Hans de Kroon

ELASTICITIES: A REVIEW OF METHODS AND MODEL LIMITATIONS

Elasticity is a perturbation measure in matrix projection models that quantifies the proportional change in population growth rate as a function of a proportional change in a demographic transition (growth, survival, reproduction, etc.). Elasticities thus indicate the relative “importance” of life cycle transitions for population growth and maintenance. In this paper, we discuss the applications of elasticity analysis, and its extension, loop analysis, in life history studies and conservation. Elasticity can be interpreted as the relative contribution of a demographic parameter to population growth rate. Loop analysis reveals the underlying pathway structure of the life cycle graph. The different kinds of results of the two analyses in studies of life histories are emphasized. Because elasticities quantify the relative importance of life cycle transitions to population growth rate, it is generally inferred that management should focus on the transitions with the largest elasticities. Such predictions based on elasticities seem robust, but we do identify three situations where problems may arise. The mathematical properties and biological constraints that underlie these pitfalls are explained. Examples illustrate the additional information that needs to be taken into account for a sensible use of elasticities in population management. We conclude by indicating topics that are in need of research.

VEGETATION PATTERN FORMATION IN SEMI-ARID GRAZING SYSTEMS

Hypotheses about the origin of vegetation pattern formation in semi-arid areas around the world almost all include a common feature of semi-arid areas: the presence of a positive feedback between plant density and water infiltration. We investigate whether this positive feedback and the spatial redistribution of runoff water are sufficient to explain vegetation pattern formation. For this purpose, we analyze a spatially explicit model con- sisting of partial differential equations using a method for demonstrating pattern formation (Turing analysis). Our analysis reveals that pattern formation can occur in semi-arid areas given only the positive feedback between plant density and local water infiltration coupled with the spatial redistribution of runoff water. Thus, slope and underlying heterogeneity are not essential conditions. Other factors in the model, such as herbivory, plant dispersal, rainfall, and drought tolerance of plants, appear to determine under what conditions pattern formation is likely but are not the primary factors that generate the patterns. The model is in agreement with field observations and indicates the conditions for which vegetation pattern formation can be expected in arid and semi-arid grazing systems.

Declines in insectivorous birds are associated with high neonicotinoid concentrations

Recent studies have shown that neonicotinoid insecticides have adverse effects on non-target invertebrate species. Invertebrates constitute a substantial part of the diet of many bird species during the breeding season and are indispensable for raising offspring. We investigated the hypothesis that the most widely used neonicotinoid insecticide, imidacloprid, has a negative impact on insectivorous bird populations. Here we show that, in the Netherlands, local population trends were significantly more negative in areas with higher surface-water concentrations of imidacloprid. At imidacloprid concentrations of more than 20 nanograms per litre, bird populations tended to decline by 3.5 per cent on average annually. Additional analyses revealed that this spatial pattern of decline appeared only after the introduction of imidacloprid to the Netherlands, in the mid-1990s. We further show that the recent negative relationship remains after correcting for spatial differences in land-use changes that are known to affect bird populations in farmland. Our results suggest that the impact of neonicotinoids on the natural environment is even more substantial than has recently been reported and is reminiscent of the effects of persistent insecticides in the past. Future legislation should take into account the potential cascading effects of neonicotinoids on ecosystems.

Soil nutrient heterogeneity alters competition between two perennial grass species

Differences in root foraging behavior between species have been well documented, but the effects of these differences on belowground competitive ability are only beginning to be studied. Here we report the results of a competition experiment in homogeneous and heterogeneous soils between two species that differ in their ability to acquire nutrients from patchy environments. The perennial grasses Festuca rubra and Anthoxanthum odoratum have comparable growth rates, but results of previous studies with isolated plants designated the latter species as the more effective forager, probably due to its higher physiological plasticity (stronger increase in nutrient uptake rates per unit root mass in enriched nutrient patches). We introduced nutrient soil heterogeneity at two spatial scales. In a fine-grained heterogeneous treatment, the nutrient-rich patches were smaller and more concentrated than in a coarse-grained heterogeneous (checkerboard) treatment. Overall, the level of nutrient availability in these hete...

Effects of nutrients and shade on tree‐grass interactions in an East African savanna

Savanna trees have a multitude of positive and negative effects on understorey grass production. but little is known about how these effects interact. We report on a fertilization and shading experiment carried out in a Tanzanian tropical city savanna around Acacia tortilis trees. In two years of study there was no difference in grass production under tree canopies or in open grassland, Fertilization, however, indicate that trees do affect the nutrient limitation of the grass layer with an N-limited system in open grassland to a P-limited system under the trees. The NT ratios of grass gave a reliable indication of the nature of nutrient limitation, but only when assessed at the end of the wet season, Mid-wet season nutrient concentrations of grasses were higher under than outside the tree canopy, suggesting that factors other than nutrients limit grass production. A shading experiment indicated that light may be such a limiting factor during the wet season when water and nutrients are sufficiently available. However, in the dry season when water is scarce, the effect of shade on plant production became positive. We conclude that whether trees increase or decrease production of the herbaceous layer depends on how positive effects (increased soil fertility) and negative effects (shade and soil water availability) interact and that these interactions may significantly change between wet and dry seasons.

The evolution of the worldwide leaf economics spectrum

The worldwide leaf economic spectrum (WLES) is a strikingly consistent pattern of correlations among leaf traits. Although the WLES effectively summarizes variation in plant ecological strategies, little is known about its evolution. We reviewed estimates of natural selection and genetic variation for leaf traits to test whether the evolution of the WLES was limited by selection against unfit trait combinations or by genetic constraints. There was significant selection for leaf traits on both ends of the WLES spectrum, as well as significant genetic variation for these traits. In addition, genetic correlations between WLES traits were variable in strength and direction. These data suggest that genetic constraints have had a smaller role than selection in the evolution of the WLES.

A modular concept of plant foraging behaviour: the interplay between local responses and systemic control

In this paper we examined the notion that plant foraging for resources in heterogeneous environments must involve: (1) plasticity at the level of individual modules in reaction to localized environmental signals; and (2) the potential for modification of these responses either by the signals received from connected modules that may be exposed to different conditions, or by the signals reflecting the overall resource status of the plant. A conceptual model is presented to illustrate how plant foraging behaviour is achieved through these processes acting in concert, from the signal reception through signal transduction to morphological or physiological response. Evidence to support the concept is reviewed, using selective root placement under nutritionally heterogeneous conditions and elongation responses of stems and petioles to shade as examples. We discussed how the adoption of this model can promote understanding of the ecological significance of foraging behaviour. We also identified a need to widen the experimental repertoires of both molecular physiology and ecology in order to increase our insight into both the regulation and functioning of foraging responses, and their relationship with the patterns of environmental heterogeneity under which plants have evolved.

More than 75 percent decline over 27 years in total flying insect biomass in protected areas

Global declines in insects have sparked wide interest among scientists, politicians, and the general public. Loss of insect diversity and abundance is expected to provoke cascading effects on food webs and to jeopardize ecosystem services. Our understanding of the extent and underlying causes of this decline is based on the abundance of single species or taxonomic groups only, rather than changes in insect biomass which is more relevant for ecological functioning. Here, we used a standardized protocol to measure total insect biomass using Malaise traps, deployed over 27 years in 63 nature protection areas in Germany (96 unique location-year combinations) to infer on the status and trend of local entomofauna. Our analysis estimates a seasonal decline of 76%, and mid-summer decline of 82% in flying insect biomass over the 27 years of study. We show that this decline is apparent regardless of habitat type, while changes in weather, land use, and habitat characteristics cannot explain this overall decline. This yet unrecognized loss of insect biomass must be taken into account in evaluating declines in abundance of species depending on insects as a food source, and ecosystem functioning in the European landscape.

Loop Analysis: Evaluating Life History Pathways in Population Projection Matrices

A new way of analyzing life histories is presented, based on the use of elasticities as derived from the limit properties of population projection matrices. In general there is not a simple relation between life history traits and matrix elements and their corresponding elasticity values. By recognizing the fact that a life history trait always is part of a life history pathway that can be contrasted to other pathways, this problem can be overcome. The various life history pathways form loops within the life cycle graph, the graphical representation of the projection matrix. It is shown that one can calculate the elasticities of such loops from the single elasticity values in the matrix in an unequivocal way. This presents a new and meaningful decomposition of total elasticity in population projection matrices and provides a potentially powerful tool to evaluate the importance of alternative life history options in demographic studies. This point is demonstrated using two previously published sets of data.

Size hierarchies of shoots and clones in clonal herb monocultures: do clonal and non-clonal plants compete differently?

In order to test whether predictions of mortality and size hierarchy development, as derived from studies of non-clonal plants, are valid for clonal plants, an experiment was carried out with Brachypodium pinnatum and Carex flacca, two rhizomatous herbs, planted at three densities. Competition was studied at two levels of organization, the shoot (ramet) and the clone (the individual plant consisting of all interconnected ramets). Size inequalities (measured as Gini coefficients), size-dependent growth and mortality of both shoots and clones were analysed over a period of two years. The species were grown in monocultures in three treatments that differed in clone density. At the end of both the first and the second year of the experiment there were not significant differences in Gini coefficients of shoot height or weight between treatments (...)