Pulsed resources and community dynamics of consumers in terrestrial ecosystems

Short-term transformation of matrix into hospitable habitat facilitates gene flow and mitigates fragmentation

Habitat fragmentation has major implications for demography and genetic structure of natural plant and animal populations as small and isolated populations are more prone to extinction. Therefore, many recent studies focus on spatial fragmentation. However, the temporal configuration of suitable habitat may also influence dispersal and gene flow in fragmented landscapes. We hypothesize that short-term switching of inhospitable matrix areas into suitable habitat can mitigate effects of spatial fragmentation in natural and seminatural ecosystems. To test our hypothesis, we investigated the hairy-footed gerbil (Gerbillurus paeba, Smith 1836), a ground-dwelling rodent, in fragmented Kalahari savannah areas. Here, rare events of high above mean annual rainfall suggest short-term matrix suitability. During the field survey in 'matrix' areas in the Kalahari (shrub encroachment by heavy grazing) we never observed the hairy-footed gerbil in years of average rainfall, but observed mass occurrences of this species during rare events of exceptionally high rainfall. In a second step, we developed an agent-based model simulating subpopulations in two neighbouring habitats and the separating matrix. Our mechanistic model reproduces the mass occurrences as observed in the field and thus suggests the possibly underlying processes. In particular, the temporary improvement in matrix quality allows reproduction in the matrix, thereby causing a substantial increase in population size. The model demonstrates further how the environmental trigger (rainfall) impacts genetic connectivity of two separated subpopulations. We identified seasonality as a driver of fragmentation but stochasticity leading to higher connectivity. We found that our concept of temporal fragmentation can be applied to numerous other fragmented populations in various ecological systems and provide examples from recent literature. We conclude that temporal aspects of fragmentation must be considered in both ecological research and conservation management.

Indirect effects of prey swamping: differential seed predation during a bamboo masting event

Resource pulses often involve extraordinary increases in prey availability that "swamp" consumers and reverberate through indirect interactions affecting other community members. We developed a model that predicts predator-mediated indirect effects induced by an epidemic prey on co-occurring prey types differing in relative profitability/preference and validated our model by examining current-season and delayed effects of a bamboo mass seeding event on seed survival of canopy tree species in mixed Patagonian forests. The model shows that predator foraging behavior, prey profitability, and the scale of prey swamping influence the character and strength of short-term indirect effects on various alternative prey. When in large prey-swamped patches, nonselective predators decrease predation on all prey types. Selective predators, instead, only benefit prey of similar quality to the swamping species, while very low or high preference prey remain unaffected. Negative indirect effects (apparent competition) may override such positive effects (apparent mutualism), especially for highly preferred prey, when prey-swamped patches are small enough to allow predator aggregation and/or predators show a reproductive numerical response to elevated food supply. Seed predation patterns during bamboo (Chusquea culeou) masting were consistent with predicted short-term indirect effects mediated by a selective predator foraging in large prey-swamped patches. Bamboo seeds and similarly-sized Austrocedrus chilensis (ciprés) and Nothofagus obliqua (roble) seeds suffered lower predation in bamboo flowered than nonflowered patches. Predation rates on the small-seeded Nothofagus dombeyi (coihue) and the large-seeded Nothofagus alpina (rauli) were independent of bamboo flowering. Indirect positive effects were transient; three months after bamboo seeding, granivores preyed heavily upon all seed types, irrespective of patch flowering condition. Moreover, one year after bamboo seeding, predation rates on the most preferred seed (rauli) was higher in flowered than in nonflowered patches. Despite rapid predator numerical responses, short-term positive effects can still influence community recruitment dynamics because surviving seeds may find refuge beneath the litter produced by bamboo dieback. Together, our theoretical analysis and experiments indicate that indirect effects experienced by alternative prey during and after prey-swamping episodes need not be universal but can change across a prey quality spectrum, and they critically depend on predator-foraging rules and the spatial scale of swamping.

Cohort effects in red squirrels: the influence of density, food abundance and temperature on future survival and reproductive success

1. Environmental conditions experienced early in life may have long-lasting effects on individual performance, thereby creating 'silver-spoon effects'. 2. We used 15 years of data from a North American red squirrel (Tamiasciurus hudsonicus Erxleben) population to investigate influences of food availability, density and spring temperature experienced early in life on reproduction and survival of female squirrels during adulthood. 3. We found that spring temperature and food availability did not affect female survival after 1 year of age, whereas higher squirrel densities led to lower survival, thereby affecting longevity and lifetime fitness. 4. In addition, both food availability experienced between birth and weaning, and spring temperature in the year of birth, had long-lasting positive effects on female reproductive success. These results emphasize the critical effect environmental conditions during the early life stages can have on the lifetime performance of small mammals. 5. These long-term effects of early food and temperature were apparent only once we controlled for conditions experienced during adulthood. This suggests that silver-spoon effects can be masked when conditions experienced early in life are correlated to some environmental conditions experienced later in life. 6. The general importance of silver-spoon effects for adult demographic performance might therefore be underestimated, and taking adult environment into account appears to be necessary when studying long-term cohort effects.

The interaction of parasites and resources cause crashes in a wild mouse population

1. Populations of white-footed mice Peromyscus leucopus and deer mice Peromyscus maniculatus increase dramatically in response to food availability from oak acorn masts. These populations subsequently decline following this resource pulse, but these crashes cannot be explained solely by resource depletion, as food resources are still available as population crashes begin. 2. We hypothesized that intestinal parasites contribute to these post-mast crashes; Peromyscus are infected by many intestinal parasites that are often transmitted by density-dependent contact and can cause harm to their hosts. To test our hypothesis, we conducted a factorial experiment in natural populations by supplementing food to mimic a mast and by removal of intestinal nematodes with the drug, ivermectin. 3. Both food supplementation and the removal of intestinal nematodes lessened the rate and magnitude of the seasonal population declines as compared with control populations. However, the combination of food supplementation and removal of intestinal nematodes prevented seasonal population crashes entirely. 4. We also showed a direct effect on the condition of individuals. Faecal corticosterone levels, an indicator of the stress response, were significantly reduced in populations receiving both food supplementation and removal of intestinal nematodes. This effect was observed in autumn, before the overwinter crash observed in control populations, which may indicate that stress caused by the combination of food limitation and parasite infection is a physiological signal that predicts low winter survival and reproduction. 5. This study is one of the few to demonstrate that the interaction between resource availability and infectious disease is important for shaping host population dynamics and emphasizes that multiple factors may drive oscillations in wild animal populations.

Acorn mast drives long-term dynamics of rodent and songbird populations

Resource pulses can have cascading effects on the dynamics of multiple trophic levels. Acorn mast is a pulsed resource in oak-dominated forests that has significant direct effects on acorn predators and indirect effects on their predators, prey, and pathogens. We evaluated changes in acorn mast, rodent abundance, raptor abundance, and reproductive success of a ground-nesting songbird over a 24-year period (1980-2004) in the southern Appalachian Mountains in an effort to determine the relationships among the four trophic levels. In particular, we examined the following: acorn mast from red oaks (Quercus rubra) and white oaks (Q. alba), abundance of white-footed mice (Peromyscus leucopus) and deer mice (P. maniculatus), population estimates of seven raptor species from three feeding guilds, and nest failure and number of juveniles of dark-eyed juncos (Junco hyemalis). Finally, we recorded seasonal temperature and precipitation to determine the effects of weather on each trophic level. We found that weather patterns had delayed effects of up to 3 years on these trophic interactions. Variation in acorn mast, the keystone resource in this community, was explained by weather conditions as far back as 2 years before the mast event. Acorn mast, in turn, was a strongly positive predictor of rodent abundance the following year, whereas spring and summer temperature and raptor abundance negatively affected rodent abundance. Dark-eyed junco nests were more likely to fail in years in which there were more rodents and raptors. Nest failure rate was a strong predictor of the number of juvenile juncos caught at the end of the summer. Our results improve our understanding of the complex ecological interactions in oak-dominated forests by illustrating the importance of abiotic and biotic factors at different trophic levels.

Long-term reproductive behaviour of woody plants across seven Bornean forest types in the Gunung Palung National Park (Indonesia): suprannual synchrony, temporal productivity and fruiting diversity

For 68 months, we observed the reproductive behaviour of 7288 woody plants (172 figs, 1457 climbers and 5659 trees) spanning major soil and elevational gradients. Two 2-3 month community-wide supra-annual fruiting events were synchronized across five forest types, coinciding with ENSO events. At least 27 genera in 24 families restricted their reproduction to these events, which involved a substantial proportion of tree diversity (> 80% of phylogenetic diversity). During these events, mean reproductive levels (8.5%) represented an almost four-fold increase compared with other months. These patterns indicate a strong behavioural advantage to this unusual reproductive behaviour. Montane forest experienced a single, separate fruiting peak while the peat swamp forest did not participate. Excluding these events, no temporal reproductive pattern was detectable, at either the landscape or forest type. These phenological patterns have major implications for the conservation of frugivore communities, with montane and swamp forests acting as 'keystone' forests.

Extreme environmental variation sharpens selection that drives the evolution of a mutualism

The importance of infrequent events for both adaptive evolution and the evolution of species interactions is largely unknown. We investigated how the infrequent production of large seed crops (masting) of a bird-dispersed tree (whitebark pine, Pinus albicaulis) influenced phenotypic selection exerted by its primary avian seed predator-disperser, the Clark's nutcracker (Nucifraga columbiana). Selection was not evident during common years of low seed abundance, whereas it was replicated among areas and favoured traits facilitating seed dispersal during infrequent years of high seed abundance. Since nutcrackers act mostly as seed predators during small seed crops but as seed dispersers during the largest seed crops, trees experienced strong selection from nutcrackers only during infrequent years when the interaction was most strongly mutualistic. Infrequent events can thus be essential to both adaptive evolution and the evolutionary dynamics of species interactions.

Prolonging the arctic pulse: long-term exploitation of cached eggs by arctic foxes when lemmings are scarce

1. Many ecosystems are characterized by pulses of dramatically higher than normal levels of foods (pulsed resources) to which animals often respond by caching foods for future use. However, the extent to which animals use cached foods and how this varies in relation to fluctuations in other foods is poorly understood in most animals. 2. Arctic foxes Alopex lagopus (L.) cache thousands of eggs annually at large goose colonies where eggs are often superabundant during the nesting period by geese. We estimated the contribution of cached eggs to arctic fox diets in spring and autumn, when geese were not present in the study area, by comparing stable isotope ratios (delta(13)C and delta(15)N) of fox tissues with those of their foods using a multisource mixing model in Program IsoSource. 3. The contribution of cached eggs to arctic fox diets was inversely related to collared lemming Dicrostonyx groenlandicus (Traill) abundance; the contribution of cached eggs to overall fox diets increased from < 28% in years when collared lemmings were abundant to 30-74% in years when collared lemmings were scarce. 4. Further, arctic foxes used cached eggs well into the following spring (almost 1 year after eggs were acquired) - a pattern that differs from that of carnivores generally storing foods for only a few days before consumption. 5. This study showed that long-term use of eggs that were cached when geese were superabundant at the colony in summer varied with fluctuations in collared lemming abundance (a key component in arctic fox diets throughout most of their range) and suggests that cached eggs functioned as a buffer when collared lemmings were scarce.

Interacting effects of climate change, landscape conversion, and harvest on carnivore populations at the range margin: marten and lynx in the northern Appalachians

Assessing the effects of climate change on threatened species requires moving beyond simple bioclimatic models to models that incorporate interactions among climatic trends, landscape change, environmental stochasticity, and species life history. Populations of marten (Martes americana) and lynx (Lynx canadensis) in southeastern Canada and the northeastern United States represent peninsular extensions of boreal ranges and illustrate the potential impact of these threats on semi-isolated populations at the range margin. Decreased snowfall may affect marten and lynx through decreased prey vulnerability and decreased competitive advantage over sympatric carnivores. I used a spatially explicit population model to assess potential effects of predicted changes in snowfall by 2055 on regional marten and lynx populations. The models' habitat rankings were derived from previous static models that correlated regional distribution with snowfall and vegetation data. Trapping scenarios were parameterized as a 10% proportional decrease in survival, and logging scenarios were parameterized as a 10% decrease in the extent of older coniferous or mixed forest. Both species showed stronger declines in the simulations due to climate change than to overexploitation or logging. Marten populations declined 40% because of climate change, 16% because of logging, and 30% because of trapping. Lynx populations declined 59% because of climate change, 36% because of trapping, and 20% in scenarios evaluating the effects of population cycles. Climate change interacted with logging in its effects on the marten and with trapping in its effects on the lynx, increasing overall vulnerability. For both species larger lowland populations were vulnerable to climate change, which suggests that contraction may occur in the core of their current regional range as well as among smaller peripheral populations. Despite their greater data requirements compared with bioclimatic models, mesoscale spatial viability models are important tools for generating more biologically realistic hypotheses regarding biotic response to climate change.

Effects of flood suppression on natricine snake diet and prey overlap

Organisms may become adapted to disturbances when these disturbances occur periodically and at intermediate intensity. To investigate the effects of flood suppression, this study compared diet and competition of semi-aquatic snakes during flood (2000–2001) and no flood (2003–2005) years. Three natricine species, Nerodia erythrogaster (Forster in Bossu, 1771), Nerodia fasciata (L., 1766), and Thamnophis proximus (Say in James, 1823), were palpated for prey items in an east Texas floodplain under both conditions. Prey items were classified as crayfish, salamanders, anurans, or fish. Simpson’s diversity index of prey, frequency of consumed prey type, and prey importance values were compared between flood and no flood years. Pianka’s index of niche overlap was used to compare changes in diet overlap between species in the years with floods and those without. In the absence of floods, the number of prey types consumed by N. erythrogaster and T. proximus decreased. The frequency of prey types consumed during flood years was significantly different from the period of flood suppression for all three species. The order of prey importance also changed in the absence of floods for all three snake species. Without floods, diet overlap decreased between N. erythrogaster and the other two species; however, overlap between N. fasciata and T. proximus doubled.

A perfect storm: the combined effects on population fluctuations of autocorrelated environmental noise, age structure, and density dependence

While it is widely appreciated that climate can affect the population dynamics of various species, a mechanistic understanding of how climate interacts with life-history traits to influence population fluctuations requires development. Here we build a general density-dependent age-structured model that accounts for differential responses in life-history traits to increasing population density. We show that as the temporal frequency of favorable environmental conditions increases, population fluctuations also increase provided that unfavorable environmental conditions still occur. As good years accumulate and the number of individuals in a population increases, successive life-history traits become vulnerable to density dependence once a return to unfavorable conditions prevails. The stronger this ratcheting of density dependence in life-history traits by autocorrelated climatic conditions, the larger the population fluctuations become. Highly fecund species, and those in which density dependence occurs in juvenile and adult vital rates at similar densities, are most sensitive to increases in the frequency of favorable conditions. Understanding the influence of global warming on temporal correlation in regional environmental conditions will be important in identifying those species liable to exhibit increased population fluctuations that could lead to their extinction.

The extended Moran effect and large-scale synchronous fluctuations in the size of great tit and blue tit populations

1. Synchronous fluctuations of geographically separated populations are in general explained by the Moran effect, i.e. a common influence on the local population dynamics of environmental variables that are correlated in space. Empirical support for such a Moran effect has been difficult to provide, mainly due to problems separating out effects of local population dynamics, demographic stochasticity and dispersal that also influence the spatial scaling of population processes. Here we generalize the Moran effect by decomposing the spatial autocorrelation function for fluctuations in the size of great tit Parus major and blue tit Cyanistes caeruleus populations into components due to spatial correlations in the environmental noise, local differences in the strength of density regulation and the effects of demographic stochasticity. 2. Differences between localities in the strength of density dependence and nonlinearity in the density regulation had a small effect on population synchrony, whereas demographic stochasticity reduced the effects of the spatial correlation in environmental noise on the spatial correlations in population size by 21.7% and 23.3% in the great tit and blue tit, respectively. 3. Different environmental variables, such as beech mast and climate, induce a common environmental forcing on the dynamics of central European great and blue tit populations. This generates synchronous fluctuations in the size of populations located several hundred kilometres apart. 4. Although these environmental variables were autocorrelated over large areas, their contribution to the spatial synchrony in the population fluctuations differed, dependent on the spatial scaling of their effects on the local population dynamics. We also demonstrate that this effect can lead to the paradoxical result that a common environmental variable can induce spatial desynchronization of the population fluctuations. 5. This demonstrates that a proper understanding of the ecological consequences of environmental changes, especially those that occur simultaneously over large areas, will require information about the spatial scaling of their effects on local population dynamics.

Anticipatory reproduction and population growth in seed predators

Mast seeding, the intermittent, synchronous production of large seed crops by a population of plants, is a well-known example of resource pulses that create lagged responses in successive trophic levels of ecological communities. These lags arise because seed predators are thought capable of increasing reproduction and population size only after the resource pulse is available for consumption. The resulting satiation of predators is a widely cited explanation for the evolution of masting. Our study shows that both American and Eurasian tree squirrels anticipate resource pulses and increase reproductive output before a masting event, thereby increasing population size in synchrony with the resource pulse and eliminating the population lag thought to be universal in resource pulse systems.

META-ANALYSIS: TROPHIC LEVEL, HABITAT, AND PRODUCTIVITY SHAPE THE FOOD WEB EFFECTS OF RESOURCE SUBSIDIES

Studies of the effects of cross-habitat resource subsidies have been a feature of food web ecology over the past decade. To date, most studies have focused on demonstrating the magnitude of a subsidy or documenting its effect in the recipient habitat. Ecologists have yet to develop a satisfactory framework for predicting the magnitude of these effects. We used 115 data sets from 32 studies to compare consumer responses to resource subsidies across recipient habitat type, trophic level, and functional group. Changes in consumer density or biomass in response to subsidies were inconsistent across habitats, trophic, and functional groups. Responses in stream cobble bar and coastline habitats were larger than in other habitats. Contrary to expectation, the magnitude of consumer response was not affected by recipient habitat productivity or the ratio of productivity between donor and recipient habitats. However, consumer response was significantly related to the ratio of subsidy resources to equivalent resources in the recipient habitat. Broad contrasts in productivity are modified by subsidy type, vector, and the physical and biotic characteristics of both donor and recipient habitats. For this reason, the ratio of subsidy to equivalent resources is a more useful tool for predicting the possible effect of a subsidy than coarser contrasts of in situ productivity. The commonness of subsidy effects suggests that many ecosystems need to be studied as open systems.

Indirect interactions between browsers and seed predators affect the seed bank dynamics of a chaparral shrub

Interactions between herbivores and seed predators may have long-term consequences for plant populations that rely on persistent seed banks for recovery after unpredictable fires. We assessed the effects of browsing by deer and seed predation by rodents, ants and birds on the densities of seeds entering the seed bank of Ceanothus cuneatus var. rigidus, a maritime chaparral shrub in coastal California. Ceanothus produced many more seeds when protected from browsers in long-term experimental exclosures than did browsed plants, but the seed densities in the soil beneath browsed and unbrowsed Ceanothus were the same at the start of an intensive one-year study. The density of seeds in the soil initially increased in both treatments following summer seed drop: while densities returned to pre-drop levels within a few weeks under browsed plants, soil seed densities remained high for 5-8 months beneath unbrowsed plants. Rodent abundance (especially deer mice) was higher near unbrowsed plants than >30 m away, and rodents removed Ceanothus seeds from dishes in the experimental plots. At least in the short term, rodent density and rates of seed removal were inversely related to the intensity of browsing. Our data have management implications for maintaining viable Ceanothus populations by regulating the intensity of browsing and the timing, intensity and frequency of fires.

Spillover edge effects: the dispersal of agriculturally subsidized insect natural enemies into adjacent natural habitats

The cross-edge spillover of subsidized predators from anthropogenic to natural habitats is an important process affecting wildlife, especially bird, populations in fragmented landscapes. However, the importance of the spillover of insect natural enemies from agricultural to natural habitats is unknown, despite the abundance of studies examining movement in the opposite direction. Here, we synthesize studies from various ecological sub-disciplines to suggest that spillover of agriculturally subsidized insect natural enemies may be an important process affecting prey populations in natural habitat fragments. This contention is based on (1) the ubiquity of agricultural-natural edges in human dominated landscapes; (2) the substantial literature illustrating that crop and natural habitats share important insect predators; and (3) the clear importance of the landscape matrix, specifically distance to ecological edges, in influencing predator impacts in agroecosystems. Further support emerges from theory on the importance of cross-boundary subsidies for within site consumer-resource dynamics. In particular, high productivity and temporally variable resource abundance in agricultural systems are predicted to result in strong spillover effects. More empirical work examining the prevalence and significance of such natural enemy spillover will be critical to a broader understanding of fragmentation impacts on insect predator-prey interactions.

HIGH SURVIVAL IN POOR YEARS: LIFE HISTORY TACTICS ADAPTED TO MAST SEEDING IN THE EDIBLE DORMOUSE

Edible dormice (Glis glis) reproduce in years with beech mast seeding, but entire populations may skip reproduction in years when tree seeds, a major food resource of this small hibernator, are absent. We tested the hypothesis that the year-to-year variability in reproductive effort caused by this breeding strategy should lead to detectable differences in yearly survival rates. Therefore, we analyzed capture-recapture data from animals occupying nest boxes, collected over nine years at two study sites in Germany. Among fully grown adults (aged two years or older), survival probabilities were significantly lower (0.32 +/- 0.04) after reproductive years (n = 5) compared to years (n = 4) with absent or below-average reproduction (0.58 +/- 0.07) on both study sites. This trade-off between reproduction and subsequent survival was observed in both females and males and appears to be a relatively rare case in which costs of reproduction in terms of longevity are detectable at the population level. Effects of reproduction on survival were less pronounced when yearlings (with a generally lower reproductive effort) were included and were more distinct in a suboptimal habitat. Of those females breeding in nest boxes, 96.5% had only one or two litters within the study period. Considering these and previously published results, including a report of extremely high mean longevities (9-12 years) of dormice in a habitat with infrequent mast seeding, we conclude that edible dormice flexibly adjust life history tactics to local mast patterns. Long stretches of mast failures can in fact lead to relative semelparity, i.e., a strategy in which dormice "sit tight" for several years until environmental conditions are favorable for reproduction.

Interactions between a detrital resource pulse and a detritivore community

Detritivore communities influence the decomposition of detrital resources in virtually all natural systems. Conversely, detrital resources can also have considerable bottom-up effects on detritivore communities. While many investigations have examined detritivory and decomposition processes, few have considered interactions between detritivores and detritus as concurrent processes in the same system, or in the context of natural detrital pulses. In many systems, resource pulses contribute substantial detrital inputs to belowground systems. These detrital pulses may influence interactions between the detritivore community and detrital decomposition. I conducted field experiments to investigate interactions between detrital resource pulses of periodical cicada (Magicicada spp.) carcasses and scavenging detritivorous macroarthropods. Cicada litterfall pulses influenced several broad groups in the macroarthropod community, including relatively specialized necrophilous taxa and relatively generalized detritivores, omnivores and predators. Conversely, detritivore activity increased the rate of cicada carcass decomposition by 4,082% compared to caged control carcasses. These results suggest that interactions between pulses of cicada detritus and the detritivore community influence both the persistence of ephemeral detrital resources, and the distribution, abundance and behavior of detritivore populations.

Periodical Cicadas as Resource Pulses in North American Forests

Resource pulses are occasional events of ephemeral resource superabundance that occur in many ecosystems. Aboveground consumers in diverse communities often respond strongly to resource pulses, but few studies have investigated the belowground consequences of resource pulses in natural ecosystems. This study shows that resource pulses of 17-year periodical cicadas (Magicicada spp.) directly increase microbial biomass and nitrogen availability in forest soils, with indirect effects on growth and reproduction in forest plants. These findings suggest that pulses of periodical cicadas create "bottom-up cascades," resulting in strong and reciprocal links between the aboveground and belowground components of a North American forest ecosystem.

Spatial dynamics of specialist seed predators on synchronized and intermittent seed production of host plants

Masting, the synchronized and intermittent seed production by plant populations, provides highly variable food resources for specialist seed predators. Such a reproductive mode helps minimize seed losses through predator satiation and extinction of seed predator populations. The seed predators can buffer the resource variation through dispersal or extended diapause. We developed a spatially explicit resource-consumer model to understand the effect of masting on specialist seed predators. The masting dynamics were assumed to follow a resource-based model for plant reproduction, and the population dynamics of the predator were represented by a spatially extended Nicholson-Bailey model. The resultant model demonstrated that when host plants reproduce intermittently, seed predator populations go locally extinct, but global persistence of the predator is facilitated by dispersal or extended diapause. Global extinction of the predator resulted when the intermittent reproduction is highly synchronized among plants. An approximate invasion criterion for the predators showed that negative lag-1 autocorrelation in seeding reduces invasibility, and positive lag-1 cross-correlation enhances invasibility. Spatial synchronization in seeding at local scale caused by pollen coupling (or climate forcing) further prevented invasion of the predators. If the predators employed extended diapause, extremely high temporal variability in reproduction was required for plants to evade the predators.

Hierarchy of responses to resource pulses in arid and semi-arid ecosystems

In arid/semi-arid ecosystems, biological resources, such as water, soil nutrients, and plant biomass, typically go through periods of high and low abundance. Short periods of high resource abundance are usually triggered by rainfall events, which, despite of the overall scarcity of rain, can saturate the resource demand of some biological processes for a time. This review develops the idea that there exists a hierarchy of soil moisture pulse events with a corresponding hierarchy of ecological responses, such that small pulses only trigger a small number of relatively minor ecological events, and larger pulses trigger a more inclusive set and some larger ecological events. This framework hinges on the observation that many biological state changes, where organisms transition from a state of lower to higher physiological activity, require a minimal triggering event size. Response thresholds are often determined by the ability of organisms to utilize soil moisture pulses of different infiltration depth or duration. For example, brief, shallow pulses can only affect surface dwelling organisms with fast response times and high tolerance for low resource levels, such as some species of the soil micro-fauna and -flora, while it takes more water and deeper infiltration to affect the physiology, growth or reproduction of higher plants. This review first discusses how precipitation, climate and site factors translate into soil moisture pulses of varying magnitude and duration. Next, the idea of the response hierarchy for ecosystem processes is developed, followed by an exploration of the possible evolutionary background for the existence of response thresholds to resource pulses. The review concludes with an outlook on global change: does the hierarchical view of precipitation effects in ecosystems provide new perspectives on the future of arid/semiarid lands?