Changes in sooty shearwater (Puffinus griseus) abundance and harvesting on the Rakiura Titi Islands

Abstract

We estimated the change in abundance of sooty shearwater (titi, Puffinus griseus) at six Rakiura Titi Islands, New Zealand, by comparing historical and recent surveys of the density of entrances to breeding burrows. We found evidence that entrance density between 1994 and 2006 was lower than it was between 1961 and 1976. Our overall estimate of the annual rate of change in burrow entrance density is ‐1.0% (95% CI ‐2.3 to ‐ 0.1%). Declines have been slower on four islands where Rakiura Maori maintain a traditional harvest of sooty shearwater chicks ("muttonbirding") compared with three unharvested islands. Density‐dependent population processes may explain this difference: rates of decline have been faster in areas of relatively high initial entrance density, and historically the harvested islands have had lower initial density. There was a strong, apparently linear, relationship between entrance density and chick density on breeding colonies, so changes in entrance density probably do indicate a real population decline. The western side of Taukihepa, the largestof the Titi Islands, first became accessible for muttonbirding with the advent of helicopters in the 1970s, but it is unknown whether this has caused an increase in the number of sooty shearwaters harvested by Rakiura Maori.     

Direct and indirect effects of rats: does rat eradication restore ecosystem functioning of New Zealand seabird islands?

Introduced rats (Rattus spp.) can affect island vegetation structure and ecosystem functioning, both directly and indirectly (through the reduction of seabird populations). The extent to which structure and function of islands where rats have been eradicated will converge on uninvaded islands remains unclear. We compared three groups of islands in New Zealand: islands never invaded by rats, islands with rats, and islands on which rats have been controlled. Differences between island groups in soil and leaf chemistry and leaf production were largely explained by burrow densities. Community structure of woody seedlings differed by rat history and burrow density. Plots on islands with high seabird densities had the most non-native plant species. Since most impacts of rats were mediated through seabird density, the removal of rats without seabird recolonization is unlikely to result in a reversal of these processes. Even if seabirds return, a novel plant community may emerge.     

Variation in abundance and harvest of sooty shearwaters (Puffinus griseus) by Rakiura Maori on Putauhinu Island,New Zealand

Abstract

Sooty shearwater (Puffinus griesus, titi) abundance, harvest levels and chick mass were monitored repeatedly on Putauhinu Island, south‐west of Rakiura (Stewart Island) between 1997 and 2005. Putauhinu is the second largest of the Titi Islands and has a relatively high density of chicks distributed over most of the island, so it supports what is likely the second‐largest population of sooty shearwaters in the Rakiura region (after Taukihepa, Big South Cape Island). Rakiura Maori harvested chicks from five “manu” (family birding areas) that covered 56% of the 128.4 ha of breeding colony of the island. Chick density was lower on the unharvested area in the interior of the island than on harvested areas. Burrow entrance density was higher where there was more ground cover (mainly fern) vegetation, but these areas had lower burrow occupancy, so overall chick density was similar at different levels of ground cover. Twenty‐six harvesters present on Putauhinu in 2005 took 31 280 chicks in total, equivalent to 8.4% (95% CI = 6.6–12%) of the available chicks on the entire island. Seasonal variation in total chicks harvested (CV 15–22%) was not related to chick abundance or mass. Refuges, including impenetrable patches of vegetated ground within manu, the unharvested centre of the island, and even nearby unharvested islands, will ameliorate localised impacts of harvest if density‐dependent immigration is operating.     

Bioturbation of Burrowing Crabs Promotes Sediment Turnover and Carbon and Nitrogen Movements in an Estuarine Salt Marsh

Ecological functions of bioturbation in ecosystems have received increasing attention over the recent decades, and crab burrowing has been considered as one of the major bioturbations affecting the physical and chemical processes in salt marshes. This study assessed the integrated effects of crab excavating and burrow mimic trapping on sediment turnover and vertical C and N distributions in a Chinese salt marsh in the Yangtze River estuary. Crab burrowing increased soil water content and the turnover of carbon and nitrogen and decreased bulk soil density. Vertical movement of materials, nutrient cycling and reuse driven by crab burrowing might be obstructed by vegetation (Phragmites australis and Spartina alterniflora communities). The amount of soil excavated by crab burrowing was higher than that deposited into burrow mimics. In Phragmites marshes, Spartina marshes and unvegetated mudflats, net transport of soil to the marsh surface was 171.73, 109.54, and 374.95 g m⁻² d⁻¹, respectively; and the corresponding estimated soil turnover time was 2.89, 4.07 and 1.83 years, respectively. Crab burrowing in salt marshes can mix surface and deeper soil over a period of years, accelerating litter decomposition and promoting the efficient reuse of nutrients by plants. Therefore, bioturbation affects soil physical processes and functioning of ecosystems, and needs to be addressed in ecosystem management.     

Invasive rats alter woody seedling composition on seabird-dominated islands in New Zealand

Invasive rats (Rattus rattus, R. norvegicus, R. exulans) have large impacts on island habitats through both direct and indirect effects on plants. Rats affect vegetation by extirpating burrowing seabirds through consumption of eggs, chicks, and adults. These seabirds serve as ecosystem engineers, affecting plant communities by burying and trampling seeds and seedlings, and by altering microclimate. Rats also directly affect plant communities by consuming seeds and seedlings. We studied the direct and indirect impacts of rats on the seedlings of woody plants on 21 islands in northern New Zealand. We compared seedling densities and richness on islands which differed in status with respect to rats: nine islands where rats never invaded, seven islands where rats were present at the time of our study, and five islands where rats were either eradicated or where populations were likely to be small as a result of repeated eradications and re-invasions. In addition, we compared plots from a subset of the 21 islands with different burrow densities to examine the effects of burrowing seabirds on plants while controlling for other factors that differ between islands. We categorized plant communities by species composition and seedling density in a cluster analysis. We found that burrow densities explained more variation in seedling communities than rat status. In areas with high seabird burrow density seedling densities were low, especially for the smallest seedlings. Species richness and diversity of seedlings, but not seedling density, were most influenced by changes in microclimate induced by seabirds. Islands where rats had been eradicated or that had low rat populations had the lowest diversity and richness of seedlings (and adults), but the highest seedling density. Seedling communities on these islands were dominated by Pseudopanax lessonii and Coprosma macrocarpa. This indicates lasting effects of rats that may prevent islands from returning to pre-invasion states.     

Effects of invasive rats and burrowing seabirds on seeds and seedlings on New Zealand islands

Rats (Rattus rattus, Rattus norvegicus, Rattus exulans) are important invaders on islands. They alter vegetation indirectly by preying on burrowing seabirds. These seabirds affect vegetation through nutrient inputs from sea to land and physical disturbance through trampling and burrowing. Rats also directly affect vegetation though consumption of seeds and seedlings. Seedling communities on northern New Zealand islands differ in composition and densities among islands which have never been invaded by rats, are currently invaded by rats, or from which rats have been eradicated. We conducted experimental investigations to determine the mechanisms driving these patterns. When the physical disturbance of seabirds was removed, in soils collected from islands and inside exclosures, seedling densities increased with seabird burrow density. For example, seedling densities inside exclosures were 10 times greater than those outside. Thus the negative effects of seabirds on seedlings, by trampling and uprooting, overwhelm the potentially beneficial effects of high levels of seed germination, seedling emergence, and possibly seed production, which result from seed burial and nutrient additions. Potential seedling density was reduced on an island where rats were present, germination of seeds from soils of this island was approximately half that found on other islands, but on this island seedling density inside exclosures was 7 times the density outside. Although the total negative effects of seabirds and rats on seedling densities are similar (reduced seedling density), the differences in mechanisms and life stages affected result in very different filters on the plant community.     

An estimate of population sizes of burrowing seabirds at the Diego Ramirez archipelago, Chile, using distance sampling and burrow-scoping

The Diego Ramirez Islands lie 60 nautical miles southwest of Cape Horn and are the breeding site for three species of burrowing seabirds: blue petrels (Halobaena caerulea), common diving petrels (Pelecanoides urinatrix) and sooty shearwater (Puffinus griseus). Burrowing seabirds are highly vulnerable to predation by introduced vertebrate pests, and Diego Ramirez is an important breeding site because it is one of a few remaining subantarctic island groups with no introduced predators. Diego Ramirez is the only known breeding site for blue petrels in the southeast Pacific region, holding about 80% of the global population of that species, and with a population ten times larger than any other population in the world. We estimated the population size in 2002, using a novel application of the distance sampling technique to determine burrow density, and a burrow-scope with excavations to determine occupying species. We found that density was correlated with slope angle and soil wetness. Burrow densities in flatter terrain with drier soils were 2.03 burrows/m² (95% confidence intervals: 1.82–2.27) and 1.11 burrows/m² (0.84–1.48) in steeper terrain with wetter soils. The occupation rate of burrows were significantly different between habitat types (t=2.74, d.f. 11, P<0.05); in flatter drier habitats the proportion of burrows that led to a nest was 0.85 (0.74–0.96), in steeper wetter habitats this decreased to 0.64 (0.50–0.78). We used a digital elevation model to calculate true area rather than planar area for the two habitat types on the main island of Bartolome, and charts to calculate planar area for the remainder of the archipelago. There were 1.35 (1.15–1.54) million pairs of blue petrels and 99,000 (65,000–134,000) pairs of common diving petrels on the archipelago. These are similar figures to those from the only previous estimate, made in 1980. We found breeding sooty shearwaters for the first time, and estimated a population of several thousand pairs. We emphasise the facility of distance sampling as an unbiased technique with practical advantages over commonly used area search methods for monitoring populations of burrowing seabirds. These advantages include increased survey efficiency allowing a larger sample size for a given effort and a correspondingly tighter estimation of density.     

The Contribution of Crab Burrow Excavation to Carbon Availability in Surficial Salt-marsh Sediments

Geomorphology, vegetation and tidal fluxes are usually identified as the factors introducing variation in the flushing of particulate organic matter (POM) from tidal marshes to adjacent waters. Such variables may, however, be insufficient to explain export characteristics in marshes inhabited by ecosystem engineers that can alter the quantity and quality of POM on the marsh surface that is subject to tidal flushing. In this study we evaluated the balance between transfer of buried sedimentary organic carbon (C) to the marsh surface due to crab excavation (measured from the mounds of sediment excavated from burrows) and outputs of C from the surface due to sediment deposition within crab burrows (estimated from sediment deposited within PVC burrow mimics), in a Southwestern Atlantic salt marsh supporting dense (approximately 70 ind m⁻²) populations of the crab Chasmagnathus granulatus. C excavation by crabs was much greater than deposition of C within crab burrow mimics. Per area unit estimates of the balance between these two processes indicated that crabs excavated 5.98 g m⁻² d⁻¹ and 4.80 mg m⁻² d⁻¹ of total and readily (10 d) labile C, respectively. However, sediments excavated by crabs showed a significantly lower content of both total and readily-labile C than sediment collected in burrow mimics. This indicates that ecosystem engineering by burrowing crabs causes a net decrease in the concentration of C in the superficial sediment layers and, thus, an overall decrease in the amount of C that can be washed out of the marsh by tidal action. Incorporating the in situ activities of ecosystem engineers in models of marsh export should enhance understanding of the function of marshes in estuarine ecosystems.     

Video camera count of burrow‐dwelling fairy prions,sooty shearwaters,and tuatara on Takapourewa (Stephens Island), New Zealand

Abstract

Video camera technology is an increasingly common tool in the study of burrow‐dwelling animals. A camera used to observe the inhabitants of narrow burrows on Takapourewa (Cook Strait, New Zealand) is described. Population densities (animals per m²) of up to 1.1 for fairy prions {Pachyptila turtur), 0.06 for sooty shearwaters (Puffinus griseus), and 0.15 for tuatara (Sphenodon punctatus) were estimated in four different habitats. These data are similar to other estimates collected in more labour intensive studies on the island, and provide baseline information for future conservation work on Takapourewa.     

Predictive habitat modelling to estimate petrel breeding colony sizes: Sooty shearwaters (Puffinus griseus) and mottled petrels (Pterodroma inexpectata) on Whenua Hou Island

Abstract

Between 2001 and 2006, we systematically sampled the entire coast of Whenua Hou, a rugged offshore island in southern New Zealand, to estimate the population densities of sooty shearwaters (Puffinus griseus) and mottled petrels (Pterodroma inexpectata) by counting the entrances of breeding burrows. A two‐step regression modelling process using binomial errors was used to predict the presence of a colony, and a normal general linear model was used to predict the density of entrances within colonies. Aerial photography, GIS and a Digital Elevation Model were used to extract relevant habitat and location variables, and a combination of both regression models was used to predict the density of breeding burrows within each 5.32 m² pixel on the island. This complex GIS and habitat prediction modelling approach gave population estimates very similar to a more traditional simple area extrapolation method and gave no improvement in precision. However, correction for the slope of the land increased our simple area estimates of population size by 11%. We estimate populations of sooty shearwater and mottled petrel breeding pairs at 173 000 (162 000–190 000) and 160 000 (123 000–197 000) respectively. Based on this number of breeding pairs, we calculate that Whenua Hou supports a total population of 868 000 (554 000–1 270 000) sooty shearwaters. Our estimate of the total mottled petrel population 202 000 pairs (162 000–242 000) is comparable with the only published estimate, but could be an underestimate because mottled petrels are sometimes found in large burrows. More research for robust estimation of population trends is needed to assess the conservation status of mottled petrels.     

Effect of streaked shearwaterCalonectris leucomelas on species composition ofPersea thunbergii forest on Kanmurijima Island,Kyoto Prefecture,Japan

The effects of trampling and burrowing by streaked shearwaters were studied on thePersea thunbergii forest in Kanmurijima Is. The vegetation data were analysed using both the phytosociological tabulation method and principal component analysis (PCA). The vegetation ordination on the first axis reflected the environmental gradient from light to heavy disturbance by the streaked shearwater, and the vegetation ordination on the second axis represented the gradient from the latter phase of forest succession to retrogressive succession. The position of the three vegetation groups in the ordination diagram successfully explained the relation between the habitat and the species composition. The disturbance caused by activities of the streaked shearwater such as burrowing and trampling causes a decrease of species, especially character species of Camellietea japonicae, and also causes retrogressive succession: patches ofMallotus japonicus develop and many heliophytes occur.     

Estimating and Correcting for Bias in Population Assessments of Sooty Shearwaters

Abstract: We investigated the precision and accuracy of an infrared burrowscope for detecting sooty shearwater (Pufffinus griseus) chicks at 13 plots from 3 islands in southern New Zealand in 2003. We partially excavated burrows systems to reveal the entire burrow contents after 2 teams of observers had prospected all burrow entrances. Accuracy was similar between islands and observer teams at approximately 85%. The majority of the inaccuracy stemmed from failure to detect some chicks. Logistic regression modeling identified 4 burrow characteristics occurring between the entrance and the nest-site that influenced detection of burrow occupants. Detection was lower at nest-sites further from burrow entrances, in burrows with a high rate of burrow division, and in burrows with a high level of curvature. There was a positive relationship between the interaction of rate of division and curvature and detection of chicks. Distance from the burrow entrance was the only parameter that could be reliably used as a predictor of detection rate, so a reduced model containing only this variable was constructed to correct for burrowscope bias. The correction factor performed well on The Snares and Bench Island where predicted bias was very similar to observed levels (within 5%), but bias was overestimated on Putauhinu by up to 19.1%. Consistent bias, lack of damage to burrows from excavation, and the successful application of a correction factor all indicate the value of further testing burrowscope accuracy on other burrow-nesting seabird species.     

Estimating regional population size and annual harvest intensity of the sooty shearwater in New Zealand

Abstract

Recent comprehensive survey data from multiple New Zealand offshore islands were combined with demographic population models to produce the first formal estimate of the total population of sooty shearwaters within New Zealand territory. We estimated the total population over 1994–2005 to be 21.3 (19.0–23.6) million individual birds in the New Zealand region. This population consisted of 12.8 (12.0–13.6) million adults, 2.8 (2.5–3.1) million chicks, and 4.4 (4.2–4.7) million breeding pairs. Breeding sooty shearwaters were concentrated primarily around the southern islands of New Zealand, with 53% breeding in the Titi Islands surrounding Rakiura (Stewart Island). Rakiura Maori muttonbirders were estimated to harvest 360 000 (320 000–400 000) sooty shearwaters per year, equivalent to 18% of the chicks produced in the harvested areas and 13% of chicks in the New Zealand region. Overall, 11% of the chicks within the Titi Islands live on unharvested ground. Systematic and widespread surveys of breeding colonies in South America are needed before a reliable global sooty shearwater population estimate can be calculated.     

Effect of available burrow densities of plateau pika (Ochotona curzoniae) on soil physicochemical property of the bare land and vegetation land in the Qinghai-Tibetan Plateau

The available burrow densities of plateau pika (Ochotona curzoniae) regulate the soil physicochemical property in alpine meadow. A field survey was conducted to investigate the effect of available burrow densities of plateau pika on soil physicochemical property of bare land (bare patch produced by burrowing behavior of plateau pika) and vegetation land (land covered with vegetation). This study indicated that the increase in available burrow of plateau pika caused the soil water content at 0–10 cm layer of bare land and that at 10–20 cm of vegetation land to reduce, and caused the soil water content at 10– 20 cm layer of bare land and that at 0–10 cm layer of vegetation land to firstly increase and then decline. In the increasing process of available burrow of plateau pika, the soil silt content firstly increased and then decreased, and soil sand content firstly decreased and then increased. With the increase of available burrow of plateau pika, the soil porosity at 0–10 cm layer of bare land and that at 10–20 cm layer of vegetation land decreased, while the soil porosity at 10–20 cm layer of bare land and that at 0–10 cm of vegetation land firstly increased and then decreased. Soil pH value, soil organic matter, total nitrogen and total phosphorus content firstly increased and then decreased, peaking at 14 available burrows per 625 m², while total soil potassium content did not respond to available burrow densities of plateau pika. This study suggested that the proper available burrows existing in the alpine meadow increased soil permeability, accelerated soil moisture to penetrate deeply, increased the proportion of soil silt, and improved the soil nutrient; however, this beneficial effect was strongly influenced by the available burrow density of plateau pika, implying that plateau pika did not benefit soil structure when its available burrow was over 34 number/625 m².     

The activity of a subterranean small mammal alters Afroalpine vegetation patterns and is positively affected by livestock grazing

Subterranean rodents can act as ecosystem engineers by shaping the landscape due to soil perturbation and herbivory. At the same time, their burrow density is affected by environmental conditions, vegetation and anthropogenic factors. Disentangling this complex interplay between subterranean rodents and their environment remains challenging. In our study, we analysed the interplay of abiotic conditions, vegetation patterns and human land-use and the burrow density of the giant root-rat (GRR; Tachyoryctes macrocephalus), a subterranean rodent endemic to the Afroalpine ecosystem of the Bale Mountains in south-east Ethiopia. Specifically, we examined the effects of GRR on plant species richness and vegetation cover and vice-versa, and how these reciprocal effects might be modulated by temperature, habitat wetness and grazing. Our results showed that increasing GRR burrow density led to decreased vegetation cover, and that effects of GRR on vegetation cover were slightly stronger than vice-versa. Considering the reciprocal causation models, we found that increasing plant species richness led to increased GRR burrow density, while GRR burrow density decreased as vegetation cover increased. Increases in habitat wetness and livestock grazing intensity also directly led to increased GRR burrow density. Our results stress the importance of subterranean ecosystem engineers on vegetation and highlight the vulnerability of these complex interactions to human activity.     

What determines the way of deposition of excavated soil

Subterranean rodents continuously extend their burrow systems primarily in search of food, which has an important impact on the ecosystem in which they live. Excavated soil may be pushed either into aboveground mounds or into tunnels underground. Factors affecting the amount of burrowing and the preference of aboveground or underground soil deposition are, nevertheless, little known. We investigated the influence of food supply, soil hardness, and the animal’s body mass on the mode of soil deposition in ten burrow systems of free ranging silvery mole-rats Heliophobius argenteocinereus Peters, 1846. In each burrow system, we estimated the volume of backfilled tunnels and the volume of soil deposited aboveground. The highest amount of variation in these parameters was explained by the interaction of food supply and soil hardness. The ratio of the volume of backfilled tunnels to the volume of mounds was not significantly dependent on any of the explanatory variables. The proportion of backfilled tunnels decreased with the increasing volume of the complete burrow system. We propose that both low food supply and soft soil lead to an increased amount of burrowing, which results in a larger volume of soil deposited both above ground and under ground over a given period of time.     

Denning behaviour of ship rats (Rattus rattus) on Taukihepa,a seabird breeding island

Abstract

Den sites of 14 ship rats (Rattus rattus) were located daily during the rat breeding season on Taukihepa (Big South Cape), a seabird island southwest of Rakiura (Stewart Island). In contrast to other New Zealand studies, no arboreal dens were found. Den sites on Taukihepa were in ferns, under logs, in woodpiles, or underground in sooty shearwater (Puffinusgriseus) breeding burrows. The number of times known den sites used was positively related to the amount of leaf litter and woodpiles near the den sites. Overall, 24% of radio‐tagged rats were sharing den sites on any given day. While there was considerable individual variation in the number of times den sites were used, female rats tended to reuse den sites more than males. Many rats were found in dens alone, but frequently males and females shared. Occasionally two females and one male denned together, as did two females, whereas males never denned with another male.     

Nutrition and burrowing energetics of the Cape mole-rat Georychus capensis

Summary At 22°C the resting oxygen consumption of G. capensis is 1.13±0.05 cm³O₂·g^-1·h^-1 (mean± S.E.). In loose sandy soil the burrowing metabolic rate was approximately three times that of resting (3.41±0.19 cm³O₂·g^-1· h^-1). Rate of oxygen consumption while burrowing bears a linear relationship with rate of burrowing. The equation of the regression line describing this relationship was used to construct a model for calculating energy expenditure of burrowing in free-living mole-rats. The diet of G. capensis consists of some green plant material and geophyte corms. The latter has a mean gross energy content of 16.36 kJ·g^-1 dry weight. The digestibility coefficient for captive G. capensis fed on sweet potato, was 97.42±0.41%. Data collected from an excavated burrow system revealed that the total energetic cost of constructing the burrow amounted to 79% of the estimated digestible energy available from geophyte corms in the area. A food store in the same burrow system was sufficient to meet the maintenance requirements of an adult G. capensis, resting at 22°C, for approximately 80–85 days. Soil samples taken at random adjacent to the burrow contained corms with a mean estimated digestible energy value of 2084 kJ per m³ of soil. A comparison of energetic cost of burrowing and randomly available digestible energy in the field suggests that foraging patterns are not random.     

Can land crabs be used as a rapid ecosystem evaluation tool? A test using distribution and abundance of several genera from the Seychelles

Habitat destruction and the introduction of exotic species are the primary causes of biodiversity loss in tropical island ecosystems. Conservation efforts on oceanic islands are often biased towards charismatic vertebrate faunas, neglecting invertebrate assemblages. We sampled the land crab community on five islands in the central Seychelles; in the intertidal zone, in the supralittoral zone and in nine different inland habitat types to explore the impacts of exotic vegetation and environmental variables on land crab abundance and community composition, and investigate whether land crabs can be used as a tool for the rapid assessment of habitat quality on tropical oceanic islands. We found that species richness and the abundance of the dominant ghost crab Ocypode cordimana was higher in native habitat types than habitats dominated by exotic vegetation. Available ground substrate suitable for burrowing may be a limiting factor for O. cordimana in exotic habitat types. Coenobita rugosus, the dominant crab in the supralittoral zone is largely absent where there is no supralittoral vegetation. These results suggest that land crabs could be reliable indicators of habitat quality on oceanic islands. The abundance of land crabs could be used in the rapid assessment of ecosystem perturbation and identification of sites requiring restoration or management.     

When seasnake meets seabird: Ecosystem engineering,facilitation and competition

Ecosystem engineers such as burrowing seabirds can increase habitat availability for sympatric taxa – but only if the burrow's owner allows other species to use the newly created shelter site. Our studies on a small Pacific island suggest that an avian burrower (the wedge‐tailed shearwater Puffinus pacificus) is both a facilitator and a competitor for amphibious seasnakes. Video camera inspection of 102 burrows revealed frequent usage of these burrows as retreat sites by the snakes, with Laticauda laticaudata restricted to burrows <4 m from the water's edge, whereas Laticauda saintgironsi often used burrows further inland. Snakes never occupied burrows that contained adult shearwaters, suggesting active burrow defence by the birds. Model snakes that we inserted into burrows were attacked, especially on the head and upper body, and we found one snake pecked to death outside a burrow. Wedge‐tailed shearwaters act as facilitators, creating a thermally favourable microhabitat and substantially enhancing habitat suitability for snakes; but they are also competitors, aggressively competing with snakes for occupancy of the resource that has been created.