Interactive disturbance effects of two disparate ecosystem engineers in North American shortgrass steppe

Abstract

Disturbances such as fire, grazing, and soil mixing by animals interact to shape vegetation in grassland ecosystems. Animal-generated disturbances are unique in that they arise from a suite of behaviors that are themselves subject to modification by external factors. The manner in which co-occurring animal taxa interact to alter vegetation is a function of their respective behaviors, which shape the characteristics (e.g., the magnitude or extent) of their disturbances. To determine whether prairie dogs (Cynomys ludovicianus) and harvester ants (Pogonomyrmex occidentalis) interactively alter vegetation structure and heterogeneity on the Colorado shortgrass steppe, we characterized the size, dispersion, and vegetation of prairie dog burrow mounds and ant nests (located on and off prairie dog colonies) and vegetation growing beyond mound and nest perimeters. Ants located on prairie dog colonies engineered significantly larger nests and disturbed nearly twice as much total soil area as their off-colony counterparts. Ant nests were overdispersed both on and off prairie dog colonies, while prairie dog mounds were randomly dispersed. Where harvester ants and prairie dogs co-occur, the overdispersed pattern of on-colony ant nests is in effect "overlaid" onto the random pattern of prairie dog mounds, resulting in a unique, aggregated pattern of soil disturbance. Ant nests on prairie dog colonies had significantly less vegetation and lower plant species diversity than did prairie dog mounds, while off-colony nests were similar to mounds. These results suggest that ant nests are more highly disturbed when located on prairie dog colonies. Beyond nests proper, ants did not appear to alter vegetation in a manner distinct from prairie dogs. As such, the interactive effects of prairie dogs and ants on vegetation arise mainly from the disturbance characteristics of mounds and nests proper.

Pacific salmon effects on stream ecosystems: a quantitative synthesis

“…For example, nesting activities of seabird colonies and alteration of plant communities by large flocks of geese could be considered engineering effects, although these organisms have typically been viewed chiefly as resource subsidies (Polis and Hurd 1995;Kitchell et al 1999). Similarly, some well-known ecosystem engineers, such as beaver and prairie dogs (Naiman et al 1988;Whicker and Detling 1988;Wright and Jones 2004;Alba-Lynn and Detling 2008), may simultaneously transport resources, such as vegetation, from one ecosystem to another. If Pacific salmon are any indicator, the observed effects of these and other ecosystem engineers and resource subsidies on their surroundings are likely to vary with environmental conditions and study design.…”

Adopting a spatially explicit perspective to study the mysterious fairy circles of Namibia

“…One of the most popular hypotheses on the origin of fairy circles is grass or seed harvesting by social insects such as termites (Moll 1994, Becker and Getzin 2000, Albrecht et al 2001, Juergens 2013 or ants (Picker et al 2012). It is believed that the clearing of vegetation results in disc-like structures, similar to those observed for harvester ants in North America (Wiernasz andCole 1995, Alba-Lynn andDetling 2008), and previous field studies have demonstrated correlation of fairy circles with ants or termites (Picker et al 2012, Juergens 2013. Abiotic gas leakage has also been proposed as causal agent.…”

Geology drives the spatial patterning and structure of termite mounds in an African savanna

“…There were clear signs of density‐dependent thinning on granite where fewer large mounds were present around other large mounds, compared with small mounds around large mounds. This indicates that as mounds grow larger, they become over‐dispersed, and also the mark correlation function partly indicates negative size correlation of the diameters up to small scales of 20 m. The over‐dispersion of large mounds at small spatial scales can be attributed to competition (Alba‐Lynn and Detling ). The high density of small mounds around large mounds cannot be interpreted as facilitation because self‐thinning was evident, but can rather be attributed to chance events leading to colony establishment by queens.…”