Ecology of Desert Systems

Walter G. Whitford, Benjamin D. Duval

• Publisher: Academic Press
• Year: 2019
• ISBN:
  9780081026557
• Edition: 2
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

Nearly one-third of the land area on our planet is classified as arid or desert. Therefore, an understanding of the dynamics of such arid ecosystems is essential to managing those systems in a way that sustains human populations. This second edition of Ecology of Desert Systems provides a clear, extensive guide to the complex interactions involved in these areas.

This book details the relationships between abiotic and biotic environments of desert ecosystems, demonstrating to readers how these interactions drive ecological processes. These include plant growth and animal reproductive success, the spatial and temporal distribution of vegetation and animals, and the influence of invasive species and anthropogenic climate change specific to arid systems. Drawing on the extensive experience of its expert authors, Ecology of Desert Systems is an essential guide to arid ecosystems for students looking for an overview of the field, researchers keen to learn how their work fits in to the overall picture, and those involved with environmental management of desert areas.

• Highlights the complexity of global desert systems in a clear, concise way
• Reviews the most current issues facing researchers in the field, including the spread of invasive species due to globalized trade, the impact of industrial mining, and climate change
• Updated and extended to include information on invasive species management, industrial mining impacts, and the current and future role of climate change in desert systems

Chapter 1

Conceptual Framework, Paradigms, and Models

Abstract

Aridity is defined by the relationship between precipitation (P) and evapotranspiration (ET):(1) Hyperarid zone (P/ET < 0.03)(2) Arid zone (P/ET > 0.03 and < 0.20)(3) Semiarid zone (P/ET > 0.2 and < 0.5)(4) Subhumid zone (P/ET > 0.5 and < 0.75)Causes of aridity include latitude (descending Hadley cells), distance from oceans, mountain ranges, and near-shore cold oceanic currents. The conceptual model for drylands focuses on soil and landscape level processes (wind and water) that modify the availability of soil water and nutrients. Soil modifies the effectiveness of precipitation, water availability to plants, nutrient availability, and the physical environment for establishment of plants.

Keywords

Ecosystem; Landscapes; Potential evapotranspiration; Precipitation; Drylands; Hadley cells; Pulse; Soil; Legacies; Models

Outline

• 1.1 Introduction
• 1.2 Pattern and Process
• 1.3 Definition of Ecosystem
• 1.4 Landscape Units
• 1.5 Defining Deserts
• 1.6 Causes of Aridity of Global Deserts
• 1.7 The Pulse-Reserve Paradigm/Model
• 1.8 The Soil Resource
• 1.9 Rooting Patterns
• 1.10 Long-Term Legacy Effects
• 1.11 Legacies in Ecological Time
• 1.12 Conceptual and Simulation Models
  • 1.12.1 Precipitation and Soil Layers
• 1.13 Ecosystem Processes
• 1.14 Problems of Scaling
• References
• Further Reading

1.1 Introduction

Nearly one-third of the land area of this planet is classified as arid or semiarid and virtually none of the drylands have escaped the impacts of humans. These lands are areas where rainfall limits productivity and/or is so unpredictable that cropping is not feasible. Of the approximately 37,000,000 km² classified as arid and semiarid lands, 25,560,000 km² are used as rangelands: lands used by pastoralists for domestic livestock production (Verstraete and Schwartz, 1991). Despite these limitations, humans have inhabited these lands for millennia, using the limited and varied productivity to support nomadic pastoralism. In the 20th century, nomadic pastoralism has largely been replaced by pastoral industries (commercial livestock production) in many areas of the world. Commercial livestock production has had very different impacts on arid lands than nomadic pastoralism. Nearly coincident with the development of commercial ranching or pastoral industry was the realization that these lands were fragile and that when degraded, recovery was slow or did not occur (Dregne, 1986). Along with the recognition that degradation of rangelands was occurring, came the realization that we lacked sufficient knowledge about how the pastoral and livestock industries could develop sustainable management strategies. During the last half of the 20th and first two decades of the 21st century, a considerable amount of literature has developed on most aspects of the ecology of deserts and of desert organisms. Despite this explosion of information, problems of sustainable use and management of arid lands remain nearly as intractable as they were more than a century ago.

Deserts are also increasingly urbanized and for many of the urban inhabitants deserts are valued as places for recreation and solitude. The need for desert landscapes to be productive for pastoralism and/or the livestock industry and to retain the biota that attracts tourism adds a level of complication for scientists and land managers who need to develop a sound basis for sustainable management. These needs require conceptual frameworks that incorporate data from species populations, community ecology, ecosystem structure and dynamics, and landscape ecology.

1.2 Pattern and Process

Ecology is generally considered at seven levels of organization: organism, population, community, ecosystem, landscape, biome, biosphere. However, our thinking about ecology and ecological relationships must not be constrained by a conventional hierarchy. It is necessary to keep temporal and spatial scale ordering separate from ecological organization constraints. Scale ordering of time and space addresses the physical side of ecological systems. Both scale-dependent physical fluxes and human intellectual constructs such as community, ecosystem, and landscape are necessary to address the full range of phenomena that contribute to the ecology of desert systems. Ecological processes do not function in a way that is limited to physical and chemical mechanisms (Allen and Hoekstra, 1990). For example, the behaviors of organisms in a landscape are distinctive and are dependent on the genetic characteristics of the species population. A landscape feature such as a riverbed may be a corridor for some species and a barrier for others and some species see the riverbed as neutral. These behaviors cannot be understood as physical and chemical mechanisms but are important for understanding ecological systems (Allen and Hoekstra, 1990).

Levels of organization above the population are conceptual constructs that facilitate communication among ecologists and help focus research. Populations consist of a single species with birth and death rates affecting the numbers of individuals within a subject population. Communities involve many species and have been identified as discernible aggregations of organisms occupying specified areas on the ground. Ecosystems involve integration of water and nutrient fluxes over wide ranges of time and space and include the influence of the abiotic environment on feedbacks between the biotic and abiotic components. Ecosystems should not be defined by an area of landscape.

Landscapes are composed of distinct bounded units that are differentiated by biotic and abiotic structure and composition (Pickett and Cadenasso, 1995). Landscapes are generally recognized by the life form and dominant species. In arid, semiarid, and subhumid environments, landscapes are heterogeneous and composed of patches and mosaics (e.g., grassland/steppe, shrubland, open woodland/savanna). A patch may be a single species but a mosaic is always two or more species in close spatial proximity. Patches may also be identified as unvegetated areas with unique features such as stony surfaces, gravel surfaces, lava boulder fields, and so on.

The biota in deserts is composed of many species that are “living on the cusp of disaster.” Many, but not all desert species, exist very close to environmental tolerance thresholds or very close to the maxima or minima of limiting factors. When extreme conditions occur, such as long time periods with little or no rainfall or temperatures well above or below thermal maxima or minima, there may be local extinctions of some species. Such episodic climatic events contribute a temporal component to structural heterogeneity.

Before we can understand ecosystem processes from the patch to landscape scale, it is necessary to examine the characteristics of the structural components and how these components serve as determinants of ecosystem and landscape properties. The characteristics of dominant plant species in a patch, mosaic, or landscape unit determine the way in which that entity interfaces with weather and how the physical environment is modified by that entity. Life form and morphological characteristics must be considered in terms of general life history parameters in order to understand the contributions of species to ecosystem and landscape properties and processes. For example, the way in which plants interface with wind, precipitation, and sunlight modifies the amounts, quality, and intensity of precipitation reaching the soil surface. These interactions establish spatial patterns of soil moisture and temperature that affect the fate of materials such as plant litter and seeds.

1.3 Definition of Ecosystem

In functional terms, an ecosystem is all of the living organisms in a place interacting with each other and with the physical environment. Those interactions include basic ecosystem processes such as energy flow and nutrient cycling, plus population attributes such as competition and predation. Animal activities have effects on vegetation structure, soil properties, and water distribution which are essential modifiers of ecosystem processes. Ecosystems are more than the sum of the component parts, are dynamic, and vary through time. Interactions among species in ecosystems produce rate-modifying feedbacks that affect the responses of component populations over time.

The ecosystem is a concept that needs no defined physical boundary. Conceptually, an ecosystem can be as simple as a tussock of grass, the soil in which it is rooted, the organisms in the soil around the roots, the physical and chemical characteristics of the soil, and the atmosphere around the aboveground parts. A grass tussock ecosystem is dynamic, grows, matures, and senesces in response to weather and soil conditions, animals feed on the foliage, seeds, and roots eliciting physiological responses of the grass species. The soil microflora and microfauna change the physical and chemical properties of the soil generating feedbacks that modify the state of the system through time. An ecosystem is not a closed system, therefore it experiences inputs of atmospheric gasses, precipitation, air-borne dust, and materials brought in by mobile consumers; outputs include gasses, materials transported by wind and water, and materials carried by consumers (Fig. 1.1).

Fig. 1.1 Examples of a simple ecosystems or patches (delineated by broken lines) with surrounding patches. Arrows indicate direction and magnitude of inputs and outputs. Interpatch exchanges occur as the result of differences in microtopography.

Ecosystems are perceived by society as sources of goods and services for human populations. However, exploitation of ecosystems for goods can compromise the nature of services provided by ecosystems. In dryland ecosystems, there are many conflicts among different segments of society concerning appropriate harvesting of goods and nonconsumptive uses that support healthy lifestyles of urban dwellers. Understanding ecosystem processes while essential for sustainable management will do little to resolve conflicts between proponents for drylands as a source of ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Acknowledgments
• Chapter 1: Conceptual Framework, Paradigms, and Models
• Chapter 2: Landforms, Geomorphology, and Vegetation
• Chapter 3: Characterization of Desert Climates
• Chapter 4: Wind and Water Processes
• Chapter 5: Patch—Mosiac Dynamics
• Chapter 6: Adaptations
• Chapter 7: Primary Production
• Chapter 8: Consumers and Their Effects
• Chapter 9: Decomposition and Nutrient Cycling
• Chapter 10: Nonnative, Exotic, or Alien Species
• Chapter 11: Anthropogenic Climate Change in Deserts
• Chapter 12: Desertification
• Chapter 13: Rehabilitation of Degraded Landscapes
• Chapter 14: Monitoring and Assessment
• Chapter 15: The Human Footprint (Roads; Urbanization; Energy Developments)
• Glossary
• Index