Control of invasive alien weeds averts imminent plant extinction

Control of invasive alien weeds averts imminent plant extinction Cláudia Baider & F. B. Vincent Florens Biological Invasions ISSN 1387-3547 Volume 13 Number 12 Biol Invasions (2011) 13:2641-2646 DOI 10.1007/s10530-011-9980-3 1 23 Your article is protected by copyright and all rights are held exclusively by Springer Science+Business Media B.V.. This e-offprint is for personal use only and shall not be selfarchived in electronic repositories. If you wish to self-archive your work, please use the accepted author’s version for posting to your own website or your institution’s repository. You may further deposit the accepted author’s version on a funder’s repository at a funder’s request, provided it is not made publicly available until 12 months after publication. 1 23 Author's personal copy Biol Invasions (2011) 13:2641–2646 DOI 10.1007/s10530-011-9980-3 INVASION NOTE Control of invasive alien weeds averts imminent plant extinction Cláudia Baider • F. B. Vincent Florens Received: 19 May 2010 / Accepted: 2 March 2011 / Published online: 13 March 2011 Ó Springer Science+Business Media B.V. 2011 Abstract Invasive alien species constitute a major threat to biodiversity and cases of extinction caused by invasive alien animals are abundant. However, while invasive alien plants also harm native biota there exists a lack of cases demonstrating their ability to cause extinction of native plant species. Different alien species (vertebrates, invertebrates, pathogens etc.) commonly deliver different simultaneous impacts like predation, disease or competition. In such situations, assessing the contribution of plant invasion in causing decline of a given plant population in its natural habitat can be difficult, yet is desirable to avoid or minimize wastage of managers’ resources. Using native angiosperms in lowland wet forests of Mauritius, we first compared native seedling diversity in forest areas that have been weeded of invasive alien plants about a decade previously, with adjacent similar but non-weeded areas. Then, using the weeded area, we compared results of native plant surveys carried out around the time that invasive alien plants were controlled, with the same community about a decade latter. Species richness and abundance of seedlings were higher in the weeded areas compared to the adjacent non C. Baider Mauritius Herbarium, MSIRI, Réduit, Mauritius F. B. V. Florens (&) Department of Biosciences, Faculty of Sciences, University of Mauritius, Réduit, Mauritius e-mail: [email protected] weeded forest. We also found that several species that were presumed extinct or critically threatened with extinction had recovered dramatically as a consequence of the sole removal of invasive alien plants. This shows that the threat posed by invasive alien plants can be overwhelmingly important in driving native plant population declines in tropical forests and that imminent plant extinctions can be averted by timely control of alien plants. Keywords Conservation management
Critically threatened plants
Invasive alien plants
Lowland wet forest
Mauritius
Restoration Introduction Invasive alien species constitute a major threat to biodiversity (Mack et al. 2000; Mooney et al. 2005) and there exist many cases of extinction caused by invasive alien animals (Savidge 1987; Blackburn et al. 2004). However, while invasive alien plants can also harm native biota (Mack et al. 2000; Hejda et al. 2009), there still exists a lack of cases demonstrating their ability to cause extinction of native plant species (Gurevitch and Padilla 2004; Sax and Gaines 2008). Documented plant species declines and extinction have often coincided as much with habitat destruction and fragmentation or other threats, as with the invasion of alien species themselves, stressing the dangers of using simple correlation to imply 123 Author's personal copy 2642 causation (Simberloff 1995; Gurevitch and Padilla 2004). Furthermore, different alien species (vertebrates, invertebrates, pathogens etc.) commonly deliver different simultaneous impacts like predation, disease or competition (Caujapé-Castells et al. 2010). In such situations, assessing the contribution of plant invasion in causing decline of a given plant population and its eventual extinction in its natural habitat can be difficult, yet is desirable to avoid or minimize wastage of managers’ resources (Gurevitch and Padilla 2004; Baider and Florens 2006). One approach to probe the potential of invasive alien plants (IAP) in causing native plant extinction may be to carry out controlled experiments where IAP are removed from the habitats they invade and the subsequent community changes monitored over a period of time which is meaningfully long to detect eventual recovery of the native species impacted by the IAP. Here, we report on such a study, triggered by the observation that two native species previously taken as extinct on Mauritius ‘reappeared’ within areas that have been cleared of dense stands of invasive alien plants. We collected new data to compare with past findings to investigate whether such reappearances are likely to have been triggered by the removal of invasive alien plants. C. Baider, F. B. V. Florens pigs. However, damage done by these large alien mammals within and outside the fenced areas does not differ. This was shown by a comparison of physical damage to the ground flora using the method developed by Clark and Clark (1989) at both locations that were thus studied, and which included our present study site (Florens 2008). This lack of effectiveness of the fencing seems linked to animals meant to be excluded being, despite attempts to maintain the fence, in fact able to enter the CMA through the numerous and continuously appearing gaps either dug underneath the fence by pigs or caused for example by fallen trees or storm streams. Our study site was the forest of Brise Fer centered on the point 20° 220 3500 S and 57° 260 4000 E within the Black River Gorges National Park. The site harbors one of the earliest CMAs created in a well preserved patch of forest over 1.2 ha in 1987 and later expanded to 19.3 ha mainly in 1996 when adjacent poorer quality forest patches were at some places incidentally included (Fig. 1). Thorough qualitative and quantitative botanical surveys at the site before and shortly after the main weeding of 1996 yielded over 160 species of native angiosperms—the richest site for the Mascarene region (Lorence and Sussman 1986; Strahm 1993; Page and D’Argent 1997; Florens et al. 1998a). Context and study site Materials and methods Mauritius is a 1865 km2 tropical oceanic island in one of the world’s biodiversity hotspots (Myers et al. 2000). The island also harbors exceptionally high levels of endemism like most other such insular ecosystems (Kreft et al. 2008) and has one of the most threatened floras in the world (Walter and Gillet 1998). In attempts to conserve local biodiversity, active management has been ongoing for over three decades targeting mainly certain bird species but also sometimes their habitats in particular through the control of alien plants. This is done in selected ‘Conservation Management Areas’ (CMAs) (Cheke and Hume 2008). CMAs, (totaling 70 ha or \1% of remaining mainland native habitats of Mauritius) are typically located within well preserved remnants of native vegetation where native plant diversity is greatest. They range in size from 0.3 to 19.3 hectares, are typically regularly weeded of invasive alien plants and are fenced against alien deer and feral 123 For the comparison through time, we thoroughly resurveyed the CMA for native angiosperm species about 10 years after the main weeding of 1996 using similar techniques as previous workers, namely through a combination of random walks through the area alongside a more quantitative sampling of the seedlings (defined here as 50–130 cm in height) and larger woody plants (C1.3 m tall) in a series of plots (30 plots of 5 9 5 m for seedlings and 250 plots of 10 9 10 m for woody plants). To compare these results with the non-weeded area, we selected a forest zone immediately adjacent to the CMA and which had, except for alien plant presence, a very similar forest structure (e.g. 1,448 native trees C10 cm dbh per ha for the non-weeded forest against 1,330 similar sized stems per ha for the weeded forest). We surveyed its plant community in a similar manner, that is through random walks over an area of about Author's personal copy Control of invasive alien weeds averts imminent plant extinction A B C Fig. 1 a: Location of Mauritius within the south West Indian Ocean. b: Mauritius, showing the Black River Gorges National Park (shaded) and Brise Fer (black dot). c: Study site of Brise Fer showing the weeded area (shaded) and the fence to exclude feral deer and pigs (solid lines); ‘Well preserved plot’ represents the first 1.2 ha Conservation Management Area (CMA) created in 1986 and which is set within a larger fenced off area representing the limits of the extension of 1996; dashed lines represent vehicle track; dotted lines represent the vegetation survey plots (1 ha for the non-weeded and 2.5 ha for the weeded forest) where quadrats to sample seedlings and larger woody plants were placed 20 ha alongside 30 square plots of 25 m2 each to survey seedlings, and 100 plots of 10 9 10 m for the larger plants (C1.3 m tall). Results and discussion Total native angiosperm flora Excluding species misidentified by previous workers on the basis of comparisons of our samples to their vouchers deposited at the Mauritius Herbarium (e.g. Florens et al. 2008), we found all but one species that had previously been recorded from the Brise Fer region (that is both inside the CMA and in a roughly equivalent area outside it). We also recorded an additional 34 species of native angiosperms within the CMA compared to what was known previously in the same area at around the time the main IAP control 2643 was carried out in 1996. Several of these same newly recorded species were also found outside the CMA, in addition to two new records that were exclusively from there, both of which consisting of large and old adults that must have been missed by previous surveys. Some of the new records made inside the CMA were again represented by at least one large and old individual that must have been already present before the 1996 weeding and must therefore be taken as species genuinely missed by previous workers. However, most (63%) of these new records are of species either known to date only as juveniles or as young adults that, given their size and growth rate, must have germinated post-weeding indicating that adult plants are either so rare as to have escaped all surveys or grow elsewhere and their seeds dispersed to the site where they were recorded to germinate and grow for the first time (Table 1). The newly recorded species also include a few currently non-threatened pioneer or post-pioneer native species of lesser conservation concern like Harungana madagascariensis and Bremeria landia which now occur in relative abundance as individuals belonging mainly to a single cohort and growing in those areas where the forest had previously been sufficiently degraded for substantial canopy gaps to have been opened when the alien plants were weeded. Understorey and herb species Of all the new records made at the site, two were presumed extinct: the understorey endemic tree Ixora vaughanii and the native ground orchid Nervilia bicarinata (Strahm 1993; Page and D’Argent 1997; Roberts 2001) and both species are now vigorously regenerating in the CMA (Table 1). The Critically Threatened species (sensu IUCN 2001) that recovered most dramatically are all understory or ground flora plants. Alien plant invasion in wet lowland forests of Mauritius is virtually confined to the understory and comprise mainly the Strawberry guava (Psidium cattleianum Sabine) ([95% of all alien woody stems) which grows to about eight m in height in a 15–18 m tall forest. About 4,000 individuals with diameter at breast height (dbh) C1 cm were recorded in 15 random plots of 100 m2 of invaded forest or 26,607 per ha (Florens 2008). While native trees overtopping the weed are recorded to have reduced fitness (Baider and Florens 2006), it appears 123 Author's personal copy 2644 C. Baider, F. B. V. Florens Table 1 Estimated or known (marked *) number of adults and juveniles of selected native species and their respective families recorded in the 19.3 ha of weeded forest of the Brise Family Fer Conservation Management Area by the time of the alien plant weeding of 1996 and about a decade later Species Records by 1996a Records in 2006–2008 Adult Juvenile Adultb Juvenile A Hitherto presumed extinct Rubiaceae Ixora vaughaniic (Verdc.) Mouly & B. Bremer 0 0 4(1) 170 Orchidaceae Nervilia bicarinatad (Blume) Schltr. 0 0 5(?)* 160* 50 B Critically endangered understory trees Rubiaceae Chassalia grandifolia DC. 1* 0 5(1)* Rubiaceae Chassalia lanceolata (Poiret) A. Chev. \50 0 85(40) 560 Rubiaceae Rubiaceae Gaertnera hirtiflora Verdc. Gaertnera pendula Bojer 2* 1* 0 0 14(10)* 3(3)* 318* 12* Canarium paniculatum (Lam.) Benth. ex Engl. \300 0 \300 100 C Endangered trees Burseraceae Ebenaceae Diospyros nodosa Poiret \50 0 \50 40 Elaeocarpaceae Elaeocarpus integrifolius Lam. \160 0 \160 117* Euphorbiaceae Macaranga mauritiana Bojer ex Muell. Arg. \20 0 \20 10* Sapotaceae Sideroxylon grandiflorum DC. \150 0 \150 81* Monimiaceae Tambourissa sieberi (Tul.) A. DC. \5 0 \5 60 Rutaceae Zanthoxylum heterophyllum (Lam.) Smith 3* 0 3* 1* D Woody species appearing post weeding a Rubiaceae Bremeria landia (Poir.) Razafim. & Alejandro 0 0 770 1570 Sapindaceae Dodonaea viscosa (L.) Jacq. 0 0 0 3* Erythroxylaceae Erythroxylum hypericifolium Lam. 0 0 0 50 Moraceae Ficus rubra Vahl. 0 0 0 70 Clusiaceae Phyllantaceae Harungana madagascariensis Lam. ex Poiret Phyllanthus casticum Willemet f. 0 0 0 0 1440 1* 615 4* Pittosporaceae Pittosporum ferrugineum Aiton f. 0 0 0 16 Araliaceae Polyscias dichroostachya (Bojer) Baker 0 0 0 7* Araliaceae Polyscias maraisiana (Marais) Lowry & Plunkett 0 0 0 1* Araliaceae Polyscias sp. 0 0 0 2* Asteraceae Psiadia penninervia DC. 0 0 1* 20* Asteraceae Psiadia viscosa (Lam.) A. J. Scott 0 0 0 1* Apocynaceae Secamone dilapidans F. Friedmann 0 0 0 1* Urticaceae Urera acuminata (Poiret) Decne Br. 0 0 Present Present In conservation management area weeded the same year b Numbers within brackets represent aged adults whose presence is estimated to predate the weeding c Last record 1927 d Last record in Mauritius 1769 that the invasion is mainly detrimental to other native understory species which enter in fiercer competition with the weeds. Besides, understory species tend to be shorter lived than canopy species, a factor that, relative to longer lived species, is expected to result 123 in a faster conclusion of the outcomes of competitive interaction, including exclusion. Indeed, during a survey of native plants in Macchabé, one of the best preserved remnants of Mauritian forest in the 1930s, Vaughan and Wiehe (1941) noted that Chassalia Author's personal copy Control of invasive alien weeds averts imminent plant extinction capitata (an endemic small tree of 2–4 m in height) dominated the understory with 284 individuals in 0.1 ha—or 2,840 per ha—(with mean dbh of 3.5 cm). Seventy years later, despite intensive surveys (Strahm 1993; Page and D’Argent 1997; Florens 2008), C. capitata seems extirpated from that forest, whose understory is now highly invaded by Strawberry guava (6,229 stems of dbh C1 cm recorded in 15 random plots of 100 m2—or 41,527 stems per ha— (Florens 2008). C. capitata is today considered Critically Threatened with less than 250 adults estimated to survive over the island. Newly regenerating trees In addition to the recovery of understory plants, seven larger tree species already known at the site but of which no juveniles or young adults were recorded before the weeding, are now known in the CMA by up to over a hundred seedlings and saplings that sprouted after weed removal (Table 1). In the adjacent but non-weeded areas, despite thorough surveys (estimated at least at 20 person days for 20 ha) seedlings and saplings of these same species are either not known to occur or are exceeding rare despite the presence of roughly equivalent and in some cases even higher densities of their adults as compared to within the CMA. Native seedling plots Surveys of native seedlings showed that the CMA had much higher mean species richness and density than the adjacent non-weeded area of comparable quality with 11.1 (SD 3.5) species at a density of 17.0 (SD 9.3) seedlings per 25 m2 plot against 2.0 (SD 1.5) species at a density of 2.9 (SD 1.7) individuals per 25 m2 plot respectively. Conclusion If alien plant invasion, instead of the many possible other threats like habitat fragmentation, alien animal impacts, or lost mutualisms had caused precipitous population declines and extirpations like that of C. capitata, then removal of the threat by weeding the alien plants should elicit dramatic population recovery (D’Antonio et al. 2001). Our 2645 data documented such recovery for two species hitherto presumed extinct and several others considered Critically Threatened or Endangered (sensu IUCN 2001). In addition, in situ germination of several species, including the Tambalacoque, sometimes misnamed ‘Dodo-tree’, (Sideroxylon grandiflorum) was observed for the first time ever and exclusively in weeded areas (Baider and Florens 2006). The fact that many species, including threatened ones, can recover dramatically as a consequence of the sole removal of invasive alien plants, shows that the threat these pose can be overwhelmingly important in driving native plant population declines. Our findings also indicate that imminent plant extinctions can be averted by little more than timely control of the invading plants. Given the severity of alien plant invasion in Mauritius (Cheke and Hume 2008), the island can not only be seen as a relevant model for a whole swath of other island nations and territories around the world particularly in the Pacific and Indian Oceans (e.g. Cronk and Fuller 1995; Meyer and Florence 1996), but it can also be regarded as reflecting what awaits much of the world given the worsening problem of alien invasion worldwide (Mooney and Hobbs 2000). Finally, it is also encouraging that the native butterfly community recovers well after alien plant weeding (Florens et al. 2010) and that a presumed extinct endemic animal has also been relocated within CMAs underlying the wider benefits of such restoration efforts (Florens and Baider 2007) although some apparently detrimental effects of alien weed removal on this animal group had also been documented (Florens et al. 1998b). We hope that the conservation success presented here will also help managers who, in countries like Mauritius, still require encouragement to shift emphasis away from intensive species-centric conservation to the generally more all-encompassing and financially sustainable ecosystem approach to stem population declines and extinction. Acknowledgments We thank the National Parks and Conservation Services for granting permission to conduct research in the Black River Gorges National Park and two anonymous reviewers for improving the manuscript. Part of the surveys was enabled with funding from the British Ecological Society through its Overseas Bursary Scheme of 2003 to FBVF. 123 Author's personal copy 2646 References Baider C, Florens FBV (2006) Current decline of the ‘Dodotree’: a case of broken-down interactions with extinct species or the result of new interactions with alien invaders? In: Laurance WF, Peres CA (eds) Emerging threats to tropical forests. 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