Island theory, matrix effects and species richness patterns in habitat fragments

Island biogeography theory, created initially to study diversity patterns on islands, is often applied to habitat fragments. A key but largely untested assumption of this application of theory is that landscape matrix species composition is non‐overlapping with that of the islands. We tested this assumption in successional old field patches in a closely mowed matrix, and because our patches are appropriately viewed as sets of contiguous habitat units we studied patterns of species richness per unit area. Previous studies at our site did not find that diversity patterns on patch ‘islands’ conformed to predictions of island biogeography theory. Our results indicate that when matrix species are removed from the patch samples, diversity patterns conform better to theory. We suggest that classical island theory remains an appropriate tool to study diversity patterns in fragmented habitats, but that allowances should be made for spill‐over colonization of ‘islands’ from the ‘sea’.     

Altitude modifies species richness–nutrient indicator value relationships in a country-wide survey of grassland vegetation

Nutrient enrichment is a threat to botanical diversity in Europe, and its assessment is part of biodiversity monitoring schemes. In Switzerland, this is done by calculating the average nutrient (N) indicator value of the vegetation based on a country-wide systematic vegetation survey. However, it is questionable whether N values indicate eutrophication and resulting species loss equally well across an entire country, which includes wide topographic gradients and distinct biogeographic regions. Here we analyze vascular plant species lists from 415 grassland plots (10 m²) between 365 and 2770 m a.s.l. throughout Switzerland to investigate how the relationship between N value and species richness differs with altitude and among regions. The N value strongly decreased with altitude (piecewise regression: r² = 0.77), particularly between 800 and 2000 m a.s.l., where this decrease was related to a decreasing proportion of fertilized grasslands. In the alpine belt, lower N values were associated with a greater frequency of acidic soils and a restricted species pool. Vascular plant species richness was maximal at intermediate altitude (piecewise regression: r² = 0.33) and intermediate N value (polynomial regression: r² = 0.46). When analyzed separately by altitudinal belt, the relationship between species richness and N value was negative in the lowlands and montane belt but unimodal in the subalpine belt. In the alpine belt, soil pH (R indicator values) explained most of the variation in species richness. Two indices of between-plot diversity (floristic dissimilarity and the contribution of individual plots to total species richness) were negatively related to N values from the lowlands to the subalpine belt but not in the alpine belt. All relationships differed little among the biogeographic regions of Switzerland, but they might be modified by changes in management and by the expansion of common lowland species into mountain grasslands.     

Mobility and resource use influence the occurrence of pollinating insects in restored seminatural grassland fragments

After habitat restoration, species need to recolonize from existing populations. The ability of species to recolonize restored habitats likely depends on their traits. This study aimed to test if species traits and isolation from source habitat can explain the presence of insects in restored grasslands. We surveyed the occurrence of hoverflies and bees in 14 restored seminatural pastures as well as in intact seminatural grasslands in the surrounding landscape. We tested how connectivity, time since restoration, and species traits influence if species that are present in the surrounding landscape also occur in restored pastures. Solitary bee species present in the landscape were less likely to occur in restored pastures compared to bumblebees and hoverflies. The occurrence of bumblebees, but not solitary bees or hoverflies, decreased with time since restoration. The occurrence of solitary bees increased but the occurrence of hoverflies decreased with high connectivity. Migratory hoverflies were more likely to occur in restored pastures than nonmigratory hoverflies, especially in pastures with low connectivity. Among both bumblebees and solitary bees, the occurrence was influenced by nesting traits, with the lowest occurrence of parasitic species and of species digging nests in the ground. The subset of the landscape's species pool that occurs in restored pastures has a contrasting set of traits compared with species in intact source habitats. Both mobility and resource use act as filters that influence the assembly of pollinator communities after restoration. A full recovery of pollinator communities is more likely if source populations are available nearby.     

From plant neighbourhood to landscape scales: how grazing modifies native and exotic plant species richness in grassland

The interactive effect of grazing and soil resources on plant species richness and coexistence has been predicted to vary across spatial scales. When resources are not limiting, grazing should reduce competitive effects and increase colonisation and richness at fine scales. However, at broad scales richness is predicted to decline due to loss of grazing intolerant species. We examined these hypotheses in grasslands of southern Australia that varied in resources and ungulate grazing intensity since farming commenced 170 years ago. Fine-scale species richness was slightly greater in more intensively grazed upper slope sites with high nutrients but low water supply compared to those that were moderately grazed, largely due to a greater abundance of exotic species. At broader scales, exotic species richness declined with increasing grazing intensity whether nutrients or water supply were low or high. Native species richness declined at all scales in response to increasing grazing intensity and greater resource supply. Grazing also reduced fine-scale heterogeneity in native species richness and although exotics were also characterised by greater heterogeneity at fine scales, grazing effects varied across scales. In these grasslands patterns of plant species richness did not match predictions at all scales and this is likely to be due to differing responses of native and exotic species and their relative abundance in the regional species pool. Over the past 170 years intolerant native species have been eliminated from areas that are continually and heavily grazed, whereas transient, light grazing increases richness of both exotics and natives. The results support the observation that the processes and scales at which they operate differ between coevolved ungulate—grassland systems and those in transition due to recent invasion of herbivores and associated plant species.     

Invertebrate conservation at the interface between the grassland matrix and natural Afromontane forest fragments

Diversity patterns of amphipods, carabid beetles and ants were investigated in five naturally-fragmented Afromontane forest remnants, and in the surrounding grassland matrix. Forests were architecturally similar. In contrast, grasslands surrounding these forests are subject to great differences in anthropogenic impacts. Consequently, transition from forest to grassland ranged from being abrupt (heavy disturbance) to gradual (little disturbance). Significantly different mean numbers of carabid individuals and species were captured between sites and multivariate analyses showed clear separation in carabid assemblage-structure with level of disturbance. Carabids were, furthermore, significantly more diverse in forests, compared to grasslands. Ants, however, were equally species rich between sites but were significantly more abundant and species rich in grasslands than forests. Amphipods, represented here by a single species, Talistroides africana, was significantly less abundant at highly disturbed sites and significantly more abundant in forests than grasslands. Results support the hypothesis that the dynamics of remnants are influenced by their surrounding landscape. Here, the dynamics of amphipods and carabids (predominantly forest taxa) were influenced by different disturbance regimes in grasslands surrounding these forests. Epigaeic ants, a predominantly grassland taxon here, also showed significant differences in assemblage-composition between sites with varying disturbance. Conserving Afromontane grasslands should be of prime concern because this will include the protection of forest/grassland ecotones and forest remnants.     

Ancient origin of high taxonomic richness among insects

Insects are a hyper-diverse group, comprising nearly three-quarters of all named animal species on the Earth, but the environmental drivers of their richness and the roles of ecological interactions and evolutionary innovations remain unclear. Previous studies have argued that family-level insect richness increased continuously over the evolutionary history of the group, but inclusion of extant family records artificially inflated the relative richness of younger time intervals. Here we apply sampling-standardization methods to a species-level database of fossil insect occurrences, removing biases present in previous richness curves. We show that insect family-richness peaked 125 Ma and that Recent values are only 1.5–3 times as high as the Late Palaeozoic. Rarefied species-richness data also tentatively suggest little or no net increase in richness over the past 125 Myr. The Cretaceous peak in family richness was coincident with major radiations within extant groups but occurred prior to extinctions within more basal groups. Those extinctions may in part be linked to mid-Cretaceous floral turnover following the evolution of flowering plants. Negligible net richness change over the past 125 Myr implies that major radiations within extant groups were offset by reduced richness within groups that are now relict or extinct.     

Size-related deterioration of semi-natural grassland fragments in Sweden

Abstract. One of the most dramatic landscape changes during the 20th century in Sweden, like in most of Europe, has been the reduction and fragmentation of semi-natural grasslands. Using a set of remnant semi-natural grasslands, chosen to be as similar as possible, but differing in size, we have examined whether size of remnant fragments of traditionally managed semi-natural grasslands in Sweden is related to patterns of species richness and composition. We focused on edge-to-interior relationships, since we expected that a possible impact from invasive habitat generalists would be manifested in a gradient from the edge of fragments to their interior. We found no relationship between size of grassland fragments and (a) overall species richness, (b) species richness at different spatial scales, and (c) abundance of some typical invader species or species characteristic of semi-natural grasslands. However, the results indicated that larger grasslands have a comparatively larger number of species in the edges, whereas the opposite pattern was found in smaller grasslands. The similarity in species composition between the edge and the interior of the pastures also increased with grassland size. Thus, even though the overall species richness is still unaffected by reduction in grassland fragment size, the edges of smaller grasslands show signs of degradation, i.e. reduction in species richness and a decreased similarity to the grassland interior. We suggest that these kinds of effects may be early signs of fragmentation effects that in the future will result in species loss even if the present distribution of semi-natural grasslands is maintained.     

Impact of flooding on potential and realised grassland species richness

In order to reduce flood risk, river management policies advise floodplain restoration and the recreation of water retention areas. These measures may also offer opportunities for the restoration of species-rich floodplain habitats through rewetting and the restoration of flood dynamics. The potential to enhance biodiversity in such flood restoration areas is, however, still subject to debate. In this paper we investigate whether flooding along a small altered lowland river can contribute to the potential and realised species richness of semi-natural meadows. We compare the seed bank and vegetation composition of flooded and non-flooded semi-natural meadows and test the hypothesis that flooding contributes to an input of diaspores into the meadow seed banks, thereby promoting seed density and potential species richness. Furthermore we hypothesise that, where habitat conditions are suitable, flooding leads to a higher realised species richness. Results showed that seed densities in flooded meadows were significantly higher than in non-flooded meadows. The seed banks of flooded meadows also contained a higher proportion of exclusively hydrochorous species. However, the seed bank species richness, as well as the species richness realised in the vegetation did not differ significantly between flooded and non-flooded meadows. Finally, the seed bank and standing vegetation of flooded sites showed larger differences in species composition and Ellenberg nitrogen distribution than non-flooded sites. From these results we conclude that, although flooding does contribute to the density and composition of the seed bank, most imported seeds belong to only a few species. Therefore, it is unlikely that flooding substantially enhances the potential species richness. Furthermore, even if new species are imported as seeds into the seed bank, it seems unlikely that they would be able to establish in the standing vegetation. However, it is unclear which factors impede the establishment of imported species in the vegetation. The implications of our findings for flood meadow restoration are discussed.     

Modelling species richness and diversity in grassland communities of the Central Caucasus

In this study I used small squares (4 cm×4 cm) as a sampling technique within plots (128 cm×128 cm) of different elevation, aspect and slope angle in grassland communities (20 plots examined). Then I used a rectangular hyperbole equation (the Michaelis-Menten model) to describe species richness and the Inverse of Simpson Concentration (ISC) as functions of sample size. I checked robustness and precision of the model both by interpolation and extrapolation. Interpolation was similarly good in both cases, while extrapolation produced reliable predictions of ISC but underestimated species richness. Dominance analysis indicated that the underestimation of richness depends on the proportion of bottom species, and that the predicted values of richness roughly coincide with the numbers of dominant species found in plots. Therefore, the model may be used to assess number of dominant species when precision is less important than saving time during a survey. However, the rectangular hyperbole equation appears to be precise and robust in the prediction of ISC, at least in grassland communities. This property may also be employed for extrapolation of diversity indices with a limited sampling effort.     

Climate-driven diversity dynamics in plants and plant-feeding insects

The origin of species-rich insect-plant food webs has traditionally been explained by diversifying antagonistic coevolution between plant defences and herbivore counter-defences. However, recent studies combining paleoclimatic reconstructions with time-calibrated phylogenies suggest that variation in global climate determines the distribution, abundance and diversity of plant clades and, hence, indirectly influences the balance between speciation and extinction in associated herbivore groups. Extant insect communities tend to be richest on common plant species that have many close relatives. This could be explained either by climate-driven diffuse cospeciation between plants and insects, or by elevated speciation and reduced extinction in herbivore lineages associated with expanding host taxa (resources). Progress in paleovegetation reconstructions in combination with the rapidly increasing availability of fossil-calibrated phylogenies provide means to discern between these alternative hypotheses. In particular, the 'Diffuse cospeciation' scenario predicts closely matching main diversification periods in plants and in the insects that feed upon them, while the 'Resource abundance-dependent diversification' hypothesis predicts that both positive and negative responses of insect diversity are lagged in relation to host-plant availability. The dramatic Cenozoic changes in global climate provide multiple possibilities for studying the mechanisms by which climatic shifts may drive diversity dynamics in plants and insect herbivores.     

Plant patch structure modifies parasitoid assemblage richness of a specialist herbivore

1. The spatial structure of plant patches has been shown to affect host–parasitoid interactions, but its influence on parasitoid diversity remains largely ignored. Here we tested the prediction that parasitoid species richness of the specialist leafminer Liriomyza commelinae increases in larger and less isolated patches of its host plant Commelina erecta. We also explored whether parasitoid abundance and body size affected the occurrence of parasitoid species in local assemblages. 2. A total of 893 naturally established C. erecta patches were sampled on 18 sites around Córdoba city (Argentina). Also, two experiments were performed by creating patches differing in the number of plants and the distance from a parasitoid source. For these tests, plants were infected with the miner in the laboratory prior to placement in the field. 3. Plant patch size, independently of host abundance, positively affected the number of parasitoid species in both survey observations and experimental data. However, plant patch isolation did not influence parasitoid species richness. 4. The probability of finding rare parasitoid species increased with patch size, whereas occupation of isolated patches was independent of dispersal abilities (body size) of parasitoid species. 5. Overall, the results highlight the importance of considering spatial aspects such as the size of plant patches in the study of parasitoid communities.     

Biogeography and species richness of endophagous insects associated with Proteaceae in South Africa

The endophagous insects associated with Proteaceae of the Cape fynbos were compared to endophage assemblages from more northern non-Capensis Proteaceae. Insects were collected from Proteaceae in the Cape on a regular basis and additional records obtained from insect collections. Northern samples were collected more opportunistically or records were obtained from collections or through personal communication. The Cape fynbos genus Protea is utilized by many more insect taxa than the non-fynbos Protea species. The fynbos Proteaceae has very few species in common with the northern Proteaceae, yet each has many of their own distinct species. This suggests that the fynbos endophage insect fauna is distinct from that of the other regions. It appears that the high diversity of host plants in the fynbos has contributed to generating high, local endophagous insect diversity.     

Importance of local habitat conditions and past and present habitat connectivity for the species richness of grassland plants and butterflies in power line clearings

Agricultural intensification has caused drastic declines in the area and species richness of semi-natural grasslands across Europe. Novel habitats, such as power line clearings, provide alternative habitats and niches for grassland species, and might therefore mitigate these declines. However, it is not fully understood which environmental factors determine the occurrence of grassland species in the clearings. Identifying the most important drivers for grassland species occurrence would help understand the value of the clearings for grassland conservation and target enhanced management into clearings with most potential as grassland habitat. We studied the effects of local environmental conditions, and past and present connectivity to semi-natural grasslands, on the species richness of grassland plants and butterflies in 43 power line clearings in Finland. The results of generalized linear models and hierarchical partitioning showed that increasing time since clear-cut and amount of clearing residue decreased the species richness of both species groups, while the cover of mesic habitats increased it. However, the two species groups showed also divergent responses. Present-day local environmental conditions appeared to be the sole driver of grassland butterfly species richness, whereas the richness of grassland plants was related both to current conditions and historical connectivity to grasslands in 1870–1880s. This suggests the presence of an extinction debt in the studied grassland plant communities, emphasizing the need for enhanced management to increase suitable grassland habitat in the clearings. This would diminish the potential future losses of grassland plant species in the clearings and create valuable habitat for grassland butterflies as well.     

The role of species richness for recruitment in a seminatural grassland

I examined two aspects of how recruitment is influenced by species richness in a seminatural grassland: effects of species richness among colonisers in a seed mixture, and effects of species richness, ramet density, grass and moss cover in the colonised vegetation. The results suggest that recruitment is higher in more species-rich seed mixtures (for three of the four target species: Anthyllis vulneraria, Centaurea jacea, Filipendula vulgaris and Primula veris ). Average recruitment of the target species was negatively related to ramet density in the colonised vegetation, but not affected by grass or moss cover. The results imply that mechanisms other than interspecific competition among species are important in structuring the community during the recruitment phase. The negative effects of ramet density and the positive effects of species richness may be a result of strong intra-specific competition, or species-independent competition in combination with diversity effects enhancing microsite variability.     

The dynamics of species richness in an experimentally restored calcareous grassland

Abstract. This paper concerns the hypothesis that shoot (light) competition is the main interaction determining the community change during succession from a relatively species-poor deciduous forest (an overgrown former grassland) to a species-rich grassland, while root (nutrient) competition is of little importance. In a 4-yr restoration experiment, clearcutting, mowing and root trenching were used as treatments. The results did not reject the hypothesis. However, the significance of separating two kinds of shoot competition - ‘coarse-scale (between different growth forms) and ‘fine-scale’ (between similar growth forms) became evident. Release from the ‘coarse-scale’ shoot competition (between different growth forms) increased species richness at the beginning of the experiment. This change was interpreted as the replacement of one species pool (shade-tolerant herbaceous perennials) by another (light-demanding herbaceous perennials), the second pool containing considerably more species. The importance of ‘fine-scale’ shoot competition increased gradually - the levelling of competition by mowing resulted in a more pronounced increase in species richness during successive years. The elimination of ‘coarse-scale’ root competition seemed to be important to some extent only in combination with another treatment - mowing. Initial colonization of the cleared area by individual species was a stochastic process which had little relevance to life-history traits. True grassland species were able to colonize quickly. On the community scale, the developing community still remained relatively poor in species. In all plots which were cleared but not mown, succession already started to reverse towards woody vegetation in the third year.     

Chemical preservation of plants and insects in natural resins

The morphological preservation of fossils in amber is remarkable, but their chemical composition is largely unknown. The likelihood of DNA preservation in amber has been questioned but, surprisingly, the fate of more decay-resistant macromolecules such as ligno-cellulose in plants or the chitin–protein complex in insect cuticle has not been investigated. Here we report the results of investigations using pyrolysis-gas chromatography/mass spectrometry (py-GC/MS) of the tissues of insects and the plant Hymenaea from ancient and sub-fossil resins (2–20 ka) from Kenya, and from Dominican amber (25–30 Ma). The volatile components of the resin have penetrated even the internal tissues, resulting in the exceptional three-dimensional preservation of amber inclusions. Chitin is preserved in the bee and ligno-cellulose in the Hymenaea leaf from the Kenyan resins. There was no trace, however, of these macromolecules in tissues in Dominican amber. The presence of aliphatic polymer and sulphur-containing moieties in these tissues indicates that they have undergone diagenetic alteration; in view of this, the preservation in Dominican amber of a macromolecule as labile as DNA would be extraordinary.