ecology 2

Life history

A set of co-evolved traits that affect an individual's survival and reproductive potential. It sets the schedule of growth, development, reproduction and survival.

What does a life history represent, in terms of resources?

An allocation of limited time and resources to achieve maximum reproductive success.

Fecundity

The number of offspring produced by an organism per reproductive episode.

Parity

The number of reproductive episodes an organism experiences.

Parental investment

The amount of time and energy given to an offspring by its parents.

Longevity

The life span of an organism; also called life expectancy.

Traits at the FAST ("r-selected") end of the life-history continuum

Shorter life span, faster growth, earlier sexual maturation, earlier reproduction, more offspring, smaller parental investment.

Traits at the SLOW ("K-selected") end of the life-history continuum

Longer life span, slower growth, later sexual maturation, delayed reproduction, fewer offspring, greater parental investment.

A mouse vs. an elephant: which end of the fast–slow continuum is each?

Mouse = fast (r-selected). Elephant = slow (K-selected).

Grime's three environmental axes for plant life histories

Stress, competition, and frequency of disturbance.

Grime's three plant life-history categories

Stress tolerators, competitors, and ruderals.

Stress tolerators: growth rate, age at maturity, energy to seeds, vegetative reproduction

Slow growth; late maturity; low energy to seeds; vegetative reproduction frequently important. Example: woody lousewort (small, long-lived herbs).

Competitors: growth rate, age at maturity, energy to seeds, vegetative reproduction

Fast growth; early maturity; low energy to seeds; vegetative reproduction often important. Example: goldenrod.

Ruderals: growth rate, age at maturity, energy to seeds, vegetative reproduction

Fast growth; early maturity; HIGH energy to seeds; vegetative reproduction rarely important. Example: weeds such as Canada thistle.

Which Grime category puts the highest proportion of energy into seeds?

Ruderals. Stress tolerators and competitors both put low energy into seeds.

Why do many stress tolerators rely on vegetative reproduction?

Seeds are costly; reproducing from roots and stems avoids that cost in a stressful environment.

A plant invades a frequently plowed field, grows fast and makes many seeds. Grime category?

Ruderal (high disturbance, low stress).

Three fish life-history strategies shown on the 3-D plot (generation time, fecundity, juvenile survival)

Opportunistic, periodic, and equilibrium.

Opportunistic strategy

Short generation time, low fecundity, low juvenile survival. Small, fast-maturing species.

Periodic strategy

Long generation time, high fecundity, low juvenile survival. Large broadcast spawners.

Equilibrium strategy

Long-ish generation time, low fecundity, HIGH juvenile survival (heavy parental care).

Complex life cycle

A life history in which an individual lives in different habitats or undergoes a substantial change in morphology or physiology (e.g., tadpole to frog).

Principle of allocation

When resources go to one body structure, physiological function, or behavior, they cannot go to another.

The most common trade-off in offspring production

Number of offspring vs. size of those offspring.

Three trade-offs covered in lecture

1) Offspring number vs. offspring size; 2) mating/parental effort vs. parental survival; 3) offspring number (fecundity) vs. parental growth.

European magpie experiment: what happened when 1–2 eggs were added or removed from the usual 7?

Fewer chicks survived to fledge either way, suggesting 7 eggs is the optimal clutch size.

Kestrel brood experiment: chick survival in removal/control vs. enlarged nests

98% survived in removal and control nests; only 81% in enlarged broods (chicks were malnourished).

Kestrel experiment: effect of enlarged broods on the PARENTS

Extra hunting effort lowered adult survival into the next breeding season, reducing future offspring. Shows parental care vs. parental survival.

Determinate growth

Individual stops growing once it starts reproducing. Common in many birds and mammals.

Indeterminate growth

Individual keeps growing after it starts reproducing. Common in many plants, invertebrates, fish, reptiles, amphibians.

Per the lecture, long-lived organisms should favor which growth type, and why?

Determinate: grow first, reproduce later.

Per the lecture, short-lived organisms should favor which growth type, and why?

Indeterminate: reproduce quickly before death while still growing.

Benefit of delaying sexual maturity

The individual grows larger and can produce more offspring per year once it starts reproducing.

Across species, how is age of sexual maturity related to adult life span?

Positively: later maturity goes with more years of survival after maturity.

Trinidad guppies in HIGH-predation streams: 4 evolved life-history changes

(a) males mature at smaller size, (b) more energy to offspring (% of mother's mass), (c) more offspring, (d) smaller offspring.

Why do high-predation guppies reproduce earlier and more?

High predation shortens life span, so selection favors reproducing early and heavily before death.

Semelparity

Reproducing only once in a lifetime. Rare in vertebrates, common in insects and plants.

Iteroparity

Reproducing multiple times in a lifetime. Common in birds, reptiles, mammals, amphibians.

Annual vs. perennial

Annual: life span of one year. Perennial: life span of more than one year.

When does semelparity arise?

When reproduction requires a massive amount of energy.

Two semelparous plant examples

Bamboos and agaves: live many years, reproduce once, then die.

Periodical cicadas: life history

13- and 17-year cicadas are semelparous: years underground, emerge to breed, die soon after. (e.g., Magicicada septendecim)

Senescence

A gradual decrease in fecundity and an increase in probability of mortality with age.

Two explanations for why senescence exists

1) Inevitable natural wear and tear; 2) accumulation of unrepaired molecular defects (e.g., UV damage).

How do long-lived animals slow senescence?

Better mechanisms for reducing reactive oxygen species and for repairing damaged DNA and proteins.

Why should only long-lived species invest heavily in repair mechanisms?

Investment in repair pays off only if the individual is expected to live long. Allocation depends on expected life span.

Photoperiod

The amount of light in each day; a cue for many life-history events in virtually all organisms.

Daphnia diapause: Michigan vs. Alaska photoperiod cue

Michigan: mid-September, when days drop below 12 h of light. Alaska: August, when days drop below 20 h.

Why do Alaskan and Michigan Daphnia use different photoperiod cues?

Each cue tracks when local temperatures fall below a hospitable threshold, which happens earlier and at longer day lengths in Alaska.

Barking treefrog: high food vs. low food at metamorphosis (points A, B, C, D)

High food: large mass, young age (A). Low food: tadpoles compromise and metamorphose at a somewhat smaller mass AND somewhat later age (C), not B (same age, smaller) or D (same mass, later).

Life-history traits predation can affect

Time to and size at hatching, metamorphosis, and sexual maturity.

Red-eyed tree frog embryos and snake predators

Embryos sense snake vibrations on the leaf and hatch early, escaping the snake but hatching smaller and more vulnerable to aquatic predators.

Thoreau's Concord flowering data: what changed?

Local temperatures rose 2.4°C and first flowering now occurs about 7 days earlier.

Boston-area spring events since the 1850s

Bird arrivals, first bees and butterflies, flowering and leaf-out all occur earlier, due to warming.

Snowshoe hares and climate change

Hares change coat color by day length, but later snowfall and earlier melt leave white hares on brown ground, raising predation. A cue–environment mismatch.

Sexual reproduction

Progeny inherit DNA from two parents.

Gonads

The primary sexual organs in animals.

How are gametes produced, and what ploidy are they?

By meiosis; haploid (one full set of chromosomes). Two haploid gametes fuse into a diploid zygote.

Where does genetic novelty come from in sexual reproduction?

Random distribution of parental chromosomes into gametes, plus mixing of two parents' chromosomes.

Asexual reproduction

Progeny inherit DNA from a single parent.

Vegetative reproduction

Asexual reproduction where a new individual grows from nonsexual tissue of a parent (leaf, root, rhizome). Example: walking ferns.

Clones

Individuals descended asexually from the same parent with the same genotype.

Binary fission

Genes are duplicated, then the cell divides into two identical cells.

Parthenogenesis

Asexual reproduction where an embryo develops without fertilization.

Parthenogenesis: clones or variable offspring?

Either: clones if no meiosis; genetically variable offspring with full or partial meiosis.

Typical sex of parthenogenetic animals; vertebrate examples

Typically all female. Rare in vertebrates, but increasingly found (boas, pythons, iguanas, condors).

Three costs of sexual reproduction

1) Sexual organs cost energy and resources; 2) mating behaviors cost time/energy and raise risk of herbivory, predation, parasitism; 3) the cost of meiosis.

Cost of meiosis

The 50% reduction in a parent's genes passed to each offspring via sex compared with asexual reproduction, because gametes are haploid.

Two ways the cost of meiosis is offset

1) Hermaphroditism (pass genes via both male and female function); 2) male helps care for offspring, lowering female costs.

Kalutas (Antechinus): mating cost example

Males live ~11 months, then breed frantically in a 2-week window with very high testosterone; all males die by the end.

Benefit of sex #1: coping with environmental variation

Offspring meet conditions different from their parents'; genetic variation raises the chance some have well-suited gene combinations.

Benefit of sex #2: purging mutations

Through random assortment in meiosis, many gametes lack deleterious mutations, so sex can remove them.

Red Queen hypothesis

Sexual reproduction lets hosts evolve fast enough to counter rapidly evolving parasites. ("It takes all the running you can do to keep in the same place.")

Snail–trematode evidence for the Red Queen

Trematodes are common in shallow water (from duck feces). Shallow snails reproduce more sexually; deep snails more asexually and are infected more easily. Sex helps resist parasites.

Which groups mostly have separate sexes vs. hermaphroditism?

Most vertebrates and some plants have separate sexes; most plants and some vertebrates are hermaphrodites.

Perfect flowers

Flowers containing both male and female parts.

Simultaneous hermaphrodites

Have male and female reproductive function at the same time.

Sequential hermaphrodites

Have one sexual function, then switch to the other. Example: blue-headed wrasse.

Monoecious

Separate male and female flowers on the SAME individual. Example: hazelnut.

Dioecious

Each individual has only male OR only female flowers. Example: white campion.

Hermaphroditic plant example from the plant mating slide

St. John's-wort: flowers with both male and female parts.

Mnemonic: monoecious vs. dioecious

Mono = one house (both sexes on one plant). Di = two houses (sexes on separate plants).

Selfing (self-fertilization)

An individual's male gametes fertilize its own female gametes.

Why should selection favor outcrossing?

Selfing causes inbreeding depression.

Two ways hermaphrodites avoid selfing

1) Sequential hermaphroditism (separate sexual functions in time); 2) self-incompatibility genes.

When is hermaphroditism favored over separate sexes?

When adding the other sexual function causes only a small decline in the existing function, so total fitness rises.

When are separate sexes favored over hermaphroditism?

When adding the other sexual function causes a large decline in the existing function.

Mixed-mating strategy

Ability to switch between outcrossing and selfing, often in response to low resources.

Orange jewelweed and herbivory

After leaf loss, plants spend energy regrowing tissue and increase self-fertilization rates (a mixed-mating response).

Genetic sex determination

Sex determined by inheritance of sex-specific chromosomes (mammals, birds, many others).

Environmental sex determination

Sex determined largely by the environment (e.g., temperature).

Why does XY sex determination give ~1:1 sex ratios?

The sex with two different chromosomes (XY) makes about equal numbers of X and Y gametes, so half of offspring are male.

Haplodiploidy

Sex determined by fertilization: fertilized (diploid) eggs become females; unfertilized (haploid) eggs become males. Bees, ants, wasps.

Jacky dragon: temperature and sex

Low and high temperatures give females; intermediate temperatures give both. Female fitness is highest at high temperature; male fitness highest at intermediate.

Loggerhead turtle nests: depth and sex

Deeper eggs are cooler and more likely male; eggs near the surface are warmer and more likely female.

With temperature-dependent sex determination, how are both sexes still produced?

Environmental variation: early vs. late nests, cool bottoms vs. warm tops, open vs. shaded sites.

Hymenopteran females control offspring sex how?

By choosing whether or not to fertilize each egg.

Red deer: young vs. older mothers' offspring sex ratios

Male fawns are costly; yearling mothers can selectively abort male embryos (female-biased). Older mothers with enough resources produce ~1:1.

Frequency-dependent selection

Selection where the rarer phenotype is favored.

How does frequency-dependent selection keep sex ratios at 1:1?

The rarer sex faces less competition and has higher fitness, so mothers producing it are favored until it is no longer rare and the ratio evens out.