Structure Of Terrestrial Communities Research Articles

Temperature effects on the temporal dynamics of a subarctic invertebrate community.

Climate warming is predicted to have major impacts on the structure of terrestrial communities, particularly in high latitude ecosystems where growing seasons are short. Higher temperatures may dampen seasonal dynamics in community composition as a consequence of earlier snowmelt, with potentially cascading effects across all levels of biological organisation. Here, we examined changes in community assembly and structure along a natural soil temperature gradient in the Hengill geothermal valley, Iceland, during the summer of 2015. Sample collection over several time points within a season allowed us to assess whether temperature alters temporal variance in terrestrial communities and compositional turnover. We found that seasonal fluctuations in species richness, diversity and evenness were dampened as soil temperature increased, whereas invertebrate biomass varied more. Body mass was found to be a good predictor of species occurrence, with smaller species found at higher soil temperatures and emerging earlier in the season. Our results provide more in-depth understanding of the temporal nature of community and population-level responses to temperature, and indicate that climate warming will likely dampen the seasonal turnover of community structure that is characteristic of high latitude invertebrate communities.

Biostratigraphy of the Lystrosaurus declivis Assemblage Zone (Beaufort Group, Karoo Supergroup), South Africa

Abstract The earliest Triassic (Induan) Lystrosaurus Assemblage Zone has long been recognised as a particularly significant biozone in palaeoecological and palaeoenvironmental studies as it contains a community assemblage that records the survival and recovery from the end-Permian mass extinction (EPME). Here renamed the Lystrosaurus declivis Assemblage Zone it represents the best record globally of the ecological changes in terrestrial community structure and stability during this time period. The assemblage is dominated by two species of small to medium-sized herbivorous dicynodonts L. declivis and L. murrayi that co-occur in equal abundance, along with a range of smaller and less common faunivorous and insectivorous taxa. The latter comprise cynodonts (Thrinaxodon, Galesaurus, and Platycraniellus), therocephalians (Olivierosuchus, Regisaurus and Promoschorhynchus), the diminutive parareptiles (Saurodektes, Sauropareion, Colleta, Phonodus and Procolophon) and eureptilian arrivals possibly representing immigrant taxa (Prolacerta, Heleosuchus and Noteosuchus), among others. The attendant large carnivores were sabre-toothed Moschorhinus and the long-snouted archosauromorph Proterosuchus. In the aftermath of the mass extinction, new small temnospondyl taxa established their first occurrences i.e. Broomistega, Lydekkerina, and Micropholis, and have relatively high abundances compared to earlier temnospondyl records in the Karoo. Lithostratigraphically, the biozone for the most part spans the upper Palingkloof Member of the Balfour Formation and the overlying Katberg Formation in the western part of the basin, and the Normandien Formation in the east. The Lystrosaurus declivis Assemblage Zone of the main Karoo Basin hosts the type locality of the global Lootsbergian land-vertebrate faunachron. The biozone is one of the most widespread terrestrial faunal assemblages of western Gondwana with closely related species occurring in India and Antarctica. Similar, but much more distantly related taxa, occur in Induan-aged strata of Russia, China and Brazil.

Drought and Root Herbivory Interact to Alter the Response of Above-Ground Parasitoids to Aphid Infested Plants and Associated Plant Volatile Signals

Multitrophic interactions are likely to be altered by climate change but there is little empirical evidence relating the responses of herbivores and parasitoids to abiotic factors. Here we investigated the effects of drought on an above/below-ground system comprising a generalist and a specialist aphid species (foliar herbivores), their parasitoids, and a dipteran species (root herbivore).We tested the hypotheses that: (1) high levels of drought stress and below-ground herbivory interact to reduce the performance of parasitoids developing in aphids; (2) drought stress and root herbivory change the profile of volatile organic chemicals (VOCs) emitted by the host plant; (3) parasitoids avoid ovipositing in aphids feeding on plants under drought stress and root herbivory. We examined the effect of drought, with and without root herbivory, on the olfactory response of parasitoids (preference), plant volatile emissions, parasitism success (performance), and the effect of drought on root herbivory. Under drought, percentage parasitism of aphids was reduced by about 40–55% compared with well watered plants. There was a significant interaction between drought and root herbivory on the efficacy of the two parasitoid species, drought stress partially reversing the negative effect of root herbivory on percent parasitism. In the absence of drought, root herbivory significantly reduced the performance (e.g. fecundity) of both parasitoid species developing in foliar herbivores. Plant emissions of VOCs were reduced by drought and root herbivores, and in olfactometer experiments parasitoids preferred the odour from well-watered plants compared with other treatments. The present work demonstrates that drought stress can change the outcome of interactions between herbivores feeding above- and below-ground and their parasitoids, mediated by changes in the chemical signals from plants to parasitoids. This provides a new insight into how the structure of terrestrial communities may be affected by drought.

Primary Consumer Body Size and Food-Chain Length in Terrestrial Communities

Using 21 community food webs, we tested Elton's two hypotheses regarding the main factors limiting food-chain length in terrestrial communities, namely, energy (energy limitation hypothesis—ELH) and body size (size limitation hypothesis—SLH). As predators tend to be larger than their prey, food-chains are size-structured: animal size increases with trophic position. We found a negative correlation between the size of the primary consumer and the length of the chain. Food-chains based on small primary consumers are longer than those based on large primary consumers, and size rather than energetic efficiency is the main contributing factor. We found no correlation between habitat productivity and mean food-chain length. All these findings support the SLH over the ELH. Our results suggest that, as in aquatic communities, a single factor—a predator/prey size-ratio greater than 1—governs the structure of terrestrial communities.

Cellulose Centered Perspective on Terrestrial Community Structure

How can the earth be so green when it teems with plant-eating animals ranging in size from aphids to elephants, including half of the ca. 800,000 species of insects which accounts for one-quarter of living species excluding algae and microorganisms (Strong et al. 1984)? Plant-eating animals kill more plants than drought or logging does, yet they do not wipe out the entire plant community (Howe and Westley 1988). As for the evolution of plants in relation to phytophagous animals, especially insects, much attention has been paid to the chemical defenses that plants exert with their secondary compounds (Ehrlich and Raven 1964, Whittaker and Feeny 1971, Feeny 1975, Rosenthal and Janzen 1979, Strong et al. 1984, Howe and Westley 1988). In contrast, very little attention has been paid to the fact that most animals, unlike certain microorganisms (Nielsen 1962, Janzen 1981, Begon et al. 1986), lack the ability to produce the enzymes necessary for decomposing cellulose, the primary cell-wall component of higher plants and the most abundant organic compound on earth. The limited ability to digest cellulose places a definite limit on the consumption of plants. Thus, cellulose provides a basic defense for plants. The green on earth would be best preserved if a living plant is not consumed by heterotrophs, rather consumed promptly upon death thereby returning nutrients back to the living portion of plants. Therefore, the spatial distribution pattern of cellulolytic organisms (organisms that can digest cellulose) plays a crucial role. In terrestrial communities, microorganisms that can produce cellulases, the cellulose-decomposing enzymes, are largely confined to the soil and thus, in most cases, they decompose the cellulose not in living plants but in fallen dead plants; the important exception being fungi that break down the roots of living trees (Bell 1981). This restricted distribution and feeding habit of cellulose-digesting organisms, supplemented with chemical defenses by secondary compounds, may possibly provide one answer to the question: how can the earth be so green? Hairston et al. (1960) addressed this question of a green earth and concluded that green plants of the earth are abundant because populations of herbivores are controlled by predation. This general conclusion was criticized by Murdoch (1966) and Ehrlich and Birth (1967); in particular, Murdoch pointed out that all of the green plant material may not be edible. Those recent studies on the chemical defenses of plants to which we have already referred support Murdoch's point. The argument that we have just presented is one possible answer to the question of a green earth. It extends Murdoch's view by identifying cellulose as a capsule that protects plants while alive and lets them dissolve upon death. The real situation is, however, more complicated than the general picture that we have described, mainly because of the associations that microorganisms make with some animals, such as termites and other cellulosedigesting insects, which expand beyond soil the distribution of microorganisms. There are also those exceptional microbes that break down living plant tissue. After examining the significance of cellulose, we will return in the last section with a new insight to this basic problem of the maintenance of a green earth. Here a remark is in order: there may not necessarily be a single answer to this problem, and those different hypotheses that have been proposed and the one that we will present in the last section are not mutually exclusive. Thus, it is possible, and we suspect very likely, that these causes work together for the maintenance of the green plants of the earth, or different causes act in varied habitats on the earth.