Environmental Engineering Reference
In-Depth Information
CHAPTER TWO
Metallophytes: the unique biological
resource, its ecology and conservational
status in Europe, central Africa and
Latin America
ALAN J.M. BAKER, WILFRIED H. O. ERNST,
ANTONY VAN DER ENT, FRAN¸OIS MALAISSE
AND ROSANNA GINOCCHIO
Introduction
Metalliferous soils provide very restrictive habitats for plants due to phytotoxicity,
resulting in severe selection pressures. Species comprising heavy-metal plant
communities are genetically altered ecotypes with specific tolerances to,
e.g., cadmium, copper, lead, nickel, zinc and arsenic, adapted through micro-
evolutionary processes. Evolution of metal tolerance takes place at each specific
site (Ernst
2006
). A high degree of metal tolerance depends on the bioavailable
fraction of the metal(loids) in the soil and the type of mineralization. At extremely
high soil metal concentrations, especially on polymetallic soils, evenmetal-tolerant
genotypes are not able to evolve extreme tolerances to several heavy metals
simultaneously. Adapted genotypes are the result of the Darwinian natural selec-
tion of metal-tolerant individuals selected from surrounding non-metalliferous
populations (Antonovics et al.
1971
;Baker
1987
;Ernst
2006
). Such selection can
lead ultimately to speciation and the evolution of endemic taxa. Heavy-metal
tolerance was first reported by Prat (
1934
)inSilene dioica and demonstrated experi-
mentally in grasses by Bradshaw and co-workers in Agrostis spp. and by Wilkins
in Festuca ovina in the late 1950s and 1960s (see Antonovics et al.
1971
)andfrom
the early 1950s onwards in the herb Silene vulgaris by Baumeister and co-workers
(see Ernst
1974
). Metal-tolerant plants avoid intoxication by an excess of heavy
metals by means of special cellular mechanisms, as long as the soil metal levels
do not exceed the levels of metal tolerance (Ernst
1974
;Ernstet al.
2004
). They
can thus thrive on soils that are too toxic for non-adapted species and ecotypes.
These unique plants with an ability to tolerate metal toxicities and survive and
reproduce on metalliferous soils are called metallophytes.
