Seabird Group Seabird Group

Habitat categorisation and mapping of a seabird reserve: Ilhéu da Praia, Azores

Sarah E. Long1*, Jack J. Devlin2,3, Pedro Raposo4, Ben J. Porter1 and Hannah F. R. Hereward2

1Independent researcher

2School of Biosciences, Cardiff University, Cardiff, Wales, UK;

3University of Kentucky, Lexington, Kentucky, USA;

4Graciosa Natural Park, Graciosa Island, Azores, Portugal.

Full paper

Abstract

The removal of invasive species and mitigation of habitat loss are vital conservation tools for species such as seabirds. We surveyed habitats across the entirety of a small seabird islet reserve, Ilhéu da Praia, in the Azores, two decades on from mammalian eradication and floral restoration. Here, we present a fine-scale map, to a 12 metre resolution, representing the first full habitat survey of this islet. This resource can serve as a baseline for monitoring future habitat changes to this internationally important seabird islet. Following restoration efforts in the 1990s, the islet’s dominant habitats are meadow, grassland dominated by Festuca petraea, or a combination of the two. We also report a novel observation of both Band-rumped Hydrobates castro and Monteiro’s H. monteiroi Storm Petrels breeding in the cavities of Tamarix africana, an invasive tree species.

Introduction

Islands provide a crucial resource for rare and endangered taxa. Islands are particularly important to species such as seabirds, which require undisturbed land for nesting and have historically used islands as refuges, with fewer predators, competitors, and anthropogenic activities compared to mainland sites (Bellingham et al. 2010). In the Anthropocene, however, colonies of seabirds on islands have become increasingly threatened, primarily driven by invasive species, e.g. mammals, particularly rodents (Harper & Bunbury 2015; Brooke et al. 2017; Jones et al. 2019), and habitat loss, e.g. erosion and overgrazing (Monteiro et al. 1996; Cadiou et al. 2009; Gizicki et al. 2018; Dias et al. 2019). Ground- and burrow-nesting seabirds that use such islands are highly vulnerable to invasive species and habitat degradation, which is amplified by their low fecundity (Ratcliffe et al. 2009). Additional anthropogenic threats have also emerged for pelagic species such as seabirds: mainly climate change, overexploitation of fishing resources, entanglement in discarded fishing gear, and plastic ingestion (Bertrand et al. 2012; Gr millet et al. 2016; Savoca et al. 2016; Donnelly-Greenan et al. 2019). Procellariiformes are one of the most threatened groups of seabirds, their decline driven by many of these threats (Dias et al. 2019; Solothurnmann 2019). The removal of invasive species and habitat restoration at crucial breeding sites are valuable conservation tools for seabirds, leading to increased nesting success and enhanced adult survival (Cadiou et al. 2009; Brooke et al. 2017).

The Azores is an archipelago of islands within the mid-Atlantic Ocean. While all nine main islands of the Azores have been deeply transformed by almost six centuries of continued human presence, it is still possible to find many endemic species (> 70 species of vascular plants, > 260 species of arthropods, and two species of birds (Borges et al. 2010; Rodrigues et al. 2010; Silva et al. 2010)). It is the islets (smaller islands) of the Azores, however, which are particularly important, as their remote nature has enabled them to host unique ecosystems, particularly due to the absence of herbivorous grazers, which deplete and erode the vegetation and soil that many species depend on for food or shelter (Boersma et al. 2002). One such example is Ilhéu da Praia (39°3’N 27°57’W, hereafter ‘Praia’), a volcanic islet located 1 km east of Graciosa Island, classified in 2007 as a Biosphere Reserve, due to its importance as a breeding site for seabird species such as the endemic Monteiro’s Storm Petrel Hydrobates monteiroi (UNESCO 2007).

Until the late twentieth century, Praia was used by local farming communities of neighbouring Graciosa as pasture for grazing domestic Goats Capra argagrus hircus. Praia was also used as a recreational area by Graciosa residents, mostly during the summer months. Consequently, the disturbance level was too high for many seabirds to breed, particularly terns. Grazing by mammals, including European Rabbits Oryctolagus cuniculus hereafter ‘Rabbits’, (introduced pre-1830s (Andrade 1831)), contributed to continued habitat loss and soil erosion. By 1995, the vegetation over most of Praia was highly degraded, primarily through overgrazing and trampling by domesticated animals and humans, leading to soil compaction and vegetation suppression (Boersma et al. 2002; Bried et al. 2009). However, due to its proximity to rich marine feeding grounds, absence of mammalian predators, and breeding colonies of several IUCN Red-listed seabirds, Praia is arguably an internationally important site for seabird conservation (Monteiro et al. 1996; BirdLife International 2018). Recognising the existing damage, and ongoing threats to this important seabird nesting community, access was restricted and significant restoration work was carried out on Praia between 1996 and 2004: Rabbits were successfully eradicated, soil erosion mitigation measures implemented, much non-native flora removed, and native flora reintroduced (Figure 1) (Bried et al. 2009). Invasive Hottentot Fig Carpobrotus edulis, which is native to South Africa and forms vast ‘mats’ to the exclusion of all other plants, was removed, along with Wild Sage Lantana camara, which chemically prevents other plants growing adjacent (Taylor et al. 2012; Smyth et al. 2013). Between 2000 and 2003, various native plants were translocated onto Praia. These included: four Azorina vidalii, 450 Festuca petraea, 165 Carex hochstetteriana, 120 Tolpis succulenta, 42 Erica azorica, and 32 Morella faya (Bried et al. 2009). In addition, Graciosa Natural Park frequently controls the non-native tree, Tamarix africana.

Alongside the vegetation restoration work, 50 artificial nest boxes were also installed in 1996 to increase suitable breeding areas for terns along the western coast of Praia, the area most deprived of vegetation (Bried et al. 2009). The terns on Praia are not observed to use these boxes, unlike in the UK (Morrison & Gurney 2004). In 2000– 01, 150 plastic cavity nesting boxes were installed for storm petrels, mostly along the eastern and northern coasts, which have been successfully used (Bolton et al. 2004).

Through these conservation approaches of both habitat restoration and nest box installation, there was a rapid increase in breeding seabird numbers and diversity, which now total eight species. Despite not inhabiting the nest boxes, habitat restoration measures led to an increase in Common Tern Sterna hirundo and Roseate Tern S. dougallii, with populations increasing by 200 and 570 breeding pairs respectively by 2015 (Bried et al. 2009; Neves et al. 2016). In 2013, Bandrumped Storm Petrel H. castro active nests totalled 101, signifying a four-fold increase in breeding attempts from 2000. Similarly, 95 active nests of the endemic Monteiro’s Storm Petrel represented a six-fold increase in breeding attempts over the same period. For both storm petrel species, recorded successful breeding attempts in natural nests were in natural crevices in bare rock, and burrows in the ground (Bolton et al. 2004). Barolo Shearwaters Puffinus baroli now breed on Praia, with over nine attempts in 2013 (Bried & Neves 2015). Cory’s Shearwater Calonectris borealis breeding pairs number between 120 and 150, although breeding success has not been monitored closely (Bried & Neves 2015). Other birds have been recorded breeding in a variety of the reintroduced flora: Roseate Terns breed in the shelter of the larger A. vidalii plants, with chicks observed sheltering under F. petraea (Bried & Neves 2015).

Following the conservation measures described above, the flora of Praia has now had approximately 20 years to establish and recover from anthropogenic pressures (Figure 2).

In this study, we surveyed the entire islet and characterised vegetation complexes in 12 x 12 m resolution cells to create the most complete, accurate, and up-to-date vegetation map of Praia. We discuss the conservation implications of these modifications in floral communities, particularly for the recovering seabird communities present. Finally, we present potential uses for this resource as a conservation tool.

Acknowledgements

We thank Direção Regional do Ambiente, Região Autónoma dos Açores for granting access to Praia Islet (HFRH licence number: SAI-DRA/2019/1821 Proc. 116.14.03/43) and Graciosa Natural Park for helping us access and stay on Praia Islet. Thank you to Verónica Neves for providing contacts and information, and to Joël Bried for kind permission for use of his data. Figure 1 was reprinted by courtesy of University of Wisconsin Press. Figure 2 featured a photograph courtesy of Elizabeth Bell/Wildlife Management International Ltd. Thanks to Robert Thomas and Renata Medeiros-Mirra for their initiation of this paper. This work was conducted in conjunction with HFRH’s PhD who is supervised by: Robert Thomas, Frank Hailer, Renata Medeiros-Mirra, Sean Rands and Verónica Neves, and advised by Mark Bolton and Joël Bried. HFRH is supported by a NERC GW4+ Doctoral Training Partnership studentship from the Natural Environment Research Council [NE/L002434/1] and is thankful for the support and additional funding from CASE partner, Eco-explore Community Interest Company and Cardiff University. Finally, we thank our two anonymous reviewers, whose constructive comments greatly improved the quality of this paper.

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