conway (1976) cites the codling moth (cydia
pomonella) as a k-pest : it represent an excellent example of a k-pest's normal
harvesting having no real impact on the species being harvested, but the level
being intolerable to man (clark et al. 1967). elephants always have more impact
on their habitat than the codlin moth does on its habitat, but in forest situations
in both africa and indonesia the natural populations do not seem to cause
significant degradation. however when man cultivates small areas in the pest
caregory. tha skylark (alauda arvensis) is a third example of a relatively
k-species becoming a pest because, economically, man cannot tolerate its
feeding. in the spring in east anglia skylarks often remove a small number of seedling
of sugar beet. formerly there were excees seedlings and the stand with
specially bred sugar beet fruit containing a single fertile seed have made this
bird, the poet's 'blithe spirit', a pest.
a striking instance of the damage caused by
a k-species after man has disturbed its regulation is provided by the african
elephan (loxondonta african) when is population is confied to game parks. this
compression of the population, together with the shootingof the leader
matriarchs of the individuals : these mill around on the peripheryb of the park
where their activities are causing srious destruction of the trees and other
vegetation (laws et al.1975)
a slingtly diffrentn scenario is provided
by the bullfinch (pyrrhula pyrrhula) which is now regarded as one of the major
pests of fruit growing in souther britain. its large size for a finch and
territorial, non-migratory behaviour are k-characteristics. for much of the
year the adults are seed feeders, but in the spring the flowering buds of trees
form a major part of their diet ; where they are availabel the buds of certain
fruit trees are favoured. an individual will take between 15 and 45 buds/minute
and will feed for about five hours/day (newton 1964) . thus a singel bullfinch
may take some thoussnds of buds per day and a relatively low population may cause
a market reduction in the subsequent yield of fruit, a characteristic of a
k-pest. it seems that shorteage of food resources in the spring often limits
the size of the bullfinch populationb (newton 1964, lack 1966) and thus we may
suppose that the development and planting by man of fruit trees with large buds
has increased the survival and hence population level of the bullfinch in fruit
growing and garden areas. it has been recognized as a pest for over 300 years,
but the upsurge in its importance in the last two decades may reflect a further
increase in population size due to reduced predation, especially by man
(shooting and egg-collecting) but also by raptores another reason for its
increased importance in modern crops may be relative because the amount of
damage sustained from other causes has been greatly breduce in the last thirty
years.
in tropical forests ants frequently play a
dominant role in the invertebrate ecosystem: modification of their distribution
may allow the increase of other insecta, such as the coreids amblypelta and pseudotheraplus
on coconuts, which then cause extensive damage to their hosts (o'connor 1950
way 1953 a and b)
undoubtedly the introduction of plants and
animal to new areas is the commonest cause of k-pests. the cocoa tree has been
introduced into west africa and the fas east and in both areas among the
indigenous insects that have become pest area centain large-sized mirids:
sahlbergella singularis , distantiella theobroma, pseudodoniella spp. and
platyngomiroides apiformis (leston 1970, conway 1971). in west africa the two
former cause considerable damage at relatively low densities (2-4 per tree) to
the young shoots of cocoa; their native hosts are various trees, cola spp.
adansonia and other malvales, and these suffer less than cocoa (entwistle 1972).
the transfer to cocoa in west africa seems to have occurred around 1900 and is
undoubtedly a reflection of the general relationship between the number of
insect species attached to a tree and its abundace ( southwood 1961, strong
1974): it is noteworthy that there is now some evidence that there may be
separate races (entwistle1972). platyngomiroide apiformis was first found on
cocoa in sabah in 1962 feeding on pods, later young shoots (chupons) were
attacked (lesions axtending up to a foot from the feeding site ) and in 1964
servere and extensive damage to the trunk and branches was seen ( conway 1971).
the sensitivity of the new host, to the extent that trees may be killed by relatively
small numbers, is a feature of k-pests arising by this mechanism and it is noteworthyvthayvin
brazil, where the relationship with the mirid monalonion presumably has a long
evolutionary history, the major damage is the pods: unless this is at a very
high level it is unlikely to influence the food supply of subsequent
generations a feature of k-strategisst.
turning to a completely different part of
the trophic web many zoonoses may be regarded as examples of k-pets. these are
discussed by garnham (1971, 1977) : for the purpose of this argument it is
necessary only to point out the good adaptation of the parasite tp its natural
host whose numbers it seldom depletes (the k-strategy); and the extreme
sensitivity of the new (unnatural or adventitious) host, in which the parasite
may develop new races (c.f.cocoa mirids) and where it behaves essentially as an
r-strategist
extremly intersting examples of two k-pests
are provided by the african trypanosomes and their vectors, tsetse flies
(glossina spp.) conway (1976) has already pointed out the k-strategy characters
of tsetse flies and
it is noteworthy that
they are considered to have evolved in association with that K- group par
excellence, the large reptiles: on the latter's extinction (apart from
crocodiles) tsetse transferred to suidae, which occupied similar habitats and
were abundant in the eocene and oligocene, or to other k-strategists-elephants
and rhinoceros (ford 1971). The association with antelopes and other mammals is
believed to have come later buffalo may have been the first, again because of
habitat similarities. The salivaria group of trypanosomes probably arose in
leeches, were passed to the suidae and from them infected glossima. The high
degree of adjustment to infection of pigs and buffaloes (parasitaemia is
infrequent), supports this view (ford 1971). Although some other game animals
are more sensitive, especially when young, for most of those regularly
bitten by tsetse trypanosomiasis is not lethal. Into this situation man and
domestic animals intruded, firstly the negroid races, subsequently the arabs
and europeans. In a fascinating account ford (1971) shows how the present
status of trypanosomiasis in africa may be related to the distrubances
occasioned by the arrival of europeans, their associated diseases (rinderpest,
smallpox) and politicial actions.Man and his domestic animals, being the most
recently acquired hosts, are the most sensitve to infection by the trypanosomes
and in these hosts New races have developed (garnham 1971).
Elm disease (ceratocystis ulmi) has generally
behaved like an r-strategist killing most of the trees it attacks its origin is
uncertain but we do know that as it is carried around by man more virulent strains develop. Both the fungus and its beetle vector have been
carried to the north american continent, exposed to different species of elm
and now a new aggressive strain introduced to europe (brasier & gibbs
1973). Perhaps the suckering habit of elm allowed such a lethal parasite to
survive or perhaps it was originally more of a k-startegist, killing limbs on
old trees over a long period of time. Whichever the case, the present pandemic
may enable us to witness an illadapted over virulent race of a parasite
exterminating its host and itself, the race resulting from the disturbances to
evolved relationships caused by man's movement of flora and fauna around the
world.
When consideration has been made
for the different biological circumstances of the cocoa mirids, the
trypanosomes and elm disease fungus the paralles in the icreased
sensitivity the new races and the exploitation (r-strategy) after disturbance
are striking.
Pest control strategy and r-k continuum
Applied
bilogists have recognized that habitat charcters are important indicators for
pest management strategies ( southwood & way 1970, fig.4) and conway (1976)
has shown that as the r-k continnum is related to habitat
characteristics it is relevant to decisions on the choice of control strategy,
control being used in the sense of reducing the impact of the pest to a level
when it no longer justifes pest status. Normally this is done by reducing the
population but it may be achieved by breeding for more tolerance (less
sensivity) in the victin.
R-PESTS
the levels of these populations are always
fluctuating therefore any appeal to the stability of natural ecosystem is
foredoomed to failure. With arable crop pests in the annual re-invasion is the
key population pathway (southwood 1975) and a powerful control strategy is to
maximize the difficulties in this pathway. This is analysed by way (1977) the
degree of isolation may be increased by greatly enlarging the scale of the crop
monoculture and so minimizing the edge effect in the crop and changes can be
made in the overwintering, aestivating or alternate hosts sites. The
manipulation of planting dates to minimize the synchronization of host plant
and pest migration time is another cultural control technique. Attention should
also be given to the ecological bottlenecks in the regional population dynamics
of such species, these may be manipulated to make them narrower direct
reduction the removal of alternate hosts is one of the oldest techniques of
pest and disease control. More subtly the pest could be overcrowded in its
refuge so bringing intranspecifc mecmanisms into play and natural enemies might
be encouraged in these sites as way (1976) suggests for thr aphid myzus
persicae. If pest numbers cross the economic injury threshold
(norton&conway 1977) pesticides will have to be used to protect the crop
the pest population will of course bounce back this is the nature of
r-startegists. The crop may be harvested before this happens. A more serious
long-tern problem is the potential development of resistance to the pesticide.
The high reproductive rates of r-species will favour this but, as comins (1976)
shows, provided much of the development of resistance. From economic and
biological viewpoints pesticide application must be related to need, this
highlights the importance of methods of forecasting and monitoring the pest
population (smith 1970). Sometimes forecasts can be based on information about
the pest in its overwintering site (refuge or foci of infectio) as with the
russia work on the functional morphological condition of the bug eurygaster integriceps
(fedotov&bocharova 1955) or the studies on aphis fabae in southern britain
(way&cammell 1973). More frequently variations in the success of
hibernation, migration and colonization require.monitoring the invading
population (taylor 1974) or that in the field (matthews & tunstall 1968)
intermedieate pests
the essence of the dyanamics of these
special is that there is a natural enemy ravine : the frequency with which the
pest is able to cross this will depend on many factors, particularly its width
(which up to a certain point is proportional to the duration stability of the
habitat). the degree of climatic instability (i.e. the envronmental noise) and
the organisn's own instrinsic rate of increase. it is assumed that the lower equilibrium
point (see fig. 2b) is suffciently low that the organism is no longer a pest.
this could be invalidated by the increasing emphasis on cosmetic control ( norton
& conway 1977). and when thus happens the application of integrated pest
control will be difficult. host resistance to attack inevitably lowers the
instrinsic rate of increase and so strengthens the ravine.
when the pest is not endemic then the
introduction of appropriate natural enemies will establish the ravine. most
successful examples of biological control fall into this category and accur in habitats
characteristic of intermediete to k-species, rather that r-pests. this is show
in table 1.
when intermediate pests are native species
it is important that management strategies keep the natural enemy revine intact
and widen it if possible : this is the essence of integreted control.
k-pests
the extreme k-strategist is a specialist
and will be sensitive to the disturbance of its habitat. therefore k-pests may
be reduced by cultural procedures that change thenform of the habitat, for
example altering the amount of shade or, for many parasites, reducing the
numbers or longevity of another organism (the vector). the majority of
k-strategists fail to make the transition fron natural to agroecosystems. the
low recruitment rate of these organisms to methods of reproductive control, e.g.
sterile male techniques.
canclusion
population dynamics theory has now reached
the stage where centain general izations are beginning to appear. the synoptic
model is one way of expressing these. it is based on equations for natality, intraspecific
competition and natural enemy action which are varied along an axis related to
the duration of the habitat and to various bionomic charactera of the organism
(size, atc). conveniently described as the r-k continuum. it allows the diversity
of nature to be expressed whilst maintaining the uniformity of the underlying
equations. pest organisms from anthrax to elephants can be viewed against this
framework. it gives useful insights into their origins and the probable success
of potential control strategies.
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