Metabolism drives demography in an experimental field test

Metabolism should drive demography by determining the rates of both biological work and resource demand. Long-standing "rules" for how metabolism should covary with demography permeate biology, from predicting the impacts of climate change to managing fisheries. Evidence for these rules is almost exclusively indirect and in the form of among-species comparisons, while direct evidence is exceptionally rare. In a manipulative field experiment on a sessile marine invertebrate, we created experimental populations that varied in population size (density) and metabolic rate, but not body size. We then tested key theoretical predictions regarding relationships between metabolism and demography by parameterizing population models with lifetime performance data from our field experiment. We found that populations with higher metabolisms had greater intrinsic rates of increase and lower carrying capacities, in qualitative accordance with classic theory. We also found important departures from theory-in particular, carrying capacity declined less steeply than predicted, such that energy use at equilibrium increased with metabolic rate, violating the long-standing axiom of energy equivalence. Theory holds that energy equivalence emerges because resource supply is assumed to be independent of metabolic rate. We find this assumption to be violated under real-world conditions, with potentially far-reaching consequences for the management of biological systems.

Going cheap: determinants of bird price in the Taiwanese pet market

BACKGROUND

International wildlife trade is the largest emerging source of vertebrate invasive alien species. In order to prevent invasions, it is essential to understand the mechanics of trade and, in particular, which traded species are most likely to be released or escape into the wild. A species' economic value is a key factor, because we expect cheaper species to be less assiduously secured against escaping, and more likely to be deliberately released. Here, we investigate determinants of the price of species in the Taiwanese bird trade. Taiwan is an international hub for bird trade, and several native species are threatened by alien bird species.

METHODOLOGY

We investigated the relationship between the traded species sale price in Taiwan and the species availability for trade (the number of birds for sale, geographic range size and their origin, conservation and CITES status) and traits (body size, coloration, song attractiveness). We used phylogenetic generalized least squares models, with multi-model inference, to assess the variables that are best related to the price of birds in the Taiwanese pet trade.

PRINCIPAL FINDINGS/CONCLUSIONS

We found that species available for sale in larger numbers, native to Taiwan, not globally endangered, and small-bodied are all relatively cheaper, as too are species lacking yellow coloration and without attractive songs. Our models of price revealed high levels of phylogenetic correlation, and hence that closely related species tended to be sold for similar prices. We suggest that, on the basis of price, native species are more likely to be deliberately or accidentally released than alien species. Nevertheless, our survey of bird shops recorded 160 species alien to Taiwan (7,631 individuals), several of which are for sale cheaply and in large numbers. Alien bird species in trade therefore present an ongoing, non-trivial invasion risk on the island.

Relations between abundance, body size and species number in British birds and mammals

British birds and mammals are compared in terms of their frequency distributions of abundance and body mass and in respect of the relation between abundance and body mass. Body masses of non-flying mammals are greater than those of resident birds which are, in turn, heavier than migrants; bats are lightest. The frequency distribution of masses are close to log-Normal for each of these groups, though their variances and skews differ. Differences in mean abundances (which are log-Normally distributed) parallel those in body mass. In each group, abundance declines with body mass: the exponent of the relation is close to the value of —0.75 predicted by the ‘energetic equivalence’ rule though not significantly different from the value of — 1.0 predicted by the ‘biomass equivalence’ rule. At comparable masses, species of non-flying mammals are more abundant than resident birds, migrant birds and bats by approximately 45, 300 and 200 times, respectively. The similarity between birds and bats in this regard may be no more than coincidental but it may be related to ecological similarities related to flight. The metabolic rates of non-flying mammals may be generally lower than those of birds and bats but not sufficiently to account for their much greater abundances.

Spatial patterns in the geographic range sizes of bird species in the New World

The attempt to identify and explain pattern in the extent of species’ geographical distributions at regional scales has been central to macroecology. However, with the exception of abundance, consistent relations between other variables and species geographic extent have not been forthcoming. One reason may be that studies often encompass the entire geographic ranges of only a fraction of the species in the taxon under consideration, setting biologically artificial boundaries to the area of study, and only revealing part of the pattern in question. Here, we examine patterns in the geographic range sizes of birds in the New World. By testing for patterns in the entire avifauna of a geographically isolated region (95% of species are endemic), we avoid many of the problems of previous studies. Most New World bird species have small geographic ranges, although the frequency distribution of logarithmically transformed ranges is left-skewed. The geographic range size-body size relation is approximately triangular. Small-bodied species may have either large or small ranges, whereas large-bodied species have only large ranges. Species threatened with extinction more often fall nearer to (or below) the lower edge defined by the majority of species in this triangle than do non-threatened species, suggesting that this represents the minimum area needed to sustain viable populations of species of different sizes. The maximum range size attained by species is relatively constant across body sizes, but falls short of the maximum possible given the land area of the New World, and so cannot be limited by this constraint. What does limit maximum range size is thus unclear. There is a latitudinal gradient in the size of species geographic ranges. Species which have the latitudinal mid-point of their geographic ranges at high latitudes either side of the equator tend to have large range sizes, whereas those with mid-points at lower latitudes tend to have small range sizes (as expected from Rapoport’s rule). However, this pattern is not symmetrical about the equator, but rather, at about 17° N. It appears to be a consequence of the biogeography of the New World, and implies that mechanisms suggested to explain Rapoport’s rule based on climatic variability are incorrect. Migrant birds have larger geographic ranges, on average, than do residents. They are also larger-bodied, and tend to inhabit more northerly latitudes than residents, but their larger ranges are not the simple consequence of these other patterns. The patterns we demonstrate, in particular those relating to maximum range size across body sizes and to latitudinal variation in range size, have significant consequences for the understanding of what determines species geographic range sizes.