Ecology of the Black Stilt (Himantopus novaezelandiae) in a century of industrialization: consequences of indirect human impact
The Black Stilt feeding in its natural waterway habitat. Image Credit: Glenda Rees from Gore, NZ. http://www.flickr.com/photos/nzsamphotofanatic/
Contents
Introduction
This article addresses the ecology of the Black Stilt (Himantopus novaezelandiae) in a century of industrialization: consequences of indirect human impact. The Black Stilt (Himantopus noveazelandiae) is a critically endangered species that requires advanced conservation methods to effectively sustain the species. Endemic to New Zealand, the Black Stilt is found in riverbed, delta, and open wetland habitats of the Mackenzie Basin where a braided river gravel substratum is exposed for part of the year. Like many other island species, over the past century, the Black Stilt has declined a result of habitat destruction and increased predation by introduced species. In the case of the Black Stilt, these changes have also resulted in detrimental inbreeding. Investigators who have attempted to develop conservation plans have yet to develop a viable plan that reduces both human impacts and predation. However, all investigators are able to agree that conservation plans need to begin with the monitoring and adjustment of interbreeding and predation impacts. In this aspect, the study of the Black Stilt is a model species for the study of endangered species; its recovery will require that primary and secondary causes of species endangerment are well understood, and the development of appropriate conservation plans are based on this understanding.
Environment and Distribution of the Black Stilt
The Black Stilt is endemic to the North and South Islands of New Zealand. The preferred habitat of the Black Stilt is the wet substrata of open wetlands. Here, the Black Stilt is able to readily harvest Oligochaeta, Chironomidae, and other invertebrates. These are harvested by the Black Stilt using probing techniques within the wet substrata. The single population of the Black Stilt is a remnant population from a period in which the species was widely dispersed. The population is located around Lake Tekapo, the Ohau River, Ahuriri River, and the Tasman River region. Geographical distribution of the Black Stilt is dependent upon the particular breeding seasons which coincide with specific months of the year. This time period consists of a breeding season, followed by a post-breeding season, and a wintering season. During breeding season from August through December, the Black Stilt occupies exposed riverbank habitats for locating mates and mating. During the post-breeding season (December to April), the Black Stilt moves to a stagnant waterway or swamp habitat until the wintering season. During this season, they seek to obtain adequate amounts of energy for post-breeding activity. During the wintering season, the Black Stilt inhabits various delta and exposed waterway environments containing exposed substrata. The exposed substratum provides a feeding ground for the Black Stilt during this season.
The Black Stilt in its natural waterway habitat. Image Credit: Glenda Rees. http://www.flickr.com/photos/nzsamphotofanatic/
Biotic Interactions with Other Species
The diet of the Black Stilt is mainly made up of invertebrate species, such as Oligochaeta and Chironomidia. Introduced species, such as ferrets (Mustela furo), weasels (Mustela nivalis), stoats (Mustela erminea), and house cats (Felis catus) prey heavily on birds, including the Black Stilt. Currently, introduced predator species are the main control on the population size of the Black Stilt.
Due to the Black Stilt’s characteristic of breeding with the same partner for its lifespan, there are often an inadequate number of individuals to form breeding pairs in the current population. To resolve this problem, the Black Stilt will cross breed with the Pied Stilt (Himantopus himantopus leucocephalus) if no Black Stilt mate is available. Offspring from these two species are considered hybrid Stilts, and often do not possess the fitness required to exist in the modified New Zealand environment.
Demography of the Black Stilt Population
Population dynamics of the Black Stilt have been highly variable in previous years, and are still considered variable in recent studies. Black Stilt populations were stable up to the end of the nineteenth century, when the results of introduced mammalian predator interaction and habitat modification caused population sizes to plummet. At the beginning of the twentieth century, no nesting pairs of the Black Stilt were identified on New Zealand’s North Island, and nesting pairs of Black Stilt on the South Island were considered location-specific, only being found in riverbed habitats in Canterbury and Otago. While still common within these areas, the population declined, setting the population size between 500 and 1000 individuals. Shortly after the middle of the twentieth century, the total Black Stilt population exhibited a second sharp decline, leaving 50 to 100 wild adult Black Stilts in a single population in New Zealand’s South Island. The single population appeared to remain relatively stable for the next thirty years, until a 1983 census of wild Black Stilts revealed a low count of 23 adults. As a response to the results of gradual population decline seen in the 1983 census, various intensive conservation plans have been implemented for the Black Stilt to increase the wild population of the species. A 2010 census showed a positive response to the intensive conservation efforts of the past thirty years, revealing 85 adults present within the Waitaki Basin region of New Zealand. The population within the past thirty years is best described as dynamic with an upward trend, while still considered critically endangered.
Genetics of the Black Stilt
Although plumage of the Black Stilt is identical in male and female individuals, the species exhibits sexual dimorphism. Differences in call and body size exist between male and female individuals, however DNA sampling and testing is currently the only accurate means of sexing the Black Stilt. Restriction fragments found in Taq 1 Digest reports from DNA analysis reveal males containing two restriction fragments, while females containing four restriction fragments. Genetic tests to determine sex greatly aid conservation efforts, commonly resulting in the reduction of inbreeding in captive settings. Though the Black Stilt is known to cross-breed with the Pied Stilt (Himantopus himantopus leucocephalus) as a result of population decline, little evidence exists to suggest any significant morphological differences beyond bill and body size in the hybrid vs. purebred individuals.
The Black Stilt and Pied Stilt (Himantopus himantopus leucocephalus) often interbreed. Interbreeding significantly lowers birth and survival rates, and lowers the fitness of the hybrid offspring Stilts. Due to mating habits of the Black Stilt, a hybrid mating pair has a low chance of contributing to the Black Stilt population.
The hybrid Black Stilt. Image Credit: Glenda Rees. http://www.flickr.com/photos/nzsamphotofanatic/
Human Interactions with the Species
European colonial expansion of New Zealand during the early nineteenth century led to extensive land modification and alteration of biological communities. Most notably, the European Rabbit (Oryctolagus cuniculus) became invasive shortly after introduction. Various species including ferrets (Mustela furo), weasels (Mustela nivalis), stoats (Mustela erminea), and house cats (Felis catus) were introduced in the middle of the nineteenth century to control the European Rabbit population. During European colonization, foreign vegetation was introduced to New Zealand in an effort to replicate European landscapes. Some species of introduced vegetation became invasive to the shores of lakes and wetlands shortly after introduction, creating large modifications to the habitat of the Black Stilt. Early land modifications to allow for agricultural expansion focused upon the drainage of wetlands to de-saturate soil. The de-saturation of wetland soil destroys the wet substratum feeding grounds of the Black Stilt, and alters the vegetation of the land to produce an exotic habitat. The development of hydroelectric power plants has also directly affected the Black Stilt by raising some major lakes and inundating Black Stilt foraging and nesting habitats. Deltas also exhibit higher or lower levels as a result of hydroelectric power plant interference, potentially flooding feeding and breeding grounds of the Black Stilt.
Synthesis
The Black Stilt has been close to extinction several times, but has been rescued by intensive conservation efforts. The Black Stilt was considered a common species in New Zealand before European colonization. Since human arrival, indirect human interactions have caused the population to decline. However, mammalian predator introductions in particular, and possibly invasive species introductions, and human development have contributed towards an exponential population decline. As a result of these environmental changes, the species was not able to effectively sustain an adequate population size. A reduced population size in the species results in inbreeding, or cross-breeding with the Pied Stilt. Either circumstance will produce offspring with a reduced level of fitness. Offspring with reduced fitness levels generally have difficulty surviving in their natural habitat, and will often die before they are able to reproduce. While the individuals purposely try to avoid these breeding patterns by choosing the darkest colored mate and laying multiple egg clutches, the limited population occasionally requires the species to avoid a natural breeding scenario. Current efforts to genetically sex and identify the Black Stilt in captive populations help to eliminate inbreeding and unbalanced populations in captivity. Genetic sexing is also able to be used in the wild if no hybrid species exist in the population.
Conservation Implications
If the Black Stilt population is to become successful in the wild, predation and crossbreeding between species must be reduced. It is essential to increase the natural population of the Black Stilt to allow for an adequate sex ratio for appropriate breeding pairs. Open wetlands intended to assist the Black Stilt should be returned to optimal condition in which all exotic plants are removed, predators are managed, and human disturbance via waterway volume fluctuation is not evident.
While habitat conservation is an important prerequisite for population restoration, mammalian predator control poses the largest threat to the Black Stilt and should receive the largest amount of focus in population restoration. Before the introduction of foreign predators to New Zealand, the Black Stilt had no foreign predator interaction. Upon the introduction of predators, the Black Stilt was negatively affected both directly as a result of mammalian predation, and also a side effect of the European Rabbit population control. While removing all introduced predators from New Zealand is difficult, the method of monitoring and controlling predator population is feasible. Controlling and monitoring predator populations would imply observing predator interactions circa the Black Stilt’s environment, and capturing predators within the Black Stilt’s environment for removal. Assuming that all requirements of this restoration plan are met, it is expected that the Black Stilt will have little environmental constraint. With little environmental constraint, the species will be able to effectively mate and reproduce without predator limitation. Once the species has recovered to an acceptable extent, the issue of inbreeding and crossbreeding will expectedly resolve itself as there will be ample numbers of individuals to form breeding pairs.
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