Oikos

doi: 10.1111/j.0030-1299.2007.15846.xpmid: N/A

Van Teeffelen, Astrid J.A.; Ovaskainen, Otso

doi: 10.1111/j.2006.0030-1299.15131.xpmid: N/A

Species distributions often show an aggregated pattern, which can be due to a number of endo‐ and exogenous factors. While autologistic models have been used for modelling such data with statistical rigour, little emphasis has been put on disentangling potential causes of aggregation. In this paper we ask whether it is possible to infer sources of aggregation in species distributions from a single set of occurrence data by comparing the performance of various autologistic models. We create simulated data sets, which show similar occupancy patterns, but differ in the process that causes the aggregation. We model the distribution of these data with various autologistic models, and show how the relative performance of the models is sensitive to the factor causing aggregation in the data. This information can be used when modelling real species data, where causes of aggregation are typically unknown. To illustrate, we use our approach to assess the potential causes of aggregation in data of seven bird species with contrasting statistical patterns. Our findings have important implications for conservation, as understanding the mechanisms that drive population fluctuations in space and time is critical for the development of effective management actions for long‐term conservation.

Thomsen, Meredith A.; D'Antonio, Carla M.

doi: 10.1111/j.2006.0030-1299.14929.xpmid: N/A

Resistance to the invasion of exotic plants may sometimes result from the strong effects of a relatively small number of resident species. Understanding the mechanisms by which such species resist invasion could provide important insights for the management of invaded ecosystems. Furthermore, the individualistic responses of community members to resource availability and environmental gradients could drive spatial variation in resistance at the local to landscape scales. We tested the resistance of monoculture plots of three native perennial grasses from the California coastal prairie to the invasion of the European perennial grass Holcus lanatus. We also used a watering treatment that increased early summer water availability and a natural elevational gradient in resource availability and soil texture to evaluate how resident identity interacted with abiotic resistance to affect Holcus establishment. Two native species, Festuca rubra and Calamagrostis nutkaensis, exhibited strong resistance, correlated with their negative effects on light availability. A third native grass, Bromus carinatus var. maritimus, had either no effect or a weakly facultative effect on Holcus performance relative to bare plots. Water addition did not alter the resistance of these species, but the elevation gradient did. Holcus invasion increased with improving abiotic conditions towards the slope bottom in bare and Bromus plots, but invasion decreased towards the bottom in Calamagrostis plots, where better conditions favored competitive residents. These results support the idea that resistance to invasion can sometimes be due to a subset of native species, and that the resistance provided by even a single species is likely to vary across the landscape. Identifying the mechanisms by which species resist invasion could facilitate the selection of management strategies that at best increase, or at worst do not decrease, natural resistance.

Richards, Lora A.; Coley, Phyllis D.

doi: 10.1111/j.2006.0030-1299.15043.xpmid: N/A

Herbivore populations are influenced by a combination of food availability and predator pressure, the relative contribution of which is hypothesized to vary across a productivity gradient. In tropical forests, treefall gaps are pockets of high productivity in the otherwise less productive forest understory. Thus, we hypothesize that higher light availability in gaps will increase plant resources, thereby decreasing resource limitation of herbivores relative to the understory. As a result, predators should regulate herbivore populations in gaps, whereas food should limit herbivores in the understory. We quantified potential food availability and compared arthropod herbivore and predator densities in large forest light gaps and in the intact understory in Panama. Plants, young leaves, herbivores and predators were significantly more abundant per ground area in gaps than in the understory. This pattern was similar when we focused on seven gap specialist plant species and 15 shade‐tolerant species growing in gaps and understory. Consistent with the hypothesis, herbivory rates were higher in gaps than the understory. Per capita predation rates on artificial caterpillars indicated higher predation pressure in gaps in both the dry and late wet seasons. These diverse lines of evidence all suggest that herbivores experience higher predator pressure in gaps and more food limitation in the understory.

Miller, Alison M.; McArthur, Clare; Smethurst, Philip J.

doi: 10.1111/j.2006.0030-1299.15331.xpmid: N/A

Consumption of a focal plant by herbivores depends, not only on the physical and chemical characteristics of that plant, but also on the characteristics of the neighbouring vegetation. Consumption of focal plants has been related to their own characteristics and to the quality of the neighbouring vegetation, but the two have not been combined to examine the relative importance of focal plant and neighbouring vegetation characteristics. We conducted a series of feeding trials to examine the relative importance of focal plant and various characteristics of neighbouring vegetation to browsing of a focal plant within vegetation patches. We planted Eucalyptus nitens seedlings of high and low nutrient status amongst vegetation patches differing in palatability, abundance and height. Generalist mammalian herbivores, red‐bellied pademelons (Thylogale billardierii), were allowed to feed in each of these patches one at a time, and seedling consumption was recorded. Results were considered in light of the attractant‐decoy and apparency hypotheses, which focus on the outcome to plants, and in terms of foraging theory, which is process‐focussed. Seedling and vegetation characteristics were both important. Seedlings of high nutrient status were preferred over those of low nutrient status. The relative quality, abundance and height of neighbouring vegetation all influenced browsing of a focal plant. Seedlings were more vulnerable amongst vegetation that was of low palatability, of low abundance, or was short. Seedling and vegetation effects were additive in two of three trials. Results were consistent with both the attractant‐decoy and apparency hypotheses, and could be explained in terms of maximising foraging efficiency. They demonstrate the need to consider characteristics of both the focal plant and its neighbouring vegetation when predicting the vulnerability of the former to browsing by generalist herbivores.

Pineda‐Krch, Mario; J. Blok, Hendrik; Dieckmann, Ulf; Doebeli, Michael

doi: 10.1111/j.2006.0030-1299.14940.xpmid: N/A

Periodic predator–prey dynamics in constant environments are usually taken as indicative of deterministic limit cycles. It is known, however, that demographic stochasticity in finite populations can also give rise to regular population cycles, even when the corresponding deterministic models predict a stable equilibrium. Specifically, such quasi‐cycles are expected in stochastic versions of deterministic models exhibiting equilibrium dynamics with weakly damped oscillations. The existence of quasi‐cycles substantially expands the scope for natural patterns of periodic population oscillations caused by ecological interactions, thereby complicating the conclusive interpretation of such patterns. Here we show how to distinguish between quasi‐cycles and noisy limit cycles based on observing changing population sizes in predator–prey populations. We start by confirming that both types of cycle can occur in the individual‐based version of a widely used class of deterministic predator–prey model. We then show that it is feasible and straightforward to accurately distinguish between the two types of cycle through the combined analysis of autocorrelations and marginal distributions of population sizes. Finally, by confronting these results with real ecological time series, we demonstrate that by using our methods even short and imperfect time series allow quasi‐cycles and limit cycles to be distinguished reliably.

Christensen, Søren; Bjørnlund, Lisa; Vestergård, Mette

doi: 10.1111/j.2006.0030-1299.15178.xpmid: N/A

Quantification of the organic carbon released from plant roots is a challenge. These compounds of rhizodeposition are quickly transformed into CO2 and eventually bacterial biomass to be consumed by bacterivores (protozoa and nematodes). Microbes stimulate rhizodeposition several‐fold so assays under sterile conditions give an unrealistic value. Quantifying bacterial production from 3H‐thymidine incorporation falls short in the rhizosphere and the use of isotopes does not allow clear distinction between labeled CO2 released from roots or microbes. We reduced rhizodeposition in 3–5 week old barley with a 2 week leaf aphid attack and found that biomass of bacterivores but not bacteria in the rhizosphere correlated with plant–induced respiration activity belowground. This indicated top‐down control of the bacteria. Moreover, at increasing density of aphids, bacterivore biomass in the rhizosphere decreased to the level in soil unaffected by roots. This suggests that difference in bacterivore biomass directly reflects variations in rhizodeposition. Rhizodeposition is estimated from plant‐induced increases in bacterial and bacterivore biomass, and yield factors, maintenance requirements, and turnover rates from the literature. We use literature values that maximize requirements for organic carbon and still estimate the total organic rhizodeposition to be as little as 4–6% of the plant‐induced respiration belowground.

Catenazzi, Alessandro; A. Donnelly, Maureen

doi: 10.1111/j.2006.0030-1299.15230.xpmid: N/A

How can terrestrial animals survive in a desert with scant primary productivity? The Peruvian coastal desert is hyper‐arid, but faces one of the world's most productive marine ecosystems, the Peru–Chile cold current. Given the stark difference in productivity between these adjacent ecosystems, we expected to find strong linkages connecting the terrestrial and marine food web. We investigated how marine resources are incorporated in the diet, and influence the distribution of terrestrial consumers (geckos, scorpions, solifuges and darkling beetles). Stomach contents from geckos, and δ13C and δ15N values of geckos and other terrestrial consumers suggest that marine green algae of the genus Ulva provide energy and nutrients to the terrestrial food web. Isotopic values suggest that amphipods, which feed on stranded Ulva, make marine resources available to terrestrial predators by moving between the intertidal and supratidal zones. The relative contribution of terrestrial and algal carbon sources varied among terrestrial predators, because scorpions assimilated a lower proportion of energy from Ulva than did geckos and solifuges. These δ13C patterns reflected differences in the spatial distribution of consumers. Our study supports the idea that in places where ecosystems with contrasting productivity levels are spatially juxtaposed, it is not possible to understand the structure and dynamics of food webs without taking into account the effects of energy and nutrients flowing from adjacent ecosystems. In contrast to other studied systems, especially those in Baja California, our site in Peru receives very little rainfall and the amount of precipitation is not affected by El Niño events. The near absence of rainfall promotes an extreme dependence of terrestrial consumers on marine resources, and causes permanent indirect food‐web effects that are affected by temporal variability in marine productivity, rather than temporal patterns of plant growth.

Losure, David A.; Wilsey, Brian J.; Moloney, Kirk A.

doi: 10.1111/j.2006.0030-1299.15341.xpmid: N/A

Experiments that have manipulated species richness with random draws of species from a larger species pool have usually found that invasibility declines as richness increases. These results have usually been attributed to niche complementarity, and interpreted to mean that communities will become less resistant to invaders as species go locally extinct. However, it is not clear how relevant these studies are to real‐world situations where species extinctions are non‐random, and where species diversity declines due to increased rarity (i.e. reduced evenness) without having local extinctions. We experimentally varied species richness from 1 to 4, and evenness from 0.44 to 0.97 with two different extinction scenarios in two‐year old plantings using seedling transplants in western Iowa. In both scenarios, evenness was varied by changing the level of dominance of the tall grass Andropogon gerardii. In one scenario, which simulated a loss of short species from Andropogon communities, we directly tested for complementarity in light capture due to having species in mixtures with dissimilar heights. We contrasted this scenario with a second set of mixtures that contained all tall species. In both cases, we controlled for factors such as rooting depth and planting density. Mean invader biomass was higher in monocultures (5.4 g m−2 week−1) than in 4‐species mixtures (3.2 g m−2 week−1). Reduced evenness did not affect invader biomass in mixtures with dissimilar heights. However, the amount of invader biomass decreased by 60% as evenness increased across mixtures with all tall species. This difference was most pronounced early in the growing season when high evenness plots had greater light capture than low evenness plots. These results suggest that the effect of reduced species diversity on invasibility are 1) not related to complementarity through height dissimilarity, and 2) variable depending on the phenological traits of the species that are becoming rare or going locally extinct.

Murphy, Helen T.; Lovett‐Doust, Jon

doi: 10.1111/j.2006.0030-1299.15050.xpmid: N/A

Ecological niche modeling has become an increasingly important tool to address issues in many fields of basic and applied ecology. The ecological niche space occupied across the geographic range, particularly for wide‐ranging species, may vary for a variety of evolutionary and non‐evolutionary reasons. However, ecological niche models are often applied over large geographic areas without regard for the potential effects of regional variation in adaptation, environmental conditions and their interactions, and species responses, thus significantly reducing their accuracy and utility. We develop regionally partitioned ecological niche models, using GARP, for the wide‐ranging North American tree Gleditsia triacanthos (Fabaceae). Models were constructed based on known tree occurrences at peripheral and range‐centre locations, as well as across the geographic range as a whole. Our results suggest that the niche space occupied by G. triacanthos varies regionally and that between some regions in particular there may be a complete absence of niche overlap. In particular, while there is some overlap between the niche space occupied by trees in the western and central regions of the range, there appears to be virtually no overlap in the niche space occupied by trees in the south of the range and that occupied by western and central trees. This lack of overlap appears to be driven primarily by regional differences in abiotic conditions, rather than regional adaptation per se. The results of our study have several important implications for the future development of habitat suitability models over large geographic areas. Spatial partitioning of data is clearly necessary to improve predictions of models where regional niche variation occurs. For wide‐ranging species in particular, regional differences in ecological characteristics may cause apparent niche variation.