Showing papers in "Oikos in 1994"

Organisms as ecosystem engineers

Abstract: Interactions between organisms are a major determinant of the distribution and abundance of species. Ecology textbooks (e.g., Ricklefs 1984, Krebs 1985, Begon et al. 1990) summarise these important interactions as intra- and interspecific competition for abiotic and biotic resources, predation, parasitism and mutualism. Conspicuously lacking from the list of key processes in most text books is the role that many organisms play in the creation, modification and maintenance of habitats. These activities do not involve direct trophic interactions between species, but they are nevertheless important and common. The ecological literature is rich in examples of habitat modification by organisms, some of which have been extensively studied (e.g. Thayer 1979, Naiman et al. 1988).

Effects of habitat fragmentation on birds and mammals in landscapes with different proportions of suitable habitat: a review

Abstract: Habitat fragmentation implies a loss of habitat, reduced patch size and an increasing distance between patches, but also an increase of new habitat. Simulations of patterns and geometry of landscapes with decreasing proportion of the suitable habitat give rise to the prediction that the effect of habitat fragmentation on e.g. population size of a species would be primarily through habitat loss in landscape with a high proportion of suitable habitat. However, ast the proportion of suitable habitat

A functional group approach to the structure of algal-dominated communities

Abstract: We suggest that relatively few species attributes are of overriding importance to the structure of benthic marine algal communities and that these are often shared among taxonomically distant species. Data from the western North Atlantic, eastern North Pacific and Caribbean suggest that patterns in algal biomass, diversity and dominance are strikingly convergent when examined at a functional group level relative to the productivity and herbivore-induced disturbance potentials of the environment. We present a simple graphical model that provides a way to predict algal community composition based on these two environmental axes. This predictability stems from algal functional groups having characteristic rates of mass-specific productivity, thallus longevity and canopy height that cause them to «behave» in similar ways (...)

What Do Species Do in Ecosystems

Abstract: Three aspects of the role of species in ecosystems are reviewed. (1) Theoretically, what are the possible relationships between ecosystem processes (Likens' (1992) transformation and flux of energy and matter) and the species richness of communities ? (2) Summaries of two experiments with artificially constructed terrestrial ecosystems in the controlled environment facility known as the Ecotron are described. The first draws attention to the role of earthworms as ecosystem engineers; the second explores changes in ecosystem processes in mesocosms assembled with three different levels of biological diversity

Carotenoids, parasites, and sexual selection

Abstract: In recent years carotenoids have become increasingly important in the study of sexual selection. Endler (1980) found that the colour pattern diversity and conspicuousness of males of a population of guppies (Poecilia reticulata) increased after a few generations without predation. He argued that males, emancipated from the constraints of predation, were able to respond better to sexual selection brought about by female preference for colourful males. He subsequently (Endler 1983) showed that in the early stages of courtship females prefer males with more red spots. Kodric-Brown (1985) and Houde (1987) later confirmed these results. Finally, Kodric-Brown (1989) showed that, as previously thought, dietary carotenoids affect male guppy coloration and, as a consequence, female preference. Analogous results have been found in other taxa. Female house finches (Carpodacus mexicanus) prefer males with redder plumage (Hill 1990). Zuk et al. (1990a) found that red jungle fowl (Gallus gallus) females prefer males with redder combs. Milinski and Bakker (1990) found a positive correlation between the intensity of male coloration and female preference in the three-spined stickleback (Gasterosteus aculeatus). In these three species the expression of the traits being used by females to select males depends partially on dietary carotenoids (Brush and Power 1976, Rothschild 1973, Brush and Reisman 1965, respectively). Endler's (1980) explanation has always been embraced when interpreting these results. Endler (1980) suggested that, by choosing mates based on a carotenoid-dependent trait, females choose superior mates. He argued that, because these males are able to obtain carotenoids from a carotenoid-poor environment, they are better foragers, and, because they are more conspicuous, they must also be better at avoiding predators. In recent years carotenoids have also aroused the attention of nutritionists, immunologists and oncologists. Carotenoids are currently recognized not only as an essential part of a normal diet (e.g. Simpson and Chichester 1981, Goodwin 1986), but also as free radical scavengers (e.g. Burton 1989, Olson 1989, Sueki 1991), stimulants of the immune system (e.g. Alexander et al. 1985, Bendich and Shapiro 1986, Olson 1989, Prabhala et al. 1990, Jyonouchi et al. 1991, Sueki 1991, Watson et al. 1991; see reviews by Bendich 1989a, b), and even as potential prophylactics against cancer (e.g. Modan et al. 1981, Mathews-Roth 1982, Santamaria et al. 1983, MathewsRoth and Krinsky 1985, Trechsel et al. 1985, Schoen and Watson 1988, Schwartz and Shkalr 1988, Olson 1989, Wang et al. 1989, El Attar and Lin 1991, Sueki 1991; see reviews by Ames 1983, Olson 1986, Ritenbaugh 1987, Temple and Basu 1988, Connett et al. 1989, Krinsky 1989, Ziegler 1989). This rise in the interest in carotenoids in nutritional and medical circles not only parallels, but might also be closely related to the study of sexual selection. In terms of sexual selection an intricate picture emerges if these functions of carotenoids are considered. By choosing males based on carotenoid-dependent traits, females may not be merely choosing males who are good foragers and efficient at avoiding predators. Females might also be increasing the likelihood that their mates are not only healthy, but also better able to deal with any future health problems. The benefits to females would be a lower risk of getting pathogens from their mates, and a greater likelihood that males will be able to provide adequate paternal care if such care is required. Ties with Hamilton and Zuk's (1982) hypothesis on the role of parasites in sexual selection are immediately apparent. For the purposes of this discussion, parasites are broadly and functionally defined as any organisms living in or on, as well as lowering the fitness of a heterospecific animal. Hamilton and Zuk (1982) proposed an explicit mechanism by which Zahavi's (1975) handicap principle could work: they suggested that male secondary sexual traits used by females to choose males indicate heritable variation in parasite resistance. If the immuno-stimulant effects of carotenoids are considered (see references

The riverine productivity model: an heuristic view of carbon sources and organic processing in large river ecosystems

Abstract: Our current views of the structure and function of large river ecosystems are based primarily on three influential and still valuable riverine models: (1) the river continuum concept, or RCC (e.g., Vannote et al. 1980); (2) the serial discontinuity concept (Ward and Stanford 1983), which integrated the effects of large dams and reservoirs on the RCC; and (3) the flood pulse concept in river-floodplain systems (Junk et al. 1989) and its relationship to the RCC (Sedell et al. 1989). With regard to food webs in large rivers, these models accentuated the importance of nutrients derived from either headwater streams or seasonal floodplain pulses and downplayed or virtually ignored the role of local instream primary production and riparian litterfall. We believe that the general portrayal of ecosystem function within large rivers needs substantial revision because previous models relied too heavily on data from either lower-order streams, floodplain rivers (thereby exluding large rivers with constricted channels), or main channel habitats with their dominant collector feeding guild (thus de-emphasizing nearshore areas where species in many feeding guilds congregate). We propose an alternative hypothesis the riverine productivity model (RPM) which stresses the varying importance to large river food webs of local autochthonous production and direct organic inputs from the riparian zone. The RPM's portrayal of ecosystem function differs most significantly from that of previous models for rivers characterized by constricted channels, such as the upper two-thirds of the Ohio River.

Reaction norms for age and size at maturity in response to temperature: A puzzle for life historians.

Abstract: Ectotherms mature later at smaller size when growth rates are lowered by reductions in food quality but they mature later at larger sizes when growth rate is lowered by reductions in temperature. We argue that this general pattern has been neglected by life history theorists and suggest that an explanation for these strikingly different responses to two environmental cues might be found by considering correlations between the growth coefficient of the Bertalanffy equation and asymptotic size in a model for the evolution of age and size at maturity

The relationship between species richness and community biomass: the importance of environmental variables.

Abstract: Several studies have used plant community biomass to predict species richness with varying success. In this study we examined the relationship between species richness and biomass for 36 marsh communities from two different watersheds. In addition, we measured several environmental variables and estimated the potential richness (the total number of species known to be able to occur in a community type) for each community. Above-ground living and dead biomass combined was found to be weakly correlated with species richness (R 2 = 0.02). Instead, a multiple regression model based on elevation (R 2 = 0.30) soil organic matter (R 2 = 0.18), and biomass was able to explain 82 % of the variance in species richness (...)

Vertebrate Herbivores and Northern Plant Communities: Reciprocal Influences and Responses

Abstract: Vertebrate herbivores in northern latitudes are generalists in their selection of forage species Their foraging behaviour is influenced by the availability of vegetation of high nutritional quality, abiotic disturbance events, the effect of plant species on nutrient cycling, and the degree to which herbivores can forage on a diversity of plant species In resource-limited, early successional environments their foraging activities may accelerate or delay plant succession, depending on the type of plant-animal interaction They may increase the turnover rate of nutrients, particularly nitrogen, thereby sustaining net primary production of preferred forage plants Alternatively, the movement of animals from locality to locality at the landscape level allows them to exploit available nutrient pulses in space and time in environments where high turnover rates cannot be sustained The long-term effects of herbivory on the vegetation in the latter case appear to be minimal There is also a group of plants which are unsuitable as forage because of the presence of secondary compounds The low digestion efficiency when these plants are eaten correlates well with the recalcitrant nature of the tissues to microbial decomposition, the slow turnover of nutrients in the soil and the slow growth rates of the plants Overall, herbivores do not appear to influence plant species assemblages in the different communities in the long term or alter the trajectory of plant succession at the landscape level, but they may have considerable short-term effects on vegetation, particularly at fine spatial scales (1-20 m) Exceptions to these generalizations are primarly at the local level and involve the top-down influence of human agencies in altering herbivore numbers, thereby creating trophic cascades and leading to the destruction of plant communities

Competition, facilitation, seed distribution and the origin of patches in a Patagonian steppe

Abstract: The Pataxonian steppe is composed mainly of shrubs and tussock grasses organized in two types of patches: scattered grass tussocks in a matrix of bare soil and shrubs, each surrounded by a dense ring of grass tussocks. We analyzed the variation of competition, facilitation, and seed distribution through space and time as major driving forces in the development of this patch structure. Emergence and survival of grass seedlings increased with distance from shrubs when the ring of grasses was left intact. On the contrary, when the ring of grasses was experimentally removed, seedling survival decreased with distance from the shrub. Differences in root density, soil water potential, and evaporation accounted for these patterns (...)

Bird-dispersed seed rain and seedling establishment in patchy Mediterranean vegetation

Abstract: The composition and spatial patterns of the seed rain produced by bird dispersers and of the seedlings of fleshy-fruited plants have been studied in patchy Mediterranean vegetation in southern France. The seeds of fleshy-fruited plants were collected in seed traps during a 17-month period (1981-1983) and the fecal samples of bird dispersers mist-netted on the same 2.5-ha site over a 25-month period (1981-1984) were analysed. The locations of the 5 sets of seed traps and the 6 mist-nets were chosen in order to take into account the various plant communities and the successional gradient from old fields to Quercus ilex coppice. Seedling establishment of fleshy-fruited plants was censused in 1983 adjacent to the seed traps. In 1992 seedlings were again censused in the open coppice and in the old fields (...)

Sex ratio and mating structure in the parasitoid Hymenoptera.

Abstract: Assumptions about mating structure have been important in the development of sex ratio theory. Fisher's theory of frequency dependent selection for sex ratio equality, and Charnov's theory of sex allocation when host quality varies, assume panmixis, whereas Hamilton's theory of local mate competition assumes mating occurs only at the natal patch. An important group of organisms in stimulating the development of sex ratio theory, and in its empirical testing, have been the parasitoid Hymenoptera. These exhibit many life history variations and possess a mechanism by which females can adaptively control their progeny sex ratio. Although many parasitoid species may conform closely to both the assumptions and predictions of the above models, others do not

Trade-offs and constraints in plant-herbivore defense theory: a life-history perspective

Abstract: Trade-offs and constraints are key ideas in plant defense theory. Here I argue that trade-offs, such as those between defense and growth, may be better understood by incorporating insights from life-history studies. Specifically, I set out criteria for characterizing resource based trade-offs, because these lie at the heart of current defense theory. I show that defense theory is often supported by inappropriate evidence from negative trait associations seen in the field, or from genetic studies. Such evidence is inappropriate because trade-offs grounded in genetic constraints can differ fundamentally from those conceived in terms of limiting resources. In work on resource based trade-offs there is often a failure to consider the critical issues of whether the resources involved are limiting, and whether this limitation is imposed by the assimilatory capacity of the organism or by a shortage in its environment. Another important consideration is whether a trade-off is between two traits that jointly consume all resources. Where this is not the case, unforeseen trade-offs with "third party traits" may confound analyses. I discuss the practical issues of resolving these problems with plants, and present a program for future research on resource based trade-offs involving plant defense.

Nickel hyperaccumulated by Thlaspi montanum var. montanum is acutely toxic to an insect herbivore

Abstract: Some plants growing on serpentine soils sequester (hyperaccumulate) nickel from those soils in their tissues. Several ecological functions for metal hyperaccumulation have been suggested, including defense against herbivores. This study tests the herbivore defense hypothesis using the Ni hyperaccumulator Thlaspi montanum var. montanum. Leaves differing 167-fold in Ni content (3,000 vs 18 ppm) were obtained by growing plants on high- and low-Ni soils. Leaves were fed to larvae of Pieris rapae, a generalist folivore. Larvae fed high-Ni leaves did not grow and suffered 100 % mortality after 12 d, whereas those fed low-Ni leaves quadrupled in weight with a corresponding mortality of only 21 % (...)

Plasticity in morphology and biomass allocation in Cynodon dactylon, a grass species forming stolons and rhizomes.

Abstract: Stolons and rhizomes may be important for the placement of leaves and roots in new microhabitats (foraging), as well as for the storage of resources. In a species which forms both types of structures, such as Cynodon dactylon, both such functions could be combined in a single structure, or rhizomes and stolons could exert different functions. We examine these functions in C. dactylon, by investigating plasticity in morphology and biomass allocation in response to variation in light and nutrient availability. Both stolon and rhizome branching intensities were reduced in response to lower light and lower nutrient levels. Stolons and their internodes elongated greatly under lower light levels, but slightly shorter under lower nutrient levels (...)

On the importance of sampling variance to investigations of temporal variation in animal population size

Abstract: Temporal variation in animal population size and the related concept of population stability are of substantial interest to animal ecologists (e.g., MacArthur 1955, Holling 1973). Empirical studies of variation in population size are not new (e.g., Watt 1964) but have become especially popular in recent years. Recent empirical work has included interesting research on the relationships between temporal variation in population size and such factors as extinction probability (Karr 1982, Pimm et al. 1988, Schoener and Spiller 1992, Tracy and George 1992), mean population density (Taylor and Woiwod 1980), latitude (Wolda 1978), human disturbance (Pechmann et al. 1991), body size (Gaston 1988, Gaston and Lawton 1988), food habits (Gaston and Lawton 1988, Redfearn and Pimm 1988), geographic range (Gaston 1988, Gaston and Lawton 1988), and taxonomy (Connell and Sousa 1983, Schoener 1985). However, estimation of temporal variation in population size is not a simple matter (Greenwood 1989, Mc Ardle 1989, McArdle et al. 1990). McArdle et al. (1990: 439) recently discussed "problems associated with the measurement and interpretation of population density variability that have confounded most, if not all, previous studies of the subject". These problems include "artefactual patterns" induced by commonly used transformations and misleading conclusions resulting from comparisons of populations sampled at different spatial and temporal scales. McArdle et al. (1990) also argued that the dependence of population variability on mean density should be considered in making comparisons of variability among species and populations. Recent contributions to the literature conclude with statements emphasizing the importance of properly estimating and comparing population variability and recommending additional statistical and biological work on the topic (McArdle et al. 1990, Schoener and Spiller 1992). Despite the fairly extensive literature on population variability and its estimation, there has been little discussion of the fact that virtually all of the empirical work has relied on estimates or indices to animal abundance (see McArdle and Gaston 1993). That is, estimates of population variability are not based on a time series of true population sizes measured without error, but on a series of estimates or indices (count statistics assumed to be related to population size by a proportionality constant, see Lancia et al. in press). Measures of population variability computed using point estimates of population size over time represent the sum of at least two conceptually distinct variance components. One component is temporal variation in actual population size and is relevant to interesting biological hypotheses. Another component is "sampling variation", defined here as the variation associated with the population estimation procedure. This component is sometimes called "error of estimation" and occurs whenever it is impossible to directly enumerate all individuals in the population. This component is not relevant to biological hypotheses and should not be included in measures of population variability computed for the purpose of addressing such hypotheses. We believe that the inclusion of this unwanted variance component in previous studies of population variability is at least as serious a problem as those associated with transformations and scale discussed by McArdle et al. (1990). For studies in which population size is estimated rather than indexed, it is usually possible to estimate the sampling variance of the population estimates. This component can then be subtracted from the total variance (e.g., based on point estimates) to estimate the variance component of interest. This basic approach has been used to estimate variance of survival probabilities among replicate groups of animals receiving different experimental

Chemical cues from piscivores induce a change in morphology in crucian carp

Abstract: Crucian carp (Carassius carassius) develop a deeper body in the presence of northern pike (Esox lucius) and this has been suggested to be a predator-induced morphological defense. The evolution of inducible defenses requires that prey have reliable cues of detecting the predator. Here we investigate the nature of the cues used by crucian carp in a series of laboratory experiments. We monitored the change in body depth of crucian carp when exposed to visual and chemical cues from predators and to alarm substances from conspecifics. Results show that chemical cues are sufficient to induce an increase in body depth of crucian carp. Alarm substances released by injured crucians had no effect on body depth, whereas pike, even when not feeding in the experimental aquaria, induced an increase in body depth

Tropical forest litter decomposition under seasonal drought: nutrient release, fungi and bacteria

Abstract: Irrigation was used to study the consequences of seasonal drought for nutrient release and bacterial and fungal numbers during dry season litter decomposition in tropical forest on Barro Colorado Island, Panama. Litter bags containing a single species of leaves were placed beneath conspecific trees at the onset of the dry season in December 1987 and collected at one-month intervals until the onset of the wet season in May 1988. Serial dilutions were used to quantify densities of fungi and bacteria. Nutrient concentrations in recalcitrant litter fractions showed rapid declines in the first month of exposure (K, P) followed by bioaccumulation (N) or no significant changes over the next four months (P, K, Mg and most Ca) (...)

Survival of dispersing and philopatric yellow-bellied marmots: what is the cost of dispersal?

Abstract: Understanding the evolution of dispersal has been hindered by a lack of reliable measures of the cost of dispersal. We report that survival of dispersing yellow-bellied marmots (Marmota flaviventris) (0.73) was only 16% less than survival of philopatric marmots (0.87). A cost-benefit analysis reveals that the survival cost of dispersal may be less than some other costs of philopatry; thus, dispersal is explicable as a tactic to improve individual fitness

Are large predators keystone species in Neotropical forests ? The evidence from Barro Colorado Island.

Abstract: Large cats have been hypothesized to act as keystone predators in Neotropical forests by limiting mid-sized terrestrial mammals, which in turn control forest regeneration. The evidence for this is based predominantly on cross-site comparisons involving Cosha Cashu (CC), Peru, with its complete predator complement, and Barro Colorado Island (BCI), Panama, where puma and jaguar are no longer residents. We reevaluate the evidence in light of 725 km of transect censuses performed on BCI, comparisons with additional sites throughout the Neotropics, and historical changes in the BCI mammal fauna. Several mid-sized species showed habituation to humans near the BCI laboratory compound (where previous censuses had been carried out), indicating that cross-site comparisons, despite using similar methodologies, may be biased by differentially meeting technique assumptions, and that densities of these species at BCI and CC may not differ as greatly as previously assumed. Analyses of additional sites also indicate that individual species population density estimates for BCI, while high for several species, are not extreme. Cross-site analyses indicate that most, but not all, species show no significant difference in mean densities at sites with and without large cats. On the other hand, an evaluation of the qualitative historical evidence indicates that as cats were eliminated from BCI by poaching, many potential prey species population densities increased. These increases were not sustained, however, and may reflect natural population variability rather than the status of large cats. Finally, we examine the evidence for prey species controlling forest regeneration. While extreme removal of herbivorous and frugivorous mammals will profoundly affect forest regeneration, the effects of slight decreases in densities, or the effects of increased densities (as implicated by the keystone predator hypothesis) are equivocal.

Space use and dispersal of red squirrels in fragmented habitats

Abstract: Eurasian red squirrels Sciurus vulgaris in fragmented woodlands had a similar spacing pattern as in large woodlands: females had intrasexual exclusive core-areas, while core-areas of males overlapped extensively with core-areas of other males and with core-areas of females. However, in fragmented woodlands, home ranges (site B) and core-areas (both study sites) were smaller than in large deciduous woodlands. Hence, space use and home range size in habitat fragments are strongly influenced by size and structure of the woodlots and, in case of small woodland patches, by the presence of connecting treerows. Juveniles had very small home ranges prior to dispersal, but home range size increased after the dispersal period

Species Richness of Stream Stones: An Investigation of the Mechanisms Generating the Species-Area Relationship

Abstract: In an upland fourth-order stream, the fauna dwelling on both natural and artificial stones was found to be highly correlated with stone surface area. The power function model provided a good description of the species-area relationship of the stones. For both types of stones, passive sampling was rejected as an explanation for the species-area relationship. The compilation of cumulative species richness, proceeding in order from the smallest area to the largest area and vice versa revealed a negative fragmentation effect. Groups of small stones harboured an impoverished fauna compared with large stones of the same surface area. Small stones did not harbour a distinctive fauna, but simply a sub-set of the species found on larger stones

The distribution of body sizes of the world's bird species

Abstract: Data for more than 6000 species are used to determine the form of the frequency distribution of the body masses of all extant bird species. The distribution of known masses is strongly right-skewed, even on logarithmic axes, and the median bird mass is 37.6 grammes. We present evidence that this right-hand skew is a real property of the body mass distribution, and not the result of a biased sample of bird masses

Small scale disturbances and the maintenance of species diversity in Mediterranean old fields

Abstract: In three Mediterranean old fields, one, seven, and fifteen years after the last ploughing, small scale disturbances (0.25 m 2 ) by contact herbicide associated or not with litter removal and scratching of the top 3 cm of the soil were generated in October 1988, December 1988, and March 1989. Subsequent changes in cover, species richness and composition were observed over the two following vegetation cycles (until May 1990), and compared to the dynamics in undisturbed samples. The identity of the colonizers for each disturbance date were compared to the soil readily-germinable seed content, sampled simultaneously to the disturbances. The species richness of the samples within the fields increased the first year after disturbance. Within-field heterogeneity for taxonomic composition was unchanged, but species turnover was increased by disturbances

Frugivory and seed dispersal by the lizard Gallotia galloti (Lacertidae in a xeric habitat of the Canary Islands

Abstract: The relationship between an endemic lizard (Gallotia galloti) and plants with fleshy fruits was examined in a xeric habitat on the island of Tenerife, Canary Islands. An analysis of 441 droppinps collected in May 1991, showed that fruits are an important item in the diet of this omnivorous lizard; 92% and 56% of droppinps contained fruits from two different areas. Fruits of seven plant species were found in the droppinps: Rubia fruticosa, Neochamaelea pulverulenta, Withania aristata, Lycium intricatum, Atriplex semibaccata, Opuntia dillenii and Scilla cf. haemorrhoidalis. The number of fruit species consumed at each area by G. galloti was related to their availability. Seed viability did not differ between control seeds and seeds from the droppings, except for N. pulverulenta and L. intricatum, where viability was higher in control seeds

A demonstration of guild-based assembly rules for a plant community, and determination of intrinsic guilds

Abstract: Assembly rules were sought, as relative constancy in the proportion of species from different guilds that can coexist at a point. The community used was a regularly-mown lawn, sampled at the end of the mowing/regrowth cycle. There was a significant tendency towards constant proportions of representatives from the Graminoid and Forb guilds. E.g., at 2-species points there was one graminoid species plus one forb species more often than expected. This effect was seen whether or not the two bryophyte species were included with the forbs. There was little evidence that the rule was based on grass/legume interactions. Optimisation of the Graminoid vs Forb + bryophyte classification showed that some forbs were better assigned to the «graminoid» guild, apparently because of the role of their laminae in the upper canopy (...)

Differential growth, allocation and photosynthetic responses of Polygonum viviparum to simulated environmental change at a high arctic polar semi-desert

Abstract: Growing season temperatures and precipitation, and soil nutrient status, were increased in situ at a polar semi-desert site in northwest Spitsbergen to simulate the possible impacts of climate change. During the second year of the experiment the responses of a perennial geophyte, Polygonum viviparum, were assessed both by biometric analyses of vegetative and reproductive structures and by measurements of instantaneous net photosynthesis (P n ). The objectives were to determine whether P. viviparum demonstrates conservative or opportunistic responses to increased temperature, water supply and nutrient availability, to assess whether vegetative and reproductive development show differential sensitivity, and thus whether allocation patterns are altered, and to evaluate whether changes in rate of photosynthesis underlied any changes in growth and allocation (...)

Parasite manipulation of host behaviour: should hosts always lose?

Abstract: Parasites of all kinds alter the behaviour of their hosts. In many systems, these behavioural modifications appear adaptive for the parasite, by facilitating the completion of its life cycle. However, not all parasitized hosts are under the influence of parasites. This may be due to the timing of the onset of behavioural manipulation by the parasites: changes in behaviour may coincide with completed parasite development and only appear late in the infection. In addition, certain hosts may oppose the parasite's attempts at manipulating their behaviour. Hosts with high expected future reproductive success, i.e. young hosts or hosts that have not yet reproduced, are more likely to benefit by opposing the influence of parasites, as their expected gains would outweigh the costs of opposition. Such conditional opposition would be a better alternative to resistance to infection itself if resistance is costly, and would explain the considerable variability often observed in the behavioural responses of parasitiaed hosts

An Index of Evenness and Its Associated Diversity Measure

Abstract: Evenness is an important property of all ecological communities. It may be defined as "the degree to which the abundances are equal among the species present in a sample or community" (Molinari 1989). The most accepted measures of evenness are the J index (H'/H'max) (Pielou 1969), and Alatalo's (1981) F21, a simple but important modification of Hill's (1973) E21 ratio. Molinari (1989) studied the behavior of these indices over a gradient of increasing evenness and found that both J and F21 have two important shortcomings: a) they tend to strongly overestimate evenness specially when its real value is low, and b) their behavior is decidedly nonlinear, the indices grow very fast at low equitabilities and then level off and change very slowly. Consequently, he developed G21 "a calibrated version of F21", to overcome these limitations. G21 is an empirical mathematical transformation of F21 that effectively linearizes it and greatly improves its accuracy, at least in collections of only two species. As a practical tool it is the best index already available, but it has both conceptual and practical drawbacks. The most important are: 1) It is a purely empirical measure, without any clear ecological meaning. As pointed out by Molinari (1989), the usual interpretation of F21 as a ratio of very abundant to abundant species is not correct and should be avoided. For G21 the problem is even greater, because of the complexity of the mathematical transformations involved. 2) The statistical properties of G21 are unknown. Consequently, it is impossible to make formal statistical comparisons among samples or communities. This is a problem shared by many other indices. 3) G21 (as F21) measures evenness within those species with intermediate and high proportional abundances. Its value is little influenced by the species with low abundances. Consequently, the index fails to discriminate between communities whose main difference rests in the relative proportion of rare species. This last point is of great importance. In many ecological communities the proportion of rare species is an important component of evenness, perhaps the one that most clearly distingishes one community from another (Medina and Huber 1992; and see Fig. 4 for a real example). Besides, the rare species are very sensitive to perturbations or environmental changes. Their sudden decrease or even disappearance produce changes in evenness, that a good index should be able to detect. In the past, the lack of sensitivity to rare species was considered as a desirable property in an evenness index, because it makes it "robust" against sampling bias. However, the occurrence of rare species in a sample is not an effect of sampling bias, but a consequence of important characteristics of the community that should not be ignored. This article proposes a new evenness index that tries to overcome these problems. It is much simpler than the already discussed indices. It shows linear behaviour over gradients of increasing equitability, and has an immediate and realistic ecological interpretation. It also gives equal weight to all species independently of their abundances. It allows the derivation of a simple diversity index, and its statistical distribution is known (Appendix 1).