Spread of exotic plants in the landscape: the role of time,growth habit,and history of invasiveness

We investigated the relative contribution of minimum residence time, growth habit, and history of invasiveness to the spread of exotic plants in Michigan and California. Our data include minimum residence time as estimated by earliest herbarium collection records, growth habit, and history of invasiveness for over 2000 records from two herbaria (MI = 943, CA = 1131). Our data support the hypothesis that minimum residence time is highly associated with landscape spread, explaining 39–44% of variation in the number of counties invaded. In contrast, growth habit and history of invasiveness explained a small fraction of variation in spread in California but not Michigan. Over the past 30 years exotic plant species frequently became established in Michigan and California (≥50 species per decade), suggesting that many more species will become invasive over time. There is an urgent need to develop effective policies for exotic plant management. In both states we found significant positive correlations between minimum residence time and species occurrence on state invasive plant lists. Further, we found historical information on the pest status of a plant species introduced into a similar environment to be relevant in determining landscape spread of exotic plants. We conclude that efforts to predict exotic species spread based on biological characteristics may have limited success, and instead endorse pest risk analysis for proposed new imports coupled with rapid detection and early response for unintended and unwanted introductions.     

Historical legacies and ecological determinants of grass naturalizations worldwide

The global distribution of exotic species is the result of abiotic, biotic and dispersal filtering processes that shape the movement and success of species outside their native range. In this study we aim to understand how these filtering processes drive the fluxes of grass species among regions, the factors that influence which species establish outside of their native range, and where they do so. We used national and subnational checklists of native and introduced grass species to determine the extent to which each region was a source or recipient of exotic grass species. We asked how species traits may distinguish those grass species that have naturalized outside their native range from those that have not, and how environmental conditions are related to the distribution of exotic grass species. We found that exotic grass establishment is shaped by an array of factors including characteristics of regions, traits of species and their interactions. Regions with a longer history of human occupation and larger numbers of native grass species were generally the most important sources of exotic species. Global flows of species were mostly driven by a climate match between the native and exotic ranges, but were also highly asymmetric, with regions with recent human arrival being the major hosts of exotic grass species. Tall, annual and C4 grass species exhibited particularly high probabilities of establishment outside their native range. Despite the idiosyncrasy and stochasticity characteristic of exotic species establishment, this biogeographical analysis revealed important generalities across this large plant group. Our results suggest that grass species that have co-occurred with humans for a longer time may be better adapted to living in anthropogenic landscapes, explaining the global asymmetry in species introductions.     

Species richness of herbivores on exotic host plants increases with time since introduction of the host

Aim Species richness of insect herbivores feeding on exotic plants increases with abundance as well as range size of the host in the area of introduction. The formation of these herbivore assemblages requires a certain amount of time, and the richness of insect faunas should also increase with the length of time an exotic plant has been present in the introduced range. Location Central Europe. Methods We analysed the variation in species richness of leaf‐chewing Lepidoptera larvae and sap‐sucking Auchenorrhyncha (Hemiptera) associated with 103 exotic woody plant species in Germany in relation to time since introduction, range size, growth form (trees versus shrubs), biogeographical origin (distance from Central Europe) and taxonomic isolation of the host plant (presence or absence of a native congener in the introduced area). Results Using simple correlation analyses we found for Lepidoptera and Auchenorrhyncha that species richness increased with time since introduction of the host plant. For the Lepidoptera the increase of species richness with time since introduction remained significant even after removing the effects of all other independent variables. Main conclusions Our results provide some evidence that assemblages of insects on exotic plants do not reach saturation within a time scale of few hundred years. This contrasts with previous findings for crop plants.     

Inhibitory effects of Eucalyptus globulus on understorey plant growth and species richness are greater in non‐native regions

Aim

We studied the novel weapons hypothesis in the context of the broadly distributed tree species Eucalyptus globulus. We evaluated the hypothesis that this Australian species would produce stronger inhibitory effects on species from its non‐native range than on species from its native range.

Location

We worked in four countries where this species is exotic (U.S.A., Chile, India, Portugal) and one country where it is native (Australia).

Time period

2009–2012.

Major taxa studied

Plants.

Methods

We compared species composition, richness and height of plant communities in 20 paired plots underneath E. globulus individuals and open areas in two sites within its native range and each non‐native region. We also compared effects of litter leachates of E. globulus on root growth of seedlings in species from Australia, Chile, the U.S.A. and India.

Results

In all sites and countries, the plant community under E. globulus canopies had lower species richness than did the plant community in open areas. However, the reduction was much greater in the non‐native ranges: species richness declined by an average of 51% in the eight non‐native sites versus 8% in the two native Australian sites. The root growth of 15 out of 21 species from the non‐native range were highly suppressed by E. globulus litter leachates, whereas the effect of litter leachate varied from facilitation to suppression for six species native to Australia. The mean reduction in root growth for Australian plants was significantly lower than for plants from the U.S.A., Chile and India.

Main conclusions

Our results show biogeographical differences in the impact of an exotic species on understorey plant communities. Consistent with the novel weapons hypothesis, our findings suggest that different adaptations of species from the native and non‐native ranges to biochemical compounds produced by an exotic species may play a role in these biogeographical differences.     

Abiotic conditions shape the relationship between indigenous and exotic species richness in a montane biodiversity hotspot

Montane ecosystems are more prone to invasions by exotic plant species than previously thought. Besides abiotic factors, such as climate and soil properties, plant-plant interactions within communities are likely to affect the performance of potential invaders in their exotic range. The biotic resistance hypothesis predicts that high indigenous species richness hampers plant invasions. The biotic acceptance hypothesis, on the other hand, predicts a positive relationship between indigenous and exotic species richness. We tested these two hypotheses using observational data along an elevational gradient in a southern African biodiversity hotspot. Species composition data of indigenous and exotic plants were recorded in 20 road verge plots along a gradient of 1775–2775 m a.s.l. in the Drakensberg, South Africa. Plots were 2?×?50 m in size and positioned at 50 m elevational intervals. We found a negative correlation between indigenous and exotic richness for locations with poorly developed mineral soils, suggesting biotic resistance through competitive interactions. A strong positive correlation for plots with very shallow soils at high elevations indicated a lack of biotic resistance and the possibility of facilitating interactions in harsher environments. These results suggest that biotic resistance is restricted to the lower and mid elevations while biotic acceptance prevails in presence of severe abiotic stress, potentially increasing the risk of plant invasions into montane biodiversity hotspots.

     

Rodent endemism,turnover and biogeographical transitions on elevation gradients in the northwestern Argentinian Andes

The aim of this research is to relate patterns of endemism and turnover along a local elevation gradient in northwest Argentina with continental biogeographical transitions. Specimen based records constituted the principal source of information to infer rodent distribution along the elevation gradient. I assessed elevational variation of richness, endemism and turnover by means of non-linear regression analysis. Then I identified five distributional patterns based on the overlap of species geographic range. Their frequency along elevation was used to validate biogeographical boundaries inferred by turnover rates. Eleven species out of 37 (30%) are endemic to the study area. Species richness and endemism were hump-shaped. The rate of endemism reached its maximum value at the upper limit of the forest (2500 m). By contrast, species turnover was U-shaped, with a small peak at 1500 m and a maximum at 3500 m. The species’ geographic range patterns were not randomly distributed along elevation but agglomerated at specific elevation. Species turnover and chorological analysis suggest two biogeographical boundaries, a weaker at 1500 m and a stronger at 3500 m. The 1500 m boundary marks the transition from assemblages dominated by Lowland-widespread fauna at lower elevation to Montane (Andean eastern slopes) species at middle elevation. This boundary is characterized by moderate species turnover and high species richness. The strong turnover rate at 3500 and the dominance of highland Andean and Andean-Patagonian species above this elevation suggest the occurrence of the transition between the Neotropical and Andean regions; which is characterised by an almost complete species replacement.     

Exploring associations between international trade and environmental factors with establishment patterns of exotic Scolytinae

Although invasion of exotic ambrosia beetles (fungus feeders) and bark beetles (phloem feeders) (Coleoptera: Curculionidae: Scolytinae) is considered a major threat to forest health worldwide, no studies have quantitatively investigated the anthropogenic and environmental factors shaping the biogeographical patterns of invasion by these insects across large spatial scales. The primary aim of this study was to assess the relative importance of international trade and several environmental variables of the recipient region on species richness of established exotic Scolytinae. As a reference, we also evaluated the relationships between the same environmental variables and species richness of native Scolytinae. Using an information-theoretic framework for model selection and hierarchical partitioning, we evaluated the relative importance of the potential drivers of species richness of native and exotic Scolytinae in 20 European countries and the 48 contiguous continental US states. Analyses were conducted separately for ambrosia and bark beetle species. Value of imports was a strong predictor of the number of exotic Scolytinae species in both regions. In addition, in the USA, warmer and wetter climate was positively linked to increased numbers of both native and exotic ambrosia beetles. Forest heterogeneity and climatic heterogeneity and secondarily forest area were key drivers in explaining patterns of species richness for native bark beetles but not for exotic species in both regions. Our findings suggest that if current infestation levels continue on imported plants and wood packaging material, increasing international trade will likely lead to more establishments of exotic Scolytinae with concomitant negative effects on forest health in both Europe and the USA. Compared to Europe the risk of invasion appears higher in the USA, especially for ambrosia beetles in the southeastern USA where the climate appears highly suitable for exotic establishment.     

Alternative pathways of liana communities in the forests of northwestern Argentina

Liana dynamics in secondary and mature forests are well known in tropical areas dominated by native tree species. Outside the tropics and in secondary forests invaded by exotic species, knowledge is scarce. In this study, we compare liana communities between secondary and mature forests dominated by native species in a subtropical montane area of Sierra de San Javier, Tucuman, Argentina. Additionally, we evaluate changes of liana communities in secondary forests with increasing densities of Ligustrum lucidum and Morus alba, two of the most invasive exotic trees of the area. We surveyed liana species richness and density in three 30-year secondary patches, four 60-year secondary patches, and four mature patches dominated by native tree species, to analyze changes in liana communities with forest age. Within each patch, we sampled 10–25 20 × 20 m quadrats. Additionally, we surveyed liana density and species richness in secondary forest patches with different densities of L. lucidum and M. alba. In native-dominated forests, liana species richness increased and showed a tendency of increasing basal area from 30-year secondary forests to mature forests. Liana density was highly variable, and most of the species were shared between native-dominated secondary and mature forests. Liana density and species richness decreased with L. lucidum density, whereas in secondary forests highly dominated by M. alba, lianas increased in density. Overall, lianas followed different pathways influenced by native forest succession and exotic tree invasions.     

Elevational gradients of reptile richness in the southern Western Ghats of India: Evaluating spatial and bioclimatic drivers

Exploring elevational patterns in species richness and their underlying mechanisms is a major goal in biogeography and community ecology. Reptiles can be powerful model organisms to examine biogeographical patterns. In this study, we examine the elevational patterns of reptile species richness and test a series of hypotheses that may explain them. We sampled reptile communities along a tropical elevational gradient (100–1,500 m a.s.l.) in the Western Ghats of India using time‐constrained visual encounter surveys at each 100‐m elevation zone for 3 years. First, we investigated species richness patterns across elevation and the support of mid‐domain effect and Rapoport's rule. Second, we tested whether a series of bioclimatic (temperature and tree density) and spatial (mid‐domain effect and area) hypotheses explained species richness. We used linear regression and AICc to compare competing models for all reptiles, and each of the subgroups: snakes, lizards, and Western Ghats’ endemics. Overall reptile richness and lizard richness both displayed linear declines with elevation, which was best explained by temperature. Snake richness and endemic species richness did not systematically vary across elevation, and none of the potential hypotheses explained variation in them. This is the first standardized sampling of reptiles along an elevational gradient in the Western Ghats, and our results agree with the global view that temperature is the primary driver of ectotherm species richness. By establishing strong reptile diversity–temperature associations across elevation, our study also has implications for the impact of future climate change on range‐restricted species in the Western Ghats.     

Effects of historical and contemporary factors on global patterns in avian species richness

Aim  The aim of this study was to determine how regional and historical factors influence global patterns in avian species richness.
Location  Global.
Methods  Using a comprehensive data set including 710 World Wildlife Fund terrestrial ecoregions covering nearly all the land surface of the Earth, avian species richness was compared among six biogeographical regions after accounting for sample area, elevational range and climate. Analysis of variance and multiple regressions were used. Spatial autocorrelation in model residuals was accounted for.
Results  Significant effects of region on avian species richness were found in nearly all comparisons between biogeographical regions.
Main conclusions  Regional and historical processes have played a role in regulating large-scale avian species richness patterns across the globe. Avian species richness in different regions of the world cannot be accurately predicted by a single global model. Avian species richness in areas of similar environments may differ substantially between regions, and thus avian species richness in one biogeographical region cannot be predicted using the richness–environment relationship derived from the data of another biogeographical region, even one with similar environments.     

Environmental and vegetation variables have a greater influence than habitat fragmentation in structuring lizard communities in remnant urban bushland

Abstract The expansion of urban areas and adjacent farming land into natural landscapes modifies habitats and produces small isolated pockets of native vegetation. This fragmentation of the natural habitat subdivides animal communities, reduces population sizes and increases vulnerability to extinction. In this paper we investigate whether fragmentation decreases lizard species richness, composition, overall abundance and abundance at the species level. Urban remnants consisting of five small (< 10 ha) and four large (> 10 ha) fragments of natural bushland were paired with continuous bushland areas located near Hobart, Tasmania, Australia. These remnants were surveyed six times, using pitfall traps, from November 2001 to March 2002. Lizard species richness and abundance were not significantly influenced by habitat fragmentation or fragment size. Egernia whitii was the only lizard species significantly influenced by fragment size, and was only present in large fragments and continuous bush. Vegetation type and structure as well as environmental variables (geology and aspect) influenced the structure of reptile communities. Lizard species that were able to use a number of different habitat types were found to persist at most sites, irrespective of fragment size. Edge environment did not significantly influence lizard species richness or abundance in remnant areas. Lizard species richness was significantly lower in sites that had a high ratio of exotic to native plant species. Therefore, if remnants continue to be invaded by exotic plants, lizard species that require native plant communities will become increasingly vulnerable to local extinction. Our results suggest that lizard species requiring specialized habitats, such as E. whitii, may persist in large urban remnants rather than small urban remnants because large reserves are more likely to encompass rare habitats, such as rocky outcrops. Habitat heterogeneity, rather than size, may be the key to their persistence.     

Negative native–exotic diversity relationship in oak savannas explained by human influence and climate

Recent research has proposed a scale-dependence to relationships between native diversity and exotic invasions. At fine spatial scales, native–exotic richness relationships should be negative as higher native richness confers resistance to invasion. At broad scales, relationships should be positive if natives and exotics respond similarly to extrinsic factors. Yet few studies have examined both native and exotic richness patterns across gradients of human influence, where impacts could affect native and exotic species differently. We examined native–exotic richness relationships and extrinsic drivers of plant species richness and distributions across an urban development gradient in remnant oak savanna patches. In sharp contrast to most reported results, we found a negative relationship at the regional scale, and no relationship at the local scale. The negative regional-scale relationship was best explained by extrinsic factors, surrounding road density and climate, affecting natives and exotics in opposite ways, rather than a direct effect of native on exotic richness, or vice versa. Models of individual species distributions also support the result that road density and climate have largely opposite effects on native and exotic species, although simple life history traits (life form, dispersal mode) do not predict which habitat characteristics are important for particular species. Roads likely influence distributions and species richness by increasing both exotic propagule pressure and disturbance to native species. Climate may partially explain the negative relationship due to differing climatic preferences within the native and exotic species pools. As gradients of human influence are increasingly common, negative broad-scale native–exotic richness relationships may be frequent in such landscapes.     

The effect of anthropic pressures and elevation on the large and medium-sized terrestrial mammals of the subtropical mountain forests (Yungas) of NW Argentina

We conducted a 55-day long camera-trap survey in the Yungas subtropical forest in NW Argentina, to assess the effect of human accessibility, conservation status of the area, domestic animals and elevation on the diversity and composition of the large and medium-sized native terrestrial mammal assemblage. We deployed 24 camera-trap stations at distances of ～2 km from each other. The study area is covered by continuous forest and has its center in the small community of Acambuco, in the Acambuco Provincial Reserve. The main economic activity in the area is oil/gas exploitation. Local residents raise cattle, hunt and use timber and non-timber forest products. The human impact was indirectly measured with an accessibility cost model. We used a multiple regression ANCOVA to assess the effect of elevation (range: 628–1170 masl), accessibility, protection status (reserve vs not) and frequency of records of domestic animals on the native mammal species richness and on a nonmetric multidimensional scaling (NMDS) ordination based on the frequency of records of the native mammals recorded at >3 camera-trap stations. We recorded 15 species of native mammals. Native mammal species richness decreased with elevation. Elevation was correlated with NMDS axes. Other predictive variables had no effect on species richness or the NMDS ordination, probably as a result of the relatively narrow range of conditions assessed in this study. The effect of elevation on mammal assemblages should be considered in landscape planning processes aimed at promoting biodiversity conservation.     

Species–area relationships and additive partitioning of diversity of native and nonnative herpetofauna of the West Indies

To evaluate the regional biogeographical patterns of West Indian native and nonnative herpetofauna, we derived and updated data on the presence/absence of all herpetofauna in this region from the recently published reviews. We divided the records into 24 taxonomic groups and classified each species as native or nonnative at each locality. For each taxonomic group and in aggregate, we then assessed the following: (1) multiple species–area relationship (SAR) models; (2) C‐ and Z‐values, typically interpreted to represent insularity or dispersal ability; and (3) the average diversity of islands, among‐island heterogeneity, γ‐diversity, and the contribution of area effect toward explaining among‐island heterogeneity using additive diversity partitioning approach. We found the following: (1) SARs were best modeled using the Cumulative Weibull and Lomolino relationships; (2) the Cumulative Weibull and Lomolino regressions displayed both convex and sigmoid curves; and (3) the Cumulative Weibull regressions were more conservative than Lomolino at displaying sigmoid curves within the range of island size studied. The Z‐value of all herpetofauna was overestimated by Darlington (Zoogeography: The geographic distribution of animals, John Wiley, New York, 1957), and Z‐values were ranked: (1) native > nonnative; (2) reptiles > amphibians; (3) snake > lizard > frog > turtle > crocodilian; and (4) increased from lower‐ to higher‐level taxonomic groups. Additive diversity partitioning showed that area had a weaker effect on explaining the among‐island heterogeneity for nonnative species than for native species. Our findings imply that the flexibility of Cumulative Weibull and Lomolino has been underappreciated in the literature. Z‐value is an average of different slopes from different scales and could be artificially overestimated due to oversampling islands of intermediate to large size. Lower extinction rate, higher colonization, and more in situ speciation could contribute to high richness of native species on large islands, enlarging area effect on explaining the between‐island heterogeneity for native species, whereas economic isolation on large islands could decrease the predicted richness, lowering the area effect for nonnative species. For most of the small islands less affected by human activities, extinction and dispersal limitation are the primary processes producing low species richness pattern, which decreases the overall average diversity with a large among‐island heterogeneity corresponding to the high value of this region as a biodiversity hotspot.     

Do anthropogenic corridors homogenize plant communities at a local scale? A case studied in Tenerife (Canary Islands)

Biological homogenization is defined as a process that occurs when native species are replaced by common and dominant exotic species or due to depletion and expansion of native species, reducing the beta diversity between areas or habitats. Islands are particularly vulnerable to plant invasion, and as a consequence, homogenization is a process that can become faster and more intense in islands than in continental areas. We recorded vascular plant species composition in roadside communities along a strong altitudinal gradient using plots beside the road and at two distances from the road (0–50 and 50–100 m). We analyzed the results separately for each group of plots with a Detrended Correspondence Analysis (DCA) including and excluding exotic species. The results revealed that where exotic species were most abundant, i.e., at the road edge, they can create an effect of floristic homogenization where three similar roads are compared. At a distance of >50 m from the road, where exotic species are less frequent, this effect has already disappeared, indicating that it is a local phenomenon, closely related to the highly disturbed roadside environment. Furthermore, floristic homogenization seems to be more important at higher altitudes (>1000 m), probably related to higher diversity in native plant communities and lower levels of human disturbances. Roads allow humans to reach relatively remote and sometimes well-conserved areas, and, at the same time, facilitate the spread of exotic plants and the most common native species which can locally create floristic homogenization in roadside communities on this oceanic island. A deeper understanding of the effects of these anthropogenic corridors at the local and regional scales is therefore required to integrate road planning and management with the aim of conserving the value of the natural areas.     

Human impacts, energy availability and invasion across Southern Ocean Islands

Aim Ongoing biological invasions will enhance the impacts of humans on biodiversity. Nonetheless, the effects of exotic species on diversity are idiosyncratic. Increases in diversity might be a consequence of similar responses by species to available energy, or because of positive relationships between human density, energy and propagule pressure. Here we use data from the Southern Ocean island plants and insects to investigate these issues. Location The Southern Ocean Islands ranging from Tristan da Cunha to Heard Island and South Georgia. Methods Generalized linear models are used to explore the relationships between indigenous and exotic species richness for plants and insects on two different islands. Similar models are used to examine interactions between indigenous and exotic species richness, energy availability and propagule pressure at the regional scale. Results Positive relationships were found between indigenous and exotic species richness at local scales, although for plants, the relationship was partially triangular. Across the Southern Ocean Islands, there was strong positive covariation between indigenous and exotic plant species richness and insect species richness, even taking spatial autocorrelation into account. Both exotic and indigenous plant and insect species richness covaried with energy availability, as did human visitor frequency. When two islands with almost identical numbers of human visits were contrasted, it was clear that energy availability, or perhaps differences in climate‐matching, were responsible for differences in the extent of invasion. Conclusion In plants and insects, there are positive relationships between indigenous and exotic diversity at local and regional scales across the Southern Ocean islands. These relationships are apparently a consequence of similar responses by both groups and by human occupants to available energy. When visitor frequency is held constant, energy availability is the major correlate of exotic species richness, though the exact mechanistic cause of this relationship requires clarification.     

Minimum residence time, biogeographical origin, and life cycle as determinants of the geographical extent of naturalized plants in continental Chile

The geographical extent of exotic plant species is a major component of invasiveness, which has been explained by intrinsic attributes of the plants, such as growth rate, reproductive type, life form, and biogeographical origin. We assessed quantitatively life cycle and biogeographical origin as determinants of the geographical distribution of naturalized plants in continental Chile, using minimum residence time (MRT) as an estimator of introduction date. We assembled a database with information on 428 plants (principally herbs) in continental Chile, corresponding to 61% of the exotic naturalized flora. For each species we recorded: (1) minimum residence time (introduction date or first recorded date in the country); (2) biogeographical origin (American, Eurasian, others); (3) life cycle (annual, perennial, others); (4) number of Chilean regions occupied by the plant. We found that 82 species (19%) have been recorded in only one region of Chile, while only three species have been found in all 13 regions of the country. About 89% of the species (381) have been found only in central Chile (Regions IV to VIII), while the remaining 11% (47) are found only in the northern (Regions I to III) or southern parts of the country (Regions IX to XII). We detected significant differences in regional spread of naturalized plants according to minimum residence time: those species with shorter MRT had more limited spread ranges than those with longer MRT. Biogeographical origin and life cycle did not explain geographical extent in Chile. This study shows that historical factors are more important than biological ones in determining the geographical extent of naturalized plants in continental Chile. Thus, caution should be exercised when assigning value to biological attributes that may confer invasiveness to naturalized plants.     

Birds on the move in the face of climate change: High species turnover in northern Europe

Species richness is predicted to increase in the northern latitudes in the warming climate due to ranges of many southern species expanding northwards. We studied changes in the composition of the whole avifauna and in bird species richness in a period of already warming climate in Finland (in northern Europe) covering 1,100 km in south–north gradient across the boreal zone (over 300,000 km²). We compared bird species richness and species‐specific changes (for all 235 bird species that occur in Finland) in range size (number of squares occupied) and range shifts (measured as median of area of occupancy) based on bird atlas studies between 1974–1989 and 2006–2010. In addition, we tested how the habitat preference and migration strategy of species explain species‐specific variation in the change of the range size. The study was carried out in 10 km squares with similar research intensity in both time periods. The species richness did not change significantly between the two time periods. The composition of the bird fauna, however, changed considerably with 37.0% of species showing an increase and 34.9% a decrease in the numbers of occupied squares, that is, about equal number of species gained and lost their range. Altogether 95.7% of all species (225/235) showed changes either in the numbers of occupied squares or they experienced a range shift (or both). The range size of archipelago birds increased and long‐distance migrants declined significantly. Range loss observed in long‐distance migrants is in line with the observed population declines of long‐distance migrants in the whole Europe. The results show that there is an ongoing considerable species turnover due to climate change and due to land use and other direct human influence. High bird species turnover observed in northern Europe may also affect the functional diversity of species communities.     

Breakdown of the species-area relationship in exotic but not in native forest patches

We studied the pattern of bird species richness in native and exotic forest patches in Hungary. We hypothesized that species-area relationship will depend on forest naturalness, and on the habitat specialization of bird species. Therefore, we expected strong species-area relationship in native forest patches and forest bird species, and weaker relationship in exotic forest patches containing generalist species. We censused breeding passerine bird communities three times in 13 forest patches with only native tree species, and 14 with only exotic trees in Eastern Hungary in 2003. Although most bird species (92%) of the total of 41 species occurred in both exotic and native forests, the species-area relationship was significant for forest specialist, but not for generalist species in the native forests. No relationship between bird species and area was found for either species group in the forest with exotic tree species. The comparison of native versus exotic forest patches of similar sizes revealed that only large (>100 ha) native forests harbor higher bird species richness than exotic forests for the forest specialist bird species. There is no difference between small and medium forest patches and in richness of generalist species. Thus, the species-area relationship may diminish in archipelago of exotic habitat patches and/or for habitat generalist species; this result supports the warning that the extension of exotic habitats have been significantly contributing to the decline of natural community patterns.     

Factors Controlling Compositional Changes in a Northern Andean Páramo (La Rusia,Colombia)

In this study, we aimed to assess the processes controlling compositional change in a Northern Andean páramo highly affected by human‐induced disturbances over the last few decades (La Rusia, Colombia). Along the 3000–3800 m asl altitudinal range, we randomly sampled fifty 10 × 10 m plots. Therein, we measured altitude and variables related to soil conditions (i.e., moisture, nutrient contents, bulk density, and texture), occurrence of human‐induced disturbances (i.e., fire, vegetation clearing, potato cultivation, and cattle grazing), and land‐use history. We also recorded richness and abundance of plant species, identifying them as exotic or native. We differentiated four groups of plots according to their species composition. The groups had significant differences in altitude, soil conditions, land‐use history, and particularly, in richness of exotic species and exotic/native cover ratio. They could be ascribed to shrub‐ and grass‐páramo vegetation types based on their relative dominance of woody and herbaceous species; however, these groups were not arranged according to the hypothetical composition of altitudinal belts, but rather formed a mosaic of patches. This mosaic was determined not only by altitude but also by soil conditions and disturbance history of sites. Our results corroborate recent findings which highlight shrub‐ and grass‐páramo vegetation types as patches of contrasting species composition and structure that depend on local environmental variables, as well as human‐induced disturbances as a major determinant of compositional discontinuities in these ‘high mountain’ tropical ecosystems.