Seed rain and environmental controls on invasion of <Emphasis Type="Italic">Picea abies</Emphasis> into grassland

Although changes in land-use, climate, and the spread of introduced tree species have increased the global importance of tree invasions into grasslands, our ability to predict any particular invasion is limited. To elucidate mechanisms driving tree invasions of grasslands, we studied in detail how seed dispersal and fine-scale environment control the expansion of an introduced Picea abies Karst. (Norway spruce) population into Western Carpathian grassland. We mapped invading trees and measured tree size, fecundity, seed rain, seedling density, plant community composition, and light and soil environment within a 200 × 60 m belt across the invasion front. Maximum likelihood estimates of dispersal kernels suggested peak seed deposition directly underneath tree crowns where germination was poor, but mean dispersal distances were sufficiently large to generate overlapping seed shadows from multiple trees that saturated the invasion front with seeds further away from seed-dispersing trees. Partial Mantel tests indicated that germinant density was affected considerably less by seed rain than by moss cover (r = 0.54), overstory tree influence (r = −0.32), soil moisture (r = 0.21), grass cover (r = −0.15), and diffuse radiation (r = 0.13). However, these variables were not independent but formed complex multivariate gradients within the invasion front. Moss cover and soil moisture were negatively correlated with overstory tree influence and the resulting gradient clearly affected germinant density (partial Mantel r = 0.45). In contrast, positively correlated light and grass cover defined a gradient related weakly to germinant density (partial Mantel r = 0.05) as it integrated opposing effects of these variables on germinants. Seedlings had similar environmental associations, but except for the lasting positive effects of moss these tended to weaken with seedling size. Although a few seedlings may establish and survive in the more adverse environment of the outer edges of the invasion front, a significant population expansion may require a gradual build-up of the critical density of invading trees to reduce grass cover and facilitate germination on moist mossy seedbeds within uncolonized areas. Thus, Picea abies appears more likely to spread within temperate grasslands by gradual expansion of its population frontier rather than by advanced groups.     

Dynamism in the upstream invasion edge of a freshwater fish exposes range boundary constraints

Studying the dynamics of species’ borders can provide insight into the mechanisms limiting or promoting range expansion in response to environmental change. In the John Day River, Oregon (USA), rising stream temperatures are facilitating the upstream expansion of invasive smallmouth bass Micropterus dolomieu. Here, where smallmouth bass occupy the upstream limit of its thermal tolerance, we explore population structure and seasonal movement patterns to elucidate the environmental conditions and individual traits that define front edge (where individuals reside year-round) and leading edge (where individuals colonize, but may not establish) limits to its upstream distribution. Reporting on a multi-year, spatially extensive riverscape survey, our results show dramatic ebbs and flows of seasonal occupancies due to individual movement with an overall trend of upstream expansion. We revealed distinct front and leading edge invasion extents, each constrained by different ecological conditions. The front edge is largely constrained by the ability for juveniles to survive an overwinter starvation period, whereas the leading edge is associated with adult growth potential and seasonal hydrological conditions. We also found key morphological traits associated with more mobile individuals. By providing mechanistic insight into the factors that promote or limit range expansion of an invasive riverine species, our study enhances the ability to predict future range shifts and provides critical information to managers tasked with restricting further expansion.     

Evolutionary ecology along invasion fronts of the annual grass Aegilops triuncialis

Over the last two decades, Aegilops triuncialis (barbed goatgrass) has rapidly spread into many annual grassland and serpentine soil sites within California, USA. The capacity of this species to invade edaphically stressful serpentine soil is especially unusual. It is unclear whether genetic differentiation, phenotypic plasticity, or both have allowed A. triuncialis to invade competitive (i.e. high productivity non-serpentine annual grassland) and edaphically stressful (i.e. low productivity serpentine) environments. We used a reciprocal transplant field experiment to examine the effects of plasticity and genetic variation on A. triuncialis phenology and demography along invasion fronts associated with interspecific competition and edaphic gradients. We reciprocally transplanted seeds collected behind invasion fronts (core subpopulations) and along invasion fronts (edge subpopulations). For both gradient types we measured higher reproduction and population growth at invasion front edges. This was true for both edge and core subpopulation seed sources, suggesting that phenotypic plasticity may facilitate invasive spread. Consistent planting site effects indicated that phenotypic plasticity is a primary contributor to A. triuncialis demographic responses along interspecific competition gradients. In contrast, significant seed source effects suggest genetic differentiation along invasion fronts in serpentine edaphic gradients. Although persistent maternal environmental effects cannot be ruled out entirely, seed source effects suggest genetic differences between serpentine subpopulations located behind and beyond the invasion fronts for plant survival, plant size, total seed production, and individual seed size. Rapid expansion of A. triuncialis in California may reflect an evolutionary capacity in this species for both phenotypic plasticity and genetic differentiation.     

Sex and stochasticity affect range expansion of experimental invasions

Understanding and predicting range expansion are key objectives in many basic and applied contexts. Among dioecious organisms, there is strong evidence for sex differences in dispersal, which could alter the sex ratio at the expansion's leading edge. However, demographic stochasticity could also affect leading‐edge sex ratios, perhaps overwhelming sex‐biased dispersal. We used insects in laboratory mesocosms to test the effects of sex‐biased dispersal on range expansion, and a simulation model to explore interactive effects of sex‐biased dispersal and demographic stochasticity. Sex‐biased dispersal created spatial clines in the sex ratio, which influenced offspring production at the front and altered invasion velocity. Increasing female dispersal relative to males accelerated spread, despite the prediction that demographic stochasticity would weaken a signal of sex‐biased dispersal. Our results provide the first experimental evidence for an influence of sex‐biased dispersal on invasion velocity, highlighting the value of accounting for sex structure in studies of range expansion.     

Patterns of Patchy Spread in Deterministic and Stochastic Models of Biological Invasion and Biological Control

A few spatiotemporal models of population dynamics are considered in relation to biological invasion and biological control. The patterns of spread in one and two spatial dimensions are studied by means of extensive numerical simulations. We show that, in the case that population multiplication is damped by the strong Allee effect (when the population growth rate becomes negative for small population density), in a certain parameter range the spread can take place not via the intuitively expected circular expanding population front but via motion and interaction of separate patches. Alternatively, the patchy spread can take place in a system without Allee effect as a result of strong environmental noise. We then show that the phenomenon of deterministic patchy invasion takes place ‘at the edge of extinction’ so that a small change of controlling parameters either brings the species to extinction or restores the travelling population fronts. Moreover, we show that the regime of patchy invasion in two spatial dimensions actually takes place when the species go extinct in the corresponding 1-D system.     

Spatial Assortment of Mixed Propagules Explains the Acceleration of Range Expansion

Range expansion of spreading organisms has been found to follow three types: (i) linear expansion with a constant rate of spread; (ii) bi-phase expansion with a faster linear expansion following a slower linear expansion; and (iii) accelerating expansion with a continuously increasing rate of spread. To date, no overarching formula exists that can be applied to all three types of range expansion. We investigated how propagule pressure, i.e., the initial number of individuals and their composition in terms of dispersal ability, affects the spread of a population. A system of integrodifference equations was then used to model the spatiotemporal dynamics of the population. We studied the dynamics of dispersal ability as well as the instantaneous and asymptotic rate of spread. We found that individuals with different dispersal abilities were spatially sorted with the stronger dispersers situated at the expanding range front, causing the velocity of expansion to accelerate. The instantaneous rate of spread was found to be fully determined by the growth and dispersal abilities of the population at the advancing edge of the invasion. We derived a formula for the asymptotic rate of spread under different scenarios of propagule pressure. The results suggest that data collected from the core of the invasion may underestimate the spreading rate of the population. Aside from better managing of invasive species, the derived formula could conceivably also be applied to conservation management of relocated, endangered or extra-limital species.     

Modeling and analysis of stochastic invasion processes

In this paper we derive spatially explicit equations to describe a stochastic invasion process. Parents are assumed to produce a random number of offspring which then disperse according to a spatial redistribution kernel. Equations for population moments, such as expected density and covariance averaged over an ensemble of identical stochastic processes, take the form of deterministic integro-difference equations. These equations describe the spatial spread of population moments as the invasion progresses. We use the second order moments to analyse two basic properties of the invasion. The first property is permanence of form in the correlation structure of the wave. Analysis of the asymptotic form of the invasion wave shows that either (i) the covariance in the leading edge of the wave of invasion asymptotically achieves a permanence of form with a characteristic structure described by an unchanging spatial correlation function, or (ii) the leading edge of the wave has no asymptotic permanence of form with the length scales of spatial correlations continually increasing over time. Which of these two outcomes pertains is governed by a single statistic, φ which depends upon the shape of the dispersal kernel and the net reproductive number. The second property of the invasion is its patchy structure. Patchiness, defined in terms of spatial correlations on separate short (within patch) and long (between patch) spatial scales, is linked to the dispersal kernel. Analysis shows how a leptokurtic dispersal kernel gives rise to patchiness in spread of a population. Received: 11 August 1997 / Revised version: 22 September 1998 / Published online: 4 October 2000     

Strong spatial genetic structure in peripheral but not core populations of Sitka spruce [Picea sitchensis (Bong.) Carr.

We examined spatial genetic structure within eight populations of Sitka spruce classified as core or peripheral based on ecological niche, and continuous or disjunct based on species distribution. In each population, 200 trees were spatially mapped and genotyped for eight cDNA-based sequence tagged site (STS) codominant markers. Spatial autocorrelation was assessed by estimating p(ij), the average co-ancestry coefficient, between individuals within distance intervals. The distribution of alleles and genotypes within core populations was almost random, with nonsignificant co-ancestry values among trees as close as 50 m in core populations. In contrast, the distribution of alleles and genotypes within peripheral populations revealed an aggregation of similar multilocus genotypes, with co-ancestry values greater than 0.20 among trees up to 50 m apart and significant, positive values between trees up to 500 m. The relatively high density of reproductive adults in core populations may lead to highly overlapping seed shadows that limit development of spatial genetic structure. However, in peripheral populations with a lower density of adults, the distribution of alleles and genotypes was highly structured, likely due to offspring establishment near maternal trees and subsequent biparental inbreeding, as well as more recent population establishment at the leading edge of post-Pleistocene range expansion. Conserving genetic diversity in peripheral populations may require larger reserves for in situ conservation than required in core populations. These data on spatial genetic structure can be used to provide guidance for sampling strategies for both ex situ conservation and research collections.     

Coexistence of individual and social learners during range expansion

Individual learning and social learning are two primary abilities supporting cultural evolution. Conditions for their evolution have mostly been studied by investigating gene frequency dynamics, which essentially implies constant population size. Predictions from such “static” models may only be of partial relevance to the evolution of advanced individual learning in modern humans, because modern humans have experienced rapid population growth and range expansion during “out-of-Africa.” Here we model the spatial population dynamics of individual and social learners by a reaction–diffusion system. One feature of our model is the inclusion of the possibility that social learners may fail to find an exemplar to copy in regions where the population density is low. Due to this attenuation effect, the invasion speed of social learners is diminished, and various kinds of invasion dynamics are observed. Our primary findings are: (1) individual learners can persist indefinitely when invading environmentally homogeneous infinite space; (2) the occurrence of individual learners at the front may inhibit the spread of social learners. These results suggest that “out-of-Africa” may have driven the evolution of advanced individual learning ability in modern humans.     

Evolutionarily accelerated invasions: the rate of dispersal evolves upwards during the range advance of cane toads

Human activities are changing habitats and climates and causing species' ranges to shift. Range expansion brings into play a set of powerful evolutionary forces at the expanding range edge that act to increase dispersal rates. One likely consequence of these forces is accelerating rates of range advance because of evolved increases in dispersal on the range edge. In northern Australia, cane toads have increased their rate of spread fivefold in the last 70 years. Our breeding trials with toads from populations spanning the species' invasion history in Australia suggest a genetic basis to dispersal rates and interpopulation genetic variation in such rates. Toads whose parents were from the expanding range front dispersed faster than toads whose parents were from the core of the range. This difference reflects patterns found in their field-collected mothers and fathers and points to heritable variance in the traits that have accelerated the toads' rate of invasion across tropical Australia over recent decades. Taken together with demonstrated spatial assortment by dispersal ability occurring on the expanding front, these results point firmly to ongoing evolution as a driving force in the accelerated expansion of toads across northern Australia.     

Virulence evolution at the front line of spreading epidemics

Understanding and predicting the spatial spread of emerging pathogens is a major challenge for the public health management of infectious diseases. Theoretical epidemiology shows that the speed of an epidemic is governed by the life‐history characteristics of the pathogen and its ability to disperse. Rapid evolution of these traits during the invasion may thus affect the speed of epidemics. Here we study the influence of virulence evolution on the spatial spread of an epidemic. At the edge of the invasion front, we show that more virulent and transmissible genotypes are expected to win the competition with other pathogens. Behind the front line, however, more prudent exploitation strategies outcompete virulent pathogens. Crucially, even when the presence of the virulent mutant is limited to the edge of the front, the invasion speed can be dramatically altered by pathogen evolution. We support our analysis with individual‐based simulations and we discuss the additional effects of demographic stochasticity taking place at the front line on virulence evolution. We confirm that an increase of virulence can occur at the front, but only if the carrying capacity of the invading pathogen is large enough. These results are discussed in the light of recent empirical studies examining virulence evolution at the edge of spreading epidemics.     

Dynamic expansion in recently introduced populations of fire ant parasitoids (Diptera: Phoridae)

Combating invasive species requires a detailed, mechanistic understanding of the manner and speed with which organisms expand their ranges. Biological control efforts provide an opportunity to study the process of species invasions and range expansions under known initial conditions. This study examines the rate, pattern and mechanisms of spread for two populations of the biological control agent Pseudacteon tricuspis, phorid-fly parasitoids of imported fire ants. We employ a trap-based survey method that detects phorid flies in low-density populations, and provides data on abundance. This technique allows us to differentiate between continuous population spread and effective long-distance dispersal and to examine density gradients of phorid flies across the expanding population front. We find that occupied sites in front of the leading edge of continuous populations were common; forming small populations we refer to as satellite populations. Satellite populations are tens of kilometers from the nearest possible source. Wind governs the dynamics of spread in these two central Texas populations. Population edges expanding with the wind exhibited a higher frequency of effective long-distance dispersal than did populations expanding into the wind. This enhanced effective long-distance dispersal rate translated into a five times faster rate of spread for population edges traveling with the wind. This planned invasion shares many characteristics in common with unplanned species invasions including: protracted establishment phase during which densities were below detection thresholds, and slow initial spread immediately after establishment followed by rapid, accelerating spread rates as population sizes grew.     

All quiet on the western front? Using phenological inference to detect the presence of a latent gypsy moth invasion in Northern Minnesota

Following establishment in a new environment, invasive species expand their range through stratified diffusion, the coupling of local growth and spatial spread with long-distance movement of propagules that found new colonies. Consequently, the stages of arrival and establishment serially repeat until the entire habitat susceptible to invasion is occupied. An empirical example of such a phenomenon is the gypsy moth, Lymantria dispar (L.), invasion of northeastern Minnesota. Under a regional management program, a trapping grid is deployed along the L. dispar leading edge to detect male moth presence and inform management decisions. However, the presence of moths does not always indicate reproducing populations, and moths dispersing from populations behind the front can obscure the presence of latent invasions. We analyzed trapping data during the arrival and establishment of L. dispar in this region (2000–2012), supplemented with sentinel trap data to ascertain the timing of male moth flight (2005–2009), and derived yearly phenological predictions for male moth flight. We also used a male wing morphology metric (2007–2009) to further quantify spatial and temporal patterns associated with the L. dispar invasion. We provide evidence of an established L. dispar population when analyses suggest spatial randomness in trap catch data, and how the presence of phenologically distinct, immigrating adult moths from source populations outside of Minnesota recorded from sentinel traps could lead to the overestimation of the abundance of an established population. This work highlights the complexity of the initial invasion process even in a well-studied system for which a sensitive monitoring program exists.     

Distribution and effects of tree leaf litter on vegetation composition and biomass in a forest-grassland ecotone

Aims After abandonment of grasslands, secondary succession leads to the invasion by woody species. This process begins with the accumulation of tree litter in the forest–grassland ecotone. Our objectives were to determine the relationships between litter amounts and vegetation composition and cover along natural forest–grassland ecotones and to experimentally study the initial effects of tree litter accumulation on grassland vegetation and on microsite conditions.Methods We established 11 transects varying from 12 to 15 m in length in different forest–grassland ecotones in the Lahn-Dill highlands, Germany, and measured the mass and cover of tree litter and the cover and composition of vegetation at five sequential positions along each transect by using 1 m 2 plots with five replications. In a field experiment, we established plots subjected to different litter amounts (0, 200 and 600g m ?2) and evaluated changes in grassland vegetation, soil temperature and soil nutrient availability below the litter layer.Important findings Tree litter amounts decrease from 650 to 65g m ?2 across the forest–grassland ecotone. Vegetation changed from shrubs and annual species (adapted to more stressful conditions) in the forests edge to grasses, rosettes and hemirosette species (with higher competitive abilities) in the grassland. These anthropogenic forest–grassland ecotones showed abrupt edges, and the two adjacent ecosystems were characterized by different species pools and functional groups. In the field experiment, the presence of a litter layer reduced vegetation biomass and cover; the species richness was only reduced in the treatment with high litter (600g m ?2). Additionally, adding litter on top of vegetation also reduced thermal amplitude and the number of frost days, while increasing the availability of some nutrients, such as nitrogen and aluminium, the latter being an indicator of soil acidification. Adding a tree litter layer of 600g m ?2 in grassland areas had strong effects on the composition and diversity of grassland vegetation by reducing the cover of several key grassland species. In, or near, forest edges, litter accumulation rapidly changes established vegetation, microsite conditions and soil nutrients.     

The evolution of juvenile-adult interactions in populations structured in age and space

We study the evolution of a spatially structured population with two age classes using spatial moment equations. In the model, adults can either help juveniles by increasing their survival, or adopt a cannibalistic behaviour and consume juveniles. While cannibalism is the sole evolutionary outcome when the population is well-mixed, both cannibalism and parental care can be evolutionarily stable if the population is viscous. Our analysis allows us to make two main technical points. First, we present a method to define invasion fitness in class-structured viscous populations, which allows us to apply adaptive dynamics methodology. Second, we show that ordinary pair approximation introduces an important quantitative bias in the evolutionary model, even on random networks. We propose a correction to the ordinary pair approximation that yields quantitative accuracy, and discuss how the bias associated with this approach is precisely what allows us to identify subtle aspects associated with the evolutionary dynamics of spatially structured populations.     

Woody species patterns at forest–grassland boundaries in southern Brazil

Vegetation mosaics of grassland/savanna and forest can be found in tropical and subtropical regions of the world, as in southern Brazil, where climate conditions are suitable for forest. Changes in intensity or frequency of disturbances could enable woody species encroachment in grassland communities; however, the processes are related to site conditions and life history of pioneer species. In this paper, we study transition patterns of forest to grassland in the absence of grazing, but under different site conditions related to aspect (landscape position) and time since the last burn. Data are based on shrub and tree species composition and soil variables at forest–grassland boundaries. We found 119 woody species of 42 families along transects of 27 m into the forest and 31.5 m into the grassland. Gradients from forest to grassland were analysed as compositional trajectories in ordination space and differences in the spatial patterns depicted between distinct site aspects. The time since the last fire did not influence these patterns. Inside the forest, tree species diversity was significantly higher close to the edge, independent of the density of individuals. Two main mechanisms may promote forest expansion into the grassland. First, a gradual tree encroachment near the edge and, second, a mechanism linked to the recruitment of isolated pioneer trees within the grassland matrix, most frequently near rocky outcrops, where a decrease in grass biomass leads to low-intensity fires. Despite vegetation patterns at boundaries differing according to aspect, the most important explanatory factor was the distance from the forest border, not just by itself, but with all correlated parameters that are changing along the gradient.     

Population cycles produce periodic range boundary pulses

Classical theories of biological invasions predict constant rates of spread that can be estimated from measurable life history parameters, but such outcomes depend strongly on assumptions that are often unmet in nature. Subsequent advances have demonstrated how relaxing assumptions of these foundational models results in other spread patterns seen in nature, including invasions that accelerate through time, or that alternate among periods of expansion, retraction, and stasis of range boundaries. In this paper, we examine how periodic population fluctuations affect temporal patterns of range expansion by coupling empirical data on the gypsy moth invasion in North America with insights from a model incorporating population cycles, Allee effects, and stratified diffusion. In an analysis of field data, we found that gypsy moth spread exhibits pulses with a period of 6 yr, which field data and model simulations suggest is the result of a 6‐yr population cycle in established populations near the invasion front. Model simulations show that the development of periodic behavior in range expansion depends primarily on the period length of population cycles. The period length of invasion pulses corresponded to the population cycle length, and the regularity of invasion pulses tended to decline with increases in population cycle length. A key insight of this research is that dynamics of established populations, behind the invasion front, can have strong effects on spread. Our findings suggest that coordination between separate management programs targeting low‐density spreading and established outbreaking populations, respectively, could increase the efficacy of efforts to mitigate gypsy moth impacts. Given the variety of species experiencing population fluctuations, Allee effects, and stratified diffusion, insights from this study are potentially important to understanding how the range boundaries of many species change.     

Enemy release at range edges: do invasive species escape their herbivores as they expand into new areas?

Aims We test the hypothesis that invasive plant species at their range edges experience lower herbivory and allocate less to defense at the edge of an expanding range edge than from more central parts of their distribution, during secondary invasion in a new range. Invasive plants are often able to spread rapidly through new areas. The success of invasive species in new ranges is frequently attributed to enemy release in these new areas and associated evolutionary changes minimizing allocation to defense in favor of growth and reproduction. Enemy release could also explain rapid advances of invasive species upon arriving in new habitats. If invasive species accumulate enemies over time in a new location, then these species may experience a release from their enemies at expanding range fronts. Enemy release at these range fronts may accelerate range expansion.Methods We used populations of four woody invasive species within the invaded range, and four native control species. We quantified leaf herbivory and leaf physical defense traits at both range central and range edge locations, over two 1-month sampling periods, sampled 7 months apart.Important findings Herbivory at the range edge did not differ to the range center but patterns were not consistent across species. There was a trend for lower herbivory at the range edge for Lantana camara, which was reflected in lower leaf toughness. Overall, leaf toughness was greater at the range edge location across invasive and control species. Physical defenses were different among range locations in a few species, though most species show the same trend, suggesting higher herbivory pressures at the range edge location or differences may be due to climatic factors. Leaves of L. camara were significantly less tough at range edges, suggesting that some species can potentially escape their enemies at range edges. However, our results overall do not support the hypothesis that plants at the edge of their ranges experience reduced impact from their enemies.     

The impact of forest spread on a marginal population of a protected peony (<Emphasis Type="Italic">Paeonia officinalis</Emphasis> L.): the importance of conserving the habitat mosaic

In the Mediterranean region of Europe, land-use changes have allowed for rapid colonisation of open habitats by woody species. As a result, it is critical to gather information on how protected species in open habitats respond to forest spread in such areas. Our objective is to quantify whether spatial heterogeneity of the vegetation associated with recent forest closure influences demographic structure and maternal fertility in a population of the protected Paeonia officinalis L. In closed woodland, adult plants of P. officinalis are almost exclusively vegetative, in open habitats seedlings are rare and on the woodland edge there is a relative over-representation of flowering plants and seedlings. Forest closure dramatically reduces flowering frequency, but has no significant effect on maternal fertility of flowering plants. The spatial aggregation of seedlings close to the maternal plants suggests that dispersal is spatially restricted. Together, these results suggest that the viability of the population requires a transitional habitat between open garrigues or grassland with spaced trees and woodland. A management programme incorporating tree and shrub thinning and cutting of parcels in rotation to maximise the length of the forest edge could maintain a habitat mosaic that favours the persistence of this species in the study site.     

Multiple factors determine the rate of increase of an invading non-native tree in New Zealand

We reconstructed the invasion of a non-native tree (hawthorn, Crataegus monogyna) into fire-induced grassland in montane South Island, New Zealand. Using the relationship between height and age to reconstruct the rate of increase of the population, we identified three distinct invasion phases. We hypothesised that these related to the abundance of woody vegetation and therefore of non-native blackbirds (Turdus merula), the primary disperser of hawthorn in this environment. From the 1930s to 1959, increase was relatively slow, with hawthorn spread probably constrained due to browsing of seedlings by European rabbits (Oryctolagus cuniculus) and the low abundance of a native N-fixing shrub (matagouri, Discaria toumatou) providing few perches and nesting sites for blackbirds and safe sites for hawthorn establishment. Subsequently, from 1959 to 1976, hawthorn showed greater than six fold acceleration in its rate of population increase, resulting largely from intensive rabbit control, less frequent fires, and aerial topdressing of phosphate fertilizer promoting the growth of matagouri scrub. In addition, maturing hawthorn trees provided additional food and nesting sites for blackbirds. Hawthorn population increase slowed again from 1976, possibly because most suitable habitat in the immediate vicinity of the population neared saturation. From this case study we have constructed a general hypothesis for the factors determining the rate of invasion of matagouri—grassland habitats by bird-dispersed non-native woody plants in montane New Zealand.