The evidence-based conservation
Whichever position is taken regarding big or small debates, conservation (like any other discipline) needs evidence whenever possible to prove if a management action gave the expected outcome or not (Sutherland et al. 2004a; Sutherland 2015).
Such evidence can be examples of good or bad experiences from nature reserve/species management actions, which can inform other conservation managers’ actions (Sutherland 2015).
However, to produce conservation evidence, an important amount of baseline information is required on population size, habitat selection and use, demography, species distribution and movements.
First, the knowledge of the size or density of a population is often an essential prerequisite to manage it effectively (Fryxell et al. 2014), therefore it is required to investigate whether a population is too small, too big, increasing or decreasing in order to decide if any action is required. For some cases a population index may be enough if the main aim is just to have a trend. In fact, indices provide measures of relative density and are used only for comparison purposes. More detailed information than indices may be required if information on the species is lacking, the species is suspected to be declining or if it counts a small number of individuals or populations (i. e. endemic species) or simply because it is important to know its distribution within a certain area for management purposes. Management decisions often require information on density, and/or trends in density, and many factors can drive the decision on the methods to undertake to obtain these information (Bibby et al. 2012).
Second, it has been recognized that habitat structure is fundamental in influencing its use by animals (MacArthur & MacArthur 1961). Therefore, habitat assessment is an important part of wildlife ecology. In fact, good understanding of habitat selection allows appropriate management decisions regarding different forms of land use and can improve the success in case reintroduction actions are required (Fryxell et al. 2014). Habitat use information is important during all the different stages of a species life cycle, during breeding and non-breeding periods and for both foraging and breeding sites.
Third, describing and predicting stochastic population dynamics in time and space is fundamental to ecology and conservation biology (Lande et al. 2003). Population dynamic can be influenced by different demographic factors including social structure, life history variation caused by environmental fluctuations, dispersal in spatially heterogeneous environments and local extinction and colonisation (Lande 1988). The assessment of populations’ demographic parameters is therefore very important to understand what happens within a population and identify the key stages that demography is most sensitive to, in order to guide direct management (Sutherland et al. 2004a; Bibby et al. 2012; Fryxell et al. 2014).
Finally, animal movement is directly dependent on environmental conditions (such as climate, resources, presence of partners/predators/competitors) but it is also the result of complex evolutionary mechanisms driving physiological and behavioural responses (Nathan et al. 2008). Given that, the analysis of animal movement can be used as a tool to identify such conditions and to assess the capacity of animals to respond to rapid changes in ecosystems (Urbano & Cagnacci 2014). With the use of telemetry to track animal movement, it is now possible to assess the environmental conditions favourable for a species during different stages of its life cycle and to identify/predict suitable areas. Knowledge of suitable habitat has become an important tool in conservation for both management (Gurnell 2002) and identification of threats (i. e. Arcos et al. 2012).
The nature reserve context
As underlined above, nature reserves remain a cornerstone of global conservation efforts (Lopoukhine et al. 2012). Over 100 000 protected areas have been established worldwide, covering over 12% of the Earth’s land surface (based on the World Database on Protected Areas: : https://www.iucn.org/about/work/programmes/gpap_home/ gpap_biodiversity/gpap_wdpa/) and representing one of the most significant human resource use allocations on the planet. The importance of protected areas is reflected in their widely accepted role as an indicator for global targets and environmental assessments (Chape et al. 2005).
However, establishing and properly managing nature reserves is not the first priority for most of the world countries, especially the developing ones. Therefore, even if a protected area is hardly instituted, it does not mean it will be well managed or it will keep its biological importance (Chape et al. 2005). The amount of funding available to a nature reserve is fundamental for its management and subsistence. A nature reserve can be managed by different entities: non-governmental organizations (NGOs), private owners or governmental bodies; in any case the managing board and legislation have the responsibility of making decisions on which actions are allowed to be undertaken within its boundaries. A nature reserve can be integral (no human activities are allowed within its borders) or, in most of the cases, a different degrees of human actions are allowed within its borders (IUCN). Integral nature reserves offer to biodiversity a refuge from disturbed areas and are often occupied by many species (Kingsland 2002).
As discussed above, management decisions are not easy to undertake especially if a nature reserve hosts many species with different levels of conservation concerns. In the “Antropocene” the little space left for species survival has reduced with years and human development (see above) and it is highly contended among species (Brussard and Tull 2007).
Organisms within a community interact continuously with each other (Begon et al. 1999) and they need food and shelter to improve their fitness (Krebs & Davies 2009). Therefore, the ensemble of direct and indirect competition, predation or mutualism interactions, can deeply modify and shape the whole community, driving evolutionary changes (Pianka 2011). These processes can be amplified/modified by the fact that species live in limited places. In fact the process of habitat loss and fragmentation brought by human activities did inevitably increase the inter- and intra-specific competition for both resources and breeding ground (Coppack & Pulido 2004).
In particular, interspecific competition mainly occurs when individuals of different species utilize common and limited resources (Fryxell et al 2014). Competition for limited resources can affect population size and distribution and the type of limiting resources also depends on species’ life history and ecology niches (Dhondt 2012). This is particularly the case on islands, where in the absence of predators, ecosystems and communities are bottom-up regulated (Polis & Strong 1996). There are at least two groups of limiting resources: space and food. Competition, to occur, must have some effect on the fitness of both parties (Fryxell et al. 2014) but competitive effects can be unequally distributed among competitors. This is the case of asymmetric competition, when a species outcompetes the other (Begon et al. 1999).
The ecological niche concept in a competition framework
The modern concept of ecological niche was proposed by George Evelyn Hutchinson (1957) and it is defined as a quantitative n-dimensions hypervolume constructed by the range of environmental features that enable a species to maintain a viable population indefinitely (Blonder et al. 2014). Independent axes, that have a biological meaning for the species, characterize the dimensions of the niche (Maire et al. 2012). If we assume that interspecific competition does truly occur among co-existing species, the ecological niche can be divided into two categories: the fundamental niche is defined in absence of competition while the realized niche is characterized by the presence of competition among species (Hutchinson 1957, Maire et al. 2012).
Based on the principle of competitive exclusion, niche theory implicitly assumes that, in order to have a stable co-existence, niches of co-occurring species must differ (Hutchinson 1957; Chesson 1991) although a certain degree of similarity is permissible (May & Mac Arthur 1972; Pianka 1974). By measuring competitors’ niche overlap it is possible to assess the effects of density-dependent competition on the tolerable upper limit of niche overlap (Young 2004). Moreover, the conceptual niche framework allows testing if competition between sympatric species is happening with the use of species assemblages modeling.
The ecological niche concept is strongly related to habitat selection (Maire et al. 2012). The latter is considered as a density dependent process. In fact, when populations are at low-density levels, individuals can freely occupy the habitat that maximizes their fitness. At the opposite, when population density levels increase, the individual fitness decreases within the most favourable habitat, making adjacent and less favourable habitats providing the same fitness. If habitat suitability can vary in function of population densities (Morris 1988), then habitat selection depends not only on resources abundance but also on the density of the same and/or different species sharing the same area. In the latter case, which is the most commonly represented, the organization of the community can be based on shared or distinct preferences (Morris 1988). In the case of shared preferences, competition for resources may occur. It is therefore possible to use habitat distribution patterns to evaluate the role of interspecific competition.
The necessary link between ecosystem knowledge and conservation
We have seen above how the loss of biodiversity is becoming an urgent issue that needs to be addressed by the conservation and scientific world before it becomes irreversible. Effective conservation measures are therefore required to guarantee the persistence of ecosystems that might provide invaluable services for human well-being (Braat & de Groot 2012), in most cases still undiscovered (Wilson 1992).
However, successful conservation management cannot be implemented without well-developed knowledge on species biology and processes occurring among species sharing the same habitat and, eventually, the same preferences.
The discipline of conservation biology has already contributed to mitigating anthropogenic actions on biodiversity at different organization levels and it has helped to reveal underlying mechanisms inducing variation in populations’ demographic parameters (Primack & Miller-Rushing 2012). Thanks to this information, policy makers were able to act, most of the time under high urgency, setting up appropriate conservation programs to save endangered species from extinction.
Most of the conservation measurements put into place to preserve particular species deal only with the management of physical or habitat features without accounting for species interactions, which, most of the time, are unknown before the management action has been undertaken (Soulé et al. 2005). This kind of procedure is a risk for the ecosystem as ecological cascades may bring a better or a worst result than the expected one. For example, the eradication of invasive rats on North Island (Seychelles) in 2005 led to an unexpected decrease in the number of invertebrates, both on ground and leaves, despite the fact that rats were known to be feeding on invertebrates. This unexpected result was probably due to the trophic cascading effects of the removal of rats, which triggered a significant increase in land birds and lizards, and also in large invertebrates, all of which are feeding on small invertebrates and limited by rats (Galman 2011; Rocamora & Henriette, in press).
Therefore, when conservationists fail to understand the interactions occurring between species, conservation measurements might not achieve the desired results (Soulé et al. 2005). A solution to improve biodiversity maintenance can therefore be found in both practice and theory. Conservation evidence provide practical examples of what works in conservation (Sutherland et al. 2004a; Sutherland 2015) i.e. which actions have been already undertaken with successful results (see above). However, empirical studies on wild communities are strongly required to improve knowledge on mechanisms driving species coexistence (Morris 2003) and to track and predict changes on populations and communities when humans alter ecosystems properties.
In summary, theory and conservation should be closely related to effectively protect and manage important ecosystems and endangered or declining populations.
The information required for conservation and management can be very difficult to obtain depending on the category of wildlife we want to study and on how complex is the surrounding habitat and ecosystem.
According to the Birdlife International/IUCN Red List assessment, the status of the world’s birds has deteriorated over the past 20 years (BirdLife International, 2013). These changes were recorded in all major ecosystems but seabirds were found to be more threatened than other groups and declining the fastest (Butchart et al. 2004); therefore enough to be considered the most threatened group of birds (BirdLife International 2013; Croxall et al. 2012). In addition, seabirds are the most threatened marine taxonomic group in the world, with ~28% of species currently listed as threatened (IUCN 2012), plus others that are considered of special concern (Sydeman et al. 2012). The population decline of many seabirds needs attention within the world of conservation studies, in particular for the important role this group occupies in the marine ecosystem (Croxall et al. 2012). In fact, as top predators, seabirds are a valuable indicator of the marine ecosystem (Frederiksen et al. 2006; Furness and Camphuysen 1997; Zador et al. 2013). They can be used to evaluate the impact of climate change (Barbraud and Weimerskirch 2001; Barbraud et al. 2008) and fisheries (Catry et al. 2009a; Einoder 2009; Le Corre et al. 2012) and as indicators of prey stock (Le Corre and Jaquemet 2005; Lyday et al. 2015; Montevecchi, 1993; Piatt et al. 2007). It has been seen that seabirds’ declines are often closely related to the worsening of the ecological conditions in marine ecosystems (Becker & Beissinger 2006; Bond & Lavers 2014). More knowledge in this field will allow to raise the understanding of basic processes linking seabirds with their environment, and to identify major threats and consequent actions oriented towards seabirds and marine ecosystem conservation.
As many seabird species home ranges are distributed widely across the world’s oceans, seabird conservation issues need to be addressed globally (BirdLife International 2015). Within the seabirds, there is a category that requires particular attention, which is the order of the Procellariiformes (del Hoyo et al. 1992). Actually, 45% of species belonging to this order is threatened and information on population estimates, trends, movements at sea and population dynamic is lacking for many species. Moreover, Procellariiformes have to face a high number of threats both in land and at sea (Cooper & Baker 2008).
To improve the conservation efforts towards this category, the agreement on the conservation of albatrosses and petrels (ACAP) was constituted in 2004 (www.acap.aq). Since then, many actions have been undertaken, in particular towards the most threatened southern albatrosses, tackling especially birds by-catch in long-line fishery and introduced predators at colony levels (see for example Wanless & Maree 2014 and ACAP). If many procellariiformes species, like albatrosses and southern petrels, have been deeply studied over the last decades, others have been neglected. Information is mostly lacking for the burrow-nesting petrels (Cooper et al. 2006).
It is indeed very difficult to study burrow-nesting petrels because of their behaviour both in the colony and at sea (Warham 1996; Scott et al. 2009). The breeding sites are often inaccessible (remote islands, steep slopes, mountain tops) and in the case of nocturnal species, fieldwork must preferentially be conducted at night when most of the birds are present. As they nest in deep burrows, occupancy cannot always be assessed (i. e. it is difficult to conduct census work) and capturing individuals can also be a difficult task to achieve (i.e. difficulties in deploying and retrieving devices) (Buxton et al. 2015). As for the other categories of seabirds, it is difficult to identify optimal foraging areas as they can depend on different environmental features that can vary in time, space and among species (Sekercioglu 2006). Luckily, with modern technology and with telemetry development, it is now relatively easy to track seabirds and identify the most exploited areas at sea (Le Corre et al. 2012). However, it is more difficult to determine if such areas are stable in time or if they move under the influence of preys-predators interactions and shifts (Corre 2001; Barrett et al. 2012)(i. e. pelagic fish predators-prey interactions) and changes in the marine ecosystems (Durant et al. 2009; Sydeman et al. 2012; Bond & Lavers 2014). Identifying seabirds’ important areas at sea is therefore an essential (Lascelles et al. 2012) but challenging task as suitable areas can change in space and time.
Birds of the tropics
Even if it is hard work, knowledge on seabirds and on nest-burrowing petrels has exponentially grown over the last decades (see a review in Rayner et al. 2007). This was mainly due to: the availability of long term datasets, the development of statistical methods and tools for spatial and population analysis, the development of telemetry and related analysing tools, and the availability of more accurate remote sensing data (Wilson et al. 2002; Wakefield et al. 2009). Despite the increment and improvement on seabirds and marine ecosystem research over the past 30 years, most of these studies are focused on high latitude species and ecosystems dominated by high productivity with cyclic and abundant resources. In fact, even if increasing, the number of studies and publications on tropical areas are still insufficient when compared to quantity of research on temperate and polar regions. In tropical areas, resources at sea are more constant through the seasons but scarcer, less predictable and patchier than in temperate and polar waters (Weimerskirch 2007). Moreover, most of the tropical seabird species are associated and rely on schools of marine fish predators like tuna to make little fish more accessible on the surface (Le Corre and Jaquement 2005, Thiers et al. 2014). These factors can make uneasy the prediction and identification of important foraging areas for tropical seabirds. A special effort is therefore still required to increase the knowledge of seabirds breeding and foraging at tropical latitudes, and to understand the processes taking place between them and the environment they live in.
General questions and objectives for conservation
We’ve seen above how conservation science should adjust the methods to deal with biodiversity loss taking into account a more pressing and expanding anthropic development (Reyers 2004). Preserving nature and endangered or declining species is indeed becoming a process that cannot only be focused on nature reserves but should act at different levels of space (inside and outside nature reserves). It should also involve local communities and provide proper education to the new generations (Berkes 2007). However, as it often happens, financial resources are limited and in many cases the sole action of running and managing nature reserves is financially challenging. In fact, especially in developing countries, funding and local support to the preservation of species are often lacking, therefore the actions to undertake in support of biodiversity have to be well pondered and ranked in priority and feasibility. In such an environment, where priorities need to be taken into account, we first have to better understand the populations’ status and the associated level of concern. The priority should be given to endemic endangered species and to key ecosystem species for which information is lacking (Wilson et al. 2006). In many countries, the most valuable biodiversity is confined in nature reserves, often considered “island refuges” for many species (Kingsland 2002). Therefore, nature reserves offer a unique opportunity to better understand what is happening to species at a more general scale, although they can be considered a particular case study (Pullin 2010). As we explained above, nature reserves often host an incredible variety of different species that can survive thanks to the “facilities” that a protected area offers (i. e. control/absence of invasive predators and limited anthropic disturbance). These species often co-exist with unnaturally high densities caused by the lack of other suitable habitats outside the reserves. This can provoke an increment of interaction among species and high levels of competition among species for which ecological niches strongly overlap. Knowledge on species distribution and abundance, trends, habitat selection, movement, behaviour and foraging ecology is therefore essential to understand what is happening at community level and which are the mechanisms driving the community structure (Weiher et al. 2011). Once the mechanisms occurring among species and individuals within a community are clearer, actions can be informed and undertaken towards the species that need more help. For example, species that are declining or endangered and which are at the same time considered key species for a whole ecosystem. Action in nature reserves (and if possible outside) should be undertaken to avoid the loss of biodiversity. Acting in nature reserves is the easiest (and most of the time the main or even the only) path to undertake strict protection measures for species that depend on delicate ecosystem balances well beyond the boundaries of the reserves. If more resources become available, it then becomes possible to undertake further actions also outside nature reserves.
Conducting research, monitoring and conservation work in protected areas has also allowed scientists to acquire long term data that can be used to analyse the temporal development of communities and to establish how they react to the modification of resources. Long term monitoring protocols should therefore be implemented to better understand how populations change in a fast changing world.
Thesis outline and objectives
In small nature reserves, the conservation debate is often associated to practical issues, in particular the debate on whether or not to take action. Some conservationists defend the idea that once an island returns to its natural vegetation state after anthropogenic disturbance, nature should then follow its course and nothing should be managed. Others believe that what was once “natural” is long gone since humans started to interfere with original ecosystems, and also that nature reserves are now the only refuges left for many species. Therefore, if actions are required to help species, and in particular those that have the more unfavourable conservation status, they should be undertaken (Wynne 1998; Green et al. 2005). In little islands, the choice is even more difficult as often the species to preserve are many and crowded on a small patch of land. This is especially the case where management actions have to be well informed, as the consequences will involve many species. The outcome of interspecific interactions may change due to habitat modification; competition can intensify due to resource shortage and the management of a species is then directly related to the increment or to the decline of another (White 1978).
Considering that conservation management choices are difficult to make, the understanding of basic mechanisms acting between species and habitat, and among species, is fundamental to inform conservation management. It is important to know what is happening in a population to decide on which best management actions need to be undertaken and with which order of priority (Sutherland 1998; Sutherland et al. 2004b; Fryxell et al. 2014).
With this research we analysed almost the full life cycle of two shearwater species breeding in a small nature reserve island in the Seychelles archipelago. We focus in particular on determining the ecological niche they occupy, both in land and at sea, and on possible competitive interactions that might occur between them. The use of a 2-competing species approach is important to highlight conservation trade-offs. Moreover, the fact that we approach it in almost the full life cycle, make our study particularly novel in the conservation framework.
To achieve the aforementioned general objective in CHAPTER 2 and 3 we investigate habitat selection and assess abundance, distribution and trends of two species of shearwaters breeding in Seychelles. At the same time, given the difficulty of census taking for nest-burrowing seabirds, we propose methods that could be also applied to other species. Furthermore, we investigate their life at sea during both breeding and interbreeding periods in order to identify the marine features selected during the birds’ dispersion (CHAPTER 4 and 5). Based on selected habitat and oceanographic features, habitat suitability maps were created per each breeding period in order to identify the areas in the Indian Ocean which are more important for the two species (CHAPTER 4). In APPENDIX I we include the data on the breeding success and nest occupancy collected during the research.
In this study, nearly all aspects of the life cycle are investigated except the stage from fledging to breeding adult (Figure 1.1). The CHAPTERS 2 and 3 describe the life in-land while the CHAPTERS 4 and 5 describe their life at sea. Given all this information, possible competition between the two species can be assessed and management actions towards their conservation informed.
Our study was carried out in the Seychelles archipelago, which is located between c.04°S to 10°S and 46°E to 54°E, in the western Indian Ocean. In 1968 these islands were already considered as a sanctuary for biodiversity and an important place to preserve when, in the first issue of the journal “Biological conservation” Stoddard and Polunin underlined the importance of the archipelago for both birds and vegetation (Polunin 1968; Stoddart 1968). These islands do not only host rarities and endemic species, but also some of the largest seabird colonies of the tropical Indian Ocean (Stoddart 1984; Skerrett et al. 2001). They are part of one of the most important hotspots for biodiversity in the world (Madagascar & Indian Ocean Islands), to which both attention and priority in terms of conservation should be given (Myers et al. 2000; Critical Ecosystem Partnership Fund 2014).
The archipelago has a total landmass of 455 km² spread across an Exclusive Economic Zone of about 1,374,000 km² (Rocamora & Skerrett 2001). It consists of 155 islands divided into two groups: the granitic islands, which form the inner islands (together with the two northern coralline islands of Denis and Bird) and 4 groups of coralline islands, called outer islands. The inner islands lie on the submerged Seychelles Bank and are the world’s only oceanic islands of continental rock. They have been isolated from any other land mass for 65 million years, before mammals evolved; hence there are no naturally occurring non-flying mammals (Skerrett et al. 2001). Such isolation has led to a high number of endemic species of both fauna and flora.
The 4 groups of outer islands are located west and southwest of Seychelles. They count a total of 115 main islands excluding the smallest islets and they host many numerous colonies of seabirds; the endemism (birds, plants, reptiles, invertebrates) are restricted to the southernmost of the 4 archipelagos, the Aldabra group (Rocamora & Skerrett 2001).
The climate of Seychelles is tropical and it is influenced by two seasonal wind systems: monsoonal wind shifts and the South Indian Ocean subtropical anticyclone. The South-east monsoon is relatively dry and blows from May until October. It is characterized by strong and unidirectional wind coming from the Southeast. The temperatures average 25°–30°C at sea level and humidity is around 80%. The Northwest monsoon blows from October to April. The wind is more changing than unidirectional and the temperature is slightly higher (up to 35°C) as are the humidity and the rainfall.
Rainfall is higher in the high granitic islands than in the flat coralline ones, due to the influence of relief. The annual mean rainfall in the inner islands is approximately of 2,400 mm; while in the outer islands it varies from 1,000 mm to 1,500 mm (Rocamora & Skerrett 2001).
The Republic of Seychelles counts about 89,000 inhabitants and its population is incrementing (World Bank 2013, www.worldbank.org). Nearly the entire population of Seychelles (99.7%) lives in the granitic group, with 90% located on the main island of Mahé (15,500 ha) and most of the rest on Praslin (2,756 ha) and La Digue (1,101 ha).
The first human settlement was in 1770, when the French colonized the islands. The islands then came under British rule in 1815 and the independence of Seychelles was obtained in 1976.
Colonization of the islands by humans resulted in a huge ecological trauma to the fragile ecosystems. The human colonization brought intense habitat destruction (e.g. forest clearance, wetland drainage), the introduction of alien, invasive plants and predators (e.g. rats, cats) and unsustainable exploitation of the fauna. Much of the original vegetation was cleared throughout Seychelles for timber production or agriculture, particularly coconut plantations (for copra) and cinnamon exploitation in the inner islands. These industries used to be the major source of income together with agriculture and traditional fishing until the 1970s. They have been replaced recently by tourism since the opening of Seychelles International Airport on Mahé in 1972, while the industrial fishing industry (mainly tuna fishing and canning industry) became the second largest source of income (Critical Ecosystem Partnership Fund 2014). For example, in 2010, c. 60 metric tons of tuna were caught in the Seychelles Economic Exclusive Zone (Seychelles Fishing Authority, 2014) representing 1.4 % of the world tuna catch based on the FAO reports (http://www.fao.org/fishery/statistics/tuna-catches/query/en). In addition, offshore waters are now dedicated to oil exploration as indicated in the 2013 model petroleum agreement (www.petroseychelles.com).
Table of contents :
1.1 GENERAL INTRODUCTION
1.2 GENERAL METHODS
CHAPTER 2 ANALYSIS OF PLAYBACK CENSUS TO ESTIMATE THE DENSITY OF CAVITY DWELLING BIRDS
2.2 MATERIALS AND METHODS
CHAPTER 3 ASSESSING POPULATION SIZE IN NOCTURNAL DWELLING SEABIRDS ACCOUNTING FOR DETECTION PROBABILITY AND HABITAT PREFERENCES: A SEQUENTIAL APPROACH
3.2 MATERIAL AND METHODS
CHAPTER 4 COMPARATIVE FORAGING DISTRIBUTION AND ECOLOGY SUGGESTS INTERSPECIFIC COMPETITION BETWEEN TWO SYMPATRIC SHEARWATERS FROM THE SEYCHELLES
4.2 MATERIALS AND METHODS
CHAPTER 5 MOVEMENT PATTERNS AND HABITAT SELECTION OF WEDGE-TAILED SHEARWATERS (PUFFINUS PACIFICUS) BREEDING AT ARIDE ISLAND, SEYCHELLES
5.2 MATERIALS AND METHODS