Are populations of the candy barrel cactus (Echinocactus platyacanthus) in the desert of Tehuacán, Mexico at risk? Population projection matrix and life table response analysis

Echinocactus platyacanthus is a candy barrel cactus endemic to Mexico and an endangered species owing to its exploitation and the destruction of its habitat. The population dynamic of this species is analyzed using matrix models. Three consecutive censuses were carried out (1997, 1998, and 1999) for six populations of this species in the Tehuacán-Cuicatlán Biosphere Reserve. Fruit contain many seeds (mean = 171 ± S.E. 11.03 seeds/fruit); seedling establishment and survival are low (2 × 10⁻⁶), and fecundity increases as the diameter of the individuals increases (62 seeds in adult 1-4322 in adult 4). The rates of population growth (λ) range from 0.9285 to 1.0005. Elasticity values for demographic processes indicate that the stasis of the adults is the greatest contribution (S = 0.982), followed by growth (G = 0.017) and fecundity (F = 0.001) to λ. The populations are located in the lower left corner of the demographic triangle; however, there are variations for a given population from one year to the next. Life table response experiments indicate that although there are local variations, the most important differences in the values of λ between populations and between years are associated with changes in the stasis of the adults. The disturbance index is not directly related to population density or to the current value of λ. The protection of adult E. platyacanthus must be taken into account for the management of this species and its conservation in the study area.     

Female biased mortality caused by anthropogenic nest loss contributes to population decline and adult sex ratio of a meadow bird

Meadow breeding birds such as the whinchat Saxicola rubetra have been declining due to increased farming intensity. In modern grassland management, the first mowing and the bird’s breeding cycle coincide, causing high nest destruction rates and low productivity of grassland bird populations. However, it is virtually unknown whether the mowing process directly affects adult survival by accidentally killing incubating females. We studied adult survival of an Alpine whinchat population during two breeding seasons using either colour-ringing or radio-tracking of 71 adults. Assessing territories, mowing phenology and nest destruction from 1988 to 2007 allowed changes in the factors associated with female mowing mortality to be estimated. Adult survival over 5-day-periods was Φ = 0.986, but during the period of mowing female survival was strongly reduced (Φ = 0.946). As a result, 80.6% of the males, but only 68.4% of the females survived the breeding season. Mowing undoubtedly killed two of 20 radio-tagged females when they were laying or incubating. In the 20-year period, an increasing proportion of nests were destroyed before the chicks hatched and this change was associated with an increased distortion of the adult sex ratio. Modelling the population growth rate showed that including the additional effect of mowing on female mortality resulted in a 1.7 times faster local population decline. These results are consistent with the hypothesis that the extinction of whinchat populations in the lowlands of central Europe was caused not only by habitat degradation and low productivity, but also by increased man-made female mortality.     

Remaining large grasslands may not be sufficient to prevent grassland bird declines

Grassland birds are in steep decline throughout many regions of the world. In North America, even some common species have declined by >50% over the last few decades. Declines in grassland bird populations have generally been attributed to widespread agricultural conversion of grasslands; more than 80% of North American grasslands have been converted to agriculture and other land uses, for example. Remaining large grasslands should thus be especially important to the conservation of grassland birds. The Flint Hills of Kansas and Oklahoma (USA) preserves the largest intact tallgrass prairie (∼2 million ha) left in the world. The Flint Hills supports a major cattle industry, however, and therefore experiences widespread grazing and frequent burning. We assessed the regional population status of three grassland birds that are considered the core of the avian community in this region (Dickcissel, Spiza americana; Grasshopper Sparrow, Ammodramus savannarum; Eastern Meadowlark, Sturnella magna). Our approach is founded on a demographic analysis that additionally explores how to model variability in empirically derived estimates of reproductive success across a large heterogeneous landscape, which ultimately requires the translation of demographic data from local (plot) to regional scales. We found that none of these species is demographically viable at a regional scale under realistic assumptions, with estimated population declines of 3-29%/year and a likelihood of regional viability of 0-45% over the two years of study. Current land-management practices may thus be exacerbating grassland bird declines by degrading habitat in even large grassland remnants. Habitat area is thus no guarantee of population viability in landscapes managed predominantly for agricultural or livestock production.     

Post-fledging dispersal, habitat use, and survival of Sprague’s pipits: Are planted grasslands a good substitute for native?

Loss and degradation of native grasslands on the North American landscape are some of the key factors influencing population declines of grassland songbirds. It is unclear what role anthropogenic grasslands play in the conservation of grassland specialists and whether demographic parameters of grassland songbirds differ between anthropogenic and native grasslands. Furthermore, there has been little research examining the overlap (or lack thereof) between nesting and post-fledging habitat. We initiated a radio-telemetry study from 2004 to 2008 on dispersal, microhabitat use, and survival of juvenile Sprague’s pipits (Anthus spragueii), a threatened grassland-obligate songbird, in native pastures and fields planted with exotic species (planted grasslands). Dispersal distance was greater in planted compared to native grasslands, but appeared to be constrained to natal fields up to 26-d post-fledging. Pipits used microhabitats in planted grasslands with vegetation that was on average 11 cm taller than used locations in native pastures, possibly due to the rapid growth of planted vegetation by the time individuals fledged. Individuals that were reared in planted grasslands had consistently lower daily survival rates compared to those in native pastures. Our observation that young birds rarely left their natal field suggests that conserving nesting habitat in native pastures would be an effective management strategy for this species. However, our results suggest that planted grasslands may act as population sinks given the lower survival of juvenile pipits reared in planted fields in our region. Demographic studies during the post-fledging period are essential to determine the conservation value of anthropogenic grasslands for North American songbirds.     

Contrasting measures of fitness to classify habitat quality for the black-throated sparrow (Amphispiza bilineata)

Habitat quality is an important consideration when identifying source and sink habitat and setting priority areas for avian conservation. The problem is that different measures may lead to different conclusions about habitat quality, and may also vary in the resources required to estimate them. Individual level measures, such as nest success, and fecundity, will often identify different high quality habitats than population level measures, such as abundance or the number of fledglings produced per unit area. We tested measures of fitness in the Black-throated Sparrow both at the individual and at the population level for six habitats in the northern Chihuahuan Desert, to explore their value as indicators of habitat quality. We compared clutch size, number of nestlings per nest, number of fledglings per successful nest, nest density, nest success, daily nest survival rate, season-long fecundity, number of fledglings produced per 100 ha, and adult abundance, in each habitat type. We also modeled source-sink dynamics to estimate the scale at which they operate, to infer survival rates, and to ascertain the relative source potential of each habitat. We found that fecundity is the best indicator of individual level habitat quality but a poor indicator of population level habitat quality. Nest success (or fecundity, if resources are available to adequately estimate it) plus nest density provide the most robust indicator of population level habitat quality, which is the level at which priority habitats for conservation should be identified. Mesa grassland and black grama grassland functioned as source habitats most consistently, and mesquite was consistently a sink but also probably a reservoir of individuals available to occupy other habitats.     

Dispersal of the endangered flightless beetle Dorcadion fuliginator (Coleoptera: Cerambycidae) in spatially realistic landscapes

Habitat destruction and degradation are the major causes for the decline of the endangered grass-feeding beetle Dorcadion fuliginator in Central Europe. In the southern part of the Upper Rhine valley (border region of Switzerland, Germany and France) the habitat suitable for this flightless beetle has been reduced to small remnants of extensively managed dry grassland, usually surrounded by intensively cultivated agricultural fields or settlements. Using a mark-release-resight technique we examined movement patterns in three D. fuliginator populations to obtain basic information on the dispersal ability and longevity of this beetle. Estimated daily survival rates ranged from 88.8% to 90.8% in the populations examined. This corresponds to a mean life span of 10.5 days. Distances moved by D. fuliginator differed among populations. The beetles walked the largest distances in the verges of a field track. Several beetles moved distances of 20-100 m along the track, with a maximum distance of 218 m (a male in 12 days). The shortest displacements were recorded in the bank of the river Rhine, a narrow habitat surrounded by tarmac roads. We also assessed the spatial arrangement of 12 patches with D. fuliginator populations in two regions and estimated the size of each population over 4 years. Data on dispersal, daily survival, population size and spatial arrangement of patches were used to simulate patch-specific migration rates. The simulations suggested that in both areas the beetles regularly moved between neighbouring patches separated by distances shorter than 100 m, whereas patches separated by distances exceeding 500 m are isolated.     

Management recommendations based on matrix projection models: The importance of considering biological limits

Matrix population models are a common tool for evaluating different management strategies. In general, under deterministic analyses, management strategies are recommended that improve those matrix transitions that are most sensitive or elastic with respect to the asymptotic population growth rate, λ. These recommendations usually ignore the biological limit for these transitions. In this paper we use the endangered Serengeti cheetah (Acinonyx jubatus) as a case study to illustrate that ignoring biological limits leads to a recommendation that will not always achieve the desired goal of an asymptotic population growth rate, λ ? 1.We estimate the survivorships of adult cheetahs in captivity using cheetah studbook data, which is a conservative estimate of the biological limit for the adult survivorship of wild cheetahs. Our analysis suggests that adult survival sharply decreases after 8.5 years. In addition, captive cheetahs older than 18 years do not reproduce. We modify a previously published population projection matrix to include the effect of senescence on survival and fecundity. Our model suggests that increasing adult survivorship alone is not sufficient to reverse population decline. However, an additional small increase in newborn survival is likely to result in a viable cheetah population. We show these conclusions hold even in the presence of relatively large parameter perturbations.     

Viability of mountain caribou in British Columbia, Canada: Effects of habitat change and population density

Population viability analyses (PVA) are frequently employed to develop recovery plans and inform management of endangered species. Translating results from PVA into meaningful management recommendations often depends on an understanding of how population parameters change with environmental conditions as well as population density. The decline of mountain caribou (Rangifer tarandus caribou) in British Columbia, Canada, is believed to be caused by apparent competition with alternative prey species following changes to the forest age structure from timber harvest and wildfire. In addition, populations have been shown to decline at faster rates at low population density. To evaluate the potential effects of habitat change and population density on population persistence, we used stochastic projection models for 10 distinct populations varying in initial size from <10 to approximately 150 females. In an initial model, we used estimates of vital rates based on information sampled from >350 radiocollared caribou between 1984 and 2004. We then compared the results of the initial model to a set of models that evaluated the effects of habitat conditions and population density via their expected relationships to female adult survival. Assuming that vital rates remain constant over a 200-year time frame, only three populations have high probabilities (>0.95) of extinction. When models incorporate the declines in adult female survival know to occur with increasing proportions of young forest and declining population densities, all 10 populations are predicted to decline to extinction within <200 years. Based on our results, we suggest that PVA models that fail to incorporate the effects of changes in vital rates with habitat and population density may lead to overly optimistic assessments of the probability of population persistence in endangered species.     

Successful reintroductions of the endangered long-lived Sargent’s cherry palm, Pseudophoenix sargentii, in the Florida Keys

The 1991-1994 reintroductions of Florida endangered Pseudophoenix sargentii to 13 Florida Keys sites represent a rare example of a successful multi-agency long-term effort to conserve a long-lived palm. To assess reintroduction success, we compared population demographics with and without reintroduced plants and conducted population viability analyses. Since 1991, the wild population has increased 6.4-fold. Survival from 2000-2004 was 94%, growth was positive (λ = 1.013), and there was no predicted extinction risk. Recent wild population growth is attributed to good seedling recruitment and removing the greatest threats. After 14 years, reintroductions had 43% survival, increased total plants in the wild by 27%, and expanded the species’ distribution. Reintroduced plants had faster maturation rates, improved population age structure, and enhanced population growth (λ = 1.032). Success varied with transplant year, location, microsite, and original transplant size. Failures in 1991 and at some historic sites emphasize the need for a multi-year, multi-site approach to reintroductions to buffer against stochastic losses. Rockland hammocks and the tops of coastal berms had greatest plant growth and survival. Large transplants had the greatest survival. Because no reintroduced plants are reproductive, transitions between stages are extremely slow, and plants may require >30 years to mature, continued institutional dedication to long-term monitoring will be required to assess whether the populations are self-sustaining. Horticultural expertise and ex situ collections complimented support of land managing agencies for the species’ preservation. These first rare plant reintroductions to Florida State Parks opened avenues for more plant conservation efforts and public interpretation.     

The effectiveness of conservation measures to enhance nest survival in a meadow bird suffering from anthropogenic nest loss

The main reason for meadow bird declines is supposed to be the intensification of grassland management with earlier first harvest dates and more frequent harvests, resulting in high nest destruction rates. To increase productivity of meadow bird populations in intensified grassland areas a delay of mowing date and individual nest protection measures have been proposed. However, for ground-nesting songbirds such as the whinchat (Saxicola rubetra) the effectiveness of such measures remains widely unknown. In particular, if nest predation rate is high, measures to protect nests from agricultural destruction alone might be questionable. Here, we quantify whinchat nest survival of (1) unprotected nests situated in early mown meadows, (2) protected nests situated in early mown meadows, and (3) nests situated in late mown meadows. Analyses considered the fact that successful and unsuccessful nests are not found with equal probabilities. Periods of reduced nest survival were associated with mowing periods in the different types of meadows. In early mown meadows nest survival rates were low (S < 0.1), and both conservation measures, individual nest protection and delayed mowing, resulted in significantly increased nest survival rates (S > 0.7). Individual nest protection cannot avoid changes in habitat quality of intensively managed meadows, and therefore is only suitable as small-scale and short-term measure to increase nest success until a high portion of late mown meadows is established. Thus, we suggest that a combination of the two measures applied to intensified grassland fields should be provided to maintain viable sizes of endangered meadow bird populations.