Mick Crawley

Pathogens and the structure of plant communities

There is increasing interest in the direct and indirect effects of pathogens and parasites on the structure of plant communities. The direct influence of pathogens is seen in cases where pathogens reduce the populations of adult and seedling plants or prevent the recruitment of seeds. Indirect effects occur when epidemic outbreaks or endemic parasites of herbivores lead to reductions in grazing pressure, which temporarily allow plants to escape from the detrimental effects of their herbivores. In both cases, the presence of pathogens can lead to changes in the relative abundance of the species in a plant community, which are discernible for many years after the initial disease outbreak has passed.

Determinants of Species Richness in the Park Grass Experiment

The Park Grass Experiment at Rothamsted in southeast England was started in 1856, making it the longest‐running experiment in plant ecology anywhere in the world. Experimental inputs include a range of fertilizers (nitrogen, phosphorus, potassium, and organic manures) applied annually, with lime applied occasionally, and these have led to an increase in biomass and, where nitrogen was applied in the form of ammonium sulfate, to substantial decreases in soil pH. The number of species per plot varies from three to 44 per 200 m2, affording a unique opportunity to study the determinants of plant species richness and to estimate the effect sizes attributable to different factors. The response of species richness to biomass depends on the amount and type of nitrogen applied; richness declined monotonically with increasing biomass on plots receiving no nitrogen or receiving nitrogen in the form of sodium nitrate, but there was no relationship between species richness and biomass on plots acidified by ammonium sulfate application. The response to lime also depended on the type of nitrogen applied; there was no relationship between lime treatment and species richness, except in plots receiving nitrogen in the form of ammonium sulfate, where species richness increased sharply with increasing soil pH. The addition of phosphorus reduced species richness, and application of potassium along with phosphorus reduced species richness further, but the biggest negative effects were when nitrogen and phosphorus were applied together. The analysis demonstrates how multiple factors contribute to the observed diversity patterns and how environmental regulation of species pools can operate at the same spatial and temporal scale as biomass effects.

RAINFALL, BIOMASS VARIATION, AND COMMUNITY COMPOSITION IN THE PARK GRASS EXPERIMENT'

We used data on grassland plant community composition over a 90-yr period in the history of the Park Grass Experiment, England to look for relationships between variation in composition and annual variation in rainfall and biomass. This was investigated by regressions of biomass and rainfall on each other, and of these variables separately on each of three different measures of variation in plant community composition. Two of these measures, principal components analysis scores based on variation in species abun- dance and the ratio by mass of nongrass/grass species, showed significant relationships with biomass variation on many experimental plots, although relationships with rainfall were relatively slight or nonexistent. The third measure employed similarity indices to detect changes in species composition in response to variation in biomass, but failed to find any. Biomass was significantly increased by rainfall on all plots. We propose that variation in community composition was more closely related to biomass variation than to rainfall because rainfall selectively favored the grasses in the community, which we believe com- peted asymmetrically (for light) with the other species when rainfall was high. The severity of this competition would depend upon biomass more directly than upon rainfall, although it is rainfall that enhanced grass growth. In effect, asymmetric competition magnified the effect of rainfall on community composition.

community stability - a 60-year record of trends and outbreaks in the occurrence of species in the park grass experiment

1 A 60-year time-series of species recorded in visual surveys of the plant communities of the Park Grass Experiment is analysed to detect changes through time in the frequency of species on seven plots with acidified soil and on 35 non-acidified plots. 2 Of 14 species recorded on the acidified plots, eight decreased with time, one ( Agrostis capillaries ) increased, four showed no trend and one ( Chamerion angustifolium ) showed an outbreak with a peak in 1946. Of the 43 species recorded on the nonacidified plots, six increased, five decreased, 10 showed outbreaks and 22 showed no trend. 3 We used discriminant analysis to try to identify combinations of seven life-history and two habitat variables that would correctly classify species according to how their frequency changed on the non-acidified plots. Habitat variables (mean pH and mean hay yield of plots) were poor discriminators, but some life history variables (notably ruderalness, mating system and flowering time) were more successful. 4 Species which increased were more outcrossing whereas outbreak species were more selfing than the average for all species, both were more ruderal than average. We speculate that the ruderal species were all well equipped to spread across plots, but that only outcrossing species possessed sufficient genetic variation to be able to sustain a broad distribution in the heterogeneous environment represented by the PGE. 5 In view of the otherwise stable nature of the Park Grass communities we suggest that the existence of outbreaks in a significant number of species calls for a reevaluation of the concept of the stable plant community.

ESTIMATING THE FUNCTIONAL FORM FOR THE DENSITY DEPENDENCE FROM LIFE HISTORY DATA

Two contrasting approaches to the analysis of population dynamics are currently popular: demographic approaches where the associations between demographic rates and statistics summarizing the population dynamics are identified; and time series approaches where the associations between population dynamics, population density, and environmental covariates are investigated. In this paper, we develop an approach to combine these methods and apply it to detailed data from Soay sheep (Ovis aries). We examine how density dependence and climate contribute to fluctuations in population size via age- and sex-specific demographic rates, and how fluctuations in demographic structure influence population dynamics. Density dependence contributes most, followed by climatic variation, age structure fluctuations and interactions between density and climate. We then simplify the density-dependent, stochastic, age-structured demographic model and derive a new phenomenological time series which captures the dynamics better than previously selected functions. The simple method we develop has potential to provide substantial insight into the relative contributions of population and individual-level processes to the dynamics of populations in stochastic environments.

Testing the distinctness of shoot ionomes of angiosperm families using the Rothamsted Park Grass Continuous Hay Experiment

• The ionome is the elemental composition of a tissue or organism. Phylogenetic variation in the ionomes of plant shoots has been widely reported based on controlled experiments, vegetation surveys and literature meta-analyses. However, environmental effects on phylogenetic variation in shoot ionomes have not been quantified. This study tests the hypothesis that phylogenetic variation in shoot ionomes is robust to environmental perturbation and that plant families can be distinguished by their shoot ionomes. • Herbage was sampled from six subplots of the Rothamsted Park Grass Experiment. Subplots had received contrasting fertilizer treatments since 1856. Herbage was separated into its constituent species (n = 21) and concentrations of eleven mineral elements were determined in dried shoot material. • Shoot concentrations of calcium (Ca), zinc (Zn), manganese (Mn), magnesium (Mg) and sodium (Na) showed significant variation associated with plant species, and responded similarly to fertilizer treatments in diverse plant species. Species × treatment interactions were indicated for phosphorus (P), potassium (K), nickel (Ni), copper (Cu) and iron (Fe). Plant families could be distinguished by their shoot ionomes. The most informative elements for discriminant analysis were Ca > Mg > Ni > S > Na > Zn > K > Cu > Fe > Mn > P. • Whilst shoot ionomes were sensitive to fertilizer treatment, phylogenetic variation in a subset of the shoot ionome (Ca, Zn, Mn, Mg) was robust to this environmental perturbation.

APPLICATION OF THE BRITISH NATIONAL VEGETATION CLASSIFICATION TO THE COMMUNITIES OF THE PARK GRASS EXPERIMENT THROUGH TIME

The Park Grass Experiment at Rothamsted consists of a series of fertilizer treatments which have been applied to a species-rich hay meadow annually since 1856. Detailed botanical analyses of the plots between 1862 and 1992 were classified to one of the National Vegetation Classification (NVC) communities using the similarity measured by Czekanowski coefficient using computer program MATCH. Results indicated that “control” unfertilized plots were a relatively stableCynosurus cristatus-Centaurea nigra dicotyledon-rich grassland. However mineral fertilized plots moved towards a species-richArrhenatherum elatius grassland MG1e after 50–80 years and remained there. Plots receiving nitrogen fertilizer moved first to MG1e then on to aLolium perenne-Alopecurus pratensis grassland MG7D. Perhaps the most interesting plot was one which received a low rate of the soil acidifying ammonium sulphate fertilizer; the species composition changed towards a MG7D (Lolium perenne-Alopecurus pratensis) community between 1856 and 1903 but since then a liming treatment has been applied to part of the plot and the species composition on that part has recovered to a MG5 community resembling the control plots, taking 70–90 years to do so.

Possible Impacts of the Invasive Plant Rubus niveus on the Native Vegetation of the Scalesia Forest in the Galapagos Islands

Originally from Asia, Rubus niveus has become one of the most widespread invasive plant species in the Galapagos Islands. It has invaded open vegetation, shrubland and forest alike. It forms dense thickets up to 4 m high, appearing to displace native vegetation, and threaten the integrity of several native communities. This study used correlation analysis between a R. niveus cover gradient and a number of biotic (vascular plant species richness, cover and vegetation structure) and abiotic (light and soil properties) parameters to help understand possible impacts in one of the last remaining fragments of the Scalesia forest in Santa Cruz Island, Galapagos. Higher cover of R. niveus was associated with significantly lower native species richness and cover, and a different forest structure. Results illustrated that 60% R. niveus cover could be considered a threshold for these impacts. We suggest that a maximum of 40% R. niveus cover could be a suitable management target.

Management of the Invasive Hill Raspberry (Rubus niveus) on Santiago Island, Galapagos: Eradication or Indefinite Control?

Abstract The eradication of an invasive plant species can provide substantial ecological and economic benefits by eliminating completely the negative effects of the weed and reducing the high cost of continuing control. A 5-yr program toward the eradication of hill raspberry (Rubus niveus Thunb.) in Santiago Island is evaluated using delimitation and extirpation criteria, as well as assessment of the ecological community response to management techniques. Currently, hill raspberry is located in the humid zone of Santiago island. It is distributed over three main infestations, small patches, and many scattered individuals within an area of approximately 1,000 ha. New infestations are constantly being found; every year, new detections add an area of approximately 175 ha. Adult and juvenile individuals are still found, both beyond and within known infestations. Both plant and seed bank density of hill raspberry decreased over time where infestations were controlled. Species composition in the seed bank and existing vegetation were significantly different between areas under intensive control and adjacent uninvaded forest. After 5 yr of intensive management, delimitation of hill raspberry has not been achieved; new populations are found every year, increasing the infested area that requires management. Off-target effects on native species resulting from control efforts seem to be substantial. Although a vast increase in economic investment would allow intensive searching that might enable all individuals to be found and controlled, the resultant disturbance and off-targets effects could outweigh the conservation benefits of eradication. Nomenclature: Hill raspberry, Rubus niveus Thunb. Management Implications: During the last two decades, the Galapagos National Park Directorate and the Charles Darwin Foundation have been carrying out control and eradication programs to restore natural areas that have been degraded by invasive plant species. Control has proved to be costly and perennial and at best has slowed the spread of invasives. Similarly, eradication has had limited success, with only four (targeting weeds with small distributions and transient seed banks) out of 30 programs reaching the goal. Hence, there is an obvious need to evaluate the current 5-yr eradication program targeting hill raspberry in Santiago Island. Eradication does not seem feasible with the current search methodology, primarily because of the failure to find all plants before they fruit. Therefore, one of the fundamental requirements for successful eradication is not being met. Increase in the frequency and extent of search and control operations would be needed to meet this requirement, with all known and potential sites being visited at 4- to 6-month intervals to prevent fruit production. Although larger infestations can be seen from a helicopter, the discovery of all individual plants would require cutting a very closely spaced network of paths through nearly impenetrable low spiny forest. This more effective search methodology would need to be implemented and maintained until the elimination of the seed bank has been achieved (a minimum of 4 yr and up to 10 yr). It is estimated that it would cost USD 10 million (USD 1 million yr−1) over 10 yr to achieve eradication, a 6.7-fold increase in investment from the current level. Additionally, although herbicide control successfully kills individual plants, it has also affected natural vegetation, and opening a tight network of tracks would also cause concomitant disturbance. If the increased investment is not available or the off-target effects are considered greater than the benefits of eradication, or both, alternative objectives and methodologies (e.g., biocontrol) must be considered for the management of hill raspberry.

Evidence for constraint on species coexistence in vegetation of the Park Grass experiment

Repeated patterns, of a type that would be expected to result from limitations to species coexistence (i.e. ‘assembly rules’) were sought in the Park Grass experiment. This classical grassland experiment was sampled in two years, using replicated biomass samples. Variance in a number of measures was examined, and compared to the variance expected under appropriate null models, the latter based on assumptions of no interactions between species. In each case, an assembly rule would result in low variance. Examining variance in species richness between quadrats within a treatment, there was no indication of constraint on species co-occurrences; variance in richness was actually greater than expected under the null model, attributable to environmental variation or perhaps positive interactions between species. However, there was control on biomass, evidenced by variance in total biomass (i.e. over all species) within a treatment being significantly lower than expected under the null model. There was no indication of community structure based on guilds (i.e. functional types). Although there was in 1991 some, non-significant, indication of a constant proportion of species from the legume guild, there was no sign of such an effect in 1992. Searches for intrinsic guilds failed to converge. There was no indication at all of constancy in the proportional representation of guilds by biomass. Thus, there is good evidence for competitive control on plant growth, but none for control of species occurrences. There is no convincing evidence for guild structure in this community at the scale sampled. Possible conflict is discussed between the existence of evidence for temporal stability but the absence of evidence for spatial uniformity. It is concluded that most of the mechanisms proposed for temporal stability will not necessarily lead to control on spatial variation. For many mechanisms, this would depend on the spatial scale examined.