Invaders: A Response to Thomas and Palmer, 2015

Recently, a number of studies have highlighted that whilst we are suffering global biodiversity losses, local diversity is actually relatively stable, or in some cases increasing. This is due to a number of reasons, including increasing habitat heterogeneity and crucially the spread of strong generalist competitors. In the analysis conducted by Vellend et al., they found that whilst post disturbance was the only truly positive driver of vegetation change (as we would expect in accordance with the intermediate disturbance hypothesis, on which much of our conservation practice is based), invasion was the only factor which stood out as driving a decline in diversity.

Postdisturbance promotes diversity, whilst invasion seems to limit it. Source: Vellend et al., 2014

However, it is arguable that as the error margins cross zero, it is weak evidence for the derogatory effect of invasive species. Indeed, recent thinking has been that the impacts of invasions are not as significant as we have traditionally thought. Often primary production can be enhanced, and resident species do not always decline, overall less than half of the impacts were found to be statistically significant.  The study which this post focuses on, published earlier this year, focuses on the impact of invasive plant species in Great Britain. Across 479, sites, Thomas and Palmer found that diversity had actually increased, and that the negative impacts of invasive plants on British biodiversity had arguably been exaggerated.

Increased diversity of native species with non-natives, Source: Thomas and Palmer, 2015

Whilst their data is sound, I am inclined to disagree with their sentiment. Invasive species, particularly fauna but some flora as well, have had well documented negative impacts on native British ecosystems. Overall diversity can be a very crude measure of the health or success of an ecosystem, the composition is far more important - particularly if we are looking to safeguard our native biota. Yes, there may have been an increase in biodiversity, but is it the biodiversity we want? These patterns are not unexpected, invaders are particularly successful in frequently disturbed areas, which are also likely to be diverse areas due to the variety of niches to be exploited by fast growing colonisers. Indeed, if these areas are regularly disturbed, it will likely prevent competitive exclusion by the invaders and maintain diversity. This kind of ecosystem is far removed from our native climatic community of oak woodland. Perhaps I'm old fashioned, but I believe that our goals in conservation should be to maintain these native conditions as much as possible, rather than simply to maximise diversity.

Many examples of invaders, outside of the infamous grey squirrel, exist and are key issues in current British conservation.  The American Mink, which has all but wiped out our native Water Voles - diminishing populations by up to 94%. The signal crayfish, larger and more aggressive than to our native white clawed crayfish, are estimated to have reduced native numbers by 95%.  The aptly named Killer Shrimp spreads rapidly throughout freshwater ecosystems and has been shown to drastically decrease invertebrate and fish diversity, leading to simplification of food webs. Plants, as well, have had severe ecological consequences as invaders. Top of the most wanted list is Japanese Knotweed, which overruns fluvial ecosystems as well as much of our urban environment. There is even an invasive mouse which eats the chicks of a critically endangered Albatross species alive, documented in the British Overseas Territory Gough Island. The list goes on. The point is that there are many, many species which have and will have severe impacts on our native biota. The local lens must focus on what is actually happening in these local ecosystems, and we should be aware of what is happening to British wildlife before accepting sweeping generalizations that invaders have "no negative consequences for native diversity".

Purposefully evil looking pictures of invasive species

Habitat Fragmentation 1: The Need for Otter-ly Integrated Aquatic Systems

Habitat fragmentation is among the most blatant and well documented human abuses of our environment. The science behind it is not difficult to get your head around either - ever more isolated populations of both plants and animals are much less able to mingle with other populations and are therefore vulnerable to local extinctions (or extirpations). Simple metapopulation dynamics! There are many stories to tell about which species have suffered from this, and in a series of posts I plan to share a few of them, perhaps those which are not as immediately obvious.

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Eurasian Otters (Lutra lutra) are among Britain's cutest and most loved species. They suffered a very well documented and publicized decline throughout the latter half of the 20th Century, largely due to pesticide use, hunting and habitat loss. Thankfully, due to the ban of some of these pesticides and the implementation of conservation policies in the 80s and 90s, Otters have recovered from near extinction in Britain. As of 2011, Natural England announced that they were once again present in all English counties.  Hooray! But what do they have to say about habitat fragmentation?

Otters provide a perfect example to support the argument for landscape scale ecology, and conservation approaches that consider freshwater systems as an integrated whole.  Otters, among other creatures, utilise all components of the freshwater landscape: ponds and rivers, lakes and backwaters. In a study of the River Glaven in Norfolk, a well kept and highly interconnected river, droppings from the recovering otter population were found in a variety of freshwater habitats - showing distinct patterns throughout the year. In the summer, otters primarily fed on fluvial creatures such as the white clawed crayfish, but during the spring and winter, whilst the crayfish were hibernating and amphibians breeding, their diet consisted of >70% of still-water creatures. The study found evidence of otters in ponds over 1km away from the main river, which shows that even seemingly detached bodies of water are indeed integrated.

Map of otters in the River Glaven catchment, source: Sayer 2014

It is arguable that the otter populations in Norfolk would not have recovered as successfully if the habitat had been more fragmented. It is not only the otters themselves, relying on the ponds to feed them in winter, but the myriad of invertebrate and amphibian species that rely on the existence of a metapopulation to stabilise their demographics. Among those pond inhabitats with metapopulations are Great Crested Newts, Crucian Carp, dragonflies, mayflies, caddis flies... anything from a pond which ends in -fly really. All of these animals and the plants they feed on need a highly connected habitat to support their metapopulations and in turn they stabilise the ecosystem and support fluffier and more high profile predators like the otters. This is the case with any habitat, not just the ponds of Norfolk and as we considered the global picture of environmental change, we must not forget the little guys on the front lines that hold everything together. To those who are opposed to landscape scale ecology....

Upset Otter, source: QuickMeme

An Explanation and an Introduction

The unfortunate pun in the blog's URL is inspired by a whimsical term coined by Robert M. Webster & Bruce Erickson in 1996 as part of a correspondence to Nature, to denote the last individual of a species - an endling. Whilst the term never gained footing in academic circles (or even the dictionary)  it has been featured in exhibitions, essays, articles and books. In contrast, many endlings have themselves become household names, 'Lonesome George' the last Pinta Island Tortoise being the best example of this. Personally, I quite like 'Celia' the last Pyrenean ibex.

The rest of the pun refers to the seemingly "never ending" list of extinctions humanity has already caused. Ofcourse, this list is entirely hypothetical as several studies have suggested that there may be upwards of 5-10 million extant unknown species in addition to the 1.2 million already documented (Miller and Spoolman, 2012). The estimated 86% (Mora et al. 2011) of unknown organisms are subject to the same anthropogenic pressures - global warming, habitat fragmentation and destruction, freshwater acidification - the list goes on. There is no knowing how many endlings have gone unnamed and species lost to all but perhaps the fossil record.

The concept of endlings, to me, reeks of human sentimentality. No doubt, there are good intentions behind bringing media attention to an endling, to try quell the relentless assault on our biosphere. But where have they got us? How many Georges and Celias, Marthas and Benjamins do we have to lose before a true change in our collective global ethos occurs? Through this blog, I hope to change or reinforce this ethos in my readers through discussion of anthropogenic extinctions  and extirpations and their consequences for our ecosystems.

R.I.P Lonesome George, source: Crowded Comics