples include: * Food: Essential for life; lack of food leads to population decline. * Water: Vital for terrestrial ecosystems, particularly in arid regions. * Light: Plants deplete light for others by shading. * Space: Needed for growth (sessile animals) or for hunting, refuge, and mates (mobile animals).

The Ecological Niche

Fundamental Niche: The full set of resources and the complete range of biotic and abiotic requirements a species can potentially use.
Realized Niche: The actual, restricted set of resources a species is limited to due to species interactions (such as competition).

Varieties of Competition

Intraspecific Competition: Competition between individuals of a single species.
Interspecific Competition: Competition between members of different species.
Exploitation Competition (Indirect): Individuals reduce the supply of a resource as they use it. (e.g., Two herbivores eating the same forage).
Interference Competition (Direct): One species directly interferes with another's ability to access a resource. * Animals: Carnivores fighting over prey; meadow voles aggressively excluding red-backed voles. * Plants: One species growing over or shading another to block light (e.g., Kudzu vine).
Amensalism: A $-/0$ interaction where one species is harmed while the other remains unaffected.

Competition Case Studies

Rodents and Ants (Brown and Davidson 1977): * Both groups eat the same seeds. When either rodents or ants were removed from a plot, the remaining group consumed as many seeds as both groups combined would have normally.
Kudzu (Pueraria montana): * Native to Japan and SE China; introduced to the US in the $1870\text{'s}$ as an ornamental plant. * It is a formidable competitor that completely covers trees and shrubs, outcompeting them for light.
Allelopathy (Garlic Mustard): * An invasive plant in North American forest understories. * It releases sulfur-containing toxins that inhibit seed germination and growth of other plants. * It indirectly harms others by killing mycorrhizal symbionts (soil fungi). * It is also toxic to the larvae of native butterflies.

Competitive Exclusion vs. Competitive Coexistence

Competitive Exclusion Principle (Gause's Principle): Two species that use a limiting resource in exactly the same way cannot coexist indefinitely; the dominant species will drive the inferior one to local extinction.
Gause’s Paramecium Experiment (1930s): * P. aurelia vs. P. caudatum: Both feed on floating yeast. When grown together, P. aurelia is dominant and drives P. caudatum to extinction. * P. caudatum vs. P. bursaria: These two were able to coexist. P. caudatum lived/fed on suspended bacteria, while P. bursaria focused on yeast cells. However, their stable densities were lower than when grown alone due to competition.

Mechanisms of Coexistence

Resource Partitioning: Species use limited resources in different ways to coexist. * Warblers (MacArthur 1958): Five species (Bay-Breasted, Myrtle, Cape May, Blackburnian, Black-throated Green) coexist in New England forests by using different parts of the habitat (trees) for nesting and feeding. * Large Predators (Miller et al. 2018): Lions prefer areas with high large-prey abundance, while Leopards prefer "hideable" bushy habitats, topographic ruggedness, and areas with human tourist activity. * Temporal Partitioning (Negev Gerbils): Gerbillus pyramidum ( $40\,g$ , dominant) forages early at night in rich patches. Gerbillus andersoni allenbyi ( $25\,g$ ) is a more efficient forager and survives on patches abandoned by the larger species.

Character Displacement

Definition: When two species compete, natural selection favors phenotypes that partition resources, decreasing competition intensity.
Galapagos Finches: * Geospiza fuliginosa (small ground finch) and G. fortis (medium ground finch). * On islands where only one species exists, their beak sizes are similar. * On islands where both coexist, beak sizes (and thus the size of seeds eaten) diverge to reduce competition.

The Lotka–Volterra Competition Model

This model is a modification of the logistic growth equation to include the effects of competition.
The Logistic Equation: $\frac{dN}{dt} = rN \left( \frac{K - N}{K} \right)$ * Where $N = \text{population size}$ , $r = \text{intrinsic rate of increase}$ , and $K = \text{carrying capacity}$ .
The Competition Model Equations: $\frac{dN_1}{dt} = r_1 N_1 \left( \frac{K_1 - N_1 - \alpha N_2}{K_1} \right)$ $\frac{dN_2}{dt} = r_2 N_2 \left( \frac{K_2 - N_2 - \beta N_1}{K_2} \right)$ * $\alpha$ and $\beta$ are competition coefficients. * $\alpha$ represents the effect of species 2 on species 1. * $\beta$ represents the effect of species 1 on species 2.