Intraspecific Interference Competition Research Articles

Intraspecific interference retards growth and development of cane toad tadpoles, but those effects disappear by the time of metamorphosis.

Competition among larval anurans can occur via interference as well as via a reduction in per-capita food supply. Previous research on intraspecific interference competition in cane toad (Rhinella marina) tadpoles found conflicting results, with one study detecting strong effects on tadpoles and another detecting no effects on metamorphs. A capacity to recover from competitive suppression by the time of metamorphosis might explain those contrasting impacts. In a laboratory experiment, we found that nine days of exposure to intraspecific interference competition strongly reduced tadpole growth and development, especially when the competing tadpoles were young (early-stage) individuals. Those competitive effects disappeared by the time of metamorphosis, with no significant effect of competition on metamorph body condition, size, larval period or survival. Temporal changes in the impact of competition were not related to tadpole density or to variation in water quality. The ability of larval cane toads to recover from intraspecific interference competition may enhance the invasive success of this species, because size at metamorphosis is a significant predictor of future fitness. Our study also demonstrates a cautionary tale: conclusions about the existence and strength of competitive interactions among anuran larvae may depend on which developmental stages are measured.

Effects of Intraspecific Competition and Larval Size on Bioconversion of Apple Pomace Inoculated with Black Soldier Fly

Waste from apple juice and cider industries (pomace) compares poorly against spent grains and other relatively high-nutrient wastes as a substrate for bioconversion by the black soldier fly (BSF: Hermetia illucens). However, global pomace production exceeds 24 million tonnes annually and novel management approaches are required to reduce waste to landfill. We examined the effects of BSF inoculation densities (intraspecific competition) and larval size categories on cohort weight gains and apple pomace waste reduction. We found that, by increasing larval densities, cohort biomass and bioconversion rates (BRs) increased; however, at very high densities (overcrowding), BRs declined and cohorts lost weight. Furthermore, larger larval size classes accelerated substrate desiccation, possibly because of greater demands for water by older larvae. Larger larvae have slower relative growth rates and BRs compared to smaller size categories and require comparatively less dry weight substrate. Our results suggest that overcrowding on low-nutrient substrates reduces BRs and could exaggerate differences between BSF relative performances in comparative studies, particularly if intraspecific interference competition for space and exploitation competition for water diminish BSF weight returns at the end of the bioconversion cycle. We make a series of recommendations for the use of BSF in pomace waste reduction.

Forest Saccharomyces paradoxus are robust to seasonal biotic and abiotic changes.

Microorganisms are famous for adapting quickly to new environments. However, most evidence for rapid microbial adaptation comes from laboratory experiments or domesticated environments, and it is unclear how rates of adaptation scale from human‐influenced environments to the great diversity of wild microorganisms. We examined potential monthly‐scale selective pressures in the model forest yeast Saccharomyces paradoxus. Contrary to expectations of seasonal adaptation, the S. paradoxus population was stable over four seasons in the face of abiotic and biotic environmental changes. While the S. paradoxus population was diverse, including 41 unique genotypes among 192 sampled isolates, there was no correlation between S. paradoxus genotypes and seasonal environments. Consistent with observations from other S. paradoxus populations, the forest population was highly clonal and inbred. This lack of recombination, paired with population stability, implies that selection is not acting on the forest S. paradoxus population on a seasonal timescale. Saccharomyces paradoxus may instead have evolved generalism or phenotypic plasticity with regard to seasonal environmental changes long ago. Similarly, while the forest population included diversity among phenotypes related to intraspecific interference competition, there was no evidence for active coevolution among these phenotypes. At least ten percent of the forest S. paradoxus individuals produced “killer toxins,” which kill sensitive Saccharomyces cells, but the presence of a toxin‐producing isolate did not predict resistance to the toxin among nearby isolates. How forest yeasts acclimate to changing environments remains an open question, and future studies should investigate the physiological responses that allow microbial cells to cope with environmental fluctuations in their native habitats.

Grazer commensalism varies across the species range edge: host chiton size influences epibiont limpet incidence and spatial segregation

Biotic interactions can determine species distributions and range limits, but little theoretical background exists regarding variation in commensalistic associations across latitudes. We estimated the geographic variation of the epibiont limpet Scurria parasitica’s association with its obligate host chiton species Enoplochiton niger across their distribution from Peru to northern Chile (12-30°S), to test the influence of host size variation on epibiont occurrence, individual spatial distribution, and body size. We analyzed the contribution of chiton body size to the pattern of abundance and distribution of occupancy of the epibiont limpet. We also examined the relationship between limpet shell size, coverage, and incidence probability function and chiton body size across latitudes. For some localities across the range edge of the host grazer’s distribution (i.e. 28-30°S), incidence and densities of the epibiont limpet were higher on larger chitons. Unoccupied host chiton proportions decreased at the poleward edge of the host-epibiont species range compared with sites located to the north. Increased variation in the epibiont limpet distribution on the host chiton plates suggests that limpets’ spatial segregation may have a role in lessening intraspecific interference competition with the host species at southern latitudes. Therefore, local and large-scale processes seem to contribute to modify the host-epibiont association pattern. Further studies are necessary to determine if this association shifts from commensalistic to antagonistic across the distributional range of both species.

Expression of Streptococcus pneumoniae Bacteriocins Is Induced by Antibiotics via Regulatory Interplay with the Competence System.

Pneumococcal bacteriocins (pneumocins) are antibacterial toxins that mediate intra-species competition within the human host. However, the triggers of pneumocin expression are poorly understood. Using RNA-sequencing, we mapped the regulon of the pneumocin cluster (blp) of Streptococcus pneumoniae D39. Furthermore, by analogy with pneumococcal competence, we show that several antibiotics activate the blp-genes. Using real-time gene expression measurements we show that while the promoter driving expression of the two-component regulatory system blpR/H is constitutive, the remaining blp-promoters that control pneumocin expression, immunity and the inducer peptide BlpC, are pH-dependent and induced in the late exponential phase. Intriguingly, competence for genetic transformation, mediated by the paralogous ComD/E two-component quorum system, is induced by the same environmental cues. To test for interplay between these regulatory systems, we quantified the regulatory response to the addition of synthetic BlpC and competence-stimulating peptide (CSP). Supporting the idea of such interplay, we found that immediately upon addition of CSP, the blp-promoters were activated in a comD/E-dependent manner. After a delay, blp-expression was highly induced and was strictly dependent on blpRH and blpC. This raised the question of the mechanism of BlpC export, since bioinformatic analysis showed that the genes encoding the putative exporter for BlpC, blpAB, are not intact in strain D39 and most other strains. By contrast, all sequenced pneumococcal strains contain intact comAB genes, encoding the transport system for CSP. Consistent with the idea that comAB mediate BlpC export, we finally show that high-level expression of the blp-genes requires comAB. Together, our results demonstrate that regulation of pneumocin expression is intertwined with competence, explaining why certain antibiotics induce blp-expression. Antibiotic-induced pneumocin expression might therefore have unpredictable consequences on pneumococcal colonization dynamics by activating genes that mediate intra-specific interference competition.

Evolution of enlarged body size of coal tits Parus ater in geographic isolation from two larger competitors, the crested tit Parus cristatus and the willow tit Parus montanus, on six Scandinavian islands.

ABSTRACTHere, we report that on six widely separated Scandinavian islands, the coal tit Parus ater has evolved morphologically in the direction of two absent competitors, the crested tit P. cristatus and the willow tit P. montanus, to the effect that it is up to 10% larger in linear dimensions than conspecifics on the adjacent Swedish mainland, where all three species coexist. The large size is genetically determined, as ascertained by clutch exchange experiments between island and mainland nests. We conclude that the increased size of P. ater in places where it is geographically isolated from its larger congeners is the result of evolutionary adaptation, due ultimately to relaxed interspecific competition. On the islands, P. ater has evolved into a medium-sized generalist, with selection pressures likely governed by the following causal relationships. When competitors are lacking, P. ater takes over the foraging space of the absentees. The enlarged food base allows higher population densities, which intensifies intraspecific interference competition. This, in turn, selects for increased body size. When P. ater coexists with its larger congeners, it occupies peripheral foraging sites in trees, which requires excellent manoeuvrability and energy-expensive locomotion modes. Reduction of body size increases locomotor capacity for mechanical and aerodynamic reasons and lowers energy consumption, so small size is favoured in sympatry. But in geographic isolation, P. ater exploits the tree periphery less and the inner tree regions more, and it also adopts the easier locomotion modes of the absent species. Therefore, selection for manoeuvrability and a small body size is relaxed. The new selection regime shifts the balance between opposing selection forces towards a larger body size. We were able to test 11 alternative hypotheses and available evidence conclusively eliminates them all. As a result, here, evolution could be predicted regarding both direction and amount of change.

Interference versus exploitative competition in the regulation of size-structured populations.

Competition is a major regulatory factor in population and community dynamics. Its effects can be either direct in interference competition or indirect in exploitative competition. The impact of exploitative competition on population dynamics has been extensively studied from empirical and theoretical points of view, but the consequences of interference competition remain poorly understood. Here we study the effect of different levels of intraspecific interference competition on the dynamics of a size-structured population. We study a physiologically structured population model accounting for direct individual interactions, allowing for a gradient from exploitative competition to interference competition. We parameterize our model with data on experimental populations of the collembolan Folsomia candida. Our model predicts contrasting dynamics, depending on the level of interference competition. With low interference, our model predicts juvenile-driven generation cycles, but interference competition tends to dampen these cycles. With intermediate interference, giant individuals emerge and start dominating the population. Finally, strong interference competition causes a novel kind of adult-driven generation cycles referred to as interference-induced cycles. Our results shed new light on the interpretation of the size-structured dynamics of natural and experimental populations.

Warming effects on consumption and intraspecific interference competition depend on predator metabolism

1. Model analyses show that the stability of population dynamics and food web persistence increase with the strength of interference competition. Despite this critical importance for community stability, little is known about how external factors such as the environmental temperature affect intraspecific interference competition. 2. We aimed to fill this void by studying the functional responses of two ground beetle species of different body size, Pterostichus melanarius and Poecilus versicolor. These functional response experiments were replicated across four predator densities and two temperatures to address the impact of temperature on intraspecific interference competition. 3. We generally expected that warming should increase the speed of movement, encounter rates and in consequence interference among predator individuals. In our experiment, this expectation was supported by the results obtained for the larger predator, P.melanarius, whereas the opposite pattern characterized the interference behaviour of the smaller predator P.versicolor. 4. These results suggest potentially nontrivial implications for the effects of environmental temperature on intraspecific interference competition, for which we propose an explanation based on the different sensitivity to warming of metabolic rates of both species. As expected, increasing temperature led to stronger interference competition of the larger species, P.melanarius, which exhibited a weaker increase in metabolic rate with increasing temperature. The stronger increase in the metabolic rate of the smaller predator, P.versicolor, had to be compensated by increasing searching activity for prey, which did not leave time for increasing interference. 5. Together, these results suggest that any generalization how interference competition responds to warming should also take the species' metabolic response to temperature increases into account.

Importance of seagrass vegetation for habitat partitioning between closely related species, mobile macrofauna Neomysis (Misidacea)

Seagrass meadows provide both habitats and a range of food sources for macrofaunal communities. These functions facilitate the coexistence of less mobile invertebrates (in comparison with mysids, such as amphipods) that are associated with seagrass leaves, and may also enhance the coexistence of highly mobile invertebrates such as mysid. We investigated the function of seagrass in supporting the coexistence of two mysid species, Neomysis awatschensis and N. mirabilis. These taxa are dominant in seagrass ecosystems of temperate coastal areas. We compared patterns of habitat use between the two species at mesoscales (among seagrass patches) and microscales (among seagrass leaves) by performing field surveys and laboratory experiments. The field survey results showed positive correlations in the abundance of the two mysid species, indicating that both species select similar habitats at the mesoscale level. In the laboratory experiments, the pattern of microhabitat selection (fundamental habitat) was similar for both species, even at increased densities and with the presence of an immobile habitat-competitor (the gastropod Barleeia angustata) on the leaves. However, this pattern changed significantly when a food source (epiphytic microalgae) was present on the leaves. This result indicates that (i) inter- and intraspecific interference competition does not affect microhabitat selection in these two mysids and (ii) both Neomysis species use similar habitats at the feeding stage. Although these two closely related mysids species may have similar requirements for microhabitat and food, the evidence that they did not act as competitors is attributable to unrestricted microhabitat and food (e.g., epiphytic algae) in the presence of seagrass vegetation.

Consumer functional responses under intra- and inter-specific interference competition

Mechanistic models of population, community and ecosystem dynamics require the mathematical description of trophic interactions in the form of functional response equations. There is a wealth of such equations developed to incorporate the effects of multitude forms of foraging behaviour including intra-specific interference competition. However, there has been no attempt to include inter-specific behaviours beyond the obvious consumer–resource relationship, and thus, mechanistic models of communities and ecosystems remain limited in their incorporation of individual behaviour. In this paper we extend existing functional response models to account for both intra- and inter-specific interference behaviours. Together with response surface experiments, these can be used to investigate the role of both types of interference for a given species’ resource acquisition efficiency. We illustrate this with data from foraging trials of guppies Poecilia reticulata in the presence and absence of a competitor species, Hart's killifish Rivulus hartii. Our results show that in the studied example, intra-specific interference is important and stronger than inter-specific competition.

Density dependence of feeding success in haematophagous ectoparasites

We studied the effect of density on feeding success of 2 fleas, Xenopsylla conformis and Xenopsylla ramesis, when exploiting rodents Meriones crassus and Gerbillus dasyurus. We tested 2 alternative hypotheses: (i) that intraspecific interference competition occurs and, thus, feeding success of a flea decreases with an increase in density and (ii) that facilitation via suppression of a host defence system occurs and thus, feeding success of a flea increases with an increase in density. The mean size of a bloodmeal and the proportion of highly engorged individuals in X. conformis feeding on both hosts were affected by density. When on G. dasyurus, both the size of a bloodmeal and the proportion of highly engorged individuals were lower at low (5-15 fleas per host) than at high (25-50 fleas per host) densities. The opposite was true when this flea fed on M. crassus. The mean bloodmeal size and proportions of highly engorged X. ramesis parasitizing either host were not affected by flea density. This study showed that the density dependence of feeding success of a flea (a) varied both between fleas and within-fleas between hosts and (b) indicated either intraspecific competition or facilitation via the host in a particular flea-host association.

Productivity gradients cause positive diversity–invasibility relationships in microbial communities

AbstractModels predict that community invasibility generally declines with species diversity, a prediction confirmed by small‐scale experiments. Large‐scale observations and experiments, however, find that diverse communities tend to be more heavily invaded than simple communities. One hypothesis states that large‐scale environmental heterogeneity, which similarly influences native and invasive species, can cause a positive correlation between diversity and invasibility, overriding the local negative effects of diversity on invasibility. We tested this hypothesis using aquatic microbial communities consisting of protists and rotifers consuming bacteria and nanoflagellates. We constructed a productivity gradient to simulate large‐scale environmental heterogeneity, started communities with the same number of species along this gradient, and subjected equilibrial communities to invasion by non‐resident consumer species. Both invaders and most resident species increased their abundances with resource enrichment, resulting in a positive correlation between diversity and invasibility. Intraspecific interference competition within resident species and the positive effect of enrichment on the number of available resources probably accounted for the higher invasibility with enrichment. Our results provide direct experimental evidence that environmental heterogeneity in productivity can cause a positive diversity–invasibility relationship.

Behavioral interactions between salamanders and centipedes: competition in divergent taxa

Red-backed salamanders, Plethodon cinereus, use territorial advertisement in the form of agonistic displays and pheromonal scent marking as a mechanism for intraspecific interference competition. Although ecological and behavioral interactions among species of salamanders have been well studied, little is known about the interactions between territorial P. cinereus and other ecologically similar species, such as large predatory invertebrates. Our field data indicate that P. cinereus and a large syntopic centipede, Scolopocryptops sexspinosus, exhibit negative spatial associations in natural habitats, possibly indicating interspecific territoriality. Only seven instances of salamander/centipede co-occurrence were recorded from a field sample of 247 occupied cover objects. Cover object size was positively correlated with salamander SVL (tip of the snout to the anterior end of the cloaca), but there was no correlation of cover object size to centipede length. Data on the ability of P. cinereus to differentiate among chemicals on the substrate suggest that visual cues are not necessary to elicit a territorial response from intruding salamanders. Although in laboratory trials salamanders behaved similarly toward intruders of both species, biting was directed only toward centipedes. Salamanders spent significantly more time approaching centipedes than they did approaching other salamanders. Approach behavior was often associated with nose tapping and may be an investigative, rather than aggressive, behavior. We suggest that territorial P. cinereus respond similarly to intruding salamanders and centipedes, but that they escalate more readily to biting centipedes because S. sexspinosus is sightless and thus unable to respond to visual signals. Key words: aggression, centipede, Plethodon cinereus, salamander, Scolopocryptops sexspinosus, spatial distribution. [Behav Ecol 15:679–686 (2004)]

Intraspecific density dependence in the dynamics of zooplankton under hypertrophic conditions

Intraspecific interactions may limit population growth of small cladoceran taxa under food-rich, hypertrophic conditions. Multiple-regression models significantly explained a large proportion of the variation in the body size adjusted fecundity and population growth rate of crustacean zooplankton taxa in a shallow, hypertrophic lake. The results of partial correlation analyses suggested exploitative competition to have only limited significance in determining the zooplankton dynamics. The analyses also revealed strong negative relationships between biomass and both body size adjusted fecundity and population growth rate within most taxa. Most of these relationships cannot be explained by food shortage or predation and suggest alternative mechanisms such as chemically mediated, intraspecific interference competition or life history shifts.

Biodiversity, Habitat Area, Resource Growth Rate and Interference Competition

For the majority of species, per capita growth rate correlates negatively with population density. Although the popular logistic equation for the growth of a single species incorporates this intraspecific competition, multi-trophic models often ignore self-limitation of the consumers. Instead, these models often assume that the predator–prey interactions are purely exploitative, employing simple Lotka–Volterra forms in which consumer species lack intraspecific competition terms. Here we show that intraspecific interference competition can account for the stable coexistence of many consumer species on a single resource in a homogeneous environment. In addition, our work suggests a potential mechanism for field observations demonstrating that habitat area and resource productivity strongly positively correlate to biodiversity. In the special case of a modified Lotka–Volterra model describing multiple predators competing for a single resource, we present an ordering procedure that determines the deterministic fate of each specific consumer. Moreover, we find that the growth rate of a resource species is proportional to the maximum number of consumer species that resource can support. In the limiting case, when the resource growth rate is infinite, a model with intraspecific interference reduces to the conventional Lotka–Volterra competition model where there can be an unlimited number of coexisting consumers. This highlights the crucial role that resource growth rates may play in promoting coexistence of consumer species.

Yellow‐Footed Rock‐Wallaby Group Size Effects Reflect A Trade‐Off

As group size increases, individuals of many species modify the time allocated to anti‐predator vigilance and foraging. Group size effects can result from a reduction in predation risk or from an increase in competition as a function of aggregation. Anti‐predator models of vigilance and foraging group size effects both predict a non‐linear relationship between group size and time allocation. Linear relationships between group size and time allocation may reflect the modification of such relationships by intraspecific interference competition for limited resources, which would reveal a fundamental cost of sociality. We studied the degree to which group size effects in the yellow‐footed rock‐wallaby (Petrogale xanthopus, a macropodid marsupial) were non‐linear. Like several other macropods, yellow‐footed rock‐wallabies foraged more and looked less as group size increased. Variation in vigilance was best explained by the number of conspecifics within 10 m–a distance substantially less than the 30–50 m often used to quantify group size in macropodids. Linear regressions explained more variation than non‐linear ones, suggesting that wallabies traded‐off the benefits of aggregation with the costs of competition. Moreover, dominant yellow‐foots looked less and tended to forage more than subordinate animals. We hypothesize that competition may be relatively more important in the life‐histories of yellow‐footed rock‐wallabies than those of other macropodid marsupials.

Interaction between the solitary bee Chelostoma florisomne and its nest parasite Sapyga clavicornis − empty cells reduce the impact of parasites

Summary1. Nesting behaviour and interactions between the bee Chelostoma florisomne (L.) (Megachilidae) and its nest parasite Sapyga clavicornis (L.) (Sapygidae) were studied through continual observations of individuals and dissections of bee nests. Protection of bee offspring is based on (1) the bee’s discovery and removal of parasite eggs deposited prior to the construction of a cell closure, (2) minimising the time when fully provisioned cells might be parasitised successfully, and (3) the construction of empty cells in front of brood cells.2. An empty cell was found in front of 64.4% of all brood cells and, if the outermost brood cell in a nest was excluded, in front of 74.3% of inner brood cells. A vestibule closure is most often constructed in front of the outermost brood cell.3. Following oviposition, the bee made only five flights, which together lasted 6–13 min, to construct a cell closure. A cell closure does not prevent the nest parasite from oviposition inside the brood cell, however, and parasite eggs deposited through the cell closure are not detected and removed by the bee. Only an additional cell closure, i.e. the formation of an empty cell, may protect a brood cell when the bee is not in the nest. The nest parasite often oviposited through the additional cell closure but its offspring were then trapped in the empty cell and starved to death.4. Only 5.4% of the inner brood cells that were protected by an empty cell were parasitised, compared with 28.9% of those without an anterior, empty cell; 27.4% of the empty cells contained dead parasite offspring (eggs and larvae). Thus, the empty cells provided significant protection and, combined with additional means of protection of brood cells, led to a low degree of parasitism. More than 77% of the wasp offspring died at an early stage due to intraspecific interference competition within brood cells and as result of the wasps’ oviposition into empty cells.

Food habits of the green lizard, Lacerta bilineata, in central Italy and a reliability test of faecal pellet analysis

Food habits of the green lizard (Lacerta bilineata) were studied in some Mediterranean sites of the vicinities of Rome (Latium, central Italy), by means both of stomach dissection of individuals found already dead in the field and faeces analysis of living individuals. The taxonomic diet composition of the lizards was accurately described by both methods, thus demonstrating the reliability of faecal pellet analysis as an non‐cruel method to study lizard diets. However, by using the faeces analysis technique, it is difficult to measure the exact food intake rate (items / unit of time) as well as the size distribution of prey ingested by lizards. There was a remarkable ontogenetic change in the taxonomic diet composition of green lizards: adults essentially consumed beetles and isopods, whereas juveniles fed mainly upon orthopterans, spiders, and Rhynchota. Cannibalism and predation upon small vertebrates (lizards) were very rare events. It is suggested that (i) age‐related differences in microhabitat frequented could explain the dietary differences between young and adult lizards due to different availability of the various prey categories in the various microhabitats, and that (ii) adults and juveniles partition their habitats to reduce intraspecific interference competition.

Intraspecific competition through food robbing in the harvester ant,Messor aciculatus (Fr. Smith), and its consequences on colony survival

Intraspecific interference competition in the harvester ant,Messor aciculatus, was studied. Colonies of this species were found not to have territories. Some nests were located very close to each other, and the foraging areas of the neighbors usually overlapped. Even though the frequency with which alien and resident ants met was very high in the vicinity of the nest entrances, aggressive interactions between them rarely occurred. However, when hostile workers encountered each other, they exhibited a kind of ritualized combat and the winner ejected, but did not injure the loser. If any aliens entered the nest, some of them were pulled out, mainly by the residents. Aliens roaming near a neighbor's nest entrance ferociously attacked the residents carrying seeds in their mandibles and robbed them. On other occasions, aliens entered the nest and stole the collected seed. Although seed robbing and stealing occurred among neighboring colonies, there were remarkable differences in the frequency of their occurrence. The results of field observations and experiments suggest the existence of a dominance order among the neighbors. In one instance, extermination of an inferior colony by its neighbor was observed. The raider colony transferred the stored seeds from the nest of the inferior colony to its own and deposited the larvae and workers some distance away from the nest. The influence of ritualized combat and food robbing on colony activities, and the ecological significance of this interference behavior in terms of spatial distribution and temporal persistence of the nest sites, is discussed.

Interspecific Aggression Between the Sympatric Salamanders, Ambystoma maculatum and Plethodon cinereus

-Ambystoma maculatum and Plethodon cinereus are sympatric across much of eastern North America. The field distributions of individuals of each species are thought to be determined primarily by environmental factors and intraspecific interactions. We investigated the hypothesis that members of A. maculatum would show aggressive behavior toward individual P. cinereus which may affect the distribution of members of this species in nature. We also investigated whether this aggressive behavior is better characterized as interference competition or attempted predation. During staged encounters in laboratory chambers, both male and female resident A. maculatum responded aggressively toward introduced P. cinereus. Direct observations and analysis of videotaped encounters showed that A. maculatum used behavior patterns more similar to those used during predation attempts than to those used in intraspecific interference competition. This aggressive biting by A. maculatum resulted in the consumption of over 9% of the P. cinereus intruders, although most (62%) of the P. cinereus escaped following these strikes. This aggression, although not yet clearly classifiable as either predation or interspecific interference, caused death and injury to P. cinereus and suggested that the presence of A. maculatum may affect the distribution of P. cinereus on the