Disturbance effects on plant community diversity ... - Springer Link

Vegetatio 93: 143-155, 1991. © 1991 Kluwer Academic Publishers. Printed in Belgium.

143

Disturbance effects on plant community diversity: spatial scales and dominance hierarchies E.J. Chaneton & J.M. Facelli 1 Departamento de Ecologia, Facultad de Agronomia, Universidad de Buenos Aires, Av. San Martin 4453 (1417) Buenos Aires, Argentina. 1present address: Department of Biological Sciences, R utgers University, P.O. Box I059, Piscataway, NJ 08854, USA; All correspondence should be addressed to E.J. Chaneton, Departamento de Ecologia, Facultad de Agronomia, Universidad de Buenos Aires, Av. San Martin 4453, (1417) Buenos Aires, Argentina Accepted 5.2.1991

Keywords. Coexistence, Community organization, Flooding, Grassland, Grazing, Perception levels Abstract

It is proposed that evaluations of disturbance effects upon community diversity will be influenced by two factors currently overlooked in models addressing disturbance-diversity relationships: (1)the spatial scale of inquiry, and (2)the level of the species abundance (dominance) hierarchy at which the search for diversity is done. We analyzed how two disturbance types - cattle grazing and large flooding - affected community diversity at two spatial scales (stand and patch) and three levels of species dominance in a grassland of the Flooding Pampa, Argentina. The effect of disturbance interaction was also examined. Species diversity at the stand scale was reduced by either grazing or flooding. Both disturbances decreased community spatial heterogeneity. At the patch scale, diversity declined with flooding but was enhanced by grazing. Flooding increased diversity under grazing conditions. However, stand diversity was highest in the undisturbed grassland; pattern diversity was the salient feature in this condition. The combination of disturbances yielded the highest patch-scale diversity; grazing increased richness whilst flooding enhanced evenness. Comparisons among grassland conditions appeared scale-dependent. Moreover, the extent of disturbance effects varied with the level of dominance hierarchy considered. We point out the relevance of site history and initial conditions, encompassing the possibility of disturbances interaction, to the patterns produced by disturbance events. Effects perceived at different spatial scales, or in species positioned at separate dominance levels, may parallel meaningful changes in the relative importance of factors controlling species coexistence and community organization. Nomenclature: Cabrera, A.L. & Zardini, E.M. 1978. Manual de la Flora de los alrededores de Buenos Aires. Acme, Buenos Aires. Introduction

Species diversity constitutes a core property of biological communities since it is the manifestation of the variety of processes underlying species

coexistence (see Shmida & Ellner 1984). It relates to the patterns of resource division in space and time within ecological assemblages (Whittaker 1975; Pielou 1975; Tilman 1982). As a consequence of nonuniformity in the spa-

144 tial distribution of species abundances, community diversity changes over a nested hierarchy of spatial scales. The diversity observed at the whole-community level results from (1)local heterogeneity, the richness and evenness at the level of smaller-scale units - patches - (i.e. point diversity), and (2) spatial heterogeneity, the extent to which the component patches differ in species composition and abundance (i.e. pattern diversity) (Pielou 1966; Whittaker 1977). The relative importance of factors determinant of diversity also varies with spatial scale (Shmida & Wilson 1985; Auerbach & Shmida 1987; Ricklefs 1987). Species coexistence at the patch scale is controlled in part by resource use and habitat partitioning, and by competitive interactions (Whittaker 1972, 1975, 1977; Pielou 1975; Tilman 1982). In plant communities, processes associated with the regeneration cycle (Grubb 1986), and migration of propagules among patches (Shmida & Ellner 1984), may also contribute to the maintenance of point diversity. At larger spatial scales habitat heterogeneity becomes more important for coexistence (Shmida & Wilson 1985; Auerbach & Shmida 1987), affecting pattern diversity (Whittaker 1977). The existence of several determinants for the diversity observed at different scales in the same community points to a general mechanism whereby species may coexist (cf. Ricklefs 1987). The perception of species diversity depends on the 'depth' of the search into the rarities of the community (Hill 1973). Diversity may be investigated 'superficially' by using an index that weights mostly the common (dominant) species, or 'deeply', by indices that assign more weight to rarer species. The number of species occurring at different levels of the hierarchy of relative abundance (hereafter dominance hierarchy) concerns the processes shaping community structure (Kolasa 1989; Collins & Glenn 1990). Viewed at a particular point in time, that hierarchy would reflect the short-term relative success of species in habitat and resource utilization (Whittaker 1975; Tilman 1982; Kolasa 1989). Recent work on herbaceous communities showed that abundance patterns may correspond with competitive

hierarchies (Fowler 1982; Mitchley & Grubb 1986; Miller & Werner 1987). The occurrence of physical and biological disturbances introduces a major source of variation to community structure (Whittaker & Levin 1977; Sousa 1984). Disturbances influence the abundance and distribution of populations because they modify the physical environment, and the spatial and temporal distribution of resources availability (Tilman 1982; Bazzaz 1983; White & Pickett 1985). Interactions among species are affected both through the modification of habitat conditions and by direct impact upon individuals (Huston 1979; Tilman 1982). Hence, non-equilibrium conditions generated by disturbances interact with other mechanisms of coexistence (Whittaker & Levin 1977; Huston 1979; Pickett 1980; Sousa 1984; Denslow 1985), affecting diversity at all spatial scales (White & Pickett 1985; Auerbach & Shmida 1987; Pickett etal. 1989). In addition, disturbance events will likely have varied consequences for species positioned at distinct levels of the dominance hierarchy (Kolasa 1989). Models addressing the relationship between disturbance regime and diversity have focused on the effects of single disturbing agents (Connell 1978; Grime 1979; Huston 1979; Tilman 1982; Miller 1982; Malanson 1984) and those produced by different disturbances acting in concert (Collins & Barber 1985). The evaluation of disturbance effects, however, varies with the level of resolution of the study (Allen & Starr 1982; White & Pickett 1985; Pickett etal. 1989; O.E. Sala unpubl.). In practice, definitions of spatial and dominance perception levels are dictated by the sampling scheme (Allen et al. 1984), and the index of diversity being employed (Whittaker 1972; Hill 1973), respectively. Although the qualifications we made above on the perception of diversity had been considered in previous studies (e.g. McNaughton 1983; Persson 1984; Maurer 1985), this matter has not been specifically addressed in analyses of disturbance impact on the diversity of plant communities. We analyzed the effect of two disturbance agents - grazing and flooding - upon community

145 diversity at different spatial scales and different levels of the species hierarchy, in a subhumid grassland of the Flooding Pampa, Argentina. We have dealt with a real situation encompassing a source of disturbance hardly amenable to manipulation, i.e. a large flood, which affected a grassland subjected to cattle grazing. Our objectives were: (1)to examine the effect of disturbance on plant community diversity at different levels of a simple hierarchy of spatial scales and species dominance; (2)to evaluate how could flooding and grazing interact to determine patterns of community organization. Finally, we discuss whether our results fit into predictions derived from previous hypotheses on disturbance-diversity relationships. Detailed descriptions of floristic changes induced by grazing and flooding in these grasslands have been reported elsewhere (Facelli 1988; Sala 1988; Chaneton etal. 1988; Facelli etal. 1989).

the year (Lavado & Taboada 1987). These factors interact to promote the occurrence of light floods in winter and spring, when sustained rainfalls (900 mm on average) exceed the drainage capacity of the system. Light floods occur almost annually and cover the ground of lower areas with ca. 10 cm of standing water during a few weeks. Large floods lasting several weeks are spatially widespread and recur over longer time intervals (Paruelo & Sala 1990). Seasonal, light events cannot be considered as disturbances for the plant community because they do not cause observable changes in structural properties of interest (Pickett et al. 1989; O.E. Sala unpubl.), that is, the relative abundance of species (Chaneton unpubl, data). Conversely, extreme infrequent floods may act as disturbance at the community level (Chaneton etal. 1988; Insausti & Soriano 1988).

Vegetation sampling Methods Study area and disturbance agents

The study was conducted in a floodplain grassland located at the central zone of the Flooding Pampa (36°30 , S, 58°30 ' W), Province of Buenos Aires, eastern Argentina. In this region grazing by large ungulates constitutes a manintroduced agent of disturbance (see Milchunas et al. 1988), which altered the composition and structure of native vegetation (Sala et aI. 1986; Facelli 1988). Cattle grazing enhanced species diversity within communities but reduced floristic differences between communities (Sala etaL 1986). The natural disturbance regime of the region includes the yearly occurrence of floods of varying size and magnitude. The flat topography, the reduce slope, and the lack of an integrated drainage system are prominent features of the landscape. Soils (Typic Natraquolls) have low hydraulic conductivity, and the water table remains near the surface during several months of

The study was performed in the 'lowland community' of Mentha pulegium, Leontodon taraxacoides and Paspalidium paludivagum (Le6n 1975). We monitored two adjacent plots: a 2 h a enclosure where large herbivores had been excluded since 1972, and a portion of the surrounding grassland subjected to continuous grazing (year-round stocked at 0.5 animal/ha). Before the cattle exclusion treatment was initiated, the plots were not distinguishable with regard to vegetation structure and composition. They were mapped as belonging to the same stand in an extensive phytosociological survey carried out in 1971 (Le6n 1975; also Burkart et al. 1990). In spring 1985 the entire area was affected by a flood of unusual intensity and duration; the waterlogging period lasted about three months and standing water reached 40 cm-depth. Both study plots occupy the same topographic position and were equally inundated. The effect of flooding was assessed by comparing pre-flood (early February 1985; midsummer in the southern hemisphere) to post-flood (late January 1986) samplings, made inside and outside the enclosure. Prior to 1985 the

146 study area had been subjected to a large flood in 1980. Because plots were similar before cattle was excluded, and since grassland condition preceding the large flood was known, we assumed that differences between locations (grazed vs. ungrazed) and dates (we-flood vs. post-flood) could appropriately be assigned to the involved factors (Green 1979; Hawkins 1986). Our data thus represented four grassland conditions suitable for studying the single and combined effects of two disturbance types upon community structure (Chaneton et al. 1988; Facelli et al. 1989). We used the line interception method to measure basal cover per species (Greig-Smith 1983); 4 permanent, 5 m-long transects were laid out at random on each plot. Maximum and minimum distance between transects located in different plots were 200 and 50 m, respectively.

Diversity analysis

Community diversity was investigated at three levels of the dominance hierarchy, considering two spatial scales: stand and patch. The setting of limits to the perception levels of diversity is recognized as arbitrary, having primarily an observational meaning (sensu Allen & Starr 1982). The stand scale referred to a floristically homogeneous portion of grassland large enough (here ca. 1 ha) to contain many local patches potentially linked by dispersal (Wiens et al. 1986). The patch scale of perception referred to a relatively small collection of individuals of different species, defined by the size of the sample unit (5 m). In addition, we used diversity indices that differ in sensitivity to the presence of rare species. This highlights different aspects of the species abundance hierarchy (Whittaker 1972; Hill 1973; Peet 1974). Diversity numbers (Na, Hill 1973)were selected as descriptors to define a hierarchy of weighted abundances. Maurer (1985) applied a similar approach to analyzing the dynamics of avian community structure. Species diversity at the level of dominants was computed by N2 - the reciprocal of Simpson's index, a relative expression of dominance concen-

tration (Whittaker 1965, 1972). To estimate diversity at a hierarchical level comprising more (intermediate abundant) species we used N 1 - the exponential form of Shannon's index. Both N 2 and N1 indicate the 'apparent' number of species in the community (Hill 1973; Peet 1974), but at separate levels of the dominance hierarchy. We considered species richness (No), the total number of species in the sample, as the most inclusive measure of diversity (it weights all species equally). To analyze the influence of changing the spatial scale of resolution, we varied the aggregation form of data. The 'extent' of the study remained fix whilst the 'grain' was changed to represent stand and patch scales of inquiry (Allen etal. 1984). Community diversity at the stand scale was calculated on the basis of the mean proportional abundance of all species in the four transects. Statistical analysis of differences between grassland conditions was not possible for these estimates due to the lack of plot replicates. The 'percent change' (Peer 1974) among dates was computed for each diversity index in order to distinguish the effects of flooding at upper and lower levels of the dominance hierarchy. At the patch scale, community - point - diversity was calculated as the average diversity, richness or evenness per interception line. Hill's ratio E2,1 = N z / N 1 (1973) was used to compute evenness. Comparisons between grazed and ungrazed plots at this scale were made by the Mann-Whitney U test (Sokal & Rohlf 1969). Community pattern diversity was evaluated in two ways. Spatial variability in floristic composition within a plot was assessed by the average similarity (PS) among samples (N = 6, pairwise comparisons), where P S was the Percentage Similarity index of Czekanowski (Greig-Smith 1983). We also estimated stand heterogeneity by computing diversity (N1) for progressively pooled transects. Linear regressions of N1 on sample length were plotted for each site-by-day combination (cf. Facelli et al. 1989). Fifteen different combinations of transects were taken at random per situation, all giving similar results. A t test of Ho: b = 0 (df= 2, P = 0.05) was performed for regressions made on the average values (N = 15)

147 native graminoids (Fig. la). In the ungrazed plot (13-yr-old enclosure), native caespitose and prostrate grasses were major components of the community, whereas forbs comprised about 1% of the total basal cover (Fig. lc; see also Fig. 3). Prolonged flooding caused a replacement of dominant species in the grazed site. After the flood, no exotic forb appeared among the first four species in the rank order; dominance was shared by native grasses and sedges (Fig. lb). The relative cover of dicots decreased from 55.8 to 11.6%; their contribution to stand richness was also reduced (1985 = 54.6%, 1986 = 39.3%). Floris-

obtained per unit of increasing length (Sokal & Rholf 1969). Dominance-diversity curves (Whittaker 1965) corresponding to each transect on each plot were depicted in order to illustrate and summarize the results of the diversity analysis.

Results

Species composition Under continuous grazing, the grassland was dominated by cool-season exotic forbs and some

POST-FLOOD

PRE-FLOOD

t

G z

D '~

Mp Pm Dm Lt

Pg PI Ev

Ev Om Pm Lh Jm Pp Mp

12

7T I

04

Pd Pm Sp Ev Dm Pp Ss

U N G R A Z E D

Ev Pp Pm Pd Lh Dm Sp

SPECIES Fig. 1. Mean percent of basal cover of the seven major species (N = 4) in grazed and ungrazed plots, before and after the occurrence of the flood. Cover values are expressed in degrees (after arcsin square-root data transformation). VerticaI bars denote 1 SE. • alien forbs, [ ] cool-season and [] w a r m - s e a s o n native grasses, [] native sedges. Species code: D m = Danthonia montevidensis, Ev = Eleoeharis vMdans, Jm = Juncus microcephalus, Lh = Leersia hexandra, Lt = Leontodon taraxacoides, M p = Menthapulegium, Pd = Paspalum dilatatum, Pg = Panicum gouinii, P m = Panicum milioides, Pp = Paspalidiumpaludivagum, PI = Plantago lanceolata, Sp = Stipa philippii, Ss = Stenotaphrum secundatum.

148 GRAZED

UNGRAZED

10 Iv

8

o

......

I

z~~

X

6

O

I

5

I

10

I

1;

20

I

;

10

1;

2'0

SAMPLE LENGTH(m) Fig. 2. Relationship between diversity (N 1 = exp H ' ) and sample length as an estimate of spatial heterogeneity in the community. Different random combinations of line-transects were tested for each condition; points plotted are averages (N = 15) obtained for each sample length. Regressions are for pre-flood () and post-flood ( . . . . . . . . . ) conditions in two grazing treatments.

tic composition of the ungrazed plot was relatively less affected by flooding. There was a significant increase in the cover of some subordinate species (Fig. lc, d).

richness decreased in the grazed site but remained higher than inside the enclosure.

Point diversity Stand diversity Before the flood, species diversity at the stand scale was higher in the ungrazed plot considering either the dominant species (N2) or including the subordinate ones (N1) (Table 1). Differences between grazing conditions were larger when N 2 was used (percent differences: N2=46.3, N1 = 17.8~o), which reflected more dominance concentration under grazing (see Fig. la). Total species number was higher outside the enclosure (Table 1). In general, flooding reduced diversity in the ungrazed plot but increased diversity in the grazed plot. In the ungrazed plot, the largest percent change occurred for N1, accompanied by a marked decline of total richness (Table 1). In the grazed plot, percent increment of N2 was larger than for N1. After the flood, both sites had nearly the same diversity at the level of dominants. The N~ index, however, indicated that the grazed site was more diverse, i.e., the reverse pattern compared with pre-flood conditions (Table 1). Stand

The N1 index suggests that before flooding the community in the grazed plot contained on average more diverse patches (Table 2), but this difference was weak (P > 0.05). The use of N2 yielded quite similar values of point diversity for both plots (Table 2). Significant differences were detected between sites with respect to point richness. The grazed plot included 9 species per transect more than the ungrazed plot (Table 2). Point evenness did not differ statistically (P > 0.10) between sites. Prolonged flooding extended previous site difference in point diversity, richness, and evenness (Table 2). In the grazed plot, diversity increased mainly at the level of dominant species by 25.5 70. The diversity of the ungrazed plot was slightly affected; less than 10~ for both N 2 and N1. After the flood, diversity at the patch scale was significantly higher in the grazed community, especially at the subordinate species level (Table 2). The change in average diversities seemed associated with a reduction in the spatial variability of observations on each plot (see CV in Table 2). The

149 PRE-FLOOD

POST- FLOOD

100.

50

( ~e

10

v

cr LLI > O (_) d

co 0, t -- 9.52, P < 0.02) (Fig. 2). A mosaic made up of different quantitative mixtures of species developed with cattle exclusion (pre-flood PS = 35.6~o). The grazed area was floristically more homogeneous (PS = 63.7~o; Mann-Whitney test: U = 3, P < 0.02), and diversity estimates were not affected by the sample size (b = 0, t = 3.14, P > 0.05). The large flood exerted an homogenizing effect on both sites. Pattern diversity was substantially reduced in the ungrazed plot (b = 0 in 1986,

6.07 (23.4)

28 (15.2)

t -- 3.03, P > 0.05) and continued to be negligible in the grazed plot (t = 2.72, P > 0.05) (Fig. 2). This is consistent with the increased similarity among samples (PS---59.3 and 75.4~o, inside and outside the enclosure respec.), though differences in floristic heterogeneity between plots were maintained (U = 5, P < 0.05).

Discussion

We found contrasting patterns of community organization in the four grassland conditions studied (Fig. 2, 3). The influence of grazing and flooding upon community diversity was distinctly perceived according to the spatial scales and levels of the species hierarchy defined here (Table 1, 2).

Pre-flood community organization The 'undisturbed' grassland, i.e. pre-flood enclosure, was the most heterogeneous at the stand scale even though it comprised low diversity patches (Table 1). Pattern diversity was the main component of whole-community diversity. A similar spatial structure was observed after the exclusion of domestic grazers in meadows of Sweden (Persson 1984). We found striking differences between stand and point diversity for N 2

151 and N1, which means that community diversity was accounted for by spatial variation in both dominant and subordinate species abundances. Species coexistence in the ungrazed community was a consequence of the summation of patches which differed markedly in dominance structure (Fig. 3) and species ranking (Platt 1975; Glenn-Lewin & ver Hoef 1988). Spatial variability in dominance hierarchies would be partially associated with subtle differences in local resource availability and habitat conditions (Chaneton unpubl, data; see Fowler 1982; Tilman 1982, 1988). The grazed community had lower stand diversity but appeared more diverse at the patch scale than the ungrazed one. High point richness was the salient feature of the grassland under continuous grazing (Table 2). Large herbivores may drastically alter dominance hierarchies (see Fig. 1, 3) by selectively feeding upon grasses that concentrate dominance in ungrazed conditions (Facelli 1988; Sala 1988; Facelli etaL 1989; see also McNaughton 1983; Collins 1987; Gibson 1988). Grazing allowed for invasion (Facelli 1988) and subsequent dominance of low-growing dicots and grasses (Sala et aL 1986; Facelli et aI. 1989; Oesterheld & Sala 1990). The creation of canopy gaps also encourages the persistence of many opportunist, rare species (Fig. 3; Mitchley & Grubb 1986; Grubb 1986). Nevertheless, local equitability was low because dominance was still divided among few species in each patch. The last property along with the reduced pattern diversity determined the lower diversity of the grazed site at the stand scale (Table 1). Grazing much decreased stand diversity at the level of dominants as they appeared evenly distributed across the community (e.g. Mentha pulegium, Leontodon taraxacoides, Fig. l a). Only the rarest species showed minor spatial variation. Species ranking did not vary substantially among patches, which also had similar dominance structures (Fig. 3). Therefore, coexistence in the grazed community was related to species co-occurrence at the patch scale (Glenn-Lewin & ver Hoef 1988). In other grasslands subject to heavy grazing by native herbivores the limited dispersal of

clonal species generated patchiness in the dominance hierarchy (Belsky 1983, 1986). Furthermore, McNaughton (1983) concluded that pattern diversity was the main property of community diversity in grazed areas of the Serengeti ecosystem. The spatially generalized effect of continuous herbivory may cause patches to resemble each other in composition and dominance relations (Mitchley & Grubb 1986). In fact, grazing could be overriding the influence of habitat heterogeneity (Sala 1988; see Persson 1984; Gibson 1988). Post-flood community organization

In the ungrazed grassland, flooding decreased stand and point diversity. These changes involved primarily intermediate and rare species (Table 1). On the one hand, there was a marked reduction of species richness. Low richness - or diversity has been predicted for situations of intense environmental stress (e.g. Grime 1979). Prolonged flooding may eventually produce gaps in vegetation since the above- and below-ground organs of several species are damaged because of the stress triggered by soil anaerobiosis (Drew 1983). However, we did not find any 'new' species colonizing the newly disturbed patches (Chaneton etal. 1988), but an increase of certain species which regenerated from in situ vegetative propagules (see below). Moreover, in the ungrazed plot severe flooding did not suppress previous dominants (see Fig. 1); instead, most rare species were eliminated. A reduced pool of species will constrain the potential for attaining a high community diversity through any of its various aspects. On the other hand, there was a significant reduction in community spatial heterogeneity (Fig. 2). After the flood, the sampled patches exhibited substantial similarity of dominance structure and species rank order (Fig. 3, see CV in Table 2). This trend to a less obvious patterning was clear at all the three levels of the dominance hierarchy. Responses contributing to homogeneize the community were the decline in patchiness of dominants (Fig. id), and the patch-scale

152 decrease in the number of subordinate species (Table 2; Fig. 3). The large flood altered the species hierarchy by changing the rank order of some previously dominant tall-grasses and increasing the relative importance of subordinate low-growing species (Fig. 1, 3). At the patch scale, the latter effect and the low number of rare species resulted in higher evenness (Table2). Species like the native creeping grasses, Leersia hexandra and Paspalidium paludivagum, and the sedge Eleocharis viridans were tolerant to prolonged waterlogging (see Fig. lc, d), and may have benefitted by using freed resources (Insausti & Soriano 1988; Crawford et al. 1989). Individual species responded similarly to flooding throughout the sampled area which led to enhance among-patch similarity. Species coexistence at the local scale seemed to result from varied responses to a combination of apparently conflicting selective pressures (Menges & Waller 1983). In the grazed site, the effect of the flood was similar at both stand and local spatial scales. Major changes occurred at the level of dominant species. Overall community heterogeneity was yet mostly determined by species diversity at the patch scale. Despite a slight decline in richness, point diversity was increased by the reduction of dominance concentration in some patches (Table 2, Fig. 3). After the flood, dominance was divided among several native graminoids (Fig. 1, 3). The combination of disturbances gave rise to the highest diversity observed at the patch scale (Table 2). Changes in relative abundances would be the result of distinct species-level abilities to cope with waterlogging stress, which appeared related to species' history in the system, whether they were natives or exotics (Chaneton et al. 1988). The spatial floristic homogeneity was reinforced by the flood because the same species increased cover in all patches. The structure of dominance also varied locally less than in 1985 (Fig. 3). The joint action of continuous grazing and episodic flooding allowed small-scale diverse assemblages to be established because of interspecific differences in the response to these alternative environmental constraints (Huston 1979).

Comparison to current models

The effect of cattle grazing upon grassland heterogeneity in the Flooding Pampa depends on the spatial scale of inquiry (Sala et al. 1986; Facelli etal. 1989). Traditional models (e.g. Harper 1969; Connell 1978; Grime 1979) readily explain differences in point diversity between the two grazing conditions before the flood, and also the higher stand richness of the grazed site. However, those models do not predict the overriding effect of grazing on the spatial variability of vegetation and the resultant lower stand diversity. Contrasting responses to grazing in the diversity of grassland communities have been recently pointed out by Milchunas et al. (1988), who emphasize the role of the evolutionary history of grazing and water availability as determinants of structural responses. Their model accounts for differential changes in the abundance of canopy dominant and subordinate species (Milchunas et al. 1988). Nevertheless, no distinction was explicitly drawn between point and pattern responses to herbivores action, which obscures the spatial scale at which the effect occur. The single effect of flooding was consistent with the intermediate-disturbance hypothesis (Connell 1978; Grime 1979; Huston 1979; Miller 1982; Malanson 1984), as we studied an event of infrequent large magnitude. Yet, the results suggest that the causes of changes in point and pattern diversity are probably not the same. Disturbance effect on pattern diversity might depend on the relationship between the scale of the event and the grain of existing environmental gradients (Denslow 1985; Belsky 1986; Sala 1988). Alternatively, effects on diversity at the patch scale may vary according to the degree of dominance within local species hierarchies (Platt 1975; Auerbach & Shmida 1987). Our results also show that disturbances do not affect the abundance of dominant and subordinate species in an easily predictable manner (cf. Kolasa 1989, p. 41). Disturbance regime and initial habitat conditions determine the nature and identity of the dominant species at the moment of occurrence, which in turn will affect the response of the plant

153 community (Denslow 1980; Armesto & Pickett 1985; Collins 1987; Chaneton etal. 1988). This stresses the importance of previous site history to the patterns observed after disturbance. Relevant factors may be the time elapsed since, e.g, the last large flooding event, or the presence of any other type of disturbance operating over the community (Collins & Barber 1985; Collins 1987). We did not find a clear-cut effect of disturbance interaction on species diversity. It has been argued that plant community diversity should be maximized by the concurrence of natural disturbances (Denslow 1980; Collins & Barber 1985). In the present study, stand diversity was highest on the undisturbed plot. The interaction of disturbances had opposite effects on point and pattern diversity; therefore, results at the stand scale disagreed with the model of Collins and Barber (1985). Moreover, the site of lowest diversity varied with the dominance hierarchy level considered. It was apparent that comparisons were spatial-scale dependent. The results at the patch scale supported the hypothesis quoted above; the highest point diversity was recorded in the grazed/flooded condition. Grazing enhanced floristic richness while prolonged flooding increased species equitability. In several cases, however, for some parameters there seemed to be a cancellation of effects between the two disturbance types.

Conclusions The study provided new evidence of the central, ubiquitous role of the concurrence of disturbances in the organization of plant communities (Denslow 1980; Belsky 1983; McNaughton 1983; Collins & Barber 1985; Collins 1987). The combined use of various diversity measures enabled to focus on different community properties. It also permitted exploration of the influence of spatial scale, and of the level of dominance hierarchy addressed, upon the evaluation of disturbance effects in ecological communities. Consideration of these factors may contribute to integrating the available evidence relat-

ing natural disturbance to patterns of community structure within a hierarchical framework (Allen & Starr 1982; Pickett etal. 1989). Changes observed in different spatial scales or in species having very separate rank orders, may parallel meaningful changes in the relative importance of factors determining species coexistence and community organization (Auerbach & Shmida 1987; Kolasa 1989). Here we examined the effect of two disturbance agents which largely differ in mechanism of action and role in the evolution of grasslands of the Flooding Pampa (Chaneton et al. 1988). Accordingly, they brought about different consequences for community diversity (Denslow 1980, 1985), even when evaluated at a given hierarchial level. Our findings led to the conclusion (see also Milchunas etal. 1988, 1990) that predictions derived from current general models will be affected not only by the level of perception, but also by the type of disturbance considered, particularly with regard to the evolutionary history of the system.

Acknowledgements We thank R.J.C. Ledn for advise and constructive discussion at all stages. S. Burkart helped in data collection and C.M. Ghersa provided suggestions on the manuscript. O.E. Sala shared an unpublished chapter. The comments from an anonymous reviewer improved the readability of the paper. The study was funded by Consejo Nacional de Investigaciones Cientificas y T6cnicas de Argentina (CONICET). We dedicated this paper to the memory of Mr. A. Bordeu, who kindly permitted and facilitated our work on his land over the last two decades.

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