Tim M. Blackburn

Animal body size distributions: patterns, mechanisms and implications

Documenting the shape of the frequency distribution of species body sizes for an animal taxon appears at first sight a straightforward task. However, a variety of patterns has been reported, and a consensus is only now being reached through an understanding of how potential biases may affect observed shapes of distributions. A new body of evidence suggests that, at large scales, size distributions are right-skewed, even on logarithmic axes. If body size distributions can be described with certainty, this will allow assessment of the mechanisms proposed to generate them, and will be an important step towards understanding the structure and dynamics of animal assemblages.

Mapping Biodiversity Using Surrogates for Species Richness: Macro-Scales and New World Birds

A number of surrogates have been suggested for predicting relative levels of biodiversity (typically expressed in terms of species richness) in areas for which this information is not available. However, to date there has been little attempt to perform direct and explicit empirical comparisons of their effectiveness. Here we examine the relative predictive value of some environmental variables and of the numbers of higher taxa, using the avifauna of the New World. Numbers of genera and families are found to have the strongest correlations with species richness, and to provide the best predictions of the numbers of species in areas of Central and South America on the basis of data for North America. Their effectiveness as surrogates may result from the fact that they themselves represent an alternative currency for expressing levels of biodiversity.

The Tropics as a Museum of Biological Diversity: An Analysis of the New World Avifauna

The tropics have variously been argued to represent a cradle of diversity, a museum of diversity, or some combination of the two. Few broad scale data have been available to examine these mechanisms. For the avifauna of the New World, the tropics appear, at least, to act as a museum. The mean age of tribes is greatest at the equator and declines towards the Poles, whether or not the mean is weighted by the number of species in the tribe. The decline is asymmetrical, being much more severe in the northern hemisphere. The pattern results predominantly from a progressive loss of older tribes towards higher northern latitudes. Some mechanisms that might generate these patterns are discussed.

Prehistoric bird extinctions and human hunting.

Holocene fossils document the extinction of hundreds of bird species on Pacific islands during prehistoric human occupation. Human hunting is implicated in these extinctions, but the impact of hunting is difficult to disentangle from the effects of other changes induced by humans, including habitat destruction and the introduction of other mammalian predators. Here, we use data from bones collected at a natural sand dune site and associated archaeological middens in New Zealand to show that, having controlled for differences in body mass and family membership (and hence for variation in life–history traits related to population growth rate), birds that were more intensively hunted by prehistoric humans had a higher probability of extinction. This result cannot be attributed to preservation biases and provides clear evidence that selective hunting contributed significantly to prehistoric bird extinctions at this site.

A sideways look at patterns in species richness, or why there are so few species outside the tropics

What causes the widely observed latitudinal gradients in species richness remains one of the key unanswered questions in ecology. Despite numerous hypotheses generating a wealth of tests, no consensus has yet been reached. We do not add to the list either of tests or of hypotheses, but instead examine how some of the implicit (and occasionally explicit) assumptions of most research into the problem may be hindering its resolution. We discuss four ideas that could aid progress in this regard. First, we observe that view depends on viewpoint, and that altering the latter often leads to changes in the former. The most common view of richness gradients is that the tropics are unusually species rich. Viewing the richness of the tropics as the norm causes more than a simple rephrasing of the question: rather, it alters the perspective on other assumptions about richness gradients. Second, we suggest that the latitudinal gradient in species richness may actually arise from more than one process. The amount of energy flowing into a region must set an upper bound on the amount of the raw material of life the region can support. However, this raw material can be viewed as first being converted into individuals, and these individuals as subsequently distributed into species. Both processes will contribute to the determination of how many species inhabit a region. Third, we argue that the species richness of a region will not be independent of the mean body size and mean abundance (density) of the species, because all these variables depend on division of the same basic raw material. The energy entering a region may be viewed as being allocated to the number, abundance and size of species, but the allocation to each will not be independent. Spatial variation in mean abundance and mean body size will interact with spatial patterns in species richness. Fourth, we argue that, given the second and third points, it is unlikely that a single cause for species richness gradients will be identified. Effectively, this amounts to trying to give one answer to two quite different questions.

Evolutionary age and risk of extinction in the global avifauna

SummarySpecies at high risk of extinction are not distributed at random among higher taxa. Here we demonstrate that there is a positive relationship between the proportion of species in a taxon which are considered to be threatened and the evolutionary age of that taxon, both for the global avifauna and the avifauna of the New World. The potential mechanisms and consequences of the relationship are examined.

Which species are described first?: the case of North American butterflies

Within a taxon, some species are described before others, and some have greater numbers of synonyms. Here, we explore the correlates of date of description and numbers of synonyms for a well-known group of species, the butterflies of North America. Larger and more widespread species were described earlier. Species which were described earlier and are more widespread have greater numbers of associated synonyms. Development of an understanding both of patterns of non-random description and of their determinants is particularly important as increasing use is made of historical (museum) collections of specimens to document spatial patterns in the occurrence of individual species and levels of species richness, and large scale patterns in species-level traits. Exploration of patterns of description of well-known groups provides a point of reference for assessing potential difficulties with those which are more poorly known.

The relationship between geographic area and the latitudinal gradient in species richness in New World birds

One hypothesis for the latitudinal gradient in species richness observed in most animal taxa is that the richness of a region is determined by its geographic area. However, the relationship between geographic area and species richness across regions is generally weak. It has been suggested that this is because species from the tropics spill out of this region of high richness, artificially inflating the richness of other regions. This generates the interesting prediction that the area and richness of extra-tropical regions should be more strongly correlated if tropical species are excluded. We test this prediction using the avifauna of the New World. We find that there is indeed a relationship between the land area and species richness of a region once tropical species are excluded. This relationship is independent of the latitude and productivity of regions. Both latitude and productivity can explain variance in richness unexplained by land area. There is no relationship between land area and species richness if tropical species are not excluded from the analysis, suggesting that tropical species do indeed mask the relationship between richness and area. We conclude that our results generally support the geographic area hypothesis, although tests of its other predictions and on other land masses are required.

Aggregation and interspecific abundance-occupancy relationships.

Hartley (1998) is critical of the approach taken by recent attempts to understand the mechanisms underlying the positive relationship between the local abundances and regional occupancies of species which has been documented for many groups of organisms. Whilst he highlights important issues, some of the observations made are simplistic. Here we draw attention to the problems and to the substantial body of previous work that bears upon them.

Foraging and courtship behaviour in males of the solitary bee Anthophora plumipes (Hymenoptera: Anthophoridae): thermal physiology and the roles of body size

Abstract. 1 The effects of climate and body size on male behaviour were examined in the solitary bee Anthophora plumipes (Hymenoptera: Anthophoridae), which shows resource‐based polygyny at floral food sources in Britain in spring. 2 Larger males are able to fly at lower temperatures than smaller males, and can therefore court females under conditions in which smaller males cannot fly. This is predicted from patterns of endothermic ability at low temperatures already demonstrated within this species. 3 Video analysis of male competition for opportunities to initiate courtship with tethered females showed that larger males are also competitively superior, and can displace smaller males from favoured flight positions immediately behind females. 4 The mating system shown by male A plumipes is strongly dependent on male density. At low densities, males show exclusive territoriality at floral sources. As male density increases, this strategy is abandoned in favour of patrolling with considerable spatial overlap between males, and opportunistic Polygyny. 5 Despite high endothermic abilities, male behaviour is highly dependent on weather, and particularly ambient temperature. Males bask in the early morning and maintain high thoracic temperatures. Temperature data from freshly killed bees show that thoracic warming from solar sources effectively doubles the thermogenic power generated by the bee alone at low ambient temperatures. 6 Male strategies in A.plumipes are compared to female responses to climate. Having controlled for differences in body size there is no difference in endothermic abilities between the sexes. Males do not, however, fly under conditions in which females of the same size would remain active. These results are discussed in the light of differential dependence of reproductive success on flight activity for the two sexes.