Michael J. Crawley

Exotic plant invasions and the enemy release hypothesis

To curb the future economic and environmental impacts of invasive exotic species, we need to understand the mechanisms behind exotic invasions. One commonly accepted mechanism for exotic plant invasions is the enemy release hypothesis (ERH), which states that plant species, on introduction to an exotic region, experience a decrease in regulation by herbivores and other natural enemies, resulting in a rapid increase in distribution and abundance. The success of classical biological control has been used as support for ERH, but this observational evidence does not directly test ERH, and the more experimental evidence is equivocal. Competitive release through greater generalist enemy impact on natives seems to be an important but understudied mechanism of enemy release, but there is a serious need for experiments involving exclusion of natural enemies in invaded plant communities. With a clearer understanding of the role of enemy release in exotic plant invasions, we can begin to build a comprehensive predictive model of exotic plant invasions.

Noise and determinism in synchronized sheep dynamics

A major debate in ecology concerns the relative importance of intrinsic factors and extrinsic environmental variations in determining population size fluctuations. Spatial correlation of fluctuations in different populations caused by synchronous environmental shocks,, is a powerful tool for quantifying the impact of environmental variations on population dynamics,. However, interpretation of synchrony is often complicated by migration between populations,. Here we address this issue by using time series from sheep populations on two islands in the St Kilda archipelago. Fluctuations in the sizes of the two populations are remarkably synchronized over a 40-year period. A nonlinear time-series model shows that a high and frequent degree of environmental correlation is required to achieve this level of synchrony. The model indicates that if there were less environmental correlation, population dynamics would be much less synchronous than is observed. This is because of a threshold effect that is dependent on population size; the threshold magnifies random differences between populations. A refined model showsthat part of the required environmental synchronicity can be accounted for by large-scale weather variations. These results underline the importance of understanding the interaction between intrinsic and extrinsic influences on population dynamics.

Biodiversity and plant productivity in a model assemblage of plant species

We examined productivity as a function of biotic diversity. We manipulated plant species richness as an experimental factor to determine if productivity (net above ground primary productivity or NPP) is affected by changes in plant diversity (species richness). We constructed 164 assemblages that varied in species richness and measured their biomass at the end of one growing season. The plants were drawn from a pool of 16 species of self-pollinating annual herbs common to English weedy fields. On average, species-poor assemblages were less productive. Results also showed, however, that species-poor assemblages had wider ranges of possible productivities than more diverse assemblages.

Transgenic crops in natural habitats.

Although improved crop yields can be engineered by genetically modifying plants, there is ecological concern over whether these plants are likely to persist in the wild in the event of dispersal from their cultivated habitat. Here we present the results of a long-term study of the performance of transgenic crops in natural habitats. Four different crops (oilseed rape, potato, maize and sugar beet) were grown in 12 different habitats and monitored over a period of 10 years. In no case were the genetically modified plants found to be more invasive or more persistent than their conventional counterparts.

Population dynamics and plant community structure: competition between annuals and perrenials

A model of competition between an annual plant species invading only by seed, and a perennial species invading only by lateral growth, shows the conditions necessary for equilibrium coexistence of the two species. The annual can persist at equilibrium even when the perennial always wins in competition for a particular site, so long as the product of its fecundity and the equilibrium proportion of empty sites exceeds unity. The equilibrium proportion of empty sites depends upon the birth and death rates of perennial ramets. In stochastic simulations, the probability of invasion by the annual into an equilibrium community of perennials is lower than the probability of the annual itself surviving invasion by the perennial, even though the deterministic criteria for persistence are identical; that is, the probability that the annual will succeed in establishing itself depends on the size of the initial innoculum. Developments of the model to include more realistic assumptions about population dynamics, and extensions to multi-species cases are discussed. The model also has potential applications in the study of competition between annual and perennial modular invertebrates (e.g. on intertidal rock surfaces).

HERBIVORES AND PLANT DIVERSITY

We study spatial lottery models of competition between two plant species in which competitive ability is affected by levels of herbivory. Herbivory may enhance plant diversity in two qualitatively different ways. The first is global frequency dependence; the level of herbivory suffered by a plant decreases as the species becomes rare. Second, spatial variability in levels of herbivory can create ephemeral, local refuges for herbivory for each plant species. Both of these mechanisms operate only if there is not a negative correlation between a plants palatability and its competitive ability. Both mechanisms also require that herbivores have sufficiently strong diet preferences (or, equivalently, that the plants have sufficiently different grazing tolerances). If there is no relationship between palatability and competitive ability, then plant diversity is a monotonically increasing function of the herbivores degree of monophagy. In contrast, if there is a positive correlation between palatability and competitive ability, then the diversity/degree-of-monophagy relationship may be either monotonically increasing or humped.

Seed limitation and the dynamics of feral oilseed rape on the M25 motorway

Feral oilseed rape populations in 3658 quadrats (each of 100 m) on the verges of the M25 motorway were studied for two years. There was substantial turnover in site occupancy; 55 % of quadrats had a different population density in 1994 than in 1993; 53% of the quadrats occupied in 1993 were locally extinct in 1994; and 20% of the empty quadrats in 1993 were occupied by oilseed rape in 1994. Verges next to the carriageway carrying traffic towards the main rape seed crushing plant at Erith in Kent had significantly more plants than the opposite verge carrying traffic away from Erith. Mean rape densities were also higher in the vicinity of exit and entry slip roads than on sections of verge between motorway junctions, but densities were not affected by the presence of rape crops or rape volunteers in adjacent fields. The apparent permanence of oilseed rape populations on motorway verges clearly belies substantial turnover in patch occupancy. In the absence of soil disturbance, rapid secondary succession (principally the growth of perennial grasses) tends to lead to local extinction within three years. Given sufficient soil disturbance, however, rape population density appears to be seed limited, and seed spillage can cause a two- to fivefold increase in mean population density.

Rabbit grazing, plant competition and seedling recruitment in acid grassland

(1) Two experiments were done to determine the effect of grazing by rabbits on plant recruitment in mature grassland and on cultivated soil. The first was a factorial experiment, with and without rabbit fencing, and with and without soil cultivation, carried out between 1986 and 1989 in acid grassland with a long history of rabbit grazing. In the second, carried out between 1982 and 1985 in two contrasting arable fields, rabbits grazed crops of winter wheat, with fencing enclosures erected at different times and for different durations. (2) Rabbit grazing affected the stature and composition of the grassland throughout the year. Species that increased in cover in rabbit-grazed grassland included the grass Anthoxanthum odoratum and the forb Rumex acetosella. Species that decreased included the grasses Festuca rubra and Agrostis capillaris and the forbs Vicia sativa and Trifolium repens. There was negligible change in the total number of plant species with grazing. (3) The cultivated treatments showed that the seed bank under the acid grassland was extremely heterogeneous. (4) Eight of the twenty-three commonest ruderal species beneath acid grassland decreased in cover significantly (e.g. Capsella bursa-pastoris and Papaver dubium), and none increased significantly. In contrast, most of the ruderal species in the seed bank of nearby arable soils increased under grazing. (5) Most plant species on cultivated grassland soils regenerated from vegetative fragments (e.g. Holcus mollis, Stellaria graminea, Rumex acetosa and R. acetosella), rather than by germination of seed. Regrowth shoots outnumbered seedlings by a factor of between 3 and 23 3. (6) The extent of spatial heterogeneity within and between plots in both the composition of the buried seed bank, and in recruitment by vegetative regrowth, highlights the need for large sample sizes in this kind of study. (7) The mechanisms that determine whether a plant species increases or decreases under grazing are discussed.

Growth, reproduction and population dynamics

The relationship between individual resource use and fitness is fundamental to an understanding of population dynamics.It has been demonstrated that,in theory,reproductive thresholds have destabilizing effects on population dynamics.We review the published size-fecundity schedules and demonstrate a fundamental difference between plants and animal.Most animal schedules have large,significant thresholds for reproduction,but few plant species exhibit size-fecundity relationships that have negative intercepts.This leads to the prediction that plant populations will generally have stable dynamics whereas animal populations will display the full repertoire of dynamical behaviour

Rodent seed predation and seedling recruitment in mesic grassland

Abstract Seedling recruitment of two grasses (Arrhenatherum elatius and Festuca rubra) and two herbs (Centaurea nigra and Rumex acetosa) was measured in areas with and without rodents to which seeds of each species were sown at three seed densities (1000, 10,000 and 50,000 seeds m−2) in two seasons (spring and autumn 1995). Seed removal was measured for 10-day periods and the fate of seedlings was followed for 15 months after sowing. The proportion of seed removed ranged from 6 to 85% and increased with increasing seed density for each species. Rodents had no effect on seedling emergence or survival in the spring sowing. In the autumn sowing, rodents reduced seedling emergence of all four species sown at 1000 and 10,000 seeds m−2 but had no impact at 50,000 seeds m−2, presumably because of microsite limitation. We suggest the difference between spring and autumn arose because emergence was seed limited in autumn but microsite limited in spring; microsite availability was higher in autumn because a summer drought killed plants, reduced plant biomass and opened up the sward. Fifteen months after the autumn sowing, fewer A. elatius and C. nigra seedlings survived on plots exposed to rodents. This result reflected not only the reduced seedling emergence but also increased seedling mortality (seedling herbivory) in sites exposed to rodents. In contrast, F. rubra and R.acteosa showed density-dependent seedling survival which compensated for initial differences in seedling emergence, so that no effect of rodents remained after 15 months. The results suggest that rodent seed predation and seedling herbivory exert strong effects on seedling recruitment of A.elatius and C. nigra when recruitment conditions are favourable (conditions that lead to high microsite availability) and may contribute to both species being maintained at low densities in the grassland. The results also demonstrate that highly significant impacts of rodent seed predation at the seedling emergence stage can disappear by the time of plant maturation.