Testing biological control agent compatibility: Cyphocleonus achates and Larinus minutus on diffuse knapweed

While weed biological control success is typically achieved with one agent, multiple agents are invariably introduced. Biological control agents that share a host–plant may interact either directly or indirectly through changes in host–plant quality. Negative interactions could reduce the impacts of the agents on the density of their host–plant while positive interactions (facilitation) could improve biological control success.In the Okanagan Valley of British Columbia, Canada, initial declines in the invasive rangeland weed, diffuse knapweed (Centaurea diffusa) were attributed to the introduction of the weevil Larinus minutus. A second weevil, Cyphocleonus achates has recently become common on diffuse knapweed. We sought to determine if the recent increase of C. achates could threaten the success of L. minutus. We considered whether L. minutus colonisation or performance remained the same when C. achates was present, and whether the two agents acted independently to reduce plant performance.Neither changes in colonisation rates nor competitive interactions were apparent between C. achates and L. minutus. Both insects reduced plant performance and, for all metrics, the reduction in plant performance by one species was independent of the second. The two agents appear to be compatible and both should contribute to the control of diffuse knapweed. To assess how biological control agents interact requires understanding both their competitive interactions and their joint effects on the shared host.     

Competition between co-dominant plants of the Serengeti plains depends on competitor identity,water, and urine

Kyllinga nervosa (Steud.) and Sporobolus kentrophyllus (K. Schum.) are co-dominant plants of the Serengeti short-grass plains, Tanzania. The plains are characterized by seasonal and sporadic rainfall and currently support in excess of 1.5 million migratory ungulates. The interactive effect of simulated bovine urine and water availability were tested on the competitive interactions of these species in the laboratory. Sporobolus kentrophyllus was a superior competitor to K. nervosa over the tested treatment levels with respect to growth and reproductive effort. Sporobolus kentrophyllus exhibited rapid growth in response to urine addition, leading to a significant species × urine interaction while reduced growth by K. nervosa in response to low water availability explained the significant species × water interaction and is likely explained by K. nervosa's shallow root system. Kyllinga nervosa, however, appears to be more tolerant of low nitrogen conditions based on its similar growth with and without the urine treatment. The effect of intraspecific competition on total biomass was similar for S. kentrophyllus and K. nervosa. Competition resulted in increased size differences (asymmetry) for K. nervosa and for the interspecific competition treatments compared to the size differences observed for plants grown individually (in absence of competition).Total reproductive biomass was reduced most by competition with S. kentrophyllus, irrespective of target species. The water treatment did not influence reproduction while the urine treatment significantly increased reproductive biomass and interacted with target species, competitor species, and yielded a three-way urine × target × competitor species interaction.Results suggest that codominance of these two species in the Serengeti is regulated by water availability, nitrogen input from grazers, and local neighbor identity.     

Effects of additional illumination and fertilization on seasonal changes in fine-scale grassland community structure

Abstract. Fine-scale structure of a species-rich grassland was examined for seasonal changes caused by manipulated changes in the availability of above and below-ground resources (additional illumination with the help of mirrors and fertilization) in a field experiment. If changes induced by fertilization — which are expected to lead to a reduction in small-scale diversity —are due to intensified light competition, they should be compensated for by additional light input. Permanent plots of 40 cm × 40 cm were sampled by the point quadrat method at three angles (60°, 90° and 120° from the horizontal North-South direction), using a laser beam to position the quadrats, in early July and early September. The applied treatments did not cause apparent changes in plant leaf orientation. The degree of spatial aggregation of biomass increased seasonally in fertilized, non-illuminated plots: greater productivity at a constant light supply led to a faster growth rate of potentially dominant species, as compared to the subordinate ones. Additional illumination mitigated this effect of fertilization, indicating that the observed changes in biomass aggregation were due to increased light competition. There was a considerable seasonal decrease of variance ratio (ratio of observed variance of richness at a point and variance expected at random) in fertilized only and in illuminated only plots. In fertilized plots this was due to the positive relationship between biomass aggregation and expected variance of richness. Biomass constancy occurs to be inversely related to deficit in variance of richness. In illuminated plots, in contrast, only the observed variance of richness decreased seasonally, indicating a more uniform use of space by different species. Evidently, a deficit in variance of richness can be caused by drastically different processes, showing that the variance ratio statistic may not have a significant explanatory value in fine-scale community studies.     

Effects of insect herbivory on early plant succession: comparison of an English site and an American site

We examined the survival, size, and agonistic behaviour of faster and slower developing rainbow trout ( Salmo gairdneri ) in an artificial stream channel with a simulated redd. The analysis was performed at two levels: between three full-sib families genetically marked at a hexosaminidase locus, and within families segregating for a regulatory allele Pgml-t(b) that is associated with faster developmental rate. The families differed significantly in their hatching times but showed little variation in emergence times. Fishes with the Pgml-t(b) allele hatched and emerged significantly earlier than their full-sibs without the allele. Despite the mortality of at least 50% of emerged fishes in the selection experiment, there was no significant difference in survival between either the three families or fishes with different genotypes at the Pgml-t locus. However, fishes from the faster developing family and those with the Pgml-t(b) allele had significantly higher levels of agonistic behaviour and had a tendency to be larger than slower developing individuals.     

RUNAWAY EVOLUTION TO SELF-EXTINCTION UNDER ASYMMETRICAL COMPETITION

We analyze a popular model of the evolution of traits related to performance in exploitative competition. This model has previously been used to explain a mechanism by which interspecific competition can cause taxon cycles. We show that purely intraspecific competition can cause evolution of extreme competitive abilities that ultimately result in extinction, without any influence from other species. The only change in the model required for this outcome is the assumption of a nonnormal distribution of resources of different sizes measured on a logarithmic scale. This suggests that taxon cycles, if they exist, may be driven by within- rather than between-species competition. Self-extinction does not occur when the advantage conferred by a large value of the competitive trait (e.g., size) is relatively small, or when the carrying capacity decreases at a comparatively rapid rate with increases in trait value. The evidence regarding these assumptions is discussed. The results suggest a need for more data on resource distributions and size-advantage in order to understand the evolution of competitive traits such as body size.     

Modelling the effects of climate change on the distribution and production of marine fishes: accounting for trophic interactions in a dynamic bioclimate envelope model

Climate change has already altered the distribution of marine fishes. Future predictions of fish distributions and catches based on bioclimate envelope models are available, but to date they have not considered interspecific interactions. We address this by combining the species‐based Dynamic Bioclimate Envelope Model (DBEM) with a size‐based trophic model. The new approach provides spatially and temporally resolved predictions of changes in species' size, abundance and catch potential that account for the effects of ecological interactions. Predicted latitudinal shifts are, on average, reduced by 20% when species interactions are incorporated, compared to DBEM predictions, with pelagic species showing the greatest reductions. Goodness‐of‐fit of biomass data from fish stock assessments in the North Atlantic between 1991 and 2003 is improved slightly by including species interactions. The differences between predictions from the two models may be relatively modest because, at the North Atlantic basin scale, (i) predators and competitors may respond to climate change together; (ii) existing parameterization of the DBEM might implicitly incorporate trophic interactions; and/or (iii) trophic interactions might not be the main driver of responses to climate. Future analyses using ecologically explicit models and data will improve understanding of the effects of inter‐specific interactions on responses to climate change, and better inform managers about plausible ecological and fishery consequences of a changing environment.     

Trophic resource partitioning among five flatfish species (Actinopterygii, Pleuronectiformes) in a tropical bay in south-eastern Brazil

Five species of flatfishes in a tropical bay in south-eastern Brazil were studied to test the hypothesis that resource partitioning along the spatial and size dimensions has been used as a mechanism to enable coexistence. Three zones in the study area were defined according to environmental characteristics (inner, middle and outer). Sampling was conducted by otter trawl tows during daylight hours, between October 1998 and September 1999. Achiridae species ( Achirus lineatus and Trinectes paulistanus ) showed narrow niche width, indicating a specialized feeding strategy, preying on Polychaeta, and occurring mainly in the inner bay zone. Paralichthyidae ( Citharichthys spilopterus and Etropus crossotus ) and the Cynoglossidae ( Symphurus tessellatus ) showed broad niche width and a generalized feeding strategy preying on a large number of Crustacea. Symphurus tessellatus did not change diet with size-dimension feeding on Amphipoda and Polychaeta, whereas C. spilopterus and E. crossotus shifted diet with growth. Citharichthys spilopterus fed mainly on Mysida and secondarily on shrimps, with juvenile preying on large amounts of Calanoida, whereas adults consumed large amounts of fishes. Isaeidae amphipods were a significant prey for both small and large E. crossotus , whereas Polychaeta Errantia were used mainly by large fishes. Etropus crossotus and S. tessellatus share similar feeding resources in outer bay zone preying on Isaeidae and Polychaeta Errantia. Differences in the Pleuronectiformes diet composition along with spatial and size changes in the use of the available resources contributed to allow the organisms' coexistence in Sepetiba Bay. The high item diversity used by flatfishes indicates that the system plays an important role as a feeding ground, and that interspecific competition for food was unlikely.     

Interspecific conflict in lizards: Social dominance depends upon an individual's species not its body size

Abstract Animals frequently compete for resources (food, shelter, etc.) against conspecifics as well as against individuals of other species; larger animals typically are dominant over smaller ones. Although body size thus correlates with social dominance in interspecific as well as intraspecific encounters, the causal connection remains unclear. That is, one species might dominate another not simply because of larger size, but some other species‐specific attribute. In such a case, we expect an animal of the subordinate taxon to be subservient to all members of the ‘dominant’ species, even those too small to pose a physical threat. This scenario can be tested by staging encounters over resource use in the laboratory. We conducted such trials using a guild of sympatric montane lizards (Reptilia: Scincidae), that compete for shelter sites (rock crevices), with larger species routinely displacing smaller taxa. Remarkably, neonates of larger species effectively deterred adults of smaller species from entering occupied retreat sites – even when the neonates were much smaller than the adults they displaced. Thus, the outcomes of interspecific interactions in this system depend upon the species of the participants, not their relative body sizes. Measurements of bite force confirm that the neonates of the most dominant species posed little physical threat to heterospecific adults, so that species‐specific variation in fighting ability cannot explain this puzzling result. However, juveniles often share shelter sites with conspecific adults, so that avoidance of neonates may reduce the risk of attack by an unseen adult resident.     

Food plant and herbivore host species affect the outcome of intrinsic competition among parasitoid larvae

1. In nature, several parasitoid species often exploit the same stages of a common herbivore host species and are able to coexist despite competitive interactions amongst them. Less is known about the direct effects of resource quality on intrinsic interactions between immature parasitoid stages. The present study is based on the hypothesis that variation in the quality or type of plant resources on which the parasitoids indirectly develop may be complementary and thus facilitate niche segregation favouring different parasitoids in intrinsic competition under different dietary regimes. 2. The present study investigated whether two herbivore species, the cabbage butterflies Pieris brassicae and Pieris rapae (Pieridae), and the quality of two important food plants, Brassica oleracea and Brassica nigra (Brassicaceae), affect the outcome of intrinsic competition between their primary larval endoparasitoids, the gregarious Cotesia glomerata (Braconidae) and the solitary Hyposoter ebeninus (Ichneumonidae). 3. Hyposoter ebeninus is generally an intrinsically superior competitor over C. glomerata. However, C. glomerata survived more antagonistic encounters with H. ebeninus when both developed in P. brassicae rather than in P. rapae caterpillars, and while its host was feeding on B. nigra rather than B. oleracea. Moreover, H. ebeninus benefitted from competition by its higher survival in multiparasitised hosts. 4. These results show that both plant and herbivore species mediate the battleground on which competitive interactions between parasitoids are played out and may affect the outcomes of these interactions in ways that enable parasitoids to segregate their niches. This in turn may promote coexistence among parasitoid species that are associated with the same herbivore host.