Houses and
hotels have monopoly on Helianthemum habitats
As noted before in these
pages, significant numbers of plant species are becoming increasingly rare or
threatened by extinction. It is often difficult to know exactly how to ensure
survival of such species, but it is seems obvious that a working knowledge of
their ecology and breeding biology is essential. Thus, Javier Rodríguez-Pérez (pp. 1229–1236), based in Mallorca, one of
the Spanish Balearic Islands, has studied two Helianthemum (rock-rose) species that occur
on the islands as well as on the mainland of Spain. He performed
hand-pollination experiments to determine inbreeding and outbreeding success,
and also studied abundance of pollinators in the wild along with investigations
of floral traits, flowering times and the effects of weather. Helianthemum mariflorum is
mainly an outbreeder: fruit and seed set in self-pollinated flowers is only
about one-third of that in out-pollinated flowers. Helianthemum
caput-felis,
however, shows no difference between self- and out-pollinated flowers. Natural
pollination was mostly by hymenopterans and, in H. mariflorum, especially Apis mellifera, the honey bee. Helianthemum mariflorum, as a mainly outbreeding
species, is more vulnerable to variations in pollinator populations, a feature
that is clearly seen in comparisons between 2001 and 2002. The weather in 2002
was very wet and pollinator abundance was much lower. Helianthemum mariflorum showed
a 50 % decrease in fruit set between these 2 years. However, this was somewhat
compensated for by a much increased seedling survival in 2002. Taking all these
factors into account, the author concludes that even allowing for the vagaries
of the weather, it is not reproductive success that limits these two species.
Instead he attributes their increasing rarity to loss of habitat through
urbanization, a feature only too familiar to those of us who have visited Spain and its
islands. This emphasizes the importance of setting aside refuges large enough
to support viable populations, a policy which fortunately is already being
implemented.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
