The relative importance of predation and competition in two reef fishes

Competition and predation may both strongly influence populations of reef fishes, but the importance of these processes relative to one another is poorly understood. I quantified the effects of predation and competition on the growth and survival of two temperate reef fishes, Lythrypnus dalli and Coryphopterus nicholsii, in field experiments in which I manipulated the densities of the two species and the abundance of predators (using exclosure cages) on small replicate patch reefs. I also evaluated the influence of predators on the behavior of the two species to help interpret the mechanisms of any predatory influences on growth or survival. Predation was much more important than competition (inter- or intraspecific) in Lythrypnus. For Coryphopterus, neither competition nor predation were particularly important. Behaviorally, both species responded to predators by reducing foraging rate and hiding. This altered behavior, however, had no repercussions for growth or survival of Coryphopterus. In contrast, Lythrypnus grew more slowly and suffered greater mortality when exposed to predators. Interspecific competition did not significantly influence either species. Intraspecific competition did not affect the growth of Coryphopterus, but survival tended to be lower at high densities. Growth of Lythrypnus was depressed by intraspecific competition, but survival was not, except that, in the presence of predators, survival was density dependent. In contrast to the historical emphasis placed on the role of competition, this study indicates that predation can be more important than competition in determining patterns of abundance of some reef fishes. For example, predators not only influenced foraging of both Lythrypnus and Coryphopterus, but they also reduced growth and survival of Lythrypnus, and therefore appear to help maintain the marked habitat segregation between the two species. Received: 16 June 1997 / Accepted: 3 December 1997     

High variability in patterns of population decline: the importance of local processes in species extinctions

A fundamental goal of conservation science is to improve conservation practice. Understanding species extinction patterns has been a central approach towards this objective. However, uncertainty remains about the extent to which species-level patterns reliably indicate population phenomena at the scale of local sites, where conservation ultimately takes place. Here, we explore the importance of both species- and site-specific components of variation in local population declines following habitat disturbance, and test a suite of hypotheses about their intrinsic and extrinsic drivers. To achieve these goals, we analyse an unusually detailed global dataset for species responses to habitat disturbance, namely primates in timber extraction systems, using cross-classified generalized linear mixed models. We show that while there are consistent differences in the severity of local population decline between species, an equal amount of variation also occurs between sites. The tests of our hypotheses further indicate that a combination of biological traits at the species level, and environmental factors at the site level, can help to explain these patterns. Specifically, primate populations show a more marked decline when the species is characterized by slow reproduction, high ecological requirements, low ecological flexibility and small body size; and when the local environment has had less time for recovery following disturbance. Our results demonstrate that individual species show a highly heterogeneous, yet explicable, pattern of decline. The increased recognition and elucidation of local-scale processes in species declines will improve our ability to conserve biodiversity in the future.     

Competition between co-dominant plants of the Serengeti plains depends on competitor identity,water, and urine

Kyllinga nervosa (Steud.) and Sporobolus kentrophyllus (K. Schum.) are co-dominant plants of the Serengeti short-grass plains, Tanzania. The plains are characterized by seasonal and sporadic rainfall and currently support in excess of 1.5 million migratory ungulates. The interactive effect of simulated bovine urine and water availability were tested on the competitive interactions of these species in the laboratory.Sporobolus kentrophyllus was a superior competitor to K. nervosa over the tested treatment levels with respect to growth and reproductive effort. Sporobolus kentrophyllus exhibited rapid growth in response to urine addition, leading to a significant species × urine interaction while reduced growth by K. nervosa in response to low water availability explained the significant species × water interaction and is likely explained by K. nervosa's shallow root system. Kyllinga nervosa, however, appears to be more tolerant of low nitrogen conditions based on its similar growth with and without the urine treatment. The effect of intraspecific competition on total biomass was similar for S. kentrophyllus and K. nervosa. Competition resulted in increased size differences (asymmetry) for K. nervosa and for the interspecific competition treatments compared to the size differences observed for plants grown individually (in absence of competition).Total reproductive biomass was reduced most by competition with S. kentrophyllus, irrespective of target species. The water treatment did not influence reproduction while the urine treatment significantly increased reproductive biomass and interacted with target species, competitor species, and yielded a three-way urine × target × competitor species interaction.Results suggest that codominance of these two species in the Serengeti is regulated by water availability, nitrogen input from grazers, and local neighbor identity.     

Estimation of effective population size and detection of a recent population decline coinciding with habitat fragmentation in a ground beetle

We assess the impact of habitat fragmentation on the effective size (N(e)) of local populations of the flightless ground beetle Carabus violaceus in a small (<25 ha) and a large (>80 ha) forest fragment separated by a highway. N(e) was estimated based on the temporal variation of allele frequencies at 13 microsatellite loci using two different methods. In the smaller fragment, N(e) estimates ranged between 59 and a few hundred, whereas values between 190 and positive infinity were estimated for the larger fragment. In both samples, we detected a signal of population decline, which was stronger in the small fragment. The estimated time of onset of this N(e) reduction was consistent with the hypothesis that recent road constructions have divided a continuous population into several isolated subpopulations. In the small fragment, N(e) of the local population may be so small that its long-term persistence is endangered.     

The white colour of the Ostrich ( Struthio camelus ) egg is a trade-off between predation and overheating

Most ground nesters lay pigmented eggs, and egg pigmentation generally matches the environment. Pigmentation of eggs has evolved as a protective device against predation, but dark-pigmented eggs can be susceptible to overheating when exposed to solar radiation. The Ostrich (Struthio camelus) lays white eggs that are unattended for the first few weeks before incubation, and are quite visible to predators. To evaluate the effect of colour on the surface and core temperatures, we painted some Ostrich eggs dark brown or white, and left some unpainted (control), and exposed all of them directly to the sun during the day. The surface and core temperatures of brown eggs were significantly higher than those of the white-painted and control eggs. In addition, the core temperature of brown eggs exceeded 37.5°C, which is the temperature at which embryo mortality starts to increase. In a second experiment, we placed eggs (brown-painted and control) in various types of vegetation to study their visibility to an observer walking towards them. The white eggs were discovered from a significantly longer distance than the brown eggs, indicating that the predation risk may be much higher for white eggs. The results thus suggest that white eggs minimise overheating and allow the Ostrich to leave its eggs unattended before incubation starts, but they are more susceptible to predation.     

Interactive effect of flooding and grazing on the growth of Serengeti grasses

Summary Grazing and flooding may potentially interact in particular habitats of many grassland regions around the world. We tested the hypothesis that grazing and flooding induce different and largely opposed allocation responses in individual plants. As a result, their combined effect on plant growth would be negative. We studied the response of three grass species from the Serengeti ecosystem (Tanzania) to the effects of flooding and clipping. Plants under the combined effect of flooding and clipping had lower growth rates than plants growing under the effect of either of the two factors individually. Plants under flooding grew taller and allocated more resources to stem growth than controls; for two of the three species, flooded plants also generated a new root system above soil level. All these morphological and physiological responses conflict with the ability of a plant to respond to defoliation with minimum reduction in growth rates. The three species showed a response to flooding reflecting their distribution ranges in the field: the species from the most flood-prone habitat showed a positive effect of flooding on growth, whereas the species from dry uplands showed a strong negative effect of flooding. Flood-tolerant species were taller and less tolerant of clipping than flooding sensitive species. Our results suggest that, in ecological time, individuals subjected to both flooding and grazing have their growth reduced to a greater extent than by either of the two factors acting individually, whereas in evolutionary time, species adapted to flooding are poor grazing tolerators and species adapted to grazing are poor flooding tolerators.     

The nature of hunting around the Western Corridor of Serengeti National Park, Tanzania

In many parts of Africa, illegal hunting is considered the most pressing issue in protected areas. Poaching has remained a persistent problem through the 50 year long history of Serengeti National Park. Around 2 million people live along the borders of the park. Poverty is widespread, and the population is increasing rapidly. In this paper we examine the local perceptions of importance and reasons for hunting, gender differences and opinions about mitigating measures among villagers around Serengetis Western Corridor. The data were collected through interviewing 590 persons in eight villages close to the borders of the park. Hunting is driven by the need to not only increase food supply and cash income, but also fulfil cultural and social needs. We also identified a proactive attitude in the rural communities towards reducing illegal hunting and more constructive relationships with the management authorities of the national park. The role of hunting in community life extends beyond the immediate poverty issue, and should be seen as an element in the larger development agenda of rural Tanzania and the quest for models for sustainable wildlife management.     

Evidence that disease-induced population decline changes genetic structure and alters dispersal patterns in the Tasmanian devil

Infectious disease has been shown to be a major cause of population declines in wild animals. However, there remains little empirical evidence on the genetic consequences of disease-mediated population declines, or how such perturbations might affect demographic processes such as dispersal. Devil facial tumour disease (DFTD) has resulted in the rapid decline of the Tasmanian devil, Sarcophilus harrisii, and threatens to cause extinction. Using 10 microsatellite DNA markers, we compared genetic diversity and structure before and after DFTD outbreaks in three Tasmanian devil populations to assess the genetic consequences of disease-induced population decline. We also used both genetic and demographic data to investigate dispersal patterns in Tasmanian devils along the east coast of Tasmania. We observed a significant increase in inbreeding (F_IS pre/post-disease −0.030/0.012, P<0.05; relatedness pre/post-disease 0.011/0.038, P=0.06) in devil populations after just 2–3 generations of disease arrival, but no detectable change in genetic diversity. Furthermore, although there was no subdivision apparent among pre-disease populations (θ=0.005, 95% confidence interval (CI) −0.003 to 0.017), we found significant genetic differentiation among populations post-disease (θ=0.020, 0.010–0.027), apparently driven by a combination of selection and altered dispersal patterns of females in disease-affected populations. We also show that dispersal is male-biased in devils and that dispersal distances follow a typical leptokurtic distribution. Our results show that disease can result in genetic and demographic changes in host populations over few generations and short time scales. Ongoing management of Tasmanian devils must now attempt to maintain genetic variability in this species through actions designed to reverse the detrimental effects of inbreeding and subdivision in disease-affected populations.     

Changes in landscape composition influence the decline of a threatened woodland caribou population

1. Large-scale habitat loss is frequently identified with loss of biodiversity, but examples of the direct effect of habitat alterations on changes in vital rates remain rare. Quantifying and understanding the relationship between habitat composition and changes in vital rates, however, is essential for the development of effective conservation strategies. 2. It has been suggested that the decline of woodland caribou Rangifer tarandus caribou populations in North America is precipitated by timber harvesting that creates landscapes of early seral forests. Such habitat changes have altered the predator-prey system resulting in asymmetric predation, where predators are maintained by alternative prey (i.e. apparent competition). However, a direct link between habitat condition and caribou population declines has not been documented. 3. We estimated survival probabilities for the threatened arboreal lichen-feeding ecotype of woodland caribou in British Columbia, Canada, at two different spatial scales. At the broader scale, observed variation in adult female survival rates among 10 distinct populations (range = 0.67-0.93) was best explained by variation in the amount of early seral stands within population ranges and population density. At the finer scale, home ranges of caribou killed by predators had lower proportions of old forest and more mid-aged forest as compared with multi-annual home ranges where caribou were alive. 4. These results are consistent with predictions from the apparent competition hypothesis and quantify direct fitness consequences for caribou following habitat alterations. We conclude that apparent competition can cause rapid population declines and even extinction where changes in species composition occur following large scale habitat change.     

Lack of compensatory growth under phosphorus deficiency in grazing-adapted grasses from the Serengeti Plains

Summary Two shortgrass species (Sporobolus ioclados and Eustachys paspaloides) and two midgrass species (E. paspaloides and Pennisetum mezianum) from the Serengeti grasslands of Tanzania were grown under conditions of extreme phosphorus (P) deficiency. Production of each of these species is maintained or enhanced by defoliation under adequate nutrient supply (McNaughton et al. 1983). However, under the P-deficient conditions of our experiment, defoliation caused a reduction in biomass of all plant parts of each species. Green leaf biomass was reduced most strongly by defoliation, and crowns were least affected. Yield of biomass and nutrients to grazers (green leaves+clipped material) was enhanced by weekly defoliation in the shortgrass grazing-adapted species, whereas yield to producers (live biomass and nutrients retained by the plant) and yield to decomposers (litter) were strongly reduced by defoliation in all species. Phosphate absorption capacity (V _max) measured on excised roots was enhanced by defoliation in the grazing-adapted Sporobolus, but, due to low affinity (high K _m) of roots of defoliated plants for phosphate, absorption rate was not greatly altered at low solution concentrations. Phosphate absorption capacity was reduced or unaffected by defoliation in other species. We conclude that under conditions of P deficiency, plants are unable to acquire the nutrients necessary to replenish large nutrient losses to grazers. In low-nutrient environments, compensatory growth (stimulation of production by grazing) is not a viable strategy. Therefore, in these environments plants respond evolutionarily to herbivores by developing chemical or morphological defenses.     

Effects of herbivory and competition on an introduced plant in decline

Anthemiscotula was introduced to Denmark 500 years ago, and its distribution is presently limited and in decline. A manipulative field experiment was performed to investigate the effects of native plant competitors and native invertebrate herbivores on its performance. Generally, both herbivory and competition treatments had great impact, and when both factors were operating, the effects were additive for all variables except plant height. Although A. cotula showed plasticity in growth, resource allocation and flowering timing, it was unable to adjust to competition and compensate for losses due to herbivory sufficiently to ensure and restore its achene production. This vulnerability, combined with improved cereal cleaning techniques and thus fewer reintroductions of A. cotula seeds, may be the cause of its current decline. A. cotula responded to herbivory by prolonging its flowering period, a “bet-hedging” strategy. In Denmark this strategy is unreliable since risks of sub-optimal conditions are much greater in August–October. Received: 12 December 1997 / Accepted: 14 October 1998     

The influence of wetlands in regulating water quality in the Seronera River,Serengeti National Park,Tanzania

The distribution of temperature, salinity, visibility and dissolved oxygen was sampled from 1996 to 2002 at sites along the Seronera River. The minimum temperature decreased with distance upstream. The salinity increased up-river where occasionally hypersaline conditions prevailed. Dissolved oxygen was highly variable spatially and temporally, depending on both the level of eutrophication by animal dung and the presence of wetlands that help filter the excess nutrients. During the study period, fringing, freshwater wetlands have generally been degraded and in some cases destroyed, and this has been accompanied by significantly decreased oxygen levels, sometimes nearing anoxic conditions. Also during this period, saltwater wetlands have increased, and since wildlife impacted these wetlands little, dissolved oxygen levels remained high throughout. Visibility was highest in areas fringed by wetlands.This revised version wa published online in March 2005 with corrections to the issue cover date.     

Genetic diversity, population structure, effective population size and demographic history of the Finnish wolf population

The Finnish wolf population (Canis lupus) was sampled during three different periods (1996-1998, 1999-2001 and 2002-2004), and 118 individuals were genotyped with 10 microsatellite markers. Large genetic variation was found in the population despite a recent demographic bottleneck. No spatial population subdivision was found even though a significant negative relationship between genetic relatedness and geographic distance suggested isolation by distance. Very few individuals did not belong to the local wolf population as determined by assignment analyses, suggesting a low level of immigration in the population. We used the temporal approach and several statistical methods to estimate the variance effective size of the population. All methods gave similar estimates of effective population size, approximately 40 wolves. These estimates were slightly larger than the estimated census size of breeding individuals. A Bayesian model based on Markov chain Monte Carlo simulations indicated strong evidence for a long-term population decline. These results suggest that the contemporary wolf population size is roughly 8% of its historical size, and that the population decline dates back to late 19th century or early 20th century. Despite an increase of over 50% in the census size of the population during the whole study period, there was only weak evidence that the effective population size during the last period was higher than during the first. This may be caused by increased inbreeding, diminished dispersal within the population, and decreased immigration to the population during the last study period.     

The cultural evolution of fertility decline

Cultural evolutionists have long been interested in the problem of why fertility declines as populations develop. By outlining plausible mechanistic links between individual decision-making, information flow in populations and competition between groups, models of cultural evolution offer a novel and powerful approach for integrating multiple levels of explanation of fertility transitions. However, only a modest number of models have been published. Their assumptions often differ from those in other evolutionary approaches to social behaviour, but their empirical predictions are often similar. Here I offer the first overview of cultural evolutionary research on demographic transition, critically compare it with approaches taken by other evolutionary researchers, identify gaps and overlaps, and highlight parallel debates in demography. I suggest that researchers divide their labour between three distinct phases of fertility decline—the origin, spread and maintenance of low fertility—each of which may be driven by different causal processes, at different scales, requiring different theoretical and empirical tools. A comparative, multi-level and mechanistic framework is essential for elucidating both the evolved aspects of our psychology that govern reproductive decision-making, and the social, ecological and cultural contingencies that precipitate and sustain fertility decline.     

The effects of multiple stressors on wetland communities: pesticides,pathogens and competing amphibians

1. Anthropogenic effects have propelled us into what many have described as the sixth mass extinction, and amphibians are among the most affected groups. The causes of global amphibian population declines and extinctions are varied, complex and context‐dependent and may involve multiple stressors. However, experimental studies examining multiple factors contributing to amphibian population declines are rare. 2. Using outdoor mesocosms containing zooplankton, phytoplankton, periphyton and tadpoles, we conducted a 2 × 2 × 3 factorial experiment that examined the separate and combined effects of an insecticide and the fungal pathogen Batrachochytrium dendrobatidis (Bd) on three different assemblages of larval pacific treefrogs (Pseudacris regilla) and Cascades frogs (Rana cascadae). 3. Larval amphibian growth and development were affected by carbaryl and the amphibian assemblage treatment, but only minimally by Bd. Carbaryl delayed metamorphosis in both amphibian species and increased the growth rate of P. regilla. Carbaryl also reduced cladoceran abundance, which, in turn, had positive effects on phytoplankton abundance but no effect on periphyton biomass. Substituting 20 intraspecific competitors with 20 interspecific competitors decreased the larval period but not the growth rate of P. regilla. In contrast, substituting 20 intraspecific competitors with 20 interspecific competitors had no effect on R. cascadae. Results of real‐time quantitative polymerase chain reaction (qPCR) analysis confirmed infection of Bd‐exposed animals, but exposure to Bd had no effects on either species in univariate analyses, although it had significant or nearly significant effects in several multivariate analyses. In short, we found no interactive effects among the treatments on amphibian growth and development. 4. We encourage future research on the interactive effects of pesticides and pathogens on amphibian communities.