Inf luence of Season and a Sympatric Congener on Habitat Use by Stephens' Kangaroo Rat

We examined habitat use by the endangered Stephens' kangaroo rat ( Dipodomys stephensi) in different seasons and assessed whether this may be influenced by a sympatric congener, the Pacific kangaroo rat (Dipodomys agilis). Trapping on three plots over 2 years revealed these species were rarely captured at the same trap stations. Spatial segregation was highly significant when both species were at high density. The spatial distribution of these species was temporally stable where both species were relatively abundant, but where D. agilis was relatively uncommon the distribution of D. stephensi varied from one census to another. The abundance of three microhabitats (grass, debris, and bare ground) followed a regular seasonal pattern of variation that was consistent across 2 years of substantially different rainfall. A canonical discriminant analysis showed that the five quantified microhabitats (those above and bush and rock cover) provided highly significant discrimination between the trap stations occupied by the two species. Dipodomys stephensi was associated with trap stations where grass cover and bare ground were abundant but where bush and rock were uncommon. Dipodomys agilis was associated with stations that had large amounts of bare ground and average abundances of bush and rock cover. The spatial segregation of these species appears to be mediated by habitat preferences; D. stephensi prefers grassland and D. agilis prefers sage scrub. This suggests that habitat management for D. stephensi should include (1) controlling the spread of shrubs into grassland and (2) creating dispersal corridors of open habitat to link areas of suitable habitat where none presently exist. Each of these options may be needed to maintain viable populations in all reserves designated for the conservation of D. stephensi.     

Demographic Viability of a Relict Population of the Critically Endangered Plant Borderea chouardii

Abstract:   In addition to human-caused changes in the environment, natural stochasticity may threaten species persistence, and its impact must be taken into account when priorities are established and management plans are designed. Borderea chouardii is a Tertiary relict at risk of extinction that occurs in only one location in the world, where the probability of human disturbance is low. Its persistence, therefore, is mainly linked to its response to natural threats such as stochasticity. Over 8 years I monitored up to 25% of this rupicolous small geophyte. The population had an unbalanced size structure and 90% failure in seed arrival at appropriate microhabitats, which suggests a problem with recruitment. I used matrix models to describe its population dynamics, conducted hand sowings, and performed stochastic simulations to investigate the effect of environmental stochasticity on population trend and viability. I modeled several scenarios to represent a variety of ecological situations, such as population reduction, episodic or persistent disease, and enhancement or decrease of recruitment. Population growth rate (λ) was never significantly different from unity over the study period. The risk of extinction was null over the next five centuries under current conditions. Increase of mortality and decrease of recruitment reduced stochastic population growth rate, but no factor except a persistent increase of 10% mortality resulted in extinction. These results are the consequence of the plant's extremely long life span (over 300 years) and low temporal variability of key vital rates. Even though hand sowing significantly increased the stochastic population growth rate, other approaches may be more important for the persistence of this species. The extremely slow capacity for recovery following disturbances renders habitat preservation essential. In addition, the founding of new populations would reduce the risk associated with habitat destruction.     

Recovery of the Kakerori: An Endangered Forest Bird of the Cook Islands

The Kakerori, or Rarotonga Flycatcher (Pomarea dimidiata) , is an endangered monarch flycatcher endemic to the island of Rarotonga, Cook Islands. This bird was reported to be common until the middle of the nineteenth century, but it declined before 1885 and was thought to be extinct early this century. A small population persisted in the rugged interior of Rarotonga; in 1987 the population stood at 38 birds but was in decline. We determined that introduced predators, especially ship rats (Rattus rattus) , were affecting breeding success. Through a program of experimental management, aimed mainly at rodent control, the population increased from a low of 29 birds in Spring 1989 to 60 birds in Spring 1993. The recovery can be attributed to both improved breeding success and increased adult survivorship.     

What Exactly Is an Endangered Species? An Analysis of the U.S. Endangered Species List: 1985–1991

Critics of the Endangered Species Act have asserted that is protects an inordinate number of subspecies and populations, in addition to full species, and that the scientific rationale for listing decisions is absent or weak. We reviewed all U.S. plants and animals proposed for listing or added to the endangered species list from 1985 through 1991 to determine the relative proportion of species, subspecies, and populations, and their rarity at time of listing. Approximately 80% of the taxa added to the list were full species, 18% were subspecies, and 2% were distinct populations segments of more widespread vertebrate species. The proportion of subspecies and populations was considerably higher among birds and mammals than among other groups. The median populations size at time of listing for vertebrate animals was 1075 individuals; for invertebrate animals it was 999. The median population size of a plant at time of listing was less than 120 individuals. Earlier listing of declining species could significantly improve the likelihood of successful recovery, and it would provide land managers and private citizens with more options for protecting vanishing plants and animals at less social or economic cost.     

Population Viability Analysis for an Endangered Plant

Abstract: Demographic modeling is used to understand the population viability of Furbish's lousewort, Pedicularis furbishiae , a perennial plant species endemic to the St. John River Valley in northern Maine. Environment-specific summaries of demographic parameters (survivorship, growth, and fecundity) over four years, organized into stage-based projection matrices, provide predictions of future population dynamics given a deterministic extension of past conditions. Stochastic modeling, using (I) empirically observed variation in demographic parameters, and (2) estimated rates of natural catastrophes, leads to predictions of extinction probability.
P. furbishiae viability has varied widely over the study period Viable populations with finite rates of increase > 1 are found where cover is low, woody plants do not dominate, and disturbance does not occur. Rates of increase vary over time, suggesting that stochastic analyses would be realistic. Stochastic measures of population viability incorporating environmental variation suggest that early successional environments, especially wetter sites, can support viable populations in the absence of disturbance. However; observed rates of natural catastrophe dominate viability estimates of individual populations. Metapopulation dynamics feature extinction rates that are greater than recolonization rates, and may be affected by land use in the watershed Species management needs to consider a large-scale view of the riverine corridor.     

Status of Piping Plovers in the Great Plains of North America: A Demographic Simulation Model

A stochastic population growth model using empirical demographic data confirmed that the Piping Plover population of the Great Plains of North America is declining by more than 7% annually. Unchecked, this decline would result in extirpation in approximately 80 years. When recent adult (0.66) and immature (0.60) survival rates were held constant, a 31% increase—from 0.86 to 1.13 chicks fledged per pair—was needed to stabilize the population. Annual population increases of 1% and 2% required 1.16 and 1.19 chicks per pair, respectively. Such growth would result in the Great Plains population reaching the level—(2550 pairs)—needed for delisting from the U.S. Endangered Species Act protection in 53 and 30 years, respectively. One- and five-year delays in the initiation of 1% population growth caused 13 and 67 year delays respectively in reaching recovery.     

Protected Areas and Prospects for Endangered Species Conservation in Canada

Abstract:  Reserve networks figure prominently in conservation strategies that aim to reduce extinction rates. We tested the effectiveness of the current reserve network at protecting species at risk in Canada, where relatively extensive wilderness areas remain. We compared numbers of terrestrial species at risk included in existing reserves to randomly generated networks with the same total area and number of reserves. Existing reserve networks rarely performed better than randomly selected areas and several included fewer endangered species than expected by chance, particularly in the most biologically imperiled regions. The extent of protected area and density of species at risk were unrelated at either broad (countrywide) or finer spatial scales (50 × 50 km grids), although there was a tendency for the most threatened regions of the country to have few or no protected areas (1.5% of areas with >30 endangered species were in reserves). Although reserves will play a useful role in conserving endangered species that occur within them, reducing extinction rates in a region with much of the world's remaining wilderness will require integrating conservation strategies with agricultural and urban land-use plans outside formally protected areas.     

Control of Abundant Native Vertebrates for Conservation of Endangered Species

Abundant native vertebrates, which we define as those that have increased in abundance due to human-induced changes in communities or ecosystems, have contributed to the decline of rare vertebrates through predation, competition, habitat change, disease transmission, and hybridization. Recent literature dealing with the negative effects of abundant native vertebrates on rare native vertebrates argues for population control by killing or translocating animals. We identify several potential problems with these methods, including the high cost of population control, community changes such as mesopredator release that favor other harmful vertebrate species, and increases in diseases harmful to the rare species. Also, public opposition to and lack of species specificity in population control techniques often make population control difficult. We propose alternatives to population reduction/or management of abundant native vertebrates, including techniques that prevent abundant vertebrates from causing harm, and community and ecosystem rehabilitation and restoration. The latter provide the best solutions to problems caused by abundant native vertebrates because community and ecosystem degradation are the primary factors responsible for some species becoming rare and others becoming abundant. These solutions are long term, biologically sound, and involve little direct human intervention into ecosystem processes. But population control may be necessary as a short-term solution when abundant vertebrates pose an immediate threat to the survival of a rare species. We conclude that those involved in the conservation of rare species should consider population control of abundant native vertebrates only as a last resort.     

Resembling a Viper: Implications of Mimicry for Conservation of the Endangered Smooth Snake

The phenomenon of Batesian mimicry, where a palatable animal gains protection against predation by resembling an unpalatable model, has been a core interest of evolutionary biologists for 150 years. An extensive range of studies has focused on revealing mechanistic aspects of mimicry (shared education and generalization of predators) and the evolutionary dynamics of mimicry systems (co‐operation vs. conflict) and revealed that protective mimicry is widespread and is important for individual fitness. However, according to our knowledge, there are no case studies where mimicry theories have been applied to conservation of mimetic species. Theoretically, mimicry affects, for example, frequency dependency of predator avoidance learning and human induced mortality. We examined the case of the protected, endangered, nonvenomous smooth snake (Coronella austriaca) that mimics the nonprotected venomous adder (Vipera berus), both of which occur in the Åland archipelago, Finland. To quantify the added predation risk on smooth snakes caused by the rarity of vipers, we calculated risk estimates from experimental data. Resemblance of vipers enhances survival of smooth snakes against bird predation because many predators avoid touching venomous vipers. Mimetic resemblance is however disadvantageous against human predators, who kill venomous vipers and accidentally kill endangered, protected smooth snakes. We found that the effective population size of the adders in Åland is very low relative to its smooth snake mimic (28.93 and 41.35, respectively).Because Batesian mimicry is advantageous for the mimic only if model species exist in sufficiently high numbers, it is likely that the conservation program for smooth snakes will fail if adders continue to be destroyed. Understanding the population consequences of mimetic species may be crucial to the success of endangered species conservation. We suggest that when a Batesian mimic requires protection, conservation planners should not ignore the model species (or co‐mimic in Mullerian mimicry rings) even if it is not itself endangered. Implications of mimicry for Conservation of the endangered smooth snake     

Importance of Habitat Quality and Landscape Connectivity for the Persistence of Endangered Natterjack Toads

Abstract: The natterjack toad (Bufo calamita) is endangered in several parts of its distribution, including Belgium, where it occurs mainly in artificial habitats. We parameterized a general model for natterjack population viability analysis (PVA) and tested its sensitivity to changes in the values of basic parameters. Then we assessed the relative efficiency of various conservation measures in 2 situations: a small isolated population and a system of 4 populations connected by rare dispersal movements. We based the population viability analysis on a stage‐structured model of natterjack population dynamics. We parameterized the model in the RAMAS GIS platform with vital rates obtained from our own field experience and from published studies. Simulated natterjack populations were highly sensitive to habitat quality (particularly pond drying), to dispersal from surrounding local populations, and to a lesser extent to values of fecundity and survival of terrestrial stages. Population trajectories were nearly insensitive to initial abundances, carrying capacities, and the frequency of extreme climatic conditions. The simulations showed that in habitats with highly ephemeral ponds, where premetamorphosis mortality was high, natterjack populations nearly always had a very high extinction risk. We also illustrated how low dispersal rates (<1 dispersing individual/generation) efficiently rescued declining local populations. Such source‐sink dynamics demonstrate that the identification and management of source populations should be a high priority.     

Taxonomic and Geographic Patterns of Decline for Threatened and Endangered Species in the United States

Abstract: Species listed under the U.S. Endangered Species Act (i.e., listed species) have declined to the point that the probability of their extinction is high. The decline of these species, however, may manifest itself in different ways, including reductions in geographic range, number of populations, or overall abundance. Understanding the pattern of decline can help managers assess extinction probability and define recovery objectives. Although quantitative data on changes in geographic range, number of populations, and abundance usually do not exist for listed species, more often qualitative data can be obtained. We used qualitative data in recovery plans for federally listed species to determine whether each listed species declined in range size, number of populations, or abundance relative to historical levels. We calculated the proportion of listed species in each state (or equivalent) that declined in each of those ways. Nearly all listed species declined in abundance, and range size or number of populations declined in approximately 80% of species for which those data were available. Patterns of decline, however, differed taxonomically and geographically. Declines in range were more common among vertebrates than plants, whereas population extirpations were more common among plants. Invertebrates had high incidence of range and population declines. Narrowly distributed plants and invertebrates may be subject to acute threats that may result in population extirpations, whereas vertebrates may be affected by chronic threats that reduce the extent and size of populations. Additionally, in the eastern United States and U.S. coastal areas, where the level of land conversion is high, a greater percentage of species’ ranges declined and more populations were extirpated than in other areas. Species in the Southwest, especially plants, had fewer range and population declines than other areas. Such relations may help in the selection of species’ recovery criteria.     

Experimental Rock Outcrops Reveal Continuing Habitat Disturbance for an Endangered Australian Snake

Abstract: Protected areas are commonly viewed as safe havens for endangered species. To test this notion, we experimentally constructed small rock outcrops for the endangered broad-headed snake (   Hoplocephalus bungaroides ) within a national park near Sydney, Australia. Rock outcrops provide vital shelter sites during the cooler months of the year. Constructed rock outcrops (3 × 5 m) were placed at 11 paired sites located near (≤250 m) and far from (>400 m) walking tracks and roads. Eight of our 22 rock outcrops were disturbed by people over a 15-month period. Disturbance consisted of displacement of some rocks or complete destruction of the outcrop. Disturbed outcrops occurred up to 450 m from a walking track or road. Disturbance to natural outcrops has also been observed in this park. This demonstrates a continuing decline in the quality of this snake's habitat. Twenty of our rock outcrops were colonized by velvet geckos ( Oedura lesueurii ), the primary prey of this snake. One broad-headed snake was found in one outcrop. According to these findings, attempts to restore the habitat of this endangered snake should be centered on sites located ≥500 m from a walking track or road. Our study highlights the value of targeted experiments that precede larger-scale habitat restoration.