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Keywords:

  • anthropogenic disturbance;
  • Banksia;
  • flowering phenology;
  • hybridization

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References

Abstract Putative hybrids between Banksia hookeriana and B. prionotes were identified among 12 of 106 populations of B. hookeriana located at or near anthropogenically disturbed sites, mainly roadways, but none in 156 undisturbed populations. Morphometrics and AFLP markers confirmed that a hybrid swarm existed in a selected disturbed habitat, whereas no intermediates were present where the two species co-occurred in undisturbed vegetation. Individuals of both species in disturbed habitats at 12 sites were more vigorous, with greater size and more flower heads than their counterparts in undisturbed vegetation. These more fecund plants also showed a shift in season and duration of flowering. By promoting earlier flowering of B. hookeriana plants and prolonging flowering of B. prionotes, anthropogenic disturbance broke the phenological barrier between these two species. We conclude that anthropogenic disturbance promotes hybridization through increasing opportunities for gene flow by reducing interpopulation separation, increasing gamete production and, especially, promoting coflowering.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References

Hybridization between plant species can provide the raw material for adaptive evolution in rapidly changing environments (Arnold, 1997; Rieseberg & Carney, 1998). Hybrids are often associated with disturbed habitats (Anderson, 1948; Arnold, 1997). Disturbances can foster co-colonization by normally allopatric species, creating opportunities for hybridization and suitable habitats for the survival of hybrids. Human disturbance of natural habitats has increasingly promoted hybrid establishment between previously isolated species (Loneragan, 1975; Neuffer et al., 1999; Bleeker & Hurka, 2001). Should there be hybrid genotypes with superior competitive abilities or wider environmental tolerances, they may gradually replace one or more parent species, depending on the abundance of parents and opportunities for hybridization (Levin et al., 1996; Bleeker & Hurka, 2001).

Mechanisms that reduce or prevent gene exchange between otherwise compatible species include selective pollinator behaviour (Hopper & Burbidge, 1978), spatial barriers (Grant, 1963) and asynchronous flowering (McIntosh, 2002). Although co-occurrence is not essential for hybridization (pollen may be wind-dispersed, many pollinators are strong fliers), coflowering is essential. We demonstrate here that a previously unrecognized side-effect of anthropogenic disturbance is that it can induce coflowering between species normally displaying asynchronous flowering in undisturbed sites. Together with increased gamete production in disturbed sites, this increases the chance of gene exchange between closely related species even when they do not co-occur.

Banksias are dominant components of the extensive sandplain flora of Australia (Taylor & Hopper, 1988; Low & Lamont, 1990). Their reproductive parts are a major food resource for indigenous animals, and their spectacular blooms are a focus for the ecotourism and floriculture (wildflower-picking) industries. Banksia hookeriana Meissn. is a shrub, up to 4 m tall, that is restricted to the upper slopes and crests of deep sand dunes in a 75 × 25 km area, 300 km north of Perth, Western Australia (Lamont et al., 1989). These are among the poorest soils known, with low levels of all nutrients and low water holding capacity (Lamont, 1995). Banksia prionotes Lindl. is a tree up to 10 m tall, in the lower parts of dune systems, on calcareous uplands or along drainage lines, with a distribution area of 815 × 125 km (extending 600 km north and 360 km south-east from Perth). Although their distribution ranges overlap, B. hookeriana and B. prionotes usually occupy different parts of the landscape, although they are sometimes interspersed. Both species are pollinated by the same nectar-feeding birds (Meliphagidae), including the white-cheeked honeyeater (Phylidonyris nigra) and brown honeyeater (Lichmera indistincta) (Taylor & Hopper, 1988; personal observations). Even where populations are separated by several hundred metres, they are well within the daily feeding ranges of these birds (Collins & Rebelo, 1987).

On morphological and phylogenetic grounds, B. prionotes and B. hookeriana are considered sister species (George, 1981; Thiele & Ladiges, 1996). Flowering times are mainly January to May for B. prionotes, and June to October for B. hookeriana (Taylor & Hopper, 1988; Sedgley et al., 1994). Artificial crossing between B. hookeriana and B. prionotes has shown that the two species can hybridize readily, with similar levels of seed set as for natural pollination of the two species (Sedgley et al., 1996). George (1984) noted that presumed hybrids among banksias are rare in Western Australia: ‘only recently have they been reported … possibly between B. prionotes and B. hookeriana’. From the above discussion, asynchronous flowering appears to be the most likely cause for the rarity of these hybrids.

In a recent survey of the distribution of B. hookeriana, we identified several populations with apparent hybrids between these two species at disturbed sites (roadways, tracks, abandoned mine pits, railway lines, firebreaks). It was noted earlier that individuals of banksias along roadways are much more fecund than in undisturbed vegetation (Lamont et al., 1994a,b). We now noted that the flowering seasons were also extended, potentially removing asynchronous flowering as the major barrier to hybridization. We therefore tested the hypothesis that anthropogenic soil disturbance promotes hybridization between these two species by altering their biology; specifically, that gene exchange is fostered by inducing coflowering and promoting gamete production.

Study sites

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References

Our study was carried out in the Eneabba Plain, 240–320 km north of Perth, Western Australia. The climate in this area is extra-dry Mediterranean (Beard, 1976), characterized by winter rains and summer drought. Mean annual rainfall for Eneabba (29°52′S, 115°15′ E) is 506 mm with a mean minimum monthly (July) temperature of 9.2 °C, and a mean maximum monthly (January) temperature of 38.8 °C. The vegetation is scrub-heath dominated by banksias on the sand dunes, with low heath lacking banksias in the intervening swales. The weak to moderately acid sands are extremely nutrient-impoverished (Lamont, 1995). The vegetation is fire-prone and both species are killed by fire (Groeneveld et al., 2002).

Field surveys were conducted from March to June 2002. We surveyed populations of B. hookeriana at 262 locations and recorded whether they were disturbed or undisturbed and the occurrence of putative B. hookeriana-prionotes hybrids based on morphological attributes (see below). The survey sought to identify as many hybrid (morphologically intermediate) plant localities as possible within the full geographical range of B. hookeriana. Intensive studies of selected populations were undertaken in nature reserve 39 744 (beekeepers), 15 km north of Eneabba. The entire area was burnt in a wildfire 5.5 years before. Suitable stands of B. prionotes for the transect study were difficult to locate (they tended to run along disturbed sites rather than across them and rarely flowered at this age) and so sites up to 15 km north of the reserve were used.

Morphological and phenological analyses

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References

We selected for intensive assessment: (a) the largest apparent hybrid swarm located and the nearest two populations of the parent species, and (b) an undisturbed site where the two species were interspersed, and the nearest populations to these species. Fifteen typical plants were assessed in the hybrid swarm, eight paired plants of each species in the interspersed population, and six plants from each of the remaining four populations. The following were measured: plant height, crown width, maximum number of branches from a node, level of flowering (1 = early bud to 5 = finished flowering), and whether florets were persistent on old flower heads (a feature of B. hookeriana but absent in B. prionotes). Samples of 20 leaves from the previous growing season were returned to the laboratory and the mean length and mid-length width of five intact leaves were taken. Leaves were then dried in a fan-forced oven at 65 °C for 1 week and bulk weighed. Data for the six quantitative attributes were placed in five groups (hybrid swarm, two combined nearest populations of B. hookeriana and B. prionotes, and interspersed populations of B. hookeriana and B. prionotes) and were subjected to canonical variates analysis using SYN-TAX 5.0, Mac version (Podani, 1995). In April and June, numbers of plants with open flower heads were noted on 75 B. hookeriana and 240 B. prionotes plants in the interspersed populations.

Genetic analysis

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References

Eighteen individuals of presumed ‘pure’B. hookeriana, 16 individuals of ‘pure’B. prionotes and 15 individuals of their putative hybrids all measured above for their morphological characteristics were DNA-fingerprinted using amplified fragment length polymorphism (AFLP) markers (Vos et al., 1995). Another two putative F1 hybrids, not included in the morphological analysis, from another two populations were also analysed. Terminal (actively growing) leaves from each individual were harvested and placed in air-tight plastic bags with silica gel in the field, and then kept at room temperature until analysed.

The DNA was extracted from dried-leaf tissue following a modified protocol (Doyle, 1991) involving RNAase (final concentration 20 μL mL−1). The quality and quantity of the extracted DNA was assessed by agarose gel electrophoresis and spectrophotometry (GeneQuant, Pharmacia, Cambridge, UK), respectively. DNA samples were diluted with 0.1 × TE to obtain concentrations of between 150 and 200 ng μL−1. AFLP markers were resolved using the AFLP TM Plant Mapping Kit protocol from Perkin Elmer Corporation (Carlsbad, CA, USA) and ABI 377 Auto-sequencer (ABI 377XL; Applied Biosystems, Foster City, CA, USA). The procedure followed Krauss & Peakall (1998). Arithmetic hybrid index scores using polymorphic AFLP markers were calculated as described in Fritz et al. (1994) and Hardig et al. (2000), which extended the method of maximum likelihood proposed by Rieseberg et al. (1998). The hybrid index was calculated using a computer program written by Hardig et al. (2000). All scores were standardized to range between zero (pure B. prionotes) and unity (pure B. hookeriana).

Transect study

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References

Flowering stage and plant size were assessed in relation to distance from the edge of disturbance. Populations were chosen to represent a range of soil-disturbance types, with plants both in the disturbed site and spreading 40 m back into undisturbed vegetation. In April 2002, when B. prionotes was in full flower and B. hookeriana was in bud, individuals along three B. hookeriana transects (rehabilitated road gravel pit, two highway sites) and three B. prionotes transects (unsealed road, two highway sites) were assessed for distance from disturbance, height, crown width, and numbers of open flower heads and late buds (and thus total flower heads). All stands had regenerated 5.5 years ago, except two stands of B. prionotes where plants were up to 30 years old. In June 2002, when B. hookeriana was in substantial flower and B. prionotes almost finished, the same flowering attributes were recorded for another three B. hookeriana (railway line, two highway sites) and three B. prionotes (unsealed road, two highway sites) transects. At least 60 individuals were assessed at each site.

Index of flowering was calculated as [2 (no. open flower heads) + no. late buds] to weight flowering stage and also minimize the occurrence of zeros (for statistical reasons) for plants yet to flower. Biovolume of individual plants was calculated from (4π/3 × 6 × 6 × 6) (height +2 × width)3 that assumes plant shape is ellipsoid (Lamont et al., 1994a). Results per species per time of assessment were combined for consecutive 5-m intervals along the transects. They were also analysed using canonical variates analysis after linearizing and placing into disturbed and undisturbed groups. Based on the expected spread of their root systems, we considered plants to be influenced by disturbance if they were up to 6 m (B. hookeriana) and 14 m (B. prionotes) away (Lamont & Bergl, 1991).

Occurrence of putative hybrids

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References

Of 262 sites with B. hookeriana populations, 156 were in undisturbed vegetation and 106 were in disturbed sites. Putative hybrids were present at 12 of the disturbed sites, whereas no putative hybrids were observed in the undisturbed vegetation (inline image = 17.38, P < 0.001). Four B. hookeriana and B. prionotes populations overlapped in undisturbed vegetation and two (both with hybrids) in disturbed vegetation. The remaining 10 populations had either B. hookeriana (two) or B. prionotes (eight) present.

Morphological and phenological analyses

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References

At plant age of 5.5 years (all arose from seedlings), there was no overlap between B. hookeriana and B. prionotes for seven of eight attributes measured (Table 1, Fig. 1). Banksia hookeriana was characterized by its shorter stature, more branches per node, smaller leaves, later seasonal flowering and retention of old florets, compared with B. prionotes. The putative hybrids were intermediate overall for all attributes, although some individuals were indistinguishable from some individuals of either parent species (Table 1, Fig. 1).

Table 1.  Morphology (mean ± SE) of the two Banksia species (n = 20) and their putative hybrid swarm (n = 15) at Beekeepers Reserve. Eneabba in May 2002.
 Height (cm)No. of branchesLeaf length (mm)Leaf width (mm)Leaf weightLevel of floweringRetention of dead florets
B. hookeriana113.1 ± 5.323.6 ± 2.7166.4 ± 4.17.7 ± 0.20.18 ± 0.001.41 ± 0.15
Putative hybrids145.3 ± 11.614.0 ± 1.9205.2 ± 12.811.4 ± 1.10.37 ± 0.052.91 ± 0.18+/–
B. prionotes177.1 ± 8.19.9 ± 0.8224.9 ± 6.317.7 ± 0.60.57 ± 0.033.67 ± 0.24+
image

Figure 1. Canonical variates analysis of B. hookeriana (BH) and B. prionotes (BP) and their putative hybrids. Group A (squares): two populations of BH; B (diamonds): population of BH interspersed with group D; C (circles): putative hybrid swarm; D (triangles): population of BP; E (inverted triangles): two populations of BP. Vector 1: plant height; 2: crown width; 3: level of flowering (in April); 4: leaf length; 5: leaf width; 6: leaf weight; 7: number of branches (square root). Individuals represented by filled symbols retained their old florets, whereas those with open symbols did not.

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The interspersed populations of B. hookeriana and B. prionotes lacked overlap of all attributes except crown width (Fig. 1). Some individuals were larger than those in the other two conspecific populations assessed, but they were embedded among them on the basis of the other six attributes. In April, no B. hookeriana individuals at this site flowered, whereas all B. prionotes were, or had finished, flowering. By June, < 5% of B. prionotes were in flower, whereas all B. hookeriana were flowering.

Genetic analysis

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References

Using three pairs of primers, 290 polymorphic AFLP markers were obtained to calculate the hybrid index. The index ranges from 0 to 1, with B. prionotes having a mean of 0.161 (all < 0.2) and B. hookeriana a mean of 0.864 (all > 0.8) (Fig. 2). The putative hybrids had a mean of 0.516 (all 0.3–0.7). No marker was consistently expressed in the hybrids, with 10 plants having an index close to 0.5; seven plants deviated further and could be higher-order crosses or introgressives. BP-29 is noteworthy because it was identified as a pure B. prionotes in the field but its index of 0.49 indicated that it was a hybrid.

image

Figure 2. Histogram of standardized hybrid index for putative hybrids (HY, 17 plants) and individuals from the nearest three populations of B. prionotes (BP, 17 plants) and B. hookeriana (BH, 19 plants), based on 290 AFLP markers.

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Transect study

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References

For both species, individuals in or near the disturbed sites were larger (greater crown volume), produced more flowers, and had more flower heads open or about to open (higher index of flowering) (Fig. 3). All variables were highly correlated with each other (r = ±0.675–0.915 on 178 and 239 d.f.). In April, B. prionotes had 2.6 times the level of flowering (flowers open or soon to open) in and up to 15 m from the disturbance as individuals further from the disturbance (Fig. 4). The only significant flowering by B. hookeriana at this early stage in its seasonal cycle was in the disturbed sites. By June, the only B. prionotes still showing some flowering were in or up to 5 m from the disturbance. Flowering by B. hookeriana was strongly related to distance from disturbance at this time, with 3.4 times the level of flowering up to 10 m from the disturbance as beyond. Assuming constant distances between species, there were 10 times more opportunities (B. hookeriana × B. prionotes both flowering) per plant for interspecific crossing in disturbed than undisturbed sites in April and 54 times in June.

image

Figure 3. Canonical variates analysis of three transects combined for each species. Vector 1: location of plant along transect; 2: loge biovolume; 3: loge total flowers; 4: loge index of flowering.

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image

Figure 4. Index of flowering (number of open flower heads or late bud) per plant of B. hookeriana (BH) and B. prionotes (BP) in relation to distance from edge of disturbance (0) (mean ± SE). (a) April assessment. (b) June assessment. Three transects each with 60–100 plants combined for each species and month of assessment. Each flower head possessed 1500–2000 florets.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References

For natural interspecific hybridization to occur, the parent species must be in physical proximity (dependent on pollinator range), they must flower at the same time, utilize the same pollinators at least occasionally, and have some degree of pollen compatibility (Whelan & Burbidge, 1980; Maguire & Sedgley, 1998). Although they are routinely pollinated by the same bird species, and readily cross when hand-pollinated, we have shown that hybrids are not present even among interspersed plants of B. hookeriana and B. prionotes in undisturbed areas. However, the parent species rarely coflower. Thus, we conclude that the divergence of flowering times is the critical barrier to hybridization between the two species. However, anthropogenic disturbance does two things relevant to promoting hybridization: the plants produce more flowers (earlier work supported here) and their flowering season is extended (shown here for the first time). By promoting earlier seasonal flowering of B. hookeriana plants and prolonging flowering of B. prionotes, disturbed sites break the phenological barrier between them. The net result is that production of gametes increases and the likelihood of gene exchange is enhanced.

Soil disturbance also creates colonization opportunities and, as seeds are wind-dispersed (Lamont et al., 1993), decreases interpopulation distances (we often noted both species spread along roadways and firebreaks). Although the distance between adjacent populations of the two species in the absence of anthropogenic soil disturbance may exceed 500 m, the closest populations to account for the hybrids were still on average 112 m from each other (range of 5–250 m). This gives a clue to the distances that bird pollinators may fly without losing their current pollen load completely.

Both changes in reproductive properties can be attributed to the fact that plants in disturbed sites are larger (Fig. 3) (Canham & Marks, 1985; Laska, 2001). Larger plants produce more flowers (Weiner & Thomas, 1986; Herrera, 1993; Lamont et al., 1994a,b) and plants with more flowers have a longer flowering season (Lamont & Watkins, 1985; Ollerton & Lack, 1998). Why the plants should be larger in disturbed sites has been attributed to (a) greater access to resources through reduced competition and (b) greater levels of resources. Lamont et al. (1994a) showed that soil conductivity and levels of four nutrients, as well as water availability, were higher in sands supporting B. hookeriana beside roadways than in undisturbed vegetation.

Greater flowering in disturbed sites not only increases the chances of crossing simply through higher gamete production, but it is also likely to attract more pollinators (Willson & Price, 1977). In particular, B. prionotes, as one of the few trees in the area, is likely to dominate roadside vegetation, maximizing its floral display relative to other species. For a rare Banksia species, B. tricuspis, 80 km south of our study area, van Leeuwen (1997) showed that large plants in dense stands were preferentially visited by honeyeaters.

Sedgley et al. (1996) were only able to induce B. prionotes × hookeriana hybrids when B. prionotes was the pollen donor. However, we found hybrids among otherwise pure B. prionotes stands in nine cases, with the nearest B. hookeriana stand 30–250 m away. Only four hybrid populations were located among B. hookeriana stands. The most likely explanation is that B. hookeriana usually acts as the pollen donor. If that is so, then early flowering by B. hookeriana, rather than late flowering by B. prionotes, is more critical for hybridization. This requires further investigation.

The AFLP analysis showed that our assignment of individuals to either hybrid or ‘pure’ species status on morphological grounds was correct in 52 of 53 cases. Our return to the location of the B. prionotes exception (an old gravel pit) revealed a B. hookeriana-dominant and an intermediate hybrid present <10 m away, showing that it was, in fact, a B. prionotes-dominant hybrid (Fig. 2). This gives us confidence that our field identification of the 12 putative hybrid populations was correct, and supports our conclusion that hybridization is essentially a disturbance response.

General lack of introgression in morphology and phenology, and the fact that even the hybrid swarm had an intermediate hybrid index, support the impression that the high incidence of hybrids we observed here is a recent phenomenon. Only three populations could be considered hybrid swarms (i.e. beyond the F1 generation), although the oldest populations were ∼30 years old. The major highway, beside which much of our work was conducted, was built in the 1960s. The firebreaks, rail and gas lines, many minor roads and mine pits are post-1970. This highlights the recent occurrence of anthropogenic disturbance in promoting hybridization between these species.

The F1 hybrid is fully fertile (Sedgley et al., 1996; personal observations). At present, distribution of the two species in the dune landscape is under strong edaphic control (Lamont et al., 1989). Combined with increasing opportunities for hybridization, it raises the possibility of gradual loss of genetic integrity of the parent species and selection favouring those individuals with wider habitat tolerance than either parent. This would require some form of hybrid advantage. At 5 years, the hybrids are of intermediate stature, so have no size advantage over B. prionotes but they might over B. hookeriana. Personal observations indicate that the F1 and later hybrids are more fecund than B. prionotes but less than B. hookeriana. As hybrid seeds may be blown at least 80 m from their parents (unpublished), it is possible that they could reach soils of intermediate properties with that occupied by the parents that might enable them to flourish. The net effects of these differences in fitness, and possible changes in environmental tolerances, await investigation.

With escalating anthropogenic disturbances, including climate change, occurring throughout the world, we can expect increasing cases of hybridization to be reported. No doubt some of the causes will be attributable to associated changes in the biology of the parents, as we have demonstrated here. Where the parent species are of restricted distribution, as is the case with B. hookeriana, then increasing opportunities for hybridization, introgression and selection might threaten the conservation of genetic resources. This will occur should any combination prove to have greater competitiveness or environmental tolerances than either parent species (Levin et al., 1996). The creation of disturbed sites should now be seen as having biological, as well as habitat consequences on existing species.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References

We thank the Australian Research Council (Discovery) for financing this project, the Department of Conservation and Land Management for permission to work in its reserves, Robyn Taylor, Allan and Lorraine Tinker, Laurton McGurk, and Meifang Zhao for technical and logistic assistance, and Scott Armbruster and another reviewer for their helpful comments on the manuscript.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study sites
  6. Morphological and phenological analyses
  7. Genetic analysis
  8. Transect study
  9. Results
  10. Occurrence of putative hybrids
  11. Morphological and phenological analyses
  12. Genetic analysis
  13. Transect study
  14. Discussion
  15. Acknowledgments
  16. References
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