Thomas A. Waite

Forecasting global biodiversity threats associated with human population growth

Abstract The size and growth of the human population are often cited as key factors in threats to Earth’s biodiversity, yet the extent of their contribution to the endangerment and extinction of other species has remained unclear. Moreover, it could be valuable to know what additional threats may arise from continued human population growth. Here we quantify a model of the relationship between human population density and the number of threatened mammal and bird species by nation. Our multiple regression analysis revealed that two predictor variables, human population density and species richness (of birds and mammals), account for 88% of the variability in log-transformed densities of threatened species across 114 continental nations. Using the regression model with projected population sizes of each nation, we found that the number of threatened species in the average nation is expected to increase 7% by 2020, and 14% by 2050, as forecast by human population growth alone. Our findings strongly support the notion that abating human population growth is a necessary, if not sufficient, step in the epic attempt to conserve biodiversity on the global scale.

FLUCTUATING POPULATION SIZE AND THE RATIO OF EFFECTIVE TO CENSUS POPULATION SIZE

The effective size of a population (Ne) quantifies the rate at which genetic diversity is eroded by genetic drift (i.e., 112Ne per generation), a fundamental process of evolutionary change. Genetic diversity and its rate of decay have been linked with key components of population fitness (Allendorf and Leary 1987; Ralls et al. 1988; Briscoe et al. 1992; Newman and Pilson 1997; but see Britten 1996). Ne is thus a central parameter both in studies aimed at understanding evolution (Falconer and Mackay 1996) and in the field of conservation genetics (Lande and Barrowclough 1987; Nunney and Campbell 1993; Nunney and Elam 1994). Unfortunately, accounting for all factors that influence Ne is notoriously difficult (reviewed by Caballero 1994). This difficulty is apparently responsible for significant disagreement between theoretical (Nunney 1993) and observed values of the ratio, Ne/N (Frankham 1995). Here we investigate whether this disagreement can be reconciled by incorporating the effect of a factor long known to reduce Ne, namely temporal fluctuations in population size (FPS; Wright 1938). More specifically, we consider the extent to which Ne/N is depressed by FPS over the range of fluctuations observed in wild animal populations. In addition, we present a method for predicting Ne/N from a standard measure of population variability, and we discuss the implications of this theoretical relationship. Several factors affect the effective size of a population: fluctuations in size, variance in fecundity, sex ratio, and the degree to which generations overlap (Crow and Kimura 1970). One difficulty in estimating Ne is that no single formula simultaneously accounts for all these factors. This difficulty would be largely inconsequential if the ratio Ne/N were known to fall consistently within a narrow range. Estimating Ne would be trivial because N is often relatively easily estimated. Theoretical and empirical studies have searched for such a range of Ne/N. Theoretical studies have explored the plausible range of Ne/N through analysis of a reparameterized version of Hills (1972) expression for Ne (Nunney 1991, 1993, 1996). This reparameterization provides several advantages. Ne is expressed in parameters that are biologically interpretable, and for which typical ranges are known. In addition, the parameters can be estimated from data commonly available from single-season studies of real populations (Nunney and Elam 1994). Through thorough numerical exploration of the parameter space, these studies led to the conclusion that Ne/N is usually close to 0.5 and only rarely outside the range 0.250.75 (Nunney 1991, 1993, 1996; hereafter, referred to as the theoretical expectation.) In contrast with this theoretical expectation, a review of 192 empirical estimates (based on a variety of demographic and genetic methods) revealed that Ne/N was usually less than 0.5 (Frankham 1995; hereafter, referred to as empirical estimates.) In fact, approximately one-third of the Ne/N estimates were less than 0.25, and a subset of these estimates (37 from animal taxa) accounting for all factors that influence Ne had an average Ne/N of 0.15 (median = 0.08). By contrast, a subset of estimates (27 from animal taxa) accounting for all factors except FPS had an average Ne/N of 0.38 (median = 0.38). The discrepancy between theoretical expectation and empirical estimates may thus be largely attributable to the fact that the theoretical expectation is based on the assumption of constant N. The theoretical expectation may provide a reasonable estimate of the short-term Ne/N, but the longerterm ratio may often be less than 0.25, owing to the effect of FPS. A long-term estimate of Ne that accounts for FPS is obtained by transforming a series of short-term effective sizes (Wright 1938; see also Crow and Kimura 1970; Lande and Barrowclough 1987):

Population limitation and the wolves of Isle Royale

Population regulation for gray wolves in Isle Royale National Park, Michigan, was examined in 1987–1995 when wolves were in chronic decline following a crash of the population in 1981–1982. Canine parvovirus (CPV-2) was probably influential during the crash, but it disappeared by the late 1980s. High mortality abruptly ceased after 1988, but low recruitment in the absence of disease and obvious shortage of food prevented recovery of the wolf population. In 1983–1995, with a comparable number of moose ≥10 years old as potential prey, wolves were only half as numerous as in 1959–1980. A simulation of annual fluctuations in effective population size ( N e ) for wolves on Isle Royale suggests that their genetic heterozygosity has declined ca. 13% with each generation and ca. 80% in the 50-year history of this population. Inbreeding depression and stochastic demographic variation both remain possible explanations for recent low recruitment.

Controlling the false discovery rate and increasing statistical power in ecological studies

ABSTRACT Ecologists routinely use Bonferroni-based methods to control the alpha inflation associated with multiple hypothesis testing, despite the aggravating loss of power incurred. Some critics call for abandonment of this approach of controlling the familywise error rate (FWER), contending that too many unwary researchers have adopted it in the name of scientific rigour even though it often does more harm than good. We do not recommend rejecting multiplicity correction altogether. Instead, we recommend using an alternative approach. In particular, we advocate the Benjamini–Hochberg and related methods for controlling the false discovery rate (FDR). Unlike the FWER approach, which safeguards against falsely rejecting even a single null hypothesis, the FDR approach controls the rate at which null hypotheses are falsely rejected (i.e., false discoveries are made). The FDR approach represents a compromise between outright refusal to control for multiplicity, which maximizes alpha inflation, and strict adherence to FWER control, which minimizes power. We review the multiplicity problem, illustrate the advantage of the FDR approach, and promote this approach for widespread adoption in ecology.

Raven scavenging favours group foraging in wolves

Wolves, Canis lupus, routinely live in large packs that include unrelated individuals and mature offspring. Studies show that individual wolves that live in large packs suffer reduced foraging returns. Therefore, group hunting and group living (sociality) in wolves is generally thought to be favoured by indirect fitness gains accrued through kin-directed altruism. However, we show that kin-directed altruism cannot account for groups that include mature offspring or unrelated individuals. We also present an analysis that incorporates a previously ignored feature of wolf foraging ecology, namely the loss of food to scavenging ravens, Corvus corax. By accounting for this process, we show that individuals in large packs do indeed accrue foraging advantages. In the hypothetical absence of this scavenging pressure, an individual would maximize its rate of prey acquisition, and minimize its risk of energetic shortfall, by foraging with just one other individual. However, incorporating the effect of scavenging by ravens leads to a dramatic increase in the predicted group size. Our analysis indicates that per capita gains are highest in the largest observed packs. The greater food-sharing costs in a larger pack are more than offset by smaller losses to scavengers and increased rates of prey acquisition. Thus, in contrast with previous interpretations, the selfish benefits of social foraging appear to contribute to the maintenance of sociality in wolves after all. We explore whether such benefits favour group living in various social carnivores that hunt large prey and are thus vulnerable to scavenging.

MONOGAMY IN LEACH'S STORM-PETREL: DNA-FINGERPRINTING EVIDENCE

We used multilocus minisatellite DNA fingerprinting to estimate the frequency of extrapair fertilizations in a population of Leachs Storm-Petrel (Oceanodroma leucorhoa) on Kent Island, New Brunswick, Canada. Leachs Storm-Petrel is a member of Procellariiformes, an order of long-lived pelagic birds characterized by long-term pairbonds, single-egg clutches, and extended periods of parental care. We found no evidence of extrapair fertilizations in 48 families (42 full families and 6 partial families consisting of the putative father and the single offspring). Thus, our results indicate that the breeding system (genetic monogamy) matches the mating system (social monogamy) in our study population, a condition that no longer can be assumed in socially monogamous bird species. Genetic monogamy in Leachs Storm-Petrels may be maintained by last-sperm precedence and frequent copulation by mates during the females fertile period. Such tactics employed by a male may yield a high prob- ability of fertilizing the single egg laid by his mate. Received 15 June 1994, accepted 31 August

Vigilance in the White-breasted Nuthatch: effects of dominance and sociality

-Using captive pairs of White-breasted Nuthatches (Sitta carolinensis), I tested the nonexclusive hypotheses that the vigilance of social foragers has two components, (1) vigilance for predators and (2) vigilance for dominant conspecifics, and thus is mediated by the size of the foraging group as well as by an individuals dominance status. Evidence from pairs of nuthatches, tested while solitary and while together, revealed that males increased their vigilance when solitary, that females were subordinate to and more vigilant than males when both sexes were housed together in the aviary, and that the advantage of foraging socially in terms of a reduction in vigilance was greater for males. My results demonstrate that vigilance in White-breasted Nuthatches was sex specific and that male social dominance was the responsible mechanism. I suggest that subordinate social foragers have the additional constraint on their foraging time of keeping higher-ranking flock mates under surveillance. Received 27 June 1986, accepted 26 January 1987. THE two commonly hypothesized advantages of foraging socially rather than solitarily are improved predator avoidance and enhanced foraging efficiency (reviewed by Wilson 1975, Moriarity 1976, Rubenstein 1978, Morse 1980, Krebs and Davies 1981). There is a growing realization that these two benefits are not independent. In fact, the size of a foraging group influences how individuals apportion their time budgets among such incompatible activities as scanning for predators and searching for food (e.g. Pulliam 1973; Powell 1974; Caraco 1979; Barnard 1980; Bertram 1980; Caraco et al. 1980a, b; Elgar and Catterall 1981; Lendrem 1983; Studd et al. 1983; Elgar et al. 1984). Several studies have demonstrated that as flock size increases, individual birds can allocate more time to foraging activities while the per capita vigilance level is maintained (e.g. Powell 1974, Siegfried and Underhill 1975, Caraco 1979, Jennings and Evans 1980, Sullivan 1984a). A common shortcoming of these studies was their failure to consider that the costs and benefits associated with social foraging may be sex specific, dominance specific, or both [but cf. Moore 1972 (in Caraco 1979), Caraco 1979, Ekman and Askenmo 1984, Waite 1986]. Based partly on evidence that birds increased their vigilance in areas of higher predation risk (Barnard 1980, Caraco et al. 1980a) or when a predator was present (Caraco et al. 1980b), previous workers assumed that the principal function of vigilance was predator detection. Thus, these studies overlooked the possibility that the time devoted to foraging was more constrained for subordinates because higher-ranking conspecifics interfered with their foraging (but cf. Robinson 1981, Knight and Knight 1986). I tested experimentally the possibility that females of a heterosexual pair are more vigilant than males because they must keep their dominant mate under surveillance while also maintaining a certain level of vigilance for predators. I used captive mated pairs of White-breasted Nuthatches (Sitta carolinensis) to examine the nonexclusive hypotheses that the vigilance of animals that forage in social groups is comprised of both vigilance for predators and vigilance toward other members of the foraging group, and, hence, is influenced by two proximate factors, sociality and dominance status. One prediction follows from each of these hypotheses, respectively. First, a male nuthatch should be more vigilant when alone than when with his mate. Second, a female nuthatch should be more vigilant than her (dominant) mate when they forage together. No prediction regarding the vigilance levels of solitary vs. social females is made because females, owing to their need to keep their dominant mates under surveillance, actually may be more vigilant when social than when solitary.

Genetic (RAPD) diversity in Peromyscus maniculatus populations in a naturally fragmented landscape

We assessed the effects of long‐term habitat fragmentation on genetic (random amplified polymorphic DNA) diversity in 11 Peromyscus maniculatus populations in the Lake Superior watershed. We analysed genetic structure at two spatial scales and the effect of island size and isolation on genetic diversity. At the regional scale, island populations differed from mainland populations (FST = 0.36), but mainland populations did not differ from each other (FST = 0.01). At the local scale, populations of the main island of Isle Royale differed from adjacent islet populations (P < 0.001; Monte Carlo approximation of Fisher’s exact test), but not from each other (combined P = 0.63). Although geographical distance and genetic distance were positively correlated (P < 0.01; Mantel test), cluster analysis revealed some inconsistencies. Finally, genetic diversity was inversely related to isolation (P = 0.01), but had an unexpectedly negative relationship with island area (P = 0.03). The genetic structure of P. maniculatus populations in portions of the Lake Superior watershed appears to have been affected by long‐term habitat fragmentation.

Absence of female conspecifics induces homosexual behaviour in male guppies

Social environment can have dramatic effects on the expression of species-typical sexual behaviour. Using guppies, Poecilia reticulata, we asked how an all-male social environment affects male sexual behaviour. Males were assigned to either single-sex or mixed-sex groups. After 15 weeks, their sexual behaviour was assayed in the presence of three novel individuals of each sex. Males from single-sex groups performed higher rates of sexual displays and sneak copulation attempts towards stimulus males than did males from mixed-sex groups. Males from mixed-sex groups directed the majority of their sexual behaviour towards females using typical heterosexual behaviour. The social environment for each subject was then reversed for 2 weeks, after which subjects in both treatments showed a stronger tendency to display or sneak towards males. These findings reveal that homosexual behaviour is not readily extinguished when females become accessible and that it can be induced even after the ontogeny of heterosexual behaviour. While guppies may rarely occur in all-male groups in nature, these findings highlight the capacity for social environment to shape mating behaviour, which is particularly relevant for captive populations used in behavioural studies or conservation breeding programmes.

Efficacy of Land-Cover Models in Predicting Isolation of Marbled Salamander Populations in a Fragmented Landscape

Amphibians worldwide are facing rapid declines due to habitat loss and fragmentation, disease, and other causes. Where habitat alteration is implicated, there is a need for spatially explicit conservation plans. Models built with geographic information systems (GIS) are frequently used to inform such planning. We explored the potential for using GIS models of functional landscape connectivity as a reliable proxy for genetically derived measures of population isolation. We used genetic assignment tests to characterize isolation of marbled salamander populations and evaluated whether the relative amount of modified habitat around breeding ponds was a reliable indicator of population isolation. Using a resampling analysis, we determined whether certain land-cover variables consistently described population isolation. We randomly drew half the data for model building and tested the performance of the best models on the other half 100 times. Deciduous forest was consistently associated with lower levels of population isolation, whereas salamander populations in regions of agriculture and anthropogenic development were more isolated. Models that included these variables and pond size explained 65-70% of variation in genetically inferred isolation across sites. The resampling analysis confirmed that these habitat variables were consistently good predictors of isolation. Used judiciously, simple GIS models with key land-cover variables can be used to estimate population isolation if field sampling and genetic analysis are not possible.