Nontrophic Species Interactions

Competition among and within species has been recognized as an important force that shaped the communities in all mesic environments and the question whether this is also true for deserts is a natural one, however one that has not been answered univocally. Some researchers conclude that biomass production and densities in desert are typically below a threshold that would necessitate competition for resources. Observing the same density pattern, other researchers state that because such low densities indicate strong resource limitation in desert, strong competition should ensue.

Based on studies of spatial plant community structure, it appears that current competition in deserts is rare; most studies show clumped or neutral patterns - itself a sign of the lack of competition - while only few studies show a clear regular pattern (a sign of past competition). Experimental removal of individual plants in the Mojave Desert, on the other hand, demonstrated interspecific competition among dominant desert shrubs. Spatial studies that assess the size distributions in dependence of distance between desert shrubs typically detect signs of negative association; larger shrubs tend to be spaced farther from each other than smaller ones. Removal experiments with granivorous rodents commonly result in density increase of the remaining species, thereby indicating current competition. The fact that character displacement, the evolution of divergent body features in coexisting species, has been demonstrated for desert rodents is another sign that competition has been of importance at least at one time.

Ecological theory predicts that negative interactions (such as resource competition) decrease in importance with increasing abiotic stress, and positive interactions (such as facilitation) increase. Following this, it should be possible to observe along a mesic to arid gradient a waning of competitive interaction and an increase of facilitative interactions. Indeed, a clear indication of this has been observed in a survey of positive effects among plants that resulted in a proportionally large number of cases from arid regions. In many deserts of the world, one can easily observe the positive association of either young perennials with adult perennials or herbaceous plants with larger perennial plants. Experimentally, it had been shown that the perennials had net positive effect on the smaller sheltered plants. Examples for these so-called 'nurse plant effects' are the associations of young succulent plants (often cacti), trees, and shrubs and the prevalent, close association of annual plants with desert shrubs. Typically the larger nurse plant provides canopy shading and increased soil fertility (see above discussion on islands of fertility), and sometimes protection from herbivorous animals to the sheltered plants. In accordance with this prediction, shrub-annual associations tend to be strongly positive in arid sites and less so (or even negative) in less arid sites (Figure 15). As nothing ever in nature is one-sided, these unidirectional facilitative effects are countered by negative effects as the nursed plants can have negative, competitive effects on their benefactor. Competition for water has been shown between annuals and sheltering shrubs and such negative effects are typical once sheltered young succulents outgrow the nurse plant.

Tradeoffs in competitive/facilitative interactions are also found between taxonomically very distant groups. One example is the complex nature of interaction between microbial crusts and vascular plants. For one, these crusts can have very contrasting effects on seed placement. Cold deserts tend to have very rough crust surfaces that facilitate seed deposition and establishment, while the smooth crusts typical to hot deserts decrease such seed entrapment. Because of these differences, no general effect of desert crusts on the performance of vascular plants has been recognized. Nitrogen fixation

Figure 15 Clear associations of annual plants with shrubs (here Ambrosia dumosa) are common in deserts. Annual plants benefit from nutrient enrichment and shade provided by the shrub canopy and since they usually only provide little benefit to the shrub (e.g., thatch-induced increase in water infiltration and lower soil surface evaporation), they can compete with the shrub for resources. Owens Valley, California, USA, March 1997. Photograph by C. Holzapfel.

Figure 15 Clear associations of annual plants with shrubs (here Ambrosia dumosa) are common in deserts. Annual plants benefit from nutrient enrichment and shade provided by the shrub canopy and since they usually only provide little benefit to the shrub (e.g., thatch-induced increase in water infiltration and lower soil surface evaporation), they can compete with the shrub for resources. Owens Valley, California, USA, March 1997. Photograph by C. Holzapfel.

by cyanobacteria increases nitrogen availability, thereby favoring plant growth; however, the creation of crusts can result in runoff and water redistribution that in turn locally reduces plant performance.

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