Spatial heterogeneity and plant species richness at different spatial scales under rabbit grazing

Herbivores influence spatial heterogeneity in soil resources and vegetation in ecosystems. Despite increasing recognition that spatial heterogeneity can drive species richness at different spatial scales, few studies have quantified the effect of grazing on spatial heterogeneity and species richness simultaneously. Here we document both these variables in a rabbit-grazed grassland. We measured mean values and spatial patterns of grazing intensity, rabbit droppings, plant height, plant biomass, soil water content, ammonia and nitrate in sites grazed by rabbits and in matched, ungrazed exclosures in a grassland in southern England. Plant species richness was recorded at spatial scales ranging between 0.0001 and 150 m(2). Grazing reduced plant height and plant biomass but increased levels of ammonia and nitrate in the soil. Spatial statistics revealed that rabbit-grazed sites consisted of a mixture of heavily grazed patches with low vegetation and nutrient-rich soils (lawns) surrounded by patches of high vegetation with nutrient-poor soils (tussocks). The mean patch size (range) in the grazed controls was 2.1 +/- 0.3 m for vegetation height, 3.8 +/- 1.8 m for soil water content and 2.8 +/- 0.9 m for ammonia. This is in line with the patch sizes of grazing (2.4 +/- 0.5 m) and dropping deposition (3.7 +/- 0.6 m) by rabbits. In contrast, patchiness in the ungrazed exclosures had a larger patch size and was not present for all variables. Rabbit grazing increased plant species richness at all spatial scales. Species richness was negatively correlated with plant height, but positively correlated to the coefficient of variation of plant height at all plot sizes. Species richness in large plots (<25 m(2)) was also correlated to patch size. This study indicates that the abundance of strong competitors and the nutrient availability in the soil, as well as the heterogeneity and spatial pattern of these factors may influence species richness, but the importance of these factors can differ across spatial scales.     

Grazing and an invasive grass confound spatial pattern of exotic and native grassland plant species richness

Previous work has shown exotic and native plant species richness are negatively correlated at fine spatial scales and positively correlated at broad spatial scales. Grazing and invasive plant species can influence plant species richness, but the effects of these disturbances across spatial scales remain untested. We collected species richness data for both native and exotic plants from five spatial scales (0.5–3000 m²) in a nested, modified Whittaker plot design from severely grazed and ungrazed North American tallgrass prairie. We also recorded the abundance of an abundant invasive grass, tall fescue (Schedonorus phoenix (Scop.) Holub), at the 0.5-m² scale. We used linear mixed-effect regression to test relationships between plant species richness, tall fescue abundance, and grazing history at five spatial scales. At no scale was exotic and native species richness linearly related, but exotic species richness at all scales was greater in grazed tracts than ungrazed tracts. Native species richness declined with increasing tall fescue abundance at all five spatial scales, but exotic species richness increased with tall fescue abundance at all but the broadest spatial scales. Severe grazing did not reduce native species richness at any spatial scale. We posit that invasion of tall fescue in this working landscape of originally native grassland plants modifies species richness-spatial scale relationships observed in less disturbed systems. Tall fescue invasion constitutes a unique biotic effect on plant species richness at broad spatial scales.     

Water and fertilizer have opposite effects on plant species richness in a mesic early successional habitat

Herbaceous plant species richness typically declines with increasing productivity, but differences in the resources underlying these gradients are often ignored. This study adds to the small number of studies examining the effects of water and mineral nutrients on biomass and richness in oldfield communities. We established 60 4 m² plots in a goldenrod-dominated oldfield to test the differential effects of water and mineral nutrients on community properties. Species richness declined with added nutrients, but increased with added water. Aboveground biomass increased only when both nutrients and water were added. Leaf area index increased with added nutrients alone, although the increase was greater when water was also added. Understory light levels decreased with added nutrients, but not with added water; however, the per-gram effect of biomass on understory light levels did not vary significantly among nutrient and water treatments. Our results suggest that water tends to enhance productivity, but does not cause the common decline in species richness that typically accompanies nitrogen additions. Rather, water increased richness through positivie effects on either germination and establishment, or increased survival. These results are consistent with either increased light limitation or increased water limitation causing loss of species from nitrogen-rich habitats.     

The resource balance hypothesis of plant species diversity in grassland

Abstract: We hypothesize that plant species diversity is favoured when actual resource supply ratios are balanced according to the optimum resource supply ratios for the vegetation as a whole. This ‘resource balance hypothesis of plant species diversity’ (RBH) follows from two different mechanisms of plant species coexistence, namely: ‘differential resource limitation’, which allows species to coexist in a competitive equilibrium in a homogeneous environment and ‘micro-habitat differentiation’, which builds on spatial heterogeneity. Both mechanisms require that resource supply ratios are intermediate between the optimum supply ratios of the species present in the species pool. Additional conditions, concerning the resource acquisition and requirement ratios of the species, are easier to meet for the second mechanism than for the first. To test the RBH we measured species diversity parameters in 74 grassland plots, as well as the N, P and K concentrations in the above-ground biomass. We used a new ceiling detection algorithm to examine the relationship between maximum observed diversity and the N/P-, P/K- and K/N-ratios in the biomass. Most of these ceiling relationsips could be described by parabolic curves with significant quadratic terms. This indicates that high diversity does not occur at the extremes of the observed ranges of nutrient ratios. This supports the RBH.     

Spatial variation in seed limitation of plant species richness and population sizes in floodplain tallgrass prairie

The relative importance of seed availability versus biotic interactions that affect early life stages in limiting plant population sizes and determining composition of plant communities is a central debate in plant ecology. We conducted a seed addition experiment in restored tallgrass prairie in central Kansas to determine (1) whether addition of seed of 18 native forb species produced persistent (three growing seasons) increases in the species' population sizes and plant species richness, (2) what properties of recipient communities best explained spatial variation in added species' establishment, and (3) whether seed size explained interspecific patterns in establishment success. Adding seed led to persistent increases in the number of added species present and in plant species richness at one of three sites. Increased species richness at the one site where community composition was structured by seed availability largely resulted from greater densities of four species. Seed size did not predict species' establishment success. Pre-existing plant species richness was correlated with added species' establishment success, but the direction of the relationship (positive vs. negative) varied among sites. Living aboveground plant biomass in experimental plots in the year of seed addition was negatively correlated with the number of added species established three years later. Our results provide further evidence for large spatial variation in seed limitation of plant community composition. Surprisingly, mean light availability and heterogeneity in light, both important parameters in conceptual models of grassland plant coexistence, did not predict the response of the recipient plant community to seed addition as well as pre-existing plant species richness and living aboveground biomass.     

Soil nutrients and variation in biomass rather than native species richness influence introduced plant richness in a semi-arid grassland

Several different hypotheses account for the success of introduced species in new environments. Experimental studies show a negative native-exotic richness relationship (NERR), while observational studies suggest that this relationship is usually positive. Increased resource availability and environmental variation can also enable introduced species to establish in new environments. We conducted an observational study in a semi-arid grassland in the Thompson-Nicola District of British Columbia to examine the biotic and abiotic factors that account for variation in introduced and native species richness.In each of 12 sites, an 8 × 8 m area was set up, containing 64, 1-m² plots. We identified and categorized plant species in each site into introduced and native species. We tested the relationship between introduced species richness and native species richness at the 1-m² sampling grain and at sampling grains up to 64 m². We also analysed the relationship between native and introduced species, and within-plot biomass, and between native and introduced species and variation in biomass. For a representative subset of four sites, we tested the relationship between introduced and native species richness and nitrogen, phosphorus and potassium.We found no NERR at the 1 m² sampling grain, nor for the other sampling grains up to 64 m². Introduced species richness increased with phosphorus and nitrogen availability, and was also positively related to biomass heterogeneity.Our results indicate that introduced species richness in these grasslands is likely influenced by phosphorus and nitrogen, and by variation in vegetation biomass, but not by native species. More non-native plants are likely to occupy nutrient-rich plots compared to nutrient-poor plots in these grasslands. Variation in biomass can leave gaps for the establishment of introduced species. These results should inform management considerations for the control of invasive species to optimize preservation of grasslands.     

Clonal plants in a spatially heterogeneous environment: effects of integration on Serengeti grassland response to defoliation and urine-hits from grazing mammals

It has been hypothesized that clonal integration between ramets inenvironments with spatially variable rates of herbivory and nutrientavailabilities leads to increased growth and fitness in the genet. An increasein genet growth could potentially influence ecosystem processes such as primaryproductivity and nutrient cycling. I tested the idea that clonal integrationwould lead to greater aboveground productivity, compensatory response todefoliation, and N re-distribution in a factorial experiment in fieldplots in the Serengeti Ecosystem, Tanzania. Each plot had either all stolonssevered or left intact, had repeated defoliation (to simulate grazing bythe African buffalo) or none, and was either located next to a plot thatreceived urea (to simulate a urine-hit) or next to a plot thatremained untreated. Plots that received stolon severing treatments had32% less peak biomass than did connected control plots, and this suggeststhat plants grew better when ramets remained connected. However, compensatoryresponse by plots to repeated defoliation was inconsistent with the hypothesizedbenefits of ramet integration at the ecosystem level: productivityresponse to defoliation was similar between plots with connected vs. severedramets (i.e. no stolon severing × defoliation interaction wasfound). When averaged across other treatments, defoliated plots hadincreased productivity compared to nondefoliated plots during the growingseason. Thus, ramet connection and defoliation did increase productivity whenthey were considered alone, but productivity response to defoliation wasunaffected by ramet connections. Urea additions, which led to a 78%increase in productivity in adjacent urine-hits, had noconsistent effect on productivity but did increase leaf percent N in adjoiningstudy plots. Thus, in the Serengeti, urine hits probably have very localizedeffects on productivity during the initial growing season.     

From competition to facilitation: how tree species respond to neighbourhood diversity

Studies on tree communities have demonstrated that species diversity can enhance forest productivity, but the driving mechanisms at the local neighbourhood level remain poorly understood. Here, we use data from a large‐scale biodiversity experiment with 24 subtropical tree species to show that neighbourhood tree species richness generally promotes individual tree productivity. We found that the underlying mechanisms depend on a focal tree's functional traits: For species with a conservative resource‐use strategy diversity effects were brought about by facilitation, and for species with acquisitive traits by competitive reduction. Moreover, positive diversity effects were strongest under low competition intensity (quantified as the total basal area of neighbours) for acquisitive species, and under high competition intensity for conservative species. Our findings demonstrate that net biodiversity effects in tree communities can vary over small spatial scales, emphasising the need to consider variation in local neighbourhood interactions to better understand effects at the community level.     

Experimental habitat fragmentation and invertebrate grazing in a herbaceous grassland species

A field experiment was conducted to examine the effects of habitat fragmentation on herbivore damage to individually tagged leaves of Betonica officinalis rosettes. Fragments of different size and corresponding control plots were established at three study sites in nutrient-poor calcareous grasslands in the northern Swiss Jura mountains. Leaf damage was recorded three times over the growing season (late spring, summer and early autumn). Five years after the initiation of the fragmentation, the density of rosettes did not differ between fragments and control plots. The number of leaves per rosette was higher in fragments than in control plots in summer but not in late spring and early autumn. The extent of leaf damage, expressed as proportion of leaf area removed by invertebrate herbivores, increased over the vegetation period. Leaf damage was greater in fragments than in control plots at two study sites, whereas the opposite (less strongly expressed) was found at the third site. Number of species and density (individuals per m²) of potential herbivores (gastropods and grasshoppers) were recorded in all fragments and control plots. Effects of fragmentation on the number of species and densities depended on plot size and differed between gastropods and grasshoppers. Leaf damage in fragments increased with increasing density of gastropods if the third site, which had lowest leaf damage, was excluded. Such a positive relationship was neither found in control plots nor for grasshopper densities. Thus, movement of gastropods in fragments was probably restricted which resulted in increased feeding pressure at least in two sites. However, even if our fragmentation experiment was well designed and replicated, the interpretation of these experimental results remains difficult because there was large site-to-site and seasonal variation.     

Effects of grazing exclusion on plant species richness and phytomass accumulation vary across a regional productivity gradient

Question: Does long‐term grazing exclusion affect plant species diversity? And does this effect vary with long‐term phytomass accumulation across a regional productivity gradient? Location: Lowland grassy ecosystems across the state of Victoria, southeast Australia. Methods: Floristic surveys and phytomass sampling were conducted across a broad‐scale productivity gradient in grazing exclusion plots and adjacent grazed areas. Differences in species richness, evenness and life‐form evenness between grazed and ungrazed areas were analysed. The environmental drivers of long‐term phytomass accumulation were assessed using multiple linear regression analysis. Results: Species richness declined in the absence of grazing only at the high productivity sites (i.e. when phytomass accumulation was >500 g m^?2). Species evenness and life‐form evenness also showed a negative relationship with increasing phytomass accumulation. Phytomass accumulation was positively associated with both soil nitrogen and rainfall, and negatively associated with tree cover. Conclusions: Competitive dominance is a key factor regulating plant diversity in productive grassy ecosystems, but canopy disturbance is not likely to be necessary to maintain diversity in less productive systems. The results support the predictions of models of the effects of grazing on plant diversity, such as the dynamic equilibrium model, whereby the effects of herbivory are context‐dependent and vary according to gradients of rainfall, soil fertility and tree cover.     

Modelling of rare plant species richness by landscape variables in an agriculture area in Finland

A multivariate linear regression model is proposed for predicting and mapping rare vascular plant species richness in Finnish agricultural landscapes according to landscape variables. The data used in developing the model were derived from a floristic inventory from 105 0.5 km × 0.5 km grid squares. Using a stepwise multiple regression technique, four landscape variables were found to explain 71.8% of the variability in the number of rare plant species. The results suggest that the local `hotspots' of rare plants (squares with 5 rare taxa) are mainly found in heterogeneous river valleys, where extensive semi-natural grasslands and herb-rich forests occur on the steep slopes. According to other similar studies, intermediate human disturbance increases the number of rare species in agricultural landscapes. It appears that empirical models based on landscape variables derived from digital maps can provide relatively accurate surrogates for extensive field surveys and fine-scale observations on the distributions of rare taxa in agricultural landscapes. Potential reasons for the performance of the model and the ecology and habitats of the species concerned are discussed.     

The effect of diversity and spatial arrangement on biomass of agricultural cultivars and native plant species

Various spatial arrangements can be found within natural grassland plant populations and communities. In contrast, spatial arrangement diversity is typically not observed in agroecosystems. Little is known about the influence of spatial arrangement on the productivity and success of agricultural or native plants. Such information is of interest to farmers who want to increase yield and restorationists working with native ecosystems. Agricultural and native plants were planted in two-way mixtures that included combinations of cultivars, species, genera, or functional groups. Each combination was arranged as a random mixture within rows, alternating rows of the different genotypes or species, and as alternating pairs of rows. Aboveground biomass was determined for each mixture component and compared to monoculture controls. Though plot composition had the most consistent influence on aboveground biomass, spatial arrangement appeared to have some influence among agricultural cultivars. Whether native or agricultural, biomass was greater for mixtures containing both a grass and a legume.     

Differentiation between native and exotic plant species from a dry grassland: fundamental responses to resource availability,and growth rates

Abstract Species from a mixed native/exotic dry grassland community in New Zealand were experimentally assessed for three ecophysiological parameters: nutrient response, water response and maximum relative growth rate (RGR_max). These are parameters that relate to factors proposed as important in structuring plant communities in dry environments. Native and exotic species did not differ consistently in water response. Exotic species tended to have a greater response to nutrients, but there was considerable overlap between native and exotic guilds. However, exotic species did have a higher intrinsic growth rate, and this effect was not attributable to differences in life histories. The results suggest that the exotic species are more competitive and more generalist than the native species. These traits are compatible with the concept of the ‘ideal invader’, and suggest the C‐R strategy of Grime’s theory. The native species showed characteristics consistent with stress tolerance (sensu Grime). The paucity of evidence for ecophysiological differentiation between the native and exotic guilds, except in intrinsic growth rate, indicates that the exotic species were able to invade not because they had superior adaptation to the physical environment, but because they possessed, by pre‐adaptation, the same ways of coping with that environment as the existing species. However, their ability to invade can be related to their growth rates.     

Differentiation between native and exotic plant species in a dry grassland: realized responses to perturbation,and comparison with fundamental responses

Abstract An area of dry grassland in New Zealand, comprising an equal mixture of native and exotic species, was subject to perturbations of irrigation, fertilization and cessation of grazing. The vegetation response was recorded for 3 years. Total cover, and the contribution of native species to that cover, fluctuated between years even in the control plots. Irrigation increased total cover, but decreased the cover of native species. Fertilization produced the same effects, only less strongly, and also reduced species richness, the loss being in native species. In spite of overall effects of treatments on native and exotic cover, when individual species’ responses to irrigation, fertilization or exclosure were calculated, there was no significant difference between the native and exotic plant guilds. Species differed in their responses, but the native and exotic guilds overlapped. When grouped by morphology, the only significant difference between the responses to perturbation was that forbs and graminoids responded more positively to irrigation than woody and cryptogamic species. The realized responses of the species to the perturbations described here showed little correlation with their physiological responses as determined in previous greenhouse experiments. It is suggested that the realized responses are strongly, and currently unpredictably, influenced by competition from the other species present. Soil nutrients and soil water were both important controls on the community. The relative similarity in the nature of the response to these two factors – nutrients and water – suggests that they affect species in similar ways, possibly because the greater growth rate of the exotic species mediates the short‐term response to both. Grazing has less effect on the current community than either nutrients or water, although it may have been historically important in shaping the species pool. From the poor predictability of field responses from morphological guilds or from ecophysiological responses, it is suggested that the ‘functional types’ approach, although conceptually attractive, lacks experimental support in these grasslands. It is concluded that the exotic species have invaded by being pre‐adapted to the environment with the same environmental responses as the natives, but with the advantage of generally higher growth rates.     

Effects of restoration with cattle grazing on plant species composition and richness of semi-natural grasslands

Restoration of semi-natural grasslands by cattle grazing is among the most practical options for reversing the decline of northern European floristic diversity, but no studies on this subject are available. In this work the success of restoration of abandoned, privately owned mesic semi-natural grasslands by farmers receiving support from the EU agri-environmental support scheme was studied in southwestern Finland. Three kinds of grasslands were compared: old (continuously cattle grazed), new (cattle grazing restarted 3–8 years ago) and abandoned pastures (grazing terminated >10 years ago). Plant species composition of the three pasture types was floristically different in multivariate analyses (non-metric multidimensional scaling). Total species richness, richness of grassland plants, indicator plants and rare plants were highest in old and lowest in abandoned pastures in all studied spatial scales (0.25–0.8 ha, 1 and 0.01 m²). The results were congruent with different scales and species list definitions, suggesting that species density scale (1 m²) can be used as a partial surrogate for large scale species richness. Species richness of new pastures was 20% higher on 0.25–0.8 ha, 40–50% higher on 1 m² and 30% higher on the 0.01 m² scale compared to abandoned grasslands. Rare species showed insignificant response to resumed grazing. Despite problems in management quality, this study showed promising results of restoration of abandoned grasslands by cattle grazing on private farms. However, populations of several rare grassland plants may not recover with present cattle grazing regimes. Management regulations in the agri-environmental support scheme need to be defined more precisely for successful restoration.