Report – ZL2104 – Possum & Eucalyptus

ZL2104, 2000-08-11

A Study of the Possum, Trichosurus vulpecula, Living in Mangrove and

Eucalyptus Habitats on Magnetic Island in Tropical Northern Queensland.

Introduction

To overseas visitors the Possums are as Australian as Eucalyptus, Koalas and Kangaroos.

The lively brushtail Possum is one of Australia’s most familiar marsupials. They are our most common possum species and largest arboreal (tree-dwelling) marsupial herbivore (plant-eater). It is the size of a domestic cat with a pointed face, long oval ears, pink nose and bushy black tail. Their fur is prized for its thickness and warmth which a small industry in Tasmania utilizes (Anon., 1997)

The order Marsupialia contains 14 genera of arboreal herbivores distributed widely over the Papua New Guinean and Australian regions. The Common Brushtail Possum (Trichosurus vulpecula) has adopted successfully to the European settlement in Australia, that some marsupial species have not been able to do and therefore declined in numbers and some even gone extinct (Pieters, 1985).

The brushtail possums are one of just a few species of mammals on Magnetic Island but occur in a variety of woodland habitats around the island. Possums are one of the few animals that are able to eat Eucalyptus leaves in spite of the high level of toxins. Even though an obvious abundance as a potential food resource, only three mammals – the Koala; the Greater Glider; and the Possum – are known to use this abundance in more than 60% of their diet. These animals are highly selective of how much, what, and when they will eat the Eucalyptus leaves (Watson, 1998).

Mangrove habitats are comprised of many unrelated plant species, the one thing they have in common is the tolerance for saline concentrations. Studies has shown that brushtail possums are able to live and feed in mangrove habitats, some mangrove plant species were pointed out as ‘favored’ food species, these were Lumnitzera racemosa and Excoecaria agallocha (Dalla Pozza, 1993).

The possums in this study occurred in two very different habitats on Magnetic Island, a mangrove area and a woodland area.

AIMS:

  • To compare the physical condition of possums in two habitats (woodland vs mangrove).
  • Compare breeding rates and sex ratio of the possums between the habitats.
  • Compare abundance of the possums in the two habitats.

Method

A. Site description

This study was conducted during week 29 – 2000 on Magnetic Island, located (19° 10.8’S, 146° 50.6’E), 5km north offshore from the city of Townsville. Magnetic Island is located in the dry tropics. I.e. relatively dry winters, and monsoonal rains during the summer months which can result in very low salinities offshore in Picnic Bay (Walker, 1981). The to sites were located on either side of a road leading to Cockle Bay.

B. Data Collection

Fifteen wire-cage traps in each habitat were baited in the afternoon with a mixture of rolled oats, honey, peanut butter and vanilla. The difference in number of trapped individuals at each site was used as an index of population abundance. The traps were checked early the next morning, so that the animals would not overheat or suffer from dehydration.

All possums caught (which were initial captures) were sexed, measured, weighted, marked and then released. The possums were marked with a transponder tag, implanted subcutaneously, i.e. in the skin of the neck. These kinds of tags are humane and permanent. Some of the possums had a cotton spool attached which provide a trace of their movements to reveal the track to their dens. Reproductive status was assessed by inspecting pouches of the females and measuring any young present, and measuring the testis of captured males.

Body conditions was compared by looking at the weight-for-size of individuals and was used as an index of “wealthyness”.

Results:

The results are presented in Figures 1 to 5 and Tables 1 to 3.

Figure 1. Difference in mean head length between male and female.

Fig 1No significant difference was found between male and female head lengths, with a mean of the males std = 0.39 and std = 0.28 for females. The T-test value of 0.100 assumed unequal variances.

Figure 2. Difference in weight between woodland and mangrove habitat.

Fig 2There was no statistical significance found between the weights of the two habitats, the T-test assumed equal variance (t=0.0858) std for woodland: 0.598 and std for mangrove: 0.177. However, the small sample size can be the reason for assumed equal variance, though the data tends to move towards to becoming significant.

Figure 3. Difference in mean head length between woodland and mangrove habitat.

Fig 3

The T-test indicated again that there was no significant difference between head length and habitat (T-test: 0.63). Woodland std: 0.285; Mangrove: std: 0.42.

Figure 4. Comparing testis of possums in mangroves vs. woodland

Fig 4

The mean volume of the possum testis were found to be slightly greater in the woodland habitat than in the mangroves but with a T-test value at; 0.714321, the testis was not significantly larger. Woodland std: 1.54; Mangrove std: 3.40.

Lincoln index

The population index (Lincoln) assumed no immigration / emigration due to the possums were equally cacheable, no births / deaths and they were found to be randomly distributed.

Figure 5. Regression of weight on the square root of head size and head size.

Fig 5

Generally, the values above the line are animals from the woodland habitat and the values below the line are from the mangrove habitat.

Table 1. Sex ratio in- and between the habitats

Females

Males

Sex distribution of caught animals in Woodland habitat.

60% (6 of 10)

40% (4 of 10)

Sex distribution of caught animals in Mangrove habitat.

50% (7 of 14)

50% (7of 14)

Table 2. Number of recaptures, indication of population size.

Woodland

Mangrove

Number of recaptures

11

7

Table 3. Number of pouch young in the different habitats.

of which were…

females

and males

Mangrove habitat 5of 5 females

1

4

Woodland habitat 7 of 7 females

4

3

Discussion:

The physical condition of Brushtail possums in two habitats (woodland and mangrove) was assessed by using the weight and the head length measurement.

Interestingly there was not a statistical significant difference in the mean weights between habitats (Fig. 2) but the study indicated that the populations may gradually become isolated due to increasing population pressure and intraspecific aggregation from the woodland area. (see further comments below) forcing sub adults to migrate into other habitats and perhaps this can be shown with some more data and after some more thorough studies are conducted of the populations (Dr. C. Johnson, School of Tropical Biology, J.C.U. 2000 pers. comm.).

One could think that living in the mangrove area would affect the possum population negatively due to the harsh conditions compared to the woodland area. Previous studies has shown (e.g. Watson, 1998) that the animals are highly selective of how much, what, and when they will eat leaves. But the mangroves have large numbers of nutrient-rich leaves that the possums have adapted to use (Dalla Pozza, 1993).

The difference in mean head length of a possum between the habitats (Fig. 3) showed that there was no difference in size, but in the woodland habitat were slightly larger than in the mangrove.

Figure 1 indicates that there was no significant difference in the physical condition of the sexes. No significant difference could be found in testis size. Although testis of male possums in the woodland habitat were larger (Fig 4).

The sex ratio between the habitats indicated that there were more females in the woodland habitat (Table 1). In the mangrove habitat the sex ratio was 1:1. Males disperse further than females do and have a higher mortality rate which results in more females in the adult population (Anon., 1996).

This supports that males have larger home ranges than females and goes between the habitats and are thereby not bound to one habitat. Tasmanian Parks and Wildlife Service (1996) stated that the survival of young is high in Brushtail Possums but when juveniles of both sexes disperse from the area of their birth an increase in mortality occurs. During this study, one female and one male that was caught had serious injuries, the female had lost the right eye and had a bad infection on the right side of her head, while the male had an old wound on the left shoulder and a damage to the left eye. These injuries probably caused by “domestic fights” with larger adults. However, Magnetic Island have a large problem with feral cats and dogs, that can harm and seriously injure possums (Dr. A. Webb, School of Tropical Biology, J.C.U. 2000 pers. comm.).

Domestic fights within a population are not the only population regulator though not all marsupial species have adopted successfully to European settlement in Australia, some marsupial species have not been able to do so and therefore declined in numbers and some even gone extinct (Pieters, 1985).

All the females in both habitats had pouch young (Table 3), in the mangrove habitat 80% of the females carried male joeys while in the woodland habitat the sex ratio of pouch young was approximately 1:1. Previous studies have shown that reproductive female pressures produce higher ratios of male to female pouch young in successional habitats but a higher ratio of female to male pouch young ratio in older more established habitats (Tasmanian Parks and Wildlife Service, 1996).

The animals in the woodland habitat tended to be above the trend-line in Fig. 5, indicating that they were fat, healthy animals while the animals below the trend-line tended to be skinny and little under nourished belonged to the mangrove habitat.

This study was conducted during a five day period, 24 possums were caught and many of them recaptured, but to get better data set a more substantial study has to be conducted under a longer period of time, perhaps with more traps. Other problems during the study was following the spools to the dens, were the thread often broke and it could be really problematic sometimes to follow the thread through the mangroves. Double measurement of the same individual with different values and incomplete data was other small emerging problems.

Reference:

Anon., Tasmanian Parks and Wildlife Service (1996) MANAGEMENT PROGRAM FOR THE BRUSHTAIL POSSUM Trichosurus vulpecula (Kerr) IN TASMANIA – REVIEW OF BACKGROUND INFORMATION URL: http://www.biodiversity.environment.gov.au/plants/wildlife/possm01.htm

Anon., Tasmanian Parks and Wildlife Service (1997) Brushtail possum Trichosurus vulpecula URL: http://www.parks.tas.gov.au/wildlife/mammals/btposs.html

Dalla Pozza, S.A. (1993) The use of mangroves by brushtailed possums Trichosurus vulpecula, and the subsequent effects on the water and sodium turnover rates., Honors Theses, Townsville, Queensland, James Cook University.

Pieters, C.W. (1985), The ecology of common brushtail possum Trichosurus vulpecula (Kerr, 1792) on Magnetic Island with particular reference to its adaptability to a modified environment., Ph.D. Theses, Townsville, Queensland, James Cook University.

Walker, T.A. (1981) Seasonal salinity variations in Cleaveland Bay, northern Queensland. Aust. J. Mar. Freashwater Res., 32:143-149.

Watson, S. (1998) Living on toxic diet ingestive behavior and nutritional physiology in the common ringtail possum (Pseudocheirus paeginus). Townsville, Queensland, James Cook University.

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