Abstract: Mounds built in pastures by the subterranean ant Lasius flavus
often have a vegetation which differs markedly from their surroundings.
I set out to compare the relative abundances of plant species on and off
ant-hills in some grasslands in southern Britain, and to define those
characteristics of species which mainly determine their success or
failure on the mounds.
Large ant-hills and the surrounding grassland ware sampled in detail
at five acid grassland sites in the Gower peninsula, Glamorgan, and
thirteen chalk grassland and chalk heath sites in southern Britain.
Mounds were often dominated by perennials which could grow up through
heaped soil, like Polytrichum piliferum, P. juniperinum on the Gower
and Thymus drucei and Helianthemum chamaeciatus on the chalk. Annuals
such as Aira praecox and Arenaria serpyllifolia were frequently confined
to ant-hills. However, many species, especially rosette or semi-rosette
hemicryptophytes, were consistently uncommon on ant-hills, including
Luzula campestris, Cirsium acaule, Poterium sanguisorba and Felipendula
vulgaris. The pleurocarpous mosses Pleurozium schreberi and Pseudo-scleropodium
purum were more abundant on north-facing than south-facing
aspects of the mounds.
Two main factors influenced plant patterns. Firstly, the heaping
of soil over the plants by the ants often caused considerable seedling
mortality. Secondly, species differed in their ability to disperse 'seed'
onto the mounds. When I compared the viable 'seed' 'banks' present in
equal weights and surface areas of ant-hill and pasture soils, by
keeping the samples moist and counting the emergent seedlings, the
'seeds' of those species uncommon on ant-hills were much less frequent
in ant-hill than pasture soil. Most seedlings on the mounds probably
result from 'seed' dispersal onto the mound surfaces.
Soil analyses show no large and consistent differences in nutrient
concentrations between ant-hill and pasture soils and thus it seems unlikely
that nutrient levels greatly influence the striking distributions of many
species.
The vegetation on and off two hundred and six wounds of a wide range
of size and activity was recorded in detail on a 1.2ha plot at Aston
Rowant N.N.R., Oxon. Mapping showed that species rarely occurred
on ant-hills when they were absent from the sward nearby. From
scatter diagrams of the abundance of each species in relation to ant-hill
volume and ant activity on a mound, it was possible to deduce
how succession of some species on ant-hills might occur, if ant-hill
volume were proportional to ant-hill age. The following hypothesis
is consistent with the evidence.
'Young' mounds are heaps of soil which smother existing vegetation.
They are invaded in particular by Festuca rubra, Lotus corniculatus,
Carex flacca, Leontodon hispidus and Helianthemum chamaecistus (if in the
sward nearby). Thymus drucei invades the mounds vegetatively as they
expand in basal area, and gradually becomes a more important component of
the vegetation. Carex flacca and Hieracium pilosella rosettes become
restricted to the edges of ant-hills because the continual heaping of soil
onto the tops of the mounds causes their burial and death there. However,
Leontodon hispidus and Plantago lanceolata, often present inside the margins
of medium and large mounds, set abundant seed there and their seedlings
sometimes become established on the bare soil on top.
By this time, the summits of ant-hills tend to be occupied by Helianthemum
chamaecistus and Thymus drucei, which can grow up through the heaped soil.
Small mounds are often deserted by the ant colony. Their vegetation
becomes closed and the relative abundances of species approach those of the
surrounding sward more closely than on active ant-hills of the same
volume. In the absence of burial, Carex flacca, Leontodon hispidus,
Plantago lanceolata, Hieracium pilosella become established over the whole
surface; H. pilosella rosettes are far more dense on abandoned mounds
than active ones of the same size. When mounds are abandoned the chambers
and channels disappear, organic natter accumulates, an A0 horizon is formed
and the previously structureless soil becomes aggregated into peds.
The life cycles of the species with the most marked patterns la
relation to ant-hills were studied more fully in field and laboratory
experiments. The species characteristic of ant-hills usually have either
vigorous vegetative growth when smothered, the ability to flower on the
mounds in too year after germination or both. For instance, established
plants of Helianthemum chamaecistus and Thymus drucei, woody chamaephytes
which often dominate ant-hills, can grow up through 3-4cm of suddenly-heaped
soil. Yet they rarely establish themselves from seed on the mounds. In
Arenaria serpyllifolia, Veronica arvensis and other winter annuals, the flowers
are predominantly self-pollinated, seed-set is high, the seeds are light
(60 - 270 ug) and inhibited from germinating under only 5mm of soil, a
high proportion (40) of those released onto the mounds germinate each
year, and the seedlings rapidly decline in number. For instance, a cohort
of 2078 seedlings of Arenaria serpyllifolia followed from August 1971 to
July 1972 had a half-life of 0.14 years, the seedlings dying from rabbit-scraping,
soil heaping, self-thinning, fungal infection, rainwash and so on.
Nevertheless, almost every surviving seedling flowered in the late spring of the
first year. The minute seed weights of these species are insufficient to
allow them to become established in closed vegetation. Their seeds may be
inhibited from germinating under swards by the higher ratio of far red to
red light there; Arenaria serpyllifolia, Veronica arvensis and Myosotis
ramosissima seeds did not germinate under two layers of Tilia leaves in a
laboratory experiment. Cerastium holosteoides resembles Thymus and
Helianthemum in being a perennial chamaephyte able to grow up through newly-heaped
soil to some extent, and resembles the winter annuals in flowering the
year after germination and producing abundant small (180 ug) seeds which
hardly germinate under two layers of Tilia leaves.
The species with roughly equal abundances on and off ant-hills are
frequently stoloniferous and able to grow up through heaped soil to some
degree (e.g. Agrostis stolonifera, Festuca rubra, Campanula rotundifolia,
Galium verum) or short-lived and able to flower on the mounds (e.g. Euphrasia
officinalis agg, Gentianella amarella, Linum catharicum and Medicago lupulina).
All the species which are considerably more abundant in the sward than on
the mounds are perennials, with very slow vegetative reproduction. Thus they
depend on invasion by seed to become established on ant-hills. Only a small
proportion of the rosettes in the sward flower each year, the flowers are
cross-pollinated, the fruits are comparatively heavy (600 - 4000 ug), the
seedlings are susceptible to burial, their development on the mounds is slow
and they do not flower within the first two years. Their seedlings are
robust enough to establish themselves in swards. Nevertheless, within this
framework, the factors affecting their abundances on the mounds differ from
species to species. For instance, in Cirsium acaule the number of fruits
produced is reduced to 10-14 per capitulum per year by the weather, predation
and parasitism. Their dispersal is hindered by the high frequency of pappi
lost before dispersal, the heavy achenes and the weather. The production of
vigorous seedlings on the mounds may be affected by fungal parasitism, and
their survival is probably poor because they are susceptible to burial. Seedling
development is slow on ant-hills and no flowering plants were seen there. In
Leontodon hispidus, however, dispersal of achenes onto the mounds is efficient,
and 5-9 of achenes germinate on ant-hills, so that seedlings are frequent on
the mounds in autumn. Seedlings develop slowly, remain avail, and are
susceptible to burial, but mature rosettes inside the perimeter of active
ant-hills often flower profusely and reproduce vegetatively. Thus Leontodon
maintains larger populations on ant-hills than Poterium sanguisorba, and
Cirsium acaule. On the other hand, Carex flacca maintains high densities of
rosettes around the edges of 'active' ant-hills because of vigorous vegetative
reproduction; reproduction from seed is negligible. Poterium sanguisorba and
Plantago lanceolata were also investigated.
Thus the ant-hill environment acts as a sieve, letting through those
species which can flower early on or grow up through heaped soil, and
excluding those which cannot flower on the mounds, with poor dispersal and
low seedling development. The flora of 'active' ant-hills is maintained at a
lower stage of succession than the surrounding vegetation by the constant
addition of soil to the surface. The vegetation resembles that of fixed
calcareous dunes and of Breckland grass-heath before myxomatosis, two cowaunities
in which sand movements also provide an element of environmental 'unpredictability'.
Those species which invest a high proportion of their total net assimilation
in reproduction ere pre-adapted to colonise and maintain themselves on ant-hills.
Those species selected for efficiency in maintaining themselves in a closed
community, with heavy fruits and an inability to flower early in life,
cannot maintain themselves at a high density in a community of an earlier
successional stage.
Publication Year: 1972
Publication Date: 1972-01-01
Language: en
Type: dissertation
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Cited By Count: 1
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