Large Mammalian Herbivores and the Conservation of Arid Acacia Stands in the Middle East

Abstract: High mortality and poor recruitment in Acacia tree populations in many parts of the Middle East may result in loss of biodiversity. Human activities in recent decades have caused drastic changes in the water regime and the abundance of wild and domestic ungulates, which may negatively affect these trees. We studied the effects of large mammalian herbivores on the establishment of young Acacia raddiana and A. tortilis in the Arava Valley between the Red Sea and the Dead Sea, Israel. Seed accumulation under trees was high when large herbivores were excluded, and direct observations confirmed that ungulates were the main seed dispersers of these Acacia species. Insect seed predators (bruchid beetles) damaged more than 95% of seeds not consumed by ungulates. Seed germination was facilitated by gut passage through ungulates. Seedling survival was determined largely by water availability and was independent of herbivore density. Under present conditions, several years of high rainfall are necessary for successful establishment of young trees. Under severe browsing pressure by ungulates, growth in juvenile trees was delayed and smaller sizes were overrepresented. There was no evidence, however, of effects on the long-term demography of Acacia trees, perhaps because ungulates increased seedling densities by enhancing the viability and germination of seeds. We conclude that large mammalian herbivores are essential components of arid Acacia savannas and that wild and domestic ungulates must be included in future conservation plans.     

Herbivory reduces plant interactions with above- and belowground antagonists and mutualists

Herbivores affect plants through direct effects, such as tissue damage, and through indirect effects that alter species interactions. Interactions may be positive or negative, so indirect effects have the potential to enhance or lessen the net impacts of herbivores. Despite the ubiquity of these interactions, the indirect pathways are considerably less understood than the direct effects of herbivores, and multiple indirect pathways are rarely studied simultaneously. We placed herbivore effects in a comprehensive community context by studying how herbivory influences plant interactions with antagonists and mutualists both aboveground and belowground. We manipulated early-season aboveground herbivore damage to Cucumis sativus (cucumber, Cucurbitaceae) and measured interactions with subsequent aboveground herbivores, root-feeding herbivores, pollinators, and arbuscular mycorrhizal fungi (AMF). We quantified plant growth and reproduction and used an enhanced pollination treatment to determine if plants were pollen limited. Increased herbivory reduced interactions with both antagonists and mutualists. Plants with high levels of early herbivory were significantly less likely to suffer leaf damage later in the summer and tended to be less attacked by root herbivores. Herbivory also reduced pollinator visitation, likely due to fewer and smaller flowers, and reduced AMF colonization. The net effect of herbivory on plant growth and reproduction was strongly negative, but lower fruit and seed production were not due to reduced pollinator visits, because reproduction was not pollen limited. Although herbivores influenced interactions between plants and other organisms, these effects appear to be weaker than the direct negative effects of early-season tissue loss.     

The high cost of mutualism: effects of four species of East African ant symbionts on their myrmecophyte host tree

Three recent meta-analyses of protective plant-ant mutualisms report a surprisingly weak relationship between herbivore protection and measured demographic benefits to ant-plants, suggesting high tolerance for herbivory, substantial costs of ant-mediated defense, and/or benefits that are realized episodically rather than continuously. Experimental manipulations of protective ant-plant associations typically last for less than a year, yet virtually all specialized myrmecophytes are long-lived perennials for which the costs and benefits of maintaining ant symbionts could accrue at different rates over the host's lifetime. To complement long-term monitoring studies, we experimentally excluded each of four ant symbionts from their long-lived myrmecophyte host trees (Acacia drepanolobium) for 4.5 years. Ant species varied in their effectiveness against herbivores and in their effects on intermediate-term growth and reproduction, but the level of herbivore protection provided was a poor predictor of the net impact they had on host trees. Removal of the three Crematogaster species resulted in cumulative gains in host tree growth and/or reproduction over the course of the experiment, despite the fact that two of those species significantly reduce chronic herbivore damage. In contrast, although T. penzigi is a relatively poor defender, the low cost of maintaining this ant symbiont apparently eliminated negative impacts on overall tree growth and reproduction, resulting in enhanced allocation to new branch growth by the final census. Acacia drepanolobium is evidently highly tolerant of herbivory by insects and small browsers, and the costs of maintaining Crematogaster colonies exceeded the benefits received during the study. No experimental trees were killed by elephants, but elephant damage was uniquely associated with reduced tree growth, and at least one ant species (C. mimosae) strongly deterred elephant browsing. We hypothesize that rare but catastrophic damage by elephants may be more important than chronic herbivory in maintaining the costly myrmecophyte habit in this system.     

Spillover of Agriculturally Subsidized Predators as a Potential Threat to Native Insect Herbivores in Fragmented Landscapes

Abstract:  Habitat loss and fragmentation can have strong negative impacts on populations of some native species. Spillover of generalist natural enemies from the surrounding landscape matrix is one mechanism potentially generating such effects, yet this has been rarely studied in insects. We examined the influence of habitat conversion to agriculture on the abundance and potential effects of predatory coccinellid beetles on native insect herbivores within 12 grassland remnants in central Nebraska (U.S.A.). Results of sweep sampling revealed that coccinellids were three to six times more abundant at native grassland sites embedded within cropland-dominated landscapes compared with control sites in grassland-dominated landscapes over the 3 years of the study. Exclusion experiments further demonstrated that predation intensity was strongly related to coccinellid abundances across sites and that coccinellids can dramatically reduce densities of a native aphid herbivore. In contrast to studies of specialized insect parasitoids, which have generally found reduced enemy pressure in fragmented landscapes, our results suggest that native herbivores may in some cases experience increased consumer pressure in landscapes with increasing habitat loss because of spillover of generalist predators from surrounding cropland habitats.     

Savanna tree density, herbivores, and the herbaceous community: bottom-up vs. top-down effects

Herbivores choose their habitats both to maximize forage intake and to minimize their risk of predation. For African savanna herbivores, the available habitats range in woody cover from open areas with few trees to dense, almost-closed woodlands. This variation in woody cover or density can have a number of consequences for herbaceous species composition, cover, and productivity, as well as for ease of predator detection and avoidance. Here, we consider two alternative possibilities: first, that tree density affects the herbaceous vegetation, with concomitant "bottom-up" effects on herbivore habitat preferences; or, second, that tree density affects predator visibility, mediating "top-down" effects of predators on herbivore habitat preferences. We sampled sites spanning a 10-fold range of tree densities in an Acacia drepanolobium-dominated savanna in Laikipia, Kenya, for variation in (1) herbaceous cover, composition, and species richness; (2) wild and domestic herbivore use; and (3) degree of visibility obstruction by the tree layer. We then used structural equation modeling to consider the potential influences that tree density may have on herbivores and herbaceous community properties. Tree density was associated with substantial variation in herbaceous species composition and richness. Cattle exhibited a fairly uniform use of the landscape, whereas wild herbivores, with the exception of elephants, exhibited a strong preference for areas of low tree density. Model results suggest that this was not a response to variation in herbaceous-community characteristics, but rather a response to the greater visibility associated with more open places. Elephants, in contrast, preferred areas with higher densities of trees, apparently because of greater forage availability. These results suggest that, for all but the largest species, top-down behavioral effects of predator avoidance on herbivores are mediated by tree density. This, in turn, appears to have cascading effects on the herbaceous vegetation. These results shed light on one of the major features of the "landscape of fear" in which African savanna herbivores exist.     

Functional identity and functional structure change through succession in a rocky intertidal marine herbivore assemblage

Despite the great interest in characterizing the functional structure and resilience of functional groups in natural communities, few studies have examined in which way the roles and relationships of coexisting species change during community succession, a fundamental and natural process that follows the release of new resources in terrestrial and aquatic ecosystems. Variation in algal traits that characterize different phases and stages of community succession on rocky shores are likely to influence the magnitude, direction of effects, and the level of redundancy and complementarity in the diverse assemblage of herbivores. Two separate field experiments were conducted to quantify per capita and population effects and the functional relationship (i.e., redundancy or complementarity) of four herbivore species found in central Chile during early and late algal succession. The first experiment examined grazer effects on the colonization and establishment of early-succession algal species. The second experiment examined effects on the late-successional, dominant corticated alga Mazzaella laminarioides. Complementary laboratory experiments with all species and under natural environmental conditions allowed us to further characterize the collective effects of these species. We found that, during early community succession, all herbivore species had similar effects on the ephemeral algae, ulvoids, but only during the phase of colonization. Once these algae were established, only a subset of the species was able to control their abundance. During late succession, only the keyhole limpet Fissurella crassa could control corticated Mazzaella. The functional relationships among these species changed dramatically from redundant effects on ephemeral algae during early colonization, to a more complementary role on established early-successional algae, to a dominant (i.e., keystone) effect on late succession. This study highlights that functional relationship within consumer assemblages can vary at different phases and times of community succession. Differentiation in herbivore roles emphasizes the need to evaluate consumer's impacts through different times of community succession, and through experimental manipulations to make even broad predictions about the resilience or vulnerability of diverse intertidal assemblages to human disturbances.     

Counterintuitive effects of large-scale predator removal on a midlatitude rodent community

Historically, small mammals have been focal organisms for studying predator-prey dynamics, principally because of interest in explaining the drivers of the cyclical dynamics exhibited by northern vole, lemming, and hare populations. However, many small-mammal species occur at relatively low and fairly stable densities at temperate latitudes, and our understanding of how complex predator assemblages influence the abundance and dynamics of these species is surprisingly limited. In an intact grassland ecosystem in western Montana, USA, we examined the abundance and dynamics of Columbian ground squirrels (Spermophilus columbianus), deer mice (Peromyscus maniculatus), and montane voles (Microtus montanus) on 1-ha plots where we excluded mammalian and avian predators and ungulates, excluded ungulates alone, or allowed predators and ungulates full access. Our goal was to determine whether the relatively low population abundance and moderate population fluctuations of these rodents were due to population suppression by predators. Our predator-exclusion treatment was divided into two phases: a phase where we excluded all predators except weasels (Mustela spp.; 2002-2005), and a phase where all predators including weasels were excluded (2006-2009). Across the entire duration of the experiment, predator and/or ungulate exclusion had no effect on the abundance or overall dynamics of ground squirrels and deer mice. Ground squirrel survival (the only species abundant enough to accurately estimate survival) was also unaffected by our experimental treatments. Prior to weasel exclusion, predators also had no impacts on montane vole abundance or dynamics. However, after weasel exclusion, vole populations reached greater population peaks, and there was greater recruitment of young animals on predator-exclusion plots compared to plots open to predators during peak years. These results suggest that the impacts of predators cannot be generalized across all rodents in an assemblage. Furthermore, they suggest that specialist predators can play an important role in suppressing vole abundance even in lower-latitude vole populations that occur at relatively low densities.     

Quantitative threat analysis for management of an imperiled species: Chinook salmon (Oncorhynchus tshawytscha).

Chinook salmon (Oncorhynchus tshawytscha) have declined dramatically across the Pacific Northwest because of multiple human impacts colloquially characterized as the four "H's": habitat degradation, harvest, hydroelectric and other dams, and hatchery production. We use this conceptual framework to quantify the relative importance of major threats to the current status of 201 Chinook populations. Current status is characterized by two demographic indices: population density and trend. We employ path analytic models and information theoretic methods for multi-model inference. Our results indicate that dams most strongly affect variation in population density, while harvest and hatchery production most strongly affect variation in population trend. Comparable results arise when the sample size of the analysis is reduced to 22 Chinook populations within a smaller region typical of the scale at which salmon recovery planning is conducted. Results from these threat analyses suggest that recovery strategies targeting specific demographic indices, and those considering natural and human-mediated interdependencies of major threats, are most likely to succeed.     

Insect herbivores, density dependence, and the performance of the perennial herb Solanum carolinense

How insect herbivores affect plant performance is of central importance to basic and applied ecology. A full understanding of herbivore effects on plant performance requires understanding interactions (if any) of herbivore effects with plant density and size because these interactions will be critical for determining how herbivores influence plant population size. However, few studies have considered these interactions, particularly over a wide enough range of densities to detect nonlinear effects. Here we ask whether plant density and herbivores influence plant performance linearly or nonlinearly, how plant density affects herbivore damage, and how herbivores alter density dependence in transitions between plant size classes. In a large field experiment, we manipulated the density of the herbaceous perennial plant Solanum carolinense and herbivore presence in a fully crossed design. We measured plant size, sexual reproduction, and damage to plants in two consecutive years, and asexual reproduction of new stems in the second year, allowing us to characterize both plant performance and rates of transition between plant size classes across years. We found nonlinear effects of plant density on damage. Damage by herbivores and plant density both influenced sexual and asexual reproduction of S. carolinense; these effects were mostly mediated via effects on plant size. Importantly, we found that herbivores altered the pattern of linear density dependence in some transition rates (including survival and asexual reproduction) between plant size classes. These results suggest that understanding the ecological or evolutionary effects of herbivores on plant populations requires consideration of plant density and plant size, because feedbacks between density, herbivores, and plant size may complicate longer-term dynamics.     

Herbivory by an introduced Asian weevil negatively affects population growth of an invasive Brazilian shrub in Florida

The enemy release hypothesis (ERH) is often cited to explain why some plants successfully invade natural communities while others do not. This hypothesis maintains that plant populations are regulated by coevolved enemies in their native range but are relieved of this pressure where their enemies have not been co-introduced. Some studies have shown that invasive plants sustain lower levels of herbivore damage when compared to native species, but how damage affects fitness and population dynamics remains unclear. We used a system of co-occurring native and invasive Eugenia congeners in south Florida (USA) to experimentally test the ERH, addressing deficiencies in our understanding of the role of natural enemies in plant invasion at the population level. Insecticide was used to experimentally exclude insect herbivores from invasive Eugenia uniflora and its native co-occurring congeners in the field for two years. Herbivore damage, plant growth, survival, and population growth rates for the three species were then compared for control and insecticide-treated plants. Our results contradict the ERH, indicating that E. uniflora sustains more herbivore damage than its native congeners and that this damage negatively impacts stem height, survival, and population growth. In addition, most damage to E. uniflora, a native of Brazil, is carried out by Myllocerus undatus, a recently introduced weevil from Sri Lanka, and M. undatus attacks a significantly greater proportion of E. uniflora leaves than those of its native congeners. This interaction is particularly interesting because M. undatus and E. uniflora share no coevolutionary history, having arisen on two separate continents and come into contact on a third. Our study is the first to document negative population-level effects for an invasive plant as a result of the introduction of a novel herbivore. Such inhibitory interactions are likely to become more prevalent as suites of previously noninteracting species continue to accumulate and new communities assemble worldwide.     

Modelling the spatial population dynamics of the green oak leaf roller (Tortrix viridana) using density dependent competitive interactions: Effects of herbivore mortality and varying host-plant quality

Cyclic population dynamics of forest insects with periods of more than two generations have been discussed in relation to a variety of extrinsic and intrinsic forces. In the present study, we employed the selection pressure of density dependent competitive interactions according to Witting's equations (Witting, 2000) as driver for a discrete spatiotemporal model of the green oak leaf roller (Tortrix viridana). The model was successfully parameterised to rebuild the cyclic population dynamics of an empirical data set of a 30-year leaf roller monitoring in Russia. Our analysis focussed on the role of herbivore mortality and host plant food quality, which have a significant effect on T. viridana population dynamics. An additional egg or larvae mortality lowers population density and can lead to selection pressures that favour individuals with higher growth rate. This increased population growth rate can not only compensate the additional mortality, but also can lead to higher average moth abundances in subsequent generations. Furthermore, we analysed the effect of inter- and intraspecific variation in host plant quality on herbivore population dynamics and the spatial distribution of abundance and defoliation patterns. We found significant effects of the qualitative composition of a trees neighbourhood on the herbivore population of the respective tree. Also, the patchy damage patterns observable in reality have been reproduced by the present model. The applicability of the model approach and the putative genetic processes underlying Witting's model are discussed.     

Sources of variation in herbivore preference: among-individual and past diet effects on amphipod host choice

Understanding which factors affect the feeding preferences of herbivores is essential for predicting the effects of herbivores on plant assemblages and the evolution of plant–herbivore interactions. Most studies of marine herbivory have focussed on the plant traits that determine preferences (especially secondary metabolites), while few studies have considered how preferences may vary among individual herbivores due to genetic or environmental sources of variation. Such intraspecific variation is essential for evolutionary change in preference behaviour and may alter the outcome of plant–herbivore interactions. In an abundant marine herbivore, we determined the relative importance of among-individual and environmental effects on preferences for three host algae of varying quality. Repeated preference assays were conducted with the amphipod Peramphithoe parmerong and three of its brown algal hosts: Sargassum linearifolium, S. vestitum and Padina crassa. We found no evidence that preference varied among individuals, thus constraining the ability of natural selection to promote increased specialisation on high-quality S. linearifolium. Most of the variation in preference occurred within individuals, with amphipod preferences strongly influenced by past diet. The increased tendency for amphipods to select alternate hosts to that on which they had been recently feeding indicates that amphipods are actively seeking mixed diets. Such a feeding strategy provides an explanation for the persistence of this herbivore on hosts in the field that support poor growth and survival if consumed alone. The effects of past diet indicate that herbivore preferences are a function of herbivore history in addition to plant traits and are likely to vary with the availability of algae in space and time.     

Glucosinolates and other metabolites in the leaves of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> from natural populations and their effects on a generalist and a specialist herbivore

Because many secondary metabolites in plants act as defense against herbivores it has been postulated that these compounds have evolved under selective pressure by insect herbivores. One explanation for the within-species variation in metabolite patterns in a particular species is that different populations are under selection by different herbivores. We tested this hypothesis, using Arabidopsis thaliana plants that originated from dune and inland areas. We analyzed Arabidopsis thaliana leaves using NMR spectroscopy and multivariate data analysis. Major differences in chemical composition were found in water-methanol fractions and were due to higher concentrations of sinigrin and fumaric acid in dune plants. Inland plants showed lower levels of glucose. Quantitative analysis of glucosinolates was performed with HPLC. Individual plants and populations demonstrated differences in glucosinolate composition and concentration. In growth chamber experiments, the generalist herbivore, Spodoptera exigua grew significantly better on the inland plants, while the specialist herbivore Plutella xylostella performed equally well on plants of both origins. Aliphatic glucosinolate as well as total glucosinolate concentrations negatively correlated with larval mass of Spodoptera exigua. No significant correlations, however, were found between larval mass of Plutella xylostella and glucosinolates in the leaves. A specialist and a generalist herbivore were responding differently to plant secondary chemistry, as was also found in several other studies. This is an important indication that differences in glucosinolate concentrations among populations may result from differential selection by different guilds of herbivores.     

Irruptive population dynamics in Yellowstone pronghorn.

Irruptive population dynamics appear to be widespread in large herbivore populations, but there are few empirical examples from long time series with small measurement error and minimal harvests. We analyzed an 89-year time series of counts and known removals for pronghorn (Antilocapra americana) in Yellowstone National Park of the western United States during 1918-2006 using a suite of density-dependent, density-independent, and irruptive models to determine if the population exhibited irruptive dynamics. Information-theoretic model comparison techniques strongly supported irruptive population dynamics (Leopold model) and density dependence during 1918-1946, with the growth rate slowing after counts exceeded 600 animals. Concerns about sagebrush (Artemisia spp.) degradation led to removals of >1100 pronghorn during 1947-1966, and counts decreased from approximately 700 to 150. The best models for this period (Gompertz, Ricker) suggested that culls replaced intrinsic density-dependent mechanisms. Contrary to expectations, the population did not exhibit enhanced demographic vigor soon after the termination of the harvest program, with counts remaining between 100 and 190 animals during 1967 1981. However, the population irrupted (Caughley model with a one-year lag) to a peak abundance of approximately 600 pronghorn during 1982-1991, with a slowing in growth rate as counts exceeded 500. Numbers crashed to 235 pronghorn during 1992-1995, perhaps because important food resources (e.g., sagebrush) on the winter range were severely diminished by high densities of browsing elk, mule deer, and pronghorn. Pronghorn numbers remained relatively constant during 1996-2006, at a level (196-235) lower than peak abundance, but higher than numbers following the release from culling. The dynamics of this population supported the paradigm that irruption is a fundamental pattern of growth in many populations of large herbivores with high fecundity and delayed density-dependent effects on recruitment when forage and weather conditions become favorable after range expansion or release from harvesting. Incorporating known removals into population models that can describe a wide range of dynamics can greatly improve our interpretation of observed dynamics in intensively managed populations.     

Insect behavioural ecology and other factors affecting the control efficacy of agro-ecosystem diversification strategies

In the last decade there is an increased interest in the design and use of diversified pest-suppressive agro-ecosystems. A diversification approach aims to manipulate the spatial dynamics of herbivores by adding a trap crop that attracts and retains herbivores in the non-crop area or by adding a disruptive crop that induces an emigration response. Empirical studies have shown that there is a wide variation in insect herbivore response to vegetation diversification. To increase the predictability and reliability of this approach it is necessary to understand the mechanisms underlying herbivore population response to diversified agro-ecosystems. We use a spatially explicit, individual-based, simulation framework, with a strong emphasis on the behavioural ecology of insects, to explore the factors that influence the population regulatory effect of agro-ecosystem diversification. The reported wide variation in population response of herbivores to diverse agro-ecosystems is replicated in this study. In our simulations we found that the population regulation effect of diversification can be positive, negative or negligible. Behavioural factors that influenced the spatial dynamics of herbivore populations were the colonisation pattern, movement speed and sensory mode of finding host plants. Simulations show that the strength of inhibition of flight by the trap crop (i.e. arrestment) is the most important parameter to manipulate the spatial dynamics of insects with post-alighting host recognition behaviour. For herbivore species that use olfactory or visual cues to find host plants, the mechanism of aggregation in the trap crop is a combination of attraction and arrestment and hence the population regulatory effect of the trap crop is higher compared to herbivores with post-alighting host recognition behaviour. An important factor that influences the efficacy of the disruptive cropping strategy is the strength of the emigration-inducing effect of the vegetation. The simulation framework is a valuable tool to test hypotheses on insect behaviour and dynamics in heterogeneous environments and can be used to determine optimal diversification systems and hence generate guidance for establishing environmentally benign pest control strategies.     

Density dependence vs. independence, and irregular population dynamics of a swallow-wort fruit fly

Although most long-term studies of consumer-resource (e.g., predator-prey) interactions select species showing cyclic population dynamics, strong consumer-resource interactions can also produce irregular, noncyclic dynamics. Here, we present a case in which a seed predator, the tephritid fruit fly Euphranta connexa, shows fluctuations in density of more than two orders of magnitude over a 22-year period. To explain these fluctuations, we analyzed a stage-specific data set to quantify the density-dependent and density-independent components of larval survivorship and realized fecundity. Both larval survivorship and realized fecundity were strongly density dependent. Larval survivorship dropped from 0.62 at low larval density to 0.081 at high larval density, whereas fecundity dropped from 84.3 to 0.32 eggs per individual, more than a 100-fold decrease. We divided density-independent variation in E. connexa population dynamics into components for variability in (1) larval survivorship, (2) realized fecundity, and (3) annual fruit abundance. Of these components, 96% of the density-independent variance in per capita population growth rates was caused by fluctuations in fruit abundance. This highlights the importance of the strong consumer-resource interactions in driving fluctuations in E. connexa abundance. It also demonstrates that E. connexa dynamics are remarkably simple, and aside from the 4% of unexplained variance in per capita population growth rates, our understanding of E. connexa dynamics is remarkably complete.     

Demographic importance of ecological interactions: how much do statistics tell us?

Many investigators demonstrate the existence of intra- and interspecific interactions through rigorous statistical hypothesis testing. Statistical significance of an interaction, however, does not necessarily imply that it controls a species population size or distribution. Other analytical methods must be used to make these important determinations. This paper demonstrates one method, with an example, to quantify the demographic importance of statistically significant interactions. In our example, some of the statistically significant interactions with the polychaete Clymenella torquata (Leidy) have large impacts on the population growth rate of the bivalve Gemma gemma (Totten), while others do not. This demonstrates the necessity of distinguishing between statistical significance and demographic importance. In some cases, the demographic importance of the same interaction changes through time because of changes in the bivalve's life history. Interactions affecting juvenile bivalves often have a greater demographic importance than those affecting adults; but this is not always the case.     

Remote analysis of biological invasion and the impact of enemy release

Escape from natural enemies is a widely held generalization for the success of exotic plants. We conducted a large-scale experiment in Hawaii (USA) to quantify impacts of ungulate removal on plant growth and performance, and to test whether elimination of an exotic generalist herbivore facilitated exotic success. Assessment of impacted and control sites before and after ungulate exclusion using airborne imaging spectroscopy and LiDAR, time series satellite observations, and ground-based field studies over nine years indicated that removal of generalist herbivores facilitated exotic success, but the abundance of native species was unchanged. Vegetation cover <1 m in height increased in ungulate-free areas from 48.7% +/- 1.5% to 74.3% +/- 1.8% over 8.4 years, corresponding to an annualized growth rate of lambda = 1.05 +/- 0.01 yr(-1) (median +/- SD). Most of the change was attributable to exotic plant species, which increased from 24.4% +/- 1.4% to 49.1% +/- 2.0%, (lambda = 1.08 +/- 0.01 yr(-1)). Native plants experienced no significant change in cover (23.0% +/- 1.3% to 24.2% +/- 1.8%, lambda = 1.01 +/- 0.01 yr(-1)). Time series of satellite phenology were indistinguishable between the treatment and a 3.0-km2 control site for four years prior to ungulate removal, but they diverged immediately following exclusion of ungulates. Comparison of monthly EVI means before and after ungulate exclusion and between the managed and control areas indicates that EVI strongly increased in the managed area after ungulate exclusion. Field studies and airborne analyses show that the dominant invader was Senecio madagascariensis, an invasive annual forb that increased from < 0.01% to 14.7% fractional cover in ungulate-free areas (lambda = 1.89 +/- 0.34 yr(-1)), but which was nearly absent from the control site. A combination of canopy LAI, water, and fractional cover were expressed in satellite EVI time series and indicate that the invaded region maintained greenness during drought conditions. These findings demonstrate that enemy release from generalist herbivores can facilitate exotic success and suggest a plausible mechanism by which invasion occurred. They also show how novel remote-sensing technology can be integrated with conservation and management to help address exotic plant invasions.     

Using Modeling to Improve Monitoring of Structured Populations: Are We Collecting the Right Data?

Abstract:  Population monitoring is central to most demographic studies and conservation efforts, but it may not always be directed at the most appropriate life stage. We used stochastic simulation modeling to evaluate the effectiveness of a monitoring program for a well-studied population of Eastern Imperial Eagles ( Aquila heliaca ) in Kazakhstan. Specifically, we asked whether the most appropriate data were being collected to understand system state and population dynamics. Our models were parameterized with data collected over the course of 25 years of study of this population. We used the models to conduct simulation experiments to evaluate relationships between monitored or potentially monitored parameters and the demographic variables of interest—population size ( N ) and population growth (λ). Static analyses showed that traditional territory-based monitoring was a poor indicator of eagle population size and growth and that monitoring survivorship would provide more information about these parameters. Nevertheless, these same traditionally monitored territory-based parameters had greater power to detect long-term changes in population size than did survivorship or population structure. Regardless of the taxa considered, threats can have immediate impacts on population size and growth or longer-term impacts on population dynamics. Prudently designed monitoring programs for any species will detect the demographic effects of both types of threats.