Spatial heterogeneity and interspecific competition

A model of two competing species is presented in which each species is able to disperse over a single spatial axis. The spatial axis is composed of two intervals with different carrying capacities. We ask the question: If species one is alone and at population dynamic equilibrium, then when can species two successfully invade when rare? We say that an interval is “suitable” if the interval can be invaded by species two in the absence of dispersal by both species, and we say an interval is “unsuitable” if the interval cannot be invaded by species two in the absence of dispersal by both species. We offer three findings: (I) If one interval is suitable and the other is unsuitable, then the success of invasion depends upon the length of the suitable interval. Invasion succeeds if the suitable interval is larger than a threshold minimum and fails otherwise. (II) It is possible for species two to invade even though both intervals are unsuitable. (III) It is possible for species two to fail to invade even though both intervals are suitable.     

Predaceous ants, beach replenishment, and nest placement by sea turtles

Ants known for attacking and killing hatchling birds and reptiles include the red imported fire ant (Solenopsis invicta Buren), tropical fire ant [Solenopsis geminata (Fabr.)], and little fire ant [Wasmannia auropunctata (Roger)]. We tested whether sea turtle nest placement influenced exposure to predaceous ants. In 2000 and 2001, we surveyed ants along a Florida beach where green turtles (Chelonia mydas L.), leatherbacks (Dermochelys coriacea Vandelli), and loggerheads (Caretta caretta L.) nest. Part of the beach was artificially replenished between our two surveys. As a result, mean beach width experienced by nesting turtles differed greatly between the two nesting seasons. We surveyed 1,548 sea turtle nests (2000: 909 nests; 2001: 639 nests) and found 22 ant species. S. invicta was by far the most common species (on 431 nests); S. geminata and W. auropunctata were uncommon (on 3 and 16 nests, respectively). In 2000, 62.5% of nests had ants present (35.9% with S. invicta), but in 2001, only 30.5% of the nests had ants present (16.4% with S. invicta). Turtle nests closer to dune vegetation had significantly greater exposure to ants. Differences in ant presence on turtle nests between years and among turtle species were closely related to differences in nest placement relative to dune vegetation. Beach replenishment significantly lowered exposure of nests to ants because on the wider beaches turtles nested farther from the dune vegetation. Selective pressures on nesting sea turtles are altered both by the presence of predaceous ants and the practice of beach replenishment.     

Intra- and interspecific relationships between nest size and immunity

Nest-building behavior has been suggested to represent a postmatingsexually selected signal in passerine birds, an hypothesis thathas received both comparative and experimental support. Becauseselection pressure due to parasites and diseases should be particularlyhigh during nest building, mainly due to energetic costs anddepression of the immune system associated with this reproductivephase, we predicted a positive association between nest-buildingeffort and immunity. Nest-building effort would reflect theability to produce efficient immune responses of builders onlyif individuals with a superior immune system would display exaggeratednest-building effort. We tested this prediction by studyingthe relationship between volume of nest material used for nestconstruction and, at the intraspecific level, estimates of innatehumoral immune response in barn swallows Hirundo rustica. Atthe interspecific level, we used responses to the mitogenicphytohemagglutinin as an indicator of adaptive immune responseof European passerine species. As predicted, we found, aftercontrolling for several potential confounding factors, thatvolume of nest material was positively related to immune responseboth at the intra- and at the interspecific level. Alternativehypotheses explaining the comparative results are discussed.     

The size–grain hypothesis and interspecific scaling in ants

1. The size–grain hypothesis maintains that as terrestrial walking organisms decrease in size, their environment becomes less planar and more rugose. The benefits of long legs (efficient, speedy movement over a planar environment) may thus decrease with smaller body size, while the costs (larger cross-sectional area limiting access to the interstitial environment) are enhanced.
2. A prediction from this hypothesis – that leg size should increase proportionately with body mass – is examined. Ants are among the smallest walking animals and extend the size gradient five orders of magnitude beyond the traditional 'mouse to elephant' curve. The mass of 135 species of worker ants spans 3·7 orders of magnitude (0·008–53 mg). Larger ants tended to be slimmer and longer legged. Ant subfamilies varied in their scaling relationships, but four out of five showed a positive allometry for hind leg length ( b > 0·33). Mammals, in contrast, show isometry for leg length over six orders of magnitude.
3. It is suggested that ants make a transition from living in an interstitial environment when small to a planar environment when large, a habit continued by most terrestrial mammals. Head length and pronotum width are robust estimators of mass in ants.     

63Environmental change, interspecific interactions, and shifting algal distributions on rocky shores

Because intertidal organisms live close to their environmental tolerances, intertidal species are considered early harbingers of the effects of global change. Using a combination of spatial environmental gradients and long-term datasets, I examine the likely consequences of global warming on the distribution of Mazzaella parksii (formerly Mazzaella cornucopiae ), a turf-forming red alga. On Tatoosh Island, WA, M. parksii is restricted almost entirely to north-facing surfaces. On south-facing slopes, the upper limit of M. parksii , which is set by abiotic stress, occurs much lower on the shore. However, its lower limit, which is set by herbivores, does not vary with aspect. Because its upper and lower limits are independent across the aspect gradient, M. parksii is effectively squeezed out of the system as one moves from a north-facing to a south-facing slope. This general pattern is observed at several spatial scales across gradients of wave exposure and air temperature. The vertical distributional patterns of M. parksii also shift across temporal gradients. Over the past 25 years, the upper limit of M. parksii on Tatoosh Island has shifted downshore by approximately 25 cm. Most of this change was driven by three consecutive harsh summers in the mid 1990's. These temporal patterns, combined with the insight gained from spatial patterns, suggest that M. parksii may go locally extinct in many parts of its range as its upper and lower limits converge. Furthermore, because vertical distributional shifts are linked to stochastic climatic effects, temporal change in marine algal populations may be rapid and unexpected.     

The influence of nest availability on the abundance and diversity of twig-dwelling ants in a Papua New Guinea forest

Tropical ant communities are frequently diverse, but highly patchy in nature. The availability of suitable nest sites may be a regulating force in structuring litter ant communities. Our aim was to examine ant resource utilization in naturally occurring twigs, and to modify the availability of these resources in order to quantify the influence of nest availability on ant communities in a Papua New Guinean forest. First, we compared ant communities that assemble in artificial twigs (drilled, wooden dowels), naturally occurring twigs, and the leaf litter. A total of 55 ant species were captured: 33 from the leaf litter, 29 from naturally occurring twigs, and only 12 from artificial nests. Significantly different communities formed in each of the three nest types. Second, we examined how the density of natural or artificial nest material influenced the ant abundance and species richness. Plots had between 5 and 96 potential nest sites. An average of only 11.2% of these twigs was colonized. Both species richness and the total abundance of adult ants were significantly positively correlated with increasing naturally occurring twig density. Conversely, increasing the availability of artificial nests from 5 to 20 per plot had no significant effect on the proportion of artificial nests colonized, species richness, or the colony size. We observed that ant species richness and abundance increased with natural twig density, at least for naturally occurring communities. But why so many twigs remain vacant and available for ant colonization remains unknown. Other biotic and abiotic factors likely influence the use of nesting habitat in these ant communities.     

On the origin and rarity of interspecific nest parasitism in birds

Interspecific nest parasitism is surprisingly rare in birds given the potential advantages for the parasite of exploiting the parental care of other species. One possibility is that chicks will not thrive with the parental care and food of heterospecifics. I simulated parasitism in nonparasitic congeners by switching eggs between nests of three species of titmice (great tit Parus major, blue tit Parus caeruleus, and coal tit Parus ater). The experiment showed that compatibility of parental care was not a constraint preventing parasitism. I also used the model system to compare fitness consequences of inter- and intraspecific nest parasitism, addressing the problem of which form is ancestral. Fledging success (body mass, survival) was higher when an egg was added to the nest of a smaller species than to the nest of a conspecific and also higher when the parasitic chick hatched early rather than late relative to host chicks. This suggests that interspecific nest parasitism may not require a stage of intraspecific nest parasitism before evolving but may start from a larger species directly exploiting the parental care of a smaller species or a species with shorter incubation period directly exploiting a species with longer incubation period.     

Analysis of nest occupancy and nest reproduction in two sympatric raptors: common buzzard Buteo buteo and goshawk Accipiter gentilis

Nest site selection can have important fitness consequences in birds. I analysed the habitat characteristics of 392 nests of two sympatric raptor species (common buzzard Buteo buteo and goshawk Accipiter gentilis ) in Germany and their relation to nest occupation rate and nest reproductive success. For common buzzard, multivariate models explained only small proportions of the variance in nest occupation rate and nest reproductive success (13–19%). Important variables related to nest occupation rate were human disturbances, intra- and interspecific neighbour density, the amount of forested area and nest tree crown cover. Variables related to nest site reproductive success also included human disturbance, intra- and interspecific neighbour density and nest tree crown cover as well as nest distance to the nearest forest edge. In contrast, models for the goshawk explained a much higher proportion of the variation in nest occupation rate and nest reproductive success (41–43%). Important variables related to nest occupation rate were the remoteness of the nest site and direct human disturbance. Variables related to nest site reproductive success were remoteness of the nest site and good hunting habitat. Goshawks seem to be more sensitive to human disturbance than buzzards. A multiple discriminant analysis showed that nest site characteristics substantially overlapped between the species and there is a good evidence that competition for optimal nest sites occurs. Thus, buzzards might be constrained by the dominant goshawk in their nest site selection.     

The carrying of ants (Cataglyphis bicolor Fab.) by others of the same nest

The carrying of ants from the nest by others of the same species ( Cataglyphis bicolor Fab.) is described. Two types of carrying are distinguished, systematic removal of many adults and larval stages to a new nest, and carrying of individuals, alive or dead, that are dropped at distances up to 35 metres from the nest. The rate of carrying of individual ants from a nest varied from 0–15 per day in a population of foragers estimated as about 200, making 1500–2000 exits per day. Live and dead ants appeared to be treated similarly. Live ants, in the cases examined, went back to the nest. Evidence was obtained by marking ants that carriers were distinct in habits from non-carriers, and also excavated earth from the nest.     

Mechanisms of interspecific competition among an invasive and two native fire ants

The mechanisms of interspecific competition among an invasive and two native Solenopsis fire ant forms were investigated in a series of laboratory experiments. In separate trials each with a different food resource, the native S. geminata × xyloni retrieved the greatest amount of a protein- and lipid-rich artificial food resource and a high protein natural food resource, and the native S. geminata retrieved the greatest amount of a high carbohydrate food resource. In trials investigating aspects of interference competition at the colony level, the invasive S. invicta proved to be initially more aggressive than S. geminata , but less aggressive than S. geminata × xyloni . Solenopsis invicta eventually controlled more of the foraging arenas against both native forms when colonies were equivalent by worker biomass, but not when colonies were equivalent by worker number. When paired with S. invicta , S. geminata suffered a significantly greater proportional reduction in both workers and entire colonies when colonies were initially standardized by worker biomass, but not when colonies were standardized by worker number. When paired with S. invicta , a significantly greater proportional reduction of workers occurred in S. geminata × xyloni , regardless of how colonies were standardized. In pairwise trials at the individual level, majors always exhibited significantly less mortality than minors, regardless of the Solenopsis form. The majors of both native forms suffered significantly less mortality than those of S. invicta . Superiority in colony-level interference ability appears to be an important mechanism allowing S. invicta to displace native Solenopsis forms. The ability of S. invicta to reach high population densities, because of intrinsic biological characteristics or an escape from natural enemies, plays an important contributory role. Similar mechanisms may underlie the success of other invasive ant species.     

Speed-versus-accuracy trade-offs during nest relocation in Argentine ants (Linepithema humile) and odorous house ants (Tapinoma sessile)

Animals are often forced to accommodate disturbance to their territories or nests. When nest relocation becomes necessary, it is important to efficiently evaluate alternative nest sites to choose the one most suitable under current conditions. However, if time is limiting, species may experience a speed-versus-accuracy trade-off when searching for a new home. We examined nest site selection under duress (in the form of flooding) in two species of ants: Linepithema humile and Tapinoma sessile. We predicted that if ants are able to assess and evacuate to the most suitable location, colonies should move to higher elevation, relative to their current nest site, in response to flooding. To test for a speed-versus-accuracy trade-off, we presented colonies with new nest chambers that were either higher, lower, or at the same height as their current nest and examined if their ability to efficiently choose a new site was influenced by the rate of flooding. When flooding rates were slow, both species favored the highest nest site and nearly always moved their entire nest to the same chamber. However, when the rate of flooding was doubled, colonies of T. sessile less often chose the highest nest site and were also more likely to split their nests between two of the available chambers. These results demonstrate a trade-off between speed and accuracy in nest site selection for odorous house ants, while L. humile retained their ability to adequately assess new nest sites under the conditions we presented. These patterns may arise from differences in exploratory behavior and activity between the two species. Despite having identical colony sizes, L. humile had approximately ten times more workers exploring the alternate nest sites 30 min into the experiment than did T. sessile.     

Social information in nest colonisation and occupancy in a long-lived, solitary breeding bird

Recent work increasingly reveals the importance of social information in individual dispersal decisions, population dynamics and conservation. Much of the knowledge gained to date comes from studies on short-lived and/or densely breeding species. In contrast, our understanding of the processes involved in nest-site selection for long-lived, solitary breeding species is insufficient. We increased nest-site availability by nest-box supplementation over a 5-year period in a population of a long-lived, solitary, secondary-cavity nesting bird, the European roller Coracias garrulus, breeding in natural cavities and human constructions. We tested the nest limitation and the inadvertent conspecific social information hypothesis in order to study the dynamics and mechanisms of abandonment of previously used nests and the colonisation of new ones. Our data lend support to the nest-limitation hypothesis both in terms of quantity-population and the size of breeding clusters increased, and suitability--the majority of pairs used and re-occupied nest-boxes. Nevertheless, the use of natural cavities did not decrease after 5 years. At the between-patch scale, rollers were revealed to colonise nest-boxes based on conspecific social attraction, namely distance to the nearest neighbour in the same season. Despite the unpredictability of patch productivity, at the within--patch scale, the selection of previously unoccupied cavities was consistent with the performance-based conspecific attraction hypothesis. Philopatry could account for the repeated use of cavities, because nests that were used for two successive years were more likely to also be reused in the subsequent season.     

Habitat selection,interspecific interactions and landscape composition

Summary I argue here that, from the perspective of any individual, most landscapes are composed of only three basic types of habitats. These are: (1) source habitat in which reproduction exceeds mortality and the expected per capita growth rate is greater than one; (2) sink habitat, in which limited, reproduction is possible but will not on average, compensate for mortality and the per capita rate of growth is between zero and one; and (3) unusable habitat, which comprises the matrix of all habitats that are never exploited by the species in question, and in which patches of source and sink habitats are embedded. Unlike earlier source-sink models, this model explicitly considers the effects that substituting one type of habitat for another has on the equilibrium size of a population and the interactions between species which can use both source and sink habitats. The model demonstrates that the equilibrium size of a species' population can sometimes be increased by substituting unusable habitat for sink habitat. Thus, even though the average patch quality in the landscape may be decreased, the overall quality of the landscape can increase. For two species with distinct habitat preferences, interactions between species can vary qualitatively as well as quantitatively as a function of the relative abundances of each of the habitat types. The model also shows that the interactions between species are particularly sensitive to the relative costs of moving between patches and sampling patches to determine their quality. Recent fragmentation of natural landscapes may increase the cost of searching for usable (source or sink) patches. Under some conditions, the interspecific interactions may be substantially more negative (competitive) than the interactions that evolved in the original natural landscape, further reducing population sizes and increasing the likelihood of competitive exclusion in fragmented modern landscapes.