Differences In Population Trends Research Articles

Recent Population Trends of Mountain Goats in the Olympic Mountains, Washington

Mountain goats (Oreamnos americanus) were introduced in Washington's Olympic Mountains during the 1920s. The population subsequently increased in numbers and expanded in range, leading to concerns by the 1970s over the potential effects of non-native mountain goats on high-elevation plant communities in Olympic National Park. The National Park Service (NPS) transplanted mountain goats from the Olympic Mountains to other ranges between 1981 and 1989 as a means to manage overabundant populations, and began monitoring population trends of mountain goats in 1983. We estimated population abundance of mountain goats during 18–25 July 2011, the sixth survey of the time series, to assess current population status and responses of the population to past management. We surveyed 39 sample units, comprising 39% of the 59,615-ha survey area. We estimated a population of 344 ± 72 (90% confidence interval [CI]) mountain goats in the survey area. Retrospective analysis of the 2004 survey, accounting for differences in survey area boundaries and methods of estimating aerial detection biases, indicated that the population increased at an average annual rate of 4.9% since the last survey. That is the first population growth observed since the cessation of population control measures in 1990. We postulate that differences in population trends observed in western, eastern, and southern sections of the survey zone reflected, in part, a variable influence of climate change across the precipitation gradient in the Olympic Mountains.

Population trends of birds across the iron curtain: Brain matters

One approach to assess human impact on species’ population dynamics is to correlate ecological traits of species with their long-term population trends. Yet, few studies investigated population trends in multiple regions that differ in human impact to reveal which traits explain population trends over larger geographic areas and which only regionally. We examined the relationship between various species traits and long-term population trends of 57 common passerine bird species from 1991 to 2007 in three adjacent regions in central Europe that experienced differences in socioeconomic history: North-Western Germany, Eastern Germany and the Czech Republic. We tested effects of habitat, dietary and climatic niche, migratory strategy and cognitive ability, measured as relative brain size. We predicted that traits reflecting socioeconomic and land-use change had stronger effects in former communist countries than in North-Western Germany due to marked changes in these countries after 1990. We found that climatic niche and migratory strategy affected bird abundances similarly in all regions suggesting their influence is invariant across central Europe. In contrast, brain size showed regionally varying effects. The effects were negligible in North-Western Germany, slightly positive in Eastern Germany and strongly positive in the Czech Republic. Increases of species with large brains suggest that species with good cognitive abilities might have been better able to adapt to rapid socioeconomic change and make use of novel opportunities after the end of communism. Regional differences in population trends among species thus appear to be driven by an interaction between cognitive abilities and socioeconomic change.

Assessing Long-Term Population Trends of Wood Frogs Using Egg-Mass Counts

In North America, most efforts to monitor pond-breeding anurans have focused on call surveys. Egg-mass counts offer an alternative monitoring strategy that has been used extensively in Europe because this technique can produce precise and accurate estimates of annual reproductive effort at many study sites. We surveyed egg masses of Wood Frogs (Lithobates sylvaticus) at 18 ponds for up to 16 years from 1993–2008 in the largest contiguous forest tract in southern New England. We detected an average of 441.5 ± 343.7 egg masses per pond. Based on annual egg-mass counts, coefficients of variation (CV) were slightly higher than previous estimates for this species. We detected no relationship between mean annual population size and CV or between length of time series and CV. Population fluctuations in these ponds exhibited evidence of annual synchrony, in part because annual fluctuations at individual ponds were large enough that it was difficult to assess differences in population trends among ponds. However, the overall trend suggests this population was probably increasing slightly, which was expected because ponds were located in contiguous forest that remained intact during the study. Egg-mass counts appear to represent a feasible technique to monitor Wood Frog populations, given that all local breeding ponds are monitored.

Life-history and ecological correlates of population change in Dutch breeding birds

Predicting relative extinction risks of animals has become a major challenge in conservation biology. Identifying life-history and ecological traits related to the decline of species helps understand what causes population decreases and sets priorities for conservation action. Here, we use Dutch breeding bird data to correlate species characteristics with national population changes. We modelled population changes between 1990 and 2005 of all 170 breeding bird species using 25 life-history, ecological and behavioural traits as explanatory variables. We used multiple regression and multi-model inference to account for intercorrelated variables, to assess the relative importance of traits that best explain interspecific differences in population trend, and to identify the environmental changes most likely responsible. We found that more breeding birds have increased than decreased in number. The most parsimonious models suggest that ground-nesting and late arrival at the breeding grounds in migratory birds are most strongly correlated with decline. Increasing populations are mainly found among herbivores, sedentary and short-distance migrants, herb- and shrub-nesting birds and large species with a small European range. Declines in ground-nesting and late arriving migrant birds suggest that agricultural intensification, eutrophication and climate change are most likely responsible for changes in Dutch breeding bird diversity. We illustrate that management strategies should primarily focus on the traits and causes responsible for the population changes, in order to be effective and sustainable.

Bird-habitat associations predict population trends in central European forest and farmland birds

Recent studies show differences in population trends between groups of species occupying different habitats. In Czech birds, as well as in many other European countries, populations of forest species have increased, whereas populations of farmland species have declined. The aim of our study was to test whether population trends of particular species were related to finer bird-habitat associations within farmland and forest birds. We assessed bird-habitat associations using canonical correspondence analysis based on data from a 400 km long transect across the Czech Republic. We calculated population trends of 62 bird species using log-linear models based on data from a large-scale annual monitoring scheme, which covers the time series from 1982 to 2005. Within forest birds, species with a closer association with lowland broad-leaved forest have had more positive population trends, whereas species with a closer association with montane and coniferous forest revealed more negative population trends. We attribute these opposite trends to the gradual replacement of coniferous forests by deciduous ones, which took place in the Czech Republic during recent decades. Our analyses revealed a hump-shaped relationship within farmland birds, species most closely associated with farmland habitat revealing the most negative trends, whereas species with intermediate association to farmland habitat showed the most positive population trends. Such a pattern can be explained by the abandonment of previously cultivated areas followed by the spread of unmanaged meadows and scrubland. Changes in quantity or quality of preferred habitats may thus represent major drivers of observed bird population changes.

Interspecific differences in population trends of Spanish birds are related to habitat and climatic preferences

ABSTRACTAim Animal monitoring programmes have allowed analyses of population trends, most of which now comment on the possible effect of global climate change. However, the relationship between the interspecific variation in population trends and species traits, such as habitat preferences, niche breadth or distribution patterns, has received little attention, in spite of its usefulness in the construction of ecological generalizations. The objectives of this study were: (1) to determine whether there are characteristics shared among species with upwards or downwards trends, and (2) to assess whether population changes agree with what could be expected under global warming (a decrease in species typical of cooler environments).Location The Spanish part of the Iberian Peninsula (c. 500,000 km2) in the south‐western part of the Mediterranean Basin.Methods We modelled recent breeding population changes (1996–2004), in areas without apparent land use changes, for 57 common passerine birds with species‐specific ecological and distributional patterns as explanatory variables.Results One‐half of these species have shown a generalized pattern towards the increase of their populations, while only one‐tenth showed a significant decrease. One half (54%) of the interspecific variability in yearly population trends is explained considering species‐specific traits. Species showing more marked increases preferred wooded habitats, were habitat generalists and occupied warmer and wetter areas, while moderate decreases were found for open country habitats in drier areas.Main conclusions The coherent pattern in population trends we found disagrees with the proposed detrimental effect of global warming on bird populations of western Europe, which is expected to be more intense in bird species inhabiting cooler areas and habitats. Such a pattern suggests that factors other than the increase in temperature may be brought to discussions on global change as relevant components to explain recent changes in biodiversity.

Demographic independence along ecosystem boundaries in Steller sea lions revealed by mtDNA analysis: implications for management of an endangered species

Previous genetic studies indicate Steller sea lions ( Eumetopias jubatus (Schreber, 1776)) comprise three phylogeographically distinct populations. However, differences in population trends and ecology and the limited extent of recorded dispersal suggest structure may be present at smaller scales. We examined sequence variation within a longer segment (531 bp) of the mtDNA control region in greater numbers (n = 1654) of sea lions from across Alaska than earlier investigations to investigate fine-scale dispersal patterns in Steller sea lions. We detected high levels of haplotypic diversity (h = 0.934) and confirmed phylogeographic differentiation between southeastern and western Alaska (Φst= 0.23, P < 0.0001), but also found significant differentiation at regional and local scales. Rookeries in the Gulf of Alaska, eastern Bering Sea, and eastern Aleutians were distinct from rookeries in the central and western Aleutians (Fst= 0.021, P < 0.0001; Φst= 0.017, P < 0.0001). The location of this split coincides with an oceanographic divergence between continental shelf and ocean basin waters and with differences in sea lion foraging ecology and population trends. A number of rookeries were also significantly differentiated from nearby rookeries (Fst= 0.02–0.025, P < 0.05), signifying substantial female-mediated philopatry, in some cases, at local scales. These findings have important implications for understanding the ecology of Steller sea lions in relation to marine ecosystems and the causes of population declines, and they provide guidance for management, including the identification of management stocks.

Population Dynamics of Greater Scaup Breeding on the Yukon-Kuskokwim Delta, Alaska

Abstract ABSTRACT Populations of greater scaup (Aythya marila) remained relatively stable during a period when populations of lesser scaup (A. affinis) have declined from historic levels. To assist in describing these differences in population trends, from 1991 through 2000, we studied the survival, nesting ecology, and productivity of greater scaup on the Yukon-Kuskokwim Delta (Y-K Delta), Alaska, to develop a model of population dynamics. We located nests, radio-marked females for renesting studies, estimated duckling survival, and leg-banded females to examine nest site fidelity and annual survival. Greater scaup initiated egg laying later than other species, and most clutches (>80%) were initiated over 20 days each year. We located 1,056 nests; nest success ranged from 7 to 61% among years. Following loss of their first clutch, 51% of radio-tagged females attempted to renest. Duckling survival to 30 days of age was 37.5%. Our best model suggested that annual survival did not vary among years and avera...

Time to extinction in relation to mating system and type of density regulation in populations with two sexes

Summary Population viability models usually consider only the female segment of the population. However, random variation in sex ratio as well as the mating system may also affect variation in fitness among females. Here we develop population models incorporating demographic stochasticity in both sexes. Furthermore, we consider the effects on the estimated time to extinction and whether density regulation acts only on females or on total population size. We applied these models to two populations of polygynous great reed warblers Acrocephalus arundinaceus L. with differences in population trends to investigate the importance of considering sex in population viability models. Demographic stochasticity was larger in a polygynous than in a monogamous mating system. The estimated time to extinction was considerably shorter for a monogamous than for a polygynous mating system, particularly if density regulation acted only on females than rather on the total population. This study demonstrates that structure of mating system must be included when making population viability analysis based on counts of total population sizes. It is especially important to model the specific effects of density regulation on the two sexes.

Dynamics of hunted and unhunted mountain goat Oreamnos americanus populations

Native populations of mountain goats Oreamnos americanus are sensitive to harvest. To assess the potential effects of limited hunting on population dynamics, we analysed long‐term data obtained from aerial counts of 12 native mountain goat herds in Alberta, Canada, during 1973–2001. Seven herds were hunted until 1987 and five were not hunted. Despite a decrease in the number of permits issued, mountain goat numbers declined in most hunted herds between 1980 and 1983. Hunting was closed in 1987. Only three of seven herds increased after hunting was closed. Unhunted herds also showed substantial among‐herd differences in population trends. Our results suggest that factors other than sport hunting contributed to the population decline. Future harvests should target adult males, but the adult sex ratio of one intensively studied population was heavily biased in favour of females. A herd of 100 goats may only sustain the harvest of 1–2 adult males per year.

A demographic comparison of two southern elephant seal populations

Summary To estimate concurrent age‐specific survival for southern elephant seals at Macquarie and Marion islands, seals were marked from 1993 to 1997 in the first 3 weeks of life and resighted (recaptured) on return to their natal islands (1994–2001). These recaptures formed the basis for the survival analysis in the mark–recapture program mark. Weaning masses were collected at each location. Recapture probabilities were (= 376·480, P < 0·0001) higher at Marion Island than at Macquarie Island. There are two possible reasons: (1) the population at Marion Island is smaller and less dense than at Macquarie Island and (2) seals hauled out along a smaller section of the coast at Marion Island than at Macquarie Island, which: (1) facilitates the detection and individual identification of seals and (2) increases access to hauled out seals. Age‐specific survival estimates (corrected for preweaning mortality and tag loss) differed ( = 22·264, P < 0·05) at the two islands and were consistently higher at Macquarie Island. The survival estimates for male and female seals were different at Macquarie Island (= 34·657, P < 0·0001) and Marion Island (= 20·373, P = 0·002). Female survival estimates were higher than male survival estimates. The combined survival estimates for juvenile seals (1–3 years) differed between islands but survival of older seals (4–6 years) did not. The inclusion of gender in the survival models did not improve model performance and hence male and female estimates were considered jointly. The mean wean masses of male and female seals combined from 1993 to 1998 were not different between islands (T6837 = 1·169, P = 0·242). At Macquarie Island the mean annual wean mass was 118·8 kg (SD = 27·2, n= 6504) while at Marion Island it was 120·6 kg (SD = 24·7, n= 335). The mean age at first breeding was different (P < 0·001) at the two island populations. At Macquarie Island the mean age of first breeding was 4·68 years, and at Marion Island it was 3·95 years. More (= 67·39, P < 0·0001) 3‐year‐old females breed at Marion Island (28·7%) than at Macquarie Island (1·2%) and the proportion of seals that had bred at least once by age 7 was greater at Marion Island than at Macquarie Island. We conclude that the observed decreases in elephant seal numbers between the 1950s and 1990s in the Pacific and Indian Ocean sectors were driven principally by resource limitation in the Southern Ocean. A conglomerate of factors including local predation by killer whales and intraspecific resource competition is postulated as a cause for the inter‐island (regional) differences in population trends. It appears that more resources are available per capita to the Marion Island population than are available to the Macquarie Island population.

The Effects of Illegal Killing and Destruction of Nests by Humans on the Population Dynamics of the Hen Harrier Circus cyaneus in Scotland

Breeding productivity, natal dispersal and survival of hen harriers Circus cyaneus were studied between 1988 and 1995 on moorland managed for sport shooting of red grouse, other heather moorland and young conifer forests in the uplands of Scotland. Nest success was much lower on grouse moors than on other land management classes. Annual productivity was 0.8 fledglings breeding female -1 year -1 on grouse moors compared with 2.4 on other moorland and 1.4 in young conifer forests. Human interference was recorded on half of the grouse moor estates studied and accounted for at least 30% of breeding failures in this land management class. It was much less frequent in the other land management classes. Annual survival of female hen harriers which bred on grouse moors was about half that of females breeding on other moorland. On grouse moors, survival of females which bred unsuccessfully was much lower than that of females which reared at least one fledgling. Survival of breeding females on other moorland was high and unrelated to breeding success. The difference in survival of breeding females between grouse moors and other moors was attributed to killing by humans. On average, 55-74 females were killed each year, 11-15% of the total population of breeding females in Scotland, excluding Orkney. The population of breeding females on grouse moors was estimated to decline rapidly without immigration. Harriers breeding on the other habitats were producing a surplus of female recruits approximately sufficient to compensate for the losses on grouse moors. Most females started to breed at I year old and most males at 2 years old. The percentage of breeding males which were I year old was higher on grouse moors than on the other land management classes. The median natal dispersal distance of both sexes exceeded 10 km. Harriers fledged from one land management class were often found breeding in another. Natal dispersal resulted in net movements of 1-year-old females between land management classes which were sufficient to reduce the differences in population trend which would otherwise have occurred. Moorland managed for grouse shooting was a sink habitat which received two-thirds of its female recruits from other habitats. The difference in productivity and survival between grouse moors and other habitats was attributed to illegal human interference. It is speculated that, without persecution, the hen harrier population in Scotland would increase, initially by about 13% year -1 , until a new, but unknown, equilibrium level was reached.

Use of rodeo® and X‐77® spreader to control smooth cordgrass (Spartina alterniflora) in a southwestern washington estuary: 2. Effects on benthic microflora and invertebrates

AbstractIn August 1992, we conducted an intensive short‐term (within 119d)experiment in southern Willapa Bay, Washington, to evaluate the potential effects on mudflat benthic communities of herbicide control of smooth cordgrass, Spartina alterniflora Loisel. A mixture of glyphosate (Rodeo®; 4.7 L ha−1) and an associated surfactant, alkylarylpolyoxyethylene (AAPOE, X‐77® Spreader; 1 L ha−1) was applied aerially to three mudflat sites with invasive S. alterniflora. Sediment structure (grain size), edaphic microalgal biomass (chlorophyll a), and densities of benthic and epibenthic meiofauna and benthic macrofauna were sampled systematically in treated and adjacent control (untreated) plots 1 d before, immediately after, and 1, 14, 28 and 119 d after spraying. These mudflat biota showed no definitive differences in population trends that would indicate acute responses to the herbicide and surfactant applications over the 119‐d duration of the experiment. Two‐way ANOVA tests of differences in slope of linear regressions of mean plot microalgal biomass and invertebrate density of 19 taxa groups or species testing short‐term (2 weeks) and long‐term (17 weeks) trends in response to the experimental treatment tests indicated no significant (p < 0.1) treatment and only three site effects. Natural variability in the standing stocks (in the case of benthic microalgae) or densities (invertebrates) of most of the 19 indicator taxa prior to spray application was sufficiently high within and between treatment and control plots and among sites to preclude strong inferential tests of acute effects. Although differences in mudflat habitats (e.g., tidal elevation, sediment structure) inherent in the sites prior to treatment affect the power of our ability to test direct effects, there were no indications of either short‐ or long‐term effects on the mudflat community of aerially applying this concentration of herbicide and surfactant. This study did not explicitly address either sublethal or indirect ecological effects, such as associated with an observed decrease in the exotic eelgrass Zostera japonica, which might appear as a longer‐term, more subtle response by the mudflat community.

Distribution, habitat and conservation status of the Green and Golden Bell FrogLitoria aurea(Lesson 1829) (Anura: Hylidae) in Victoria

An overview of the distribution of Litoria aurea in Victoria is presented. Habitats in which the species has been recorded are described and its current conservation status is assessed. Litoria aurea is restricted to east Gippsland where it occurs predominantly at low altitudes along the coast and hinterland. The species is usually associated with stationary water bodies, mostly permanent, in both forested and cleared habitats. The species has also been found in a range of terrestrial habitats, often considerable distances from waterbodies; suggesting a high dispersive ability and importance of terrestrial habitats. Little evidence of a decline in the distribution or abundance of L. aurea was found based on comparisons of historical and recent information. Much of the habitat and range of the species was found to be secure within the region. Possible reasons for the apparent differences in population trends between New South Wales and Victoria are discussed.

Population Trends of Common British Butterflies at Monitored Sites

1. In recent decades, many of the common and widespread butterflies in Britain which do not already occupy the whole of the country have expanded in range. 2. Populations of the species which have expanded in range, monitored for varying periods during 1974-92 have, overall, shown significantly more increases than declines in abundance, as have the species with ranges which already occupy most of Britain. 3. Nearly all of the common species have increased in abundance more in the east of Britain than in the west. 4. Although the reasons for recent range expansions and increases in abundance of common butterflies are not known, it seems likely that changes in weather have played a role. 5. It is argued that the geographical differences in population trends suggest that a factor (or factors) with a similar eastern bias is also implicated in the changes in abundance and perhaps of range. Some possible factors are discussed briefly

Demography of an Increasing Population of Sparrowhawks

SUMMARY (1) The recolonization of an area in east-central England by sparrowhawks Accipiter nisus (L.) was studied after the local population had been eliminated, almost certainly by organochlorine pesticides. Over a 10-year period, nest numbers increased rapidly to begin with and then more slowly, with a mean rate of 21% per year. (2) Over the whole 10 years, the estimated annual survival of breeders was at least 76% for males and 74% for females, and mean productivity was 2*1 young per nest. Throughout, the breeding population contained a large proportion of first-year birds, with an overall figure of 35% of all male breeders and 25% of all female breeders. As the breeding population grew, young raised in the area tended to disperse ever greater distances from their natal sites. (3) Comparison of the increasing population with stable and decreasing populations in southern Scotland showed that differences in population trend were associated mainly with differences in the recruitment of new breeders (greatest in the increasing and lowest in the decreasing population) and in age of first breeding (earliest in the increasing and latest in the decreasing population). There were also differences in dispersal distances (shorter in the increasing population), and in the annual survival of breeders (greater in the increasing population). In contrast, differences in breeding success between areas were slight, and non-significant.

Grass grub (Costelytra zealandica) population collapse in the northern North Island

Abstract Annual surveys of grass grub populations in early autumn from 1979 to 1982 at Tihoi near Lake Taupo and at Waingaro in the western Waikato have revealed that populations in old pastures (at least 7 years old) increased to a peak by 1979 or 1980 and subsequently declined naturally to low levels. Populations in newly sown pastures increased to a peak in the second or third autumn after sowing and then declined. Application of lindane at Tihoi in 1979 reduced grass grub numbers by an average of 840/0 in the year after treatment, but 3 of the 5 treated populations have since increased to higher levels than the untreated. A life table study of the 1981-82 grass grub generation at Tihoi showed that variation in summer mortality was responsible for differences in population trend. Weather, pasture composition, gcazing management, depletion of the food supply, and predation did not appear to be major causes of summer mortality. This mortality was not significantly related to population density on a conti...

Intraspecific variation in food plant favourability to phytophagous insects: psyllids on Eucalyptus blakelyi M.

Abstract. A combination of variables reflecting leaf quality provided the best predictive model explaining differences in population trend of Glycaspis spp. among individuals of the food plant Eucalyptus blakelyi, measured under field conditions on an egg to egg basis. The equation incorporated the product of: (1) proportion of foliage in each age class; (2) probability of foliage escape from herbivory; (3) preference for the foliage class measured as the proportion of eggs oviposited on it by the female; these variables being summed over the young and mature age classes. In addition, significant and consistent differences in insect survivorship among trees were demonstrated by transfer experiments. These experiments established that young foliage was preferable to mature foliage for first instar survival. Differences in insect survivorship also showed a consistent pattern among the study trees. The most probable explanation for this consistent pattern was the influence of the trees on the insects, particularly physiological or biochemical differences among trees. It was not, however, possible to identify the cause of the differences.