Population ecology

From migration to nomadism: movement variability in a northern ungulate across its latitudinal range

by Luca Börger

Ecological Applications (in print) Co-authored with Navinder Singh (first author), Nils Bunnefeld, Holger Detki, and Goran Ericsson.

This is the first proper application of my net-squared displacement approach for modelling animal movements (Borger & Fryxell 2012).
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Understanding the causes and consequences of animal movements is of fundamental biological interest because any... more Understanding the causes and consequences of animal movements is of fundamental biological interest because any alteration in movement can have direct and indirect effects on ecosystem structure and function. It is also crucial for assisting spatial wildlife management under variable environmental change scenarios. Recent research has highlighted the need of quantifying individual variability in movement behavior and how it is generated by interactions between individual requirements and environmental conditions, to understand the emergence of population level patterns. Using a multi-annual movement dataset of 213 individual moose (Alces alces) across a latitudinal gradient (from 56° to 67° N) that spans over 1,100 km of varying environmental conditions, we analyze the differences in individual and population level movements. We tested the effect of climate, risk and human presence in the landscape on moose movements. The variation in these factors explained the existence of multiple movements (migration, nomadism, dispersal, sedentary) among individuals and seven populations. Hence, heterogeneity in the immediate environment can result in multiple movements within a species. Population differences were primarily related to latitudinal variation in snow depth and road density. Individuals showed both fixed and flexible behaviors across years, and were less likely to migrate with age in interaction with snow and roads. For the predominant movement strategy, migration, the distance, timing and duration at all latitudes varied between years. Males traveled longer distances and began migrating earlier in spring than females. Our study provides strong quantitative evidence for the dynamics of animal movements in response to changes in environmental conditions along with varying risk from human influence across the landscape. For moose, given its wide distributional range, changes in the distribution and migratory behavior are expected under future warming scenarios.

Demographic Heterogeneity Impacts Density-Dependent Population Dynamics

by Gordon Fox

Theoretical Ecology 2012

Among-individual variation in vital parameters
such as birth and death rates that is unrelated to age, stage,more Among-individual variation in vital parameters
such as birth and death rates that is unrelated to age, stage,
sex, or environmental fluctuations is referred to as demographic
heterogeneity. This kind of heterogeneity is prevalent
in ecological populations, but is almost always left out
of models. Demographic heterogeneity has been shown to
affect demographic stochasticity in small populations and
to increase growth rates for density-independent populations.
The latter is due to “cohort selection”, where the most
frail individuals die out first, lowering the cohort’s average
mortality as it ages. The importance of cohort selection to
population dynamics has only recently been recognized.We
use a continuous time model with density dependence, based
on the logistic equation, to study the effects of demographic
heterogeneity in mortality and reproduction. Reproductive
heterogeneity is introduced in three ways: parent fertility,
offspring viability, and parent-offspring correlation. We
find that both the low-density growth rate and the equilibrium
population size increase as the magnitude of mortality
heterogeneity increases or as parent-offspring phenotypic correlation increases. Population dynamics are affected by
complex interactions among the different types of heterogeneity,
and trade-off scenarios are examined which can
sometimes reverse the effect of increased heterogeneity. We
show that there are a number of different homogeneous approximations to heterogeneous models, but all fail to capture
important parts of the dynamics of the full model.

VARIATION AMONG INDIVIDUALS IN CONE PRODUCTION IN PINUS PALUSTRIS (PINACEAE)

by Gordon Fox

American Journal of Botany 99: 1-6

• Premise of the study: Reproductive output varies considerably among individuals within plant populations, and this... more • Premise of the study: Reproductive output varies considerably among individuals within plant populations, and this is especially
so in cone production of conifers. While this variation can have substantial effects on populations, little is known about
its magnitude or causes.
• Methods: We studied variation in cone production for 2 years within a population of Pinus palustris Mill. (longleaf pine;
Pinaceae). Using hurdle models, we evaluated the importance of burn treatments, tree size (dbh), canopy status (open, dominant,
subordinate), and number of conspecifi c neighbors within 4 m (N 4 ).
• Key results: Cone production of individuals — even after accounting for other variables — was strongly correlated between
years. Trees in plots burned every 1, 2, or 5 years produced more cones than those burned every 7 years, or unburned. Larger
trees tend to produce more cones, but the large effects of the other factors studied caused substantial scatter in the dbh-cone
number relationship. Among trees in the open, dbh had little explanatory power. Subordinate trees with three neighbors produced
no cones.
• Conclusions: Tree size alone was a weak predictor of cone production. Interactions with neighbors play an important role in
generating reproductive heterogeneity, and must be accounted for when relating cone production to size. The strong betweenyear
correlation, together with the large variance in cone production among trees without neighbors, suggests that still more of
the variance may be explainable, but requires factors outside of our study.

Population-Level Metrics of Trophic Structure Based on Stable Isotopes and Their Application to Invasion Ecology

by Jonathan Grey

PLoS One 2012

Biological invasions are a significant driver of human-induced global change and many ecosystems sustain sympatric... more Biological invasions are a significant driver of human-induced global change and many ecosystems sustain sympatric invaders. Interactions occurring among these invaders have important implications for ecosystem structure and functioning, yet they are poorly understood. Here we apply newly developed metrics derived from stable isotope data to provide quantitative measures of trophic diversity within populations or species. We then use these to test the hypothesis that sympatric invaders belonging to the same functional feeding group occupy a smaller isotopic niche than their allopatric counterparts. Two introduced, globally important, benthic omnivores, Louisiana swamp crayfish (Procambarus clarkii) and carp (Cyprinus carpio), are sympatric in Lake Naivasha, Kenya. We applied our metrics to an 8-year data set encompassing the establishment of carp in the lake. We found a strong asymmetric interaction between the two invasive populations, as indicated by inverse correlations between carp abundance and measures of crayfish trophic diversity. Lack of isotopic niche overlap between carp and crayfish in the majority of years indicated a predominantly indirect interaction. We suggest that carp-induced habitat alteration reduced the diversity of crayfish prey, resulting in a reduction in the dietary niche of crayfish. Stable isotopes provide an integrated signal of diet over space and time, offering an appropriate scale for the study of population niches, but few isotope studies have retained the often insightful information revealed by variability among individuals in isotope values. Our population metrics incorporate such variation, are robust to the vagaries of sample size and are a useful additional tool to reveal subtle dietary interactions among species. Although we have demonstrated their applicability specifically using a detailed temporal dataset of species invasion in a lake, they have a wide array of potential ecological applications.

High site fidelity and low site connectivity in temperate salt marsh fish populations: a stable isotope approach

by Jonathan Grey

Oecologia 2012

Adult and juvenile fish utilise salt marshes for food and shelter at high tide, moving into adjacent sublittoral... more Adult and juvenile fish utilise salt marshes for food and shelter at high tide, moving into adjacent sublittoral regions during low tide. Understanding whether there are high levels of site fidelity for different species of coastal fish has important implications for habitat conservation and the design of marine protected areas. We hypothesised that common salt marsh fish species would demonstrate a high site fidelity, resulting in minimal inter-marsh connectivity. Carbon (13C) and nitrogen (15N) stable isotope ratios of larvae and juveniles of five common salt marsh fish (Atherina presbyter, Chelon labrosus, Clupea harengus, Dicentrarchus labrax, Pomatoschistus microps), seven types of primary producer and seven secondary consumer food sources were sampled in five salt marshes within two estuary complexes along the coast of south-east England. Significant differences in 13C and 15N signatures between salt marshes indicated distinct sub-populations utilising the area of estuary around each salt marsh, and limited connectivity, even within the same estuary complex. 15N ratios were responsible for the majority of inter-marsh differences for each species and showed similar site-specific patterns in ratios in primary producers, secondary consumers and fish. Fish diets (derived from isotope mixing models) varied between species but were mostly consistent between marsh sites, indicating that dietary shifts were not the source of variability of the inter-marsh isotopic signatures within species. These results demonstrate that for some common coastal fish species, high levels of site fidelity result in individual salt marshes operating as discrete habitats for fish assemblages.

Modeling the eradication of invasive mammals using the sterile male technique

by Sergios-Orestis Kolokotronis

Biological Invasions

Hatching asynchrony and growth trade-offs within barn swallow broods

by Jonathan Grey

Condor 2009

Hatching asynchrony results in age and size hierarchies within broods, and the subsequent asymmetric competition among... more Hatching asynchrony results in age and size hierarchies within broods, and the subsequent asymmetric competition among siblings has important consequences for nestlings’ fitness. In this study, we compare the growth of Barn Swallow (Hirundo rustica) nestlings in relation to their order of hatching. The aim was to test the prediction that early-hatched nestlings develop differently from late-hatched nestlings, which should be under greater pressure to trade investment in growth in favor of traits important to simultaneous fledging. Early-hatched nestlings were always larger than late-hatched nestlings, but when the age difference was taken into account, the two classes of nestling gained mass and head–bill length in similar ways, including having similar asymptotes, as predicted by nonlinear curve models. For wing length, however, late-hatched nestlings reached the inflection point of growth sooner than early-hatched nestlings, and although scaled rates of wing growth were similar, earlyhatched nestlings had significantly longer wings, both before fledging, when the oldest nestling was 14 days old, and as suggested by the asymptotic, age-independent values derived from the nonlinear curve models. This finding suggests that nestlings hatched later preferentially develop body mass and the skeleton at the expense of wing feathers. As swallows rely on their wings for foraging and avoiding predators, this pattern of resource allocation is likely to have negative consequences for the late-hatched nestlings.

Population-level metrics of trophic structure based on stable isotopes and their application to invasion ecology

by Michelle Jackson

Chaos in a long-term experiment with a plankton community

by Klaus Joehnk

Benincà, E., Huisman, J., Heerkloss, R., Jöhnk, K.D, Branco, P., Van Nes, E.H., Scheffer, M. & Ellner, S., 2008. Chaos in a long-term experiment with a plankton community. Nature 451: 822-825.

What do matrix population models reveal about the sustainability of non‐timber forest product harvest?

by Lisa Mandle

Schmidt, I. B., L. Mandle, T. Ticktin and O. G. Gaoue. (2011) What do matrix population models reveal about sustainability of non-timber forest product (NTFP) harvest? Journal of Applied Ecology. 48(4):815-826.

The Comedy of the Commons

by Nolan Bensen

An attack on Garett Hardin's classic, closely read, and much vulgarized 1968 article "The Tragedy of the Commons." I argue that while the vulgarizations have tended to misinterpret the article, they have not failed to spread mild and reserved versions of its thinly veiled racism and totally unveiled advocacy for eugenics.

Plover: a Subpopulation-Based Model of the Effects of Management on Western Snowy Plovers

by Michele M. Tobias

Endangered Species Update, 2007, 24(4): 104-109; Tables and Figures printed in 25(1) of 2008 (missing from original publication)

Plover is a program written in VB.net to model a subpopulation of Western Snowy Plovers (Charadrius alexandrinus... more Plover is a program written in VB.net to model a subpopulation of Western Snowy Plovers (Charadrius alexandrinus nivosus).  The model is based on data and observations in scientific literature.  It was intended to assist managers in understanding how management decisions could potentially affect a local population.  The results of this model underscore the need to protect Western Snowy Plovers from human impacts.

Kendall, B. E., G. A. Fox, M. Fujiwara, and T. Nogeire. 2011. Demographic heterogeneity, cohort selection and population growth.

by Gordon Fox

Ecology 92:1985-1993.

Demographic heterogeneity—variation among individuals in survival and reproduction—is ubiquitous in natural... more Demographic heterogeneity—variation among individuals in survival and reproduction—is ubiquitous in natural populations. Structured population models address heterogeneity due to age, size, or major developmental stages. However, other important
sources of demographic heterogeneity, such as genetic variation, spatial heterogeneity in the environment, maternal effects, and differential exposure to stressors, are often not easily measured and hence are modeled as stochasticity. Recent research has elucidated the role of demographic heterogeneity in changing the magnitude of demographic stochasticity in small populations. Here we demonstrate a previously unrecognized effect: heterogeneous survival in long-lived species can increase the long-term growth rate in populations of any size. We illustrate this result using simple models in which each individual’s annual survival rate is independent of age but survival may differ among individuals within a cohort. Similar models, but with nonoverlapping generations, have been extensively studied by demographers, who showed that, because the more ‘‘frail’ individuals are more likely to die at a young age, the average survival rate of the cohort increases with age. Within ecology and evolution, this phenomenon of ‘‘cohort selection’’ is increasingly appreciated as a confounding factor in studies of senescence. We show that, when placed in a population model with overlapping generations, this heterogeneity also causes the asymptotic population growth rate k to increase, relative to a homogeneous population with the same mean survival rate at birth. The increase occurs because, even integrating over all the cohorts in the population, the population becomes increasingly dominated by the more robust individuals. The growth rate increases monotonically with the variance in survival rates, and the effect can be substantial, easily doubling the growth rate of slow-growing populations. Correlations between parent and
offspring phenotype change the magnitude of the increase in k, but the increase occurs even for negative parent–offspring correlations. The effect of heterogeneity in reproductive rate on
k is quite different: growth rate increases with reproductive heterogeneity for positive parent–offspring correlation but decreases for negative parent–offspring correlation. These effects of demographic heterogeneity on k have important implications for population dynamics, population viability analysis, and evolution.

Molecular differentiation of species of the genus Zungaro (Siluriformes, Pimelodidae) from the Amazon and Paraná-Paraguay River basins in Brazil

by Andre Padial

Fish species of the Zungaro genus (Siluriformes, Pimelodidae) are amongst the largest migratory fish in Latin America... more Fish species of the Zungaro genus (Siluriformes, Pimelodidae) are amongst the largest migratory fish in Latin America and have considerable economic importance for commercial fishing in Brazil. However, natural populations of this large catfish are experiencing a severe decline. There are significant taxonomical inconsistencies for this fish. Two geographically separated species of the fish were initially described, one endemic in the Amazon and another in the Paraná-Paraguay River basins. A taxonomic review had recently proposed that there is only one Zungaro species in Brazil, based on morphological data. We made a molecular study of Zungaro populations in an attempt to solve taxonomical inconsistencies and to analyze genetic diversity in natural populations of this genus. We analyzed two regions of the mitochondrial DNA (the control region and the ATPase 6 gene region) of individuals sampled from the Paraná-Paraguay River and Amazon River basins. Analyses based on p-distances and maximum likelihood phylogenetic models showed a genetic difference between populations corresponding to different species. Genetic differentiation between Zungaro populations was at the same level as that observed between other Siluriformes species, using the same DNA sequences. We conclude that Zungaro species of the Paraná-Paraguay River basin do not belong to the same species found in the Amazon basin. This finding has a significant implication for conservation of this fish, given that populations are disappearing at a high rate in the Paraná-Paraguay River basin, mainly due to impoundments.

Sources of variation in growth, form, and survival in dwarf and normal-stature pitch pines (Pinus rigida, Pinaceae) in long-term transplant experiments

by Gordon Fox

Variation in recruitment and early demography in Pinus rigida following crown fire in the pine barrens of Long Island, New York

by Gordon Fox

Consequences of heterogeneity in survival probability in a population of Florida scrub‐jays

by Gordon Fox

Unstructured individual variation and demographic stochasticity

by Gordon Fox

Spatial structure, environmental heterogeneity, and population dynamics: analysis of the coupled logistic map

by Gordon Fox

Consequences of flowering-time variation in a desert annual: adaptation and history

by Gordon Fox