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Wildlife Research Volume 24, 1997 © CSIRO Australia 1997

A journal for the publication of original scientific research in the biology and management of wild native or feral introduced vertebrates

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© CSIRO Australia 1997

Wildlife Research, 1997, 24, 613Ð629

Fragmentation of a Small-mammal Community by a Powerline Corridor through Tropical Rainforest Miriam GoosemAB and Helene MarshA ADepartment

of Tropical Environment Studies and Geography, James Cook University of North Queensland, Townsville, Qld 4811, Australia. BCooperative Research Centre for Tropical Rainforest, Ecology and Management, Cairns, Qld 4870, Australia.

Abstract Trapping was used to investigate small-mammal community composition of a cleared powerline corridor compared with that of surrounding tropical rainforest in the wet tropics of north-eastern Queensland and to determine whether movements from the rainforest across the corridor were inhibited. The dense exotic grassland of the cleared powerline corridor supported a small-mammal community composed mainly of the grassland species Melomys burtoni (73á3%) and Rattus sordidus (15á0%) with rainforest small mammals being restricted to woody-weed thickets along the rainforestÐpowerline corridor edge. The rainforest species Rattus sp. (80á3%), Melomys cervinipes (10á9%) and Uromys caudimaculatus (8á8%) comprised the small-mammal community of the forest interior. These rainforest species also inhabited rainforest edge habitat and regrowth rainforest connections across gullies. Movements of rainforest species across the grassland corridor were almost completely inhibited even under bait inducement, a result attributable to the substantial structural and microclimatic habitat differences within the clearing and to interspecific competition with the better-adapted species of the grassland community. Rainforest species used regrowth connections along gullies to cross the powerline corridor. Mitigation of the fragmentation effects caused by powerline grassy swathes can best be achieved by strengthening extant canopy connections in regrowth gullies, and by establishing new connections across the clearings.

Introduction The main rainforest areas in north-eastern Queensland are subdivided into much smaller habitat blocks by a network of service corridors that have been cleared for powerlines, highways, roads and logging access. Studies of linear clearings in Northern Hemisphere conifer and hardwood forests, grassland and desert habitats have demonstrated that crossing movements by small mammals are inhibited (Oxley et al. 1974; Schreiber and Graves 1977; Wilkins 1982; Garland and Bradley 1984; Mader 1984; Swihart and Slade 1984; Bakowski and Kozakiewicz 1988). In Australia, roads have been the only potential linear barrier examined, with inhibition of small-mammal movements demonstrated in heathland (King 1978) and subtropical (Barnett et al. 1978) and tropical rainforest (Burnett 1992; Goosem 1997). The effects of powerline clearings on movements by Australian wildlife are entirely unknown. In the United States, powerline clearings that are maintained as grassy or low, shrubby swathes allow the penetration of faunal species alien to surrounding forest habitat (Anderson et al. 1977; Johnson et al. 1979; Kroodsma 1982). Middleton (1993) also reported that small-mammal species usually associated with grassland intruded into an Australian tropical rainforest area along the habitat afforded by a deforested powerline corridor. Fragmentation of wildlife populations by powerline corridors due to inhibition of movements and exclusion from unsuitable habitat could have serious consequences, potentially reducing population numbers below an ecologically viable level or resulting in long-term genetic isolation. In this study, we investigated differences between the small-mammal community composition of a cleared powerline corridor and that of the surrounding tropical rainforest. 10.1071/WR96063

1035-3712/97/050613$10.00

614

M. Goosem and H. Marsh

CaptureÐmarkÐrecapture methods were also used to examine movements of ground-dwelling and scansorial small mammals and to determine the magnitude of the linear barrier effect. Measures with the potential to mitigate population fragmentation including the maintenance of rainforest connectivity in gullies and the provision of artificial structures were also investigated. Materials and Methods Study Sites

PO

SI O TIO N N LY A L

Four study sites were established in Wooroonooran National Park (17¡378S, 145¡488E) approximately 35 km west of Innisfail. The sites were chosen along a 60-m-wide high-voltage powerline clearing established in 1954, which passes through Complex Mesophyll Vine Forest (Tracey 1982). At two sites, the vegetation within the powerline corridor consisted of a dense exotic grassland dominated by Melinus minutiflora (molasses grass) and Panicum maximum (Guinea grass) with several patches of dense, woodyweed thickets [Lantana camara (lantana) and Rubus alceifolius (wild raspberry)] adjacent to the rainforest edge. The other two sites were in gullies where remnant and regrowth rainforest created a continuous rainforest canopy connection (10Ð20 m in height) from one side of the powerline clearing to the other. The two rainforest connections across the clearing ranged in width from 15 to 60 m.

Fig. 1. The design of the grid-trapping and baiting experiments. ○, Elliott trap only (grid-trapping experiment); @, Elliott trap + wire cage trap (grid-trapping experiment); Á, Elliott trap only (bait-inducement experiments); ª, Elliott trap + wire cage trap (bait-inducement experiments). During the bait-inducement experiments, each side of the powerline corridor was trapped in consecutive months.

Grid-trapping Experiment A trapping grid (280 ´ 100 m) incorporating 66 trap positions was established at each site (Fig. 1). On each side of the powerline corridor, the grid consisted of four rows running parallel to the clearing at distances of 10, 30, 90 and 110 m from the rainforest edge. The perpendicular distance for animals to move between the two rows closest to the rainforest edge and the two rows furthest away was at least 60 m. Each

Fragmentation of a Rainforest by a Powerline Corridor

615

row consisted of six Elliott Type A small-mammal traps placed 20 m apart (Fig. 1). In order to capture larger mammals such as Uromys caudimaculatus, one wire cage trap (45 ´ 20 ´ 15 cm) was placed on each row with the Elliott trap at either the second or fifth trap position (Fig. 1). In addition, three rows without a cage trap were placed within the powerline corridor at distances of 10 and 30 m from the rainforest edge to maintain the 20-m spacing of the rows near the edge. In each of the rainforest gully sites, one of these three rows was omitted because of steep terrain and grassland intrusions, and wire cage traps were included on both rows. On each night at each grassland site, 74 traps were set, while 70 traps were set at each rainforest gully site. Trapping over four nights was conducted at four-weekly intervals in October, November and December 1992 and January 1993. Traps were baited with a mixture of rolled oats and vanilla essence. All animals were released at point of capture. Rattus fuscipes (bush rat), R. leucopus (Cape York rat), R. sordidus (canefield rat), Melomys cervinipes (fawn-footed melomys), M. burtoni (grassland melomys), Antechinus flavipes (yellow-footed antechinus) and Mus musculus (house mouse) were individually marked with monel metal fingerling tags through the ear pinna. Uromys caudimaculatus (white-tailed rat), Hypsiprymnodon moschatus (musky ratkangaroo), Perameles nasuta (long-nosed bandicoot) and Isoodon macrourus (northern brown bandicoot) were marked individually by tattooing of the ear pinna. Two rainforest rodent species, Rattus leucopus and R. fuscipes, could not be separated in the field, as the only satisfactory method of distinguishing them in the Palmerston area is by skull features (L. Moore, personal communication). Both were present on the sites. These two species are designated in this paper as Rattus sp. The effect of location on number of captures of rainforest and grassland species was examined by means of analysis of variance. Data were pooled over the four sampling times. Data collected from traps were pooled into five locations: (1) within the corridor; (2) the two rainforest rows closest to the rainforest edge (10 and 30 m) on either side of the corridor; and (3) the two rainforest rows furthest from the rainforest edge (90 and 110 m) on either side. Aggregated data were transformed to achieve constant variance within groups using log10 (x + 0á1). Four orthogonal contrasts were used to partition the four degrees of freedom associated with the location effects and thus determine the major contributions to the location effects. The orthogonal contrasts examined were (1) rainforest close to edge v. rainforest further from edge, (2) corridor habitat (either grassland or regrowth rainforest) v. intact rainforest, (3) rainforest on one side of the corridor v. rainforest on the other, and (4) the difference between rainforest close to the edge and rainforest further from the edge on one side of the corridor v. the equivalent difference on the other side. The mean distance between successive recaptures was calculated for each species as the mean of the distances travelled by all individuals from one capture to the next during each trapping period. Movements between the final capture in one trapping period and the first capture in the next were excluded from these calculations. The effect of habitat type within the corridor (grassland or rainforest regrowth) on distance moved by Rattus sp. and M. cervinipes was tested by ANOVA with logl0-transformed data pooled over the four sampling periods. The distances of the first movement for each animal for each trapping period between different traps was used as the response variable to reduce the bias due to repeated measures of very mobile or trappable individuals. Sedentary animals were excluded from this analysis. Contingency analyses of the proportion of movements in which individuals were sedentary or highly mobile tested for similarity between sites for Rattus sp. or between habitats (performed on pooled data from replicate sites because of small sample size) for M. cervinipes. The effect of the powerline corridor on movements of rainforest species was tested by FisherÕs Exact tests. For any one trapping period of four nights, only the first movement of 80 m or more by an individual was considered in the analysis, as this was the distance an individual would need to travel for a corridor crossing to be recorded (although the corridor was 60 m wide, the traps adjacent to the clearing on either side were placed 10 m from the edge). Excluding subsequent movements in trips reduced the effects of repeated measures. Movement data for all species during the four trips and between trips were pooled for the two grassland corridor sites and for the two sites with rainforest canopy connection. Bait-inducement Experiments Baiting experiments were conducted in February and March 1993 following the four grid-based trapping sessions. At all four sites, 12 Elliott Type A traps were placed at 10-m spacings along the two rows closest to the rainforest edge on one side of the clearing only (Fig. 1). In order to capture larger small mammals such as U. caudimaculatus, wire cage traps were placed with the first and seventh Elliott traps on the row

616


closest to the rainforest edge and with the fourth and tenth Elliott traps on the second row. The traps were baited with rolled oats and vanilla essence and set for four consecutive nights in February on one side of the powerline clearing and for four consecutive nights in March on the other side of the clearing. The proportion of movements of at least 80 m that were corridor crossings was compared for all species combined by FisherÕs Exact tests and 2 ´ 2 tables for the following treatment combinations: (1) traps baited on both sides of the corridor (standard grid trapping) v. traps baited on one side of the corridor only, and (2) grassland corridor v. regrowth corridor sites. Tunnel Experiment After completion of the grid-trapping and baiting experiments, a shadecloth tunnel was erected in the powerline clearing at one of the two grassland corridor sites. The second grassland site remained without a tunnel, as a control. The tunnel experiment was designed to determine whether the presence of an artificial structure with light regimes similar to those found beneath the rainforest canopy could induce greater crossing rates and thereby mitigate the effects of fragmentation. Although replication was impossible because of financial constraints (resulting in experimental design inadequacies), the experiment served as a pilot for the development of potential artificial crossing routes. The tunnel was designed as far as possible to emulate the dimensions of smaller highway underpasses (culverts) that are used as road-crossing routes by small mammals (Goosem, unpublished data). Grass was completely cleared across the 60-m width of the powerline corridor for a width of 5 m. The tunnel framework consisted of wooden stakes (2á5 ´ 2á5 ´ 200 cm) driven into the ground to a depth of 100 cm. The tops of the wooden stakes were connected with thick plastic agricultural tubing (5 cm diameter and 0á5 cm thick). Shadecloth was used to cover the framework and was fixed to the ground with aluminium pegs 1á5 m on either side of the framework. The result was a grass-free triangular tunnel 1 m tall and 3 m wide at ground level. The 1Ð2-m high exotic grassland community was allowed to regrow adjacent to the tunnel and was completely reinstated after two months. The 80-m-long tunnel crossed the 60-m-wide clearing and extended 10 m into the rainforest on either side. Trapping recommenced following a 2-month habituation period after erection of the tunnel. Grid trapping continued in June, August, September and October 1993 at the two grassland sites (with and without shadecloth tunnel). Baiting experiments were repeated on one side of the corridor in November 1993 and on the other side in December 1993. The initial shadecloth covering provided 32% shade. Following each trapping period, the shade provided was increased by changing the shadecloth covering to 50, 82 and 100%. Black woven weed-matting provided 100% shade. Variation in the degree of shade was intended to test for shade level preference in animals that used the tunnel as a crossing conduit. Data were analysed as described for the grid-trapping and bait-inducement experiments by contingency analysis and ANOVA.

Results Community Structure The four sites were grid-trapped for 4600 trap-nights during the first four trapping sessions. Eight species of small mammal were trapped, including 819 captures of 224 individual Rattus sp., 167 captures of 58 M. cervinipes, 54 captures of 31 U. caudimaculatus, 109 captures of 46 M. burtoni, 12 captures of 9 R. sordidus, 7 captures of 6 A. flavipes and 2 captures of 1 P. nasuta (Table 1). Recaptures of A. flavipes and P. nasuta were too few to be considered in data analysis. The small-mammal community structure within intact rainforest differed from that found within the powerline corridor (Fig. 2). Only species recognised as preferring rainforest habitats were found on the two rows most distant from the rainforestÐpowerline corridor edge (90 and 110 m). Five of the six individual A. flavipes were also captured in the trapping rows most remote from disturbance. In the two trapping rows within intact rainforest but closest to the edge (10 and 30 m), M. cervinipes formed a greater proportion of individuals known to be alive than it did further away from the edge (Fig. 2). This trend continued into the remnant and regrowth rainforest in the gullies of the powerline corridor, where the proportion of M. cervinipes was even higher.

Grid-trapping data, pooled over the first four trapping sessions, in which traps were placed on both sides of the powerline corridor for grassland corridor sites and regrowth corridor (rainforest canopy connection) sites Although the powerline corridor was only 60 m wide and therefore 60 m is used for determination of ability to cross, for a crossing to be recorded an individual had to move 80 m or more (i.e. from a row at least 10 m inside the rainforest on one side to a similar position on the other side), and therefore 80 m is used for comparative purposes. Ð , not available Species

No. of individuals

No. of captures

No. of recaptures

Maximum movement (m)

1st movements ³ 60 m (% of all 1st) movements)

1st Crossings (% of 1st movements ³80 m)

Grassland habitat in corridor Rattus sp. Melomys cervinipes Uromys caudimaculatus Melomys burtoni Rattus sordidus Antechinus flavipes

111 28 13 46 9 1

420 87 17 109 12 1

248 66 4 77 3 0

128á1 107á7 Ð 100á0 0 Ð

18 (9á7%) 1 (5á6%) Ð 5 (5á6%) Ð Ð

0 0

15 (7á1%) 3 (9á1%) 5 (71á4%) Ð Ð Ð Ð

2 (14á3%) 0 1 (25á0%) Ð Ð Ð Ð


Table 1.

Ð 0 Ð Ð

Rainforest regrowth habitat in corridor Rattus sp. Melomys cervinipes Uromys caudimaculatus Antechinus flavipes Perameles nasuta Melomys burtoni Rattus sordidus

113 30 18 5 1 0 0

399 80 37 6 2 0 0

288 50 19 1 1 0 0

201 107á7 208á8 20á0 Ð Ð Ð

617

618


Fig. 2. Composition of the small-mammal community in each of four habitats: rainforest interior 90Ð110 m from the powerline corridor edge; rainforest close (10Ð30 m) to the powerline corridor edge; regrowth and remnant rainforest in gullies of the powerline corridor; and grassland in the cleared powerline corridor. Data (individuals known to be alive) were pooled over all times of the first four grid-trapping periods and over all trapping rows from each of the four habitats.

In the cleared powerline corridor dominated by dense exotic grassland with occasional woody-weed thickets, the small-mammal community structure was completely different (Fig. 2). In this habitat, most individuals were from two grassland species, M. burtoni and R. sordidus. The habitat specificity of the grassland species is demonstrated in Fig. 3. Only once was an individual of a grassland species (M. burtoni) captured away from the grassland corridor and this animal was captured on the row closest to the clearing. The grassland species were never captured under the canopy afforded by the remnant and regrowth rainforest in the gullies of the powerline corridor. The habitat preferences of the rainforest species are also obvious in Fig. 3, where there was a general decrease in numbers of individuals of Rattus sp. from the rows furthest from the rainforest edge towards the grassland powerline corridor, with very few being caught within the clearing itself. Where individuals of this species were caught within the grassy corridor, they were invariably associated with the woody-weed thickets of Lantana camara and Rubus alceifolius. Where gully rainforest remained, this general decrease from intact rainforest, through edge and into the highly disturbed corridor was less pronounced. Particularly at Site 3 (Fig. 3), Rattus sp. was prominent within the gully rainforest. Numbers of individuals of M. cervinipes, generally increased from the less disturbed area towards the rainforest edge (Fig. 3). This preference for disturbed rainforest habitat continued into the disturbed rainforest of the powerline corridor gullies. M. cervinipes was, however, very seldom captured in the grassland of the cleared powerline corridor. U. caudimaculatus was also absent from grassland habitat but occurred sporadically in the disturbed regrowth habitat (Fig. 3).

Fig. 3. Grid-trapping data from the first trapping series. Data [individuals known to be alive (KTBA)] for each of three species of rainforest small mammal and two species of grassland small mammal at each site were pooled over the four trapping periods of the first grid-trapping series. Means and standard errors are shown for each trapping row. The powerline corridor is depicted by the shaded area.


619

620


The effects of location and corridor habitat type on the presence of the four species for which sufficient data were available for analysis is shown in Table 2. The results confirmed the general trend evidenced in Fig. 3, the interaction between location and corridor habitat type being significant in the case of M. burtoni, due to their restriction to the grassland corridor (Fig. 3). This interaction approached significance for the rainforest species, Rattus sp. and M. cervinipes, both of which were seldom captured within the grassland corridor habitat. The distributions of Rattus sp., U. caudimaculatus and M. burtoni showed significant location effects. For M. cervinipes location was not significant, possibly due to the anomalous distribution of the species at Site 3, where it was almost completely absent on one side of the corridor (Fig. 3). Significant contributions to the location effects after partitioning of the four degrees of freedom through four orthogonal contrasts are shown in Table 3. The contrast between corridor habitats and intact rainforest habitats was the major contributor to the location effects for Rattus sp. and M. burtoni, together with the significant interaction between corridor v. non-corridor in the regrowth habitats and corridor v. non-corridor in the grassland habitats. The presence of rainforest habitat, either within the disturbed corridor and consisting of regrowth or comprising intact habitat on either side of the corridor, was therefore the major determinant of the distribution of these species with respect to the powerline corridor, with M. burtoni confined to the grassland and Rattus sp. predominantly in the rainforest. For M. cervinipes, the contrast between corridor v. non-corridor in the regrowth sites and corridor v. non-corridor in the grassland sites was the only significant contributor to location effects, demonstrating a rainforest habitat preference. The sporadic occurrence of the small numbers of U. caudimaculatus is highlighted by the significance of the contrast between the two sides of the corridor for this species.

Table 2. Results of ANOVAs to test the effects of location and corridor habitat on the number of Rattus sp., M. cervinipes, U. caudimaculatus and M. burtoni known to be alive The number of individuals of each species known to be alive on each row of the trapping grid was tested for the effects of five locations (furthest from rainforest edge, i.e. 110 and 90 m on either side of the powerline corridor; close to edge, i.e. 30 and 10 m on either side; and within the corridor itself) and the effects of two corridor habitats (grassland v. regrowth rainforest). Corridor habitat and location were fixed factors. Significant effects are shown in bold owing to the multiplicity of tests, a = 0á01 was used Species

Source of variation

d.f.

F

P

Rattus sp.

Location Corridor habitat Location ´ corridor habitat Site nested in corridor habitat

4 1 4 2

13á02 0á22 5á43 5á51

0á001 0á668 0á021 0á031

Melomys cervinipes


4 1 4 2

1á85 0á02 4á58 1á28

0á213 0á908 0á032 0á329

Uromys caudimaculatus


4 1 4 2

8á29 1á24 2á28 2á05

0á006 0á382 0á149 0á191

Melomys burtoni


4 1 4 2

35á05 20á10 39á60 1á92

0á000 0á046 0á000 0á209


621

Table 3. Significant orthogonal contrasts that partition the four degrees of freedom of the effects of location on the presence of Rattus sp., M. cervinipes, U. caudimaculatus and M. burtoni The number of individuals of each species known to be alive on each row of the trapping grid was tested for the effect of four orthogonal contrasts (see methods) that partitioned the four degrees of freedom of the location effects (Table 2). Owing to the multiplicity of tests, a = 0á01 was used Species

Source of variation

d.f.

F

P

Rattus sp.

Corridor habitat v. intact rainforest Interaction between type of corridor habitat and corridor v. intact rainforest contrast

1 1

46á16 17á72

0á000 0á003

Melomys cervinipes

Interaction between type of corridor habitat and corridor v. intact rainforest contrast

1

12á51

0á008


Rainforest on one side of clearing v. rainforest on the other side of clearing

1

14á11

0á006

Melomys burtoni

Corridor habitat v. intact rainforest habitat Interaction between type of corridor habitat and corridor v.intact rainforest contrast

1 1

138á03 154á03

0á000 0á000

Movements and Powerline-corridor Crossings The mean distances travelled between successive recaptures were less than the 60-m width of the powerline corridor for Rattus sp. (grassland corridor sites, mean = 25á8 m, s.e. = 2á02 m; regrowth corridor sites, mean = 29á7 m, s.e. = 2á12 m), and M. cervinipes (grassland corridor sites, mean = 23á8 m, s.e. = 6á90 m; regrowth corridor sites, mean = 29á4 m, s.e. = 4á59 m). In the case of Rattus sp., there was no significant difference between sites in the proportion of movements where an individual was recaptured in the same trap or where non-sedentary movements were repeated (x2 = 6á726, d.f. = 6, P < 0á347). Similarly, when data were pooled over corridor habitats for M. cervinipes, there was no significant difference in these repetitive effects (x2 = 1á846, d.f. = 2, P < 0á397). For both species, no significant difference could be detected in the mean distances of the first non-sedentary movements during any one trapping period between sites with grassland habitat and sites with rainforest regrowth in the corridor (Table 4). The maximum distance travelled approximated at least twice the width of the powerline corridor for both Rattus sp. and M. cervinipes (Table 1), demonstrating that it is physically possible for these species to move the distance required to cross the corridor. However, such crossings would be expected to occur relatively infrequently, as fewer than 10% of movements by Rattus sp. or M. cervinipes were greater than the 60-m corridor width (Table 1). Few movement data were available for U. caudimaculatus at the grassland sites. However, the mean distance moved at the sites with regrowth rainforest in the corridor was greater than the width of the corridor (mean = 72á7 m, s.e. = 25á75 m), with more than 70% of movements being of distances equal to, or greater than, the 60 m required to cross the corridor. This suggests no distance restriction to corridor crossings by U. caudimaculatus. No crossings of the grassland powerline corridor were recorded during any trapping period by any species, although 17 movements sufficient for a corridor crossing to be recorded (i.e. ³ 80 m) were made by rainforest species at the sites with grassland corridor habitat (Table 1). During the same period, three within-trip crossings of the corridor were recorded at regrowth corridor sites by two Rattus sp. and one U. caudimaculatus (Table 1). These three crossings comprise 14á3% of first movements of at least 80 m for Rattus sp and 25% of first movements or at least 80 m for U. caudimaculatus. When crossings between trips are included, the number of

622


Table 4. (a) Results of ANOVAs to test the effects of type of habitat in the corridor and site on the mean distances moved by Rattus sp. and M cervinipes for the first trapping series. (b) Results of ANOVAs to test the effects of trapping series and site on the mean distances moved by Rattus sp. and M. cervinipes for grassland sites Habitat in corridor (i.e. grassland or regrowth rainforest), site (two replicate sites in each corridor habitat), and trapping series (pooled data from the first four grid trapping periods for each grassland site v. pooled data from the second four grid trapping periods) were fixed factors. Data were log10 transformed. Significant effects are shown in bold; a = 0á05 Species

Source of variation

d.f.

F

P

(a) Rattus sp.

Corridor habitat Site nested in corridor habitat

1 2

1á824 0á876

0á179 0á418

Melomys cervinipes

Corridor habitat Site nested in corridor habitat

1 2

1á062 1á198

0á313 0á318

Rattus sp.

Trapping series Site Trapping series ´ site interaction

1 1 1

0á656 0á196 1á253

0á428 0á664 0á265

Melomys cervinipes

Trapping series Site Trapping series ´ site interaction

1 1 1

0á016 16á657 0á178

0á900 0á000 0á679

(b)

crossings of the grassland corridor (0 of 34 movements ³ 80 m) and of the regrowth corridor (8 of 35 movements ³ 80 m) made by all rainforest species were significantly different (FisherÕs Exact test, x2 = 5á202, d.f. = 1, P < 0á007). Bait-inducement Experiments The bait-inducement experiments were conducted over 896 trap-nights. The total of 294 captures included 116 Rattus sp., 96 M. cervinipes, 28 U. caudimaculatus, 2 A. flavipes and 2 Hypsiprymnodon moschatus. Baiting-induced movements of 80 m or greater are shown in Table 5 together with corridor crossings. No crossings of the grassland corridor could be induced by baiting, whereas six crossings (four by Rattus sp. and two by U. caudimaculatus) of the regrowth corridor took place under bait inducement. The proportion of movements ³ 80 m consisting of crossings of the regrowth corridor during and between trapping periods increased during the baiting experiment for Rattus sp. (57á1 v. 18á1%) although not significantly (x2 = 0á960, d.f. = 1, FisherÕs Exact P < 0á163). Tunnel Experiment The tunnel experiment was continued for 2368 trap-nights. Capture and movement data are summarised in Table 6. The composition of the trappable rainforest small-mammal community was significantly different during the tunnel experiment, when compared with the first trapping series (x2 = 51á481, d.f. = 4, P < 0á001). The proportion of the trappable rainforest small-mammal community composed of M. cervinipes increased from 18á3% during the first series to 33á1% in the tunnel experiment. The corresponding decrease in Rattus sp. was from 72á5 to 55á6%. If only Rattus sp., M. cervinipes and U. caudimaculatus are considered (these three species form 99á3% of the initial trappable community), the change in trappable rainforest community structure becomes even more significant (x2 = 61á062, d.f. = 2, P < 0á001).


Table 5.

623

Captures, movements of sufficient distance to be recorded as a crossing of the corridor, and actual corridor crossings during the baiting experiments Movement and crossing data include first captures only

Species

February, March 1993 Rattus sp. Melomys cervinipes Uromys caudimaculatus November, December 1993 Rattus sp. Melomys cervinipes Uromys caudimaculatus

No. of captures

No. of movements ³80 m

No. of crossings

Grassland corridor habitat 80 58 8

8 1 1

9 0 3

No. of movements ³80 m

No. of crossings

Regrowth corridor habitat 0 0 0

Grassland corridor habitatÐtunnel site 89 6 7

No. of captures

3 0 0

86 38 20

7 1 4

4 0 2

Grassland corridor habitatÐno tunnel 65 13 7

11 0 2

3 0 0

The composition of the trappable grassland small-mammal community also changed significantly (x2 = 17á103, d.f. = 2, FisherÕs Exact P < 0á001) because of the intrusion of six Mus musculus into the clearing. There was no significant difference between trapping series in proportions of sedentary and highly mobile animals for Rattus sp. (x2 = 5á103, d.f. = 2, P < 0á078) or for M. cervinipes (x2 = 0á606, d.f. = 2, P < 0á739). Similarly, no significant difference was detected in mean first-movement distances between the two grid-trapping series for M. cervinipes or for Rattus sp. (Table 4), although there was a significant difference between sites for M. cervinipes (Table 4), reflecting greater mobility of this species at one of the sites (tunnel site, mean = 22á5 m, s.e. = 4á92 m, n = 17; control site, mean = 16á3 m, s.e. = 1á50 m, n = 96). Only one crossing of the grassland powerline corridor (by a U. caudimaculatus) was recorded within a trapping period and this occurred at the control site without a tunnel. Three additional crossings were recorded between trapping periods by two U. caudimaculatus and one Rattus sp., two occurring at the tunnel site. There was no difference between treatments. The small number of crossings of the tunnel site caused tests of preference for degree of shade to be abandoned. Data for the bait-inducement experiments performed after the tunnel grid-trapping experiment, with the tunnel still in position, are shown in Table 5. At both sites, three crossings were recorded, all by Rattus sp. There was no difference between treatments. Discussion This study demonstrates that the presence of a wide artificial linear clearing alters smallmammal community structure both within the clearing and to either side of the clearing. Two distinct small-mammal communities occurred. The distribution of small-mammal species was dependent on the type of habitat within the corridor, with small-mammal communities composed either of grassland specialists (M. burtoni and R. sordidus) in the case of the cleared powerline corridor or rainforest specialists (Rattus sp., M. cervinipes and U. caudimaculatus) in the cases of the intact rainforest and the disturbed remnant and regrowth gully rainforest of the powerline corridor. The evidence clearly demonstrates that the presence of a cleared powerline corridor passing through rainforest allowed intrusion by species alien to the natural habitat. Faunal intrusions and displacement of forest-interior species by grassland species of small mammals (Johnson et al. 1979) and birds (Anderson et al. 1977; Kroodsma 1982) have also been demonstrated along

624

Table 6.

Grid trapping data, pooled over the second four grid-trapping sessions, in which traps were placed on both sides of the corridor for grassland corridor sites Although the powerline corridor was only 60 m wide and therefore 60 m is used for determination of ability to cross, for a crossing to be recorded an individual had to move 80 m or more (i.e. from a row at least 10 m inside the rainforest on one side to a similar position on the other side), and therefore 80 m is used for comparative purposes. Ð , not available Species

Rattus sp.

No. of individuals

No. of captures

No. of recaptures

Maximum movement (m)

1st Movements ³60 m (% of all movements)

Crossings (% of 1st movements ³80 m)

143

504

393

102á0

15 (6á5%)

0

Melomys cervinipes

85

280

222

82á5

1 (1á0%)

0


28

61

41

243á3

2 (15á4%)

1 (25á0%)

Melomys burtoni

47

214

172

44á7

0

0

Rattus sordidus

11

37

26

40á0

0

Melomys musculus

6

8

2

63á2

1

Ð

0

Perameles nasuta

1

2

1

60á0

1

Ð

0

0



625

powerline clearings in the United States. In Tennessee, Schreiber and Graves (1977) found that populations of two small-mammal species in a 91-m-wide powerline clearing were distinct and separate from those found in adjacent forest habitat, while Johnson et al. (1979) demonstrated that two alien small-mammal species had penetrated into the same clearing. Such faunal intrusions can result in rapid range extensions by grassland rodent species, as occurred in Illinois along an interstate highway (Getz et al. 1978). Anderson et al. (1977) found that the width of the corridor was a significant factor in allowing bird species to intrude along powerline corridors in Tennessee, where a 12-m-wide corridor was inhabited primarily by edge specialists, while corridors of a width similar to those in this study allowed the intrusion of many more grassland species and displacement of forest-interior birds. An encouraging aspect of this study was the almost complete exclusion of grassland species from the rainforest interior. In contrast, studies in the United States have found that species with excellent dispersal abilities and that are capable of invading and colonising disturbed habitats can penetrate into the core of natural habitats along intruding linear clearings. In bird communities of Maine, Tennessee and New Jersey, edge specialists, which are often nest predators and brood parasites, penetrated into forest communities from more suitable habitat along highways and powerline clearings (Ferris 1979; Kroodsma 1982), thereby potentially reducing the breeding success of forest-interior species (Rich et al. 1994). Intruding grassland small-mammal species did not demonstrate similar rainforest invasive behaviour in this study, an indication that these species are unlikely to become environmental pests of intact rainforest habitats, analogous to the feral pig, Sus scrofa, and the cane toad, Bufo marinus. Another feral species, the house mouse, Mus musculus, intruded along the grassland of the powerline corridor during the second trapping series but similarly did not penetrate intact rainforest, although the species had no difficulty in colonising open forest at Naringal in Victoria, particularly favouring disturbed vegetation (Bennett 1990). The mechanism by which rainforest species are excluded from the grassland habitat of the powerline corridor is debatable. Intuitively, it would be expected that terrestrial and scansorial rainforest small mammals that are adapted to habitats of high structural complexity and relatively constant microclimate would find the more open, simple structure of dense grassland and greater variability in microclimatic factors (e.g. light, temperature, humidity) an environmental barrier and simply avoid the habitat discontinuity this represents. Several rainforest species, including three investigated in this study (R. fuscipes, M. cervinipes and U. caudimaculatus) have been shown to prefer rainforest vegetation over open forest (Hockings 1981; Smith 1985; Williams 1990) or pine plantations (Barnett et al. 1978). Because the difference in microclimate and vegetation structure between grassland and rainforest is much greater than that between open forest and rainforest, these rainforest species would be expected to demonstrate an even greater preference for rainforest over grassland. Alternatively, interspecific competition between the better-adapted small mammals of the grassland and attempted colonisers from the rainforest could form a mechanism for exclusion. As there was no potential competitor of similar size and niche utilisation, it appears that differences in habitat alone were sufficient to cause exclusion of the large scansorial rodent U. caudimaculatus. This exclusion was unexpected as this species is known to use the dense vegetation along creeks in pastures adjacent to rainforest remnants of the Atherton Tableland near Millaa Millaa (Laurance 1989). The intrusion into the grassland by Rattus sp. via the canopy closure afforded by woody-weed thickets is consistent with the results of Laurance (1989) in pastures adjacent to rainforest remnants of the Atherton Tableland, where R. fuscipes penetrates into similar dense edge habitats. The grassland species R. sordidus captured in this study is similar in size and niche utilisation to the rainforest Rattus sp., but did not have a large trappable population, potentially reducing the effects of any intrageneric competition, particularly in those edge habitats where Rattus sp. is better adapted.

626


Melomys cervinipes was almost never captured within the grassland of the powerline corridor, although this species appeared to prefer the disturbed habitat of the rainforest edge and the secondary regrowth rainforest of the gullies over the less disturbed forest interior (Fig. 3). The mechanism of exclusion from the cleared powerline corridor in this case therefore appears unlikely to be habitat differences. A large population of the congener M. burtoni inhabited the grassland of the clearing: intrageneric competition between these two species may have caused exclusion of M. cervinipes from the powerline corridor. Strengthening this argument is the common occurrence of M. cervinipes within grassy open forest understorey and pasture close to rainforest fragments in areas of the nearby Atherton Tablelands, where M. burtoni is absent (Laurance 1989; Williams 1990). It is likely that interspecific competition with grassland small-mammal species that are better adapted to the habitat in the cleared powerline corridor as well as the severe habitat discontinuity of the grassland combined to cause the almost complete exclusion of rainforest species from the clearing. Despite the capability of all rainforest species to move distances far greater than the width of the corridor, the cleared powerline corridor caused severe inhibition of movements across the corridor by Rattus sp. and U. caudimaculatus, while no crossing movements by M. cervinipes were recorded. Even when bait inducement was used, crossings of the grassy clearing by M. cervinipes were not recorded. Several authors have observed inhibition of small-mammal movements caused by linear environmental discontinuities (Oxley et al. 1974; Schreiber and Graves 1977; King 1978; Wilkins 1982; Garland and Bradley 1984; Mader 1984; Swihart and Slade 1984; Bakowski and Kozakiewicz 1988; Burnett 1992), but only three have failed to record any crossings of the barrier such as occurred for M. cervinipes in this study. Oxley et al. (1974) demonstrated that a highway in Canada, more than twice the width of the powerline corridor in this study, completely prevented crossings of small rodents, while in Germany a narrower road (6á5Ð8-m-wide) isolated populations of the small rodent Apodemus flavicollis (Mader 1984), and a 5-m-wide road similarly affected Polish bank voles, Clethrionomys glareolus (Bakowski and Kozakiewicz 1988). In North Queensland upland tropical rainforest, Burnett (1992) demonstrated that a narrow (6Ð12 m) bitumen road inhibited crossing movements of R. fuscipes, A. flavipes and U. caudimaculatus. Burnett (1992) emphasised that the road did not form a complete physical barrier since, besides the occasional crossings observed in routine grid-trapping on both sides of the road, individuals of all three species could easily be induced to cross the road by translocating them to the other side of the road or by baiting traps on one side of the road only. This easy induction of narrow-road crossings observed by Burnett (1992) is analogous to the readiness of rainforest small mammals to cross the powerline corridor in this study, particularly under bait inducement, where gully remnant and regrowth forest afforded a canopy connection. Other similarities in results include the greater crossing ability of the larger and more mobile U. caudimaculatus. However, there is no precedent in tropical rainforest studies for the barrier formed by the grassland corridor to M. cervinipes and the severe inhibition of crossings by Rattus sp. and U. caudimaculatus. Alignment of home ranges along environmental barriers has been proposed as one potential causative mechanism for linear barrier effects (Burnett 1992). Evidence supporting this hypothesis comes from the largely sedentary nature of many individuals even where the potential for large-scale movements was provided by the presence of connectivity of habitat. Most (86á5%) of the individuals from the two rows closest to the regrowth rainforest did not attempt to cross or enter the corridor, although suitable regrowth habitat was available. A similar disinterest in crossing through suitable habitat was observed by Schreiber and Graves (1977) for rodents, Peromyscus leucopus, and shrews, Blarina brevicauda, of Tennessee pine and hardwood forests adjacent to a powerline clearing. However, such an alignment of home ranges would not form a boundary preventing movements such as those for dispersal or those induced by baiting or translocation.


627

The grassland-corridor movement barrier must be caused by a greater effect than mere alignment of home-range boundaries. If unfavourable habitat or interspecific competition prevents rainforest species from colonising the clearing, the same factor(s) may also prevent rainforest species from entering the clearing in order to cross. The habitat differences that prevented colonisation by U. caudimaculatus were not of sufficient magnitude to completely prevent crossings by the species during the second trapping series. Rattus sp. individuals also crossed in the second trapping series aided by the dual advantages of itinerant residence along the edge of the clearing and only a small population of the speciesÕ potential interspecific competitor. Intrageneric competition for M. cervinipes created a more formidable barrier than habitat differences alone caused for the other rainforest species. For M. cervinipes, and to a lesser extent for Rattus sp. and U. caudimaculatus, it appears that whatever the mechanism of movement inhibition, the fragmentation of populations caused by the grassy swathe of the powerline corridor may have serious long-term effects on gene flow and therefore population viability. Although the ability to cross a powerline corridor would be species-specific, dependent on such factors as size, mobility, behaviour and interspecific competitors in the corridor habitat, species and guilds that may be most susceptible to such fragmentation are as follows: (1) species that do not move at ground level (e.g. some rainforest possum species); (2) species with probable intrageneric competitors within the grassland (e.g. M. cervinipes); and (3) species of low mobility with strong rainforest habitat preferences (e.g. microhylid frogs, some rainforest skinks). Unfortunately, the construction of the artificial shadecloth tunnel did not increase the crossing rate of any of the rainforest species and was unsuccessful as a mitigatory measure. Artificial constructions have been successful elsewhere in Australia in allowing wildlife to cross linear barriers by restoring habitat continuity (Mansergh and Scotts 1989). Tunnel design, particularly tunnel diameter and the presence of cover at tunnel entrances, appears to be important in encouraging wildlife to use an artificial route (Hunt et al. 1987). It is possible that further experimentation with shadecloth-tunnel design may achieve greater success, particularly for areas where re-establishment of canopy connections is impractical. From the results of this study, fragmentation caused by the cleared grassy swathe of a powerline corridor and the exclusion of rainforest species from the alienated habitat may best be mitigated by the following: (1) the maintenance and strengthening through replantings of gully remnant and regrowth vegetation that provide canopy connectivity and demonstrated crossing routes; (2) the provision of more potential crossing points by rehabilitation and replanting of rainforest species across the corridor wherever this is feasible, particularly in areas that do not have extant gullies nearby; and (3) the reclamation of areas of alienated habitat by replanting of edges along cleared corridors to create a scalloping along the swathe where the swing of the powerline cannot touch the vegetation (i.e. close to powerline support towers, the swathe can be much narrower as there is negligible powerline swing near the supports). New powerlines should be positioned in areas that avoid clearing of natural vegetation or, if this is impossible, should involve clearing of areas only large enough for each of the powerline supports. Tall towers would then allow the powerline to swing above the canopy, thereby negating the requirement for wide clearings. Such construction methods have already been proposed for new powerlines through the rainforests of the Wet Tropics of Queensland World Heritage Area (Queensland Electricity Commission 1995). Acknowledgments This study was supported by research grants from the Queensland Electricity Commission, the Wet Tropics Management Authority and James Cook University of North Queensland.

628


Miriam Goosem was supported by a part-time Australian Postgraduate Research Award. The authors would like to acknowledge the assistance in statistical analysis provided by Glenn DeÕath and Steve Delean. Greatly valued field assistance was provided by Mike Trenerry. Rodent species were identified by Les Moore, who confirmed that two species of Rattus could not be distinguished in the field. Dr Stephen GoosemÕs help in most phases of the study is deeply appreciated, particularly in site preparation, tunnel construction and constructive discussion and comments on the manuscript. References Anderson, S. H., Mann, K., and Shugart, H. H. (1977). The effect of transmission-line corridors on bird populations. American Midland Naturalist 97, 216Ð221. Bakowski, C., and Kozakiewicz, M. (1988). The effect of forest road on bank vole and yellow-necked mouse populations. Acta Theriologica 33, 345Ð353. Barnett, J. L., How, R. A., and Humphries, W. F. (1978). The use of habitat components by small mammals in eastern Australia. Australian Journal of Ecology 3, 277Ð285. Bennett, A. F. (1990. Land use, forest fragmentation and the mammalian fauna at Naringal, south-western Victoria. Australian Wildlife Research 17, 325Ð347. Burnett, S. (1992). Effects of a rainforest road on movements of small mammals: mechanisms and implications. Wildlife Research 19, 95Ð104. Ferris, C. R. (1979). Effects of Interstate 95 on breeding birds in northern Maine. Journal of Wildlife Management 43, 421Ð427. Garland, T., and Bradley, W. G. (1984). Effects of a highway on Mojave Desert rodent populations. American Midland Naturalist 111, 47Ð56. Getz, L. L., Cole, F. R., and Gates, D. L. (1978). Interstate roadsides as dispersal routes for Microtus pennsylvanicus. Journal of Mammalogy 59, 208Ð212. Goosem, M. W. (1997). Internal fragmentation: the effects of roads, highways and powerline clearings on movements and mortality of rainforest vertebrates. In ÔTropical Forest Remnants: Ecology, Management, and Conservation of Fragmented CommunitiesÕ. (Eds W. F. Laurance and R. O. Bierregaard, Jr.) pp. 241Ð255. (University of Chicago Press: Chicago.) Hockings, M. (1981). Habitat distribution and species diversity of small mammals in south-east Queensland in relation to vegetation structure. Australian Wildlife Research 8, 97Ð108. Hunt A., Dickens, H. J., and Whelan, R. J. (1987). Movement of mammals through tunnels under railway lines. Australian Zoologist 24, 89Ð93. Johnson, W. C., Schreiber, R. K., and Burgess, R. L. (1979). Diversity of small mammals in a powerline right-of-way and adjacent forest in East Tennessee. American Midland Naturalist 101, 231Ð235. King, D. (1978). The effects of roads and open space on the movements of small mammals. B.Sc. Honours Thesis, Australian National University, Canberra. Kroodsma, R. L. (1982). Edge effect on breeding forest birds along a power-line corridor. Journal of Applied Ecology 19, 361Ð370. Laurance, W. (1989). Ecological impacts of tropical forest fragmentation on nonflying mammals and their habitats. Ph.D. Thesis, University of California, Berkeley. Mader, H. J. (1984). Animal habitat isolation by roads and agricultural fields. Biological Conservation 29, 81Ð96. Mansergh, I. M., and Scotts, D. J. (1989). Habitat continuity and social organization or the mountain pygmy possum restored by tunnel. Journal of Wildlife Management 53, 701Ð707. Middleton, J. (1993). The intrusive effects of a powerline clearing on the small mammal community of a tropical rainforest. B.Sc. Honours Thesis, James Cook University of North Queensland, Townsville. Oxley, D. J., Fenton, M. B., and Carmody, G. R. (1974). The effects of roads on populations of small mammals. Journal of Applied Ecology 11, 51Ð59. Queensland Electricity Commission (1995). Chalumbin-Woree 275kV Transmission Line. Preliminary Impact Assessment Report. Sinclair, Knight, Merz, Brisbane. Rich, A. C., Dobkin, D. S., and Niles, L. J. (1994). Defining forest fragmentation by corridor width: the influence of narrow forest-dividing corridors on forest-nesting birds in southern New Jersey. Conservation Biology 8, 1109Ð1121. Schreiber, R. K., and Graves, J. H. (1977). Powerline corridors as possible barriers to movements of small mammals. American Midland Naturalist 97, 504Ð508.


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Smith, G. C. (1985). Biology and habitat usage of sympatric populations of Melomys cervinipes and M. burtoni. Australian Zoologist 21, 307Ð326. Swihart, R. K., and Slade, N. (1984). Road crossing in Sigmodon hispidus and Microtus ochrogaster. Journal of Mammalogy 65, 357Ð360. Tracey, J. G. (1982). ÔThe Vegetation of the Humid Tropical Region of North Queensland.Õ (CSIRO: Melbourne.) Wilkins, K. T. (1982). Highways as barriers to rodent dispersal. Southwest Naturalist 27, 459Ð460. Williams, S. E. (1990). The interaction between vegetation and the small mammal community of the rainforest ecotone in north Queensland. B.Sc. Honours Thesis, James Cook University of North Queensland, Townsville.

Manuscript received 12 June 1996; revised and accepted 12 November 1996