Biotic Homogenisation

Since my last post about harlequin ladybirds as invasive species here in the UK, I've been reading more widely about this topic. My curiosity takes me on all sorts of twists and turns. I just love it!

Along my travels, I stumbled upon the concept of 'Biotic Homogenisation'. This isn't a term I knew previously, so I thought others might be in the same boat. It is the phenomenon of two or more ecological communities (assemblages of species) becoming more alike over time. 

This occurs through invasions of species into new areas triggering extinctions of others. You're probably already seeing how this connects back to the beautiful, but worrisome, harlequin ladybird in my last post. I'll show you how this concept works through the simplified sequence of foody photos below.

First, imagine the orange and green saucers below are two islands. The communities of species, symbolised by the fruit and veg, on each of these islands evolved in splendid isolation from one another. Due to this, the two islands share no species in common, an unrealisic scenario but it keeps things simple. As you can see, this World houses a grand total of five species.

Biotic Homogenisation
Then, unexpectedly, some individuals from the orange island invade the green island, maybe a bunch get blown over in a freak storm or something. Lucky for them, they find a verdant land and they prosper, forming new populations on the green island as depicted below.

Biotic Homogenisation
Interestingly, as you can see, the green island's local diversity has increased. It now possesses more species than ever before. Superficially, this seems like a good thing. Right? Hmmm... Let's find out...

By chance, these newcommers are good. Really good. They outcompete one of the native species for food resources and it goes extinct. 
Biotic Homogenisation
The diversity of the green island is still high though. In fact, even after this extinction event, the island's overall diversity is still higher than pre-invasion. Phew! Right?

Wrong! Local diversity on the island might still be higher, but take a look overall. This little World of five species is now only four and those communities still remaining are more homogenous. If a global shock occurs, such as climate change, this little Earth now has less diversity overall for natural selection to use to weather the storm.
 
Back in the real World now, you can see the consequences of our actions. Introducing weird and wonderful exotic species into our gardens might superficially appear to increase diversity on a local scale. If these introductions cause extinctions of native species however, the World as a whole becomes a more uniform place and as such a less evolutionarily resilient one.
 
I promise my next post will be more uplifting!

Comments

Post a Comment

Please feel free to comment. Thank you.

Popular posts from this blog

Vegetable Growing Calendar

Image
I've always liked those little gantt chart style sowing calendars you see every so often. You know the ones I'm talking about, those little calendars that show you when to sow and harvest your vegetables, if everything goes according to plan of course. I always look at these calendars and think, gosh this has such potential... But, this one is too difficult to read, that one is missing the vegetables I like, and these ones I'm never going to have in my allotment shed or garage or greenhouse when I need them. Yet despite all that, they still have such potential, if only... Well, this month I took the plunge. Taking inspiration from all the others, I built my own.  I published it here: Vegetable Growing Calendar freely available to all. I'll let it speak for itself.  Before leaving you to explore, I should mention that over the winter you will notice the list of varieties in there grow. This project has encouraged me to slowly begin working my way through my massively di
Read more

Allotment Waterlogging Mitigation

Image
In the last month, our City Council offered us a different allotment plot, one less windswept and possessing a thriving Allotment Association. Hurray! The timing of this couldn't be better from the point of view of observing the pressures the ground faces from Devon's wet winter weather. The shallow sloping plot possesses a thin layer of heavy clay soil over what appears to be shillet. Bad for drainage, it must be said. After rain, the site becomes a quagmire of mud with pools of standing water. Not ideal, but equally not insurmountable. The plot design must mitigate waterlogging while catering for the periods of dry summer weather we occasionally, yes occasionally, get too. This clearly requires careful consideration. After contemplating this question over several nights of fitful sleep, this is the plan I'm actioning. A photo of my progress to date describes it nicely. As you can see, it's a lot of graft, but hopefully worth it. The approach consists of building up a
Read more

Pea and Bean Weevil Mitigation

Image
I'm working a new allotment plot this season and, as I normally do, I planted broad beans. I watched the little seedlings grow, but my glee quickly turned to concern when I noticed a curious problem develop. All the young leaves acquired these little notches along their edges. Below, you can see what I mean, like little peck marks, but they seemed too uniform and neat for pigeon damage... I felt puzzled. So, as I do, I commenced some research and RHS quickly offered the answer: pea and bean weevils ( Sitona lineatus). The leaf damage is caused by adult weevils. They overwinter in field margins and then fly in spring seeking out legumes, preferably beans and peas, on which to feed. Once fed, they lay their eggs in the soil around these same plants. Their larvae then feed on the plant's nitrogen fixing root nodules (housing rhizobia ). To quench your curiosity, you can see clusters of rhizobia filled root nodules growing on the broad bean roots below. My natural next question wa
Read more