Parasite lec 12

Parasites and Communities

Introduction to Parasites in Ecosystems

Parasites are often viewed negatively due to their harmful effects on hosts, including morbidity, mortality, and their role as vectors of zoonotic diseases. However, their ecological roles are complex and multifaceted, influencing biodiversity, energy flow, and ecosystem stability.

• Conventional View: Parasites are harmful and should be eradicated to maintain biodiversity.

• Modern Perspective: Not all parasites are the same; generalists often cause more problems than specialists, and parasites can play essential roles in ecosystem function.

Defining a Healthy Ecosystem

A healthy ecosystem is characterized by persistence, productivity (vigor), organization (biodiversity and predictability), and resilience to change (recovery time).

• Vigor: High productivity of biomass.

• Organization: High biodiversity and predictable interactions.

• Resilience: Ability to recover from disturbances.

Case Study: Carpinteria Salt Marsh

Classical Food Web Structure

The Carpinteria salt marsh in California is a model system for studying the ecological roles of parasites. The classical food web includes algae, horn snails (Cerithideopsis californica), killifish (Fundulus parvipinnis), and birds.

Role of Trematode Parasites

Adding parasites, such as the trematode Euhaplorchis californiensis, reveals their significant impact on the ecosystem:

• Life Cycle: Eggs in bird feces are eaten by snails, which become castrated and produce cercariae. Cercariae infect killifish, which are then eaten by birds, completing the cycle.

• Ecological Effects: Infected snails continue to graze algae, but their population is reduced, affecting energy flow and population dynamics.

• Behavioral Manipulation: Infected killifish display altered behaviors, increasing their predation risk by birds.

Energy Flow and Trophic Structure

Parasites influence energy transfer and population sizes within the food web, often increasing the availability of prey for top predators and altering the structure of trophic interactions.

Food Web Complexity and Connectance

Parasites increase the connectance of food webs, leading to greater stability and resilience. In Carpinteria, parasites constitute about half of all species and can represent a significant portion of biomass.

Parasites as Drivers of Biodiversity and Evolution

Shaping Host Population Dynamics

Parasites can restrict host population sizes, alter species composition, and drive evolutionary changes in host defenses.

• Example: In tropical forests, environmental DNA sequencing reveals high diversity of parasitic protists, especially Apicomplexa, which infect arthropods and other invertebrates.

Altering Interspecific Competition

Parasites can mediate competition between host species, sometimes favoring one over another and influencing community structure.

• Example: The nematode Heterakis gallinarum is transmitted by pheasants to native partridges, leading to declines in partridge populations.

• Example: The tick Dermacentor albipictus affects moose more severely than deer, leading to population impacts.

Influencing Energy Flow

By increasing biodiversity, parasites can enhance ecosystem productivity and buffer against the impacts of specialist parasites.

Driving Biodiversity and Evolution

Parasites promote genetic and phenotypic diversity in host populations, driving evolutionary arms races and increasing ecosystem complexity.

Parasites as Indicators of Ecosystem Health

Measuring Ecosystem Health

Parasite abundance and diversity can serve as indicators of ecosystem health, reflecting balance and resilience within communities.

Case Study: Caribbean Coral Ecosystem

Gnathiid isopods, parasites of reef fish, are used as indicators of coral reef health. Their abundance is negatively correlated with live coral cover, and they are preyed upon by cleaner fish and corals.

Parasites in Restoration and Contaminant Monitoring

Parasites such as trematodes can be used to assess restoration success in habitats like Carpinteria. Some parasites, like tapeworms, can accumulate contaminants and serve as bioindicators of pollution.

Parasites in Disturbed Systems and Invasive Species

Invasive Species and Parasite Dynamics

Invasive species often escape their native parasites, leading to unchecked population growth and ecological damage. Occasionally, invasive species encounter native parasites, which can regulate their populations.

• Example: The cassava mealy bug in Africa was controlled by introducing a specific parasitoid wasp from South America, demonstrating the importance of parasite-mediated biocontrol.

Parasite Conservation and Climate Change

Threats to Parasite Diversity

Climate change and habitat loss threaten parasite biodiversity, which can have cascading effects on ecosystem function and host populations. Conservation efforts may need to consider preserving parasite species as part of overall biodiversity.

Interactions Within Parasite Communities

Competition and Hyperparasitism

Multiple parasite species often infect the same host, leading to competition and sometimes the evolution of hyperparasitism (parasites of parasites). These interactions can influence parasite virulence and host outcomes.

• Example: Copidosoma floridana is a parasitoid wasp with soldier larvae that kill other invaders within the host.

• Example: Some flukes develop non-reproductive soldier castes to defend against competing fluke species in snail hosts.

Hyperparasitism Examples

Hyperparasitism is observed in various systems, such as microsporidians parasitizing trematodes within fish hosts, and in plant systems where one mistletoe species parasitizes another.

Myxozoans: An Evolutionary Puzzle

Discovery and Classification

Myxozoans are endoparasites with complex life cycles involving both invertebrate and vertebrate hosts. Initially classified as protozoa, they were later shown to be highly reduced cnidarians based on morphological and molecular evidence.

• Key Features: Multicellular spores, polar capsules similar to cnidarian nematocysts, and significant diversity within the phylum Cnidaria.

Summary Table: Roles of Parasites in Communities

Role	Example	Effect
Shape host population dynamics	Trematodes in Carpinteria	Regulate host densities, alter behavior
Alter interspecific competition	Nematodes in pheasants/partridges	Mediate competition, affect native species
Influence energy flow	Parasites in food webs	Change energy transfer, increase top predators
Drive biodiversity	Protist diversity in tropical soils	Promote host and parasite diversity
Indicators of ecosystem health	Trematodes, gnathiid isopods	Reflect restoration, pollution, coral health

Additional info: Parasites are integral to ecosystem function, influencing evolutionary processes, community structure, and ecosystem health. Their conservation is increasingly recognized as vital for maintaining biodiversity and ecological resilience.