Effect of habitat subdivision on the population dynamics of herbivorous and predatory insects in central Chile

ABSTRACT Habitat subdivision may facilitate the persistence at low densities of both predator and prey populations. But habitat subdivision may also allow herbivores to escape from predators and therefore reach higher local densities. The outcome will depend on the dispersal behaviour of organisms. The effect of habitat subdivision on populations of aphidophagous coccinellids Eriopis connexa and Hippodamia variegata and their prey Brevicoryne brassicae was evaluated in crops of Brassica o/eraceae. In particular, the following responses were analysed: a) density/plant of herbivorous and predatory insects, b) colonisation of herbivorous and predatory insects, c) movement between plant patches and residence time of predators, d) in situ recruitment of predators, and e) final weight of plants. Nine continuous and subdivided plots of B. oleracea were set up following a 3 x 3 completely randomised block design. The three treatments were: a) continuous, b) subdivided plus weeds, and c) subdivided plus cloth barriers. Density per plant of herbivorous and predatory insects was lower in subdivided plus barriers habitats. Colonisation by herbivores and predators was more rapid in continuous or subdivided plus weeds habitats. Residence time and in situ recruitment of predators was greater in continuous habitats. Plant weight was greater in subdivided than continuous habitats. Therefore, habitat subdivision, particularly when patches were more isolated, negatively affected the population densities of both herbivorous and predatory insects. This is likely due to lesser colonisation and in situ recruitment, and greater emigration of insects in those subdivided habitats. Key words: Brevicoryne brassicae, Eriopis connexa, Hippodamia variegata, spatial subdivision, predator-prey relationships. RESUMEN La subdivision del hábitat puede facilitar la persistencia a bajas densidades de herbfvoros y sus depredadores. Pero la subdivision del hábitat puede tambien permitir que Ios herbfvoros escapen de sus depredadores, alcanzando altas densidades a nivel local. El resultado dependera de la conducta de dispersion de Ios organismos involucrados. En este trabajo se evaluo el efecto de la subdivision del habitat sobre las dinamicas poblacionales de Ios coccinelidos Eriopis connexa e Hippodamia variegata y de su presa, el áfido Brevicoryne brassicae asociados a Brassica o/eraceae. En particular se analizaron !as siguientes respuestas: a) densidad/planta de insectos herbfvoros y depredadores, b) colonizacion de insectos herbfvoros y depredadores, c) movimiento entre parches de plantas y tiempo de residencia de depredadores, d) reclutamiento in situ de Ios depredadores, y e) peso final de !as plantas. Se implementaron nueve habitats continuos y subdivididos siguiendo un disefio de bloques al azar de 3 x 3. Los tres tratamientos fueron: a) continuo, b) subdividido más malezas y c) subdividido más barreras de tela. La densidad/planta de insectos herbfvoros y depredadores fue menor en Ios habitats subdivididos más barreras. La colonizacion por herbfvoros y depredadores fue más rapida en habitats continuos o subdivididos más malezas. El tiempo de residencia y el reclutamiento in situ de Ios depredadores fue mayor en habitats continuos. El peso final de !as plantas fue mayor en hábitats subdivididos. La subdivision del habitat, particularmente cuando Ios parches de plantas están más aislados, afecta negativamente !as densidades poblacionales de insectos herbfvoros y depredadores. Esto se deberfa a la menor colonizacion, menor reclutamiento in situ y mayor emigracion de insectos en aquellos habitats más subdivididos. Palabras clave: Brevicoryne brassicae, Eriopis connexa, Hippodamia variegata, subdivision del habitat, relacion depredador-presa

Mechanisms affecting population density in fragmented habitat

We conducted a factorial simulation experiment to analyze the relative importance of movement pattern, boundary-crossing probability, and mortality in habitat and matrix on population density, and its dependency on habitat fragmentation, as well as inter-patch distance. We also examined how the initial response of a species to a fragmentation event may affect our observations of population density in post-fragmentation experiments. We found that the boundary-crossing probability from habitat to matrix, which partly determines the emigration rate, is the most important determinant for population density within habitat patches. The probability of crossing a boundary from matrix to habitat had a weaker, but positive, effect on population density. Movement behavior in habitat had a stronger effect on population density than movement behavior in matrix. Habitat fragmentation and inter-patch distance may have a positive or negative effect on population density. The direction of both effects depends on two factors. First, when the boundary-crossing probability from habitat to matrix is high, population density may decline with increasing habitat fragmentation. Conversely, for species with a high matrix-to-habitat boundary-crossing probability, population density may increase with increasing habitat fragmentation. Second, the initial distribution of individuals across the landscape: we found that habitat fragmentation and inter-patch distance were positively correlated with population density when individuals were distributed across matrix and habitat at the beginning of our simulation experiments. The direction of these relationships changed to negative when individuals were initially distributed across habitat only. Our findings imply that the speed of the initial response of organisms to habitat fragmentation events may determine the direction of observed relationships between habitat fragmentation and population density. The time scale of post-fragmentation studies must, therefore, be adjusted to match the pace of post-fragmentation movement responses

Predators and parasitoids of the harlequin ladybird, Harmonia axyridis, in its native range and invaded areas

The harlequin ladybird Harmonia axyridis (Coleoptera: Coccinellidae) has rapidly spread in several continents over the past 30 years and is considered an invasive alien species. The success of H. axyridis as an invader is often attributed to weak control by natural enemies. In this paper, we provide an overview of current knowledge on predators and parasitoids of H. axyridis. The common feature of predators and parasitoids is that they directly kill exploited organisms. Currently available data show that H. axyridis, displaying a variety of chemical, mechanical, and microbiological anti-predator defenses, is usually avoided by predators. However, some birds and invertebrates can eat this ladybird without harmful consequences. The primary defenses of H. axyridis against parasitoids include immune response and physiological and nutritional unsuitability for parasitoid development. These defenses are probably relatively efficient against most ladybird parasitoids, but not against flies of the genus Phalacrotophora. The latter are idiobiont parasitoids and hence can evade the host’s immune response. Indeed, rates of parasitism of H. axyridis by Phalacrotophora in the Palaearctic region (both in the native range in Asia and in Europe) are relatively high. While strong evidence for enemy release on the invasive populations of H. axyridis is lacking, several cases of parasitoid acquisition have been recorded in Europe, North America, and South America. We conclude that enemy release cannot be excluded as a possible mechanism contributing to the spread and increase of H. axyridis in the early stages of invasion, but adaptation of parasitoids may lead to novel associations which might offset previous effects of enemy release. However, further work is required to elucidate the population-level effects of such interactions

Factors determining variation in colour morph frequencies in invasive Harmonia axyridis populations

The Harlequin ladybird Harmonia axyridis Pallas, native to eastern Asia, is an invasive, non-native species that has recently achieved an almost worldwide distribution. A conspicuous feature of this species is colour polymorphism of the elytra. In its native area, the populations consist of a recessive non-melanic morph, several dominant melanic morphs and small numbers of other (rare) morphs. The morph proportions in native populations have been intensively studied and vary with geographic area, climate and time. In contrast, colour polymorphism in invaded regions has been little studied. We examine and try to account for the morph frequencies observed across the different invaded regions. In America, monomorphic populations consist of the non-melanic morphs while European populations contain also melanic morphs. In particular geographic areas of Europe, the average percentage of the non-melanic morphs varied between 78 and 99%. It was highest in the lowlands of northern Italy and central and northern Europe and decreased in the Alps and western (Spain, UK) and eastern (southeast Russia) margins of the recently invaded area. In central Europe the frequency of the non-melanic morphs decreased over the course of the year but increased over the years from 2010 to 2018. The local differences might thus arise through gradual change of the morph composition of the founder invasive, non-native population. However, the variation in non-melanic morph frequency was not correlated with climatic characteristics that might affect coccinellid polymorphism. The observed rate of change in morph proportions in our data was too small to explain the diversification of what was supposedly a uniform invasive, non-native population at the point of introduction

The harlequin ladybird, Harmonia axyridis: global perspectives on invasion history and ecology

The harlequin ladybird, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), is native to Asia but has been intentionally introduced to many countries as a biological control agent of pest insects. In numerous countries, however, it has been introduced unintentionally. The dramatic spread of H. axyridis within many countries has been met with considerable trepidation. It is a generalist top predator, able to thrive in many habitats and across wide climatic conditions. It poses a threat to biodiversity, particularly aphidophagous insects, through competition and predation, and in many countries adverse effects have been reported on other species, particularly coccinellids. However, the patterns are not consistent around the world and seem to be affected by many factors including landscape and climate. Research on H. axyridis has provided detailed insights into invasion biology from broad patterns and processes to approaches in surveillance and monitoring. An impressive number of studies on this alien species have provided mechanistic evidence alongside models explaining large-scale patterns and processes. The involvement of citizens in monitoring this species in a number of countries around the world is inspiring and has provided data on scales that would be otherwise unachievable. Harmonia axyridis has successfully been used as a model invasive alien species and has been the inspiration for global collaborations at various scales. There is considerable scope to expand the research and associated collaborations, particularly to increase the breadth of parallel studies conducted in the native and invaded regions. Indeed a qualitative comparison of biological traits across the native and invaded range suggests that there are differences which ultimately could influence the population dynamics of this invader. Here we provide an overview of the invasion history and ecology of H. axyridis globally with consideration of future research perspectives. We reflect broadly on the contributions of such research to our understanding of invasion biology while also informing policy and people

Population spatial structure, human-caused landscape changes and species survival, Revista Chilena de Historia Natural

ABSTRACT Population survival depends on the spatial structure of the population, which is defined as the set of local populations that make up the population and the probability of exchange of individuals among them. Therefore, population spatial structure depends on the interaction between the landscape spatial pattern and the dispersal characteristics of the organisms. Human activities have profound effects on population spatial structure. Habitat loss decreases the number of local populations and therefore decreases overall population size. This results in a decrease in the number of dispersers available for recolonization and rescue of local populations. which further reduces overall population size. Eventually local extinctions can accumulate to regional extinctions. Habitat fragmentation per se exacerbates this problem by increasing the probability of local extinction. However. the effects of habitat loss far outweigh the effects of fragmentation per se. Species interactions are a critical component of ecosystem function. and alterations of population structure by human activity have significant effects on them. Mutualisms may be disrupted. coexistence may be either increased or decreased. predator-prey interactions may be destabilized. and new predator-prey interactions may be introduced. Realistic predictions of the effects of changes in landscape pattern on population survival and population interactions depend on an accurate understanding of their effects on population spatial structure. Key words: habitat fragmentation, habitat loss. population spatial structure, species interactions, metapopulations. RESUMEN La sobrevivencia de !as poblaciones depende de la estructura espacial de estas, la que se define como el conjunto de poblacioncs locales que conforman una poblacion y la probabilidad de intercambio de individuos entre ellas. Asf, la estructura espacial de una poblaci n depende de la interaccion entre el patron espacial del paisaje y las caracterfsticas de dispersion del organismo. Las actividades humanas tienen efectos profundos en la estructura espacial de !as poblaciones. La perdida de habitat disminuye cl n mero de poblaciones locales y con ello el tamafio total de una poblacion. Esto resulta en un decremento en el mero de dispersores disponibles para la recolonizacion y rescate de poblaciones locales, lo que reduce a n m s el tamafio pnblacional. Eventualmente. !as extinciones locales pueden causar extinciones regionales. La fragmentacion del h bitat per se cxacerba este problema a! incrementar la probabilidad de extinciones locales. Sin embargo. Ios efectos de la perdida de habitat son fuertcs que Ios efectos de la fragmentacion del habitat per se. Las interacciones entre especies pueden verse afectadas por altcraciones en la estructura poblacional debido a la actividad humana. Los mutualismos pueden ser alterados negativamente. la coexistencia de especies puede incrementar o decrecer seg n la magnitud de la perturbacion antropica. !as interacciones depredador-presa pueden desestabilizarse y nuevas interacciones depredador-presa pueden incorporarse a! ecosistema. Predicciones m s reales sobre Ios efectos de Ios cambios en el paisaje sobre la sobrevivencia de !as poblaciones y !as interacciones entre especies dependen de un entendimiento preciso de sus efectos en la estructura espacial de !as poblaciones. Palabras clave: estructura espacial poblacional. fragmentacion del habitat, perdida de habitat, interacciones entre especies, metapoblaciones

Composición, riqueza de especies y abundancia de insectos defoliadores de actividad nocturna asociados a Aristotelia chilensis (maqui) en el bosque maulino fragmentado Composition, species richness and abundance of nocturnal folivorous insects associated with Aristotelia chilensis (maqui) in the fragmented Maulino forest

En el bosque maulino, la herbivoría sobre Aristotelia chilensis (maqui) es negativamente afectada por la fragmentación del bosque, siendo mayor en el bosque continuo que en los fragmentos, particularmente a inicios de la temporada de crecimiento. Este fenómeno puede deberse a cambios en la dinámica de las poblaciones de defoliadores, esencialmente insectos. En este trabajo se evaluó la abundancia, riqueza de especies y composición de insectos defoliadores de actividad nocturna presentes en A. chilensis en un bosque continuo (600 ha) y en ocho fragmentos remanentes (0,4-20 ha). Los muéstreos se realizaron mensualmente, entre agosto de 2005 y febrero de 2006, en 32 ejemplares adultos de A. chilensis en el bosque continuo y en 32 ejemplares en los fragmentos. Los insectos fueron muestreados durante las cinco primeras horas de la noche. Se recolectaron 890 insectos defoliadores, pertenecientes a 17 familias y 77 especies pertenecientes a los órdenes Coleóptera, Orthoptera y Lepidoptera, siendo todas nativas. La abundancia total no varió según el habitat. Sin embargo, la fragmentación incrementó o disminuyó la abundancia de algunas especies. La riqueza de especies por árbol tampoco fue afectada por la fragmentación del bosque, aunque el número total de especies fue considerablemente mayor en los fragmentos que en el bosque continuo. La similitud de especies fue mayor dentro del bosque continuo que entre el bosque continuo y los fragmentos o que entre los fragmentos. A principios de la temporada de crecimiento de A. chilensis (septiembre), la abundancia de Sericoides obesa fue significativamente mayor en el bosque continuo que en los fragmentos. Al avanzar en la temporada, Sericoides viridis se hizo más abundante en los fragmentos. Por el tamaño y la voracidad de los insectos del género Sericoides ellos serían los principales responsables de los patrones de defoliación de A. chilensis en el bosque maulino.At the Maulino forest, herbivory upon Aristotelia chilensis (maqui) is negatively affected by forest fragmentation, being higher in the continuous forest than in the small remnant fragments, particularly at the beginning of the growing season (spring). This phenomenon may be attributable to changes in the population dynamics of herbivores, mostly insects. In this work, the effect of the Maulino forest fragmentation on the abundance, species richness and composition of nocturnal defoliating insects associated with A. chilensis was evaluated. The insects associated with A. chilensis were sampled monthly, between August 2005 and February 2006, in 32 trees located in a continuous forest (600 ha) and in 32 trees located in eight forest fragments (0.4-20 ha). Insect sampling was carried out during the first five night hours. 890 folivorous insects from 17 families and 77 species of Coleóptera, Orthoptera and Lepidoptera were collected. All species were native. The total abundance did not vary with habitat fragmentation. Nevertheless, some species increased their abundance with fragmentation, while others were depressed. The species richness of folivorous insects per tree did not vary with forest fragmentation, though the total number of species was considerably higher in the forest fragments. The species similarity was higher within the continuous forest than between the continuous forest and fragments or among fragments. At the beginning of the season (September), the abundance of Sericoides obesa was significantly higher in the continuous forest than in the forest fragments. Later in the season, Sericoides viridis became more abundant in the forest fragments. Because of the body size and the voracity of Sericoides sp., these insects seem to be responsible of the herbivory patterns of A. chilensis at the Maulino forest