Interspecific Competition: Lotka-Volterra equations

 

I.  Assessment-some general features of interspecific competition

A.  mechanisms

A.      consumptive or exploitative — using resources (most common)

B.      preemptive — using space, based on presence

C.      overgrowth — perhaps a combination of the first two

D.      chemical — antibiotics or allelopathy

E.      territorial — comparable to preemptive, but based on behavior

F.      encounter — chance interactions

 

B.  frequently highly asymmetric, competitive exlusion (displacement)

·         dung beetles replace flies in Australia

·         dingos replaced tasmanian tiger in Australia

 

C.  competition for one resource affects competition for others

 

II.  Competitive exclusion or coexistence

A. Lotka-Volterra: A logistic model of interspecific competition of intuitive factors

 

dN/dt = rN(1- (N/K) )   or ... rN(K-N)/K,    … where K is the upper limit to N

 

…but now we need a competition effect …

 

Kitching 1985:

 

dN1/dt = r1[(K1 - N1 - a12N2)/K1]N1

dN2/dt = r2[(K2 - N2 - a21N1)/K2]N2

 

r = rates of increase

K = carrying capacities

a = competition coefficients

 

dNi/dt = ri{[Ki - Ni - ∑j=1i≠j(aijNj)]/Ki}Ni

 

A.  the competition coeffecient (a)

B. Lotka-Volterra model: logistic model for 2 species

C. the behaviour of the Lotka-Volterra model is investigated using 'zero isoclines'

 

 

 

 

left: K1 = N2·a12 ...so: K1/a12 = K2' right: K2 = N1·a21 ...so: K2/a21 = K1'

 

lets assume that Ki' represents the individuals of species i that the environment would support IF THEY WERE ACTUALLY TRANSLATED INTO UNITS OF SPECIES J (given a 2-species system of interest)

D.  there are four to arrange the two zero isoclines

 

 

E. strong interspecific competitors out-complete weak interspecific competitors (top graphs)

 

left:          N1 always out-competes N2           

K1/a12 > K2 and K1 > K2/a21,                 

or K1> K2a12 and K2 < K1a21                                       

right: N2 always out-competes N1

K2 > K1/a12 and K1< K2/a21,

or K2a12 > K1 and K1a21 < K2

(K1/a21 = K2' and K2/a21 = K1')

 

A.      when interspecific competition is stronger than intraspecific, the outcome depends on the species' densities (unstable coexistence possible)

 

      right:           K2 > K1/a12 and K1> K2/a21,

or K2a12 > K1 and K1a21 > K2

 

G.  when inter- is weaker than intra-, the species coexists (stable coexistence)

 

left:             K1/a12 > K2 and K2/a21 > K1,

or K1 > K2a12 and K2 > K1a21

 

III. Tilman’s model of competition for specific resources (ZINGIs)

 

 

            left: B goes extinct                                             right: A goes extinct

 

 

 

            left: stable coexistence                                       right: unstable coexistence

 

IV. Coexistence: reducing competition by dividing resources

A. Niche: the n-dimensional hypervolume

1)       Niche: n-dimensional

2)       Niche: fundamental vs. realized

3)       Niche breadth

 

B. Species packing

1)       Resources can be subdivided

2)       Species packing may occur where resources exceed the minimum for survival see Tilman’s model

3)       what is the definitive set of resource dimensions that must separate two species?

 

C. The role of morphology

1)       character displacement: different sized organisms use different resource pools

2)       Not all morphological features are directly associated with aquiring resources

3)       Not all attributes of an organism directly associated with acquiring resources are physically apparent (behavior); hawks vs. owls


D. Predator-mediated coexistence

1)       competition for predator-free space

2)       prey switching = ƒ(abundance)

 

E.  r and K selection: from the logistic growth equation (see table 8.6 Stiling)

1) r-selected: reproduction (r-effect) dominates at low densities

a)       high reproduction

b)       weak competitors

c)       high mortality

2) K-selected: sensitivity to carrying capacity

a)       low reproduction

b)       strong competitors

c)       low mortality

F. heterogeneous, inconstant or unpredictable enviroments

·         Unpredictable gaps: the poorer competitor is a better colonizer

·         Unpredictable gaps:  the pre-emption of space (first come, best served

·         Fluctuating enviroments

·         Ephemeral patches with variable life-spans

·         Competitive release— competition removed

 

 

PRACTICAL APPLICATIONS TO DAILY LIFE?

 

PC Kangas and PF Risser. Species packing in the fast-food restaurant guild. Ecological Bulletin

 

What has ecology given to economics?  Competition Theory.

 

A. The commercial strip:

1) structure: a highway lined with commercial establishments

2) function: goods are provided in exchange for money

a) sessile consumers filtering resources from the flow of humanity

b) like an oyster bed or web-building spiders

 


Table 1. Cross-section of survey: SE 29th St., Del city, Oklahoma (1974)

 

Category

Number

Fast-food restaurants

13

Gas stations

11

Appliance stores

8

Finance dealers

7

Insurance dealers

7

Real estate

6

Clothing

6

Medical services

5

Barber shops

5

Restaurants

5

Beauty

5

Gifts

5

Totals: 81 Categories

196 Services

 

Question: How do these commercial establishments coexist?

 

Answer: Species packing!

 

B. Fast-Food Restaurant Guilds (FFR)

1)       most diverse guild present

2)       analogous to species populations

3)       customers are the resource (ultimately $)

4)       does competition structure the community?

5)       if so, patterns should be apparent in how FFRs utilize the resource base

 

C. Two tests applied

1) One guild: West Lindsey St., Norman, Oklahoma, detailed study

2) Ten guilds: were surveyed in less detail (menu analysis)

a) menus: analogous with diets of consumer organisms

b) measurements: menu overlap and width

 

Table 2. Fast-food restaurants along Lindsey St, Norman, Oklahoma (1974).

 

Restaurant

hours

open

time

(sec)

Park-ing

spaces

seats

inside

% menu overlap

menu width

Taco Bell

102

100

11

24

34

14

Taco Tico

91

123

35

92

49

29

Long John Silver

84

128

20

64

28

25

Chubby's

103

221

10

43

43

31

KFC

73

78

28

10

32

18

Sonic

99

389

28

0

47

35

Del Rancho

69

326

30

0

34

43

Roy Rogers

102

110

53

52

36

25

McDonalds

128

70

64

178

30

30

 

D. Results: see figures...clearly, the restaurants utilize resources in different ways, i.e., have different resource utilization strategies...back to table

1)       Sonic drive-in has intermediate hours and parking, high menu overlap, broad width, no seating and long serving time.

2)       MacDonalds has long hours, much parking, high menu overlap, broad width, much seating and short serving time.

3)       Are these relevant parameters?

a)       Customers were serveyed and corroborated the validity of these niche parameters

b)       Advantage of FFR ecology: one may converse with both resource and competitor!

 

E. Second test: general study of 10 FFR guilds (2-13 FFRs)

1)       result: menu overlap and width decreases with increasing numbers of FFRs

2)       consistent with compression hypothesis...niche overlap and width decreases with numbers of species, i.e., resource partitioning organizes the guild by minimizing overlap and compressing niches

3)       inconsistencies: particularly among generalists with large overlap and wide niche (like coyotes, alligators, humans)

a)       explanation: generalist FFR guilds dominate small towns with stable (small) resource base...while individuals may vary order...there are too few to justify specialization

b)       with larger resource base, more customers are willing to order a specialty

 

F. Types of strips

1) eutrophic: high volume flow...diverse specialist guilds are found

2) oligotrophic: low volume flow...generalist quilds dominate

 

G. Evolution of FFRs

1) ancestral FFR (advent of automobiles after WW II); primitive features:

a) generalist menu—wide niche

b) no inside seating

c) intercoms or carhops

2) first radiation (hamburger generalist): cheap, easy to prepare, convenient in cars, no franchise

3) second radiation (hamburger specialist, e.g., MacDonalds)

4) other radiations: various specalties, including hamburger size-specialists

5) future radiations: vegetarian, Vietamese, Thai, Chinese

6) menu evolution: MacDonalds breakfast (related to changing lifestyles/increased mobility, i.e., resource base)

 

 

 

 

 

Predation

 

Predators

 

I. The Nature of Predation

 

A. types of predators

1) predators: eat other living organisms

2) taxonmic and functional classifications

a) true predators:

i) kill living prey,

ii) > prey/lifetime (lions, tigers and bears)

b) grazers:

i) small amounts of living prey

ii) prey/lifetime (ungulants, leeches, mosquitos)

c) parasites:

i) small amounts of living prey

ii) 1 or few prey/lifetime (aphids, flukes, worms)

d) parasitoids:

i) kill living prey

ii) 1 per lifetime

iii) developmental stage (wasps, nematodes)

 

B. effects of true predation on prey (herbivory & parasitism, later)

1) attack weakest prey: low survival and reproduction

2) impacts of predation offset by reduced competition (somewhat)

 

C. effects of predation on predators

1) minima consumption for (zone of tolerance):

a) survival (lowest)

b) growth (higher)

c) reproduction (highest)

2) food quality ≠ quantity (especially plants)

3) predator satiation: more food cannot be assimilated

a) mast years, flocking: a prey strategy

4) effects of predator density : behavioral interactions

a) interference: reduces consumption rate

b) facilitation: social interaction enhances capture

5) response of a predator is limited by generation time

a) short GT: may track prey numbers

b) long GT: may not

 

II. Predators as foragers

 

A.  diet widths and compositions

1) food preferences

a)  preference: proportion of prey in diet exceeds relative availability

b) "availability" of prey is difficult to assess

2) ranked and balanced preferences

a) ranked preferences; assumption is that all foods are equal value

i) indicates the most preferred of all foods consumed

ii) relates best to perfectly substitutable resources

iii) most true predators; maximize benefit:cost ratio

b) balanced preferences: complimentary resources

c) many consumers exhibit a mixed approach

i) the human “food group concept”

 


A.  diet widths and compositions (cont)

3) switching

a) preference for common food types > relative availability

b) circumstances of switching

i) high concentrations of prey

ii) improved ability to capture/processing

iii) developing search images

 

 

 

4)  Diet width and design constraints

a) monophagy: prey abundant, accessible and predictable

i) maxmimize resource utilization efficiency

ii) reduces competition for resource

b) polyphagy: unpredictable prey or environment

i) food is diverse, easy to find (search cost low)

 

III. Predator-Prey Dynamics

 

A.  Possibilities

1) prey controlled by predator and both are stable

2) prey virtually eliminated by predator

3) prey virtually unaffected by predator

4) predator-prey oscillations

 

B.  Cyclical tendencies

1)  The Lotka-Volterra model

a) the Lotka-Volterra prey equation:

 

dN/dt = rN - a'CN

 

b) Lotka-Volterra predator equation:

 

dC/dt = fa'CN - qC

 

c) Zero isoclines

i) because dN/dt = rN - a'CN,

      if dN/dt = 0,

      then rN = a'CN,

      or C = r/a'

 

(predator isocline)

 

           


ii) bcause dC/dt = fa'CN - qC,

      if dC/dt = 0,

      then fa'CN = qC,

      or N = q/fa'

 

(prey isocline)

 

d) the model exhibits indefinite, neutrally stable fluctuations

i) requires stable environment

ii) coupled predator-prey osicillations

 

 

 

 

e) time delays: ramifications? why? biological mechanisms?

 

2)  Delayed density-dependence

a) regulatory effects of predators are difficult to demonstrate

b) oscillations in predator and prey densities vary K-the two populations are not synchronized, so K varies

3)  Predator-Prey cycles - fact or fantasy?

a) cycles are seldom clear

b) heterogenity vs homogeneity environment

c) Hudson Bay Company: plant-hare-lynx-grouse

i) food quality limits hares

ii) lynx track the hare populations

iii) grouse are more directly controlled by lynx in hare "off years"

 

C.  The effect of self-limitation on predators

1) a predator zero isocline with predator self-limitation

a) vertical line unrealistic: model artifact

b) minimally a linear relationship for prey:predator

c) probably nonlinear, due to predator interference

d) other factors limit predators

 

2) self-limitation has a stabilizing effect

a) possibly for inefficient predators

b) density-dependent aggression

c) territoriality

 

 


D.  Heterogeneity can stabilize prey populations

1) partial refuges lead to vertical prey isoclines at low prey densities

a) less prone to predation

b) stable prey abundances with efficient predator

2) Aggregation, another wrinkle in reality

a) prey 'escape'

b) increase efficiency of predation

 

E.  Allee effect (low density reduces recruitment)

1) Predator switching can stabilize prey abundance

a) predation can be heavy and efficient once a prey threshold is reached

b) predator populations can be maintained at high level by multiple prey

 

Foraging & Patch Dynamics

 

A. Optimal foraging - behaviors

1) diet width model : 'searching or handling'

a) searchers (generalists): unpredictable environment, diverse prey (polyphagous)

b) handlers (specialists): productive environment, monophagous

c) suboptimal foraging may result from other activities

i) organisms do more than forage: mating can reduce survival

 

2) functional responses: consumption rate vs food density

a) Type 1: zero handling time, finite capacity

b) Type 2: handling time and searching efficiency

i) handling time: constant

ii) searching efficiency: variable

iii) alternatively, predator confusion

c)  Type 3: (sigmoid)

i) switching (absolute vs relative prey densities)

ii) (decr) handling time or (incr) searching efficiency

 

 

B. resource patches in space and time

1) aggregative responses and partial refuges

a) preference for high prey density

i) enhances predator aggregation

ii) low-density patches = partial refuge from predation

b) patches in time: hide and seek

i) exploitation time

2)  Behaviors enhancing aggregation

a) consumers must perceive and locate the patch (vultures)

b) consumer response within patch

i) spend more time in profitable patches

ii) thresholds and giving up times (density vs. success)

Figure: For example, if each individual can harvest 1% of the available resource in one unit of time, then the rate of resource depletion depends on the number of consumers.

 

 

Note: Giving-up times decrease from 18 to 9 to 6 when the number of foragers increase from 1 to 2 to 3, respectively, if the giving-up threshold is 70% of the original resource level.

 

3) ideal free distribution: aggegation vs interference

a) balance attractive and repellent forces

b) all patches should be exploited to a similar level

c) optimal foraging: patches differ in value — so should foraging strategy

•time minimizers, energy maximizers, risk avoiders

4)  marginal value theorem: minimum threshold of resource extraction/time

a) when should a forager leave a patch?

i) function of patch quality

ii) function of other patch quality (experience)

b) maximize overall resource capture: including traveling time

c) predictions of the marginal value theorem

i) stay longer in more productive patches

ii) stay increases in lower productive environments

iii) stay increases when travel time is higher

iv) short (or zero) time in low productive patches

v) leave when extraction rate approaches minimum, regardless

 

C.  The optimal foraging approach to diet width

1) The diet width model-'searching and handling'

a) searchers (generalists): unpredictable environment

b) handlers should be specialists: productive environment

 

            Lotka-Volterra prey equation: dN/dt = rN - a'CN

            Lotka-Volterra predator equation: dC/dt = fa'CN - qC

 


Pe = a'TsN, where                       Pe = prey eaten by a predator

                                                a' = attack rate

                                                Ts = search time

                                                N = prey density

 

            So … Lotka-Volterra prey equation:

dN/dt = rN - a'C(Ts)N

            And … Lotka-Volterra predator equation:

dC/dt = fa'C(Ts)N - qC

 

Ts = T - ThPe, where                    T = total time

                                                Th = handling time/prey

                                                … remember: T = Ts + Th

 

so: Pe = a'(T - ThPe)N

 

or: Pe =                a'NT­             

                                         1+a'ThN

 

 ...a type 2 functional response

 

D.  Functional responses: consumption rate vs food density

1) Type 1: zero handling time; linear but finite capacity

 

… because as Th approaches 0,

 

Pe = a'NT­                

                         1+ 0

 

Pe = a'NT­                = a’NT

                             1

2) Type 2: Th and Ts

d)       Th: constant

e)       Ts: variable

f)         or, predator confusion

 

… because as Ts approaches 0, Th approaches T, because T = Th + Ts

 

Pe = a'NT­               

                          1+ a'NT

 

… as N gets large,

 

Pe approaches a'NT­ = 1.0

                                          a'NT

 

… a non-linear model, with maximum value

 

3)  Type 3: (sigmoid)

a) switching (absolute vs relative prey densities)

b) (decr) Th or (incr) searching efficiency = learning

 

 

E.  effects of predator or prey aggregations in time or space

1) intraspecific interactions: e.g., interference, facilitation

2) predator-prey interactions: ∆N/C ratios modify behaviors

 

F.  optimal foraging: patches of different value exist in space-time

1)  marginal value theorem: there exists some minimum threshold value of energy extraction/time

a) when should a forager leave a patch that it is depleting (when Pe is too low)?

a) function of patch quality (Pe = ƒ prey  =  N)

b) function of other patch quality (Ni; where i = habitat type)

c) required decisions based on the experiences of predator

b) maximize rate of overall energy intake: including traveling time (contributes to Ts)

c) predictions of the marginal value theorem

a) stay-times greater in more productive patches (minimize Ts)

b) also greater in lower productive environments or when travel time is higher (minimize Ts)

c) short (or zero) time in low prodictive patches (Th too high)

d) should leave when extraction rate approaches minimum, regardless of other factors

 

Grazers = Predators

 

I.  Predation includes grazing

 

A. predators eat other living organisms

B. classifications

1) “true” predators: kill living prey, many/lifetime

2) grazers: small amounts of living prey, many/lifetime

3) parasites: small amounts of living prey, one/lifetime

4) parasitoids: kill living prey, one/development

 

II.  The effects of herbivory on plants

 

A  Nature of herbivory

1) reduction in plant biomass

2) ... but the world is GREEN!

3) what controls herbivores?

 

B  Herbivory and plant survival

1) repeated defoliation kills plants ƒ(frequency)

2) seedlings are particularly sensitive

3) intense seed predation may not affect abundance

 

C  Herbivory and plant growth:

1) timing is important

2) grasses are resistant (coevolved with grazers?)

 


D  Herbivory and plant fecundity

1) smaller plants bear fewer seeds

2) herbivory can delay flowering...

3) ... and increase plant longevity

4) herbivores often destroy reproductive structures

5) ...but much pollen and fruit 'herbivory' is mutualistic

6) seed-eaters benefit plants by scattering, hoarding

 

E Disproportionate effects on plants

1) ring-barking and meristem consumption kills plants

2) herbivores may be disease vectors

3) not all effects are obvious (e.g., sap-suckers)

 

F Synergisms combined effects

1) herbivory and competition aren’t random or uniform

2) herbivores and pollutants

a) acid rain degrades cuticle increasing herbivory

b) herbivore damage can increase succeptibility to pollutants

 

III.  Responses of plants to herbivory

 

A  Plant compensation; herbivory can

1) reduce selfshading, increaseing net production

2) mobilize stored carbohydrates: roots to new shoots

3) alter photosynthate allocation

4) increase photosynthetic activities per unit leaf

5) reduce senescence if older parts are consumed

 

B  Defensive responses of plants — not helpless

1) plant defenses include

a) chemistry: tannins, phenols, alkaloids, etc....

i) secondary chemistry: no metabolic use

a) plant defensive chemistry (cont)

ii) accidental? coincidence? not likely!

•expensive compounds

•not randomly occurring within or among plants

•grazing may stimulate production

•considerable degree of coevolution

b) mechanical structures: spines, hairs, hooks

i) megafauna coevolution: thick hides, large size

c) hormonal control of pest development

d) behavioral adaptations: masting

e) mutualisms: especially ants

2) impacts on herbivores

a) specialists: coevolved herbivores — long-lived plants (less overtly toxic)

b) generalists: shorter-lived plants (many of the most toxic plants)

 

IV.  Models (basic predator/prey interaction)

1) plants (V): dV/dt = A - B

a) A = aV

b) B = bNV

c) so ... A = aV(K-V)/K

d) and ... dV/dt = aV(K-V)/K - bNV

2) grazers (N): dN/dt = C - D

a) D = dN

b) C = cNV

c) so ... dN/dt = cNV - dN (no carrying capacity)


3) at equilibrium: dV/dt = dN/dt = 0

a) so ... aV(K-V)/K - bNV = cNV - dN

b) so ... V = d/c

c) so ... N = a[K-(d/c)]/bK

 

Mutualisms

 

Biotic interactions take many forms: Symbioses are based on interspecific intimacy, involving a dependency

 

A. Types of Interspecific Interactions

1) Mutualisms: both species benefit

2) Commensalism: one benefits, one is unaffected

3) Herbivory: one species benefits at the other’s expense

4) Predation: one species benefits at the other’s expense

5) Parasitism: one species benefits at the other’s expense

6) Competition: both species loose / gain

7) Allelopathy (what?)

 

The world is strongly influenced by mutualistic relationships (corals, higher plants, etc...)

 

... sub-cellular structures may have evolved from symbioses

1) Serial endosymbiosis theory of evolution: eukaryotic cell

a) prokaryotic aerobe (mitochondria): detoxifying O2

b) motile surface bacteria (bacteria and cilia)

c) cyanophyta (chloroplasts, other plastids as well)

 

B. Behavioral Mutualisms

1) The honey guide and the honey badger: finder and muscle

2) Shrimps and gobiid fish: cover and scout

3) Clown fish and anemones: mutual protection

4) Cleaner fish/shrimp and customers or cowbirds and buffalo: parasites for food

5) Ants and acacia: food and shelter for protection

6) Domestication: The ultimate state of behavioral mutualism?

a) Homo sapiens: crops and livestock

b) farming of aphids by ants: who manipulates who?

 

C. Mutualisms with plants

1) pollination: flowers and animals

a) coevolution: flowering plants (insects, bats, birds)

b) specialists vs. generalists (tradeoffs?)

2) dispersal: fruit

a) coevolution: flowering plants (birds and mammals)

 

D. Nitrogen fixation (prokaryotes): evolved in unrelated groups

1) nitrogen fixation: nitrogen is often in limiting supply

2) yet only very few organisms can fix atmospheric N2

3) Rhizobium and leguminous plants (Leghaemoglibin, haem from bacteria dn globin by plant-> to reduce oxygen)

a) the energy costs of dinitrogen fixation

i) rising atmospheric CO2 may stimulate associations

c) nitrogen-fixing mutualisms may be 'suicidal'

i) Mt. St. Helens

4)  The evolution of nitrogen-fixing mutualisms: nif gene complex is essentially the same in all N-fixers (recent origin for independent lines of evolution?)

 


E. Mutualisms of algae

... with animals

1) Hydra and Chlorella

2) corals

3) kidnapping of chloroplasts: marine gastropods

...with fungus

1) the 'lichen habit' evolved in many unrelated groups

2) most lichen algae are Chlorophyta (not N-fixers), those that are Cyanophyta do fix N

 

F. Mutualisms of fungi

1) Sheathing forms (ectomycorrhizae)

a) mycorrihizal fungi require soluble carbohydrates

b) stimulates morphological changes in roots

c) hyphae penetrate between root cortical cells

d) absorb and transfer minerals to root

2) Vesicular arbuscular mycorrhizae

a) no sheathing or changes in root morphology

b) penetrates individual cells (arbuscles)

c) phosphate is transported by the fungus

 

G. Microbial consortia: many groups may live together where none may live alone

1) cryptoendoliths: extremely demanding environmentals!

a) fungus: miner / engineer

b) algae: producer / farmer

c) bacteria: decomposer / fertilizer

 

H. Some general features of the lives of mutualists

1) simple life cycles

2) suppressed sex in endosymbiont mutualists

3) limited dispersal in endosymbionts

4) no endosymbiont epidemics

5) constant numbers of endosymbionts/host

6) mutualism extends ecological niche breadth of both organisms

7) host specificity is often flexible

8) population dynamics of “interspecific help”:

a) dNi/dt = riNi(Ki - Ni + aijNj)/Ki

b) inherently unstable system (both mathematically and in reality?)

 

I. Commensalism (opportunists or cheaters?)

1) phoresy: mites and a “free ride” (Marily Houck)

2) remoras and sharks

3) burrs and stickers

... a precursor to parasitism?

 

 

Parasitism and Disease

 

I.  Introduction

1) definitions: harm, intimacy and dependence

2) differ from predators...how?

 

II.  The diversity of parasites

A.  Microparasites (multiply within host)

1) virus, bacteria, protozoa

a) directly transmitted: STDs, influenza

b) vector-transmitted: sleeping sickness, malaria

 

 

B.  Macroparasites (grow in/on host & infect other hosts)

1) external and internal

2) directly-transmitted macroparasites:

a) intestinal nematodes: roundworms, hookworms

b) lice and fleas: external, mobile

c) mildews and other plant-infecting fungi

3) indirectly transmitted macroparasites:

a) tapeworms: ingested by carnivores

b) schistosomes: aquatic snail host->swim to human

c) filarial nematodes: by blood-sucking insects

 

III.  Transmission and distribution

1) the appropriate units of study:

a) number of macroparasites

b) number of hosts infected with microparasites

 

A.  Hosts as islands: transmission requires agent

1)       transmission by contact: host densities & agent characteristics are important

2)       Mixtures of species and genotypes: hosts vary … barriers to parasite transmission

 

B.  The distribution of parasites and infected hosts

1) parasites are usually aggregated

a) many hosts have few or no parasites

b) few hosts have many

2) prevalence (proportion of host population infected)

3) intensity (number of parasites in a host)

4) mean intensity of infection (average number of parasites per host, including noninfected hosts)

5) distribution of parasites: depends on prevalence and mean intensity

 

IV.  The responses of hosts

A.  Necrotrophic parasites: kill host and live (blowfly)

1) pioneering detritivores: the first to dead matter

2) host immunity, resistence and response are limited

3) often a 'poor' or generalist parasite

 

B. The immune response of hosts

1) transient: microparasites (response strong)

2) persistent infections: macroparasites (weak response)

 

C. Responses to biotrophic parasites:

1) many biotrophs are tolerated: most effective??

2) some alter host morphogenesis: gall formation

3) some alter host behaviour: fungus/ant

 

D.  survivorship, growth and fecundity of hosts reduced

 

V.  The population dynamics of parasitism: among earliest mathematical models

 

A.  Directly transmitted microparasites

1) the basic reproductive rate

      (Rp: number of new infected hosts per infected host) and the transmission threshold ...

      (Rp = 1.0; minimum level for disease to spread)


2) the determinants of Rp:

a) density of susceptible hosts: N

b) transmission rate of disease:

      ß = ƒ(frequency of host contact, infectiousness)

c) infectiousness =

      #successful infections / #contacts

d) number of infected hosts surviving to become infectious (ƒ)

e) L: average time infected host remains infectious

 

Rp = ßNƒL

 

3) critical threshold densities (if Rp = 1.0); NT = 1/(߃L)

4) these equations help us to understand a number of patterns: various strategies of parasites to increase Rp

 

B. Vector-transmitted microparasites (forget equation)

1) vector-to-host ratio is vital with microparasites

2) contact transfers parasite both ways

 

Rp = ß2(NV/NTvƒhLvLh

 

(NV/NT) = 1/(ß2ƒvƒhLvLh) … (if Rp = 1.0)

 

 ß2; because contact transfers parasite both ways

 

C.  Directly transmitted macroparasites (forget eq.)

 

Rp = (lLaƒa) · (ßNLiƒi)

 

l; rate of egg production per adult

a = adult

i = infective stage

 

1) density-dependence within the host is crucial

2) plant macroparasites: a latent period

3) three phases in a plant macroparasite epidemic

a) exponential phase (parasite rarely observable)

b) second phase (individual growth)

c) terminal phase (greatest damage & untreatable)

4) these epidemics depend on:

a) initial dispersal of parasites to host population

b) speed of infection

c) proximity of susceptible hosts

 

VI.  Polymorphism and genetic change in parasites & hosts

1) gene-for-gene relationship: virulence & resistance

2) pathogens and dynamic polymorphisms in natural populations: Malaria, sleeping sickness & sickle cell anemia

 

Controls on Population Dynamics

 

I.  Introduction--the interpretation of census data

A. abundance is affected by many factors acting in concert

1) biotic

2) abiotic

3) synergisms abound

 

B. census information may hide vital details

1) because stages of the life cycle are hidden

2) because techniques are not always appropriate

 

C. mortality varies

1) age or sex of organism

2) time of year (seasonal)

3) interannual climatic variations (el niño, la niña)

4) disasters / catastrophes (hurricanes, forest fires)

 

D. populations may be divided into subpopulations

1) metapopulations are the sum of subpopulations

2) subpopulations may persist in places unsuitable for continued survival

 

II.  Fluctuation, stability or both?

A. Examples:

1) Swallows of Capistrano, southern California

2) British herons: stability despite catastrophe

3) apple blossom thrips: a truly fluctuating, albeit persistent, population

 

B.  Theories of species abundance

1) Intrinsic factors (density dependence)

2) Environmental factors (density independent)

 

III.  Mortality patterns: Key-factor analysis

A.  K (generation mortality) = log Nt - log N(t+1)

1) K is the mortality of N between t and t+1

2) K = k1 + k2 + k3 + ... ki, where i are the various causes of mortality

3) the KEY FACTOR is the k-value that is most closely related to K

 

B. Example: Colorado potatoe beetle (BHT Chap 15)

1) first estimate the mortality factors for a given cohort

2) next compare the mortality factors among cohorts

3) interpretation:

a) mean values

b) coefficient of regression

c) density-dependent regulation

 

1) first estimate the mortality factors for a given cohort.

 

Stage

Number

# Dying

Cause

log(N)

k

definition

Eggs

11799

2531

undeposited

4.072

0.105

k1a

 

9268

445

infertile

3.967

0.021

k1b

 

8823

408

rainfall

3.946

0.021

k1c

 

8415

1147

cannibalism

3.925

0.064

k1d

 

7268

376

predation

3.861

0.023

k1e

Early larva

6892

0

rainfall

3.838

0.000

k2

Late larva

6892

3722

starvation

3.838

0.337

k3

Pupa

3170

16

parastisim

3.501

0.002

k4

Summer adults

3154

-126

sex ratio

3.499

-0.017

k5

2 x Females

3280

3264

emigration

3.516

2.312

k6

Hibernating

16

2

freezing

1.204

0.058

k7

Spring adults

14

 

 

1.146

 

 

 

 

 

 

 

2.926

K(total)

 


2) next compare the mortality factors from several cohorts over time.

 

 

 

 

 

density

dependent

regulation

definition

Cause

mean

r

slope

intercept

r2

k1a

undeposited

0.095

-0.020

-0.050

0.27

0.27

k1b

infertile

0.026

-0.005

-0.010

0.07

0.86

k1c

rainfall

0.006

0.000

0.000

0.00

0.00

k1d

cannibalism

0.090

-0.002

-0.010

0.12

0.02

k1e

predation

0.036

-0.011

-0.030

0.15

0.41

k2

rainfall

0.091

0.010

0.030

-0.02

0.05

k3

starvation

0.185

0.136

0.370

-1.05

0.66

k4

parastisim

0.033

-0.029

-0.110

0.37

0.83

k5

sex ratio

-0.012

0.004

0.010

-0.04

0.04

k6

emigration

1.543

0.960

2.650

-6.79

0.89

k7

freezing

0.170

0.010

0.002

0.13

0.02

K(total)

 

2.263

 

 

 

 

 

3)  An example of estimating relationships among K and various factors:

 

Year

K

k1

k2

k3

k4

1

5.31E-02

8.82E-03

1.03E-02

1.04E-02

2.35E-02

2

1.33E-01

2.11E-02

4.15E-02

7.63E-05

7.00E-02

3

8.02E-02

1.29E-02

2.79E-03

2.45E-03

6.21E-02

4

7.50E-02

5.79E-03

2.72E-02

1.06E-02

3.15E-02

5

1.47E-01

2.04E-02

2.79E-02

2.83E-02

7.04E-02

6

1.00E-01

6.26E-04

2.36E-02

7.13E-03

6.89E-02

7

8.12E-02

1.18E-02

9.15E-03

2.48E-04

6.00E-02

8

1.51E-01

1.52E-03

1.31E-03

3.35E-02

1.15E-01

9

4.55E-01

6.69E-02

4.63E-03

1.13E-01

2.71E-01

10

 

 

 

 

 

 

R2 =

0.8454

0.0543

0.9268

0.9625

 

 


IV Other factors affecting population dynamics

A. Time lags

1) biological processes take time

2) adult migration in potatoe beetle overcompensates

 

B. Density vagueness

1) variation may enhance stability

a) predator / prey or host / parasite

b) too much success can lead to ultimate extinction

 

C. Spatial considerations

1) mobile organisms integrate resources over space

2) not all areas are equally valuable

3) ... but not all resources are substitutable

4) time is space

 

D. Intrinsic characteristics of biota and systems

1) mobile / non-mobile

2) size / space

3) longevity

4) trophic considerations

a) HSS theory

b) trophic complexity is related to productivity

5) stress

a) high stress: no herbivory, little competition

b) moderate stress: little herbivory, high competition

c) low stress: herbivory dominates vegetation

 

E. Categorizing controls

1) Top-down (extrinsic)

a) abiotic: climate

b) biotic: predation

2) Bottom-up (intrinsic)

a) resource limitations - ramify throughout system

 

F. Indirect effects

1) mutualists

2) predation on competitors

3) synergisms

 

VI.  Examples & Experiments

A.  Cycles

1) Canadian lynx and snowshoe hare

a) lynx track hares

b) hares track willow

c) ptarmigan track willow, pressured by lynx

2) lemmings in Alaska — boom and bust

a) low populations grow rapidly with high plant production

b) higher populations increase aggression and reduce food resources

c) higher populations attract predators & support pathogens

d) the “crash”

e) it takes time for nutrients to cycle to the plants

 


B.  The experimental perturbation of populations

1)  Introduction of new species

2)  Augmenting resources

3)  Removal of possible competitors

4)  Removal of predators (including herbivores)

5)  Introduction of a predator

 

Part I. Natural Pest Control & Management

 

Pest species are a natural component of ecosystems and, therefore, environmental resistance to their increase exists.  It is far more logical (and cost effective) to let nature control them as much as possible BEFORE resorting to expensive (and dangerous) biocides.

 

I. Pest Control

 

A. Pests (a point of view)

1. pathogens (humans or domestics)

2. harrassment (flies, etc... may transmit pathogens)

3. competitive herbivores (especially insects)

4. competitive carnivores (prey on domestic animals)

5. decomposers (cf. wood, leather, etc)

6. competitive plants (weeds)

 

B. Control value (human competitors)

1. herbivores (e.g., insects, deer, rabbits)

a. background losses

b. outbreaks (locusts)

2. weeds

a. total crop losses

3. carnivores

a. domestic livestock

 

C. Crop Losses

1. plant insects, weed, pathogens

a. 37% of potential agricultural production

b. annual losses of $64 billion

2. herbicides and pesticides

a. 500,000 MT annual production

b. $4 billion annual costs

3. industrial agriculture

a. increasing losses to pests

 

D. Philosophies of pest control

1. chemical technology; "magic bullet"

a. goal: pest eradication

b. side effects: many

2. ecologically based pest control

a. goal: protect human interests AND reduce pests

b. approach: minimize (not eliminate) chemicals

3. IPM: incorporates both methods

a. goal: cost effective and ecologically sound

b. approach: minimize negative impacts, maximize control

 

II. Promises and problems of chemical pesticides

A. Development and Application

1. historic: epidemics or plagues of insects

2. first generation pesticides

a. inorganics, e.g., heavy metals, cyanide

3. second generation pesticides

a. synthetic organics, e.g., chlorinated hydrocarbons

i. DDT: 1938 (first magic bullet): nobel prize

•highly toxic to insects

•apparently nontoxic to humans

•cheap, broad-spectrum & persistent

•banned in most industrial countries by 1970

ii. Silent Spring (Rachel Carson 1960s)

•non-target toxicity to birds

•initiated environmental awareness and debate

•Carson died of cancer

 

B. Problems with chemical pesticides

1. Resistance

a. how does this happen?

i. adaptation/evolution

•a stringent selective breeding program

ii. what effect on gene pool?

b. multiple resistences are possible (e.g., antibiotics)

2. Resurgences and secondary outbreaks

a. pest may return at higher numbers after being destroyed

i. why: release from compeition/predation

b. secondary pests may become the primary pest

i. why: release from competition

c. the approach is unstable (too broad-spectrum)

i. not pest-specific

ii. ecologically unbalanced

iii. favors the “weedy” species

3. Adverse effects

a. Ecological

i. DDT bioaccumulates in the food chain

•fat-soluble

•first observed in birds of prey: osprey, eagle, falcon, pelicans (especially in fish-eaters: long food chain)

•egg-shell thinning

4. the “Pesticide Treadmill”

a. pest->application->resurgence->//->application->resistence

b. application of pesticides is only a stop-gap mechanism: cannot solve the problem

c. Costs

i. the cheapest pesticides don't work or are banned

ii. cost/benefit

iii. Human Health: threatened the same as other organisms

 

C. Nonpersistent pesticides

1. DDT: 20-yr half-life

2. safer?

a. environmental impact = f(toxicity, dosage, location)

i. many nonpersistent pesticides are extremely toxic

•must also be applied more frequently

ii. non-target organisms still at risk

•directly or indirectly (food web)

iii. still no better conceptual approach

 


III. Alternative Pest Control Methods: BE SMARTED THAN BUGS!

Insect Life Cycle (egg->larvae->pupae->adult)

Vulnerable Points: can be attacked at different points

 

A. Cultural Controls: alter environmental factors

1. Human Pests and diseases

a. sewage disposal

b. bathing, brushing hair

c. clean clothing

d. window screens

e. remove stagnant water

2. Lawns, Gardens and Crops

a. Selecting species and sites

i. plant under appropriate circumstances

ii. natural vigor will compensate for some attack

iii. select alternative species

•spidermites love marigolds

•cabbage butterflies love snapdragons

•monarch butterflies love dill

b. Lawn and pasture

i. weed problems: let the grass grow longer

c. Water and fertilizer (alter limiting factors)

i. too much or too little may stress different plants

d. Timing of planting

i. linked to natural cycle of the pest

ii. avoid timing the plant's most vulnerable periods with the pest's most destructive period

e. Crop residues

i. sites for spores, eggs

ii. protect dormant individuals

iii. plowing or burning

(drawbacks??)

f. Adjacent crops and weeds

i. eliminate attractants

ii. plant repellants

iii. remove intermediate hosts

g. Crop rotation

i. alternating crops reduces plant-specific pests

h. Polyculture:

i. many species at once

i. Customs and quarantines

i. efforts to minimize mixing (ultimately fail?)

 

B. Natural Enemies

1. Examples:

a. predators: ladybird beetles on scale

b. parasites: wasps on caterpillars

c. disease: bacteria on Japanese beetles and gypsy moths

d. herbivores: prickly pear by moths

2. Use existing control organisms

a. enhance their populations (don’t kill with pesticides)

3. Exotics: adquate control and specificity required

 


C. Genetic Control: enhance pest resistence

1. Chemical Barriers

a. lethal or repellent to attackers

b. can defeat purpose if affects humans or domestics

2. Physical Barriers

a. barriers to eating or movement

3. Sterile Males

a. screwworm fly: females only mate once

•release millions of sterile males

4. Biotechnology

a. transgenic crops: natural resistences

•Ellen Peffley (TTU): onion resistent to fungal disease

•herbicide-resistent crops

b. viruses and bacteria (BT)

c. sort of an evolutionary arms race

 

D. Natural Chemical Control

1. hormones/pheromones

a. highly specific and nontoxic

2. can interrupt metamorphosis

3. sex attractants can trap or confuse

 

IV. Socioeconomic Issues

A. Causes of pesticide overuse

1. at what point is a species actually a pest?

a. define according to economic threshold (cost/benefit)

2. Machismo Factor: the only good bug...

a. narrow-minded ignorant perspective

i. high and frequent uses are counterproductive

3. Aesthetics

a. in search of the "perfect" fruit or vegetable

i. do you really know what you're looking for?

b. the Snow White syndrome

c. cosmetic spraying (Alar on apples)

i. avoid damage to product's APPEARANCE

d. same with lawns and ornamentals

4. Insurance spraying ... just in case ...

a. response to an imaginary threat

5. Profits

a. big business

i. advertising

 

B. Organically Grown … what’s in a name?

 

C. Integrated Pest Management: control not eradiction

1. Use ecological principles against pest, for crop

a. competition

b. predation

c. parasites

d. abiotic factors, too

2. Specialized advising

a. need specific information from knowledgable sources

i. other than pesticide manufacturers

b. pest-loss insurance (often high priced)

3. Economics of natural controls

4. cost/benefit analysis: market demand

5. maximize the profit margin, i.e., maximize yield with minimal investment

 


D. Integration of natural controls

1. the medfly program

a. customs

b. sex bait traps

c. sterile males

d. fruit stripping

e. pesticide (malathion)

f. 1990 whoops!  Much of San Diego had to be sprayed with malathion

 

E. Prudent use of pesticides is unavoidable and cost-effective

 

Part II. Biota: Background & Conservation

 

Biota: all living things (collectively); Only 1.5 M of the estimated 5-10 M species have been described.  Many/most of the others in oceans and tropics.

 

I. Wildlife: preservation or not? hunter-gatherer -> agriculture -> ?

A. Game animals

1. traditional food

a. no longer necessary

b. market hunting unlawful

2. ... now a sport

a. aesthetic value

b. sportsman support = conservation

3. non-game animals

a. only recent support $$

•song-birds

•butterflies

4. current problems

a. road kill exceeds hunting losses

b. public hazard / nuisance populations

•rural: deer, raccoon, coyote

•urban: fox, opossum, raccoon

c. habitat destruction = overcrowding

 

B. Endangered species act (1973) one hot tamale!

1. endangered: imminent peril of extinction

2. threatened: approaching endangered status

3. 1995 Supreme Court: act extends to private property

4. criticisms

a. too little, too late?

•no help for species until endangered

•too much time to enact protection

b. too much, too soon?

•limited exploitation of public lands

•limited uses of private lands

c. key questions: who owns a species?  what is a species worth?

 

II. Selfish (Practical?) Values: we STILL depend on natural biota

 

A. Biological Wealth: a seldom perceived resource

B. Instrumental (self-centered) vs. Intrinsic (moral) values

1. Underly agriculture and forestry: what were the origins of the cultivated species?

a. compare genetic diversity of wild and domestic species

b. how do you determine a non-valuable trait?

c. what is the origin of "new" traits for cultivated species?

d. how many of the potential species do we cultivate?

e. the natural biota = genetic bank


2. Medicines (25% of all drugs from plants)

a. the rosy periwinkle (Madagascar):

•childhood leukemia & Hodgkin’s disease

b. almost all drugs are from plants/fungi/animals

c. ethnobotany: study of medicinal plants

3. Commercial interests (Ecotourism)

a. sporting goods

b. resorts

c. travel

d. fishing

e. logging

4. Recreational, aesthetic and scientific values

a. enormous potential and real income

b. vast number of particular pursuits

i. even off-road vehicles!

5. Intrinsic Value: moral issue

6. Natural services

a. water cycle

i. runoff and erosion control

ii. infiltration and groundwater recharge

iii. flood control

b. wetlands: recycling nutrients (filters)

i. $100K per year to duplicate 1 acre of wetlands

c. soils: maintain fertility

 

III. Biodiversity: the numbers and kinds of living organisms

 

A. losses

1. Known and Unknown: most species remain undescribed

 

B. Reasons for loss

1. Habitat loss: single greatest impact

a. conversions: clearing forest, plowing grassland, draining wetlands

i. loss of entire community with all species impacted

ii. migrants: birds require adequate habitat each step of the way

•eastern song bird populations down 90% due to habitat loss in tropics

b. fragmentation: divided habitats

i. partial conversions

ii. barriers to dispersal

c. simplification: human uses reduce environmental complexity

i. managed systems are monocultures

2. Population problems

i. growing human population

ii. remember the concept of the "critical number"

3. Pollution: inadvertant

a. widespread and severe

b. the rate of global change will be far too fast for many species to adapt … note synergisms

4. Foreign species (exotics): competitors and predators

a. Americal chestnut blight

b. Australian rabbits and prickly pear

c. CATS!!! ... carp, zebra mussles, kudzu, eurasian tumbleweed, etc ...

5. Overuse: overharvest

a. whales and fish

b. rare and exotic woods, animals or animal products

i. who bears to blame, hunter or buyer?

c. firewood, over-harvest of habitat


6. Synergism and environmental degradation

a. as expected, combinations of factors can be very deadly

 

C. Consequences ... poorly known but partly extrapolative

1. loss of gene pool ... so what ...

2. ... which species are showing greatest adaptive potential?

 

D. International efforts towards conservation

1. CITES: convention on Trade in Endangered Species

a. international agreement regarding traffic in animal products

b. international ban on ivory trade

2. Convention on Biological Diversity (Biodiversity Treaty)

a. recognition of diversity as important

b. money from industrial countries will help developing countries to converve

c. access to genetic resources will be controlled by resident nation

d. technologies based on genetic resources will be provided to originating country

e. not ratified by USA

i. agriculture and biotech industries oppose it

ii. ... what else do developing countries have of worth?

 

E. tragedy of the commons

1. roughly equated to "free market" economics

a. unwilling to take a loss, individuals (corporations) will ultimately destroy their livelihood

•the golden goose?

 

Part III. Biota: Use, Management & Harvest

 

I. Pressures, Impacts and Ramifications of Harvest

A. Logging: Forests and Woodlands

1. Impacts

a. reduced productivity

b. reduced nutrient pool (standing stock)

c. reduced biodiversity

d. increased erosion

e. altered hydrological cycle

i. runoff-infiltration ratio

f. reduced C-fixation (global CO2)

2. Ecological Services

a. sustainable logging asks about …

i. timing of harvest

ii. which trees

iii. landscape considerations

b. monospecific plantations

i. loss of biodiversity

ii. retention of structure

c. extractive reserves: semi-natural

d. ecotourism

e. local control: villagers & stewardship

i. education needed

 

B. Oceans: mostly biological deserts

1. Marine Fisheries: tragedy of the commons

a. 12 mile limits extended to 200 miles

b. shifted overfishing from international to national problem

c. heavy govt subsidies (your taxes) spent to support overfishing

d. key species are overfished (cod, haddock, sea bass)

e. new species are being exploited (orange roughy, sharks ... krab salad)

f. overfishing is UNDENIABLE ... the catch data are empirical

g. recovery may be possible in many cases, but not rapid

h. establishing commercial "fishing rights" may be an approach

i. "by-catch" often outweighs catch (Texas shrimp)

•marine turtles are accidental kill

2. Whaling

a. overexploited for centuries (empirical catch data)

b. 1980s: international ban

c. Japan, Norway, Iceland ("scientific" studies)

d. Inuits (traditional harvest)

e. conservation / moral / ethical considerations

3. coral reefs and mangroves

a. shallow coastal areas in warm waters

b. filter feeders — support large, complex communities

c. perhaps more diverse than tropical rainforests

d. recent diebacks

i. bleaching

ii. eutrophication

iii. pollution

iv. overexploitation — people on tropical islands

•food

•tropical fish trade: cyanide & dynamite

e. mangrove systems: tropical shores

i. shoreline stabilization

ii. spawning grounds / juvenile fisheries

iii. marine wetlands

 

III. Conservation ≠ Preservation

A. Patterns of Use

1. Privatization: avoid the Tragedy of the Commons

a. private ownership CAN limit some public abuses

b. private ownership can CAUSE other abuses

c. in both cases: short-term profits can destroy the resource

•especially if the value cannot be extracted

•e.g., endangered species, natural services

d. resolution: knowledge and reasonable limitations

2. Restoration projects

a. to return the degraded environment to original form

i. sometimes impractical: old growth forests

ii. sometimes impossible: heavily eroded sites

iii. sometimes attainable!

•what type of systems?

•what type of conditions?

3. Maximum Sustainable Yield (MSY)

a) harvesting to avoid BOTH over- and underexploitation

b) theory: limit harvest to (maximum) sustainable level

i. in nature, most young organisms die ... why not harvest?

ii. in nature, these deaths drive the food chain

iii. in nature, disease, starvation and parasites set upper population limits

iv. in nature, conditions vary over time

c) reality check: difficult to determine MSY, much less enforce regulation

4. Approaches to Harvesting

4)       simple harvest model: a fixed quota

G.     Assumption: harvest = peak net recruitment

5)       fixed quotas: harvest the average MSY

·         Assumption: MSY is known and stable


 

6)       regulation of harvest effort

g)       Assumption: varying catch for fixed effort, e.g., regulated number of fishermen or hunters

7)       multiple equilibria harvest model

h)       Assumption: recruitment curves are highly variable

8)       regulated-percentage (take) or regulated-escapement (leave) harvest

d)       Assumption: maintain critical minimum population

9)       population structure: dynamic pool and surplus yeild models:

a)       Assumption: differential harvesting effort within the population, usually based on age structure