Herbivore effects on plant species density at varying productivity levels

Laura Gough

INTRODUCTION

The relationship between biomass and species density (the number of species per unit area) is described as a hump-shaped curve, with a peak in species density at a low to intermediate level of biomass (Grime 1973). This pattern has been found by several researchers (reviewed by Tilman and Pacala 1993, but see Gough et al. 1994), with the most consistent result being a decrease in species density at the highest biomass levels (Marrs et al. 1996). However, there is little consensus as to the mechanisms involved (Rosenzweig and Abramsky 1993, Tilman and Pacala 1993, Abrams 1995). Other factors such as disturbance and environmental stress may explain most of the variation in species density in a particular habitat separately from biotic factors such as biomass and competition (e.g., Shipley et al. 1991, Gough et al. 1994, Grace and Pugesek 1997).

When productivity and biomass are artificially increased through nutrient additions, species density consistently declines in a wide variety of habitats, including arctic tundra, old fields, grasslands, and other herbaceous communities (Bakelaar and Odum 1978, Silvertown 1980, Vermeer 1986, Gurevitch and Unnasch 1989, Carson and Pickett 1990, DiTommaso and Aarssen 1991, Wilson and Tilman 1993, Chapin et al. 1995; for an exception, see Goldberg and Miller 1990). This result suggests a direct correlation between these two variables and supports the idea that, with increased biomass, competitive exclusion becomes more important in structuring the plant community.

If nutrient enrichment causes increased competitive exclusion, how do herbivores affect this process? Herbivores can regulate diversity in various ways (Crawley 1983). Whether or not herbivores function as disturbance agents and maintain higher diversity than would occur without them is debatable (Milchunas et al. 1988, Pacala and Crawley 1992), but many authors believe that disturbances such as herbivory cause a decline in the importance of competition (e.g., Grime 1979, Keddy 1990, Huston 1994). The few studies examining the interaction between herbivory and competition have produced conflicting results (reviewed in Goldberg and Barton 1992; Swank and Oechel 1991, Belsky 1992, Reader 1992, Clay et al. 1993, Burger and Louda 1995). Because there is still debate about herbivory’s effect on primary productivity (reviewed in Milchunas and Lauenroth 1993), perhaps it is not surprising that we cannot draw solid conclusions as to the effect of herbivory on competitive interactions (e.g., Taylor et al. 1997). Herbivory must be examined both alone and as it interacts with fertilization in order to tease apart the effects animals may have on plant interactions (Louda et al. 1990).

The coastal wetlands of the northern Gulf of Mexico include marsh ecosystems in which plant species diversity varies widely with variation in standing biomass, disturbance regime, and abiotic stress factors such as salinity and flooding (Chabreck 1972, Mitsch and Gosselink 1986, Gough et al. 1994, Grace and Pugesek 1997). Multivariate analyses of the factors associated with species density indicate that competitive exclusion limits species density at very high levels of community biomass, whereas disturbance and vertebrate herbivore grazing may offset competitive effects (Grace and Pugesek 1997). Herbivores in these habitats tend to selectively eat certain species and have a range of effects on species density, depending on additional factors such as community type, intensity of herbivory, and frequency of fire (Llewellyn and Shaffer 1993, Nyman et al. 1993, Taylor et al. 1994, 1997, Taylor and Grace 1995, Ford 1996). Little is known about the effects of fertilization on community composition and species density in these systems. Thus, although multivariate models suggest an important, interactive influence of community biomass and herbivory on plant species density for coastal wetlands, experimental evaluation of this effect is missing.

The research presented here examined the effects of fertilization and herbivory on two coastal marsh plant communities of similar productivity levels, but differing species pools due to different ambient salinity levels. Two levels of fertilization were used: commercial fertilizer and sediment application. Numerous studies have shown that substantial amounts of sediment are deposited in coastal marshes in conjunction with floods, storms, and hurricanes (e.g., Rejmaneck et al. 1988, Guntenspergen et al. 1995). Experimental studies indicate that these sediment deposition events act as a natural form of fertilization (DeLaune et al. 1990). Our objectives were to address the following questions. (1) Will fertilization reduce species density while increasing productivity? (2) Is herbivory affecting community structure, specifically species density? (3) Will herbivory influence the response of the community to fertilization? We hypothesize that herbivory may affect plant community structure both directly, by selective removal of palatable species, and indirectly, by affecting biomass accumulation and, thus, competition. To examine these questions, nutrient additions, alluvial sediment additions, and herbivore exclosures were applied to two marsh communities in coastal Louisiana, and responses were measured for three growing seasons from 1993 to 1995.

STUDY AREA

The study area was located within the Pearl River Wildlife Management Area on the coastal boundary between Louisiana and Mississippi, United States (White 1979, Gough 1996). The Pearl River system is composed of three main channels, the East, Middle, and West Pearl Rivers, which empty into the Gulf of Mexico. Two study sites were chosen within the river basin. The fresh/oligohaline (0-2 g/kg water salinity) Sagittaria site was located along the Middle Pearl River in a homogeneous area dominated by Sagittaria lancifolia L. and Spartina patens (Ait.) Muhl., with a variety of annual species present. The brackish S. patens site, with salinities ranging from 6 to 14 g/kg (L. Gough, unpublished data), was located along the East Pearl River where it widens to empty into Lake Borgne and drains into the Gulf of Mexico; this site was dominated by S. patens and Scirpus americanus Pers. (formerly Scirpus olneyi Gray), with mostly grasses and sedges composing the rest of the vegetation.

Several mammalian herbivores were present at both sites, including wild boar (Sus scrofa), rabbit (Sylvilagus sp.), muskrat (Ondatra zibethicus), and, most abundantly, nutria (Myocaster coypus). Muskrat were originally reported in coastal Louisiana in 1870 (O’Neil 1949) and were the dominant herbivores until the 1950s. Muskrats prefer Scirpus americanus, which composes 80-90% of their diet. Nutria, rodents larger than muskrats, were introduced into Louisiana from South America in the 1930s (Lowery 1974). As nutria populations increased due to lack of natural predators, short gestation periods, and lower trapping rates, their numbers dramatically increased to a peak of 20 x [10.sup.6] animals in 1956 (Lowery 1974). By 1995, trapping rates had decreased to only 200 000 nutria/yr, allowing the populations to continue to rise (Louisiana Department of Wildlife and Fisheries; G. Linscombe, personal communication). Nutria have eating habits similar to those of muskrat, preferring Scirpus americanus and concentrating in areas where this species is available (Chabreck et al. 1981).

METHODS

Experimental design

Three levels of nutrient enrichment were applied to 1-[m.sup.2] plots. First, the control was a marked, unmanipulated 1-[m.sup.2] plot. Second, commercial fertilizer (Osmocote 20-10-5 NPK tree and shrub planting tablets; Grace-Sierra, Milpitas, California, United States) was applied to 1-[m.sup.2] plots to obtain a level of 17 g/[m.sup.2] available nitrogen as ureaformaldehyde. Twenty-four pellets were placed in each plot and inserted [approximately]10 cm into the ground in a uniform grid pattern, modified by rhizomes and roots. These tablets are designed for 1-yr slow release, but due to saturated soil conditions, fertilizations were performed in August 1993, June 1994, and April 1995. Third, sediment was applied to 1-[m.sup.2] plots in August 1993 to mimic the natural process of nutrient enrichment in coastal wetlands. Plots were covered with 2 cm of fluid sediment collected from adjacent channel edges and bottoms; care was taken to avoid damaging the plants during sediment application.

Fenced exclosures were constructed in June and July of 1993 to prevent mammalian herbivory by nutria, muskrat, rabbit, and wild boar. Wooden corner posts and 1.2 m wide plastic-coated fencing wire with 5 x 5 cm openings were used to construct the [approximately]7 x 7 m exclosures. The wire was sunk [approximately]15 cm into the soil. All plots were located [greater than or equal to] 1 m from the fences to avoid edge effects. Plots within each fenced or unfenced area were fertilized, had sediment added, or were unmanipulated. Eight replicate fenced or unfenced areas were located in each of the two marshes, for a total of 96 plots.

TABLE 1. Source table for repeated-measures analysis of species

density over time.

Source df MS F P

Marsh 1 1805.03 124.38 0.0001

Fence 1 0.17 0.01 0.91

Marsh x fence 1 1.78 0.12 0.73

Replicate (marsh x fence) 28 14.51

Enrichment 2 20.49 3.98 0.02

Marsh x enrichment 2 2.24 0.43 0.65

Fence x enrichment 2 15.66 3.04 0.06

Marsh x fence x enrichment 2 5.04 0.98 0.38

Error 55 5.15

Time 6 66.89 51.56 0.0001

Time x marsh 6 5.36 2.46 0.03

Time x fence 6 4.05 1.86 0.09

Time x marsh x fence 6 3.76 1.72 0.12

Time x replicate (marsh x fence) 168 2.18

Time x enrichment 12 3.53 2.72 0.002

Time x marsh x enrichment 12 1.13 0.87 0.58

Time x fence x enrichment 12 1.73 1.33 0.20

Time x marsh x fence x enrichment 12 0.73 0.56 0.87

Error (time) 330 1.30

Data collection

The plots were established in June 1993 and sampled in July 1993 for an initial, pretreatment census. Subsequently, they were censused in October 1993, April, July, and October 1994, and April and July 1995. These nondestructive censuses involved recording the species found in each plot, estimating aerial percent cover, counting numbers of stems, and measuring the average heights of each species. For one species, Sagittaria lancifolia, average leaf width was also recorded. Vine species were censused for percent cover only, because it is impossible during the growing season to determine rooting location and stem length.

In early August 1995, destructive harvests were performed by removing all aboveground biomass (including dead biomass composed of standing dead and litter trapped by plant stems) from the center 0.1 [m.sup.2] of the plot, using a circular plot guide. Samples were brought to the laboratory and sorted by species. Those species with easily identifiable dead ramets were divided into live and dead, whereas litter that could not be identified to species was lumped into one category. The plants were dried for [greater than or equal to]48 h at 80 [degrees] C and then weighed. For biomass analysis, the dry mass was divided by 0.1 [m.sup.2] to approximate results on a per square meter basis. Percent cover and number of stems revealed patterns similar to those from biomass results; only biomass data are presented here.

Data analysis

Species density was analyzed as a split-split plot design; the source table is presented in Table 1. The whole plot was the marsh x fence interaction, because each treatment was replicated within each marsh x fence combination. The fertilization treatments were the first split and time was the second. Error terms are shown in Table 1, directly beneath the effects that they tested. PROC GLM (SAS Institute 1995) with a repeated statement was used to perform the overall analysis, determine sphericity, and generate univariate results for each date of the census. In addition, the number of species present in the plots in 1995 was calculated by combining the census results from April and July; the same analysis was run without repeated measures. All effects were evaluated for significance using Type III sums of squares. LSMEANS with Tukey’s hsd test were used for pairwise comparisons. Alpha levels were judged significant at 0.05 unless otherwise specified.

The same analysis was performed on biomass at harvest without repeated measures. The ANOVA was run separately for total, live, and dead biomass, and for each individual species. Normality was evaluated using the Shapiro-Wilks statistic, stem and leaf plots, and normal probability plots, and homogeneity of variances was determined from residual plots. Biomass data for individual species were log-transformed as necessary to meet model assumptions.

RESULTS

Species density

As expected based on previous studies (Gough et al. 1994), species density was significantly higher in the Sagittaria marsh than in the Spartina patens marsh throughout the entire experiment (Table 1; [ILLUSTRATION FOR FIGURE 1 OMITTED]). There was no significant interaction between marsh type and fencing, and fencing did not alter species density as a main effect.

Species density peaked in all treatments in July of each year, causing the significant overall time effect, and the different responses to nutrient and sediment addition at each date caused the significant time X enrichment interaction (Table 1, [ILLUSTRATION FOR FIGURE 1 OMITTED]). (Because the repeated-measures analysis generally met sphericity requirements [Mauchly’s criterion = 0.56; P = 0.06]), the univariate results for effects of time are reported [Moser et al. 1990].) The two marshes responded differently over time, although they responded similarly to nutrient and sediment addition (time x marsh X enrichment, P [greater than] 0.10; time x marsh, P = 0.03; Table 1). In October 1993 and April 1994, the fenced Sagittaria plots had significantly higher species density than did the unfenced Sagittaria plots; this difference was not evident at any other time.

The effects of nutrient and sediment addition on species density depended, to a modest degree, on the presence of herbivores (fence x enrichment, P = 0.06; Table 1). Nutrient or sediment addition had no significant effect in the unfenced plots in either marsh at any date [ILLUSTRATION FOR FIGURES 1B, D OMITTED]. However, in the fenced plots in both marshes, nutrient addition plots contained significantly fewer species than did the control plots [ILLUSTRATION FOR FIGURES 1A, C OMITTED]. Thus, over the length of the study, enrichment remained a significant effect. Species density in July 1995 showed a significant enrichment effect as well, with the nutrient plots containing fewer species than the control plots (P [less than] 0.01; [ILLUSTRATION FOR FIGURE 2A OMITTED]). Results for the analysis of species found in April and July 1995 were nearly identical (fence x enrichment, P = 0.08; enrichment, P [less than] 0.01; marsh, P [less than] 0.01; all other effects, P [greater than] 0.10), so we focused on species density in July 1995 in order to compare those results with biomass obtained at the peak growing season.

Community biomass

Overall, nutrient enrichment increased biomass accumulation (P = 0.0001; Table 2, [ILLUSTRATION FOR FIGURE 2B OMITTED]); however, the magnitude of this effect was greater in the Spartina patens marsh and may have depended on herbivory ([ILLUSTRATION FOR FIGURE 3 OMITTED]; marsh x enrichment, P = 0.07; marsh x fence x enrichment, P = 0.08; Table 2). In fenced plots in the S. patens marsh, nutrient addition increased biomass to levels greater than those of the control and sediment addition plots [ILLUSTRATION FOR FIGURE 3A OMITTED]. This did not occur in the Sagittaria marsh, contributing to the three-way interaction (Table 2). In the S. patens marsh, the fenced plots contained less biomass than the unfenced plots, whereas the opposite was true for the Sagittaria site ([ILLUSTRATION FOR FIGURE 3 OMITTED]; marsh x fence, P = 0.06; Table 2).

Live biomass showed similar results, with nutrient addition increasing biomass more in the S. patens than in the Sagittaria marsh (pairwise P [less than] 0.01; Table 2, [ILLUSTRATION FOR FIGURE 3 OMITTED]). The fenced, sediment plots in the S. patens marsh contained less biomass than the fenced, nutrient plots (P = 0.01). Nutrient enrichment was highly significant for live biomass (Table 2, [ILLUSTRATION FOR FIGURE 3 OMITTED]).

TABLE 2. Source table for analysis of total, live, and dead

biomass from the August 1995 harvest.

P

Source df Total Live Dead

Marsh 1 0.94 0.43 0.07

Fence 1 0.85 0.29 0.01

Marsh x fence 1 0.06 0.12 0.47

Replicate (marsh x fence) 28

Enrichment 2 0.0001 0.004 0.0001

Marsh X enrichment 2 0.07 0.03 0.52

Fence X enrichment 2 0.72 0.44 0.60

Marsh x fence x enrichment 2 0.08 0.13 0.40

Error 54

There was significantly more dead biomass inside the fences than outside and significantly more in the nutrient treatment than in the other two treatments in both marshes (Table 2, [ILLUSTRATION FOR FIGURE 3 OMITTED]). This was not an artifact of the fences, as no wrack buildup was observed along the fences at any time during the study (L. Gough, personal observation). Most of the dead plant material included in the “dead” category was standing dead. This pattern is most evident when examining the fenced, nutrient plots and unfenced, nutrient plots in relation to the two control plots in the S. patens marsh. More dead biomass accumulated overall in the fresher marsh (P = 0.07; Table 2).

Biomass of individual species

Only a few species were present in enough plots to be tested statistically for fertilization effects on biomass. Polygonum punctatum Elliott, a common dicot that occurred in the Sagittaria marsh, had significantly greater biomass in the nutrient treatments than in the control or sediment plots (a threefold increase; P [less than] 0.05). Scirpus americanus, a clonal dominant sedge found only in the Spartina patens marsh, had slightly more total biomass inside the fences than outside, due primarily to more standing dead biomass accumulating when the plants were protected from herbivory (P = 0.08; [ILLUSTRATION FOR FIGURE 4 OMITTED]). There was also more biomass in the nutrient plots than in the control or sediment plots (P = 0.008), due to the substantial increase in the fenced, nutrient plots. S. americanus responded to fencing most dramatically in the spring by growing up to 40 cm taller inside the fences than out (data not shown).

Two species occured at both sites in suitable abundance for detailed analysis: Spartina patens and Sagittaria lancifolia. The latter, a clonal herbaceous monocot, was dominant at the fresher site and codominant at the brackish site. It had greater biomass in the Sagittaria marsh (P [less than] 0.01; [ILLUSTRATION FOR FIGURE 5 OMITTED]), but there was no significant difference between fertilized and control plots in either marsh, nor was there an effect of fencing.

Spartina patens, a clonal grass, was dominant in both marshes. The results for this species are complex: the three-way interaction was significant (P [less than] 0.05), although individual pairwise comparisons within each marsh were not different [ILLUSTRATION FOR FIGURE 6 OMITTED]. This interaction can be better understood by examining the significant marsh x enrichment and marsh x fence interactions (both P [less than] 0.05) and the graph of the means [ILLUSTRATION FOR FIGURE 6 OMITTED]. In the S. patens marsh, fencing decreased biomass relative to the unfenced plots; fertilization had a slight effect on the unfenced plots, but there were no significant differences among the three enrichment treatments overall. In the Sagittaria marsh, a different pattern emerged. Fencing had a minimal effect on biomass; fertilization caused a decrease in biomass relative to the controls, although not significantly. The main effect of enrichment was not significant for S. patens, and was influenced by these complex interactions.

Discussion

This study was conducted to examine the effects of fertilization treatments on the species composition of plots located in two marshes of different salinity regimes, and to determine how vertebrate herbivory might alter the outcome of these manipulations. Species density differed between these two marshes (6.8 [+ or -] 0.4 species/[m.sup.2] for the brackish marsh and 9.3 [+ or -] 0.3 species/[m.sup.2], mean [+ or -] 1 SE, for the fresher marsh in July of each year of the study), probably due to salinity constraints on germination and survival in the brackish marsh. A decrease in species number along a salinity gradient has been documented in other studies at the Pearl River (White 1979, Gough et al. 1994) and throughout coastal Louisiana (Chabreck 1972). Species density also changed with the seasons, causing a significant effect of time in our experiment, as spring annuals increased species density in April and July and senescence decreased species density in October [ILLUSTRATION FOR FIGURE 1 OMITTED]. Somewhat surprisingly, both marshes behaved in similar ways in response to the treatments, despite these differences in average species density.

The amount of sediment applied in this study did not produce consistent results; in almost all cases, however, the sediment-treated plots behaved similarly to the control plots in terms of species density and biomass (with exceptions noted in Results). A previous sediment addition study in a Louisiana saltmarsh used much greater amounts of sediment and found an increase in Spartina alterniflora biomass and leaf area (DeLaune et al. 1990, Pezeshki et al. 1992). In those studies, up to 94 kg/[m.sup.2] of sediment was used.

Nutrients and herbivory interactions influence species density and biomass

Community biomass and species density did not respond consistently with each other in this study, due to the influence of herbivores. In both marshes, biomass increased with fertilization in the fenced and unfenced plots; only in the fenced and fertilized plots did species density significantly decrease. This suggests that herbivore activity prevented competitive exclusion despite increased biomass, probably by suppressing one of the dominant plant species and preventing dead biomass accumulation. Inside the fences in both marshes, the significant increase in dead biomass accumulation may have suppressed seedling establishment and germination, because most of the species lost over the three growing seasons were annuals (Table 3). Several recent studies directly implicate dead biomass accumulation as the agent responsible for decreased species density under enriched conditions (Brewer et al. 1997, Foster and Gross 1997, 1998).

We do not believe that the fertilizer treatment selectively attracted herbivores at greater numbers, contrary to what others have seen (Tilman 1983, Nams et al. 1996) and predicted in herbivore optimization models (e.g., Oksanen et al. 1981). Community biomass did not differ between fenced, fertilized plots and unfenced, fertilized plots in either marsh at harvest [ILLUSTRATION FOR FIGURE 3 OMITTED].

Dominant species responded to nutrients and herbivory differently

In both marshes, the dominant species responded differently to the treatments, resulting in no net difference in community biomass between fenced and unfenced nutrient plots. In the brackish marsh, Scirpus americanus, the more palatable species, increased at the expense of Spartina patens inside the fences, whereas the opposite occurred when herbivores were present. This suggests an interaction between species abundance and herbivory in high fertility plots, and that inedible biomass will increase when nutrients are added in the presence of herbivores (Grover 1995), as for S. patens here. This increase in S. americanus at the expense of S. patens was noted by Linscombe et al. (1981) when nutria populations declined in an area of coastal Louisiana.

TABLE 3. Species lost from fenced, nutrient plots in both marshes.

Numbers indicate the number of plots out of eight from which each

species was lost. Species are classified as annual (a) or perennial

(p) following Tiner (1993).

Marsh type

Species Class Spartina patens Sagittaria

Aster subulatus a 2 5

Aster tenuifolius p 1 2

Cyperus haspans a 0 2

Cyperus oderatus a 2 0

Eleocharis cellulosa p 0 1

Eleocharis fallax p 1 0

Eleocharis parvula a 1 0

Galium tinctorum a 0 5

Hydrocotyle bonariensis p 0 4

Ipomoea sagittata p 0 3

Juncus roemerianus p 1 0

Lilaeopsis chinense p 1 0

Lythrum lineare p 0 1

Mikania scandens p 0 2

Phyla nodiflora p 0 2

Polygonum punctatum a 0 1

Ptilimnium capillaceum a 0 5

Sacciolepis striata p 0 1

Sagittaria lancifolia p 1 0

Scirpus americanus p 1 0

Scirpux robustus p 1 0

Scirpus validus p 1 0

Spartina alterniflora p 5 0

Vigna luteola p 0 1

In the fresh marsh, S. patens also decreased biomass in the fenced, fertilized plots. This decrease may have been due to increased accumulation of dead biomass. Although Spartina patens is a dominant coastal species throughout the eastern United States and Gulf Coast, our results show that it can be outcompeted when herbivores are not present. This species has been eliminated by other species under enriched nutrient conditions in a New England saltmarsh (Levine et al. 1998), and it proved a poorer competitor with S. americanus under flooded conditions, although it outcompeted S. americanus at high salinities (Broome et al. 1995).

CONCLUSION

Fertilization in the absence of herbivory increased biomass and decreased species density. This decrease in species density in fenced plots may have been related to increased dead biomass accumulation and to competitive exclusion by dominant species. Herbivory of the enriched plots prevented competitive exclusion without reducing community biomass at harvest.

The results presented here support predictions that, as nutrient levels are increased, species density decreases, but only when herbivores are excluded. This result was obtained in two marsh communities of different salinity regimes and different mean species density values. Because more dead biomass accumulated inside the exclosures, it is difficult to separate the effects of dead plant material from the competitive effects of living tissue. However, this study demonstrates the importance of controlling herbivore densities when manipulating productivity. Results inside and outside the fences proved similar for biomass, but quite different for species density.

ACKNOWLEDGMENTS

We thank the Louisiana Department of Wildlife and Fisheries for permission to work at the Pearl River. For assistance in the field, we thank H. Haecker, M. Ford, A. Baldwin, D. Bordelon, D. D’Abundo, J. Hayden, E. Schussler, and J. Teague. We also thank H. Haecker for invaluable lab help. The manuscript was improved by comments from J. S. Brewer, J. P. Geaghan, and I. A. Mendelssohn; constructive reviews were provided by J. S. Clark, M. J. Crawley, and an anonymous reviewer. Funding was provided by an NSF Dissertation Improvement Grant to L. Gough (DEB-9310890).

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