Density-dependent Interference Research Articles

Light intensity and seed density differentially affect the establishment, survival, and biomass of an exotic invader and three species of native competitors

Understanding how invasive and native plant species respond to their environment is essential for explaining and managing plant invasions in changing ecosystems. Light limitation and density-dependent interactions affect plant performance, but few studies have experimentally evaluated the effects of these factors on invasive and native neighbors simultaneously. Our objective was to test the hypothesis that light limitation and density-dependent interference are critical factors that influence the performance of the invasive species Bromus madritensis ssp. rubens (“B. rubens”) and three native species that commonly compete with B. rubens in North American drylands. In a full-factorial experiment, we examined the effects of (a) light limitation, (b) increasing B. rubens seed density, and (c) increasing native seed density on the establishment, survival, and biomass of each study species over a 12-week growing period in a controlled greenhouse. Light limitation reduced at least one performance metric (establishment, survival, or biomass) for all species. Increasing B. rubens seed density had no intraspecific effect on its own performance but had variable and species-specific negative effects on natives. Increasing native seed density reduced B. rubens establishment, survival, and biomass but had variable and species-specific effects on the natives themselves. These findings suggest that light intensity and density-dependent interactions can strongly but differentially influence the establishment, survival, and biomass of an important subset of invasive and native species in North American drylands. Evaluating multiple performance metrics at the species level is essential for understanding how invasive and native neighbors respond to environmental variation in changing dryland ecosystems.

Female Density-Dependent Chemical Warfare Underlies Fitness Effects of Group Sex Ratio in Flour Beetles

In animals, skewed sex ratios can affect individual fitness via sexual (e.g., intersexual conflict or intrasexual mate competition) or nonsexual (e.g., sex-specific resource competition) interactions. Because most analyses of sex ratio focus on sexual interactions, the relative importance of sexual versus nonsexual mechanisms remains unclear. We tested both mechanisms in the flour beetle Tribolium castaneum, where male-biased sex ratios increase female fitness relative to unbiased or female-biased groups. Although flour beetles show both sexual and nonsexual (resource) competition, we found that sexual interactions did not explain female fitness. Instead, female fecundity was dramatically reduced even after a brief exposure to flour conditioned by other females. Earlier studies suggested that secreted toxins might mediate density-dependent population growth in flour beetles. We identified ethyl benzoquinone and methyl benzoquinone (quinones) as components of adult stink glands that regulate female fecundity. In female-biased groups (i.e., at high female density), females upregulated quinones and suppressed each other’s reproduction. In male-biased groups, low female density and associated low quinone levels maximized fecundity. Thus, females appear to use quinones as weapons for female-specific, density-dependent interference competition. Our results underscore the importance of nonsexual interference competition that may often underlie the fitness consequences of skewed sex ratios.

Non-quantitative adjustment of offspring sex ratios in pollinating fig wasps.

Fig wasp is one of the most well known model systems in examining whether or not the parents could adjust their offspring sex ratio to maximize their gene frequency transmission in next generations. Our manipulative experiments showed that, in all of the five pollinator wasps of figs (Agaonidae) that have different averages of foundress numbers per syconium, almost the same proportions of male offspring are produced in the experiment that foundresses deposit one hour then are killed with ether (66.1%–70.1%) and over the lifespan of each foundress (14.0%–21.0%). The foundresses tend to deposit their male eggs prior to female eggs. The observed increase in the proportion of male offspring as a function of foundress number results from density-dependent interference competition among the foundresses. These results showed that the selection of gene frequency transmission through the behavioral adjustment in the evolution of sex ratio does not exist in these five fig wasps. The results here implied that genetic adjustment mechanisms of the sex ratio of fig wasps can only be triggered to be on or off and that the foundresses can not quantitatively adjust their sex ratio according to increased environmental selection pressure.

Density-dependent interference of aphids with caterpillar-induced defenses in Arabidopsis: involvement of phytohormones and transcription factors.

In nature, plants are exposed to attacks by multiple herbivore species at the same time. To cope with these attacks, plants regulate defenses with the production of hormones such as salicylic acid (SA) and jasmonic acid (JA). Because herbivore densities are dynamic in time, this may affect plant-mediated interactions between different herbivores attacking at the same time. In Arabidopsis thaliana, feeding by Brevicoryne brassicae aphids interferes with induced defenses against Plutella xylostella caterpillars. This is density dependent: at a low aphid density, the growth rate of P. xylostella was increased, whereas caterpillars feeding on plants colonized by aphids at a high density have a reduced growth rate. Growth of P. xylostella larvae was unaffected on sid2-1 or on dde2-2 mutant plants when feeding simultaneously with a low or high aphid density. This shows that aphid interference with caterpillar-induced defenses requires both SA and JA signal transduction pathways. Transcriptional analysis revealed that simultaneous feeding by caterpillars and aphids at a low density induced the expression of the SA transcription factor gene WRKY70 whereas expression of WRKY70 was lower in plants induced with both caterpillars and a high aphid density. Interestingly, the expression of the JA transcription factor gene MYC2 was significantly higher in plants simultaneously attacked by aphids at a high density and caterpillars. These results indicate that a lower expression level of WRKY70 leads to significantly higher MYC2 expression through SA-JA cross-talk. Thus, plant-mediated interactions between aphids and caterpillars are density dependent and involve phytohormonal cross-talk and differential activation of transcription factors.

Colony Development and Density-Dependent Processes in Breeding Grey Herons

The density-dependent processes that limit the colony size of colonially breeding birds such as herons and egrets remain unclear, because it is difficult to monitor colonies from the first year of their establishment, and the most previous studies have considered mixed-species colonies. In the present study, single-species colonies of the Grey Heron (Ardea cinerea) were observed from the first year of their establishment for 16 years in suburban Tokyo. Colony size increased after establishment, illustrating a saturation curve. The breeding duration (days from nest building to fledging by a pair) increased, but the number of fledglings per nest decreased, with colony size. The reproductive season in each year began earlier, and there was greater variation in the timing of individual breeding when the colony size was larger. The prolonged duration until nestling feeding by early breeders of the colony suggests that herons at the beginning of the new breeding season exist in an unsteady state with one another, likely owing to interactions with immigrant individuals. Such density-dependent interference may affect reproductive success and limit the colony size of Grey Herons.

Batch spawning decreases competition among early life stages in coastal fishes: a simulation study using red drum Sciaenops ocellatus

Batch spawning (reproducing multiple times in a single reproductive season) is mainly considered to be a bet-hedging strategy to cope with environmental uncertainty, but little attention is paid to its consequences for competitive environments of offspring. Here, we investi- gate how batch-spawning traits can affect recruitment success when offspring experience size- and density-dependent interference competition. Using an individual-based model, we simulated recruitment of a typical batch-spawning coastal fish in the Gulf of Mexico, where spawning occurs in spatially heterogeneous environments over a single spawning season. We examined the effects of competition intensity among offspring on recruitment success. As offspring were more compet- itive, higher recruitment was achieved when a population showed batch spawning compared to single spawning. Moreover, interference competition among offspring yielded an increase in total recruitment when the competition intensity was low, suggesting that offspring competition is not always detrimental to recruitment success. Our study proposes a beneficial consequence of a batch-spawning strategy—increasing recruitment by reducing competition among offspring.

The evolution of cooperation in asymmetric systems

Explaining "Tragedy of the Commons" of evolution of cooperation remains one of the greatest problems for both biology and social science. Asymmetrical interaction, which is one of the most important characteristics of cooperative system, has not been sufficiently considered in the existing models of the evolution of cooperation. Considering the inequality in the number and payoff between the cooperative actors and recipients in cooperation systems, discriminative density-dependent interference competition will occur in limited dispersal systems. Our model and simulation show that the local but not the global stability of a cooperative interaction can be maintained if the utilization of common resource remains unsaturated, which can be achieved by density-dependent restraint or competition among the cooperative actors. More intense density dependent interference competition among the cooperative actors and the ready availability of the common resource, with a higher intrinsic contribution ratio of a cooperative actor to the recipient, will increase the probability of cooperation. The cooperation between the recipient and the cooperative actors can be transformed into conflict and, it oscillates chaotically with variations of the affecting factors under different environmental or ecological conditions. The higher initial relatedness (i.e. similar to kin or reciprocity relatedness), which is equivalent to intrinsic contribution ratio of a cooperative actor to the recipient, can be selected for by penalizing less cooperative or cheating actors but rewarding cooperative individuals in asymmetric systems. The initial relatedness is a pivot but not the aim of evolution of cooperation. This explains well the direct conflict observed in almost all cooperative systems.

Consumers that are not ‘ideal’ or ‘free’ can still approach the ideal free distribution using simple patch‐leaving rules

1. The ideal free distribution (IFD) has been widely used to determine whether consumers distribute themselves optimally. However, this theory is based on three assumptions that are clearly violated in many systems. The theory assumes that all individuals know the quality of each available site, are equally free to move between all sites, and have equal competitive abilities. 2. I examine the utility of this theory to predict the distribution of the invasive European green crab Carcinus maenas, a species that likely violates all of these assumptions. I demonstrate three main findings. 3. First, understanding how density-dependent interference and size alter individual foraging behaviour is important for understanding the density and biomass distribution of C. maenas in invaded habitats. 4. Second, once behavioural mechanisms of crab foraging are accurately included in the model, the IFD does a good job of predicting the distribution of C. maenas, even though C. maenas violates the theory's fundamental assumptions. 5. Third, C. maenas' distribution can be obtained using simple decision rules and reasonable movement patterns.

The maintenance of hybrid zones across a disturbance gradient

The parapatric distribution of genetically divergent lineages in hybrid zones can be maintained by ecological differences (dispersal-independent 'ecotonal' hybrid zones), by frequency- and density-dependent interference when they intermingle and mate (dispersal-dependent 'tension' hybrid zones), or by both processes acting together. One potentially important ecological factor that has received little theoretical attention is gradients in habitat disturbance. Such gradients may be particularly important in contact zones in which the interacting lineages differ in their sexual system (e.g., self-fertile versus obligately outcrossing) because self-fertility promotes the colonization of open patches. Here we use a spatially explicit metapopulation model to examine the dynamics of a dispersal-dependent ecotonal hybrid zone across a gradient in the rate of habitat disturbance, where competing lineages differ in their sexual system. We found that self-fertility promoted the maintenance of one lineage over its outcrossing counterpart at high extinction rates, predominantly because self-fertility confers reproductive assurance. Additionally, greater seed and pollen production promoted a lineage's persistence by reducing the seed fertility of its counterpart through hybridization. Our results draw attention to the joint effects of ecological and endogenous selection in regulating the location of hybrid zones. Our study also casts new light on the maintenance of the parapatric distribution of incompatible lineages of Spanish populations of the plant Mercurialis annua. In particular, we expect the rate of movement of a contact zone in eastern Spain to increase as it moves further south, contrary to earlier predictions.

SELF-REGULATING MECHANISMS IN CANNIBALISTIC POPULATIONS OF JUVENILE SHORE CRABS CARCINUS MAENAS

Juvenile cannibalism may play an important role in population regulation of marine organisms with juvenile stages that aggregate in nursery habitats. I conducted a series of field and laboratory experiments to assess whether three potential density-dependent mechanisms could regulate local recruitment of juvenile shore crabs Carcinus maenas in Swedish nursery areas: (1) intracohort cannibalism, (2) juvenile cannibals' functional response to smaller conspecific prey densities, and (3) juvenile cannibals' numerical effect on prey survival. Juvenile cannibals in mesocosms displayed a type III (sigmoid) density-dependent functional response, resulting in significantly higher proportional mortality of smaller conspecifics at high prey densities. Consistent with the laboratory result, cage-enclosed juvenile cannibals in a field experiment eliminated conspecific settlement peaks in mussel habitats and decoupled the relationship between larval and settlement abundance within three days. Density-dependent interference between cannibals in mesocosms decreased per capita predation rates, but conspecific prey mortality still increased with cannibal densities when the abundance of cannibals passed a critical threshold, suggesting that the recruitment success of consecutive cohorts may be negatively correlated. This was supported in a field experiment where the abundance of juvenile cannibals explained 71% of the variation in settlement loss from uncaged mussel habitats. Density-dependent cannibalism also occurred within juvenile cohorts even though food was provided in excess, but the resulting mortality was too low to eliminate initial density patterns over a 30-day period. The results demonstrate that cannibalism between juvenile shore crabs can cause high and strongly density-dependent mortality at natural densities, and that both a functional response and a numerical effect of larger cannibals can regulate the recruitment of new cohorts within days of settlement. A coupling of cannibal and settlement densities on a regional scale is expected to decrease recruitment variability within and between local populations. Moreover, the interaction between the type III functional response, mutual interference, and density-dependent dispersal of cannibals is predicted to reduce cannibalistic rates at low prey and high cannibal densities, respectively, and stabilize local population dynamics in relation to the abundance of juvenile habitats, consistent with field observations of shore crab populations in the study area.