Habitat shape, metapopulation processes and the dynamics of multispecies predator-prey interactions

The Effects of Dispersal and Predator Density on Prey Survival in an Insect-Red Clover Metacommunity

Trophic interactions are often studied within habitat patches, but among-patch dispersal of individuals may influence local patch dynamics. Metacommunity concepts incorporate the effects of dispersal on local and community dynamics. There are few experimental tests of metacommunity theory using insects compared to those conducted in microbial microcosms. Using connected experimental mesocosms, we varied the density of the leafhopper Agallia constricta Van Duzee (Homoptera: Cicadellidae) and a generalist insect predator, the damsel bug (Nabis spp., Heteroptera: Nabidae), to determine the effects of conspecific and predator density and varying the time available to dispersal among mesocosms on predation rates, dispersal rates, and leafhopper survival. Conspecific and damsel bug density did not affect dispersal rates in leafhoppers, but this may be due to leafhoppers' aversion to leaving the host plants or the connecting tubes between mesocosms hindering leafhopper movement. Leafhopper dispersal was higher in high-dispersal treatments. Survival rates of A. constricta were also lowest in treatments where dispersal was not limited. This is one of the first experimental studies to vary predator density and the time available to dispersal. Our results indicate that dispersal is the key to understanding short-term processes such as prey survival in predator-prey metacommunities. Further work is needed to determine how dispersal rates influence persistence of communities in multigenerational studies.

Intermediate disturbance in experimental landscapes improves persistence of beetle metapopulations

Human-dominated landscapes often feature patches that fluctuate in suitability through space and time, but there is little experimental evidence relating the consequences of dynamic patches for species persistence. We used a spatially and temporally dynamic metapopulation model to assess and compare metapopulation capacity and persistence for red flour beetles (Tribolium castaneum) in experimental landscapes differentiated by resource structure, patch dynamics (destruction and restoration), and connectivity. High connectivity increased the colonization rate of beetles, but this effect was less pronounced in heterogeneous relative to homogeneous landscapes. Higher connectivity and faster patch dynamics increased extinction rates in landscapes. Lower connectivity promoted density-dependent emigration. Heterogeneous landscapes containing patches of different carrying capacity enhanced landscape-level occupancy probability. The highest metapopulation capacity and persistence was observed in landscapes with heterogeneous patches, low connectivity, and slow patch dynamics. Control landscapes with no patch dynamics exhibited rapid declines in abundance and approached extinction due to increased adult mortality in the matrix, higher pupal cannibalism by adults, and extremely low rates of exchange between remaining habitable patches. Our results highlight the role of intermediate patch dynamics, intermediate connectivity, and the nature of density dependence of emigration for persistence of species in heterogeneous landscapes. Our results also demonstrate the importance of incorporating local dynamics into the estimation of metapopulation capacity for conservation planning.

Explaining the host-finding behavior of blood-sucking insects: computerized simulation of the effects of habitat geometry on tsetse fly movement

BACKGROUND

Male and female tsetse flies feed exclusively on vertebrate blood. While doing so they can transmit the diseases of sleeping sickness in humans and nagana in domestic stock. Knowledge of the host-orientated behavior of tsetse is important in designing bait methods of sampling and controlling the flies, and in understanding the epidemiology of the diseases. For this we must explain several puzzling distinctions in the behavior of the different sexes and species of tsetse. For example, why is it that the species occupying savannahs, unlike those of riverine habitats, appear strongly responsive to odor, rely mainly on large hosts, are repelled by humans, and are often shy of alighting on baits?

METHODOLOGY/PRINCIPAL FINDINGS

A deterministic model that simulated fly mobility and host-finding success suggested that the behavioral distinctions between riverine, savannah and forest tsetse are due largely to habitat size and shape, and the extent to which dense bushes limit occupiable space within the habitats. These factors seemed effective primarily because they affect the daily displacement of tsetse, reducing it by up to ∼70%. Sex differences in behavior are explicable by females being larger and more mobile than males.

CONCLUSION/SIGNIFICANCE

Habitat geometry and fly size provide a framework that can unify much of the behavior of all sexes and species of tsetse everywhere. The general expectation is that relatively immobile insects in restricted habitats tend to be less responsive to host odors and more catholic in their diet. This has profound implications for the optimization of bait technology for tsetse, mosquitoes, black flies and tabanids, and for the epidemiology of the diseases they transmit.

Local competition and metapopulation processes drive long-term seagrass-epiphyte population dynamics

It is well known that ecological processes such as population regulation and natural enemy interactions potentially occur over a range of spatial scales, and there is a substantial body of literature developing theoretical understanding of the interplay between these processes. However, there are comparatively few studies quantifying the long-term effects of spatial scaling in natural ecosystems. A key challenge is that trophic complexity in real-world biological communities quickly obscures the signal from a focal process. Seagrass meadows provide an excellent opportunity in this respect: in many instances, seagrasses effectively form extensive natural monocultures, in which hypotheses about endogenous dynamics can be formulated and tested. We present amongst the longest unbroken, spatially explict time series of seagrass abundance published to date. Data include annual measures of shoot density, total above-ground abundance, and associated epiphyte cover from five Zostera marina meadows distributed around the Isles of Scilly, UK, from 1996 to 2011. We explore empirical patterns at the local and metapopulation scale using standard time series analysis and develop a simple population dynamic model, testing the hypothesis that both local and metapopulation scale feedback processes are important. We find little evidence of an interaction between scales in seagrass dynamics but that both scales contribute approximately equally to observed local epiphyte abundance. By quantifying the long-term dynamics of seagrass-epiphyte interactions we show how measures of density and extent are both important in establishing baseline information relevant to predicting responses to environmental change and developing management plans. We hope that this study complements existing mechanistic studies of physiology, genetics and productivity in seagrass, whilst highlighting the potential of seagrass as a model ecosystem. More generally, this study provides a rare opportunity to test some of the predictions of ecological theory in a natural ecosystem of global conservation and economic value.

Experimental beetle metapopulations respond positively to dynamic landscapes and reduced connectivity

Interactive effects of multiple environmental factors on metapopulation dynamics have received scant attention. We designed a laboratory study to test hypotheses regarding interactive effects of factors affecting the metapopulation dynamics of red flour beetle, Tribolium castaneum. Within a four-patch landscape we modified resource level (constant and diminishing), patch connectivity (high and low) and patch configuration (static and dynamic) to conduct a 2(3) factorial experiment, consisting of 8 metapopulations, each with 3 replicates. For comparison, two control populations consisting of isolated and static subpopulations were provided with resources at constant or diminishing levels. Longitudinal data from 22 tri-weekly counts of beetle abundance were analyzed using bayesian Poisson generalized linear mixed models to estimate additive and interactive effects of factors affecting abundance. Constant resource levels, low connectivity and dynamic patches yielded greater levels of adult beetle abundance. For a given resource level, frequency of colonization exceeded extinction in landscapes with dynamic patches when connectivity was low, thereby promoting greater patch occupancy. Negative density dependence of pupae on adults occurred and was stronger in landscapes with low connectivity and constant resources; these metapopulations also demonstrated greatest stability. Metapopulations in control landscapes went extinct quickly, denoting lower persistence than comparable landscapes with low connectivity. When landscape carrying capacity was constant, habitat destruction coupled with low connectivity created asynchronous local dynamics and refugia within which cannibalism of pupae was reduced. Increasing connectivity may be counter-productive and habitat destruction/recreation may be beneficial to species in some contexts.

Density-dependent population dynamics and dispersal in heterogeneous metapopulations

1. Metapopulation microcosms were constructed to test the effect of four different types of habitat heterogeneity on the dynamics and dispersal in spatially extended systems; homogeneity, spatial heterogeneity, temporal heterogeneity and spatio-temporal heterogeneity. Resources were distributed across discrete habitat patches in bruchid beetle (Callosobruchus maculatus) metapopulations, and long-term time series were recorded. 2. Mathematical models were fitted to the long-term time series from the experimental systems using a maximum likelihood approach. Models were composed of separate birth, death, emigration and immigration terms all of which incorporated stochasticity drawn from different probability distributions. Models with density-dependent and density-independent birth, death and emigration terms were investigated and, in each case, the model that best described the empirical data was identified. 3. At the local scale, population sizes differed between patches depending on the type of heterogeneity. Larger populations were associated with higher resource availabilities. As a result of this, the variation between local population sizes was greatest when there was spatial heterogeneity in which mean resource abundance varied from patch to patch. Variation in population sizes within patches was largest when there was temporal heterogeneity. 4. Density-dependent processes leading to the regulation of local population dynamics in our experimental systems were strongest in homogeneity or temporal heterogeneity treatments. Associated with this, we found that these systems were best described using mathematical models with density dependence acting on mortality. In contrast, spatial and spatio-temporal time series were adequately described using density-independent population processes. 5. Experimental metapopulations showed varying degrees of density-dependent dispersal. Local net dispersal each week was primarily driven by the local population size and secondarily affected by neighbourhood population density. Mathematical population models illustrated the importance of explicit description of density-dependent dispersal in all systems except the homogeneous metapopulations.

Predators reduce extinction risk in noisy metapopulations

Background

Spatial structure across fragmented landscapes can enhance regional population persistence by promoting local “rescue effects.” In small, vulnerable populations, where chance or random events between individuals may have disproportionately large effects on species interactions, such local processes are particularly important. However, existing theory often only describes the dynamics of metapopulations at regional scales, neglecting the role of multispecies population dynamics within habitat patches.

Findings

By coupling analysis across spatial scales we quantified the interaction between local scale population regulation, regional dispersal and noise processes in the dynamics of experimental host-parasitoid metapopulations. We find that increasing community complexity increases negative correlation between local population dynamics. A potential mechanism underpinning this finding was explored using a simple population dynamic model.

Conclusions

Our results suggest a paradox: parasitism, whilst clearly damaging to hosts at the individual level, reduces extinction risk at the population level.

Apparent Competition and Vector-Host Interactions

Infectious disease influences the dynamics of host populations and the structure of species communities via impacts on host demography. Species that share infectious diseases are well-known to interact indirectly through the process of apparent competition, but there has been little attention given to the role of vectors in these indirect interactions. Here we explore how vector-borne disease and host-vector interactions can drive apparent competitive interactions. We show that different facets of the ecology associated with vector-host-host interactions affect the structure of the three-species assemblage. Crucially, the patterns associated with invasion of alternative hosts, the spread of the infectious disease by the vector, and the dynamics of the community interactions are influenced by the mode of transmission. We highlight the role of alternative hosts on disease amplification, dilution and magnification and discuss the results with reference to recent developments in apparent competition and community structure.

How does spatial dispersal network affect the evolution of parasite local adaptation?

Studying patterns of parasite local adaptation can provide insights into the spatiotemporal dynamics of host-parasite coevolution. Many factors, both biotic and abiotic, have been identified that influence parasite local adaptation. In particular, dispersal and population structuring are considered important determinants of local adaptation. We investigated how the shape of the spatial dispersal network within experimental landscapes affected local adaptation of a bacteriophage parasite to its bacterial host. Regardless of landscape topology, dispersal always led to the evolution of phages with broader infectivity range. However, when the spatial dispersal network resulted in spatial variation in the breadth of phage infectivity range, significant levels of parasite local adaptation and local maladaptation were detected within the same landscape using the local versus foreign definition of local adaptation. By contrast, local adaptation was not detected using the home versus away or local versus global definitions of local adaptation. This suggests that spatial dispersal networks may play an important role in driving parasite local adaptation, particularly when the shape of the dispersal network generates nonuniform levels of host resistance or parasite infectivity throughout a species' range.

The effects of colonization, extinction and competition on co-existence in metacommunities

1. The co-existence of competitors in heterogeneous landscapes depends on the processes of colonization, extinction and spatial scale. In this study, we explore the metapopulation dynamics of competitive interactions. 2. Rather than simply evaluating the outcome of interspecific competition in the traditional manner, we focus on both the local population dynamic effects and the regional metapopulation processes affecting species co-existence. 3. We develop a theoretical model of regional co-existence to generate a set of predictions on the patterns of colonization necessary for co-existence and the regional processes that can lead to competitive exclusion. We empirically test these predictions using metacommunity microcosms of the interaction between two bruchid beetles (Callosobruchus chinensis, Callosobruchus maculatus). 4. Using well-replicated time series of the interaction between the bruchids and statistical methods of model fitting, we show how the qualitative and quantitative pattern of interspecific competition between the bruchid beetles is shaped by the structure of the metacommunity. 5. In unlimited dispersal metacommunities, the global exclusion of the inferior competitor is shown to be influenced more by the processes associated with extinction rather than low colonization ability. In restricted dispersal metacommunities, we show how the co-existence of competitors in a spatially heterogeneous habitat (patches connected through limited dispersal) is affected by Allee effects and life-history [colonization (dispersal) - competition] trade-offs.

Overcompensatory population dynamic responses to environmental stochasticity

1. To quantify the interactions between density-dependent, population regulation and density-independent limitation, we studied the time-series dynamics of an experimental laboratory insect microcosm system in which both environmental noise and resource limitation were manipulated. 2. A hierarchical Bayesian state-space approach is presented through which it is feasible to capture all sources of uncertainty, including observation error to accurately quantify the density dependence operating on the dynamics. 3. The regulatory processes underpinning the dynamics of two different bruchid beetles (Callosobruchus maculatus and Callosobruchus chinensis) are principally determined by environmental conditions, with fluctuations in abundance explained in terms of changes in overcompensatory dynamics and stochastic processes. 4. A general, stochastic population model is developed to explore the link between abundance fluctuations and the interaction between density dependence and noise. Taking account of time-lags in population regulation can substantially increase predicted population fluctuations resulting from underlying noise processes.

How to find a metapopulationThis review is one of a series dealing with some aspects of the impact of habitat fragmentation on animals and plants. This series is one of several virtual symposia focussing on ecological topics that will be published in the Journal from time to time

Where habitat loss and fragmentation is severe, many native species are likely to have reduced levels of dispersal between remnant populations. For those species to avoid regional extinction in fragmented landscapes, they must undergo some kind of metapopulation dynamics so that local extinctions are countered by recolonisation. The importance of spatial dynamics for regional survival means that research into metapopulation dynamics is essential. In this review I explore the approaches taken to examine metapopulation dynamics, highlight the analytical methods used to get the most information out of field data, and discover some of the major research gaps. Statistical models, including Hanski’s incidence function model (IFM) are frequently applied to presence–absence data, an approach that is often strengthened using long-term data sets that document extinctions and colonisations. Recent developments are making the IFM more biologically realistic and expanding the range of situations for which the model is relevant. Although accurate predictions using the IFM seem unlikely, it may be useful for ranking management decisions. A key weakness of presence–absence modelling is that the mechanisms underlying spatial dynamics remain inferential, so combining modelling approaches with detailed demographic research is warranted. For species where very large data sets cannot be obtained to facilitate statistical modelling, a demographic approach alone or with stochastic modelling may be the only viable research angle to take. Dispersal is a central process in metapopulation dynamics. Research combining mark–recapture or telemetry methods with model-selection procedures demonstrate that dispersal is frequently oversimplified in conceptual and statistical metapopulation models. Dispersal models like the island model that underlies classic metapopulation theory do not approximate the behaviour of real species in fragmented landscapes. Nevertheless, it remains uncertain if additional biological realism will improve predictions of statistical metapopulation models. Genetic methods can give better estimates of dispersal than direct methods and take less effort, so they should be routinely explored alongside direct ecological methods. Recent development of metacommunity theory (communities connected by dispersal) emphasises a range of mechanisms that complement metapopulation theory. Taking both theories into account will enhance interpretation of field data. The extent of metapopulation dynamics in human modified landscapes remains uncertain, but we have a powerful array of field and analytical approaches for reducing this knowledge gap. The most informative way forward requires that many species are studied in the same fragmented landscape by applying a selection of approaches that reveal complementary aspects of spatial dynamics.

Metapopulation extinction risk is increased by environmental stochasticity and assemblage complexity

Extinction risk is a key area of investigation for contemporary ecologists and conservation biologists. Practical conservation efforts for vulnerable species can be considerably enhanced by thoroughly understanding the ecological processes that interact to determine species persistence or extinction. Theory has highlighted the importance of both extrinsic environmental factors and intrinsic demographic processes. In laboratory microcosms, single-species single-habitat patch experimental designs have been widely used to validate the theoretical prediction that environmental heterogeneity can increase extinction risk. Here, we develop on this theme by testing the effects of fluctuating resource levels in experimental multispecies metapopulations. We compare a three-species host-parasitoid assemblage that exhibits apparent competition to the individual pairwise, host-parasitoid interactions. Existing theory is broadly supported for two-species assemblages: environmental stochasticity reduces trophic interaction persistence time, while metapopulation structure increases persistence time. However, with increasing assemblage complexity, the effects of trophic interactions mask environmental impacts and persistence time is further reduced, regardless of resource renewal regime. We relate our findings to recent theory, highlighting the importance of taking into account both intrinsic and extrinsic factors, over a range of spatial scales, in order to understand resource-consumer dynamics.