Agriculture Reference
In-Depth Information
of anecic earthworm species such as
L. terrestris
are bound to affect populations of epigeic species
adversely by reducing their food supply.
There is more general agreement on the importance of intraspecific competition in determining
earthworm populations. There may be some situations when competition is for resources other than
food, as was reported when
L. terrestris
was cultured at increasing densities with adequate food
in fixed volumes of soil (Butt et al. 1994), but in most natural habitats intraspecific competition is
likely to be for food (Satchell 1967; Daniel 1992).
PREDATION
Earthworms feature in the diets of hundreds of species of animals, vertebrates and invertebrates
(Edwards and Bohlen 1996), but the quantitative impact of predation on populations has not been
studied intensively. Most attention has been given to birds, which can be very visible and sometimes
can be severe predators on earthworms. Bengtson et al. (1976) excluded golden plovers (
Pluvialis
apricaria
) from small plots in an Icelandic hayfield in May/June and reported that the numbers
and biomass of earthworms (
A. caliginosa
and
L. rubellus
) were more than twice as high after 22
days in the protected plots than in adjacent areas exposed to predation. Observations of feeding
plovers showed that about 100 lumbricids m
−2
could be expected to be taken over the experimental
period, a figure that agreed closely with the results of the exclusion experiment.
Golden plover predation was also the subject of a study by Barnard and Thompson (1985),
who compared earthworm populations in areas enclosed by nets with those in unprotected control
areas in English pasture at the beginning and end of a 3-month period (December to March).
Earthworm populations declined by 46 and 71% in unprotected young and old pastures, respectively,
and population density increased by 11 and 14% in corresponding protected enclosures. At the
feeding rates observed, the authors estimated that golden plover predation could account for about
50% of the differences recorded; the other 50% was attributed to predation by other birds, foxes,
and badgers, which were also affected by the nets.
Earthworms were classified as a main food source for blackbirds and a regular food for song
thrushes in France (Granval and Aliaga 1988); heavy predation by gulls, starlings, and magpies
has been reported in New Zealand grassland (Moeed 1976). Earthworms comprised more than 90%
of the food mass ingested by black-headed gulls when the earthworms were readily available on
the soil surface following autumn cultivation in Switzerland (Cuendet 1983). About 10% of the
total earthworm biomass was made available by cultivation, and 25 to 33% of this was taken by
gulls. In a similar study in southwestern Ontario, Canada, the impact of ring-billed gull predation
on earthworms in freshly plowed land was also negligible (Tomlin and Miller 1988).
Many Insectivora (Mammalia) prey extensively on earthworms. Moles (
Talpa europaea
) con-
sume 18 to 36 kg of prey annually per individual in Britain; at least half, and sometimes as much
as 100%, of this prey consists of earthworms, depending on their availability (Mellanby 1966; Raw
1966; Funmilayo 1979). Large quantities of mutilated earthworms are stored in caches within mole
fortresses as future food.
Feeding on earthworms is also prevalent among shrews (Soricidae), with earthworms compris-
ing 0 to 60% of the diet of the common shrew (
Sorex araneus
) in British grassland, depending on
the season (Pernetta 1976b). At a daily consumption rate of 1.5 to 5 g fresh mass per individual
(Pernetta 1976a), a field population of 10 to 20 shrews per hectare in mixed grassland and woodland
habitat could consume 15 to 100 g fresh mass ha
−1
day
−1
or 0.5 to 3.7 g m
−2
year, equivalent to
somewhere on the order of 1 to 5% of the mean earthworm biomass in such habitats.
Among the Carnivora, the European badger (
Meles meles
) and the red fox (
Vulpes vulpes
) are
major earthworm feeders. The badger is highly specialized behaviorally as a predator of
L. terrestris
,
and its population density seems to depend heavily on earthworm availability (Kruuk and Parish
1982). Adult badgers were estimated to consume 130 to 200
L. terrestris
each per night in mixed
deciduous woodland, pasture, and arable land in England (Kruuk 1978), but annual consumption
