Metapopulation dynamics and threatened species management: Why does the broad-toothed rat (Mastacomys fuscus) persist?

Many species that exist patchily across the landscape are declining due to incremental losses of their constituent sub-populations and increasing isolation of those that remain. For threatened species with such patchy metapopulation structure, it is particularly important to identify key habitat patches and understand what factors govern their occupancy so that their management can be targeted and effective. In this paper, we describe the spatial and temporal distribution of an endangered population of broad-toothed rats (Mastacomys fuscus) at Barrington Tops, New South Wales, and model its dynamics using metapopulation theory. The study population occurs patchily in swamps on the Barrington Plateau. Using faecal pellet searches, live-trapping and data from previous surveys, we identified 12 swamps where M. fuscus persists and 13 where there has been a history of colonisation and extinction. The species now appears to be entirely absent from seven of these latter swamps. Using logistic regression and model selection procedures, we found the strongest predictor of the presence of M. fuscus to be proximity to the nearest occupied swamp. Persistence declined strongly with swamp isolation, probably due to low success of individuals dispersing through the intervening habitat. These patterns support the interpretation that swamp patches at Barrington Tops contain a functioning but fragile metapopulation of M. fuscus. We predict continued loss of remaining sub-populations in peripheral swamps if current dynamics continue, and recommend research to identify the factors that are limiting dispersal and re-colonisation so that the species’ decline can be slowed and reversed.     

Vulnerability to eutrophication of a semi-annual life history: A lesson learnt from an extinct eelgrass (Zostera marina) population

A semi-annual eelgrass (Zostera marina L.) population became extinct in 2004. It had flourished for many decades at Terschelling in the western Wadden Sea, one of the most eutrophied locations where seagrass growth has been recorded. Semi-annual populations survive the winter season by seed (annual), and by incidental plant survival (semi-annual). We compared seed bank dynamics and fate of plants between this impacted site and a reference site in the winter of 1990-1991. Seed bank density at Terschelling was extremely low (5-35 seeds m⁻²) in comparison to the reference site (>60 seeds m⁻²) and also in comparison to seed bank densities of (semi-)annual eelgrass populations in other parts of the world. Plant survival during winter was nil. Nevertheless, the population more than doubled its area in 1991, implying maximum germination and seedling survival rates. However, from 1992 onwards the decline set in and continued - while the nutrient levels decreased. To establish the cause of the low seed bank density, we conducted a transplantation experiment in 2004 to study the relationship between seed production and macro-algal cover. The transplantation experiment showed a negative relationship between the survival of seed producing shoots and suffocation by macro-algae, which is associated with light limitation and unfavourable biogeochemical conditions. The plants died before they had started to produce seeds. Thus, it is likely that macro-algal cover was responsible for the low seed bank density found in Terschelling in 1990-1991. Both the recorded low seed bank density and absence of incidental plant survival during winter were related to eutrophication. These parameters must have been a severe bottleneck in the life history of the extinct population at the impacted site, particularly as Z. marina seed banks are transient. Therefore we deduce that this population had survived at the edge of collapse, and became extinct after a small, haphazard environmental change. We argue that its resilience during these years must have been due to (i) maximum germination and seedling survival rates and (ii) spatial spreading of risks: parts of the population may have survived at locally macro-algae-free spots from where the area could be recolonised. As a consequence, the timing of the collapse was unpredictable and did not synchronise with the eutrophication process. The lesson learnt for conservation is to recognise that eutrophication may be a cause for seagrass population collapse and its eventual extinction, even years after nutrient levels stabilised, or even decreased.     

Indirect evidence for an extinction debt of grassland butterflies half century after habitat loss

Habitat loss often leads to a substantial decline in species richness. However, the extinction of species is typically not instant, but rather involves a time lag. Species richness in recently disturbed habitats is therefore expected to reflect past rather than current habitat availability, with the set of species eventually going extinct representing extinction debt. We explored current species richness of butterflies (Lepidoptera: Rhopalocera) and burnet moths (Lepidoptera: Zygaenidae) with respect to past and current habitat characteristics. The study was conducted in calcareous grasslands, a habitat that has suffered a rapid decline in Europe during the last century. We showed that species richness of habitat specialists correlated positively with both past (75 years ago) and current habitat area, and was best explained by a model incorporating both of these variables. The independent effect of past habitat area presumably indicates the presence of extinction debt in the study region. Species requiring large habitat areas were more likely to show signs of extinction debt. Species richness of other grassland butterflies (those not confined to the focal habitat type) was not associated with past or current habitat area, the pattern indicating their lower sensitivity to changes in focal habitats. The response to habitat loss in specialist butterflies appears faster than in specialist plants, studied in the same landscape earlier. We conclude that linking patterns of species diversity with the temporal dynamics of habitats substantially improves the understanding of the variation in species diversity, and thus helps to identify species of conservation concern.     

Predation history and vulnerability: Conservation of the stickleback adaptive radiation

Recent adaptive radiations offer special insight into the processes that generate biodiversity. The loss of unique members of such radiations undermines their collective evolutionary potential, as well as the ability of scientists to compare divergent populations, thereby devaluing the entire radiation as a system of study. To illustrate our case, we consider the adaptive radiation of the threespine stickleback fish (Gasterosteus aculeatus) in light of anthropogenic threats. Specifically, we consider the effects of stocking with rainbow trout, Onchorynchus mykiss, on populations of stickleback in lakes of two types in the Cook Inlet Region of Alaska, USA: (1) those in which salmonids are native and (2) those historically devoid of predatory fish. Many populations in this second class exhibit reduced pelvic armor that is, in part, a consequence of the historical absence of piscine predators that prey upon stickleback. Long-term trapping data from 77 lakes show that stocking in lakes where trout are not native is associated with large fluctuations in stickleback abundance, relative to lakes in which trout are native, or to lakes in which trout are neither native nor stocked. Pelvic-reduced populations appear to be at most risk from stocking. We discuss how these populations have responded to, and may yet respond to such threats, and how the information we have on these populations can be used to identify evolutionarily significant units (ESUs: Ryder, 1986), and certainly, distinct populations segments (DPSs: [Green, 2005], [Moritz, 2002] and [Waples, 1991]) deemed worthy of protection under Section 3(15) of the 1973 US Endangered Species Act as amended in 1978. Finally, we consider relevance of our results to the conservation of other recent adaptive radiations.