Historical perspectives on human-assisted biological invasions

Of all the things we have done to the Earth so far, which one is likely to make the most indelible mark? I have been posing this question to students and colleagues for some time, and usually receive one of three reasonable replies: habitat destruction, human overpopulation, or global climate change. Yet as bad as all these maladies are, they are, to some extent, reversible on time scales to which we can relate. The remedies for these problems, despite mammoth cultural and economic obstacles, can be visualized generally in terms of technology and scientific theory. Only the will is lacking. On longer geological and evolutionary time scales, most such human impacts, if corrected soon, would be mere blips, perturbations of the ecosphere that would leave only a few detectable traces in the fossil record a million years hence.     

Fish as vectors in the dispersal of Bythotrephes cederstroemi: diapausing eggs survive passage through the gut

• 1 Bythotrephes cederstroemi (Crustacea: Onychopoda: Cercopagidae) is an introduced invertebrate predator currently spreading through the Laurentian Great Lakes region of North America. We examined a previously unsuspected way in which B. cederstroemi may be dispersed by fish by their consumption of diapausing eggs.
• 2 Ninety‐four percentage of the mature B. cederstroemi diapausing eggs consumed by fish were egested apparently intact. This proportion is considerably above previous estimates for the ephippial eggs of Daphnia. The hatching success of diapausing eggs was compared among four categories: (a) eggs released naturally by B. cederstroemi (control, 73% hatched) (b) eggs released during ‘stressful confinement’ (46% hatched) (c) eggs dissected from dead females (13% hatched) and (d) eggs recovered from faecal pellets following consumption by fish (viable gut passage experiment, 41% hatched).
• 3 Samples of small fish and B. cederstroemi were collected simultaneously. Examination of gut contents revealed that fish contained B. cederstroemi diapausing eggs and that B. cederstroemi bearing resting eggs were consumed selectively over those without eggs. Moreover, fish selected B. cederstroemi bearing mature rather than immature diapausing eggs.
• 4 The fact that diapausing eggs survive gut passage is important for the dispersal of B. cederstroemi. Fish often move between the pelagic and littoral zones of lakes and may thus disperse diapausing eggs widely. Fish may also move between lakes connected by river systems and can be caught and passively dispersed by anglers or piscivorous birds. Our results demonstrate the potential for fish to act as vectors in the spread of B. cederstroemi.

     

How to define nativeness in organisms with high dispersal capacities? A comment on Essl et al.

Essl and colleagues documented worldwide invasion patterns in bryophytes, which so far have been neglected in invasion biology. In the absence of historical evidence, Essl and colleagues used criteria such as anomalous geographical distribution, preference for disturbed habitats, and indirect associations with some means of human transport as criteria to identify aliens. Because bryophytes exhibit high long‐distance dispersal capabilities, disjunct distribution patterns are, however, the rule rather than the exception in the group. In our opinion, none of the previously proposed criteria to characterize aliens can be satisfactorily applied to groups like bryophytes, for which historical and fossil records are extremely scarce. We suggest that, in order to validate the conclusions of Essl and colleagues, further taxonomic and phylogeographical studies are needed. This is especially true for island floras, for which recent critical taxonomic work and updated checklists, which compose the primary source of information for biodiversity, are largely missing.     

Boats, Pathways, and Aquatic Biological Invasions: Estimating Dispersal Potential with Gravity Models

Biological invaders can have dramatic effects on the environment and the economy. To most effectively manage these invaders, we should consider entire pathways, because multiple species are dispersed through the same vectors. In this paper, we use production-constrained gravity models to describe movement of recreational boaters between lakes – potentially the most important pathway of overland dispersal for many aquatic organisms. These models are advantageous because they require relatively easily acquired data, hence are relatively easy to build. We compare linear and non-linear gravity models and show that, despite their simplicity, they are able to capture important characteristics of the recreational boater pathway. To assess our model, we compared observed data based on creel surveys and mailed surveys of recreation boaters to the model output. Specifically, we evaluate four metrics of pathway characteristics: boater traffic to individual lakes, distances traveled to reach these lakes, Great Lakes usage and movement from the Great Lakes to inland waters. These factors will influence the propagule pressure (hence the probability of establishment of invasive populations) and the rate of spread across a landscape. The Great Lakes are of particular importance because they are a major entry point of non-indigenous species from other continents, hence will act as the origin for further spread across states. The non-linear model had the best fit between model output and empirical observations with r² =0.80, r² =0.35, r² =0.57, and r² =0.36 for the four metrics, respectively. For the distances traveled to individual lakes, r² improved from 0.35 to 0.76 after removal of an outlier. Our results suggest that we were able to capture distances traveled to most but not all lakes. Thus, we demonstrate that production-constrained gravity models will be generally useful for modeling invasion pathways between non-contiguous locations.     

Spatial variation in ditch bank plant species composition at the regional level: the role of environment and dispersal

Questions: Can patterns of species similarity on ditch banks be explained by environmental and dispersal factors and, if so, to what extent? Does the pattern of distance decay differ among different species groups (all species versus target species of conservation interest; species of different dispersal type)? Location: Krimpenerwaard, the Netherlands. Methods: In 2006‐2007, ditch bank vegetation data on 130 terrestrial herbaceous species were collected on 72 plots. Species similarity was measured and related to environmental distance (soil type and nutrient level) and dispersal distance (geographic distance and limitation of dispersal by water, wind and agricultural activities) as explanatory factors using multiple regression on distance matrices (MRM). Differences in rates of distance decay in species similarity among different subsets of data (species groups) were investigated using randomization tests. Results: In all species, patterns of similarity of composition are influenced mainly by variations in dispersal, while for target species these are due to combined effects of environmental and dispersal variation. Compared with species using other dispersal mechanisms, water‐dispersed species had half the rate of distance decay. Conclusions: For all species considered here, dispersal limitation seems more responsible for the spatial variation in species composition than environmental determinism. Conservation management focused on plant species diversity would be more successful in areas adjacent to those where a similar management regime is already in force. For target species of conservation interest, besides dispersal limitation, environmental determinants like nutrient level are also important. As a means of conserving such target species, therefore, focusing on reducing nutrient levels and facilitating species dispersal will be more effective than current management practices, which mainly focus on reducing fertilizer inputs.     

Are fish populations in temperate streams affected by crayfish? – A field survey and prospects

Synopsis Fish populations may be affected by predation and competition from various types of organisms, among which crayfish have been suggested as important actors. We here present results from stream surveys, suggesting that neither native noble, Astacus astacus, nor introduced signal crayfish, Pacifastacus leniusculus, necessarily affect fish population densities in temperate stream communities. Comparisons of fish densities within stream sites between years with absence and presence of crayfish showed no effect of either crayfish species. A further analysis of changes in fish densities between periods without and with crayfish in low, intermediate and high densities revealed that crayfish density did neither have an effect on fish densities. Our study is one of exceptionally few that consider the above aspects in long-term perspectives in natural systems, and we discuss that previously reported divergent results of crayfish effects on fish may be highly dependent on specific species and methods used, and that the effects of crayfish on fish populations deserve further attention to enable reliable predictions of community processes in streams.     

外来种入侵与物种多样性

在入侵生态学研究方面 ,物种多样性与生物入侵之间的关系已成为当前研究和争论的焦点。自Elton的经典假说提出以来 ,物种丰富度高的群落比物种贫乏的群落更能抵抗外来种入侵的观点得到广泛接受。一些理论模型和多样性处理实验支持了该假说。但现在越来越多的野外观测和实验研究开始对这一经典假说提出异议 ,甚至反对。同时 ,在入侵生态学广泛受到关注的今天 ,大量的实验研究也提出了一些新的观点。本文对Elton经典假说提出以来全球有关物种多样性与生物入侵关系的主要研究及其观点进行了评述 ,以期为我国有关研究工作的开展提供参考。     

外来物种入侵后的多物种竞争共存的集合种群模型

基于多物种竞争共存模型,提出外来物种与本地物种竞争共存途径的两种假想:外来物种通过插队竞争与本地物种实现共存;外来物种通过等位竞争与本地物种实现共存.并提出根据外来物种在两种竞争共存模式下占据生境斑块比例的稳定值大小来判断外来物种和本地物种的竞争共存途径.根据两种假想,分别建立了外来物种插队竞争共存模型和等位竞争共存模型.通过应用数学软件Mathematica 4.0对两个模型进行了模拟,得出以下结论:在外来物种与本地物种竞争共存状态下,如果外来物种通过插队竞争与本地物种实现共存,当本地物种竞争力差异较大时,外来物种极易对本地稀少物种构成危害.虽然外来物种不会直接造成本地稀少物种的灭绝,但是会使本地稀少物种的生境斑块急剧减少,增加本地稀少物种灭绝的可能性,而当本地物种竞争力差异较小时,外来物种对本地所有物种的影响都较小.如果外来物种通过等位竞争与本地物种实现共存,无论本地物种竞争力差异大小与否,外来物种只是影响到与其生态位相同的本地物种,影响程度取决于外来物种侵入时所占据生境斑块的比例大小.