Applications from food web and ecosystem theory - Ecological Applications: Toward a Sustainable World - page 247

risky stage for humans for three reasons - large numbers of people (including
Aretha) are hiking in forests and parks, the nymphs are small and diffi cult to detect,
and up to 40% of them carry the bacterium. The nymph drops to the ground where
it molts into an adult before taking a fi nal blood meal from a third host (often a
larger mammal such as a deer) and laying its eggs. This food web thus involves two
parasites (bacterium and tick) and a wide variety of host species. But wait, there's
more! The parasite-host web is itself embedded in a larger forest food web where
oak trees can play a key role.
The most abundant host of ticks in the eastern USA is the white-footed mouse
( Peromyscus leucopus ). Jones et al. (1998) recognized that acorns (oak seeds) are a
preferred food of the mice, and that acorns are produced in very large numbers from
time to time (known as mast years ). To simulate a mast year they added acorns to
the forest fl oor. Mice numbers increased the following year and, of more signifi cance,
the percentage of infected nymphal black-legged ticks ( Ixodes scapularis ) increased
2 years after acorn addition. Despite the complexity of this food web, it may be
possible to predict high-risk years (in relation to acorn production) and take steps
to educate and alert hikers to the danger.
It is important to realize that the potential mammal, bird and reptile hosts of ticks
show great variation in the effi ciency with which they transmit the Lyme bacterium
to the tick. The white-footed mouse is by far the most effi cient, but a plethora of
other host species harbor the bacterium but rarely transmit it. Squirrels, for example,
are abundant and provide blood meals to ticks but only a small percentage of these
acquire the bacterium. This means that squirrels, and other species that do not
transmit the bacterium effi ciently, act to 'dilute' the risk of disease to humans. With
this in mind, you might expect that high species richness of potential hosts would
result in less human disease because the high transmission effi ciency of white-
footed mice is diluted by the presence of a variety of less 'effi cient' species (LoGui-
dice et al., 2003). A related prediction is that small forest fragments, which contain
far fewer potential host species but where white-footed mice remain abundant, will
provide a greater risk of human infection than large tracts of undisturbed forest. If
this is the case, forest fragmentation, whose impact on biodiversity I have already
noted (Sections 3.4 and 4.5.1), might also be responsible for increasing the risk of
disease transmission. The prediction is supported by results from New York State
(Figure 9.4), which show an exponential decline in the percentage of nymphs
infected by the Lyme bacterium with increasing size of forest patches.
The importance of understanding human disease transmission from wild animal
populations, particularly rodents, cannot be overestimated: we can add to Lyme
disease a long list of nasties including Lassa fever, bubonic plague and various
hemorrhagic fevers. Mosquitos also carry many pathogens that are transmitted when
the female mosquito takes a human blood meal. Invasions by foreign mosquitos are
not unusual and some bring a pathogen with them, such as yellow fever transmitted
by Aedes aegypti - both arrived in the Americas on slave-ships in the sixteenth and
seventeenth centuries. Others transmit a native pathogen that was previously trans-
mitted by native mosquitos (e.g. malaria in Brazil became transmitted by Anopheles
gambiae after this arrived from Africa in 1930-40), and yet others arrive fi rst but
transmit a pathogen that turns up later (e.g. dengue fever, which arrived in infected
humans in Hawaii in 2001, but is now transmitted by the earlier arriving Aedes
albopictus ). On a somewhat br ighter note, Juliano and Lounibos (2005) note that an