The changing flora of the Shoalwater Bay Islands

23 Pages·1998·4.8 MB·

by  E Rippey

THE WESTERN AUSTRALIAN
NATURALIST
Vol. 22 30th December 1998 No. 2
THE CHANGING FLORA OF THE SHOALWATER BAY
ISLANDS
By E. R1PPEY, J. RIPPEY, N. DUNLOP, C. DURANT, B. GREEN, J. LORD
Friends of the Shoalwater Islands Marine Park
PO Box 5246, Rockingham Beach. WA 6168
ABSTRACT
A survey on the flora of the Shoalwater Bay islands was
conducted and compared with previous surveys dating
back to 1959. Vegetation cover has varied on Penguin
Island, as assessed by examination of aerial photographs, but
is now in reasonable condition. Penguin Island has shown
some turnover of species, especially annual aliens, but total
species numbers have changed little. There has been a
serious loss of species on the smaller islands with total
devegetation of West Shag Island. Reduction in species
variety seems most serious in Seal Island and Bird Island
where there has been marked loss of native perennial
species. In these two islands there has been extensive influx
of aliens with large forests of Lavatera arborea, over 2 metres
in height, and smaller stands of Ma/va parviflora which have
replaced the original vegetation. Invasion by these weeds
and others is noticeable on Penguin Island but is
proportionately less. African Boxthorn (Lycium
ferocissimum) has been noted for the first time on Penguin
Island.
far the largest, and then in
INTRODUCTION
descending order of size, Seal
The Shoalwater Bay Islands Island, Bird Island, Middle, West
include Penguin Island, which and East Shag Islands and Gull
with an area of 12.5 hectares is by Rock. There are also a few smaller
81
named rocks which have not been Department of Conservation and
considered in surveys (Figure 1). Land Management (CALM) took
Since 1992 these islands have been over the lease of Penguin Island it
had been used as a holiday island.
included in the bounds of the
Shoalwater Islands Marine Park. In October 1997 the Friends of the
Prior to 1987, when the Shoalwater Islands Marine Park
82
(FSIMP) conducted a vegetation standard field herbaria (i.e. one for
survey of the islands as a part of a each broad habitat area).
monitoring operation. The results Once the field herbaria were
of previous surveys (Storr 1961; complete the teams returned to
Abbott and Black 1980 and Chape their broad habitat areas to carry
1984) were available for out belt transect sampling.
comparative purposes. Transects were broadly orientated
east-west across the island and
consisted of a chain of plots 2m
MATERIALS AND METHODS
wide by 5m in length. Nine
Aerial photographs of all islands transects varying in length from
were obtained from the 45m to 200m were completed. The
Department of Land Adminis¬ locations of these transects are
tration, as well as photocopies of shown on Figure 2.
black and white aerial photo¬
A complete list of plant species
graphs of Penguin Island from the
was recorded for each plot and the
years 1953, 1963, 1975, 1980 and
distribution of species with high
1987 (all the years available during
coverage was sketched onto a plot
earlier vegetation surveys).
FSIMP members carried out a
complete flora survey of Penguin
Island, Seal Island, Bird Island,
Middle, West & East Shag Islands
and Gull Rock (Figure 1).
Quantitative vegetation assess¬
ment was carried out only on
Penguin Island as this larger island
had significant areas of all plant
habitats.
Field work on Penguin Island was
substantially completed on 25
and 26 October 1997. Teams of
FSIMP members were sent to
collect specimens of all plants
from four broad habitat areas on
the island, the northern plateau,
southern plateau, central dune-
field and the tombolo. Planted
species cultivated in the tombolo
area were not collected unless
there was evidence of invasion of
the natural vegetation. Specimens
were identified or given a field
number, pressed for expert
verification and the compilation
of a permanent reference
herbarium and used to create four
83
outline. Also recorded were the shrubs. Half the flora consisted of
slope, aspect, soil depth and rock introduced species including
exposure within each plot. short-lived woody perennials,
herbs and grasses.
In the week following the
Penguin Island field work all the The flora of Penguin Island has
smaller islands in Shoalwater Bay been surveyed on three previous
were surveyed by FS1MP members. occasions and the smaller islands
All plants present on each island were included in the first two of
were collected and identified by those studies (Storr 1961, Abbott
reference to the field herbaria. and Black 1980 & Chape 1984).
Species lists were then produced Table 1 compares the number of
for each island for comparison species observed on each island
with available historical surveys. during the present survey with
the number recorded in historical
surveys.
RESULTS
It shows that the number of plant
Vegetation cover of Penguin species on the larger Penguin
Island varied considerably over Island has apparently fluctuated
the years. It was much reduced in within a narrow range over time
1953 when the first aerial but has in effect changed little
photographs were taken. over the past 30 years. Conversely
There was gradual improvement plant species richness on the
until 1975. However the ground smaller islands has undergone a
cover again appeared depleted in dramatic decline. The only plant
1980 but was recovering by 1983. species discovered on the smaller
In 1997 even further improvement islands which was not present on
could be seen. Penguin Island was the native
The 1997 flora survey recorded 77 Hollyhock Lavatera plebeia.
plant species on the Shoalwater Table 2 shows the changes in the
Bay Islands (Appendices 1 & 2). Of number of indigenous species on
these 38 (+50%) were indigenous all the islands. Penguin Island has
plants, the majority of which were lost 5 (12%) indigenous taxa since
woody or succulent perennial the earliest surveys. However the
Table 1. Number of species found in each survey
Island Area (ha) 19591 19752 19843 19974
Penguin 12.5 71 81 71 76
Seal 1.2 41 32 17
Bird 0.9 31 31 14
M Shag 0.4 24 22 16
W Shag 0.16 6 12 0
E Shag 0.04 4 6 1
Gull Rock 0.04 2 5 1
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Table 2. Number of native species found in each survey
Total Species
Island 19591 19752 19843 19974 species common
to all
Penguin 42 43 39 37 51 26
Seal 24 17 8 25 8
Bird 17 16 3 18 3
M Shag 14 11 8 16 7
W Shag 6 7 0 7 0
E Shag 4 5 1 5 1
Gull Rock 2 4 1 4 1
19591 - Storr (1961)
19752- Abbott and Black (1980)
19843- Chape (1984)
19974- current study
loss of indigenous flora on the ferocissimum (Appendices 1 & 2).
smaller islands was much more The results of the strip transect
significant - 16 (67%) on Seal, 14 sampling on Penguin Island are
(82 %) on Bird, 6 (43%) on Middle presented in Table 3. For each
Shag, 6 (100%) on West Shag, 3 species recorded a percentage
(75%) on East Shag and 1 (50%) on frequency was calculated for its
Gull Rock. occurrence on each transect (eg. a
The indigenous plant species species which occurred on 3 of the
apparently lost from the 10 plots on a transect would have
Shoalwater Islands include Isolepis F% =33.3 ). An overall percentage
marginata and nodosa, Sarcocornia frequency was also calculated. The
blackiana, Calandrinia calyptrata, abundance of each species on each
Anthocercis littorea, and Angianthus transect was expressed by the
cunninghamii. mean Domin Value. This is a
simplified scale by which the
One indigenous species has
percentage cover of a species
appeared on Penguin Island since
within a transect can be
1961; this is the grass Austrostipa
estimated. The higher the Domin
elegantissima which is now
Value, the greater the percentage
abundant on the lee slope of the
cover (Slingsby and Cook, 1986).
central dune. The weeds which
have been introduced to the The important weeds on the
islands since 1961 include Urtica northern plateau of Penguin
urens, Atriplex prostrata, Island were herbaceous Malva
Chenopodium ambrosioides, Sagina parviflora, Rhaphanus raphanistrum
maritima, Fumaria capreolata, and Sonchus oleraceus, and the grass
Brassica rapa, Rhaphanus Lolium rigidum. The dominant
rhaphanistrum, Erodium moschatum, weeds on the southern plateau
Lavatera arborea, Lavatera cretica, were similar but Sisymbrium
Malva parviflora and Lycium orientate replaced Rhaphanus and
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Table 3. Plant frequencies (from transect data)
Pla Plb Pic P2a
(55m) (50m) (50m) (60m)
SPECIES Domin Domin Domin Domin
RECORDED F% Value F% Value F% Value F% Value
Acacia rostellifera
Acanthocarpus preissii
Alyxia buxifolia
Carpobrotus virescens 9 40 5.9 10
Cassytha racemosa
Clematis linearifolia
Enchylaena tomentosa 10 58.3 3
Frankenia pauciflora 20 5.9 8.3 3.9
Lepidium foliosum 50 3
Lepidosperma ghdiatum
Nitraria billardierei 8.3 3.9
Ozothamnus cordatus
R hagod ia bacca ta 54.5 3.9 70 7.4 100 7.4 100 4.6
Scaevola crassifolia
Senecio lautus
Spinifex longifolius 20
Spyridium globulosum
Austrostipa elegantissima
Austrostipa flavescens
Tetragonia implexicoma
ThrelkeId ia diffusa 9 30 3 16.6 3
*A venafatua 20 20
Bromus arenarius 30 3 20 33.3 3.9
*Ehrharta longiflora
*Euphorbia peplus
*Euphorbia terracina
*Hordeum leporinum 10
*Lavatera arborea 9
*Lavatera creiica 10 4.6
Lolium rigidum 40 10 3 66.6 3.9
*Malva parviflora 81.8 5.9 10 3.9 40 5.9 83.3 3.9
*Melilotus indica 10 10 16.6
*Raphanus raphanistrum 63.6 7.4
*Sisymbrium orientale 9 20 60 75 4.6
*Solanum nigrum 9 8.3
*Sonchus oleraceus 50 70 2.6 58.3 3.9
*Tetragonia decumbens 45.4 3 10 5.9 8.3 5.9
*T rachyandra divaricata 10 10 33.3 2.6
BARE SOIL 18 3.9 20 5.9 25 3
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P2b P3a P4a P5a P6a
(45m) (200m) (105m) (105m) (80m) OVERALL
Domin Domin Domin Domin Domin F%
F% Value F% Value F% Value F% Value F% Value
45 5.9 14.3 2.6 14
35 2.6 9
15 3 4
11.1 3.9 7.5 3 23.8 4.6 33.3 4.6 12.5 3 17
5 2.6 1
27.5 2.6 7
11.1 9.5 25 3 15
4.8 2.6 18.7 2.6 6
5
10 3 3
11.1 6.2 3 2
2.5 2.6 1
100 5.9 80 3.9 42.8 3.9 71.4 4.6 81.2 5.9 75
17.5 3.9 28.5 3.9 9
7.5 9.5 3
32.5 3.9 47 3 14.3 3 13
2.5 3 1
17.5 2.6 5
12.5 5.9 3
25 3 7
2.5 4.8 2.6 6
3
44.4 3.9 22.5 3 52.3 4.6 9.5 2.6 18.7 3 25
2.5 1
14.3 3.9 2
25 3 3
1
12.5 2.6 2
1
22.2 3.9 10 2.6 19 4.6 9.5 2.6 12.5 3 18
33.3 3 81.2 5.9 27
22.2 3.9 2.5 2.6 5
5
100 4.6 10 3.9 28.6 9.5 3 43.7 3.9 31
5 3
66.6 3.9 27.5 2.6 19 33.3 2.6 37.5 3 35
67.5 5.9 100 5.9 95.2 4.6 63
11.1 2.6 40 3 80.9 3.9 47.6 4.6 33
33.3 3.9 33.3 3.9 57.1 3.9 28.5 3 12.5 3.9 29
87
Trachyandra divaricata became the invaded area were also
important. excluded.
Introduced weeds are generally Finally further mention must be
not dominant in the sandy parts made of the African Boxthorn
of the island, especially if one (Lycium ferocissimum) on Penguin
excludes long naturalised dune Island. This may well have been
colonisers such as Tetragonia present at the time of the survey,
decumbens; however Trachyandra but was not noted until some six
divaricata, Sonchus oleraceus and months later in March 1998, when
Malva parvi flora recurred at a high one large clump of bushes of
frequency. Two of the most about lOsq m in area by 2m in
important weeds in terms of height and four smaller
ground cover Malva parviflora and neighbouring bushes were present
Rhaphanus rhaphanistrum have and were in flower. This is the
been introduced since 1961 and first record of Boxthorn on the
1984 respectively and may be in an Shoalwater Bay islands.
invasive stage.
The introduced hollyhock
Lavatera cretica has recently DISCUSSION
colonised Penguin Island and is
Vegetation cover appears variable.
currently restricted to one
It is possible that the reduced
discrete patch on the northern
cover in 1953 followed on the use
plateau. African Boxthorn (Lycium
of Penguin Island for military
ferocissimum) has been recorded in
purposes during the World War 11
the southern part of Penguin
and thereafter. There are also
Island for the first time.
reports of guano collection on the
The most significant weed on the
southern plateau. Gradual
smaller islands is the tree mallow
recovery may have taken place
(Lavatera arborea), a tall, robust
until the formation of Penguin
and relatively long-lived
Island Pty Ltd in 1969 when
coprophilous species. It forms
holiday accommodation was built
dense forest-like stands, up to 3m
(FS1MP 1995).
high, on Bird Island and has taken
By 1980 visitor pressure may have
over the core of Seal Island during
accounted for devegetation which
the past two years. The
was quite pronounced. A myriad
proliferation of this species on
of tracks criss-crossed the island
Seal Island followed the
between the eastern and western
expansion of Pied Cormorant
beaches. Following the acquisition
colonies onto and across this
of the lease in 1987 by the
island in 1995 (J.N. Dunlop pers.
government, and the intro¬
obs.). The stand on Seal Island in
duction of dune management
1997 was effectively excluding
with construction of boardwalks,
other plant species, including the
continued improvement took
native Hollyhock Lavatera plebeia.
place and vegetation cover in 1997
Birds such as Little Penguins,
appeared satisfactory.
Silver Gulls and Bridled Terns
which had formerly nested over Storr (1961) used the flora of the
88
d el islands of Shoalwater Bay to
WhitefaceStorm Petr dadreneeldvma itaopiotlnianosnstntrh sais ptpei en bc ieaetth sw isrleio cegrhne n leal-sat sin.o dlnS inslaihegreiahpart

resulted when islands had an east-
r
e
eat west rather than north-south
Littlarw orientation leading to more
he exposure and reduced sand
S accumulation. In practice the

n relationship was between the
Littleengui I+ onfu mavbaeirl aobf lpel ahntasb aitnadt st hwe hniucmh beinr
P
turn were related to area and
orientation. The Shoalwater Bay
alons Islands also show the other trends
FerPige i i 1111t otuf rinsolavnedr boifo gsepoegcriaeps htyh es ucraht ea so af

SilverGulls 1111 + + + owthheicrh t hiisn gds,e tbeyr mthien deids,t aanmceo fnrogmst
the mainland (Abbott 1977).
e Small islands with no predatory
Roseat Terns + smeacmurme alsn aerset infrge qugernotluyn duss ed bays

n seabirds. The numbers of birds at
as
pirn + + these colonies are determined by
se
aT food availability in the marine
C
environment and densities vastly
Bridled Terns emlaxrecgceeher dain sliatchnadol ss deo arm ecxaogapseet raiale nandrce eahsd.e aTvohnye
d manuring from nesting and
sterns roosting seabirds is a source of
ee
Cr T almost constant disturbance.

Gillham (1961) described the
dnt ecological changes induced by
ea
e Pimor seabird and sealion activity on the
LittlCor gvreagneittiact iiosnla ndosf ofafe sooluiathn-iwtee staenrdn

nt Australia. Burrowing and surface
a nesting seabirds were also
Piedmor observed to have slightly differ¬
or ent effects on island vegetation.
C
Table 4 lists the islands and the
current bird nesting populations.
Heavy disturbance pressures from
surface-nesting seabirds may
89
result in a form of secondary Breeding pairs retain nest sites
plant succession, in which the over successive seasons and colony
general tendency is for areas are occupied permanently
sclerophylls to become replaced (Wooller & Dunlop 1979, Dunlop
by succulents, shrubs by trailing & Jenkins 1992 & 1994). The
herbs, perennials by annuals and vegetation in these colonies tends
indigenous plants by aliens to be maintained in equilibrium,
(Gillham 1961). with the spaces between clumps
of woody or succulent shrubs
The most extreme impact on
maintained by the near nest
island vegetation occurs in the
activity of the territorial birds.
nesting areas of large body weight
Depending on the disturbance
species which form dense colonies,
pressures these spaces may be
such as Pied Cormorants and
vegetated (at least in winter and
Crested Terns. Currently Pied
spring) by Hollyhocks and/or
Cormorants are nesting on Seal
annual herbs and grasses.
Island and Middle Shag Island in
considerable numbers and it is The vegetation of seabird islands
these islands which show the is subject to high levels of
greatest depletion of native “natural” disturbance. This factor,
species in the survey. These together with elevated levels of
seabirds frequently shift or soil nitrogen and phosphorus
alternate nesting areas (Wooller & enhances colonisation by weed
Dunlop 1981, Dunlop 1987) so that species, particularly introduced
the impact on colony vegetation herbs and grasses and plants
is episodic. During a breeding which thrive in the presence of
episode woody vegetation and high concentrations of guano or
succulent shrubs are killed by a manure (e.g. Hollyhocks). The
combination of mechanical proximity of the aeolianite islands
damage and foliar scorching and to the mainland has meant that
toxic soil nitrogen levels due to these islands have been exposed to
accumulation of guano. In an ever-increasing number of
subsequent seasons the site is weed species since European
colonised by coprophilous shrubs, settlement.
including Hollyhocks Lavatera In 1997 half of the flora of the
spp. Should the site remain Shoalwater Islands consisted of
abandoned, and soil conditions introduced herbs, grasses and
ameliorate, the Hollyhocks may be short-lived coprophilous shrubs
replaced by re-establishing woody or Hollyhocks. Many of the
shrubs. However should nesting dominant weeds such as Malva
pressure continue the vegetation parviflora, Rhaphanus rhaphanistrum,
may be reduced to annual herbs,
Sisymbrium orientate, Trachyandra
most of which are introduced divaricata, Sonchus oleraceus and
weed species (Gillham 1961). Lolium rigidum are low herbs or
Another group of surface-nesting grasses which have little impact
seabirds, including Silver Gulls on habitat quality. There is also
and Bridled Terns, nest in large little evidence that these mainly
colonies but at lower densities. annual species have any
90