Speciation, the division of one species into two, has provided evolutionary biologists with a rich ensemble of questions, conundrums and revelations for over a century, and yet our understanding of many of the factors affecting this complex, multidimensional process remains limited. In this thesis, I aimed to further our understanding of speciation using divergent populations of three-spined stickleback (Gasterosteus aculeatus) on the island of North Uist, Scottish Western Isles. Firstly, I explored the degree of morphological and genetic separation between three stickleback ecotypes, showing that both strongly reproductively isolated, and admixed populations exist in close proximity. I then attempted to identify the ecological and genetic origins of strongly isolated species-pairs, testing two competing explanations for their existence. I showed that a recent 'double-invasion' is unlikely, but found stark differences in the long-term genetic history between ecotypes, indicating that the evolution of species-pairs may be related to secondary contact between anciently diverged mitochondrial lineages. I then conducted mate choice trials to assess mating preferences between ecotypes, and to test for reinforcement in species-pairs. Consistent with the idea that speciation in this case is not driven purely by ecological factors, I found no evidence that reinforcement drives assortative mating in species-pairs. Rather, it appears that extant mating preferences have developed as a by-product of other adaptations. Finally, I took a brief interlude to document and investigate an exciting chance finding, internally developed embryos retained within the ovaries of a normally oviparous species, before concluding by summing up my findings, their relevance for scientific progress, and avenues opened up for further research.

This project evaluates the phylogeographic patterns of an insect-plant interaction, using ecological and molecular data for the model system of Alkanna orientalis (Boraginaceae) and its solitary bee pollinator Anthophora pauperata (Anthophoridae) in the Middle East. Understanding the relationship between the plant and its pollinator will help with recommendations for conservation strategies. The flowering time of the plant coincided with the appearance of its main pollinator. In the northern part of the western highlands of Saudi Arabia, A. pauperata was the most common visitor for both pollen and nectar, and was also an efficient pollinator, transferring pollen to the stigmas. The females foraged for nectar and pollen early and late in the day, while the males performed territory defence in between these times. Females were shown to be able to travel long distances regardless of the harsh landscape and high mountain barriers, and hence can facilitate genetic exchange of both bee and plant amongst wadis. From a molecular point of view, I investigated genetic variation within and amongst populations of Alkanna orientalis at different spatial scales across the eastern Mediterranean and Middle East regions. I used biparentally inherited, nuclear ribosomal DNA (the ITS region) and maternally inherited, chloroplast DNA (the trnH-psbA region) to understand the rate of gene flow of pollen vs. seeds. The results showed evidence of spatial structure over a large geographical scale, which might be the consequence of historical series of expansions and constrictions. In addition to these large-scale patterns, there was evidence of spatial heterogeneity at smaller scales, which may describe more recent, localised events such as population extinction and recolonization from adjacent territories, or fluctuations in population size associated with random genetic drift when population size is small. For the pollinator, Anthophora pauperata, I describe the genetic variation within and between the only two known populations, at Jabal El-Loz in northwest Saudi Arabia and the high massif of South Sinai in Egypt. For this, I sequenced a protein-coding region of mitochondrial DNA and genotyped ten hyper-variable microsatellite loci. The results showed evidence of significant genetic differentiation across the large geographical scale between the two populations, coupled with significant gene flow within populations over smaller geographical scales, such as between wadis. One of the outcomes of this study is to suggest that some populations should be given higher priority in terms of conservation. This prioritisation is based on elements, such as the level of genetic diversity within each population, the distribution of unique genotypes, evidence that populations have been recently exposed to bottlenecks, populations that are potentially glacial refugia, and populations under extreme threat because of human activities.

This thesis presents a series of experiments on the evolutionary ecology of the reproduction, behaviour, chemical and molecular characteristics of bethylid wasps in the genus Goniozus. Part One investigates host quality by varying host age after paralysis. The quality of paralyzed hosts declines with time since paralysis negatively influences life-history characteristics of Goniozus nephantidis. Assessment of host metabolomic profiles show which chemicals change as hosts age. Part Two investigates the effect of kin recognition on contest behaviour among adult females ofGoniozus legneri. Competitive behaviour was thus used to study the basis of kin recognition mechanisms. Wasps that are genetic kin and wasps that are reared on the same host behave less aggressively towards each other than do non-kin and nonhostmates. It is likely that cuticular hydrocarbon profiles are used by wasps in kin recognition. The environmental and genetic influences on wasp cuticular hydrocarbon profiles were explored: chemical composition differed according to both wasp species and host species. Part Three investigates genetic characteristics of Goniozus legneri populations on kin recognition behaviour. A molecular genetic marker system was developed for Goniozus species. Microsatellites showed clear polymorphism in six primer pairs and are likely to be a valuable tool in the future for closely related species. One of these markers was utilized to assess sex allocation at oviposition, thus avoiding potentially biasing influences of developmental mortality. Developmental mortality does not differ between the sexes but mortality increased, sex ratio variance across offspring groups and can obscure relationships between sexual composition andg roup size that are present at oviposition. A tendency for Goniozus legneri to lay male and female eggs in spatial separation was also observed. Although the focus of these studies is on fundamental aspects of bethylid biology, advances in all of these areas have potential to enhance the deployment of these parasitoids in biological pest control.

The planktonic foraminifera are a highly abundant and diverse group of marine pelagic protists that are ubiquitously distributed throughout the worlds’ oceans. These unicellular eukaryotes are encased in a calcareous (CaCO3) shell or ‘test’, the morphology of which is used to identify individual ‘morphospecies’. The foraminifera have an exceptional fossil record, spanning over 180 million years, and as microfossils provide a highly successful paleoproxy for dating sedimentary rocks and archiving past climate. Molecular studies, using the small subunit (SSU) ribosomal (r) RNA gene are used here to investigate the biogeographical distributions and phylogenetic relationships of the planktonic foraminifera. Biogeographical surveys of two markedly different areas of the global ocean, the tropical Arabian Sea, and the transitional/sub-polar North Atlantic Ocean, revealed significant genotypic variation within the planktonic foraminifera, with some genetic types being sequenced here for the first time. The foraminiferal genotypes displayed non-random geographical distributions, suggestive of distinct ecologies, giving insight into the possible mechanisms of diversification in these marine organisms. The ecological segregation of genetically divergent but morphologically cryptic genetic types could, however, have serious repercussions on their use as paleoproxies of past climate change. Phylogenetic analyses of the foraminifera based firstly on a partial ~1,000 bp terminal 3´ fragment of the SSU rRNA gene, and secondly on the ~3,000 bp almost complete gene supported the hypothesis of the polyphyletic origins of the planktonic foraminifera, which appear to be derived from up to 5 separate benthic ancestral lineages. The almost complete gene is sequenced here in the planktonic taxa for the first time, though amplification was problematic. In a first step to addressing a pressing need for new genetic markers to support data gained from the SSU rRNA gene, a culture system was established for the benthic foraminifera, in order to provide a reliable source of DNA for EST library construction or full genome sequencing. Finally, to overcome difficulties associated with the PCR amplification of the foraminifera, a new lysis buffer and DNA extraction procedure was developed. A highly successful buffer was created, allowing high quality DNA to be extracted from foraminiferal specimens, whilst leaving the delicate calcitic shell intact for morphological reference.

Arid environments are resource-limited, with scarcity of water the key limiting factor for plants and their associated fauna. Consequentially bottom-up forces often control food webs, influencing the whole system through high levels of competition. The Sinai Baton Blue butterfly, Pseudophilotes sinaicus, is Critically Endangered, with a tiny endemic distribution in the St Katherine Protectorate, South Sinai, an arid environment. Its range is restricted to that of its sole host plant, the near-endemic endangered Sinai Thyme, Thymus decussatus, leaving the butterfly in a highly fragmented distribution. This study looks into the spatio-temporal variations in quality and abundance of the host plant and its implications for the Sinai Baton Blue. Over the past decade the butterfly has exhibited severe population cycles, with the causes still unclear; it could be due to the fluctuating resource levels with large temporal variation in the quality of thyme and density of inflorescences. The number of flowers significantly influences the larval distribution, indicating that resources play a key role in offspring survivorship. Population viability analysis has also highlighted the importance of management techniques aimed at increasing the butterfly's survivorship. The butterfly population sizes are positively correlated with the total resource area and the number of host plants but not the distance between habitat patches. Population viability analysis also suggests that habitat area is more influential than connectivity in this system driving current dynamics. Overall this project has highlighted the urgent need for conservation focused on improving plant quality within patches in order to save this fragile species.

The Giant African Snail, Achatina (Lissachatina) fulica Bowdich, is a tropical crop pest species with a widespread distribution across East Africa, the Indian subcontinent, Southeast Asia, the Pacific and the Caribbean. It is also a known intermediate host of the rat lungworm, Angiostrongylus cantonensis, which can infect humans and cause eosinophilic meningitis. The phylogenetic position of A. fulica within the Achatinoidea and the Achatinidae was investigated using segments of the nuclear ribosomal (r) RNA cluster, actin and histone 3 genes and the mitochondrial CO1 and 16S rRNA genes. Results from molecular data support the monophyly of the Achatinidae based on the taxa surveyed as well as the morphological distinction of the Eastern Achatina (Lissachatina) from the Western and Central Achatina (Achatina); Lissachatina should therefore be elevated to genus status. The results also show non-monophyly of the Coeliaxidae, Ferussaciidae and Subulinidae; the taxonomy of these families must therefore be reassessed. The extent of genetic diversity in global A. fulica populations was also determined using an SSCP molecular marker developed from the 16S rRNA gene. Results reveal only one haplotype (C) emerged from East Africa and spread globally. The rat lungworm (Angiostrongylus cantonensis) has a parallel distribution with A. fulica, and the possible role of the snail in the spread of the parasite is investigated using a molecular marker derived from the small subunit (SSU) rRNA gene. A survey of the parasite within the route of dispersal of A. fulica detected A. cantonensis only in the Philippines and the French Polynesian territory of Tahiti, the latter of which being the first reported case of A. cantonensis infection for Achatina fulica in that territory. Due to the limited sampling of the snail and the patchy distribution of the parasite, there are insufficient data at this time to assess the role of Achatina fulica in the spread of Angiostrongylus cantonensis.

Neighbouring plants in the locale of an individual plant may help or hinder it in the task of defence against herbivores, depending on their levels of defence (chemical or physical), and their interactions with potential herbivores. Such ‘neighborhood effects’ are part of the complex network of species interactions that structure ecological communities. This thesis sets out to test whether there are neighbourhood effects on insect herbivory among the plants of the Al Jabal Al Akhdar region of Libya. Having chosen to concentrate on the two main species of three study sites, Juniperus phoenicea and Pistacia atlantica, nine plots were mapped in detail and the insect herbivores sampled from focal plants, and then from all plants. Leaves were sampled for chemical analysis of their phenol (tannin) content. The set of insect herbivores collected from plants in the plots were identified to species using the expertise of the staff of the Natural History Museum in London. Some insects recorded are new to Libya, and there are several species not previously recorded as feeding on either of the two plant species studied. Tannin levels were much higher in Pistacia than in Juniperus, and there were effects of elevation as well: plants from middle elevation plots had the highest levels, while those from the lowest elevation at the coast had the lowest levels of tannins. There were clear effects of neighbouring plants on the insects of individual plants, in both Pistacia and Juniperus. These were much more complex effects in Pistacia, but both sets of predictors of insect herbivore density or species richness contained clear signs of neighbourhood effects, where the existence of close neighbours reduced the herbivore load on individual plants. There were no signs of any protective neighbourhood effects of tannin levels. Thus in the plant communities of Al Jabal Al Akhdar, associational avoidance appears to be the major mechanism of neighbourhood effects, rather than associational resistance.

Intestinal schistosomiasis, caused by the trematode parasite Schistosoma mansoni, is a disease of major public health importance in the Lake Victoria region. Accurate information pertaining to the disease's distribution can greatly assist in the maintenance and realignment of existing control strategies. Rapid mapping of disease prevalence is reliant on diagnostic technologies; in the case of intestinal schistosomiasis, traditional stool-based methods are beginning to be complimented with new, rapid diagnostic tools. However, these require extensive validation, in a variety of settings, to determine their efficacy and field-utility. Similarly, diagnostic tests are influenced by other factors, such as the level of endemicity of the parasite, or even intraspecific factors such as genetic diversity. Preliminary research has shown S. mansoni to have high levels of genetic diversity throughout East Africa, although its population dynamics within Lake Victoria have never been explored in detail. Local transmission of the parasite is determined by the presence of compatible snails of the genus Biomphalaria. which act as the intermediate host for S. mansoni. As such, basic distribution mapping of the presence and abundance of Biomphalaria around Lake Victoria will better reveal potential risk areas for transmission; these data could also lead to a greater understanding of the specific environmental conditions preferred by Biomphalaria, allowing for predictive mapping of suitable transmission environments. Sophisticated geostatistical tools have facilitated this process, although new, Bayesian models, which have yielded dividends in parasite mapping, have yet to be applied to intermediate host distribution mapping. There is evidence for species-level differences in compatibility with S. mansoni; the taxonomy of Biomphalaria is confused and confusing, with two suspected species being present in Lake Victoria, but these populations have never been subject to detailed classification scrutiny. Similarly, there may be intraspecific effects on transmission compatibility; population level analysis would elucidate patterns of genetic variation across Lake Victoria. Taking both the genetics and ecological preferences of Biomphalaria into consideration, there may be an effect of scale, which should be explored. Patterns that are found to be significant at the level of Lake Victoria may not hold true at a local scale, crucial for the local transmission biology of the parasite. For example, research has suggested that the presence of other gastropods could reduce the abundance of Biomphalaria, through competitive effects. Diversity may influence the transmission of schistosomiasis at the level of the terminal host as well; it is well known that rodents are capable of acting as reservoirs for S. mansoni, as are numerous nonhuman primates. However, within the context of Lake Victoria, non-human primates have rarely been comprehensively surveyed for the disease, despite the obvious implications for conservation, as well as the potential effect on maintaining local transmission cycles even in the face of treatment campaigns. The research presented in the following thesis sought to address the above themes, relating to the dynamics of S. mansoni and Biomphalaria in Lake Victoria. Through a series of four field expeditions to the Ugandan, Tanzanian and Kenyan shorelines of Lake Victoria, data were collected pertaining to the distribution of S. mansoni in school-age children; 27 schools in Uganda were first surveyed for prevalence and intensity of infection. Questionnaires revealed high levels of migration among the school-children, with high levels of itinerancy also associated with increased risk of being infected with S. mansoni and of missing school-based treatment with praziquantel. Later surveys in Tanzania and Kenya compared prevalence of infection as diagnosed by Kato-Katz stool thick smears against a novel, urine-based, rapid diagnostic called the cathodic cirulating antigen (CCA) dipstick. The CCA performed well, with good agreement against the Kato-Katz diagnostic and high sensitivity and specificity in this high-endemicity environment. However, when the CCA tests were used alongside Kato-Katz thick smears in the Ssesse Islands in Uganda, the agreement was not so good, suggesting that different transmission environments may affect the efficacy of diagnostic tests, and moreover, may result in widely discordant treatment recommendations being put forward. Throughout all of these field expeditions, stool samples were also used for hatching of schistosome eggs and miracidia were collected and stored on Whatman® FT A cards for later molecular analysis. Combined with DNA sequences obtained from cercariae from shedding snails and adult worms passaged in the laboratory, genetic material was obtained from 25 sites along the shoreline of Lake Victoria, representative of all three countries. Using both cytochrome oxidase sub-unit 1 (COl) and microsatellite DNA markers, population genetics analyses revealed extremely high genetic diversity, consistent with previous research, but also low levels of population structuring, with no clear geographical patterns. This contrasted with earlier work which had suggested that the populations of S. mansoni ; in Lake Albert and Lake Victoria appeared segregated; the greater and more extensive sampling presented in this thesis revealed a cross-over of haplotypes between the two lakes, potentially associated again with human migration throughout the region, but also suggesting that adaptation and compatibility with local Biomphalaria populations for successful transmission might be less crucial than previously assumed. In order to investigate the distribution and genetics of Biomphalaria, as well as relate these patterns to the transmission of S. mansoni, malacological surveys were carried out at 223 sites across the Lake Victoria shoreline. Two forms of Biomphalaria, based on shell characteristics, were observed; B. sudanica-like snails were commonly associated in marsh-like habitats whereas B. choanomphala-type snails were more usually found in the lake proper. These observations, together with a number of other environmental variables, were statistically tested using multivariate models within a Bayesian inference framework, a method which has never previously been used on snail intermediate host distributions. The models revealed different factors as being Significant predictors of B. sudanica versus B. choanomphala presence, reinforcing the hypothesis of habitat segregation between the two forms. Closer examination of the two forms of Biomphalaria found suggested intermediate shell forms as well as the more easily recognised B. choanomphala-like and B. sudanica-like forms. In order to elucidate the taxonomy of these various forms, a detailed molecular and morphological assessment was carried out on 7 populations of Biomphalaria. Morphologically, principal component analysis (PCA) of shell measurements, aperture outlines and internal anatomy measurements revealed overlapping groupings, supporting the earlier observation of intermediate forms. CrUcially, there were no clear species-level divisions in the genetic data; the groupings that were observed were not significantly associated with those according to morphology. These data combined to imply that the Biomphalaria in Lake Victoria should be considered one species, with two ecophentoypic variants: B. choanomphala var. choanomphala and B. choanomphala var. sudan;ca. Population level analysis of a total of 29 populations revealed significant levels of population structuring with strong geographical patterns; given these local variations, yet also considering the very different patterns observed for S. mansoni, these findings could suggest that local compatibility is less of a restriction on the spread of the parasite than previously thought.

Some aspects of the biology of Pacifastacus leniusculus and Astacus leptodactylus have been compared. The literature survey shows that considerably more studies have been carried out on P. leniusculus than A. leptodactylus. Although no major differences have been found in the morphology of appendages and mouthparts of the species, structural differences have been found in the abundance of setae on the second maxilliped, in the number of teeth on the mandibles and the crista dentata, and form of the chelipeds. Studies on the environmental tolerance of the species show that both species are able to survive in saline water for long periods of time but they can only increase in number in low salinities. Both species can survive over a wide range of temperatures, but they cannot tolerate temperatures of 34 °C after stepwise acclimation. Although the results do not show a clear difference in the tolerance of P. leniusculus and A. leptodactylus to low oxygen, there are some indications that A. leptodactylus is more tolerant of decreased oxygen tensions than P. leniusculus. By using a non-invasive heart beat monitor on crayfish it has been observed that the frequency of heart beats is extremely variable and can be affected by many factors, such as temperature and salinity. Juveniles of the two species can have a significant impact on plant and macroinvertebrate communities. The results also show that both species can have a negative effect on the recruitment of fish populations in freshwaters by eating fish eggs. Competition experiments show that both juveniles and adults of P. leniusculus are significantly more aggressive than those of A. leptodactylus. The results also show that A. leptodactylus would be eliminated by P. leniusculus if they met in a wild. Adults of the species prey on their juveniles, except the brooding females with stage 2s. This predation occurs in the presence of adequate nutrition. Non-predatory behaviour of the brooding females may indicate the presence of pheromones in the species. Reproductive efficiency of the populations of the species in Britain is as good as any studied elsewhere. In comparison to A. leptodactylus, P. leniusculus has more eggs, but smaller in size. Pleopodal egg development of the species can be reduced from seven months to three months with temperature acclimation, but photoperiod is not a factor in reducing pleopodal egg development. In both species sexual dimorphism was observed between males and females. Males of both species and females of P. leniusculus exhibit allometric or isometric growth during their lives but female A. leptodactylus exhibits isometric growth throughout. Comparison of body parameters shows that P. leniusculus can be considered as a morphologically better species to adapt to environmental conditions than A. leptodactylus because it has large and heavy chelae, and heavy body weight. Both species grow fast, but because P. leniusculus hatches earlier it has an advantage over A. leptodactylus and has bigger juveniles by the end of the first summer. In both species males produce significantly more claw meat than females. Although A. leptodactylus produces significantly more tail meat, males of P. leniusculus produce significantly more claw and total meat. Significant differences occur in the tail meat yield of female A. leptodactylus and in the claw meat yield of female P. leniusculus, but males produce similar amount of meat in winter and summer. The Swedish trappy is very effective method of catching both species over a certain size. Day and night catches show that both species are very active during the day and night.

The only species of crayfish native to the British Isles, i.e. Austropotamobius pallipes, is presently widely distributed throughout much of England, Wales and Ireland. The main controlling factor behind this distribution is the predominantly "bicarbonate" freshwater system of the British Isles. In geological terms, native crayfish are generally found in areas of chalk, carboniferous or magnesium limestone, or drift deposits of boulder clay. More recently, native crayfish have been affected by a general decline in the quality of Britain's freshwater system largely due to sewage effluent and agricultural. By far the majority of native crayfish records are associated with water bodies of very good or good quality, with the result that many populations of A. pallipes are isolated in small, good quality headwaters, due to the degree of pollution in the main rivers acting as "chemical barriers". The last two decades have seen a number of alien species of crayfish imported into England and Wales for aquaculture and culinary purposes. Deliberate and accidental introductions have resulted in the establishment of populations of these species in the wild, the most widespread being the North American signal crayfish Pacifastacus leniusculus and the European slender-clawed crayfish Astacus leptodactylus. An important and devastating consequence of the introduction of alien species of crayfish has been the spread of the crayfish plague fungus Aphanomyces astaci, possibly initiated by infected P. leniusculus. Both A. pallipes and A. leptodactylus are susceptible to the disease, with the result that populations of A. pallipes have been eliminated from, whole lengths of river, such as the River Kennet and the Hampshire River Avon. The nature of the disease and the ease with which it is spread means that crayfish is an ongoing problem in England and Wales, with new populations of A. pallipes regularly becoming affected. The distribution of alien species in the British Isles is an artificial one, initially determined by man, although consideration of water quality may have determined the choice of aquaculture sites. However, established populations of alien species in the wild will be subject to the same constraints as A. pallipes, which will affect their subsequent spread and distribution in the freshwater system of England and Wales. A greater tolerance of environmental factors may allow alien species to inhabit waters not currently occupied by the native species, such as polluted or estuarial waters, and possibly threaten populations of A. pallipes protected by "chemical barriers". This study investigated and compared some of the environmental tolerances of A. pallipes, A. leptodactylus and P. leniusculus and consisted of two parts: 1.) Comparison of the tolerance of A. pallipes, A. leptodactylus and P. leniusculus to four common pollutants; chloride, copper, ammonia and lindane, using lethal and sublethal toxicity tests. No species was found to have a greater overall pollution tolerance on the basis of the toxicants used in this study. Median lethal concentrations (LC50) obtained from lethal studies with stage II juveniles indicated that A. leptodactylus juveniles were most tolerant of chloride, but were very sensitive to lindane. P. leniusculus stage II juveniles were least tolerant of chloride, but of equal or greater tolerance when tested in larger juvenile stages, and were most tolerant of copper. A. pallipes juveniles were very sensitive to copper, and all three species showed a similar sensitivity to ammonia. However, episodic experiments with ammonia and copper indicated that differential short-term tolerance to toxicants, i.e. less than 24 hours, may be important in determining the outcome of episodic pollution events on crayfish populations. Salinity studies indicated high resistance to chloride, with all three species able to tolerate salinities up to 50% seawater, implying freshwater crayfish may be able to tolerate estuarine conditions. Accumulation studies with P. leniusculus inferred resistance to high levels of copper in the environment, with a substantial storage capacity for the metal and a regulatory mechanism for excretion of excess amounts. However, establishment of populations under high saline or high copper conditions would be limited by the lower tolerance of early life stages. Similarly, sublethal studies implied that sublethal toxicant effects on early life stages may be important in determining presence or absence of crayfish from a waterbody, through effects on recruitment. 2.) Comparison of the thermal relations of A. pallipes, A. leptodactylus and P. leniusculus, using tolerance, growth and respiration experiments. Results from tolerance experiments showed that P. leniusculus had a greater overall thermal tolerance, so is more resistant to changes in environmental temperature, such as in waters affected by thermal discharges or in waters affected by drought and reduced flow. In growth experiments P. leniusculus grew faster at all temperatures tested and was predicted to grow at temperatures unsuitable for the other species. Therefore, P. leniusculus would not only be able to survive and grow in conditions unsuitable for A. pallipes and A. leptodactylus, but will also grow faster where favourable conditions exist for all three species. Field experiments showed that, although P. leniusculus juveniles were smaller on release from the female, they were released earlier and their faster growth rate allowed them to maintain a distinct size advantage over A. pallipes juveniles, which was very marked by the end of the growing season. Large size is a key element in the attributes leading to competitive success in other crayfish species. Field observations on mixed populations of plague-free P. leniusculus and A. pallipes indeed show that signal crayfish are superior competitors, eventually eliminating A. pallipes. Therefore temperature effects may be important in determining the outcome of competition in mixed crayfish populations.