Foraging Intensity Research Articles

Response of a subarctic salt marsh plant community to foraging by captive lesser snow geese

Foraging intensity and faecal inputs are important determinants of plant community response to herbivory. We used captive adult lesser snow geese (Anser caerulescens caerulescens), which feed on both above- and below-ground plant tissues, to manipulate foraging intensity and faecal inputs to plots in a sedge meadow in spring, 1996. We measured plant and soil characteristics throughout the growing season of 1996 and in August 1997. We analysed three contrasts: grazed plots versus ungrazed controls, plots with a short period of feeding (3 goose-hours) versus plots with a long period of feeding (6 goose-hours), and grazed plots with faeces versus grazed plots without faeces. Although grazed plots had an order of magnitude higher foraging intensity than that imposed by wild geese in the marsh, there was no effect of feeding on biomass or nitrogen concentration in the dominant species, Carex ramenskii and Triglochin maritimum, after one and two growing seasons. The amount of forage removed by geese did not differ between plots grazed for long and short periods, indicating that geese were able to remove little additional biomass after 3 hours due to low availability. Therefore, the amount of biomass removed by geese was a better indicator of foraging intensity than the time geese fed on plots. The presence of faeces had no effect on biomass or nitrogen concentration in Carex ramenskii or Triglochin maritimum, or on rates of net nitrogen mineralization in soils. Thus, faeces did not appear to increase nitrogen availability for plants in this marsh, probably because faecal density was low.

Lowland boreal forests characterization in Algonquin Provincial Park relative to beaver (Castor canadensis) foraging and edaphic factors

Beaver (Caster canadensis) foraging and edaphic conditions can modify the vegetational characteristics of woody plant community in lowland boreal forests. Effective management of these areas requires an understanding of the relative contribution of these factors in shaping the woody plant community structure. Our objective was to quantify the effects of herbivory by beavers and edaphic conditions on woody plant community organization of lowland boreal forests surrounding beaver ponds. Woody vegetation and soils were sampled at 15 ponds occupied by beavers and one other pond abandoned by them in southern Algonquin Park, Ontario. We measured spatial variation in plant diversity, foraging rates and sapling recruitment of trees and shrubs along gradients of beaver foraging intensity and soil moisture, P, K, Mg, and pH. Beavers fed preferentially on a small number of deciduous species and the number of cut stems declined sharply with increasing distance from ponds. Conifers increased in relative dominance to deciduous species in the presence of beavers. Plant species richness and stem and basal area diversity peaked at intermediate distances (about 25 m) from ponds. Sapling recruitment by non-preferred species was positively related to foraging intensity. Total stem abundance and basal area and sapling recruitment by four preferred species (Populus tremuloides, Acer rubrum, Acer saccharum and Corylus cornuta) were negatively related to foraging intensity. However, by including Alnus rugosa and Salix bebbiana (also preferred by beavers) these patterns changed, becoming positively related to foraging intensity. There was also a pronounced gradient in soil moisture, which also decreased with distance from ponds. The other measured edaphic variables did not vary consistently with distance from ponds. Sapling recruitment in mesic versus xeric species varied consistently with hydrid conditions along the moisture gradient, such that variation in moisture also could produce the observed pattern of plant diversity. Diversity patterns changed three years after beaver abandonment of a pond, though sapling recruitment patterns in preferred and non-preferred species around the abandoned pond were similar to the occupied ponds. These observations suggest spatial variation in woody plant richness and diversity could be determined by combined effects of both herbivory (disturbance by beavers) and variable responses of different species to edaphic conditions.

Foraging and provisioning in Antarctic fur seals: interannual variability in time-energy budgets

This study examined three competing hypotheses to explain how lactating Antarctic fur seals (Arctocephalus gazella) respond to changes in the level of resource availability. Antarctic fur seals have episodic bouts of suckling (1‐3 days), alternating with foraging trips (3‐10 days). Foraging time budgets varied significantly (p , .001) among 8 consecutive years at Bird Island, South Georgia. Foraging trip duration increased during periods of relative food shortage. Time spent ashore was more consistent among years than foraging trip duration but declined during a year of particularly low food availability. In 4 of the 8 years, there was a significant positive correlation between time spent ashore and foraging trip duration. In the other years, the relationship was close to statistical significance. Energy delivery to pups during suckling bouts followed an asymptotic power function. Energy gain during foraging trips was estimated from diving behavior, which suggested that the energy gain function was linear. Distance traveled during foraging trips was correlated with foraging trip duration, and long foraging trips were associated with reduced foraging intensity. There was support for the hypothesis that lactating Antarctic fur seals compensate for reduced resources by increasing the foraging trip duration rather than working harder and increasing their energy expenditure. However, there was most support for the hypothesis that lactating Antarctic fur seals adjust time spent ashore as well as foraging trip duration, possibly to maximize the delivery of food to their offspring. Lactation appears to impose constraints on provisioning of offspring that differ from those of seabirds foraging in the same environment and often on the same prey. Key words: Arctocephalus gazella, Antarctic fur seals, lactation, optimization, provisioning, time budgets. [Behav Ecol 10:198‐208 (1999)]

Time budgets of Nuthatches Sitta europaea with supplementary food

Individual time budgets during winter and spring were studied in a population of Nuthatches Sitta europaea in south‐central Sweden. In late winter birds spent about 90% of daytime foraging, males feeding somewhat less than females, possibly due to male dominance. In early spring, foraging time decreased to about 65% of daytime activity, and sex‐related differences became more pronounced. Males then increased time spent resting and singing. Females were nestbuilding, spent less time resting and also devoted more time to foraging than did males. Prior to the observation periods, food density for part of the population was experimentally increased by supplying sunflower seed. In spite of this, time budgets in the experimental group were similar to those of the control group. It is possible that the intensity of foraging, rather than actual time devoted to it, was influenced by food density and that, as a result, birds in the experimental group were able to maintain a higher level of vigilance. Also, an increase in food supply may be less important to a hoarding bird such as the Nuthatch. However, the result remains that Nuthatch time budgets in winter and spring are not readily influenced by an increase in food density.

The development of vegetation patterns by cattle grazing at low stocking density in the Netherlands

This article discusses tthe establishment of a vegetation pattern in a cattle-grazed (0·7 Charolais heifers ha −1 in the period from May until November) former arable field on acidic humic podsol which was reclaimed from heathland in the early 1930s and was to be incorporated into a nature reserve. The structural patterns of the vegetation were quantified as a measure of foraging intensity by estimating the relative area (percentage) of short turf in each stand in both August and October 1987. A pattern of different foraging intensities had developed after four years. It consisted of stands containing over 75% coverage of short turf and stands with hardly any short turf. This pattern was significantly correlated with the patterns of terrain use. from the number of dung patches. The distribution of dung patches was significantly similar for the major part of the grazing season between 1986 and 1987 and for 1987 and 1988. The pattern of terrain use within the grazing season seeme to reach relative stability within four years. It was concluded that the pattern of different vegetation structures had also become relatively stable.

Foraging Ecology of Seed-Harvesting Ants, Pheidole spp. in a Chihuahuan Desert Ecosystem

Seasonal activity, factors affecting foraging activity and forage selection were studied in three species of Pheidole (Hymenoptera: Formicidae) on a watershed in the southern New Mexico Chihuahuan Desert. Pheidole spp. exhibited a seasonal pattern in numbers of active colonies with most colonies active in July coincident with onset of summer rains and seed drop by annual plants. Pheidole militicida occurred only on the deeper soils of the lower watershed while Ph. rugulosa and Ph. xerophila occurred on the entire watershed. The three Pheidole spp. were active only at soil temperatures between 15-35C and were most active at sunrise. Analysis of factors affecting foraging intensity using a stepwise discriminant function and regression showed evaporation and soil surface temperature to be the strongest influences and seed availability to rank second. However, the factors included in the analysis accounted for less than 50% of the variance in all three species. Pheidole militicida collected mostly forb seeds while Ph. xerophila utilized primarily grass seeds, especially the seeds of fluff grass Erioneuron pulchellum. Introduction Seed-harvesting ants (Pogonomyrmex sp., Veromessor pergandii (Mayr) and Pheidole sp.) are important components of desert ecosystems (Whitford, 1978; Davidson, 1977). The ecology of Pogonomyrmex sp. has been studied extensively by Whitford (1976, 1978), Whitford et al (1976), Whitford and Ettershank (1975) and Rogers (1974). Tevis (1958), Went et al. (1972), Clark and Comanor (1973), Wheeler and Rissing (1975a, b,) and Rissing and Wheeler (1976) reported on aspects of the ecology of Veromessor pergandii. However, studies on the foraging ecology of desert-inhabiting species of Pheidole are limited (Davidson, 1977). Studies of Pheidole are necessary to assess the relative impact of seed-harvesting ants in desert ecosystems. Brown and Davidson (1977) showed that the density of Pheidole sp. increased in areas where seed-eating rodents were excluded, indicating that these seed harvesters responded quickly to the removal of competitors whereas the large-bodysize, seed-harvesting ants of the genus Pogonomyrmex appeared not to be affected. The work of Brown and Davidson suggests potentially important differences between Pheidole and Pogonomyrmex and point to a need for information concerning the foraging ecology of desert species of Pheidole. Our studies were designed to examine the foraging ecology of Pheidole militicida (Wheeler), Pheidole rugulosa (Gregg) and Pheidole xerophila (Wheeler), the most numerous species of Pheidole in several plant communities on a Chihuahuan desert watershed. Methods Studies were conducted on the Jornada Validation Site watershed 40km NNE of Las Cruces, Dona Ana Co., New Mexico. The watershed is an alluvial fan to the ? and E of Mt. Summerford, Dona Ana Mountains, which drains into a small ephemeral lake. The upper portion of the watershed at the base of the mountain (elevation ca. 2000 m) is a black grama (Bouteloua eriopoda) (Torr.) grassland which shifts abruptly to a creosote bush (Larrea tridentata) (DC)-dominated desert pavement community on shallow soils. On the highly dissected slopes of the watershed, the creosote bush community supports a variety of annual plants which occur at low densities during summer rainy periods (ca. lO^OO'ha-1) (Whitford, 1973). 1 Permanent address : Department of Zoology and Entomology, Monash University, Clayton, Victoria, Australia.

Experimental studies on the foraging behavior of the sand fiddler crab Uca pugilator (Bosc, 1802)

The purpose of this study was to test the hypothesis that foraging sand fiddler crabs. Uca pugilator (Bosc), move through the habitat in response to low substratum food levels even though these movements may take the crabs considerable distances from the safety of the burrow area. Chl a and ATP concentrations were used as measures of food density in foraged and unforaged substratum. Field and laboratory feeding experiments showed that crab foraging intensity in a habitat patch was directly correlated with food density in the patch either in the presence or absence of alternative food patches. Other experiments showed that sand fiddlers can respond to differences in food level on a scale of millimeters and do this by probing the substratum with minor chelae. Food levels in aggregations of non-ingested particles harvested by sand fiddlers, feeding pellets, correspond to low foraging intensities predicted from foraging experiments and crabs exhibit low foraging intensities on substratum patches derived from feeding pellets. Substratum food levels in two distinct areas corresponded to high predicted foraging intensities and there was no consistent trend in the level of food in the burrow vs. the nonburrow microhabitats. These results suggest that the movements of foraging sand fiddlers are to some extent controlled by the reduction in substratum food levels due to feeding during a single foraging episode. Sand fiddlers can extract over 70% of the food from harvested substratum over a broad range of substratum food densities but harvest only 42% of the available substratum.

The Adaptive Syndromes of Two Guilds of Insectivorous Birds in the Colorado Rocky Mountains

This paper analyzes the foraging behaviors of a community of insectivorous birds breeding in a high—altitude willow floodplain. The eight species belong to two guilds. Active searchers are represented by the gleaning guild (warblers): Wilsonia pusilla, dendroica petechia, and Geothlypis trichas; passive searchers are represented by the fly—catching guild (flycatchers): Empidonax oberholseri, Empidonax tralli, Empidonax difficilis, Contopus sordidulus, and Nuttallornis borealis. The paper focuses on the following general questions: (1) What are the different components, and their correlations, of foraging behavior in the eight species? (2) What are the overall adaptive syndromes associated with predatory tactics of active and passive searchers? (3) Are the predictions of several models of optimal foraging supported by the results of the present study? Adaptive syndromes (coordinated sets of characteristics, including the specific manner of resource utilization, and an array of other, related adaptations) are described for each species. Warblers forage primarily by gleaning, concentrating on stationary prey. Their velocity (average number of perches per minute) is >10.0, their search intensity (perches visited prior to an attack) is >5.0, and their foraging intensity (number of attacks per minute) is >1.7. They have an attack radius (mean distance from perch to prey) of <0.6m. Members of the gleaning guild may thus be characterized as active searchers (they look for prey difficult to find) but passive pursuers (they attack prey easy to catch). In addition, gleaners exhibit a narrow range of preferences in habitat structure. Flycatchers forage primarily by hovering or sallying, concentrating on prey available via aerial attack. They have both a velocity and search intensity of <4.0, and a foraging intensity of <1.8. They have attack radii ranging from 1 m to >7.5 m. Members of the fly—catching guild may thus be characterized as passive searchers (they look for prey easy to find) but active pursuers (they attack prey difficult to capture). Flycatchers show a broad range of preferences in habitat structure. For all species considered together there is an inverse correlation between body size and velocity, and a positive correlation between body size and attack radius. Three core adaptations identify the adaptive syndromes of actively and passively searching predators. Active searchers have high velocities, high search intensities, and a foraging repertoire associated with passive prey. Passive searchers have low velocities, low search intensities, and a foraging repertoire associated with active prey. Additional characteristics also differentiate the two adaptive syndromes. Search/pursuit ratios critical to numerous optimal foraging models are impossible to measure reliably in the field; thus the concept of active and passive searchers is proposed as a meaningful and readily quantifiable substitute. The data presented here exhibit poor fit with the predictions of several selected models. Predictions concerning differences in diet specialization, the extent of territorial and foraging overlap, and the prevalence of sexual dimorphism are not confirmed. In general, predictions concerning morphological size gradients are upheld; those concerning habitat specialization are confirmed if habitat is measured in structural terms, but are not supported when measured by vegetation type.

Factors Affecting Foraging Activity in Chihuahuan Desert Harvester Ants

The foraging activity of 4 species of harvester ants as affected by soil surface temperature, saturation deficit at the soil surface, light and forage availability was studied in a desert grassland area 40 km NNE of Las Cruces, N.M. Pogonomyrmex sp. were capable of locomotor activity at temperatures between ca. 5° and 53°C and Novomessor cockerelli between 4° and 52°C. Pogonomyrmex sp. exhibited maximum foraging intensity at 45°C and N. cockerelli at 20°C. Forager population sizes varied among species: 1000–6000 in P. rugosus , ca. 1000 in P. californicus and N. cockerelli ; and ca. 200–600 in P. desertorum. P. rugosus exhibited peak foraging activity and had the greatest number of colonies active following drought periods, ceasing foraging when granaries were filled, and foraged at night during midsummer. P. desertorum and P. californicus were strictly diurnal and did not exhibit hoarding behavior. P. desertorum responded to soil wetting by shifting activity between watered and unwatered controls directly in response to differences in soil surface temperature. Soil wetting stimulated intense activity in P. rugosus at a time when controls were inactive. Foraging in harvester ants seemed to be primarily affected by forage availability, secondarily by microclimate, with historical factors and interspecific competition as contributing parameters in some species.

A simulation of wood consumption by the subterranean TermiteHeterotermes aureus (Snyder), in an Arizona deset Grassland

A simple model is preesented which simulates the consumption of superficial dead wood in a desert grassland by a single termite species over a period of one year. Temperature-dependent functions of termite foraging intensity and wood consumption are most critical. The model was designed to run on mean daily temperature and daily rainfall. The model predicts thatHeterotermes aureus forag ers were able to make 4.61×1010 visits to the suurf ace per hectare during the year of simulation. It estimates that there were only seven days during this period thatH. aureus did not forage. It calculates that this species remooved dead wood at the rate of 78.9 kg/ha/yr, which represents 3.7 perenct of the standing crop biomass and 17.5 percent of the annual production of superficial dead wood. The importance of this termite is emphasized by its removal of preferred species of wood.

Seasonal activity and environmental control of foraging of the subterranean termite, Heterotermes aureus (Snyder), in a desert grassland

This research was conducted on a shrub-invaded desert grassland ecotone 40 km S of Tucson, Arizona. To examine the abiotic factors affecting or regulating foraging of subterranean termites a modified bait sampling method using toilet paper rolls was developed. Observations of foraging activity of Heterotermes aureus (Snyder) were made during 24-hr periods each week for one year. Pertinent environmental data were collected at each check. H. aureus foraged day and night throughout most of the year with minimal activity from December through February. Foraging intensity increased moderately in the spring and fall and was high but erratic during the summer months. The number of foragers generally increased with increasing temperature. Rainfall had little effect on foraging when daily mean soil temperatures were below 20°C (toilet paper roll-soil interface); however, even the slightest amount of rain during the hot summer months greatly increased foraging intensity. Above 33°C, even with precipitation, foraging numbers plummeted. The number of surface foragers at any instant (Y) is best explained by the equation, In Y = −0.985 − 0.0761 T + 2.928 ln T + 0.327 ln R where T is the temperature at the roll-soil interface and R is daily rainfall. It appears that foraging activity is not endogenously controlled; rather it is exogenously controlled by temperature and moisture.