Assignment 1

•Table of data available on website- Katie emailed a few clarifications/errors

•Draft due in drop box today-- no big deal if a few days late–email Katie if need more time

•Annotated drafts will be staggered in their return as large number of these for Katie to handle

Population Ecology & Interspecific Competition

• Population Ecology– Density-dependent effects on growth, birth and

death rates– Life history patterns

• Interspecific Competition– Ecological effects of competition– Evolutionary effects of competition– Competition and community structure

Exponential & Logistic Population Growth

r = b-d

intrinsic rate of natural increase

Density-dependent birth and death rates determine K, the carrying capacity

Sea otters well below carrying capacity in a resource-rich environment:

Washington coast: N =600; increasing 10%/yr (K approx. 2400)

“Sigmoidal” or Logistic Population Growth is expected in most populations: changes in density-dependent mortality or fecundity occur as population nears K

Bacterium Lactobacillus annual Juncus modules Willow tree Salix

What determines an organism’s K? (population density)

Is this K variable from place to place & year to year?

Net recruitment curves as density increases towards K- defined as births - deaths

brown trout fruit flies, experimental herring

What population size allows greatest harvesting rate?

Life History Theory: -natural selection on “life history traits” influencing demographic variables -optimal life history evolves in ecological arena of selective pressures

• How long to live? (lifespan)

• Single bout of reproduction or many? (iteroparity/semelparity)

• Many flimsy offspring or a few robust ones? (allocation of reproductive effort into offspring size vs. litter size)

• Reproduce early or wait (and die later)? (age at first reproduction)

• How large to grow vs. allocate resources to reproduction? (allocation of resources into growth vs. reproduction)

Trade-off between growth and reproduction

Douglas fir Pseudotsuga ragwort Senecio

Trade-off between offspring number vs. size in the Australian snake Austrelaps ramsayi

Size of offspring(snout-vent length) vs.Litter size

Residual refers to statistically controlling for effect of maternal size

r-selection where adult survivorship low or uncertain

K-selection when adult survivorship high

Allometry of life history traits: + correlations with body size lifespan, survivorship (but variable!) age at first reproduction, iteroparity offspring size

- correlations with body size (lower value with increasing body size) litter size, fecundity

Humans as relatively r-selected: does high reproductive rate, relative to other great apes, indicate evolutionary history of relatively high mortality?

Large organisms (trees) in K-selected habitats:

a) iteroparous & low reproductive allocation

b) large offspring

c) delayed reproduction and long lifespans

Interspecific Competition• Occurs when individuals of one species suffer reduction in fecundity,

survivorship or growth because of exploitation of resources or interference by another species

• Therefore, these interactions affect population dynamics

• …and affect species distributions and evolution

• Therefore, competition influences the species composition of biological communities

• Competition in practice can have less impact than its potential

• Use of similar resources does not indicate competition is occurring, as resources may not be limiting to populations

Competitive exclusion in laboratory populations of two diatom species

Synedra outcompetes Asterionella even when starting population is smaller

Synedra

Asterionella

Why are Dolly Varden charr found at higher altitudes in Japanese streams compared than White-spotted charr?

Interference competition mediated by temperature-dependent aggressive behavior… but no evidence for dominance of DV charr at hi elevations (low temperature streams)

Fundamental niche larger than realized niche: expanded range where only one species

Sympatry Allopatry

Exploitative competition in Rocky Mountain Bombus:- when sympatric, specialize on preferred flower species- preference determined by feeding efficiency, matching proboscis length

- the “realized niche” are the resources & conditions for a population to exist, grow and reproduce in presence of competing population

Niche partitioning between diatoms where Si or P limited resource

Most interspecific competition is asymmetric: intertidal barnacles

…Balanaus outcompetes Chthamalus except in high dessication zone

Experimental removal of orange-crowned warblers resulted in 129% increase in Virginia warbler nestlings; and 78% reciprocally- hypothesized due to improved feeding efficiency

…but instead due to preferred nest site competition and reduced

predation

“Diffuse competition”between “guilds” of similar species with overlapping diet:

- overlap in seed size use was experimentally demonstrated to constrain relative abundances (American SW desert)

Asymmetric & diffuse competition among Bornean frugivores

The Competitive Exclusion Principle

• Coexistence of species in a stable environment depends on differences in their realized niches– Without such differentiation, one species will exclude the other

• Pattern not always linked to process: Different niches does not necessarily imply past or present competition– (species may differ from other evolutionary causes)

• Close competitors may coexist because environmental disturbance rarely allows competitive exclusion

• Disregard the Lotka-Volterra competition equations as they make simplistic assumptions about individual and environmental homogeneity, … but these indicate that coexistence possible when niches vary

The sea palm Postelsia colonizes gaps in mussel beds, and coexists by ephemeral occupation of these patches

- but coexist only if high rates of gap formation

Poorer competitors but better colonizers

Early germinating and growing plants

% Bromus depends on when germinates

Physa snails reproduce earlier and survive better in ephemeral ponds than superior competitor Lymnaea

First come, first served

Experimental plantings from seedsin different mixtures of 3-4 coexisting annual plant species

Three competitive inferiors grew better when aggregated than randomly planted

…but competitively superior Stellaria plants grew less well when seeds were aggregated illustrates coexistence in heterogeneous environments

Aggregated distributions can deflect superior species towards intraspecific competition

Character displacement: evidence of the “ghost of competition past”

-competitive release on islands

male & female Indian mongoose canine size(H. javanicus)

H.j.

H.e.

Distribution of 3 Herpestes species

Character displacement:

Stickleback fish in freshwater lakes (British Columbia)

- if two species, feeding apparatus morphology diverges

Species “packing” in communities structured by competition: Phytoplankton diversity associated with number of limiting resources (nitrogen, phosphorus, silicon & light)

n=221 samples from 3 Wyoming lakes

Species diversity increased with no. resources limiting growth

Evolutionary Effects of interspecific competition

Limiting similarity: how differentiated must niches be to allow coexistence?

- note: selection favors individuals with less overlap with competing species

- coexistence easier when individuals compete more with conspecifics

d>w, narrow niches & little competition d<<w, wide overlapping niches & high competition