Practical community ecology

◆ Complex networks of interactions◆ Practical consequences

– Agriculture

– Fisheries

– Forestry

– Human health

Lyme disease

◆ Reading– Ostfeld, R.S. 1997. Ecology of lyme-disease

risk. American Scientist 85:338-346– Jones, C. G. et al. 1998. Chain reactions linking

acorns to gypsy moth outbreaks and Lyme disease risk. Science 279:1023-1026

◆ available on Ecology course web page

Lyme disease◆ Disease symptoms

– red rash; bulls eye shape

– fever, chills, muscle aches

– untreated: arthritis, neurological disorders

◆ First identified in Europe 1909◆ Cause: Borrelia burgdorferi

– spirochaete

– idenitified in 1982

A common disease◆ 1990-1999

– 122,651 cases– Connecticut:

mean 63 cases per 100,000

◆ Reported in 48 states

◆ Most common vector borne disease in US

Lyme disease transmission

◆ Vector borne◆ Ticks◆ Northeast: Black legged

tick– Ixodes scapularis

◆ Transmitted by bites while taking bloodmeals

Tick life

cycle

Deer, rodents,

birds, canids,humans

Spring

SummerAutumn

Winter

eggs

Rodents, birds, small

mammals

Deer, canids,humans

Reservoir host

◆ Ticks become infected mainly as larvae◆ Acquire spirochaete from Peromyscus leucopus◆ Enzootic disease: cycles between mouse and

tick

Transmission to humans

◆ Both adults and nymphs can transmit spirochaete to people

◆ Nymphs believed to be greatest threat◆ Epidemic disease: outbreak in humans◆ Principal risk factor: Number of infected

nymphs in areas frequented by people in early summer

Epidemic disease

◆ Lots of infected nymphs, adults– High Tick population

– High infection rate

◆ Lots of human contact– Human-tick spatial and temporal overlap

Tick populations

◆ High tick populations associated with abundant acorns

◆ Oaks– boom / bust cycle of acorn production– Mast years

◆ Deer – eat twigs and leaves from woody plants– Mast years, eat acorns– Mast year, deer concentrated in oak woods

Deer and Ticks◆ Concentration of

deer in oak woods◆ Adult ticks in oak

woods◆ Lots of tick eggs in

oak woods◆ Following summer:

lots of tick larvae in oak woods

Experiment

◆ 1995– Acorn crop failure– 3 experimental grids (2.25 ha each)– added >800,000 acorns (>3500 kg)– 3 control grids

◆ 1996– monitor mouse density, breeding, ticks/mouse– monitor tick density

Experimental Results(Jones et al. 1998. Science)

◆ Addition grids:– Mouse densities 7x greater

– Winter: more mice in breeding condition

ExperimentalResults

(Jones et al. 1998. Science)

◆ Addition grids:– Tick abundance

8x greater

– Ticks / mouse 40% greater

High populations of infected ticks

Deer

Mice+

+

Tickegg-laying

Tick Larvae Infected

tick nymphs, adults

+

+

+

Acorns

Predicting Lyme disease risk

◆ Two years after oak masting, expect greatest risk in oak forest

◆ Test will require long-term data – several mast years

– monitor ticks, diseases incidence

◆ Where is risk in non-mast years?

Abundance of larval ticks

End 11th Lecture

Essay (20 pts)Mean = 15.0 (74.8%)SD = 2.1 (10.5%)

Overall (100 pts)Mean = 72.8 (72.8%)SD = 10.7 (10.7%)

Multiple choice (80 pts)Mean = 57.9 (72.3%)SD = 9.4 (11.8%)

Fixed Quota Harvest@1/2 MSY

NK

Rec

ruitm

ent

(# o

f in

div.

)

H

Nmed NN

MSY

Fixed Effort Harvest@ MSY

with sonar

NK

Rec

ruitm

ent

(# o

f in

div.

)

H

N

Fixed Effort Harvest

◆ H = g E N– H = harvest– g = efficiency (take per unit effort)– E = effort (e.g., trawler-days)– N = population

◆ If g is constant, H is linearly related to N, with slope dependent on E (But sonar changes g)

◆ With sonar, g is greater, so to maintain H at MSY, you need to reduce E

Answers

◆ Glass case on 3rd floor Felmley Annex◆ Figure out what you did wrong◆ Questions? … see me

Lyme disease infection

◆ Experimental manipulation of acorns– showed abundance of ticks increases after

acorn addition– Risk factor = abundance of infected ticks– data?

Longer time scale

◆ 1995 to 2000◆ Acorn crop in year t-2◆ Abundance of mice in year t-1◆ Nymphal ticks in year t

– density of nymphs– nymphal infection prevalence

❖ proportion infected

– density of infected nymphs

Effects of mice 1995 - 2000

Significant

Significant

Significant

Effects of acorns1995 - 2000

Significant

Significant

Not Significant

Acorn effects

◆ Combination of experiment and observation◆ Mast years of acorns determine human risk

of Lyme disease◆ Community interactions in this case impact

human health

Forest Fragmentation◆ Alters habitat

– shrubby species– edge

◆ Mammals– reduces diversity of

mammals– eliminates some species– increases abundance of

certain species

Peromyscus leucopus

◆ abundance increases in small patches◆ other mammal species decline in small patches◆ number of mammal species declines in small patches◆ effects on nymphal ticks and Lyme disease?◆ sample patches of different area for tick populations

Ticks vs. fragment

area

Significant

Significant

Significant

Conclusion

◆ Small fragments = Greater risk of Lyme disease

◆ Human alteration of landscape alters community and affects human health

◆ Fragmentation often a result of development – building homes– human exposure

◆ Mechanism?

Possible mechanism #1Competition and predation

Small fragment

P. leucopus

Other mammals

Infected Tick nymphs

+

-

-

Large fragment

P. leucopus

Other mammals

Infected Tick nymphs

+

+

-

Possible mechanism #2Dilution effect

Large fragment

P. leucopus

Other mammals

Infected Tick nymphs

+

+

-

Uninfected Tick nymphs

+

Small fragment

P. leucopus Other mammals

Infected Tick nymphs

+

-

Uninfected Tick nymphs

+

+

End 12th Lecture