Disease Ecology Exam 2

Disease ecology fall 2026 university of Delaware Ellis ENWC428

Fundamental niche

All the places an organism could exist

Competitive Exclusion

No two species with the same niche requirements can coexist indefinitely

Realized Niche

Subset of a species’s fundamental niche that it is restricted to due to species interactions

Examples of species interactions

Competition, predation, human interaction

Interspecific competition leads to

Reduced abundances, and smaller realized niches

Hubbell’s Neutral Theory of Biodiversity and Biogeography

Trophically similar species are “demographically identical on a per capita basis in terms of their vital rates of birth, death, dispersal”

Fecundity (life table bx)

Average number of offspring per individual produced at a particular age x

Survivorship (life table lx)

Proportion of the cohort still alive at age x

x (life table)

Age in years

Janzen-Connell Hypothesis

The probability of survival of a seedling is greater far from its parent tree because it will escape host-specific natural enemies

Conspecific Negative Density Dependence

Poor growth or survival in the presence of a conspecific

Enemy Release Hypothesis

Introduced species that leave their natural enemies behind may reach higher population densities in their introduced ranges relative to their native ranges

Apparent Competition

indirect negative interactions between species/populations mediated by a shared natural enemy

Ricklef’s Hypothesis

specialized parasites and pathogens may lead to variation in abundance among closely related species.

Some host species may deal well with their parasites and others not so well; this may be a dynamic equilibrium governed by coevolution

What happened to American Chestnuts?

nearly wiped out by chestnut blight fungus introduced in NYC around ~1900

Why do introduced parasites have such large effects on host populations?

Hosts are immunologically naive

How can you predict how demographic effects (shorter lives, fewer offspring) can impact a host population?

Life tables

When R0 is greater than 1, what happens?

The population is growing

If R0 is less than 1, what happens?

The population is shrinking

If R0 is equal to 1, what happens?

The population is stable

Coevolution

Evolution in one species in response to selection imposed by a second species, followed by evolution in the second species in response to reciprocal selection imposed by the first species

Gene-for-Gene Model

Harold Flor
Predicts universally infectious parasite phenotype and universally susceptible host, describes some host-pathogen systems, but not a universal mechanism

Matching Allele Model

• Assumes a lock-and-key mechanism

• Matching alleles prevent infection (host recognizes pathogen)

• but non-matching allows infection (pathogen escapes host immune system)

• Probably a more generally representative model of host-parasite interactions

Red Queen Hypothesis (RQH)

Host-parasite coevolution drives oscillatory changes in genotype frequencies through negative frequency-dependent selection

Negative Frequency-Dependent Selection (NFDS)

When the fitness of a phenotype/genotype increases as it becomes rarer

Why is sexual reproduction costly?

• Numerically inefficient (need 2 parents to make 1 offspring)

• Genotypes well adapted to the environment can be lost or changed through meiosis and recombination