Low Competitor Density Research Articles

An integrated experimental and mathematical approach to inferring the role of food exploitation and interference interactions in shaping life history

Abstract Intraspecific interactions can occur through many ways but the mechanisms can be broadly categorized as food exploitation and interference interactions. Identifying how intraspecific interactions impact life history is crucial to accurately predict how population density and structure influence dynamics. However, disentangling the effects of interference interactions from exploitation using experiments is challenging for most biological systems. Here we propose an approach that combines experiments with modelling to infer the pathways of intraspecific interactions in a system. First, a consumer‐resource model is built without intraspecific interactions. Then, the model is parameterized by fitting it to life‐history data from a first experiment in which food abundance was varied. Next, hypothesized scenarios of intraspecific interactions are incorporated into the model which is then used to predict life histories with increasing competitor density. Lastly, model predictions are compared against data from a second experiment which raised groups of competitors of different densities. This comparison allows us to infer the role of interference and exploitation in shaping life history. We demonstrated the approach using the smaller tea tortrix Adoxophyes honmai across a range of temperature. We investigated five scenarios of interactions that included exploitation and three pathways for interference through some effects either on energetics to represent changes in ingestion or activity, or on mortality to model deadly interactions, or on mortality and ingestion to model cannibalism. Overall, intraspecific interactions in tea tortrix are best explained by a high level of deadly interactions along with some level of interference that acts on energy such as escaping and blocking access to food. Deadly interactions increase with temperature while interference that acts on energy is strongest close to the optimal temperature for reproduction. Interestingly, exploitation is more important than interference at low competitor density. The combination of mathematical modelling and experimentation allowed us to mechanistically characterize the intraspecific interactions in tea tortrix in a way that is readily incorporated into population‐level mathematical models. The primary value of this approach, however, is that it can be applied to a much wider range of taxa than is possible with pure experimental approaches. Read the free Plain Language Summary for this article on the Journal blog.

Spoken word recognition in adolescents with autism spectrum disorders and specific language impairment

ABSTRACTSpoken word recognition, during gating, appears intact in specific language impairment (SLI). This study used gating to investigate the process in adolescents with autism spectrum disorders plus language impairment (ALI). Adolescents with ALI, SLI, and typical language development (TLD), matched on nonverbal IQ listened to gated words that varied in frequency (low/high) and number of phonological onset neighbors (low/high density). Adolescents with ALI required more speech input to initially identify low-frequency words with low competitor density than those with SLI and those with TLD, who did not differ. These differences may be due to less well specified word form representations in ALI.

Island biogeography extends to small-scale habitats: low competitor density and richness on islands may drive trait variation in nonnative plants

Previous island biogeography studies have quantified species richness on the scale of entire islands rather than smaller scales relevant to plant-to-plant competitive interactions. Further, they have not accounted for density compensation. Using mainland and island sites along the New England coast, we asked two questions. First, are both richness and density lower in small-scale habitats within islands than in similar mainland habitats? Second, do differences in competitor richness and density drive post-establishment trait variation in nonnative plant species? We used field surveys and individual-based rarefaction to estimate richness and density in 100-m2 plots and demonstrated that island sites have significantly fewer species and individuals per unit area than mainland sites. We then conducted a field study in which we removed competing neighbors from nonnative plant individuals and found that when competitors were removed, individuals in low-competition environments demonstrated a lesser increase in vegetative growth but a greater increase in reproductive effort and herbivore tolerance relative to mainland individuals whose neighbors were also removed. We found that the central concept of island biogeography, i.e., that islands host fewer species than comparable mainland habitats, can be extended to smaller-scale habitats and that this difference in competitive pressure between mainland and island habitats can act as a driver of trait variation in nonnative plants.

Effects of competition on tree radial‐growth vary in importance but not in intensity along climatic gradients

Summary 1. Plant interactions play a central role in regulating plant communities and this role can be altered by abiotic stress. With increasing stress, ecological theory predicts that the role of competition decreases whilst that of facilitation increases. Such predictions have been tested with short‐term plant removal experiments using two distinct indices evaluating the role of plant interactions: the intensity (absolute impact) and the importance (impact relative to that of other abiotic constraints) of plant interactions. 2. Using data on individual tree radial growth from more than 17 000 forest plots covering the habitat conditions of 16 species in the Alps and the Jura mountains of France, we show that non‐manipulative estimates of plant interactions provide an alternative to this experimental approach. We developed a Bayesian neighbourhood growth competition model to test theoretical predictions about plant–plant interactions with a much larger spatio‐temporal scope and set of study species than classically used in experimental studies of plant–plant interactions. 3. Our analyses revealed that competition – measured as neighbours effects on adult tree growth – varies in importance but not in intensity along two major bioclimatic gradients (degree‐day sum and water availability). Observed patterns of competition importance differed between shade‐tolerant and shade‐intolerant tree species. First, the mean importance of competition was found to be much higher for shade‐intolerant species. Second, for shade‐intolerant species the importance of competition remained high even at low crowding indices (i.e. at a low competitor density), whereas for shade‐tolerant species competition only became important at high crowding indices. 4. Synthesis. Our non‐manipulative approach to the study of plant–plant interactions allows analysing interactions among many species over large climatic gradients. Our results clearly demonstrate that a quantitative estimation of density dependence effects is key to understanding how plant–plant interactions vary along abiotic gradients. Growth predictions derived from our model can easily be integrated with other results on tree regeneration and mortality in individual‐based models to investigate how plant–plant interactions drive tree population and community dynamics under varying climatic conditions.

Group size effects in cooperatively breeding African wild dogs

In groups of cooperative breeders, individual fitness can increase as a direct consequence of group size, whereby the extent of these benefits is likely to depend, to a degree that is largely unknown, on the prevailing ecological conditions. We examined the effect of group size on pup production and survival across age classes in cooperatively breeding African wild dogs, Lycaon pictus, across 12 quasiexperimental sites in South Africa varying in ecological conditions. In this sample spanning over 43 pack-years, group size had a significantly positive effect on litter size after emergence from the den, but not on number of pups surviving to 1 year of age or survival rates for pups, yearlings and adults. Stronger relationships between these variables have been reported in other samples, so these results might be explained by low competitor density and high prey availability in our study areas. This explanation is supported by the finding that neither of these variables had any significant effect on pup production and survival across age classes in the present study. The results of our quasiexperimental study suggest that varying ecological conditions represent an important source of intraspecific variation in the impact of helpers in cooperative breeders.

Predicting the strength of interference more quickly using behaviour‐based models

Summary 1 Interference between foraging animals can be quantified directly only through intensive studies. A quicker alternative is to predict the strength of interference using behaviour-based models. We describe a field method to parameterize an interference model for shorebirds, Charadrii. 2 Kleptoparasitic attack distance is the main factor affecting the strength of interference but has rarely been measured. Attack distance is related to handling time, a frequently measured parameter, allowing the model to be parameterized for systems in which attack distance has not been measured. 3 The model accurately predicts the strength of interference between oystercatchers Haematopus ostralegus L. feeding on cockles Cerastoderma edule L. and the absence of interference between bar-tailed godwits Limosa lapponica L. feeding on lugworms Arenicola marina L. at low competitor densities. 4 We predict the strength of interference in black-tailed godwit Limosa limosa L. and oystercatcher systems in which it has not been measured previously. The strength of interference is almost entirely determined by attack distance; interference is stronger in systems with longer attacks. Interference is usually weaker in black-tailed godwits because handling time is generally shorter and this limits attack distance. 5 The interference model can be parameterized much more quickly than the alternative of measuring interference directly. Behaviour-based models have the potential to be a valuable tool for predicting the strength of interference.

Cue use affects resource subdivision among three coexisting hummingbird species

Competition for food can influence the coexistence of species via habitat selection, and learned behavior can influence foraging decisions. I investigated whether learned behavior and competition act together to influence species interactions between three coexisting hummingbird species: black-chinned (Archilochus alexandri), blue-throated (Lampornis clemenciae), and magnificent (Eugenes fulgens) hummingbirds. I found that color cue use by individuals affects not only their foraging choices but also population-level responses to competition. I presented hummingbirds two types of habitats (rich and poor feeders). All birds shared a preference for the rich feeders, but shifted preference toward poor feeders in response to competition. I used color cues to manipulate the amount of information available to birds and examined the effects of two information states (complete or incomplete) on their foraging choices. I examined hummingbirds' preferences for the rich feeders when both competitor densities and information varied. To relate foraging choices to energetic intake, I also analyzed energy gained during a single foraging bout. Males of all species exhibited strong preferences for rich feeders when they foraged with complete information and low competitor densities. Without complete information, the two subordinate species (black-chinned and magnificent) shifted preference away from rich feeders in response to high densities of the dominant species (blue-throated). Each subordinate species shifted in a unique way: black-chinned hummingbirds reduced foraging efficiency, while magnificent hummingbirds reduced foraging time. Birds foraging with complete information remained selective on rich feeders even at high competitor densities. Thus, learned information affected competitive interactions (for rich feeders) among these species.

Clonal growth and storage in relation to competition in genets of the rhizomatous perennial Amphibromus scabrivalvis

The grass Amphibromus scabrivalvis Trin. Swallen produces new ramets from the base of established ramets (to produce a phalanx growth form) and along rhizomes (to produce a guerrilla growth form). It stores resources in stem bases. This research examined genotypic variation in clonal growth and storage and its relation to the performance of replicated genets under competition. It was hypothesized that (i) genets would vary significantly in traits important to clonal growth and storage during competition, (ii) competition would reduce clonal growth capability, (iii) guerrilla-like spacing would enhance clonal growth under competition at low competitor density, and (iv) allocation to storage organs would increase under competition. Fourteen genets of A. scabrivalvis were divided into ramets and planted into pots in the glasshouse alone or in competition with four equally distributed seedlings of Lolium perenne L., a phalanx competitor. Leaf area and mass, as well as the distance of new ramets from the parental ramet, were measured at 9 and 18 weeks. Dry mass of leaves, stem bases, rhizomes, and roots was determined at 18 weeks. Relative competition intensity, based on the difference in total mass per genet between control and competition treatments relative to the control, varied between 0.69 and 0.83. Ramet production, the proportion of the total leaf area in new ramets, and ramet distances varied significantly among genets and were reduced by competition. Mean ramet distance was positively correlated with total mass. Allocation of dry mass to storage organs was significantly increased under competition and may allow genet persistence within an occupied area. The relative ranking of genets in the control and competition treatment was not different, and there were no differences in the relative degree to which the total mass of genets was reduced by competition, despite significant variation in total mass accumulated in the absence of competition.Key words: Amphibromus scabrivalvis, clonal grass, competition, genotypic variation, guerrilla strategies, phalanx strategies, rhizomatous perennial.

Clonal growth and storage in relation to competition in genets of the rhizomatous perennial <i>Amphibromus scabrivalvis</i>

The grass Amphibromus scabrivalvis Trin. Swallen produces new ramets from the base of established ramets (to produce a phalanx growth form) and along rhizomes (to produce a guerrilla growth form). It stores resources in stem bases. This research examined genotypic variation in clonal growth and storage and its relation to the performance of replicated genets under competition. It was hypothesized that (i) genets would vary significantly in traits important to clonal growth and storage during competition, (ii) competition would reduce clonal growth capability, (iii) guerrilla-like spacing would enhance clonal growth under competition at low competitor density, and (iv) allocation to storage organs would increase under competition. Fourteen genets of A. scabrivalvis were divided into ramets and planted into pots in the glasshouse alone or in competition with four equally distributed seedlings of Lolium perenne L., a phalanx competitor. Leaf area and mass, as well as the distance of new ramets from the paren...

Modelling Interference from Basic Foraging Behaviour

1. We develop an individuals-based model that predicts the strength of interference between foraging animals from basic elements of their behaviour. The model is based on the same principles as previous behaviour-based interference models, but extends and adds further realism to these models. One key difference is that in our model the responses of animals to competitors are not fixed, as is assumed in previous models. Instead, animals use optimal decision rules to determine responses which maximize their intake rate. 2. The general shape of interference function generated by the model is similar to that predicted by previous behaviour-based models. Interference is insignificant at low competitor densities, but steadily increases in intensity as density rises. However, comparison with the observed level of interference between oystercatchers, Haematopus ostralegus, feeding on mussels, Mytilus edulis, shows that the model's predictive power is substantially increased through the addition of optimal decision rules. When animals have a fixed response to encounters, too much interference occurs because dominant animals waste time avoiding subdominants and subdominants waste time attempting, but failing, to steal prey from dominants. When animals use optimal decision rules, only subdominants avoid, and only dominants initiate attacks. Interference is therefore reduced and is much closer to that observed. 3. The conditions under which optimal decision rules will lead to interference are described in terms of basic elements of foraging behaviour. Interference is predicted to occur when handling time and the probability of winning fights are high, and when prey encounter rate and the duration of fights are low. These parameters are used to predict successfully the presence or absence of interference in a range of shorebird-prey systems. 4. We suggest that behaviour-based interference models will need to incorporate optimal decision rules if they are to predict accurately the strength of interference observed in real predator-prey systems.

Interference Competition at Low Competitor Densities in Blackbirds Turdus merula

1. Interference competition in a wild population of European blackbirds was examined by comparing the feeding rates of individual birds feeding alone and with the addition of a competitor within small, experimental feeding patches. 2. Blackbirds showed type II functional responses. There was a highly significant decrease in feeding rate on addition of a competing individual to a patch: average declines in feeding rates of an individual on addition of a competitor, where nearest neighbour distance was always less than about 5m, were between 16% and 43% of the feeding rate of a lone individual. 3. The decline in feeding rate was similar whether the individuals interacted aggressively or apparently ignored each other during feeding. When patch size was doubled, the effects on feeding rate of adding a single competitor remained similar. The high levels of interference recorded may therefore have been mainly a consequence of active rather than passive interference competition. 4. The results suggest that the costs to an individual blackbird of monitoring other individuals within a group in order to avoid interactions may be sufficiently high to counter any benefits of group feeding, such as vigilance benefits.

Shape of the Interference Function in a Foraging Vertebrate

1. We provide evidence that interference between overwintering oystercatchers Haematopus ostralegus feeding on mussels Mytilus edulis is absent or has only a negligible effect on intake rate at low competitor densities, and only reduces intake rate above a threshold bird density. The threshold for interference did not differ significantly between individual birds and showed no association with their local dominance. The threshold value was c. 50 birds ha -1 for birds that opened prey by hammering and 150 birds ha -1 for those that stabbed into mussels. 2. The interference-free intake rate at densities below the threshold for interference differed significantly between individual birds but was not associated with local dominance; nor did it vary either during the tidal exposure period or between autumn and winter. 3. At competitor densities above the threshold for interference, birds of lower local dominance were more susceptible to interference than those of higher dominance, the magnitude of the difference being smaller in hammering birds than in stabbers. For birds of a given local dominance, interference was most intense towards the end of winter and, in hammerers, at the beginning of the exposure period. 4. We conclude that theoretical models in which interference operates as soon as competitor density starts to increase provide a poor description of this system. Rather, our data support those models in which interference begins only after density has reached a particular, and constant, level.

Habitat Selection by Pemphigus Aphids in Response to Response Limitation and Competition

The leaf galling aphid, Pemphigus betae can be highly resource and habitat limited and has evolved to reduce these limitations. Over a 3—day period in spring as much as 83% of the overwintering population migrates to immature leaves of Populus angustifolia where individuals are rapidly entombed by expanding leaf tissue. The size of the mature leaf is critical to ultimate fitness of the individual colonizing stem mother. Probability of stem mother failure, body weight of stem mother and progeny, number of progeny, development rate of progeny to maturity, and number of embryos in mature progeny are all correlated with leaf size. Due to these selection pressures, stem mothers search out and colonize large leaves. Thirty—two percent of the tree's leaves were so small as to result in an 80% probability of total aphid failure. Nearly all of these leaves were avoided. In comparison, only 1.6% of the tree's leaves were so large as to result in a 0% probability of failure, and 100% of these leaves were colonized. Even though most stem mothers were forced to colonize. Even though most stem mothers were forced to colonize suboptimal leaves because competitor density (35/100 leaves) was much > the availability of optimal leaves, the average stem mother colonized a leaf 60% > the mean leaf size of the tree. Resulting from this degree of selectivity, the minimum increase in fitness was 2 times > expected if leaves had been selected at random. At low competitor densities reproductive output would still be limited by energy intake such that if there were only 1 gall on a tree, the colonizing stem mother and her progeny would still be resource limited. Apparently, no leaf produced by the tree is so large that a further increase in leaf size would not result in a further increase in fitness. Thus, even at low competitor densities, resources would still be important in the habitat selection process and in determining the parasite distribution.