Impacts of seedling herbivory on plant competition and implications for species coexistence

Background and Aims

Although the causes and consequences of seedling herbivory for plant community composition are well understood, the mechanisms by which herbivores influence plant species recruitment to the established phase remain less clear. The aim was to examine how variation in the intensity of seedling herbivory interacts with growth-defence trade-offs and herbivore feeding preferences to affect plant community development.

Methods

Using 14-d-old seedlings of Trifolium pratense and T. repens, relative growth and susceptibility to herbivory by the snail Helix aspersa was quantified to elucidate putative growth-defence trade-offs for these species. Then mixed assemblages of 14-d-old Trifolium seedlings were exposed to herbivory by zero, two, five or ten snails and determined how variation in the intensity of herbivory affected competitive interactions into the mature phase (as measured by total plant biomass at 120 d old).

Key Results

In the absence of herbivory, communities were dominated by T. pratense; a result expected on the basis that it yielded larger and presumably more competitive seedlings. However, when seedlings were exposed to herbivory, the balance of competition shifted. At low levels of herbivory (two snails), both Trifolium species contributed equally to total plant biomass. More intense herbivory (five snails) resulted in almost total mortality of T. pratense and dominance of the mature community by T. repens. The most intense herbivory (ten snails) effectively removed all seedlings from the experimental community.

Conclusions

The study illustrates a mechanism whereby spatio-temporal fluctuations in seedling herbivory, when coupled with species-specific variation in competitive ability and sensitivity to herbivore attack, can differentially influence plant recruitment into the mature phase. This mechanism may be a key element in our attempts to understand plant species coexistence, since fluctuations in plant recruitment are fundamental to the many theories that view coexistence as a consequence of a spatio-temporal lottery for dominance over regeneration micro-sites.Key words: Growth-defence trade-off, lottery models, plant–animal interactions, plant size variability, seedling acceptability, seedling defence, spatio-temporal niches, Trifolium pratense, Trifolium repens     

Hydraulic lift among native plant species in the Mojave Desert

Hydraulic lift was investigated among native plants in the Mojave Desert using in situ thermocouple psychrometers. Night lighting and day shading experiments were used to verify the phenomenon. Hydraulic lift was detected for all species examined: five shrub species with different rooting depths and leaf phenologies and one perennial grass species. This study was the first to document hydraulic lift for a CAM species, Yucca schidigera. The pattern of diel flux in soil water potential for the CAM species was temporally opposite to that of C₃ species: for the CAM plant, soil water potential increased in shallow soils during the day when the plant was not transpiring and decreased at night when transpiration began. Because CAM plants transport water to shallow soils during the day when surrounding C₃ and C₄ plants transpire, CAM species that hydraulically lift water may influence water relations of surrounding species to a greater extent than hydraulically lifting C₃ or C₄ species. A strong, negative relationship between the percent sand in the study site soils at the 0.35 m soil depth and the frequency that hydraulic lift was observed at that depth suggests that the occurrence of hydraulic lift is negatively influenced by coarse-textured soils, perhaps due to less root–soil contact in sandy soils relative to finer-textured soils. Differences in soil texture among study sites may explain, in part, differences in the frequency that hydraulic lift was detected among these species. Further investigations are needed to elucidate species versus soil texture effects on hydraulic lift. This revised version was published online in June 2006 with corrections to the Cover Date.     

Invertebrate community response to fire and rodent activity in the Mojave and Great Basin Deserts

Recent increases in the frequency and size of desert wildfires bring into question the impacts of fire on desert invertebrate communities. Furthermore, consumer communities can strongly impact invertebrates through predation and top‐down effects on plant community assembly. We experimentally applied burn and rodent exclusion treatments in a full factorial design at sites in both the Mojave and Great Basin deserts to examine the impact that fire and rodent consumers have on invertebrate communities. Pitfall traps were used to survey invertebrates from April through September 2016 to determine changes in abundance, richness, and diversity of invertebrate communities in response to fire and rodent treatments. Generally speaking, rodent exclusion had very little effect on invertebrate abundance or ant abundance, richness or diversity. The one exception was ant abundance, which was higher in rodent access plots than in rodent exclusion plots in June 2016, but only at the Great Basin site. Fire had little effect on the abundances of invertebrate groups at either desert site, with the exception of a negative effect on flying‐forager abundance at our Great Basin site. However, fire reduced ant species richness and Shannon's diversity at both desert sites. Fire did appear to indirectly affect ant community composition by altering plant community composition. Structural equation models suggest that fire increased invasive plant cover, which negatively impacted ant species richness and Shannon's diversity, a pattern that was consistent at both desert sites. These results suggest that invertebrate communities demonstrate some resilience to fire and invasions but increasing fire and spread of invasive due to invasive grass fire cycles may put increasing pressure on the stability of invertebrate communities.     

Land‐use legacies and present fire regimes interact to mediate herbivory by altering the neighboring plant community

Past and present human activities, such as historic agriculture and fire suppression, are widespread and can create depauperate plant communities. Although many studies show that herbivory on focal plants depends on the density of herbivores or the composition of the surrounding plant community, it is unclear whether anthropogenic changes to plant communities alter herbivory. We tested the hypothesis that human activities that alter the plant community lead to subsequent changes in herbivory. At 20 sites distributed across 80 300 hectares, we conducted a field experiment that manipulated insect herbivore access (full exclosures and pseudo‐exclosures) to four focal plant species in longleaf pine woodlands with different land‐use histories (post‐agricultural sites or non‐agricultural sites) and degrees of fire frequency (frequent and infrequent). Plant cover, particularly herbaceous cover, was lower in post‐agricultural and fire suppressed woodlands. Density of the dominant insect herbivore at our site (grasshoppers) was positively related to plant cover. Herbivore access reduced biomass of the palatable forb Solidago odora in frequently burned post‐agricultural sites and in infrequently burned non‐agricultural woodlands and increased mortality of another forb (Pityopsis graminifolia), but did not affect two other less palatable species (Schizachyrium scoparium and Tephrosia virginiana). Herbivory on S. odora exhibited a hump‐shaped response to plant cover, with low herbivory at low and high levels of plant cover. Herbivore density had a weak negative effect on herbivory. These findings suggest that changes in plant cover related to past and present human activities can modify damage rates on focal S. odora plants by altering grasshopper foraging behavior rather than by altering local grasshopper density. The resulting changes in herbivory may have the potential to limit natural recovery or restoration efforts by reducing the establishment or performance of palatable plant species.     

Interspecific competition: A mechanism for rodent succession after fire in wet heathland

Abstract A competitive interaction between two species of sympatric native rodents was demonstrated experimentally in Myall Lakes National Park (NSW, Australia). The previously documented replacement of Pseudomys gracilicaudatus by Rattus lutreolus in areas of wet heath in this region implied that competition may be the mechanism facilitating this succession. We present experimental evidence to support this suggestion. Removal of the larger R. lutreolus caused a significant increase in abundance of P. gracilicaudatus on five experimental sites in comparison to five unmanipulated control sites. The sites used in this experiment were chosen from three ages of wet heath regenerating after fire. These three ages represent key stages in the replacement of P. gracilicaudatus by R. lutreolus and the occurrence of competition in each of these ages indicates that competition plays an important role in this succession. In the initial stages of the experiment when R. lutreolus were removed they were replaced by new R. lutreolus individuals. This has been interpreted as evidence of strong intraspecific competition. After removal of R. lutreolus, P. gracilicaudatus expanded its habitat range into microhabitats that had formerly been occupied by R. lutreolus and reduced its range in microhabitats previously occupied by P. gracilicaudatus. This leads us to believe that P. gracilicaudatus was occupying inferior microhabitat before the removal of R. lutreolus, which had excluded it from more preferable microhabitat.     

Short-term soil inorganic N pulse after experimental fire alters invasive and native annual plant production in a Mojave Desert shrubland

Post-fire changes in desert vegetation patterns are known, but the mechanisms are poorly understood. Theory suggests that pulse dynamics of resource availability confer advantages to invasive annual species, and that pulse timing can influence survival and competition among species. Precipitation patterns in the American Southwest are predicted to shift toward a drier climate, potentially altering post-fire resource availability and consequent vegetation dynamics. We quantified post-fire inorganic N dynamics and determined how annual plants respond to soil inorganic nitrogen variability following experimental fires in a Mojave Desert shrub community. Soil inorganic N, soil net N mineralization, and production of annual plants were measured beneath shrubs and in interspaces during 6 months following fire. Soil inorganic N pools in burned plots were up to 1 g m⁻² greater than unburned plots for several weeks and increased under shrubs (0.5–1.0 g m⁻²) more than interspaces (0.1–0.2 g m⁻²). Soil NO₃ ⁻−N (nitrate−N) increased more and persisted longer than soil NH₄ ⁺−N (ammonium−N). Laboratory incubations simulating low soil moisture conditions, and consistent with field moisture during the study, suggest that soil net ammonification and net nitrification were low and mostly unaffected by shrub canopy or burning. After late season rains, and where soil inorganic N pools were elevated after fire, productivity of the predominant invasive Schismus spp. increased and native annuals declined. Results suggest that increased N availability following wildfire can favor invasive annuals over natives. Whether the short-term success of invasive species following fire will direct long-term species composition changes remains to be seen, yet predicted changes in precipitation variability will likely interact with N cycling to affect invasive annual plant dominance following wildfire.     

Seed Menus: An integrated decision‐support framework for native plant restoration in the Mojave Desert

The combination of ecosystem stressors, rapid climate change, and increasing landscape‐scale development has necessitated active restoration across large tracts of disturbed habitats in the arid southwestern United States. In this context, programmatic directives such as the National Seed Strategy for Rehabilitation and Restoration have increasingly emphasized improved restoration practices that promote resilient, diverse plant communities, and enhance native seed reserves. While decision‐support tools have been implemented to support genetic diversity by guiding seed transfer decisions based on patterns in local adaptation, less emphasis has been placed on identifying priority seed mixes composed of native species assemblages. Well‐designed seed mixes can provide foundational ecosystem services including resilience to disturbance, resistance to invasive species, plant canopy structure to facilitate natural seedling recruitment, and habitat to support wildlife and pollinator communities. Drawing from a newly developed dataset of species distribution models for priority native plant taxa in the Mojave Desert, we created a novel decision support tool by pairing spatial predictions of species habitat with a database of key species traits including life history, flowering characteristics, pollinator relationships, and propagation methods. This publicly available web application, Mojave Seed Menus, helps restoration practitioners generate customized seed mixes for native plant restoration in the Mojave Desert based on project locations. Our application forms part of an integrated Mojave Desert restoration program designed to help practitioners identify species to include in local seed mixes and nursery stock development while accounting for local adaptation by identifying appropriate seed source locations from key restoration species.     

Effects of herbivory and competition on an introduced plant in decline

Anthemiscotula was introduced to Denmark 500 years ago, and its distribution is presently limited and in decline. A manipulative field experiment was performed to investigate the effects of native plant competitors and native invertebrate herbivores on its performance. Generally, both herbivory and competition treatments had great impact, and when both factors were operating, the effects were additive for all variables except plant height. Although A. cotula showed plasticity in growth, resource allocation and flowering timing, it was unable to adjust to competition and compensate for losses due to herbivory sufficiently to ensure and restore its achene production. This vulnerability, combined with improved cereal cleaning techniques and thus fewer reintroductions of A. cotula seeds, may be the cause of its current decline. A. cotula responded to herbivory by prolonging its flowering period, a “bet-hedging” strategy. In Denmark this strategy is unreliable since risks of sub-optimal conditions are much greater in August–October. Received: 12 December 1997 / Accepted: 14 October 1998     

人为不同干扰条件下荒漠啮齿动物群落格局的动态特征——动物与植物群落的典型相关分析

群落的格局和动态是群落牛态学和生态系统生态学研究中的基础问题,也是揭示群落结构和功能的核心问题。为了深入了解生态系统的结构与功能,研究啮齿动物群落格局在人为不同干扰下的动态特征,特别是研究荒漠区的脆弱生态系统中啮齿动物群落格局在人为不同干扰下的动态特征,是揭示人为干扰下荒漠生态系统结构和功能变化特征的基础。2002～2004年在内蒙古阿拉善典型荒漠区,选择了4种不同干扰条件的生境,分别为禁牧区、过牧区、轮牧区和开垦区,研究了人为不同干扰下啮齿动物群落格局的变动趋势,应用典型相关方法分析了啮齿动物群落与植物群落在不同干扰条件下的相互关系。结果表明,在两种观察尺度域上（标志样地和线路样地）动物群落变量与植物群落变量的相关关系表现为：动物群落变量与植物群落变量中的草本关系最为突出,在分析的两个尺度的8个样地中,除标志样地的过牧区外,其它7个样地均与草本关系密切,且绝大多数样地动物群落变量与草本的盖度和地上生物量呈负相关关系。这说明荒漠生态系统中,在4种干扰条件下,草本的特性,特别是草本的盖度和地上生物量对啮齿动物群落格局及其动态变化起到关键作用,其值越大,啮齿动物群落组成种的丰富度（数量）和生物量就越小。就同一种干扰条件下的两种尺度域上的情况看,动物群落和植物群落的相关性较突出的特点是：在禁牧区和开垦区,喜湿的种类（黑线仓鼠和草原黄鼠）与植被的相关性显著,在轮牧区和过牧区,喜旱的种类（小毛足鼠和三趾跳鼠）与植被的相关性显著,且均与草本呈负相关关系。     

Changing dominance of key plant species across a Mediterranean climate region: implications for fuel types and future fire regimes

Herbaceous and woody plants represent different fuel types in flammable ecosystems, due to contrasting patterns of growth and flammability in response to productivity (moisture availability). However, other factors, such as soil type, fire regimes and competitive interactions may also influence the relative composition of herbaceous and woody plants within a community. The Mediterranean climate region of south eastern Australia is transitional between two contrasting fuel systems; herbaceous dominated in the dry north, versus woody plant dominated shrublands in the relatively moist south. Across the rainfall gradient of the region, there are confounded changes in dominant soil types and fire frequency. We used model-subset selection using Akaike’s Information Criterion to examine potential driving mechanisms of community compositional change from herbaceous (e.g. Triodia scariosa, Austrostipa sp.) to woody plants (e.g. Beyeria opaca, Leptospermum coriaceum, Acacia ligulata) by measuring relative cover across combinations of rainfall, time since the last fire (TSF) and soil type. We examined the relative influence of environmental versus competitive interactions on determining the cover of perennial hummock grass, T. scariosa, and co-occurring woody shrubs. Rainfall and soil types, rather than competition, were the over-arching determinants of the relative cover of grasses and shrubs. Given the sensitivity to rainfall, our results indicate there is strong potential for the nature of fuel, flammability and fire regimes to be altered in the future via climate change in this region.     

A biogeographic model of fire regimes in Australia: current and future implications

Aim Patterns of fire regimes across Australia exhibit biogeographic variation in response to four processes. Variations in area burned and fire frequency result from differences in the rates of ‘switching’ of biomass growth, availability to burn, fire weather and ignition. Therefore differing processes limit fire (i.e. the lowest rate of switching) in differing ecosystems. Current and future trends in fire frequency were explored on this basis. Location Case studies of forests (cool temperate to tropical) and woodlands (temperate to arid) were examined. These represent a broad range of Australian biomes and current fire regimes. Methods Information on the four processes was applied to each case study and the potential minimum length of interfire interval was predicted and compared to current trends. The potential effects of global change on the processes were then assessed and future trends in fire regimes were predicted. Results Variations in fire regimes are primarily related to fluctuations in available moisture and dominance by either woody or herbaceous plant cover. Fire in woodland communities (dry climates) is limited by growth of herbaceous fuels (biomass), whereas in forests (wet climates) limitation is by fuel moisture (availability to burn) and fire weather. Increasing dryness in woodland communities will decrease potential fire frequency, while the opposite applies in forests. In the tropics, both forms of limitation are weak due to the annual wet/dry climate. Future change may therefore be constrained. Main conclusions Increasing dryness may diminish fire activity over much of Australia (dominance of dry woodlands), though increases may occur in temperate forests. Elevated CO₂ effects may confound or reinforce these trends. The prognosis for the future fire regime in Australia is therefore uncertain.     

Plant diversity,herbivory and resistance of a plant community to invasion in Mediterranean annual communities

Several components of the diversity of plant communities, such as species richness, species composition, number of functional groups and functional composition, have been shown to directly affect the performance of exotic species. Exotics can also be affected by herbivores of the native plant community. However, these two possible mechanisms limiting invasion have never been investigated together. The aim of this study was to investigate the relationships between plant diversity, herbivory and performance of two annual exotics, Conyza bonariensis and C. canadensis, in Mediterranean annual communities. We wanted to test whether herbivory of these exotics was influenced either by species richness, functional-group richness or functional-group composition. We also studied the relationship between herbivory on the exotic species and their performance. Herbivory increased with increasing species and functional-group richness for both Conyza species. These patterns are interpreted as reflecting a greater number of available herbivore niches in a richer, more complex, plant community. The identities of functional groups also affected Conyza herbivory, which decreased in the presence of Asteraceae or Fabaceae and increased in the presence of Poaceae. Increasing herbivory had consequences for vegetative and demographic parameters of both invasive species: survival, final biomass and net fecundity decreased with increasing herbivory, leading to a loss of reproductive capacity. We conclude that communities characterised by a high number of grass species instead of Asteraceae or Fabaceae may be more resistant to invasion by the two Conyza species, in part due to predation by native herbivores.     

Fire regimes and shifting community patterns: a case study and method for species with complex fire requirements

Anthropogenic alteration of fire regimes is implicated in the extinction or decline of species across the globe. Active management of fire regimes using specified guidelines can help sustain diversity and counter this loss. However, some species occur across multiple communities with differing fire regime requirements or may exploit the dynamics of associated ecotones. These complexities are not easily understood via a simple consideration of life history traits or unravelled by a simple experimental framework. Population modelling, however, may provide valuable insights in these instances, but to date this remains unexplored. A population model based on detailed demographic, habitat and fire regime data was developed for the species Prostanthera askania. The species occurs across rainforest/eucalypt forest ecotones subject to long-term alteration, fragmentation and invasion by exotic species. Modelling revealed that current recommended fire guidelines will not sustain this species under these novel conditions. Fire regimes that minimised extinction risk in some scenarios had double or five times the fire frequencies of recommended regimes. Managing fire was also more important in specific habitat (gully habitat rather than on slopes or ridges). Currently applied management actions for Prostanthera askania do not include any fire management and the regimes that apply to most habitat in which the species occurs (no fire or fire at up to 50-year intervals in gully habitat) is not optimal for the species. Management will be substantially more effective if specific fire treatments revealed by this study are employed (e.g. fire intervals of 20–30 years in gully habitat). The study demonstrates that a conceptualisation and consideration of communities as both temporally and spatially dynamic can substantially contribute to better fire management outcomes. The approach used herein can be readily adapted and applied to a plethora of species via a range of software packages and codes.     

Woody cover in African savannas: the role of resources, fire and herbivory

Aim To determine the functional relationships between, and the relative importance of, different driver variables (mean annual precipitation, soil properties, fire and herbivory) in regulating woody plant cover across broad environmental gradients in African savannas. Location Savanna grasslands of East, West and Southern Africa. Methods The dependence of woody cover on mean annual precipitation (MAP), soil properties (texture, nitrogen mineralization potential and total phosphorus), fire regimes, and herbivory (grazer, browser + mixed feeder, and elephant biomass) was determined for 161 savanna sites across Africa using stochastic gradient boosting, a refinement of the regression tree analysis technique. Results All variables were significant predictors of woody cover, collectively explaining 71% of the variance in our data set. However, their relative importance as regulators of woody cover varied. MAP was the most important predictor, followed by fire return periods, soil characteristics and herbivory regimes. Woody cover showed a strong positive dependence on MAP between 200 and 700 mm, but no dependence on MAP above this threshold when the effects of other predictors were accounted for. Fires served to reduce woody cover below rainfall‐determined levels. Woody cover showed a complex, non‐linear relationship with total soil phosphorus, and was negatively correlated with clay content. There was a strong negative dependence of woody cover on soil nitrogen (N) availability, suggesting that increased N‐deposition may cause shifts in savannas towards more grassy states. Elephants, mixed feeders and browsers had negative effects on woody cover. Grazers, on the other hand, depressed woody cover at low biomass, but favoured woody vegetation when their biomass exceeded a certain threshold. Main conclusions Our results indicate complex and contrasting relationships between woody cover, rainfall, soil properties and disturbance regimes in savannas, and suggest that future environmental changes such as altered precipitation regimes, N‐enrichment and elevated levels of CO₂ are likely to have opposing, and potentially interacting, influences on the tree–grass balance in savannas.     

Stoichiometry, herbivory and competition for nutrients: simple models based on planktonic ecosystems

Models are examined in which two prey species compete for two nutrient resources, and are preyed upon by a predator that recycles both nutrients. Two factors determine the effective relative supply of the nutrients, hence competitive outcomes: the external nutrient supply ratio, and the relative recycling of the two nutrients within the system. This second factor is governed by predator stoichiometry--its relative requirements for nutrients in its own biomass. A model with nutrient resources that are essential for the competing prey is detailed. Criteria are given to identify the limiting nutrient for a food chain of one competitor with the predator. Increased supply of this limiting nutrient increases predator density and concentration of this nutrient at equilibrium, while decreasing the concentration of a non-limiting nutrient. Changes in supply or recycling of a non-limiting nutrient affect only the concentration of that nutrient. Criteria for the invasion of a second prey competitor are presented. When different nutrients limit growth of the resident prey and the invader, increased supply or recycling of the invader's limiting nutrient assists invasion, while increased supply or recycling of the resident's limiting nutrient hinders invasion. If the same nutrient limits both resident and invader, then changes in supply and recycling have complex effects on invasion, depending on species properties. In a parameterized model of a planktonic ecosystem, green algae and cyanobacteria coexist over a wide range of nitrogen:phosphorus supply ratios, without predators. When the herbivore Daphnia is added, coexistence is eliminated or greatly restricted, and green algae dominate over a wide range of supply conditions, because the effective supply of P is greatly reduced as Daphnia rapidly recycles N.