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Mammalian herbivores can have pronounced effects on plant diversity but are currently declining in many productive ecosystems through direct extirpation, habitat loss and fragmentation, while being simultaneously introduced as livestock in other, often unproductive, ecosystems that lacked such species during recent evolutionary times. The biodiversity consequences of these changes are still poorly understood. We experimentally separated the effects of primary productivity and herbivores of different body size on plant species richness across a 10-fold productivity gradient using a 7-year field experiment at seven grassland sites in North America and Europe. We show that assemblages including large herbivores increased plant diversity at higher productivity but decreased diversity at low productivity, while small herbivores did not have consistent effects along the productivity gradient. The recognition of these large-scale, cross-site patterns in herbivore effects is important for the development of appropriate biodiversity conservation strategies. [KEYWORDS: Cross-site ; fertility ; grazing ; plant–animal ;species richness]

Retrieve original file from: http://edepot.wur.nl/121801 High herbivore densities and re-occurring fires are natural phenomenons that determine the structure and functioning of African savannas. Traditional burning practices have been intensified over the past years due to increased herbivore numbers. Insight how animals respond to fire is important to understand ecosystem functioning and for the use of fire as management tool. Until now studies have centred mainly on spatial relationships between herbivores and fires. Very little information exists on effects of temporal fire patterns on herbivore communities. To assess the effect of fire succession on herbivore species greatly differing in body mass we conducted experiments and observational studies on smaller scales and landscape scales. We investigated the response of herbivores on both short-term and long-term succession patterns. Rodent densities were affected by the presence of larger herbivores rather than by seasonal patterns and increased with the absence of large herbivores and increasing vegetation height. Both middle-sized and large herbivore species responded on short-term post-fire patterns with large species being the pioneer species on burnt patches, whereas buffalo was the only species that also responded on long-term post-fire patterns. Our results indicate that larger herbivores in interaction with fire influence rodents by both increasing and prolonging the predation risk they experience in these habitats. Furthermore grazer presence and the amount of rainfall may be two interacting factors that prolong fire patterns in the landscape. Particularly buffalo responded to fire patterns, most likely due to the higher intake levels of digestible plant material and increased forage efficiency. Herbivore response to fire history

Species abundances are undoubtedly the most widely available macroecological data, but can we use them to distinguish among several models of community structure? Here we present a Bayesian analysis of species-abundance data that yields a full joint probability distribution of each model's parameters plus a relatively parameter-independent criterion, the posterior Bayes factor, to compare these models. We illustrate our approach by comparing three classical distributions: the zero-sum multinomial (ZSM) distribution, based on Hubbell's neutral model, the multivariate Poisson lognormal distribution (MPLN), based on niche arguments, and the discrete broken stick (DBS) distribution, based on MacArthur's broken stick model. We give explicit formulas for the probability of observing a particular species-abundance data set in each model, and argue that conditioning on both sample size and species count is needed to allow comparisons between the two distributions. We apply our approach to two neotropical communities (trees, fish). We find that DBS is largely inferior to ZSM and MPLN for both communities. The tree data do not allow discrimination between ZSM and MPLN, but for the fish data ZSM (neutral model) overwhelmingly outperforms MPLN (niche model), suggesting that dispersal plays a previously underestimated role in structuring tropical freshwater fish communities. We advocate this approach for identifying the relative importance of dispersal and niche-partitioning in determining diversity of different ecological groups of species under different environmental conditions.

Current neutral theory in community ecology views local biodiversity as a result of the interplay between speciation, extinction and immigration. Simulations and a mean-field approximation have been used to study this neutral theory. As simulations have limitations of convergence and the mean-field approximation ignores dependencies between species' abundances when applied to species-abundance data, there is still no final conclusion whether the neutral theory or the traditional lognormal model describes community structure best. We present a novel analytical framework, based on the genealogy of individuals in the local community, to overcome the problems of previous approaches, and show, using Bayesian statistics, that the lognormal model provides a slightly better fit to the species-abundance distribution of a much-discussed tropical tree community. A key feature of our approach is that it shows the tight link between genetic and species diversity, which creates important perspectives to future integration of evolutionary and community ecological theory.

Aim Tropical rain forests are often regarded as pristine and undisturbed by humans. In Central Africa, community-wide disturbances by natural causes are rare and therefore current theory predicts that natural gap phase dynamics structure tree species composition and diversity. However, the dominant tree species in many African forests recruit poorly, despite the presence of gaps. To explain this, we studied the disturbance history of a species-rich and structurally complex rain forest. Location Lowland rain forest in Southern Cameroon. Methods We identified the recruitment conditions of trees in different diameter classes in 16 ha of species-rich and structurally complex 'old growth' rain forest. For the identification of recruitment preference we used independent data on the species composition along a disturbance gradient, ranging from shifting cultivation fields (representing large-scale disturbance), to canopy gaps and old growth forest. Results In nine of sixteen 1-ha forest plots the older trees preferred shifting cultivation fields for recruitment while younger trees preferred gaps and closed forest conditions. This indicates that these nine sites once experienced large-scale disturbances. Three lines of evidence suggest that historical agricultural use is the most likely disturbance factor: (1) size of disturbed and undisturbed patches, (2) distribution of charcoal and (3) historical accounts of human population densities. Main conclusions Present-day tree species composition of a structurally complex and species-rich Central African rain forest still echoes historical disturbances, most probably caused by human land use between three to four centuries ago. Human impact on African rain forest is therefore, contrary to common belief, an issue not of the last decades only. Insights in historical use will help to get a more balanced view of the 'pristine rain forest', acknowledging that the dualism between 'old growth' and 'secondary' forest may be less clear than previously thought.

Secondary forests in Central Africa are increasing in importance for biodiversity conservation as old growth forests outside the few protected areas are disappearing rapidly. We examined vegetation recovery in a lowland rain forest area in Cameroon based on a detailed botanical survey of old growth forest and different-aged logging gaps (5-27 years) and shifting cultivation fields (10-60 years). Our analysis focuses on the long-term recovery of botanical conservation values by analysing trends in vegetation structure, species composition, species diversity and levels of endemism and rarity. In the total survey (4.25 ha), we recorded 834 species of which 23% were endemic to the Lower Guinea forest region. The proportion of endemic species was high in shrubs and low in herbs. Geographic range and (local) rarity were not significantly associated. The proportion of rare species (relative frequency

In this study we analysed the effects of large herbivores on small rodent communities in different habitats using large herbivore exclosures. We studied the effects of three year grazing introduction by red deer (Cervus elaphus L.) in previously ungrazed pine and oak woodland and the exclusion of grazing by red deer, roe deer (Capreolus capreolus L.) and mouflon (Ovis ammon musimin L.) in formerly, heavily grazed pine woodland and heathland. At eight exclosure sites within each habitat type, small rodents were captured with live traps using trapping grids. At each trapping grid, seed plots of beechnuts (Fagus sylvatica L.) and acorns (Quercus robur L.) were placed to measure seed predation by rodents. Exclusion of grazing by large herbivores in formerly, heavily grazed habitats had a significant effect on small rodent communities. Inside exclosures higher densities of mainly wood mice (Apodemus svlvaticus L.) and field voles (Microtus agrestis L.) were captured. Introduction of grazing by red deer appeared to have no significant negative effects on small rodent communities. The seed predation intensity of beechnuts and acorns by small rodents was significantly higher in ungrazed situations, particularly in habitats that were excluded from grazing. The differences between grazing introduction and exclusion effects on small rodent communities can be explained by differences in vegetation structure development. The recovery of heavily browsed understory vegetation after large herbivore grazing exclusion proceeded faster than the understory degradation due to grazing introduction. Small rodents depend on structural rich vegetations mainly for shelter. We conclude that large herbivores can have significant effects on vegetation dynamics not only via direct plant consumption but also through indirect effects by reducing the habitat quality of small rodent habitats.

We studied the degree and scale of patchiness of vegetation and selected soil variables along a gradient of herbivore impact. The gradient consisted of a radial pattern of 'high', 'intermediate' and 'low' herbivore impact around a watering point in a semi-arid environment in Burkina Faso (West Africa). We hypothesised that, at a certain range of herbivore impact, vegetated patches alternating with patches of bare soil would occur as a consequence of plant-soil feedbacks and run-off-run-on patterns. Indeed, our transect data collected along the gradient showed that vegetated patches with a scale of about 5-10 m, alternating with bare soil, occurred at intermediate herbivore impact. When analysing the data from the experimental sites along the gradient, however, we also found a high degree of patchiness of vegetation and soil variables in case of low and high herbivore impact. For low herbivore impact, most variation was spatially explained, up to 100% for vegetation biomass and soil temperature, with a patch scale of about 0.50 m. This was due to the presence of perennial grass tufts of Cymbopogon schoenanthus. Patterns of soil organic matter and NH4-N were highly correlated with these patterns of biomass and soil temperature, up to r = 0.7 (P < 0.05) for the in situ correlation between biomass and NH4-N. For high herbivore impact, we also found that most variation was spatially explained, up to 100% for biomass and soil temperature, and 84% for soil moisture, with three distinct scales of patchiness (about 0.50 m, 1.80 m and 2.80 m). Here, microrelief had a corresponding patchy structure. For intermediate herbivore impact, again most variation was spatially explained, up to 100% for biomass and soil temperature, and 84% for soil moisture, with a patch scale of about 0.95 m. Here, we found evidence that vegetated patches positively affected soil moisture through less run-off and higher infiltration of rainwater that could not infiltrate into the bare soil elsewhere, which was not due to microrelief. Thus, we conclude that our findings are in line with our initial hypothesis that, at intermediate herbivore impact, vegetated patches alternating with patches of bare soil persist in time due to positive plant-soil feedbacks.

A study was conducted to analyse the effect of habitat productivity on woody species colonisation. Three soil types were distinguished: a relatively poor sandy soil type (1), a somewhat richer sandy type (2) and a relatively rich sandy loamy type (3). Chronosequences were established on these three soil types of 38 (type 1), 20 (type 2) and 54 years (type 3) after abandonment. In total 117 vegetation relevées were used to analyse life form change and species responses during old field succession via regression models. On the rich soil type the colonisation rate of woody species was slower than on the poor soil type. This can be explained by higher abundances of perennial species during the first 20 years after abandonment on the rich soil type in contrast to the poor soil type. Perennial species may delay the woody species colonisation. First they close the bare ground which inhibits germination and next they compete with woody seedlings for light, water and nutrients. The effect of habitat productivity on woody species colonisation can only be determined appropriately by taking life history traits into account. Early successional ‘pioneer’ woody species dispersed by wind have less difficulties colonising old fields than late successional ‘forest’ species; they colonise old fields prior to the development of a dense perennial sward. Forest species depend on animals to be dispersed which are attracted by vegetation structure. In ± 30 years on the poor soil type and in ± 45 years on the rich soil type woody species become dominant relative to other life forms. Forest species like Quercus robur L. invaded relatively early (

Vegetation and soil development during succession in coastal dune slacks on Terschelling island, the Netherlands, was investigated, by comparing neighbouring ecosystems on similar substrates that had been developing for 1, 5, 35 and 76 years since the vegetation and organic soil layer had been removed. In this successional sequence, soil organic matter accumulated rapidly due to the production of litter and dead roots. N mineralization was extremely low, increasing from 0.2 g m-2 yr-1 after 5 years to 0.8 g m-2 yr-1 after 76 years. It was accompanied by a decline in the pH (KCl) in the upper 10 cm of the soil from 6.8 to 4.4. Most of the above-ground biomass accumulated in the shrub species Oxycoccus macrocarpos and Salix repens. The 5- year-old plots harboured many plant species (18 species per 0.25 m2), but plant species diversity was much lower in the older plots. It is concluded that most changes in species composition and the decline in diversity occurred because early successional plant species were gradually outshaded by the thick litter layer and the accumulated shrub biomass.