Erik G. Noonburg

Scale dependent effects of biotic resistance to biological invasion

As nonindigenous species continue to displace native species and disrupt ecosystems, understanding the degree to which native species richness affects the vulner- ability of communities to nonindigenous species invasions has grown in importance. Native and exotic species diversity are often positively correlated in large-scale observational studies, but negatively correlated in small-scale experimental studies. This discrepancy suggests that the scale of invasion studies may be an important influence on their outcomes. Using a competition-based model that exhibits a negative relationship on a small scale, we show that changes in the number of available resources across communities can cause invasion success to become positively correlated with native species diversity at larger scales. The strength of the positive correlation, however, depends on the relationship be- tween niche breadth and species diversity in natural communities. Adding species to a community or removing resources has a similar effect—increasing the sum of interspecific interaction strengths, which decreases invasion success.

MORE HARM THAN GOOD: WHEN INVADER VULNERABILITY TO PREDATORS ENHANCES IMPACT ON NATIVE SPECIES

Invasion biologists typically regard susceptibility of an invasive species to native predators as a fortuitous condition that increases biotic resistance to the invasion. The line of reasoning is that predation weakens the net impact of the invader and reduces its ability to displace native competitors. However, predation on invasives is a coupled interaction; every invader consumed also enhances the predator population. If these predators also consume native species, then the invaders indirect effect via predators (i.e., apparent competition) could be more harmful to natives than the effect of resource competition from the exotic. We apply general community ecology theory to determine the conditions under which the net effect of predation on the exotic species is to extirpate the native competitor. An approximation to these conditions provides a simple metric to estimate the threat of native species extinction due to apparent competition posed by an invader.

POACHING, ENFORCEMENT, AND THE EFFICACY OF MARINE RESERVES

Marine reserves are promoted as an effective supplement to traditional fishery management techniques of harvest quotas and effort limitation. However, quantitative fishery models have ignored the impact of noncompliance (poaching). Here we link a model of a harvested fish population to a game-theoretic representation of fisherman behavior to quantify the effect of poaching on fishery yield and the enforcement effort required to maintain any desired level of reserve effectiveness. Although higher fish densities inside reserves will typically entice fishermen to poach, we show that the initial investment in enforcement efforts provides the greatest return on maintaining the benefits of the reserve to the fishery. Furthermore, we find that poaching eliminates the positive effect of fish dispersal on yield that is predicted by traditional models that ignore fisherman behavior. Our results broaden a fundamental insight from previous models of marine reserves, the effective equivalence of the harvest quota and reserve fraction, to the more realistic scenario in which fishermen attempt to maximize their economic payoffs.

Transient Dynamics Limit the Effectiveness of Keystone Predation in Bringing about Coexistence

We analyze the transient dynamics of simple models of keystone predation, in which a predator preferentially consumes the dominant of two (or more) competing prey species. We show that coexistence is unlikely in many systems characterized both by successful invasion of either prey species into the food web that lacks it and by a stable equilibrium with high densities of all species. Invasion of the predator‐resistant consumer species often causes the resident, more vulnerable prey to crash to such low densities that extinction would occur for many realistic population sizes. Subsequent transient cycles may entail very low densities of the predator or of the initially successful invader, which may also preclude coexistence of finite populations. Factors causing particularly low minimum densities during the transient cycles include biotic limiting resources for the prey, limited resource partitioning between the prey, a highly efficient predator with relatively slow dynamics, and a vulnerable prey whose population dynamics are rapid relative to the less vulnerable prey. Under these conditions, coexistence of competing prey via keystone predation often requires that the prey’s competitive or antipredator characteristics fall within very narrow ranges. Similar transient crashes are likely to occur in other food webs and food web models.

Life history and matrix heterogeneity interact to shape metapopulation connectivity in spatially structured environments

Metapopulation models have historically treated a landscape as a collection of habitat patches separated by a matrix of uniformly unsuitable habitat. This perspective is still apparent in many studies of marine metapopulations, in which recruitment variation is generally assumed to be primarily the result of variability in ocean currents and interactions with disperser behavior, with little consideration of spatial structure that can affect disperser viability. We use a simple model of dispersal of marine larvae to demonstrate how heterogeneity in dispersal habitat (i.e., the matrix) can generate substantial spatial variation in recruitment. Furthermore, we show how this heterogeneity can interact with larval life-history variation to create alternative patterns of source-sink dynamics. Finally, we place our results in the context of spatially structured matrix population models, and we propose the damping ratio of the connectivity matrix as a general and novel measure of landscape connectivity that may provide conceptual unification to the fields of metapopulation biology and landscape ecology.

Linking dynamic habitat selection with wading bird foraging distributions across resource gradients

Species distribution models (SDM) link species occurrence with a suite of environmental predictors and provide an estimate of habitat quality when the variable set captures the biological requirements of the species. SDMs are inherently more complex when they include components of a species’ ecology such as conspecific attraction and behavioral flexibility to exploit resources that vary across time and space. Wading birds are highly mobile, demonstrate flexible habitat selection, and respond quickly to changes in habitat quality; thus serving as important indicator species for wetland systems. We developed a spatio-temporal, multi-SDM framework using Great Egret (Ardea alba), White Ibis (Eudocimus albus), and Wood Stork (Mycteria Americana) distributions over a decadal gradient of environmental conditions to predict species-specific abundance across space and locations used on the landscape over time. In models of temporal dynamics, species demonstrated conditional preferences for resources based on resource levels linked to differing temporal scales. Wading bird abundance was highest when prey production from optimal periods of inundation was concentrated in shallow depths. Similar responses were observed in models predicting locations used over time, accounting for spatial autocorrelation. Species clustered in response to differing habitat conditions, indicating that social attraction can co-vary with foraging strategy, water-level changes, and habitat quality. This modeling framework can be applied to evaluate the multi-annual resource pulses occurring in real-time, climate change scenarios, or restorative hydrological regimes by tracking changing seasonal and annual distribution and abundance of high quality foraging patches.

Consequences of variable larval dispersal pathways and resulting phenotypic mixtures to the dynamics of marine metapopulations

Larval dispersal can connect distant subpopulations, with important implications for marine population dynamics and persistence, biodiversity conservation and fisheries management. However, different dispersal pathways may affect the final phenotypes, and thus the performance and fitness of individuals that settle into subpopulations. Using otolith microchemical signatures that are indicative of ‘dispersive’ larvae (oceanic signatures) and ‘non-dispersive’ larvae (coastal signatures), we explore the population-level consequences of dispersal-induced variability in phenotypic mixtures for the common triplefin (a small reef fish). We evaluate lipid concentration and otolith microstructure and find that ‘non-dispersive’ larvae (i) have greater and less variable lipid reserves at settlement (and this variability attenuates at a slower rate), (ii) grow faster after settlement, and (iii) experience similar carry-over benefits of lipid reserves on post-settlement growth relative to ‘dispersive’ larvae. We then explore the consequences of phenotypic mixtures in a metapopulation model with two identical subpopulations replenished by variable contributions of ‘dispersive’ and ‘non-dispersive’ larvae and find that the resulting phenotypic mixtures can have profound effects on the size of the metapopulation. We show that, depending upon the patterns of connectivity, phenotypic mixtures can lead to larger metapopulations, suggesting dispersal-induced demographic heterogeneity may facilitate metapopulation persistence.

Effects of life history variation on vertical transfer of toxicants in marine mammals

Toxicant bioaccumulation poses a risk to many marine mammal populations. Although individual-level toxicology has been the subject of considerable research in several species, we lack a theoretical framework to generalize the results across environments and life histories. Here we formulate a dynamic energy budget model to predict the effects of intra- and interspecific life history variation on toxicant dynamics in marine mammals. Dynamic energy budget theory attempts to describe the most general processes of energy acquisition and utilization in heterotrophs. We tailor the basic model to represent the marine mammal reproductive cycle, and we add a model of toxicant uptake and partitioning to describe vertical transfer of toxicants from mother to offspring during gestation and lactation. We first show that the model predictions are consistent with qualitative patterns reported in empirical studies and previous species-specific modeling studies. Next, we use this model to examine the dependence of offspring toxicant load on birth order, food density, and interspecific life history variation.

Alternative Stable States, Coral Reefs, and Smooth Dynamics with a Kick

We consider a computer simulation, which was found to be faithful to time series data for Caribbean coral reefs, and an analytical model to help understand the dynamics of the simulation. The analytical model is a system of ordinary differential equations (ODE), and the authors claim this model demonstrates the existence of alternative stable states. The existence of an alternative stable state should consider a sudden shift in coral and macroalgae populations, while the grazing rate remains constant. The results of such shifts, however, are often confounded by changes in grazing rate. Although the ODE suggest alternative stable states, the ODE need modification to explicitly account for shifts or discrete events such as hurricanes. The goal of this paper will be to study the simulation dynamics through a simplified analytical representation. We proceed by modifying the original analytical model through incorporating discrete changes into the ODE. We then analyze the resulting dynamics and their bifurcations with respect to changes in grazing rate and hurricane frequency. In particular, a “kick” enabling the ODE to consider impulse events is added. Beyond adding a “kick” we employ the grazing function that is suggested by the simulation. The extended model was fit to the simulation data to support its use and predicts the existence cycles depending nonlinearly on grazing rates and hurricane frequency. These cycles may bring new insights into consideration for reef health, restoration and dynamics.