Foraging Pits Research Articles

Digging up the dirt: Quantifying the effects on soil of a translocated ecosystem engineer

AbstractDigging mammals are often considered ecosystem engineers, as they affect important properties of soils and in turn nutrient exchange, vegetation dynamics and habitat quality. Returning such species, and their functions, to areas from where they have been extirpated could help restore degraded landscapes and is increasingly being trialled as a conservation tool. Studies examining the effects of digging mammals have largely been from arid and semi‐arid environments, with little known about their impacts and importance in mesic systems. To address this knowledge gap, we investigated the ecological role of a recently introduced population of eastern barred bandicoots (Perameles gunnii) on Churchill Island, Victoria, south‐eastern Australia, from which all digging mammals have been lost. We quantified the annual rate of soil turnover by estimating the number of foraging pits bandicoots created in 100‐m2 plots over a 24‐h period. Foraging pit counts could not be completed in each season, and the overall turnover estimate assumes that autumn/winter months represent turnover rates for the entire year; however, this is likely to fluctuate between seasons. Ten fresh and ten old pits were compared to paired undug control sites to quantify the effect soil disturbance had on soil hydrophobicity, moisture content and soil strength. Plots contained between zero and 64 new foraging pits each day. We estimated that an individual eastern barred bandicoot digs ~487 (95% CI = 416–526) small foraging pits per night, displacing ~13.15 kg (95% CI = 11.2–14.2 kg) of soil, equating to ~400 kg (95% CI = 341–431 kg) of soil in a winter month. Foraging pits were associated with decreased soil compaction and increased soil moisture along the foraging pit profile. Eastern barred bandicoots likely play an important role in ecosystems through their effects on soil, which adds to an increasing body of knowledge suggesting restoration of ecosystems, via the return of ecosystem engineers and their functions, holds much promise for conserving biodiversity and ecological function.

Effects of digging by a native and introduced ecosystem engineer on soil physical and chemical properties in temperate grassy woodland.

Temperate grasslands and woodlands are the focus of extensive restoration efforts worldwide. Reintroduction of locally extinct soil-foraging and burrowing animals has been suggested as a means to restore soil function in these ecosystems. Yet little is known about the physical and chemical effects of digging on soil over time and how these effects differ between species of digging animal, vegetation types or ecosystems. We compared foraging pits of a native reintroduced marsupial, the eastern bettong (Bettongia gaimardi) and that of the exotic European rabbit (Oryctolagus cuniculus). We simulated pits of these animals and measured pit dimensions and soil chemical properties over a period of 2 years. We showed that bettong and rabbit pits differed in their morphology and longevity, and that pits had a strong moderating effect on soil surface temperatures. Over 75% of the simulated pits were still visible after 2 years, and bettong pits infilled faster than rabbit pits. Bettong pits reduced diurnal temperature range by up to 25 °C compared to the soil surface. We did not find any effects of digging on soil chemistry that were consistent across vegetation types, between bettong and rabbit pits, and with time since digging, which is contrary to studies conducted in arid biomes. Our findings show that animal foraging pits in temperate ecosystems cause physical alteration of the soil surface and microclimatic conditions rather than nutrient changes often observed in arid areas.

Does the morphology of animal foraging pits influence secondary seed dispersal by ants?

AbstractSecondary seed dispersal by ants (myrmecochory) is an important process in semi‐arid environments where seeds are transported from the soil surface to an ant nest. Microsites from which ants often remove seeds are the small pits and depressions made by native and exotic animals that forage in the soil. Previous studies have demonstrated greater seed retention in the pits of native than exotic animals, but little is known about how biotic factors such as secondary seed dispersal by ants affect seed removal and therefore retention in these foraging pits. We used an experimental approach to examine how the morphology of burrowing bettong (Bettongia lesueur), greater bilby (Macrotis lagotis), short‐beaked echidna (Tachyglossus aculeatus) and European rabbit (Oryctolagus cuniculus) foraging pits and ant body size influenced ant locomotion and seed removal from pits along an aridity gradient. Ants took 3.7‐times longer to emerge from echidna pits (19.6 s) and six‐times longer to emerge from bettong pits (30.5 s) than from rabbit pits (5.2 s), resulting in lower seed removal from bettong pits than other pit types. Fewer seeds were removed from pits when cages were used to exclude large body‐sized (>2 mm) ants. Few seeds were removed from the pits or surface up to aridity values of 0.5 (humid and dry sub‐humid), but removal increased rapidly in semi‐arid and arid zones. Our study demonstrates that mammal foraging pit morphology significantly affects ant locomotion, the ability of ants to retrieve seeds, and therefore the likelihood that seeds will be retained within foraging pits.

Net effects of soil disturbance and herbivory on vegetation by a re-established digging mammal assemblage in arid zone Australia

Large-scale re-introductions of locally extinct digging mammal assemblages have been implemented at a number of arid zone sites, with the aim of conserving mammal species and restoring ecosystem function. Previous studies have focused on the ways in which foraging pits benefit plants, but the effects of herbivory by digging mammal assemblages are poorly understood.We used a randomised block design with control, procedural control and exclusion plots (n = 10) to experimentally test the net effect of a re-established mammal assemblage of bridled nailtail wallabies, Onychogalea fraenata, greater bilbies, Macrotis lagotis, brush tailed-bettongs, Bettongia penicillata, and burrowing bettongs, Bettongia lesueur, on plants in the Australian arid zone.We found that the re-established mammal assemblage limited natural seedling abundance and consumed transplanted seedlings. Plant composition between treatments did not differ, but perennial forbs and subshrubs, which are known food items of the re-introduced mammals, were most abundant in mammal exclusions. Furthermore, a landscape-scale survey showed that composition in the re-introduction area differed significantly from an adjacent control.When conceptualising the role of mammalian ecosystem engineers in arid environments, negative effects on plants from herbivory and soil disturbance must be considered alongside the better studied processes of nutrient cycling and seedling establishment.

A test of the feasibility of using glue traps to sample the invertebrate fauna in bilby Macrotis lagotis foraging pits

Interactions between mammals and invertebrates remain a gap in our knowledge of the role of fossorial foragers in Australian ecosystems. This is probably because digging mammals disappeared from the majority of their former range before they could be studied and because of the difficulties associated with sampling invertebrates in foraging pits. Bilbies were reintroduced into a rangeland restoration program in Western Australia, providing an opportunity to compare invertebrates in their foraging pits with the undisturbed surface soil. The challenge was to develop a method for collecting invertebrates while minimising disturbance of the floor of the foraging pits. This study investigated the feasibility of using folding cardboard glue traps to sample the invertebrates. The traps proved to be easy to deploy and collect and concerns that the traps on the soil surface would collect more soil than those in the foraging pits, thereby biasing the invertebrate sample, were not realised. No firm conclusions can be...

Soil-foraging animals alter the composition and co-occurrence of microbial communities in a desert shrubland.

Animals that modify their physical environment by foraging in the soil can have dramatic effects on ecosystem functions and processes. We compared bacterial and fungal communities in the foraging pits created by bilbies and burrowing bettongs with undisturbed surface soils dominated by biocrusts. Bacterial communities were characterized by Actinobacteria and Alphaproteobacteria, and fungal communities by Lecanoromycetes and Archaeosporomycetes. The composition of bacterial or fungal communities was not observed to vary between loamy or sandy soils. There were no differences in richness of either bacterial or fungal operational taxonomic units (OTUs) in the soil of young or old foraging pits, or undisturbed soils. Although the bacterial assemblage did not vary among the three microsites, the composition of fungi in undisturbed soils was significantly different from that in old or young foraging pits. Network analysis indicated that a greater number of correlations between bacterial OTUs occurred in undisturbed soils and old pits, whereas a greater number of correlations between fungal OTUs occurred in undisturbed soils. Our study suggests that digging by soil-disturbing animals is likely to create successional shifts in soil microbial and fungal communities, leading to functional shifts associated with the decomposition of organic matter and the fixation of nitrogen. Given the primacy of organic matter decomposition in arid and semi-arid environments, the loss of native soil-foraging animals is likely to impair the ability of these systems to maintain key ecosystem processes such as the mineralization of nitrogen and the breakdown of organic matter, and to recover from disturbance.

Animal foraging pit soil enhances the performance of a native grass under stressful conditions

In line with the Stress Gradient Hypothesis, studies of facilitation have tended to focus on plant–plant interactions (biotic nurses), while the relative role of abiotic nurses has been little studied. We assessed the role of biotic and abiotic nurses, and their interaction, on soil enhancement and the consequential performance of a native annual grass, Dactyloctenium radulans. We used a growth chamber study with two levels of water application to compare the performance of D. radulans growing in soil from foraging pits of the Short-beaked echidna (Tachyglossus aculeatus; abiotic nurse) and non-pit soil from either under tree canopies (biotic nurse) or surrounding open areas. All measures of plant performance were more pronounced under the high than the low water treatment. The greatest differences between pit and surface Microsites occurred under the low water application, reinforcing our view that facilitatory effects are greater in resource-limited environments. Despite tree canopy soil having greater N, there was no significant effect on plant performance, nor any significant interaction with Microsite. Our study provides strong evidence that foraging pits enhance soil properties and this soil, in turn, facilitates plant growth; and supports previous work documenting the positive effect of nurse-protege interactions under greater levels of abiotic stress.

Animal foraging as a mechanism for sediment movement and soil nutrient development: Evidence from the semi-arid Australian woodlands and the Chihuahuan Desert

An emerging area of interest in geomorphology over the past two decades has been the effects of biota on ecosystem processes. We examined the roles of a range of vertebrates on soil disturbance in two markedly different environments, the semi-arid woodland of eastern Australia and a Chihuahuan Desert grassland–shrubland in the south-western United States. Foraging pits of soil-disturbing vertebrates varied markedly from small scratchings of heteromyid (mainly Dipodomys spp.) rodents (1.8×10−4m3) to deep (1.0×10−2m3) excavations of the burrowing bettong (Bettongia leuseur) and greater bilby (Macrotis lagotis). Vertebrates moved substantial volumes of soil in both environments, and activity was highly temporally and spatially variable. At large spatial scales, soil disturbance by echidnas (Tachyglossus aculeatus) and Gould's sand goannas (Varanus gouldii) was substantially greater in communities dominated by shrubs, and where domestic livestock had been excluded. Heteromyid rodents tended to excavate more foraging pits in coarse-textured vegetation communities (both grasslands and shrublands). In both environments, foraging was concentrated close to perennial plants such as grass tussocks and tree canopies rather than in the interspaces. Foraging pits of Chihuahuan desert animals tended to be higher in labile carbon and support greater levels of infiltration, though this was plant community-dependent. Overall our results indicate that animal foraging is an important geomorphic mechanism capable of mobilizing substantial volumes of soil in arid and semi-arid environments and with potential effects on soil function.

Can the invasive European rabbit (Oryctolagus cuniculus) assume the soil engineering role of locally-extinct natives?

Habitat modifying species can play crucial roles in ecosystem function. Invasive engineers may assume these roles where native engineers have been lost from the system. We compared the dynamics of the foraging pits of an invasive engineer, the European rabbit (Oryctolagus cuniculus) with two native mammals, the greater bilby (Macrotis lagotis) and the burrowing bettong (Bettongia lesueur). Foraging pits are small surface depressions created by animals when they forage for seeds, bulbs, roots, invertebrates and fungi. We measured foraging pit density and turnover, and density and richness of plant seedlings in pits and adjacent surfaces across three landforms representing a gradient in resource availability inside (bilbies and bettongs), and outside (rabbits only) a reserve in an arid Australian shrubland over 2 years. Pits of the native engineers contained 80% more seedlings (11.2 plants m−2) than rabbit pits (6.22 plants m−2). Further, rabbit pits supported 3.6-times fewer seedlings than equivalent non-pit surfaces outside the exclosure. Only one plant species was restricted entirely to pits. The reserve had more foraging pits and greater turnover than outside, but contrary to prediction, pit effects on seedling density were no greater in the more resource-limited dunes. There were some strong temporal and landscape effects on pit density and species composition, but generally trends were similar inside and outside the reserve. Overall, despite their functional similarities, invasive rabbits created fewer pits that were less favourable patches for seedlings than those of native engineers. Our work suggests that a suite of ecosystem processes associated with fertile patch creation has potentially been lost with the extirpation of bilbies and bettongs.

Are sea otters being exposed to subsurface intertidal oil residues from the Exxon Valdez oil spill?

Twenty years after the Exxon Valdez oil spill, scattered patches of subsurface oil residues (SSOR) can still be found in intertidal sediments at a small number of shoreline locations in Prince William Sound, Alaska. Some scientists hypothesize that sea otters continue to be exposed to SSOR by direct contact when otters dig pits in search of clams. This hypothesis is examined through site-specific examinations where SSOR and otter-dug pits co-occur. Surveys documented the exact sediment characteristics and locations on the shore at the only three subdivisions where both SSOR and otter pits were found after 2000. Shoreline characteristics and tidal heights where SSOR have persisted are not suitable habitat for sea otters to dig pits during foraging. There is clear separation between areas containing SSOR and otter foraging pits. The evidence allows us to reject the hypothesis that sea otters encounter and are being exposed by direct contact to SSOR.

The resource coupling role of animal foraging pits in semi‐arid woodlands

AbstractWater is a strong driver of ecosystem function and diversity in resource‐limited drylands. Structures that alter the redistribution of water have the potential to affect vegetation pattern and productivity and the maintenance of spatial heterogeneity. We examined the role of surface foraging pits and mounds constructed by the short‐beaked echidna (Tachyglossus aculeatus) on runoff generation, sediment detention and seed capture. We speculated that pits dug into biologically crusted interspace soils would reduce overland flow and act as sinks for seed, while mounds of ejected soil would increase sediment availability. Runoff and sediment yield were examined under simulated rainfall with five levels of disturbance representing the pits and mounds of echidnas. Increasing the level of disturbance up to 47% cover of mounds and pits had no effect on time to ponding, steady‐state runoff or the coefficient of runoff. However, time to initiation of runoff, sediment removal rates and sediment concentration in runoff water increased with increasing disturbance. Pit‐to‐pit and surface‐to‐pit movements of Dodonaea viscosa seed tended to increase, while pit‐to‐surface movements tended to remain unchanged with increasing disturbance. Surface‐resident seed generally moved shorter distances with increasing disturbance. The results highlight the importance of mounds and pits of soil foraging animals for generating and trapping eroded sediment. Runoff and erosion processes may therefore provide a mechanism for coupling critical resources such as seed, water and sediment in resource‐rich patches that will contribute to the development of small‐scale heterogeneity in woodlands. Copyright © 2010 New South Wales, Australia.

Foraging pits, litter and plant germination in an arid shrubland

Many animals create soil surface depressions (pits) while foraging for subterranean resources. Foraging pits typically fill with litter, organic debris and seed, retain moisture, and become hotspots for plant germination. This study aimed to examine whether artificial foraging pits, which mimic those created by Greater bilbies ( Macrotis lagotis) and Burrowing bettongs ( Bettongia lesueur), develop into patches of enhanced plant germination due to accumulation of leaf litter, or whether physical characteristics of the pits such as temperature and soil moisture influence germination, irrespective of the presence of litter. Compared with the soil surface, significantly more plants germinated in artificially-created foraging pits, irrespective of whether they received added litter. Daytime temperatures were 17–31% (7–11 °C) cooler in foraging pits than on the adjacent soil surface, and pits retained significantly more moisture up to 5 days after rainfall. Our results suggest that the mesic conditions in foraging pits may be more important in promoting germination of vascular plants than the presence of litter.

Soil disturbance by native animals along grazing gradients in an arid grassland

Domestic grazing animals that congregate around watering points in arid rangelands create clearly-defined trampling-induced grazing gradients. Grazing and trampling alter soil and vegetation condition, often leading to substantial reductions in ecological function. We measured foraging pits and mounds created by native soil foraging animals over 12 months at three watering points in a Chihuahuan Desert grassland, and hypothesized that the density and cover of their disturbances would increase with increasing distance from water. We recorded an average of 3756 disturbances ha −1 and cover of 34.18 m 2 ha −1 across the grazing gradients, which comprised mainly pits (43%) and mounds (25%) of heteromyid rodents, ants and spiders. Soil turnover was estimated at 1.43 m 3 ha −1. We detected no differences in density, cover, soil volume or composition of disturbances in relation to distance from water, but there were significant, though ill-defined, differences across the five sampling periods, with generally more activity in the warm–wet months. Small animal-created mounds and pits are important sources of soil and sinks for litter within grazing gradients, and may represent the only sites where plants can establish given a relaxation in grazing pressure.

Density of Cape porcupines in a semi-arid environment and their impact on soil turnover and related ecosystem processes

Cape porcupines ( Hystrix africaeaustralis) appear to have an important influence on landscapes through both biotic (foraging) and abiotic (soil turnover) impacts. Porcupine abundance and abiotic effects across the landscape were investigated in Nieuwoudtville, South Africa, a semi-arid area characterized by an extraordinary diversity and abundance of geophytes. Porcupine numbers were measured in 50 burrows, and the number of burrow entrances was a good predictor of the number of porcupines it contained ( r 2=0.76). Occupied burrow density was estimated to be 2.6 burrows km −2 and porcupine density on the study farm was estimated to be 8 animals km −2, which is high relative to other Hystrix populations in semi-arid environments. This can probably be attributed to the high abundance of geophytes in the region. Annually, porcupines disturb 0.34% of the soil surface and 1.6 m 3 ha −1 yr −1 of soil is displaced across the landscape, through the excavation of 160–3463 diggings per hectare. Up to 2.2 m 3 of soil is displaced per hectare per year in the natural Dolerite Plains vegetation, but very little foraging activity occurs in previously or currently ploughed lands. The extent of disturbance in Nieuwoudtville is on a par with or higher than that recorded for other porcupine populations in arid or semi-arid regions. Other studies have shown that porcupine diggings disturb plant community structure and create fertile sites for plant germination and recruitment. Porcupine diggings in Nieuwoudtville contained significantly more seedlings than adjacent areas. Thus, through widespread foraging pits, soil turnover, and numerous burrows, porcupines modulate the availability of resources to other organisms. Therefore, Cape porcupines are likely to act as ecosystem engineers in this landscape. Porcupines should be included in any conservation actions involving the unique geophyte flora of Nieuwoudtville.