Regional patch dynamics of Cirsium arvense and possible implications for plant‐animal interactions

Abstract. Plant population biology considers the dynamics of plant modules within stands. However, stands themselves may have considerable regional turnover in space and time. These changes in the number, distribution and size of plant stands generate a dynamic spatial pattern with important implications for the spatial and temporal dynamics of phytophagous insects using these plants as a host. During five successive years we studied the regional distribution and patch dynamics of the creeping thistle Cirsium arvense and the distribution of associated populations of the herbivore Urophora cardui (Diptera: Tephritidae), a specialist stem gall former. The study conducted was in a 15 km² heterogeneous, agricultural area in northeastern Bavaria. The distribution of the number of plants per patch was skewed with many more small C. arvense patches than large ones. During the five years of study, there was a 50% increase in the number of C. arvense patches, and a decrease in the mean number of plants per patch (= patch size) to less than half the patch size of the first year. Whilst patch size was randomly distributed in space, patch density showed a consistent, non‐random spatial pattern. Patch density was spatially auto‐correlated, with areas of high or low patch density having a characteristic dimension of ca. 1 km. Patch size was predictable in time and appeared to be regulated by size dependent processes, with the extinction probability of a patch being negatively correlated with its size. Correlated with the decline of C. arvense patch size during the study, the occupancy and total numbers of the herbivore U. cardui had a marked decrease, suggesting that the regional distribution of the stem gall former is not only influenced by patch number but more importantly by the mean patch size. With decreasing patch sizes, U. cardui was faced with an increasingly dynamic landscape due to higher extinction rates of small patches, although the mean distance between host plant patches decreased.     

The larval ecology of the butterfly Euphydryas desfontainii (Lepidoptera: Nymphalidae) in SW-Portugal: food plant quantity and quality as main predictors of habitat quality

Corresponding to theory, the persistence of metapopulations in fragmented landscapes depends on the area of suitable habitat patches and their degree of isolation, mediating the individual exchange between habitats. More recently, habitat quality has been highlighted as being equally important. We therefore assess the role of habitat area, isolation and quality for the occupancy of larval stages of the regionally threatened butterfly Euphydryas desfontainii occurring in grassland habitats comprising the host plant Dipsascus comosus. We put a special focus on habitat quality which was determined on two spatial scales: the landscape (among patches) and the within-patch level. On the landscape level, occupancy of caterpillars was determined by a presence-absence analysis at 28 host plant patches. On the within-patch level, oviposition site selection was studied by comparing 159 host plants with egg clutches to a random sample of 253 unoccupied host plants within six habitat patches. The occupancy of caterpillars and presence of egg clutches on host plants was then related to several predictors such as patch size and isolation on the landscape level and host plant characteristics and immediate surroundings on the within patch level. On the landscape level, only host plant abundance was related to the presence of caterpillars, while size and isolation did not differ between occupied and unoccupied patches. However, the weak discrimination of larval stages among patches changed on the within-patch level: here, several microclimatic predictors such as sunshine hours and topography, host plant morphology and phenology as well as further potential host plants in the immediate surroundings of the plant chosen for oviposition strongly determined the presence of egg clutches. We strongly suggest promoting the presence of the host plant in topographically and structurally rich habitat patches to offer potential for microclimatic compensation for a species considered threatened by climate change.     

A comparison of the responses of two tropical specialist herbivores to host plant patch size

Summary The effects of host plant patch size on the abundances of two specialist herbivores (the chrysomelid beetle, Acalymma innubum and the pentatomid bug, Piezosternum subulatum) were investigated in a natural forest community in the Virgin Islands. Abundances were compared early and late in the season in different sized patches of the cucurbit host plant (Cayaponia americana) growing in open habitat (with no surrounding plant community) and forest habitat (with diverse surrounding plant community). For both herbivore species, adult abundances per patch were positively correlated with patch leaf area, but there was a significant patch size effect (i.e., correlation between herbivore density per unit plant and patch leaf area) only for beetles in the forest habitat. Both herbivore species were significantly affected by surrounding plant diversity, but in opposite ways: beetles were more abundant in open patches whereas bugs were more abundant in forest patches. Relationships between abundance and patch size in open and forest patches changed through the season for both herbivore species. These changing abundance patterns are discussed with respect to (1) increases in the diversity of the plant community surrounding host plant patches, and (2) differences in herbivore movement patterns.     

Temporal variation in the effects of habitat fragmentation on reproduction of the Mediterranean shrub <Emphasis Type="Italic">Colutea hispanica</Emphasis>

Habitat fragmentation poses a major threat to the viability of plant populations. However, the intensity of fragmentation effects may vary among years. We studied two possible effects of habitat fragmentation (patch size and isolation) on the reproduction and proportion of damaged fruits in 24 patches of the self-compatible shrub Colutea hispanica for three consecutive years with different climate conditions. We also studied the effect of fragmentation on the incidence of two main pre-dispersal seed predators, the butterflies Iolana iolas and Lampides boeticus. High between-year variability was found in number of viable seeds per fruit, number of fruits per plant, total number of viable seeds per plant and proportion of damaged fruits. In 2003, small, isolated patches had a higher fruit set and number of fruits per plant. The proportion of damaged fruits was significantly lower in isolated populations in 2003, while it was very high in all patches in 2004 and 2005. High between-year variability was also found in the proportion of fruits per plant with I. iolas eggs. In 2003 isolated patches had a lower proportion of fruits with I. iolas eggs, but no significant effect of patch size and isolation was found in 2004 or 2005. The proportion of fruits with L. boeticus eggs was similar in the three years of study, although it was slightly higher in large, non-isolated patches in 2003. Thus, the effects of fragmentation on plant reproduction cannot be generalized from one single-year survey. In contrast to the generally accepted idea that fragmentation reduces plant reproduction, plant fitness may increase in isolated patches in years with high fruit production and low seed predation.     

Oviposition site preference inNeacoryphus bicrucis (Hemiptera: Lygaeidae): Responses to the density and dispersion of a single host-plant species

Caged females of Neacoryphus bicrucis(Hemiptera: Lygaeidae) were permitted to settle upon and oviposit within a small or large ragwort patch, each of which contained both maturing (= adult food/water source) and dehisced (=oviposition substrate/resource for nymphs) flower heads. The proportion of females settling in the large patch was significantly greater than for the small patch but not significantly greater than expected based on the proportion of maturing flower heads contained in each patch. Also, the number of egg masses laid in each patch was proportional to the number of females that settled there. However, egg mass size was significantly skewed toward larger numbers of eggs in the larger patch. Within the large patch egg mass size was significantly larger where the number of dehisced flower heads was greater and accounts for the between-patch difference in number of eggs per mass. Thus, female settlement does not demonstrate a resource concentration effect but females do appear to evaluate patches after settlement with regard to suitability for oviposition.     

Density-dependent parasitism of Delphacodes kuscheli eggs by Anagrus flaveolus: Influence of egg patchiness and density

Many herbivorous insects display a marked tendency to aggregate and previous work has shown that the parasitoids likely to be more effective for pest control programs are those that concentrate their attacks on denser host patches. However, as herbivore density changes different spatial patterns usually emerge in their distribution. In Tucumán province, Argentina the mymarid Anagrus flaveolus is a native egg parasitoid of Delphacodes kuscheli, which oviposits endophitic eggs and is the only demonstrated vector of the MRCV virus that seriously affects maize (Zea mays). Winter is the critical period for maize colonization from oat (Avena sativa) crops where D. kuscheli overwinters. We analyzed, under field conditions and during the winter critical period, percentage parasitism by A. flaveolus when host patch density increases following two different patterns: an increase in the batch size, and an increase in the number of infested plants with similar mean batch size. In order to generate a heterogeneous egg patchiness, 144 oat plants were arranged in 36 groups of four plants each, and four types of groups were created by placing one, two, three or four infested plants per group. Each group was characterized by the number of infested plants as well as by the number of eggs. If one, two, three or all four plants were infested, the group was denominated G1, G2, G3 or G4, respectively. Considering only G1, the number of eggs per group increased only if the batch size increased. On the other hand, considering all G1, G2, G3 and G4 groups, the increase in patch density from G1 to G4, was essentially due to an increase in the number of infested plants. The groups were symmetrically distributed in the field in a homogeneous 200-m² area (the position of each group was randomly assigned), and allowed for oviposition for 48 h since A. flaveolus only attacks eggs that have three or less days of development. We found that almost all infected plants were parasitized by A. flaveolus but parasitism never surpassed 64% of the available eggs, suggesting that the parasitoid selectively attacks eggs inserted in particular places of the oat plant, and/or never saturates a batch. We also found that parasitism was spatially direct density dependent when only G1 was considered, as well as when G1, G2, G3 and G4 were considered together. In the first case, an increase in the number of eggs in a group of four plants represents an increase in the batch size, and higher batch size received higher parasitism. In the second case, it represents an increase in the number of infested plants, and higher patch density received higher parasitism. Although the proportion of infected plants parasitized was similar, infected plants with similar number of eggs received higher parasitism when placed in denser groups (G3 and G4) than when placed in the less dense groups (G1 and G2). Behavioral mechanisms leading to a restriction on re-dispersal, like an area-restricted search behavior, as well as supernormal stimulus of contiguous host egg batches in certain places that may require a higher threshold before mutual interference has an effect, could be explained these results.     

Pollinator response to within-patch spatial context determines reproductive output of a giant rosette plant

The reproductive output of many plants depends on the interaction between plant spatial pattern and pollinator behaviour. Pollinators tend to concentrate their efforts on patches of flowers offering higher rewards. The spatial relationship of an individual plant to those around it (its spatial context) is also important for its reproductive output. This study examines the effect of patch size and spatial context on the reproductive output of Puya hamata, a hummingbird-pollinated, semelparous, giant rosette plant in the Andes. Hummingbird behavioural response to Puya patch size and the effect of plant density on flowering plant size were assessed. The reproductive output (flower, fruit and seed production, seed viability, germination rates) of plants in relation to patch size and spatial context was determined. Isolated Puya inflorescences were visited by a higher diversity of mostly trap-lining hummingbirds, while plants in aggregations were almost exclusively visited by one territorial species. Spatial context did not affect Puya size at flowering, or the numbers of flowers, fruits and seeds produced. However, with respect to seed viability and germination rates, reproductive output was highest in isolated plants, and plants on the edges of patches, but lowest in plants at the centre of large patches. The effect of spatial context on reproductive output in P. hamata depends on several key conditions being met: the patches are normally made up of closely related plants and the pollinators, by switching to territoriality, restrict gene flow into patches and lower the effective reproductive output of the plants within those patches. Other plant–pollinator relationships meeting these conditions are likely to produce similar outcomes. Such situations are worthy of attention, not just for their ecological interest, but also for their implications for the management of species and genetic diversity.     

Population density and area: the role of between- and within-patch processes

The relationship between population density and the size of host plant patches was investigated for the red milkweed beetle Tetraopestetraophthalmus inhabiting unmanipulated patches of Asclepias syriaca. The resource concentration hypothesis proposes that density-area patterns, specifically that of increasing herbivore density with patch size, are primarily a function of movement between host plant patches. This research investigated the degree to which movement accounted for density-area patterns. Poisson regression analysis of beetle abundance versus milkweed patch size revealed that beetle density tended to increase with patch size. The pattern of density and patch size resulted from local reproduction and residence time. The density of emerging beetles tended to increase with patch size while emigration rates were unrelated to patch size. Immigration rates were constant with patch size for male beetles, and decreased with patch size for female beetles. Net flux of beetles (immigration – emigration) did not vary with patch size for male beetles and decreased with patch size for female beetles. Comparisons are made between this system and previously studied systems where movement plays a significant role in forming density area patterns. Additionally, several hypotheses are presented which may account for greater in situ recruitment and residence time in large patches. Received: 23 February 1996 / Accepted: 8 January 1997     

Effects of nutrients and predators on an old-field food chain: interactions of top-down and bottom-up processes

In theory, selection favours predators that select prey in order to maximise reproductive success. We studied the association between preference and performance of the generalist predator Orius laevigatus with respect to two prey species: spider mites ( Tetranychus urticae ) and western flower thrips ( Frankliniella occidentalis ). Under ample prey supply, the predators had higher maximum reproductive success (measured as intrinsic population growth rate r ) on thrips than on spider mites; hence thrips represent a higher prey quality to the bugs. This was at odds with the observed preference of the predatory bug for plants (patches) with high densities of spider mites to plants with moderate densities of thrips in release-recapture experiments. Thus, prey quality does not suffice to explain the preference of predators for plants with prey. The quality of a patch as an oviposition site (i.e. the number of eggs produced on a patch per bug per day) also did not match preference patterns. Hence, patch preference was not correlated to prey quality or oviposition rate on prey patches. However, patch productivity, i.e. the total number of offspring surviving until adulthood that can be produced by one female on a patch, was correlated with preference. This was further tested by offering the predators a choice between plants with high densities of spider mites and plants with high densities of thrips in an independent set of release-recapture experiments. These two types of prey patches were found equivalent in terms of patch productivity. Indeed, the predators showed no preference for either of the two types of patches, which is in agreement with our predictions. This suggests that the predatory bugs select patches based on patch productivity rather than on prey quality or oviposition rate on a patch.     

Host-parasitoid extinction and colonization in a fragmented prairie landscape

Few field studies of natural populations have examined the factors influencing local extinctions and colonization of empty habitat patches for a prey species and its predator. In this study, I carried out a census of planthopper (Prokelisia crocea; Hemiptera: Delphacidae) and egg parasitoid (Anagrus columbi; Hymenoptera: Mymaridae) incidence and densities in 147 host-plant patches (Spartina pectinata; Poaceae) over seven planthopper generations in a tall-grass prairie landscape. For both species, the likelihood of going extinct in a patch was related to a number of patch-specific variables: density, temporal variability in density, proportion of hosts parasitized (planthopper only), host-plant density, patch size, patch isolation, and composition of the surrounding matrix. Colonization likelihood was related only to the physical attributes of the patch. There was high patch turnover in this prairie landscape. On average, planthoppers went extinct in 23% of the patches and A. columbi went extinct in 51% of the patches in each generation. For the planthopper, extinction likelihood increased with a decrease in patch size and the proportion of the matrix composed of mudflat. Parasitism of eggs had no effect on the extinction likelihood of local P. crocea populations, suggesting that A. columbi may not play a major role in the patch dynamics of its host. The likelihood of extinction for A. columbi was dependent on factors that spanned three trophic levels. An increase in plant density, decrease in host density and decrease in parasitoid density all increased the likelihood of A. columbi extinction within a patch. The dependency on multiple trophic levels may explain the higher extinction risk for the parasitoid than its host. A. columbi extinction was also affected by the matrix habitat surrounding the patch—the effect was the opposite of that for P. crocea. Finally, vacant patches were colonized at rates of 53% and 34% per generation for the planthopper and parasitoid, respectively. For both species, colonization probabilities decreased with an increase in patch isolation. High host densities in a patch also favored high rates of colonization by A. columbi. I discuss how anthropogenic changes to the prairie landscape can affect the metapopulation dynamics and persistence time of this host-parasitoid interaction.     

Effect of plant patch shape on the distribution and abundance of three lepidopteran species associated with Brassica oleracea

• 1 Plant patch shape may affect the abundance of herbivorous insects. Patches of the same size but longer or irregular have a higher perimeter/area relationship (P/A) than square or regular ones, which may determine the immigration, emigration and abundance of individuals in the patch.
• 2 Only specialist species should be affected by plant patch shape. Those species that are more abundant in smaller patches should be more abundant in patches with higher P/A, whereas those that are more abundant in larger patches should be more abundant in patches with lower P/A.
• 3 We studied the density of eggs, larvae and pupae of Pieris brassicae, Plutella xylostella and Trichoplusia ni in square (low P/A) and I‐shaped (high P/A) patches of 144 plants of Brassica oleracea. We also estimated their immigration to these patches, and the final plant weight.
• 4 Plant patch shape affected the abundance, but not the distribution, of the two specialist species. Whereas P. brassicae was denser in I‐shaped patches, P. xylostella was more abundant in square patches. The generalist T. ni was not affected by patch shape. Immigration of P. brassicae was higher in I‐shaped patches, but immigration of P. xylostella and T. ni was not affected by patch shape. Plants were heavier in the centre of square patches.
• 5 Our results suggest that plant patch shape affects the density of herbivorous insects and should be considered independently from other plant patch variables when studying the population dynamics of these organisms.

     

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.     

野外条件下广聚萤叶甲对豚草的产卵选择

为探究影响广聚萤叶甲Ophraella communa LeSage选择豚草产卵的环境因素, 在南京城郊于2009年6月23日至9月1日豚草生长季节, 跟踪调查了60个豚草斑块上落卵数量动态, 运用跨栏模型分析了影响叶甲产卵以及产卵数量的因素。结果表明： （1）产卵发生随季节延长、 豚草被取食程度增大、 株高增长、 暴露环境中斑块增大而增加, 但随遮蔽环境中豚草斑块增大、 开花、 开花后季节延长、 季节延长情况下被取食程度增大、 开花后被取食程度增大等因素而降低; （2）落卵数量随季节延长、 豚草被取食程度增大、 暴露环境、 开花期被取食程度增大、 开花期株高增长等因素而增加, 但随豚草开花、 季节延长情况下被取食程度增大而减小; （3）落卵量随季节延长而增大的幅度在豚草营养生长期明显大于开花期, 在暴露环境中随季节延长而呈明显直线增大, 但在遮阴环境中先直线增加, 于10周后趋缓。研究结果说明, 影响产卵发生与落卵数量的因素不完全一致, 产卵发生与落卵数量主要受因子之间互作的影响, 而较少受到单因素的独立影响。     

Spatial distribution of nectar production in a natural Echium vulgare population: Implications for pollinator behaviour

The distribution of trait values in many populations is not homogenous but creates a mosaic of patches. This may lead to differences in selection on the patch level compared to selection on the population level. As an example we investigated the spatial distribution of nectar production and its effects on pollinator behaviour in a natural population of Echium vulgare. Nectar production per flower, number of flowers and total nectar production showed a hierarchy and spatial aggregation as expressed by Gini coefficients and significant Moran's I values. Plants in patches of high nectar production received significantly more pollinator visits and had a significant emanating effect on pollinator visits of neighbouring plants. The same was true for plants in patches with high number of flowers. To disentangle these effects a path analysis was applied, which suggested that the direct effect of nectar production per flower although present, seems to be small compared to the effect of the number of flowers. Nectar production per flower affected pollinator visits mainly indirectly by way of total nectar production, which includes the effect of number of flowers. Assuming a minor pollinator-mediated selection for number of flowers, pollinator-mediated selection for total nectar production equals that for nectar production per flower. If so, the observed spatial structure of nectar production and its emanating effect on pollinator behaviour is of importance for natural selection. Plants of low nectar production occurring close to patches of plants with high nectar production benefited from the enhanced pollinator service of their neighbours while saving costs of increased nectar production. Consequently, plants with low nectar production may have a selective advantage at patch level while plants with high nectar production may have a selective advantage at population level. Results presented stress the importance of small-scale patterns for ecological relationships and evolutionary change.     

Relation between plant density and arthropod density in cabbage fields

The relationship between plant density and the abundance of arthropods was examined by planting cabbages in four densities (0.25, 1, 4, or 8 plants per square meter). Four herbivorous species were examined: the small white butterfly Pieris rapae crucivora Boisduval, the diamondback moth Plutella xylostella (Linnaeus), the beet semi-looper Autographa nigrisigna (Walker), and the green peach aphid Myzus persicae (Sulzer). The number of spiders and the number of eggs of syrphid flies were also examined. The number of individuals per unit ground area increased curvilinearly with increasing plant density for most arthropods. To clarify the mechanism causing such a curvilinear increase, the oviposition behavior of Pieris rapae crucivora was examined. The oviposition process of a female was divided into three components: (1) entering the field, (2) alighting on a plant to bend her abdomen, and (3) attaching an egg on the leaf. The first and the second components curvilinearly increased with increasing plant density, while the third component was not influenced by the plant density. The analysis of the flight path of P. rapae crucivora indicated that the curvilinear increase in the frequency of abdominal bending behaviors occurs since a female flies at least about 1.3 m between successive abdominal bending behaviors, irrespective of the plant density, when the plant density is sufficiently high. Received: October 23, 1998 / Accepted: February 15, 1999     

Assessing the vulnerability of riparian vegetation to invasion by Mimulus guttatus: relative importance of biotic and abiotic variables in determining species occurrence and abundance

Riparian habitats are particularly susceptible to invasion by non‐native plants. At present, attempts to build consensus as to what the primary drivers of plant invasion in riparian ecosystems might be is hindered by the absence of common standards for data collected on plant species (e.g. occurrence, or relative abundance). Mimulus guttatus L., a non‐native riparian plant species, was used as a model to determine how environmental drivers influence two aspects of invasibility: species occurrence and abundance (assessed in relation to three variables number of patches, patch area and number of stems per patch). Mimulus occurrence and abundance, together with 20 environmental variables, were surveyed in almost 700 contiguous 50‐m‐long riverbank segments within a catchment in north‐east Scotland. More than half of the segments had been colonized by Mimulus. Occurrence and number of patches responded to similar environmental gradients, particularly bare sediment, boulders, high soil moisture, short‐statured ruderal communities, and open canopies, and tended to be highest downstream where the river was widest. In contrast to occurrence and patch number, patch area and stem number per patch were higher in the upper reaches of the catchment and were positively associated with low tree canopy and vegetation dominated by light‐demanding species and smaller‐statured species. Patch area and stem number per patch were also positively related to grazing. This study has highlighted the importance of assessing more than one measure of invasion success (occurrence or patch number and either patch area or stem number per patch), as they are each determined by a different suite of environmental variables. Abiotic factors, such as sediment availability and presence of boulders, appeared to be the major determinants of occurrence and patch number, whereas biotic factors, such as interspecific competition and grazing, were more important ecological determinants underlying area and stem number per patch.     

How discriminating are cabbage butterflies?

The egg-laying responses of cabbage butterflies Pieris rapae L. to differences in the size, quality and spatial distribution of available oviposition sites, were studied in the laboratory (Australia) and the field (Australia and Canada). Differences on three scales were investigated: between leaves within the same plant, between plants within a patch, and between patches of plants. The butterflies lay most eggs on the larger, older leaves of plants, though the oldest are sometimes avoided. Larger plants also receive more eggs than smaller ones of the same cultivar, but cultivar preferences may override the response to size. The butterflies do not discriminate against plants already bearing eggs or larvae, unless larval feeding damage to the plant is severe. They lay more eggs on young plants than old ones of the same size; plants grown in the lower of two light intensities also received more eggs. Australian butterflies discriminate not only between different species of crucifers, but also between varieties of the same species. This discrimination against less acceptable varieties is just as strong when plants of preferred varieties are not present, as when both varieties occur in the same patch. The observed responses are discussed in relation to the butterflies’ host-finding behaviour and the adaptive significance of the lack of a response to eggs and larvae is considered.     

垫状植物囊种草对群落物种多样性的影响

在甘肃盐池湾国家级自然保护区内海拔4137 m处,选择典型的囊种草垫状植被设置研究样地,研究了垫状植物囊种草对群落物种组成和群落物种多样性的影响,并且定量的研究了囊种草对群落物种丰富度的影响能力和维持潜力。研究结果表明:囊种草为群落中增加了新的植物种类,并且提高了部分生境一般种的多度;囊种草的出现提高了群落物种密度和物种丰富度,进而提高了群落物种多样性;囊种草斑块的增加将会引起景观水平物种丰富度的增加,表明囊种草具有为群落中引入新的植物种类进而提高群落物种丰富度的能力;在景观水平,囊种草所创造的生境多样性则成为一种保障,可以维持景观中物种丰富度从而降低物种损失的风险,表明囊种草具有较高的群落物种丰富度维持潜力。     

Habitat quality matters for the distribution of an endangered leaf beetle and its egg parasitoid in a fragmented landscape

Fragmentation, deterioration, and loss of habitat patches threaten the survival of many insect species. Depending on their trophic level, species may be differently affected by these factors. However, studies investigating more than one trophic level on a landscape scale are still rare. In the present study we analyzed the effects of habitat size, isolation, and quality for the occurrence and population density of the endangered leaf beetle Cassida canaliculata Laich. (Coleoptera: Chrysomelidae) and its egg parasitoid, the hymenopteran wasp Foersterella reptans Nees (Hymenoptera: Tetracampidae). C. canaliculata is strictly monophagous on meadow sage (Salvia pratensis), while F. reptans can also parasitize other hosts. Both size and isolation of habitat patches strongly determined the occurrence of the beetle. However, population density increased to a much greater extent with increasing host plant density ( = habitat quality) than with habitat size. The occurrence probability of the egg parasitoid increased with increasing population density of C. canaliculata. In conclusion, although maintaining large, well-connected patches with high host plant density is surely the major conservation goal for the specialized herbivore C. canaliculata, also small patches with high host plant densities can support viable populations and should thus be conserved. The less specialized parasitoid F. reptans is more likely to be found on patches with high beetle density, while patch size and isolation seem to be less important.     

Individually mark–mass release–resight study elucidates effects of patch characteristics and distance on host patch location by an insect herbivore

1. How organisms locate their hosts is of fundamental importance in a variety of basic and applied ecological fields, including population dynamics, invasive species management and biological control. However, tracking movement of small organisms, such as insects, poses significant logistical challenges. 2. Mass‐release and individual–mark–recapture techniques were combined in an individually mark–mass release–resight (IMMRR) approach to track the movement of over 2000 adult insects in an economically important plant–herbivore system. Despite its widespread use for the biological control of the invasive thistle Carduus nutans, the host‐finding behaviour of the thistle head weevil Rhinocyllus conicus has not previously been studied. Insects were released at different distances from a mosaic of artificially created host patches with different areas and number of plants to assess the ecological determinants of patch finding. 3. The study was able to characterize the within‐season dispersal abilities and between‐patch movement patterns of R. conicus. Weevils found host plant patches over 900 m away. Large patches, with tall plants, situated close to the nearest release point had the highest first R. conicus resights. Patch area and plant density had no effect on the number of weevils resighted per plant; however, R. conicus individuals were more likely to disperse out of small patches and into large patches. 4. By understanding how R. conicus locates host patches of C. nutans, management activities for the control of this invasive thistle can be better informed. A deeper mechanistic understanding of host location will also improve prediction of coupled plant–herbivore spatial dynamics in general.