Modeling herbivore functional responses causing boom‐bust dynamics following predator removal

Native biodiversity is threatened by invasive species in many terrestrial and marine systems, and conservation managers have demonstrated successes by responding with eradication or control programs. Although invasive species are often the direct cause of threat to native species, ecosystems can react in unexpected ways to their removal or reduction. Here, we use theoretical models to predict boom‐bust dynamics, where the removal of predatory or competitive pressure from a native herbivore results in oscillatory population dynamics (boom‐bust), which can endanger the native species’ population in the short term. We simulate control activities, applied to multiple theoretical three‐species Lotka‐Volterra ecosystem models consisting of vegetation, a native herbivore, and an invasive predator. Based on these communities, we then develop a predictive tool that—based on relative parameter values—predicts whether control efforts directed at the invasive predator will lead to herbivore release followed by a crash. Further, by investigating the different functional responses, we show that model structure, as well as model parameters, are important determinants of conservation outcomes. Finally, control strategies that can mitigate these negative consequences are identified. Managers working in similar data‐poor ecosystems can use the predictive tool to assess the probability that their system will exhibit boom‐bust dynamics, without knowing exact community parameter values.     

Rats dying for mice: Modelling the competitor release effect

Abstract Introduced vertebrate predators are one of the most important threats to endemic species throughout a range of ecosystems, in particular on islands in biodiversity hot spots. Consequently, the reduction of predator numbers is considered a key conservation action in the management of many native vertebrates vulnerable to predators. It is now established that control attempts may affect non‐target species through trophic interactions, but little is known concerning their consequences on competitive relationships. We study a mathematical model mimicking the effects of controlling introduced species in the presence of their competitors. We used two competing rodents to illustrate our study: black rats, Rattus rattus, and mice, Mus musculus. Analyses of the model show that control of only one introduced species logically results in the dramatic increase of the overlooked competitor. We present empirical data that confirm our theoretical predictions. Less intuitively, this process, which we term ‘the competitor release effect’, may also occur when both introduced competitors are simultaneously controlled. In our setting, controlling both predators can promote their coexistence. This occurs as soon as the inferior competitor benefits from the differential effect of the simultaneous control of both competitors, that is, when the indirect positive effect of control (the removal of their competitors) exceeds its direct negative effect (their own removal). Both control levels and target specificity have a direct influence on the extent of this process: counter‐intuitively, the stronger and more specific the control, the greater the effect. The theoretical validation of the competitor release effect has important implications in conservation, especially for control management.     

Legacy effects of canopy disturbance on ecosystem functioning in macroalgal assemblages

Macroalgal assemblages are some of the most productive systems on earth and they contribute significantly to nearshore ecosystems. Globally, macroalgal assemblages are increasingly threatened by anthropogenic activities such as sedimentation, eutrophication and climate change. Despite this, very little research has considered the potential effects of canopy loss on primary productivity, although the literature is rich with evidence showing the ecological effects of canopy disturbance. In this study we used experimental removal plots of habitat-dominating algae (Order Fucales) that had been initiated several years previously to construct a chronosequence of disturbed macroalgal communities and to test if there were legacy effects of canopy loss on primary productivity. We used in situ photo-respirometry to test the primary productivity of algal assemblages in control and removal plots at two intertidal elevations. In the mid tidal zone assemblage, the removal plots at two sites had average primary productivity values of only 40% and 60% that of control areas after 90 months. Differences in productivity were associated with lower biomass and density of the fucoid algal canopy and lower taxa richness in the removal plots after 90 months. Low-shore plots, established three years earlier, showed that the loss of the large, dominant fucoid resulted in at least 50% less primary productivity of the algal assemblage than controls, which lasted for 90 months; other smaller fucoid species had recruited but they were far less productive. The long term reduction in primary productivity following a single episode of canopy loss of a dominant species in two tidal zones suggests that these assemblages are not very resilient to large perturbations. Decreased production output may have severe and long-lasting consequences on the surrounding communities and has the potential to alter nutrient cycling in the wider nearshore environment.     

Trophic and functional cascades in tropical versus temperate aquatic microcosms

Inverse trophic cascades are a well explored and common consequence of the local depletion or extinction of top predators in natural ecosystems. Despite a large body of research, the cascading effects of predator removal on ecosystem functions are not as well understood. Developing microcosm experiments, we explored food web changes in trophic structure and ecosystem functioning following biomass removal of top predators in representative temperate and tropical rock pool communities that contained similar assemblages of zooplankton and benthic invertebrates. We observed changes in species abundances following predator removal in both temperate and tropical communities, in line with expected inverse effects of a trophic cascade, where predation release benefits the predator’s preys and competitors and impacts the preys of the latter. We also observed several changes at the community and ecosystem levels including a decrease in total abundance and mean trophic level of the community, and changes in chlorophyll-a and total dissolved particles. Our results also showed an increase in variability of both community and ecosystem processes following the removal of predators. These results illustrate how predator removal can lead to inverse trophic cascades both in structural and functioning properties, and can increase variability of ecosystem processes. Although observed patterns were consistent between tropical and temperate communities following an inverse cascade pattern, changes were more pronounced in the temperate community. Therefore, aquatic food webs may have inherent traits that condition ecosystem responses to changes in top-down trophic control and render some aquatic ecosystems especially sensitive to the removals of top predators.     

Do lianas shape ant communities in an early successional tropical forest?

Almost half of lowland tropical forests are at various stages of regeneration following deforestation or fragmentation. Changes in tree communities along successional gradients have predictable bottom‐up effects on consumers. Liana (woody vine) assemblages also change with succession, but their effects on animal succession remain unexplored. Here we used a large‐scale liana removal experiment across a forest successional chronosequence (7–31 years) to determine the importance of lianas to ant community structure. We conducted 1,088 surveys of ants foraging on and living in trees using tree trunk baiting and hand‐collecting techniques at 34 paired forest plots, half of which had all lianas removed. Ant species composition, β‐diversity, and species richness were not affected by liana removal; however, ant species co‐occurrence (the coexistence of two or more species in a single tree) was more frequent in control plots, where lianas were present, versus removal plots. Forest stand age had a larger effect on ant community structure than the presence of lianas. Mean ant species richness in a forest plot increased by ca. 10% with increasing forest age across the 31‐year chronosequence. Ant surveys from forest >20 years old included more canopy specialists and fewer ground‐nesting ant species versus those from forests <20 years old. Consequently, lianas had a minimal effect on arboreal ant communities in this early successional forest, where rapidly changing tree community structure was more important to ant species richness and composition.     

Extinction cascades and the distribution of species interactions

The distribution of interaction strengths among community members has important consequences for assembly processes and community responses to perturbations. Species deletion from communities can trigger cascading extinction events, with strong evidence from empirical and theoretical work. I examined model competitive communities, sequentially assembled using species drawn from a global pool with interaction strengths described by different distribution shapes (uniform or beta), with the same mean and variance. As community size increased, it became harder to assemble communities drawn from a uniform distribution compared to a beta distribution. The distribution of interaction values in the assembled communities differed from the shape of the initial distribution. The distribution shape and the relative abundance of the deleted species also had strong impacts on the probability of extinction cascades following primary species removal. Extinction cascades occurred in communities with a higher mean and variance of interaction strengths before the primary extinction. Those species lost had negative equilibrium densities and tended to be the least abundant, when assessed following the reorganisation that occurred after the primary and subsequent extinctions. Knowledge of the shape of the distribution of interaction strengths from real communities will allow us to make better predictions about which species are most at risk in extinction cascades under natural circumstances.     

Recovery of Foredune and Blowout Habitats in a Freshwater Dune Following Removal of Invasive Austrian Pine (Pinus nigra)

Invasive species removal is an important first step toward restoring invaded ecosystems; however, restoration following removal may be hindered by (1) unintended consequences of management, such as habitat destabilization, and/or (2) legacy effects of the invader, such as persistent alterations of soil structure or plant community composition. During 1956–1972, approximately 26,000 individuals of the non‐native pine, Pinus nigra, were planted into multiple freshwater sand dune habitats as a stabilization measure on the eastern shore of Lake Michigan in Allegan County, MI, U.S.A. From 2004 to 2010, we evaluated the recovery of foredune and blowout habitats following P. nigra removal in 2003–2005. We compared sand movement and plant community structure, composition, and richness between removal and control sites over the 6 years following pine removal. In addition, we evaluated the impact of litter removal on recolonization of native graminoids in foredunes. Sand movement patterns never differed between removal and control sites in foredunes; however, accumulation was more common in removal sites in blowouts 1 and 6 years following pine removal. Vegetation cover in removal sites became indistinguishable from control sites in both foredunes and blowouts, but species richness for both forb and woody species was higher in removal sites in blowouts. Removal sites in both foredunes and blowouts had higher cover by forbs and lower cover by graminoids. Pine litter did not inhibit recolonization of foredunes by native graminoids. These results suggest that high disturbance habitats, such as sand dunes, have the potential to recover from invasion if the mechanism of disturbance is restored and pioneer species are present to recolonize the system.     

Mesopredators constrain a top predator: competitive release of ravens after culling crows

Although predator control programmes rarely consider complex competitive interactions among predators, it is becoming clear that removal of larger ‘superior’ competitors often releases the ‘inferior’ ones and can precipitate trophic cascades. In contrast, our study indicates that culling hooded crows Corvus cornix appears to release a larger competitor, the common raven Corvus corax. Ravens ranged more widely, and the predation of artificial nests was significantly faster (although total predation was similar), after the removal of crows. Our study provides evidence of a novel reversal of competitive release where a larger species was freed from constraints imposed on its distribution and behaviour by a smaller species, and emphasizes the importance of considering community and ecosystem effects of predator manipulations when undertaken for conservation or game management.     

Below-ground interactions with arbuscular mycorrhizal shrubs decrease the performance of pinyon pine and the abundance of its ectomycorrhizas

Few studies have examined how below-ground interactions among plants affect the abundance and community composition of symbiotic mycorrhizal fungi. Here, we combined observations during drought with a removal experiment to examine the effects of below-ground interactions with arbuscular mycorrhizal (AM) shrubs on the growth of pinyon pines (Pinus edulis), and the abundance and community composition of their ectomycorrhizal (EM) fungi. Shrub density was negatively correlated with pinyon above- and below-ground growth and explained 75% of the variation in EM colonization. Consistent with competitive release, pinyon fine-root biomass, shoot length and needle length increased with shrub removal. EM colonization also doubled following shrub removal. EM communities did not respond to shrub removal, perhaps because of their strikingly low diversity. These results suggest that below-ground competition with AM shrubs negatively impacted both pinyons and EM fungi. Similar competitive effects may be observed in other ecosystems given that drought frequency and severity are predicted to increase for many land interiors.     

Removal of livestock alters native plant and invasive mammal communities in a dry grassland–shrubland ecosystem

The impacts of domesticated herbivores on ecosystems that did not evolve with mammalian grazing can profoundly influence community composition and trophic interactions. Also, such impacts can occur over long time frames by altering successional vegetation trajectories. Removal of domesticated herbivores to protect native biota can therefore lead to unexpected consequences at multiple trophic levels for native and non-native species. In the eastern South Island of New Zealand large areas of seral grassland–shrubland have had livestock (sheep and cattle) removed following changes in land tenure. The long-term (>10 years) outcomes for these communities are complex and difficult to predict: land may return to a native-dominated woody plant community or be invaded by exotic plants and mammals. We quantified direct and indirect effects of livestock removal on this ecosystem by comparing plant and invasive mammal communities at sites where grazing by livestock ceased c.10–35 years ago (conservation sites) with paired sites where pastoralism has continued to the present (pastoral sites). There was higher total native plant richness and reduced richness of exotic plants on conservation sites compared with pastoral sites. Further, there were differences in the use of conservation and pastoral sites by invasive mammals: rabbits and hedgehogs favoured sites grazed by livestock whereas house mice, brushtail possums and hares favoured conservation sites. Changes in the relative abundance of invasive mammal species after removal of domesticated livestock may compromise positive outcomes for conservation in successional plant communities with no evolutionary history of mammalian grazing.     

Responses of house mice to the removal of mammalian predators and competitors

Efforts to eradicate multiple mammal pests from offshore islands and fenced mainland ‘habitat islands’ often fail to remove mice, and such failures can result in a dramatic change in the food‐web whereby the removal of larger mammal pests facilitates a population explosion of mice through predator and competitor release. We investigated the ecological responses of house mice to the removal of mammalian predators from a 500‐ha fenced sanctuary at Tawharanui, northern New Zealand. Data on population structure and body condition of mice trapped in 2007, in four habitat types within the sanctuary, were compared with baseline data collected in 2001, before mammal control operations commenced. We hypothesized that: (i) in the absence of mammalian predators mouse densities would increase in all habitat types that provide vegetation cover, and (ii) in the absence of mammalian competitors mice would become heavier due to greater access to food resources. Mouse densities were significantly higher in 2007 than in 2001 in three habitat types. The high density of mice in forest – where none were trapped prior to control – suggests a competitive release, in which mice profited from the removal of ship rats. No mice were caught in the presence of ship rats on a forest trap‐line at a control site outside the sanctuary. Mice trapped in 2007 were significantly heavier than those trapped in 2001, and significantly heavier than mice trapped at the control site. Greater access to food in the absence of competing and predatory mammals probably explains the heavier body weight of Tawharanui mice. There has been a significant change in the mammalian food‐web at Tawharanui, such that the house mouse is now the primary pest. A rapid and dramatic increase in mouse numbers is likely to adversely impact invertebrates and seedling recruitment, which in turn could affect ecosystem functions.     

The impacts of different management strategies and environmental forcing in ecological communities

Understanding the effects of population management on the community a target species belongs to is of key importance for successful management. It is known that the removal or extinction of a single species in a community may lead to extinctions of other community members. In our study, we assess the impacts of population management on competitive communities, studying the response of both locally stable and unstable communities of varying size (between four and 10 species) to three different management strategies; harvesting of a target species, harvesting with non-targeted catch, and stocking of the target species. We also studied the consequences of selecting target species with different relative abundances, as well as the effects of varying environmental conditions.We show here how the effects of management in competitive communities extend far beyond the target population. A crucial role is played by the underlying stability properties of the community under management. In general, locally unstable communities are more vulnerable to perturbation through management. Furthermore, the community response is shown to be sensitive to the relative density of the target species. Of considerable interest is the result that even a small (2.5%) increase in the population size of the target species through stocking may lead to extinction of other community members. These results emphasize the importance of considering and understanding multi-species interactions in population management.     

Short‐term Effects of Single Species Browsing Release by Different‐sized Herbivores on Sand Forest Vegetation Community,South Africa

Manipulations of herbivores in protected areas may have profound effects on ecosystems. We examine short‐term effects on tree species assemblages and resource utilization by a mesoherbivore and small‐size herbivores (ungulates <20 kg) in Sand Forest, after browsing release from a megaherbivore (elephant), or both a mega‐ and mesoherbivore (nyala), respectively. Effects were experimentally separated using replicated exclosures where all trees were counted, identified to species and browsing events recorded. Tree species assemblages were impacted by both elephant and nyala, and by each herbivore species individually. Tree turnover rates were higher where both herbivore species were present than in their combined absence. Diet was segregated among elephant, nyala and small‐size herbivores. Both resource specificity and browsing pressure by nyala increased in absence of elephant; small‐size herbivores increased resource specificity in absence of elephant, and increased browsing pressure in absence of both elephant and nyala. This implies interference competition with competitive release. The indirect effect of the manipulation of herbivore populations, through the removal of one or two herbivore species, caused a shift in tree species composition and diet of smaller‐size herbivores. These indirect effects, especially on tree species composition, can become critical as they affect vegetation dynamics, biodiversity and ecosystem processes. Therefore, in order to conserve habitats and biodiversity across all trophic levels, conservation managers should consider the effects of: (1) the full herbivore assemblage present; and (2) any effects of altering the relative and absolute abundance of different herbivore species on other herbivore species and vegetation.     

Avoiding surprise effects on Surprise Island: alien species control in a multitrophic level perspective

Eradications of invasive alien species have generally benefited biodiversity. However, without sufficient planning, successful eradications can have unexpected and unwanted consequences for native species and ecosystems. In particular, the “surprise effect” is the rapid increase of hitherto unnoticed species following the sudden removal of an invasive alien that was exerting an ecological force on those species (predation, competition or herbivory, for example). The only way to prevent these undesired outcomes is to adapt the control programme following the characterization of the trophic relationships between the invasive alien species and the invaded communities, that is, to view the control with a holistic perspective. Here, we illustrate this point with the study of the role of the ship rat (Rattus rattus), which invaded a tropical pacific atoll, Surprise Island, New Caledonia. We assessed the risk of surprise effects during a pre-eradication phase of several years, and then adapted our eradication strategy accordingly.     

Plant community establishment in a restored wetland: Effects of soil removal

Question: This study investigated the establishment of wetland plant assemblages following soil removal and restored hydrology in a former agricultural field. The following questions were posed. Does plant community composition differ as a result of soil removal? Does soil removal reduce the frequency of non-wetland plants? Does soil removal reduce the frequency of non-native invasive plants? Location: The Panzner Wetland Wildlife Reserve (PWWR) in Summit County, northeastern Ohio, USA. Methods: During 2000–2001, restoration was conducted on two adjoining fields (3.9 ha total) by excavating the upper 40–50 cm of soil layer and establishing 12 10 m × 10 m undisturbed control plots. Preliminary data included seed bank composition and soil organic matter, estimated from three different soil depths on the control plots. In spring 2004, a 10 m × 10 m soil-removed plot was established adjacent to each control plot. Plant percent cover of all species was estimated within the center 5 m × 5 m of every plot. Above-ground biomass of all species from three 0.25-m² quadrats was collected. Environmental water measurements included water depth, temperature, dissolved oxygen, pH, and conductivity. Results: The top 10 cm of soil contained the most seeds, the highest species diversity, the greatest proportion of annual to perennial plants, and the lowest organic content. Obligate and facultative wetland plants were found in soil-removed plots while facultative upland and upland plants were found in control plots. The only plots with arable weeds were the control plots. However, plant communities on soil-removed plots in the North field, which had a higher elevation (ca. 15–20 cm), had a different species composition than soil-removed plots in the South field. Conclusions: The results of a controlled, replicated large-scale study on the effects of soil removal showed that soil removal altered both the biotic and abiotic environment, but that the proximity to the water table was the primary controlling factor in the assembly of plant communities.     

群落中克隆植物的重要性

综述了群落水平上克隆植物的重要性,克隆植物的生态习性,克隆植物的竞争关系等方面的研究进展,并试图在克隆植物竞争关系的背景下,结合其生境状况,探讨植物克隆性与植物物种多样性的关系。克隆植物的等级系统（基株—分株系统—分株）使其具有克隆性所赋予的多样的生活史、资源利用以及空间占据方式。列举了克隆植物的特性及其在群落中的作用。这些例子表明,克隆性强烈地影响和制约着植物群落的空间格局与竞争关系。然而克隆性也能通过权衡活动性与局部持久性来缓解对物种共存的抑制。克隆植物具有在时空尺度上分株间相互调节的机制,直接体现在群落中小尺度（个体与个体间）与大尺度（种群与种群间）间的相互作用上。丰富了传统的竞争和生态位划分理论,为群落中物种共存提供了合理解释。因此克隆植物在群落中的出现拓宽了群落系统潜在机制的范围。     

SESAM – a new framework integrating macroecological and species distribution models for predicting spatio‐temporal patterns of species assemblages

Two different approaches currently prevail for predicting spatial patterns of species assemblages. The first approach (macroecological modelling, MEM) focuses directly on realized properties of species assemblages, whereas the second approach (stacked species distribution modelling, S‐SDM) starts with constituent species to approximate the properties of assemblages. Here, we propose to unify the two approaches in a single ‘spatially explicit species assemblage modelling’ (SESAM) framework. This framework uses relevant designations of initial species source pools for modelling, macroecological variables, and ecological assembly rules to constrain predictions of the richness and composition of species assemblages obtained by stacking predictions of individual species distributions. We believe that such a framework could prove useful in many theoretical and applied disciplines of ecology and evolution, both for improving our basic understanding of species assembly across spatio‐temporal scales and for anticipating expected consequences of local, regional or global environmental changes. In this paper, we propose such a framework and call for further developments and testing across a broad range of community types in a variety of environments.     

Patterns in the structure of Asian and North American desert small mammal communities

Aim We compared assemblages of small mammal communities from three major desert regions on two continents in the northern hemisphere. Our objective was to compare these with respect to three characteristics: (1) species richness and representation of trophic groups; (2) the degree to which these assemblages exhibit nested community structure; and (3) the extent to which competitive interactions appear to influence local community assembly. Location We studied small mammal communities from the deserts of North America (N=201 sites) and two regions in Central Asia (the Gobi Desert (N=97 sites) and the Turan Desert Region (N=36 sites), including the Kara-Kum, Kyzyl-Kum, NE Daghestan, and extreme western Kazakhstan Deserts). Method To provide baseline data we characterized each desert region in terms of alpha, beta, and gamma diversity, and in terms of the distribution of taxa across trophic and locomotory groups. We evaluated nestedness of these communities using the Nestedness Temperature Calculator developed by Atmar & Patterson (1993, 1995) , and we evaluated the role of competitive interactions in community assembly and applied a null model of local assembly under varying degrees of competitive interaction ( Kelt et al., 1995, 1996 ). Results All three desert regions have low alpha diversity and high beta diversity. The total number of species in each region varied, being highest in North America, and lowest in the Turan Desert Region. The deserts studied all present evidence of significant nestedness, but the mechanism underlying this structure appears different in North American and Asia. In North America, simulations strongly implicate interspecific competition as a dominant mechanism influencing community and assemblage structure. In contrast, data from Asian desert rodent communities suggest that these are not strongly influenced by competition; in fact, they have greater numbers of ecologically and morphologically similar species than expected. These results appear to reflect strong habitat selection, with positive associations among species that share similar habitat requirements in these communities. Our analyses support earlier reports suggesting that predation and abiotic forces may have greater influences on the assembly and organization of Asian desert rodent communities, whereas interspecific competition dominates assembly processes in North America. Additionally, we suggest that structuring mechanisms may be very different among the two Asian deserts studied. Gobi assemblages appear structured by trophic and locomotory strategies. In contrast, Turan Desert Region assemblages appear to be randomly structured with respect to locomotory strategies. When trophic and locomotory categories are combined, however, Turan species are positively and nonrandomly associated. Main conclusions Very different ecological dynamics evidently exist not only between these continents, but within them as well. These small mammal faunas differ greatly in terms of community structure, but also appear to differ in the underlying mechanisms by which communities are assembled. The underlying role of history and geography are strongly implicated as central features in understanding the evolution of mammalian faunas in different deserts of the world.     

Community assembly rules affect the diversity of expanding communities

Despite centuries of interest in species range limits, few studies have taken a whole community into consideration. Actually, multiple species may simultaneously respond to environmental changes, for example, global warming, leading a series of dynamical communities toward the advancing front. We investigated multiple species range expansions through the analysis of a two‐species dispersion model and simulations of multiple species assemblages regulated by neutral and fecundity–survival trade‐offs (FSTs), respectively, and found that species assemblages regulated by different mechanisms would initiate different expanding patterns in geographic ranges in response to environmental changes. The neutral model generally predicts a higher biodiversity near the core of an expanding range, and a lower community similarity compared with a FST model. Without considering the evolution of life history traits, an assortment of the reproduction ability happens at the advancing front under FSTs at the expense of a higher death rate or lower competitive ability. These results emphasize the importance of community assembly rules to the biodiversity maintenance of range expanding communities.     

Paleoecology of some molluscan assemblages from the badenian (Miocene) marine sandy facies of Poland

Three molluscan assemblages from the Badenian (Miocene) marine sandy facies of Poland are described in terms of their taxonomic composition, diversity, and trophic structure. The structural variation between the molluscan assemblages seems to be largely controlled by a gradient in water energy which in turn results in gradients in substrate mobility and organic content of bottom sediments. Together with a subordinate factor of local sedimentation rate, these gradients control the distribution of lucinoid mucus tube feeders vs. deep burrowing siphonate suspension feeders, of browsers, and of deposit feeders vs. suspension feeders. Some structural features of the molluscan assemblages are regarded as related to biotic factors. The graphs of cumulative species frequencies vs. the logarithm of cumulative individual frequencies are used as a measure of community organization. These graphs indicate distinct differences in ecological maturity between the molluscan assemblages. On this basis, two different stages of ecological succession in an offshore sandy bottom environment are recognized. The evidence from the comparison of the Miocene molluscan assemblages with their recent counterparts may support the view that the evolutionary stability of species populations of a given community, and long-term stability of the community structure are independent. High environmental variability and stress seem to be correlated with evolutionary stability of species populations rather than with long-term permanence of community structure.