Resource Consumer Relationships Trophic Interactions

In contrast to some ecosystems, food chains in deserts can be characterized by the importance of the link between

Figure 13 Extreme, 30-fold differences of plant growth on a rocky desert slope during (a) a dry (precipitation 40 mm, NPP 0.031 ha-1 yr-1) and (b) an extremely wet year (193 mm, 0.871 ha-1 yr-1). Northern Dead Sea, Palestine, March 1991 and March 1992. Photographs by C. Holzapfel.

producers and consumers via decomposition. Less than in most mesic environments, plant material is typically not directly consumed alive; some estimation puts the amount of energy that moves via decomposition into the food web as above 90% of total primary production. Since food resources are unpredictable, many animals can opportunistically switch from one mode of consumption to another (e.g., many arthropods are either herbivores or decomposers).

Decomposition

Microbial decomposition is often limited by low water availability, resulting in the accumulation of dry plant material and seeds. For that reason, animal detrivores are more important in deserts than in more mesic environments. Examples are darkling beetles, termites, and isopods. Termites are abundant in most of the warmer deserts and are often the dominant decomposers of dead plant material (above- and belowground) that play an extraordinarily important role in nutrient cycling. Since most termites live belowground, they are also important in the formation of soils. A similar phenomenon is displayed by scavenging animals, which are comparatively abundant among the desert fauna. Examples are large mammals (hyenas, coyotes, and jackals) and many birds (Old World and New World vultures, ravens, etc.). Like smaller detrivores in the desert fauna, many of these scavengers can switch to a predatory diet when needed.

Herbivory

Similar to other ecosystems, deserts host a large variety of herbivorous animals that potentially utilize every part of the plants. Some of the drought adaptations of plants, discussed before, also function to deter herbivores. Tough outer layers, spines, and elevated leaf chemicals, all typical for desert plants, can therefore also be understood as mechanisms to protect low and therefore costly primary production. Some plants appear to employ growth forms that make them less conspicuous for herbivores. Remarkable examples are the living stones (Lithops species, Aizoaceae) of South Africa that blend with the surrounding rocky desert pavement.

Predation

Abundant detrivorous arthropods are the most important prey source in the desert and provide the base for a relatively large assembly of smaller (e.g., spiders, scorpions) and larger predatory animals (e.g., reptiles, birds). The abundance of long-term stored seeds and fruits in desert systems supports an assembly of a diverse guild of granivores (seed predators). These granivores are recruited from taxonomically much differentiated groups (e.g., ants, birds, rodents), all of them potentially competing for similar food sources. Carnivorous predators can be abundant as well. These predators are mammals, birds, and reptiles (mostly snakes). Because of the relative openness of the desert terrain, prey organisms rely on a number of predator avoidance strategies. Examples are general crypsis (camouflage), 'freezing behaviors', and nocturnal activity pattern. Active deterrents are spines (desert hedgehogs, horned lizards), hard shells (desert tortoise), and poisons that can be employed in active predation as well. Strong predator pressure combined with the need for efficient predation in a desert environment poor in prey might be the reason that some of the

Figure 14 Heavy infestation of a desert shrub (Ambrosia dumosa) by an epiphytic parasite (Cuscuta sp.). Parasitic plants can be common in deserts and their effects can add to the abiotic stresses of aridity. Panamint Valley, California, USA, April 1995. Photograph by C. Holzapfel.

most poisonous animals we know (e.g., snakes, scorpions, Gila monster) are true desert animals.

Parasitism

Parasitic interactions are often very conspicuous in desert environments. Many desert shrubs show abundant signs of an attack by gall-forming insects. For instance, Larrea tridentata, the dominant shrub in all the hot deserts of North America, is attacked by 16 specialized species of gall-forming insects. Parasitic plants, stem and root parasites alike, are common in deserts worldwide. Though detailed studies are lacking, these parasites seem to have the potential of reducing host plant production and performance (Figure 14).

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