Studies of ecosystem processes on the Jornada Experimental Range in southern New Mexico suggest that longterm grazing of semiarid grasslands leads to an increase in the spatial and temporal heterogeneity of water, nitrogen, and other soil resources. Heterogeneity of soil resources promotes invasion by desert shrubs, which leads to a further localization of soil resources under shrub canopies. In the barren area between shrubs, soil fertility is lost by erosion and gaseous emissions. This positive feedback leads to the desertification of formerly productive land in southern New Mexico and in other regions, such as the Sahel. Future desertification is likely to be exacerbated by global climate warming and to cause significant changes in global biogeochemical cycles.

https://science.sciencemag.org/content/sci/247/4946/1043.full.pdf

Biological Feedbacks in Global Desertification

Classification of streams and stream habitats is useful for research involving establishment of monitoring stations, determination of local impacts of land-use practices, generalization from site-specific data, and assessment of basin-wide, cumulative impacts of human activities on streams and their biota. This article presents a frame-work for a hierarchical classification system, entailing an organized view of spatial and temporal variation among and within stream systems. Stream habitat systems, defined and classified on several spatiotemporal scales, are associated with watershed geomorphic features and events. Variables selected for classification define relative long-term capacities of systems, not simply short-term states. Streams and their watershed environments are classified within the context of a regional biogeoclimatic landscape classification. The framework is a perspective that should allow more systematic interpretation and description of watershed-stream relationships.

https://faculty.washington.edu/cet6/pub/Temp/CFR521e/Frissell_etal_1986.pdf

A hierarchical framework for stream habitat classification: Viewing streams in a watershed context

Interactions among organisms take place within a complex milieu of abiotic and biotic processes, but we generally study them as solitary phenomena. Complex combinations of negative and positive interactions have been identified in a number of plant communities. The importance of these two processes in structuring plant communities can best be understood by comparing them along gradients of abiotic stress, consumer pressure, and among different life stages, sizes, and densities of the interacting species. Here, we discuss the roles of life stage, physiology, indirect interactions, and the physical environment on the balance of competition and facilitation in plant communities.

/pdf/competition-and-facilitation-a-synthetic-approach-to-51azsyo86c.pdf

Competition and facilitation: a synthetic approach to interactions in plant communities

Experimental evidence for positive interactions, or facilitation, among plants has increased markedly during the last 10 years. Experiments documenting facilitation have been conducted in many diverse ecological systems, which suggests that positive interactions may be fundamental processes in plant communities. Here, I review the evidence for facilitation, the mechanisms by which facilitation operates, and the effects facilitation has on community structure. Facilitative mechanisms may act simultaneously with resource competition or allelopathy, and the overall effect of one species on another may be the product of multiple, complex interactions. Positive interactions may also determine community spatial patterns, permit coexistence, enhance diversity and productivity, and drive community dynamics. Once viewed as anecdotal and idiosyncratic, facilitation is now contributing to a more complete understanding of community structure and dynamics.

Positive interactions among plants

We define disturbance in stream ecosystems to be: any relatively discrete event in time that is characterized by a frequency, intensity, and severity outside a predictable range, and that disrupts ecosystem, community, or population structure and changes resources or the physical environment. Of the three major hypotheses relating disturbance to lotic community structure, the dynamic equilibrium hypothesis appears to be generally applicable, although specific studies support the intermediate disturbance hypothesis and the equilibrium model. Differences in disturbance frequency between lentic and lotic systems may explain why biotic interactions are more apparent in lakes than in streams. Responses to both natural and anthropogenic disturbances vary regionally, as illustrated by examples from the mid-continent, Pacific northwest, and southeastern United States. Based on a generalized framework of climatic-biogeochemical characteristics, two features are considered to be most significant in choosing streams...

/pdf/the-role-of-disturbance-in-stream-ecology-4xe79wql0y.pdf

The role of disturbance in stream ecology.

Interspecific competition for space limits the distribution and abundance of many ben- thic insects on stones in a western Montana stream. The sessile caddisfly larva Leucotrichia pictipes is territorial and aggressively eliminates conspecifics and other species from its foraging territories. Territoriality produces an intraspecific pattern of regular spacing and negative spatial associations between Leucotrichia and several other sessile insects: Parargyractis confusalis, Rheotanytarsus sp. and Eukiefferiella sp. Experimental removals of Leucotrichia resulted in higher densities of other sessile species and several mobile insects, including Baetis, Glossosoma, and Simulium. These insects show broad microhabitat overlap with Leucotrichia; competition with Leucotrichia limits their dis- tributions and abundances within otherwise suitable microhabitats. Leucotrichia is the only species that monopolizes large areas of space. Physical disturbances interrupt the formation of competitive monopolies by Leucotrichia. During seasonal reduced flows, Leucotrichia is eliminated from stones and boulders in shallow water. Greater densities of a short-lived sessile species with multiple annual generations occupy this ephemeral spatial resource. On permanently submerged stones, Leucotrichia competitively affects a large part of the benthic fauna. On small stones that overturn with higher frequencies during flooding, Leucotrichia densities are reduced and species' abundances are highly equitable. Large, more stable substrates with greater densities of Leucotrichia are characterized by lower species evenness.

Competition for Space, Disturbance, and the Structure of a Benthic Stream Community

Three alluvial piedmonts (bajadas or alluvial fans) studied in the Sonoran Desert near Tucson, Arizona are complex mosaics of distinct geological landforms. These landscape mosaics have been produced through the temporally episodic and spatially discontinuous aggradation of alluvial surfaces and the destruction of other parts of the landscape by erosion. These geomorphic processes produce abrupt juxtapositions of soils of different ages and degrees of profile development. Vegetation patterns correspond closely to this geomorphic mosaic. Larrea tridentata predominates on most Holocene—aged surfaces and all parts of highly dissected, early Pleistocene surfaces. This shrub is generally excluded from Pleistocene—aged surfaces containing soils with strongly developed argillic (clay—rich) horizons. The highest species diversity is encountered on some of the most unstable, erosional slopes of early Pleistocene surfaces. Comparisons among the three study areas indicated the importance of igneous lithology (highly weatherable intrusives vs. weathering—resistant extrusives) in controlling geomorphic processes, and ultimately, vegetation patterns. The areal extent of late Holocene alluvial aggradation and patterns of erosion and dissection of older Pleistocene deposits are strongly influenced by the weatherability of different lithologies and provide a strong control over the spatial scale of ecological patterns. Processes limiting the distributions and abundances of plants are directly linked to landscape characteristics in many ways. Landform age and stability affect the structure of populations of long—lived Larrea tridentata. Individuals of this shrub species can exhibit clone—like growth and increase considerably in size (diameter) over time spans of many centuries to millennia. The growth and persistence of these long—lived clones in some parts of the landscape apparently contribute to the exclusion of other species. However, development of large clones and dominance by L. tridentata are impossible or greatly inhibited in several landscape settings including: (1) extremely young alluvial deposits that have existed for too short a time for large clones to have developed, (2) hillslopes subject to considerable erosional disturbance, and (3) extremely thin soils underlain by impenetrable petrocalcic horizons (caliche), which magnify drought conditions and apparently contribute to episodic mortality in L. tridentata. Soil horizon development as determined by landform age controls the vertical movement and distribution of soil water, in turn affecting the distribution of various plant life forms. Clay—rich (argillic) horizons that have required tens to hundreds of thousands of years to form greatly limit the downward infiltration, vertical distribution, and the temporal availability of soil water. Despite surficial stability for extremely long periods of times, sites with strongly developed argillic horizons lack L. tridentata and are instead occupied by drought—deciduous or succulent plants that are capable of highly seasonal activity in soils that exhibit high seasonal variability in water availability. Syntheses involving the study of various ecological processes (e.g., plant physiological, demographic, and interspecific interactions) with a larger landscape perspective provide a rich framework for further studies of arid land systems.

Landscape Evolution, Soil Formation, and Ecological Patterns and Processes in Sonoran Desert Bajadas

Many simple plant communities in extremely arid regions of the Sonoran Desert consist essentially of two shrub species, Ambrosia dumcsa and Larrea tridentata These species exhibit completely different recruitment patterns and rates of population turnover In these communities, the majority of space is devoid of perennial plant cover, and Ambrosia readily colonize this open space Recruitment of Larrea occurs less frequently and is largely limited to areas beneath the canopies of large, older Ambrosia individuals The dynamics of these systems can be modeled as Markov chains Probabilities of transitions among states (open space, space covered by Ambrosia canopies, and space covered by Larrea canopies) are computed from data on spatial and temporal patterns of recruitment and mortality Although the stationary distributions of states predicted by a homogeneous Markov chain are similar to those observed in undisturbed vegetation at two sites, a nonhomogeneous chain more accurately describes vegetational ch

Markovian dynamics of simple and complex desert plant communities

In the Sonoran Desert, the sahuaro cactus (Carnegiea gigantea) is commonly associated with canopies of trees and shrubs: so-called nurse plants. Although mechanisms by which nurse plants facilitate sahuaro establishment have been studied, possible competitive interactions between sahuaro cacti and nurse plants have not been conclusively demonstrated. In this paper I show that the close proximity of sahuaros leads to a relative increase in stem die-back as well as greater mortality in a common nurse tree, the foothill paloverde (Cercidium microphyllum). This interaction appears to accelerate the local loss of individual nurse trees, resulting in a predictable pattern of species replacement.

Joseph R. McAuliffe

Papers

Competition for Space, Disturbance, and the Structure of a Benthic Stream Community

Landscape Evolution, Soil Formation, and Ecological Patterns and Processes in Sonoran Desert Bajadas

Markovian dynamics of simple and complex desert plant communities

Sahuaro-nurse tree associations in the Sonoran Desert: competitive effects of sahuaros.

Herbivore‐Limited Establishment of a Sonoran Desert Tree, Cerciduium Microphyllum