Bryan Shorrocks

Making mistakes when predicting shifts in species range in response to global warming

Many attempts to predict the biotic responses to climate change rely on the ‘climate envelope’ approach, in which the current distribution of a species is mapped in climate-space and then, if the position of that climate-space changes, the distribution of the species is predicted to shift accordingly. The flaw in this approach is that distributions of species also reflect the influence of interactions with other species, so predictions based on climate envelopes may be very misleading if the interactions between species are altered by climate change. An additional problem is that current distributions may be the result of sources and sinks, in which species appear to thrive in places where they really persist only because individuals disperse into them from elsewhere,. Here we use microcosm experiments on simple but realistic assemblages to show how misleading the climate envelope approach can be. We show that dispersal and interactions, which are important elements of population dynamics, must be included in predictions of biotic responses to climate change.

Competition on a Divided and Ephemeral Resource: A Simulation Model

(2) Coexistence between two species can be extended by dividing the resource into more and smaller breeding sites. (3) Aggregation of the superior competitor also promotes coexistence, and can lead to an equilibrium between the two species if contagion is strong enough. (4) If the degree of aggregation is allowed to vary with density in a realistic way equilibrium is nearly always obtained. (5) These results may explain the high species diversity commonly observed on divided resources.

Living in a patchy environment

Introduction: Patchy environment - an overview 1. Starvation and predation in a patchy environment 2. The response of plants to patchy environments 3. Dynamic stability of a single-species population in a divided and ephemeral environment 4. Variance and patchiness in rates of population change - a planthoppers case history 5. Coexistence in a patchy environment 6. Population dynamics and community structure of parasitic helminths 7. Dung and carrion insects 8. Patchiness and community structure 9. Extinction of finite metapopulations in correlated environments 10. Conservation in a variable environment - the optimal size of reserves 11. Does interdemic group selection occur in commensal house mice (Mus domesticus)? 12. Sex determination and sex ratios in patchy environments

COMPETITION ON A DIVIDED AND EPHEMERAL RESOURCE

SUMMARY (1) Models of competition in divided or heterogeneous environments are reviewed. (2) A model of competition in a divided environment is presented. The competition coefficients, a, are reduced by a term, 0, which measures the amount of overlap between species. (3) Actual values of 0 are presented for two Drosophila communities. (4) 0 depends on the degree of aggregation of the competing species and on their densities. The outcome of competition in a divided environment must, then, be density dependent. (5) Low values of 0 improve the stability of a multi-species community and allow more

Priority Effects and Species Coexistence: Experiments with Fungal-Breeding Drosophila

1. Priority experiments were carried out using the fungal-breeding species Drosophila phalerata and D. subobscura and the mushroom Agaricus bispora forma albida. 2. Field experiments showed that an oviposition window exists for these species and that priority in increments of 1 day are suitable for laboratory experiments. 3. Priority (arriving first) had a clear effect upon three components of «fitness». When a species arrived late it had lower survival, smaller size and longer developmental time. 4. A priority model [based upon the «aggregation model» of Atkinson & Shorrocks (1981)] showed that traditional priority (both species arrive on average together, but with a range of priorities) does not significantly contribute to coexistence

Explaining Local Species Diversity

In this paper we examine the explanations for local species diversity. Using six extensive data-sets for drosophilid flies (which include both temperate and tropical species) we compare three major categories of explanation (Cornell & Lawton 1992): niche heterogeneity (resource partitioning), spatial heterogeneity (intraspecific aggregation), and the fullness of the niche space (saturation level). We conclude that these Drosophila communities are dominated by intraspecific aggregation, not by resource partitioning, and they are not fully saturated.

Guild size in drosophilids: A simulation model

(1) This paper describes a simulation model which predicts the guild size of drosophilid flies living on a divided and ephemeral resource, without any traditional resource partitioning. (2) A distribution of empirical guild sizes was obtained from fifty-three field studies collected from all over the world. The resource bases used by the flies were fruit, fungi, sap fluxes, decaying leaves and flowers. The modal guild size was seven. (3) An acceptable range of parameter values for the model was obtained from a combination of field and laboratory experiments. Within this range the model predicted distributions of guild sizes slightly less than those observed in the field, with a modal size which varied between five and six.

An ecological classification of European Drosophila species

SummaryThe associations shown between species of Drosophila collected in three European countries are analysed using a clustering method. The resulting dendrograms are combined to give a plan of associations shown by all three surveys. These general groupings are interpreted in the light of what is known about Drosophila breeding sites.One ecological group, the fungal breeding species are examined in detail and their pattern of geographical associations investigated. The three most abundant species in collections, D. transversa, D. phalerata and D. cameraria appear to replace one another in a north-south direction in western Europe. It is suggested that ecologically marginal areas may be defined using the frequency of a species within its ecological group.

Aggregation Does Prevent Competitive Exclusion: A Response to Green

Green (1986) raised a number of points concerning our simulation model of competition on a divided and ephemeral resource (Atkinson and Shorrocks 1981; Shorrocks et al. 1984), its analytical counterpart (Ives and May 1985), and the review of possible mechanisms giving rise to aggregation (Atkinson and Shorrocks 1984). This model allows a competitively inferior species to survive in probability refuges, that is, sites with no or few superior competitors that arise as a result of an aggregated distribution of individuals over breeding sites. Such refuges may occur even at equilibrium density, since aggregation increases crowding (Lloyd 1967), and global population density is limited by strong intraspecific competition in sites with high local density while low-density sites still exist. The model developed from field studies of drosophilid flies (Atkinson and Shorrocks 1977; Shorrocks 1982; Shorrocks and Rosewell 1987). In particular, Greens criticisms make use of the suggestion by Atkinson and Shorrocks (1984) that the observed negative-binomial distributions of drosophilid eggs over breeding sites could arise from a Poisson distribution of egg-laying visits by females to breeding sites, where eggs are laid in clutches, the size of which has a logarithmic distribution.

Selection, patches and genetic variation: A cellular automaton modellingDrosophila populations

SummaryMany models have been proposed in which environmental heterogeneity promotes genetic diversity. Such models describe the situation where different phenotypes have different fitness values in different types of patch and are the genetic equivalent of the traditional resource partitioning models in ecology which allow the coexistence of species. Here we construct a different type of cellular model in which polymorphisms in populations ofDrosophila can be maintained without traditional resource partitioning. Parameter values taken from laboratory and field observations represent fungal breedingDrosophila. Some stochasticity is used in the description of the migration between patches. In the model space is divided into a uniform matrix of cells each of which has the potential to contain an ephemeral resource item (fungal fruiting body). Square arenas of up to 400 cells were used. Genotypes arrive at a fresh site, breed (Hardy-Weinberg equilibrium) and lay eggs. The eggs hatch and the larvae compete using the Hassell-Comins competition equations, as if they were three different species. Adult emergents all migrate to an adjacent cell. The aggregation patterns observed in nature are produced using an ‘attraction probability’ where each fly has a chance of moving to the currently most densely populated adjacent patch. This ‘black box’ description of migration produces distribution patterns which are indistinguishable from those seen in wild populations of fungal breedingDrosophila. Results show that the ‘attraction probability’ is the key factor in the maintenance of polymorphism and that even when the competitive advantage of the superior genotype is very great, polymorphisms can be maintained.