Effects of grazers

Lawn-mowers are relatively unselective predators capable of maintaining a close-cropped sward of vegetation. Darwin (1859) was the first to notice that the mowing of a lawn could maintain a higher richness of species than occurred in its absence. He wrote that:

If turf which has long been mown, and the case would be the same with turf closely browsed by quadrupeds, be let to grow, the most vigorous plants gradually kill the less vigorous, though fully grown plants; thus out of 20 species growing on a little plot of mown turf (3 feet by 4 feet) nine species perished from the other species being allowed to grow up freely.

Grazing animals are usually more choosy than lawn-mowers, and this is clearly demonstrated by the occurrence in the neighborhood of rabbit (Oryctolagus cuniculus) burrows of plants which for chemical or physical reasons are unacceptable as food to the rabbits (including the poisonous deadly nightshade Atropa belladonna and the stinging nettle Urtica dioica). Nevertheless, many grazers seem to have a similar general effect to lawn-mowers. Thus, in one experiment, grazing by oxen (Bos taurus) and zebu cows (Bos taurus indicus) in natural pasture in the Ethiopian highlands was manipulated to provide a no-grazing control and four grazing intensity treatments (several replicates of each) in two sites. Figure 19.14 shows how the mean number of plant species varied in the sites in October, the period when plant productivity was at its highest (Mwendera et al., 1997). Significantly more species occurred at intermediate levels of grazing than where there was no grazing or heavier grazing (P < 0.05). In the ungrazed plots, several highly competitive plant species, including the grass Bothriochloa insculpta, accounted for 75-90% of ground cover. At intermediate levels of grazing, however, the cattle apparently kept the aggressive, competitively dominant grasses in check and allowed a greater number of plant species to persist. But at very high intensities of grazing, species numbers were reduced as the cattle were forced to turn from heavily grazed, preferred plant species to less preferred species, driving some to extinction. Where grazing pressure was particularly intense, grazing-tolerant species such as Cynodon dactylon became dominant.

The composition of plant communities in different grazing regimes clearly depends on a variety of species traits. First, competitively superior species can be expected to dominate in the absence of grazing. A particularly striking example

Figure 19.14 Mean species richness of pasture vegetation in plots subjected to different levels of cattle grazing in two sites in the Ethiopian highlands in October. 0, no grazing; 1, light grazing; 2, moderate grazing; 3, heavy grazing; 4, very heavy grazing (estimated according to cattle stocking rates). (After Mwendera et al., 1997.)

Index of grazing intensity

Figure 19.14 Mean species richness of pasture vegetation in plots subjected to different levels of cattle grazing in two sites in the Ethiopian highlands in October. 0, no grazing; 1, light grazing; 2, moderate grazing; 3, heavy grazing; 4, very heavy grazing (estimated according to cattle stocking rates). (After Mwendera et al., 1997.)

has been provided by Paine (2002), who reported that the exclusion of macroherbivores (urchins, chitons and limpets) from a North American rocky intertidal zone caused the multi-species kelp community to collapse to a virtual monoculture of Alaria marginata; this was 10 times more productive than its grazed counterpart (86.0 versus 8.6 kg wet mass m-2 year-1). Second, we have seen that plant species with physical or chemical characteristics that deter grazers are likely to be more strongly represented in grazed locations. Bullock et al. (2001) have also noted that while certain dominant grasses decreased in importance in response to sheep grazing, most dicotyledonous species increased in abundance, at least at certain times of year. Moreover, summer grazing produced an increased representation of plant species best able to colonize gaps.

When predation promotes the coexistence of species amongst which there would otherwise be competitive exclusion (because the densities of some or all of the species are reduced to levels at which competition is relatively unimportant), this is known generally as 'exploiter-mediated coexistence'. Many examples of this phenomenon have been reported, such as that in Figure 19.14, but grazer-mediated coexistence is far from universal. Proulx and Mazumder (1998) performed a meta-analysis of 44 reports of the effects of grazing on plant species richness from lake, stream, marine, grassland and forest ecosystems. Their conclusion was that the outcome was strongly related to whether the studies had been performed in nutrient-rich or nutrient-poor situations. All grazing can increase plant species richness (exploiter-mediated coexistence)...

... but not always exploiter-mediated coexistence is more likely in nutrient-rich situations

Exploiter Mediated

Figure 19.15 (a-c) Species richness under contrasting grazing pressure (low or high) in nonenriched or nutrient-poor ecosystems. The different lines show the results of different aquatic or terrestrial studies and are presented in three panels simply for clarity. (d-g) Species richness under contrasting grazing pressure (low, intermediate or high) in various enriched or nutrient-rich ecosystems. (After Proulx & Mazumder, 1998.)

Figure 19.15 (a-c) Species richness under contrasting grazing pressure (low or high) in nonenriched or nutrient-poor ecosystems. The different lines show the results of different aquatic or terrestrial studies and are presented in three panels simply for clarity. (d-g) Species richness under contrasting grazing pressure (low, intermediate or high) in various enriched or nutrient-rich ecosystems. (After Proulx & Mazumder, 1998.)

19 studies from nonenriched or nutrient-poor ecosystems exhibited significantly lower species richness under high grazing than under low grazing (Figure 19.15a-c). In contrast, 14 of 25 comparisons from enriched or nutrient-rich ecosystems showed significantly higher species richness under high grazing (indicating grazer-mediated coexistence) (Figure 19.15d-g). Nine of the remaining 11 nutrient-rich studies showed no difference with grazing regime whilst two showed a decline in species richness. The lack of grazer-mediated coexistence in unproductive situations may reflect the poor growth potential of the less competitive species that, in nutrient-rich circumstances, would be released from competitive domination as a result of grazing.

Osem et al. (2002) focused on the interactive effects of grazing and productivity in a study of annual herbaceous plant communities in Mediterranean semiarid rangeland in Israel. They recorded the response of the community to protection from sheep grazing in four neighboring topographic situations -south-facing slopes, north-facing slopes, hilltops and wadi (dry stream) shoulders (Figure 19.16). Annual above-ground primary productivity was measured each year for 4 years at the peak season in the four fenced subplots per site and was found to be typical of semiarid ecosystems (10-200 g dry matter m-2) except on wadi shoulders (up to 700 g dry matter m-2). The measured values were taken to represent 'potential' productivity in the adjacent grazed subplots. Grazing only increased plant species richness in the most productive site (wadi) (Figure 19.16d). In the other, less productive sites, species richness was unaffected or declined with grazing. These results are consistent with those reported by Proulx and Mazumder (1998) and support the longstanding proposal of Huston (1979) that grazing should change diversity in opposite ways in resource-poor and resource-rich ecosystems.

Figure 19.17a and b plots species richness in relation to potential productivity individually for all subplots and all years (because precipitation and productivity varied both spatially and temporally) for both grazed and ungrazed locations. Under grazing, species richness was positively related to productivity over the whole range measured. In the absence of grazers, however, a positive relationship only occurred in low-productivity sites. Osem et al. (2002) hypothesize (Figure 19.17c) that at low productivity, plant growth and diversity are limited by the soil resources of water and nutrients, while at higher productivity (with its associated larger community responses to grazing depend on productivity...

Figure 19.16 Species richness (per 20 X 20 cm quadrats) in four topographic sites in Israel in April: (a) south-facing slopes, (b) hilltops, (c) north-facing slopes, and (d) wadis. •, ungrazed subplots; o, grazed subplots. (After Osem et al., 2002.)

Osem Rooms

Figure 19.16 Species richness (per 20 X 20 cm quadrats) in four topographic sites in Israel in April: (a) south-facing slopes, (b) hilltops, (c) north-facing slopes, and (d) wadis. •, ungrazed subplots; o, grazed subplots. (After Osem et al., 2002.)

biomass) competition is predominantly for the canopy resource of light. Thus, in the low productivity range, richness was either unaffected or reduced by grazing, probably because of plant removal and trampling. In the high productivity wadi sites, however, species richness continued to increase with grazing, most likely because of a reduction in light competition through removal of the palatable larger species.

Taken overall, then, the way that plant species richness responds to grazing depends partly on grazing intensity, but also on the evolutionary history of the plant community and thus the particular plant species traits that are represented, as well as the primary productivity of the ecosystem in question. An increase in species richness in response to grazing can be expected if grazers feed preferentially on competitively dominant species, a prediction that has received support in situations as diverse as cattle grazing in Ethiopia (see above) and periwinkles (Littorina littorea) feeding on algae in rocky tide pools (Lubchenco, 1978). Conversely, a reduction in species richness can be expected if the preferred food plants are competitively inferior, as was the case for periwinkles feeding on algae on emergent substrata in Lubchenco's study.

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