Changes In Population Abundance Research Articles

Large mammal population declines in Africa’s protected areas

Protected areas (PAs) are the cornerstone of global conservation efforts but their performance in maintaining populations of their key species remains poorly documented. Here, we address this gap using a new database of 583 population abundance time series for 69 species of large mammals in 78 African PAs. Population abundance time series were aggregated to form a multi-species index of overall change in population abundance. The index reveals on average a 59% decline in population abundance between 1970 and 2005. Indices for different parts of Africa demonstrate large regional differences, with southern African PAs typically maintaining their populations and western African PAs suffering the most severe declines. These results indicate that African PAs have generally failed to mitigate human-induced threats to African large mammal populations, but they also show some successes. Further development of our index could help to measure future progress towards post-2010 targets for reducing biodiversity loss.

Resource levels, allometric scaling of population abundance, and marine phytoplankton diversity

We analyzed the relationship between population abundance and cell size in phytoplankton assemblages from coastal, shelf, and open‐ocean environments. Our results show that across the entire size spectrum considered, population abundance increases over two orders of magnitude from subtropical to coastal regions. We find a highly significant linear relationship between nutrient concentration and the intercept of the log‐log relationship between population abundance and cell size. In contrast to overall patterns reported mainly for vascular plants and animals, marine phytoplankton diversity does not show any consistent trend along either latitudinal or productivity gradients. These results imply that large‐scale (biogeographic) variations in phytoplankton standing stocks are controlled by changes in population abundances rather than by systematic variations in species richness. These findings provide a mechanistic connection among nutrient availability, population dynamics, and phytoplankton diversity over macroecological scales.

Ecosystem and competition impacts of introduced deer

Numerous deer species have been introduced beyond their native range into ecosystems around the world. Their economic value leads to further accidental and deliberate releases and lack of control is contributing to range expansion in Australia, South America and Europe. Despite localised or regional concern, the scale and generality of detrimental impacts have not been widely recognised. We review the direct and indirect impacts on ecosystems and evidence for interspecific effects on native deer. In New Zealand, where large herbivores were previously absent, severe and novel impacts have been found in susceptible forests. Even where ecosystems contain native deer, invasion by taxonomically exotic deer species carries the risk of cascade effects on spatial plant dynamics and forest composition. In Patagonia, introduced deer have disrupted forest composition, whereas in Europe, ecosystem impacts of introduced species can differ from those of over-abundant native deer. Introduced Chinese muntjac (Muntiacus reevesi) within a coniferous forestry landscape in eastern England differ from native European roe deer (Capreolus capreolus) in their distribution of herbivory among differing habitats, and provide much lower rates of endozoochorous seed dispersal. Frequent concern is expressed that introduced deer species may have detrimental effects on native deer and other ungulates, although potential epidemiological effects have not been investigated. Apparent competition, with introduced prey resulting in increased predation rates on native deer, may be occurring between South American huemul (Hippocamelus bisulcus) and southern pudu (Pudu puda). Habitat and dietary overlap is often substantial among native and introduced ungulates, including deer, and exploitation competition is likely. Evidence includes spatial responses of native to non-native deer and negatively correlated changes in population abundance, but demographic mechanisms have not been demonstrated previously. In a coniferous forestry landscape in eastern England, substantial habitat and dietary overlap occurs between native roe deer and high-density introduced Chinese muntjac. This roe deer population has shown a reduction in body weight and fertility following establishment and increasing abundance of non-native Chinese muntjac, compatible with interspecific competition. European roe deer also appear susceptible to competition from larger grazing deer, including native red deer (Cervus elaphus) and introduced fallow (Dama dama). The widely introduced fallow deer may be a particularly effective competitor in sympatry with intermediate or concentrate feeders. There is need for further investigation of potential interactions of introduced and native deer species, and a wider recognition of the ecological impacts of introduced deer.

Cladocera and copepoda fauna of Yesil Lake (Antalya, Turkey) and seasonal changes in population abundance.

Cladocera and copepoda fauna of Yesil Lake (Antalya, Turkey) and seasonal changes in population abundance.

Cryptic Population Dynamics: Rapid Evolution Masks Trophic Interactions

Trophic relationships, such as those between predator and prey or between pathogen and host, are key interactions linking species in ecological food webs. The structure of these links and their strengths have major consequences for the dynamics and stability of food webs. The existence and strength of particular trophic links has often been assessed using observational data on changes in species abundance through time. Here we show that very strong links can be completely missed by these kinds of analyses when changes in population abundance are accompanied by contemporaneous rapid evolution in the prey or host species. Experimental observations, in rotifer-alga and phage-bacteria chemostats, show that the predator or pathogen can exhibit large-amplitude cycles while the abundance of the prey or host remains essentially constant. We know that the species are tightly linked in these experimental microcosms, but without this knowledge, we would infer from observed patterns in abundance that the species are weakly or not at all linked. Mathematical modeling shows that this kind of cryptic dynamics occurs when there is rapid prey or host evolution for traits conferring defense against attack, and the cost of defense (in terms of tradeoffs with other fitness components) is low. Several predictions of the theory that we developed to explain the rotifer-alga experiments are confirmed in the phage-bacteria experiments, where bacterial evolution could be tracked. Modeling suggests that rapid evolution may also confound experimental approaches to measuring interaction strength, but it identifies certain experimental designs as being more robust against potential confounding by rapid evolution.

Population responses of small and medium sized mammals to fox control at Jervis Bay, southeastern Australia

At Jervis Bay, strategic pest programmes to control the Red Fox Vulpes vulpes have been in place on the Beecroft Weapons Range (BWR) Beecroft Peninsula, New South Wales since 1995 and Booderee National Park (BNP) Bherwerre Peninsula, Jervis Bay Territory since 1999. As an integral component of the BWR plan, monitoring terrestrial and arboreal mammals has been carried out and demonstrated a significant increase in the abundance of Common Ringtail Possum Pseudochelrus peregrinus, Long-nosed Bandicoot Perameles nasuta, and Bush Rat Rattus fuscipes. There was no significant change in the abundance of Brown Antechinus Antechinus stuartii, Sugar Glider Petaurus breviceps or Black Rat (R. raltus). At BNP changes in population abundance of mammals were not monitored, but a comparison of the abundance of terrestrial mammals between the fox baited BNP and the adjacent un-baited Jervis Bay National Park (JBNP) showed Long-nosed Bandicoots, Bush Rat and Brown Antechinus to be in higher abundance in BNP than JBNP. Scat analysis further supported this finding (Roberts et aI, 2006), We assert that these combined findings are evidence that terrestrial native fauna have responded positively to continued fox control on both peninsulas, thus proving that measuring prey responses are a positive measure of success for strategic pest control programmes.

Dramatic decline in population abundance of Salvelinus leucomaenis after a severe flood and debris flow in a high gradient stream

A dramatic change in population abundance of Salvelinus leucomaenis following a catastrophic flood event was investigated. Population density declined by c. 98% after the flood in a river section affected by debris flow. The isolation of the affected population and habitat change caused by flooding may have impeded population recovery.

The use of angler diary surveys to evaluate long-term changes in muskellunge populations on Lake of the Woods, Ontario

Reported improvements in the muskellunge angling fishery on Lake of the Woods, Ontario over the last two decades have paralleled an increase in the practice of catch and release angling, and an increase in minimum size regulations for this species. The overall status of muskellunge populations in this large, complex lake has proven difficult to monitor using standard assessment methods. A volunteer muskellunge angler diary program, established in 1988, has provided a cost effective method of gathering a large amount of information with which to track this fishery and associated populations. Results from angling diaries indicated increased angling effort, catch and success rates for muskellunge on Lake of the Woods since the early 1990s. Although the month of July accounted for the majority of angling effort and catch, angling success rates and sizes of fish reported in diaries improved monthly into the late fall. Angling success rates were consistently higher in angling diaries than from creel surveys, but both survey types showed similar long-term trends in the fishery. Angler diary data, incorporating both the numbers of fish caught and/or seen by anglers, were used to calculate catch equality indices which proved to be sensitive to changes in population abundance. Increased minimum length regulations for muskellunge during 1987–2001 have been largely responsible for a decline in harvest rates from an estimated 36% in 1986 to 0% since 1999. Although higher size limits have yet to produce more quality-sized fish in angler catches, diary survey data, supported by recent improvements in catch rates from assessment gill nets, would indicate that muskellunge recruitment has increased. This article concludes with a brief review of how muskellunge angler diary data has been used in the past, including recommendations to minimize biases associated with this survey method.

MANIFOLD INTERACTIVE INFLUENCES ON THE POPULATION DYNAMICS OF A MULTISPECIES UNGULATE ASSEMBLAGE

The dynamics of animal populations can be influenced directly by prevailing resources, population density, and environmental conditions, and through the delayed effects of trophic interactions and abiotic effects on habitat conditions. For large mammals, lagged effects can extend several years back in time. We attempted to establish the causal processes governing the population changes shown by 11 ungulate species counted annually over a 20-year period in South Africa's Kruger National Park. Kudu, waterbuck, warthog, sable, tsessebe, roan, and eland declined progressively in abundance after 1986, while zebra, wildebeest, impala, and giraffe maintained high abundance levels. To identify lagged influences, we used statistical probes indexing (1) inferred changes in predator abundance as a consequence of past food availability, (2) effects on habitat conditions of prior rainfall, and (3) competitor impacts on shared food resources. Multiple linear regression models were fit to estimates of annual population growth derived from the count totals subdivided among four regions. The temporal pattern of the population declines by five species was most consistent with a lagged effect from past predator food, in addition to the direct effects of seasonal rainfall. However, models including the lagged effect of prior rainfall fit nearly as well. Species that maintained high abundance responded mainly to an immediate or lagged density feedback. Changing rainfall conditions apparently affect the relative susceptibility of ungulate species to predation. Hence, the top-down interaction with predators cannot readily be disentangled from extrinsic influences on population dynamics, mediated through resources. Population declines by some species became extreme because a prolonged period of low rainfall, especially in the dry-season component, followed a doubling in the food base supporting lions, and was coupled with widened prey distribution as a consequence of the augmentation of water points by managers. Changes in population abundance within this multi-prey, generalist predator system arose from a complex interplay between changing climatic conditions, variable food production, shifting habitat conditions, varying vulnerability to predation, and spillover effects on other species.

Habitat selection by juvenile Atlantic salmon: the interaction between physical habitat and abundance

The effect of physical river habitat variables on the distribution of juvenile Atlantic salmon Salmo salar L. in the Rivière de la Trinité, Québec, Canada, was examined using generalized additive modelling. A survey of Atlantic salmon fry and parr densities and habitat variables (flow velocity, water column depth and substratum size) was conducted in the summer months from 1984 to 1992. Clear patterns of habitat use existed: specific ranges of habitat variables were selected, with parr preferring greater velocities, depths and substratum sizes than fry. There was a large variation, however, in juvenile densities for given velocities, depths or substratum sizes, with this variation being greatest in optimal habitats. On examination of an individual year, interaction between the variables was found to explain some of the variation. On a year‐to‐year basis the juvenile Atlantic salmon population was found to exhibit an ‘Ideal Free Distribution’, which resulted in greatest variation in optimal habitats with year‐to‐year changes in population abundance.

Influences of fertilization on population abundance, distribution, and control of Frankliniella occidentalis on chrysanthemum

AbstractWe examined the effects of fertilization on population abundance and within‐plant distribution of western flower thrips, Frankliniella occidentalis Pergande (Thysanoptera: Thripidae), on potted chrysanthemum, Dendranthema grandiflora (Tzvelev). We also investigated the effects of fertilization on the number of insecticide applications needed to control F. occidentalis on potted chrysanthemum. Under greenhouse conditions, rate of change in population abundance of F. occidentalis increased with fertilization levels from 0 to 100% of the standard fertilization level (375 ppm N) and was four times higher on plants fertilized with the standard level (rate of change = 0.14) than on plants fertilized with 0% during the first 4 weeks after thrips inoculation. Within‐plant distribution of F. occidentalis was influenced by the phenology of the plants rather than total nitrogen content of plant tissues. Prior to flower opening, more F. occidentalis were found in the middle region of the plants. When the flowers began to open, more thrips were found feeding inside the flowers than on the leaves. We further showed that production time, the time from transplantation to flower opening, shortened considerably with increased fertilization level. Production time was shortest, 12 weeks, for plants fertilized with 100% of the standard fertilization level. When the fertilization level was reduced to 20%, production time lengthened to 13 weeks. When fertilization level was reduced to 0%, production time lengthened to 14 weeks. Increased fertilization from 0 to 100% of the standard level did not result in higher numbers of insecticide applications. All three insecticides (acephate, bifenthrin, and spinosad) were effective in keeping the thrips infestation below a predetermined level, five thrips per plant, but bifenthrin required the most number of applications to do so. For chrysanthemum, a fast‐growing crop and heavy utilizer of fertilizer, fertilization influenced not only the population growth of pest insects but also plant production time. As a result, optimizing fertilization level to reduce pest population growth may be a useful tactic in an Integrated Pest Management program for managing F. occidentalis on potted chrysanthemum. However, the effect of fertilization on production time and plant quality should also be considered when implementing this tactic.

Life expectancy change in perturbed communities: Derivation and qualitative analysis

Pollution, loss of habitat, and climate change are introducing dramatic perturbations to natural communities and affecting public health. Populations in perturbed communities can change dynamically, in both abundance and age structure. While analysis of the community matrix can predict changes in population abundance arising from a sustained or press perturbation, perturbations also have the potential to modify life expectancy, which adds yet another means to falsify experimental hypotheses and to monitor management interventions in natural systems. In some instances, an input to a community will produce no change in the abundance of a population but create a major shift in its mean age. We present an analysis of change in both abundance and life expectancy, leading to a formal quantitative assessment as well as qualitative predictions, and illustrate the usefulness of the technique through general examples relating to vector-borne disease and fisheries.

Population Dynamics of Songbirds in the Boreal Mixedwood Forests of Alberta, Canada: Estimating Minimum and Maximum Extents of Spatial Population Synchrony

Ecological phenomena vary over space and time and interpretation of these processes also varies depending on the measurement scale. As the spatial scale of observation increases and decreases, changes in population abundance will likely exhibit alternating patterns of asynchrony and synchrony. While the study of how and why population dynamics change with spatial scale of measurement is intrinsically interesting, most population ecologists seek to study mechanisms of population change on a discrete study area. Our study develops methods that population ecologists can use to determine spatially appropriate sampling designs, and demonstrates how such spatial scales can be determined for 25 species of songbirds using longterm data from the boreal mixedwood forest of Alberta, Canada. To determine minimum scales of synchrony in population dynamics, we calculated the average correlation of changes in population abundance over time across different numbers of fixed-radius point-count samples. We then used a randomization test to remove the effect of number of replicates from the determination of appropriate spatial scale. The maximum extent of synchrony was estimated as the distance where population dynamics were no longer correlated. Estimates of the minimum scale of synchrony differed between species, ranging from 3.1 to 18.6 ha. The maximum scale of synchrony was estimated to be greater than or equal to 8 km for 14 of the 25 species examined, and to be greater than or equal to 70 km for Tennessee Warbler (Vermivora peregrina). Maximum spatial extents were significantly correlated with body mass and territory size.

Population models of farmed abalone Haliotis diversicolor supertexta exposed to waterborne zinc

A stage-classified demographic method was performed to investigate the effects of increased waterborne zinc (Zn) concentrations on the population dynamics of abalone Haliotis diversicolor supertexta. We reanalyzed the results of a 7-day acute and a 28-day chronic toxicity bioassays to examine the survival and growth performances when exposing abalone to different levels of zinc stresses. An energy-based biological approach was adopted to model the effects of zinc on fecundity. These data provided stage-specific schedules of vital rates that were used to parameterize a projection matrix model for abalone. Simulations were carried out to produce temporal population abundance changes under seven exposure scenarios including a control group and six treated cohorts ranged from 0.03 to 1 mg l −1 Zn. Model manipulations indicated that a reduction of individual growth rate was observed at an exposed Zn concentration greater than 0.12 mg l −1, whereas the significant influence of survivorship was occurred until the Zn concentration reached 0.25 mg l −1. For all treatments, Zn induced a slight decrease in fecundity. The asymptotic population growth rate decreased from λ=1.00 for the control group to λ=0.9968 for abalone population exposed to 1 mg l −1 Zn, indicating a potential risk of population intrinsic growth rates for abalone exposed to higher levels of waterborne Zn. These findings provide a pivotal effort for setting of the ambient water quality criterion for protecting the aquacultural species.

Magnitude of harvest of Atlantic horseshoe crabs,Limulus polyphemus, in Pleasant Bay, Massachusetts

Atlantic horseshoe crabs,Limulus polyphemus, are currently harvested for biomedical, scientific, and bait purposes. In recent years, changes in population abundance and magnitude of harvesting have raised concerns about the status of this resource. We found horseshoe crab harvest in Pleasant Bay, Massachusetts, was selective by sex and size. Biomedical harvest preferred larger individuals and females, the scientific harvest preferred smaller individuals and males, and the bait harvest preferred females. Total 2001 harvest for all purposes accounted for the mortality of ∼1–2% the adult population. Biomedical harvest accounted for the greatest loss of horseshoe crabs from Pleasant Bay, ∼1–1.6% of the total population. Although biomedical harvest had the lowest associated mortality rate (∼10–15%), many more crabs were harvested from Pleasant Bay for biomedical purposes than for other uses. The scientific harvest accounted for the mortality of ∼0.4% of the population, and bait harvest accounted for the smallest mortality at ∼0.06% of the population. Harvest mortality rate was estimated to be lower in Pleasant Bay than in other Cape Cod areas and may be lower than natural mortality in the population. This study is the first qualitative investigation of commercial harvest on horseshoe crab populations and emphasizes that harvest pressures on different populations need to be individually evaluated.

CARPSIM: stochastic simulation modelling of wild carp ( Cyprinus carpio L.) population dynamics, with applications to pest control

Common carp ( Cyprinus carpio L.) is an important pest fish species in Australasia and North America. Carp are widely implicated in freshwater aquatic resource degradation and both the resource management and broader communities are currently seeking effective control measures. We developed CARPSIM, a simple age-based model to simulate the effects of a range of management scenarios. The model simulates change in population biomass by age and sex-specific growth and simulates change in population abundance through recruitment and sex-specific mortality. Using empirical stock–recruitment data and stochastic components derived from local hydrological data or the southern oscillation index the model simulated the population dynamics of carp populations over 200 years using biological parameters previously estimated for two Victorian populations within the Murray–Darling basin. Carp management scenarios simulated included the effects of fishing the spawning stock; of fishing the whole stock; of spawning or recruitment sabotage; and of driving the population sex ratio towards male dominance. Model predictions suggests that faster growing, shorter-lived populations may be better controlled by molecular methods inducing male-dominance, or spawning sabotage type methods whereas slower growing, long-lived populations may respond best to removal type approaches. Unselective removal, such as poisoning or trapping all age-classes is more likely to cause pseudo-extinction at levels of instantaneous fishing mortality ( F>0.7); while size-selective removal at similar F levels may only be useful to reduce the biomass below 60% of virgin biomass. CARPSIM simulations show that the probability is small for any removal-based method achieving <10% of virgin biomass when F<1.4.

LATE‐QUATERNARY VEGETATION DYNAMICS IN NORTH AMERICA: SCALING FROM TAXA TO BIOMES

This paper integrates recent efforts to map the distribution of biomes for the late Quaternary with the detailed evidence that plant species have responded individualistically to climate change at millennial timescales. Using a fossil‐pollen data set of over 700 sites, we review late‐Quaternary vegetation history in northern and eastern North America across levels of ecological organization from individual taxa to biomes, and apply the insights gained from this review to critically examine the biome maps generated from the pollen data. Higher‐order features of the vegetation (e.g., plant associations, physiognomy) emerge from individualistic responses of plant taxa to climate change, and different representations of vegetation history reveal different aspects of vegetation dynamics. Vegetation distribution and composition were relatively stable during full‐glacial times (21 000– 17 000 yr BP) [calendar years] and during the mid‐ to late Holocene (7000–500 yr BP), but changed rapidly during the late‐glacial period and early Holocene (16 000–8 000 yr BP) and after 500 yr BP. Shifts in plant taxon distributions were characterized by individualistic changes in population abundances and ranges and included large east–west shifts in distribution in addition to the northward redistribution of most taxa. Modern associations such as Fagus–Tsuga and Picea–Alnus–Betula date to the early Holocene, whereas other associations common to the late‐glacial period (e.g., Picea–Cyperaceae–Fraxinus–Ostrya/ Carpinus) no longer exist. Biomes are dynamic entities that have changed in distribution, composition, and structure over time. The late‐Pleistocene suite of biomes is distinct from those that grew during the Holocene. The pollen‐based biome reconstructions are able to capture the major features of late‐Quaternary vegetation but downplay the magnitude and variety of vegetational responses to climate change by (1) limiting apparent land‐cover change to ecotones, (2) masking internal variations in biome composition, and (3) obscuring the range shifts and changes in abundance among individual taxa. The compositional and structural differences between full‐glacial and recent biomes of the same type are similar to or greater than the spatial heterogeneity in the composition and structure of present‐day biomes. This spatial and temporal heterogeneity allows biome maps to accommodate individualistic behavior among species but masks climatically important variations in taxonomic composition as well as structural differences between modern biomes and their ancient counterparts.

Is size at sex transition an indicator of growth or abundance in pandalid shrimp?

Growth and sexual development of northern shrimp Pandalus borealis was determined on the Scotian Shelf during two periods, one of low and one of high population abundance, using deviation and modal analysis. Growth rates were higher during the period of low abundance, probably due to density dependent effects and higher temperatures. Faster growth during the first period was associated with a smaller size at transition ( L t) from male to female and a smaller maximum size ( L max), while slower growth during the later period was associated with a larger L t and L max (and older age). Results are consistent with the theory of invariance in the ratio of pandalid L t/ L max and its implications, i.e. that growth can be determined from measurements of L t or L max alone. L t was not a good indicator of short-term changes in population abundance; consequently L t or L max should be used as measure of growth, not abundance. The theoretical and practical implications of the results are discussed. In particular, it is suggested that decreased fecundity resulting from an increased growth rate and decreased L t and L max is responsible for the negative lagged relationships between temperatures and abundance in stocks at the southern limit of their distribution. It is also suggested that measurements of L t and/or L max could be used in quantitative assessment models and to develop harvest limit reference points.

The influence of structure and dynamics of infaunal predator populations on predatory activity: the example of Nephtys hombergii

In an attempt to associate population structure of infaunal predators with the effect of predation in structuring benthic communities, an extensive study was conducted focusing on a population of the predatory polychaete Nephtys hombergii in the Rance estuary (western English Channel). Sampling was designed to study both spatial distributions in abundance and weight throughout the whole estuary (1995), and temporal variations of the predator population on a sublittoral mudflat (1995–1997). The results reveal that large individuals are significantly distributed on intertidal mudflats, while juveniles (G0 and G1 classes) inhabit the whole sublittoral area. Juveniles are dominant throughout the year (>90% of individuals), but are characterized by high mortality rates (70–80%), which are directly responsible for changes in population abundance. Before investigating the effect of predation, the population structure and the spatial distribution of predators must be known, an important step when considering that prey mortality induced by a 1-year-old N. hombergii was about 20% higher than rates observed with adults. Based on the example of N. hombergii, the results underline the necessity of considering the age, the weight structure and the dynamics of the predator population to understand the dynamics of marine communities in a better way.

Role of disease in abundance of a Pacific herring (Clupea pallasi) population

Disease significantly affects population abundance of Pacific herring (Clupea pallasi). Comprehensive epidemiological study of the Pacific herring population of Prince William Sound, Alaska, U.S.A., from 1994 to 2000 included complete necropsy examination of 230–500 fish each spring and 40–160 fish each fall (total n = 2983 fish). Mortality is best estimated, through modifications of an age-structured assessment model, using a disease index that combines the prevalence of viral hemorrhagic septicemia virus (VHSV) with the prevalence of ulcers. Risk factors for an epidemic include poor body condition and abundant recruitment before spawning in the spring. Prevalence of the pathogen Ichthyophonus hoferi increased as fish aged, but changes in I. hoferi prevalence were not related to changes in population abundance. Disease that caused an epidemic in 1998 (VHSV and ulcers) nearly disappeared from the population when changes in abundance were detected by traditional stock assessment methods in 1999. Disease significantly affects recruitment — the two lowest recruitment estimates on record, in 1994 and 1999, followed increased natural mortality of adults in 1993 and 1998.