ECOLOGY OF ASCLEPIAD LIANES 
109 
1956). This feature and the presence of vibratile 
hairs on the flowers suggests that pollination 
may be by flies (Whittington 1989). Both Mars- 
fa'aand Rhyncharrhena flowers are odourless, 
at least in daytime. 
All species in the family Asclepiadaceae 
studied in detail are obligate or nearly obligate 
outbreeders and exhibit low rates of fruit set, 
typically one to five per cent (see e.g. Holm 1950, 
Woodson 1954, Skutch 1988). These low rates 
can be related to low pollination rates and to 
energy limitation causing abscission of polli¬ 
nated flowers and pod abortion (see e.g. Cabin 
etal. 1991. Pleasants 1991). Low rates of fruit set 
certainly apply also to Marsdenia and 
Rhyncharrhena. For a threatened prairie species 
ot Asclepias now restricted to a few tiny refugia 
surrounded by farmland, problems may arise 
from (a) low populations of insect pollinators, 
and (b) insufficient plants to attract pollinators 
and maximize cross-pollination. Problem (b) 
can be exacerbated by extensive clones of the 
species causing extensive self-pollination (Betz 
1989). These same problems may apply to many 
isolated Marsdenia and Rhyncharrhena stands 
in largely cleared Victorian areas. 
Work on Asclepias quadrifolia shows that 
plants under the threshold of 330-340 mm stem 
height are unable to mature a fruit, any fruits 
initiated being aborted. Many small flowering 
plants apparently lack the energy resources to 
mature a fruit; only 20% of the flowering plants 
produce a fruit, production of which clearly de¬ 
pletes the plant’s energy reserves (Chaplin & 
Walker 1982). Marsdenia and Rhyncharrhena 
seem to exhibit similar behaviour. 
RESPROUTING 
fifty days after germination, ten plants each 
Aaisdenia and Rhyncharrhena were cut < 
below the cotyledons. Eight Marsdenia plar 
and a N Rhyncharrhena plants had i 
grouted 14 days later. Similarly, resprouti 
can occur in cut off 21-day-old Asclepias Syria 
seedlings (Bhowmik & Bandeen 1976). 
atches of Marsdenia are known to persist 
some areas of Victoria cleared of native ve 
etation in the 1920s; the small sucker shoots a 
repeatedly destroyed by ploughing or stock gra 
m§ but new ones continue to be produced 1 
resprouting (J. N. Macfarlane, personal comm 
mcauon). Similar behaviour occurs in asclepi; 
! eed , s ^ke Asclepias syriaca (Bhowmik 
anaeen 1976) and Morrenia odorata (Tucker 
1 ips 1974). Such resprouting behaviour al 
means that Marsdenia and Rhyncharrhena can 
occasionally be found climbing among the trees 
in commercial citrus groves, where irrigation 
can cause them to fruit prolifically (J. N. 
Macfarlane personal communication). Anal¬ 
ogously, the asclepiad Morrenia odorata is a 
major weed of Florida citrus groves (Tucker & 
Phillips 1974). 
Post-fire behaviour of Marsdenia and 
Rhyncharrhena has not been seen in Victoria. In 
central Australia, vigorous resprouting can 
make both species conspicuous after fire, they 
being among the first species to appear. Later in 
the post-fire period, increased competition from 
other perennial species can make them less con¬ 
spicuous and reduce or prevent their fruiting 
(Latz 1982 and personal communication). Re¬ 
sprouting allows other asclepiads to tolerate 
frequent low-intensity fires in monsoon areas 
further north (Russell-Smith & Dunlop 1987). 
RESPONSES TO GRAZING 
The Asclepiadaceae is noted for presence of 
toxic cardiac glycosides, alkaloids and resinoids. 
As a result, many species are extremely unpalat¬ 
able to grazing mammals and are generally 
avoided by them (Everist 1974). For example, 
severe overgrazing by goats can produce mono- 
specific communities of the poisonous asclepiad 
Calotropisprocera (Kassas 1966). Despite this, it 
is thought that mammal grazing can dramati¬ 
cally reduce Marsdenia and Rhyncharrhena 
populations. In Victoria, by 1937 Marsdenia 
was “rapidly becoming rarer” because it was 
especially palatable to rabbits, which ate the 
foliage and removed the bark from stems near 
the ground (Zimmer 1946). 
Both species are now very rare virtually 
throughout their former range in Victoria (see 
Table 1). For example, only two adult plants of 
Marsdenia are now known in Hattah-Kulkyne 
National Park and there are only three recent 
records from the whole of the Sunset Country. 
The only populations of any size are from un¬ 
stocked areas close to towns (sites 1 and 3, Red 
Cliffs; site 5, Walpeup) where rabbit (and other 
grazing mammal) numbers remain low, possibly 
due to factors like predation from domestic cats 
and dogs. Rhyncharrhena behaves similarly 
(Table 1). In some parts of the ranges of both 
species, grazing by hares may be more important 
than that by rabbits (J. N. Macfarlane personal 
communication). 
In western New South Wales Rhyncharrhena 
is grazed readily by sheep. Under grazing “the 
