Population Regulation Processes Research Articles

The pest kill rate of thirteen natural enemies as aggregate evaluation criterion of their biological control potential of Tutaabsoluta

Ecologists study how populations are regulated, while scientists studying biological pest control apply population regulation processes to reduce numbers of harmful organisms: an organism (a natural enemy) is used to reduce the population density of another organism (a pest). Finding an effective biological control agent among the tens to hundreds of natural enemies of a pest is a daunting task. Evaluation criteria help in a first selection to remove clearly ineffective or risky species from the list of candidates. Next, we propose to use an aggregate evaluation criterion, the pest kill rate, to compare the pest population reduction capacity of species not eliminated during the first selection. The pest kill rate is the average daily lifetime killing of the pest by the natural enemy under consideration. Pest kill rates of six species of predators and seven species of parasitoids of Tuta absoluta were calculated and compared. Several natural enemies had pest kill rates that were too low to be able to theoretically reduce the pest population below crop damaging densities. Other species showed a high pest reduction capacity and their potential for practical application can now be tested under commercial crop production conditions.

Autonomous Biological Control of Dactylopius opuntiae (Hemiptera: Dactyliiopidae) in a Prickly Pear Plantation With Ecological Management.

It is broadly known that the conservation of biological diversity in agricultural ecosystems contributes to pest control. This process was studied in a prickly pear plantation (Opuntia megacantha and Opuntia ficus-indica) located in central Mexico. No insecticides have been used on this plantation since 2000, and local farmers believe that the presence of different species of insects limits the growth of the wild cochineal (Dactylopius opuntiae Cockerell), which is one of the main pests in this crop. From August 2012 to November 2013, we estimated the number of cochineal per stem in the plantation and determined its spatial distribution pattern. In order to identify signs of population regulation, we obtained histograms of the frequency distribution of the size of the clusters and determined if distribution is adjusted to a power function (power law). We identified the cochineal predators and determined the correlation in their abundances. The greater abundance of cochineal occurred between summer and autumn while the minimum value was recorded in spring. The frequency distribution of the cochineal clusters had a high level of adjustment to a power function, suggesting the presence of population regulation processes. Six species that prey on cochineal were identified. Laetilia coccidivora and Hyperaspis trifurcata were the most active and their abundance was significantly correlated with the abundance of cochineal. We found that the probability of extinction of these insects in a cladode increases with its density, since the density and predator activity also increased. It is likely that, under these conditions, the cochineal have established an autonomous control.

Detecting and modelling delayed density-dependence in abundance time series of a small mammal (Didelphis aurita).

We study the population size time series of a Neotropical small mammal with the intent of detecting and modelling population regulation processes generated by density-dependent factors and their possible delayed effects. The application of analysis tools based on principles of statistical generality are nowadays a common practice for describing these phenomena, but, in general, they are more capable of generating clear diagnosis rather than granting valuable modelling. For this reason, in our approach, we detect the principal temporal structures on the bases of different correlation measures, and from these results we build an ad-hoc minimalist autoregressive model that incorporates the main drivers of the dynamics. Surprisingly our model is capable of reproducing very well the time patterns of the empirical series and, for the first time, clearly outlines the importance of the time of attaining sexual maturity as a central temporal scale for the dynamics of this species. In fact, an important advantage of this analysis scheme is that all the model parameters are directly biologically interpretable and potentially measurable, allowing a consistency check between model outputs and independent measurements.

What if it gets crowded? Density‐dependent tortuosity in individual movements of a Neotropical mammal

AbstractEffects of density dependence on animal movements have received much attention in ecology, but it is still debated to what extent dispersal and movements in general are density dependent, and their potential contribution to population regulation processes. Here, we determine the occurrence and nature of density dependence in the movements of a Neotropical marsupial, the black‐eared opossum Didelphis aurita Wied‐Neuwied 1826. Using spool‐and‐line tracking devices, we estimated the tortuosity of fine‐scale movements of 149 individuals by their fractal dimension D. We evaluated the relative importance of population size, reproductive or climatic seasons and reproductive maturity of individuals as determinants of movement tortuosity, using a model selection approach. Population size was the most important determinant of movement tortuosity, with season (climatic seasons for females, reproductive seasons for males) and reproductive maturity as secondary but also important variables. We detected a positive density‐dependent effect on movement tortuosity, resulting in more intensive use of areas by individuals during periods of high population size. This positive association between movement tortuosity and population size is more likely to result from intraspecific competition, which forces individuals to explore their environment more intensively during high‐density periods. Therefore, despite being density dependent, movements in D. aurita apparently do not contribute to population regulation mechanisms.

On the birth and death of ideas in marine science†

Abstract In this essay, I review six decades of my career in marine science and fisheries, considering the ideas that came and went in the period as “food for thought”. I describe my inspirations and successes, and my disappointments and failures. My activities were both administrative and research-oriented. As regards the former, I was part of major changes in ocean policy and new ocean research programmes that gave me a unique perspective. For example, I was responsible for the implementation of the US extended jurisdiction in fisheries under National Oceanic and Atmospheric Administration. Also I conceived and led the creation of the Global Ocean Ecosystem Dynamics Programme (GLOBEC) and guided it in many international contexts, including its integration with the International Geosphere Biosphere Programme (IGBP). From a research standpoint, my efforts leading up to GLOBEC strongly influenced the introduction of ocean physics into biological oceanography. This led me into plankton dynamics, food signals, small-scale turbulence and physical forcing, even into the stochastic geometry of the plankton. My life-long interest in the dynamics of marine fish populations was strongly influenced by the seminal thinkers in fisheries and my research explored population regulation processes as well as practical applications of statistics and operations research to fisheries management. In my last academic post, I became founding Dean of the School for Marine Science and Technology (SMAST) at the University of Massachusetts. This position required integrating administrative and research (both pure and applied) perspectives to create an institution of academic excellence which was at the same time actively responsive to issues arising in our local, nationally prominent fisheries. I end the essay with a consideration of “what has changed”.

Experimental evidence that livestock grazing intensity affects cyclic vole population regulation processes

AbstractGrazing by domestic ungulates may limit the densities of small herbivorous mammals that act as key prey in ecosystems. Whether this also influences density dependence and the regulation of small herbivore populations, hence their propensity to exhibit multi‐annual population cycles, is unknown. Here, we combine time series analysis with a large‐scale grazing experiment on upland grasslands to examine the effects of livestock grazing intensity on the population dynamics of field voles (Microtus agrestis). Using log‐linear modelling of replicated time series under different grazing treatments, we show that increased sheep densities weaken delayed density dependent regulation of vole population growth, hence reducing the cyclicity in vole population dynamics. While population regulation is commonly attributed to both top‐down and bottom up processes, our results suggest that regulation of cyclic vole populations can be disrupted by the influence of another grazer in the same trophic level. These results support the view that ongoing changes in domestic grazing intensity, by affecting small mammal dynamics, can potentially have cascading impacts on higher trophic levels, and strongly influence the dynamics of upland grassland systems.

Natal dispersal and its consequences in Black Grouse Tetrao tetrix

Data on dispersal distances of juveniles are important for understanding the genetic structure of populations, population regulation processes and the effects of landscape ecology on metapopulation dynamics. We studied dispersal in juvenile Black Grouse Tetrao tetrix in the French Alps between 1990 and 1998, by radiotracking 39 young grouse captured in August or autumn in a study area of 836 ha. Natal dispersal occurred in two discrete phases, autumn (October) and spring (mid‐April to early May), with periods of reduced mobility between. The mean distance travelled by females exceeded that of males in autumn but not in spring. The natal dispersal distance of females was greater than that of males, and resulted in 81% of females leaving the study area to nest 5–29 km from their site of capture. This emigration must have been compensated for by immigrants because the number of hens in the study area increased during the study. Males were more philopatric. By the summer after capture, only 9% had emigrated from the study area. Despite the longer dispersal distances of females, there was no difference in the survival functions of the sexes between the ages of 6 weeks and 13 months.

Testing the swash exclusion hypothesis in sandy beach populations: the mole crab Emerita brasiliensis in Uruguay

Exposed marine beaches are physically rigorous habitats in which macrofauna commu- nity patterns have been well correlated with physical factors such as grain size, beach slope and wave/swash processes. In this context, the swash exclusion hypothesis (SEH) gained wide accept- ability in explaining the control of species abundance and diversity of the sandy beach macrofauna by swash climate, determined by wave height and beach face slope. This hypothesis predicts a con- sistent increase in species richness, abundance and biomass from reflective to dissipative conditions. However, predictions of the SEH have not been adequately tested at a population level. Here, demo- graphic and life history characteristics of the intertidal mole crab Emerita brasiliensis were compared between populations of 2 microtidal exposed sandy beaches with contrasting morphodynamics dur- ing 22 consecutive months. The major prediction of the paradigm does not hold for the mole crab: total abundance, as well as that of males, females, males with spermatophores and megalops did not differ between beaches. Moreover, the reflective beach population presented higher male growth rates in size and lower natural mortality. Other population processes and life history traits gave support for the SEH: the dissipative beach population presented (1) higher abundance of ovigerous females, female growth rates, fecundity, egg production potential, size at maturity, as well as weight at size; and (2) lower female mortality and burrowing rates (for both sexes). The SEH seems to be particularly useful in explaining female somatic growth and reproductive features, which could be related to a major amount of food availability in dissipative beaches. However, other competing, but not exclusive, hypotheses are needed to explain variations in population abundance and other pro- cesses such as male growth and survivorship. The empirical evidence from several recent studies on the Uruguayan coast showed that sandy beach populations that co-occur in contrasting environments are less sensitive to variations in beach morphodynamics, which should not be considered the primary factor affecting the abundance and some life history traits. The importance of alternative population regulation processes and mechanisms are stressed.

Dynamics of fish populations with different compensatory processes when subjected to random survival of eggs and larvae

Abstract It is widely believed that abundance of fish populations is determined during the larval stage, for larval abundance is high, there is high variability in larval survival, and small changes in larval abundance could have large effects on population abundance. In this study, hypothesized population regulation processes were quantified in a mathematical model that was applied to study the importance of compensatory process during the larval stage by replacing them with random survival. The hypothesized population regulation processes were food-limited growth, size-dependent mortality, and age at maturity dependent on size. Variation in population abundance resulting from variation in egg and larval survival depends on the population regulation process. In population where mortality depends on size of individuals, variation in survival of young has little effect on abundance of adults. If age at maturity is flexible, variation in survival of young causes large fluctuations in abundance and the fluctuations follow a pattern of occasional large year classes that are slowly attrited over their lifespans. If age at maturity is not flexible, and if mortality is not size dependent, variation in survival of young causes population abundance to fluctuate widely without a clear pattern. Compensation during the larval life stage was not necessary for regulation of fish population size.